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Gallium‐Based Liquid–Solid Biphasic Conductors for Soft Electronics

Advanced Functional Materials

August 2023
33(41)

DOI:10.1002/adfm.202306453

License
CC BY-NC 4.0

Authors:

Manuel Reis Carneiro

Carnegie Mellon University

Carmel Majidi

Carnegie Mellon University

Mahmoud Tavakoli

University of Coimbra

Soft and stretchable electronics have diverse applications in the fields of compliant bioelectronics, textile‐integrated wearables, novel forms of mechanical sensors, electronics skins, and soft robotics. In recent years, multiple material architectures have been proposed for highly deformable circuits that can undergo large tensile strains without losing electronic functionality. Among them, gallium‐based liquid metals benefit from fluidic deformability, high electrical conductivity, and self‐healing property. However, their deposition and patterning is challenging. Biphasic material architectures are recently proposed as a method to address this problem, by combining advantages of solid‐phase materials and composites, with liquid deformability and self‐healing of liquid phase conductors, thus moving toward scalable fabrication of reliable stretchable circuits. This article reviews recent biphasic conductor architectures that combine gallium‐based liquid‐phase conductors, with solid‐phase particles and polymers, and their application in fabrication of soft electronic systems. In particular, various material combinations for the solid and liquid phases in the biphasic conductor, as well as methods used to print and pattern biphasic conductive compounds, are discussed. Finally, some applications that benefit from biphasic architectures are reviewed.

A) Schematic of a stretchable circuit including and LED, based on wrinkled metallic sheets (top) and highly wrinkled stretchable copper conductor in the relaxed state and stretched up to 125% (bottom). Reproduced under the terms of the CC‐BY‐NC‐ND license.[³⁰] Copyright 2014, The Authors, published by Wiley‐VCH GmbH. B ) Stretchability in noncoplanar electronics with serpentine bridge designs. FEM simulation before (35% prestrain) and after (70% applied strain) stretching (top) and fabricated circuits in unstretched (90% prestrain) and stretched (140% tensile strain) states (bottom). Reproduced under the terms of the CC‐BY license.[⁴²] Copyright 2008, The Authors, published by National Academy of Science. C) Stress distribution of periodic kirigami cuts at 0% and 58% strains. Inset shows the fabricated sample under same strain. Adapted with permission.[⁴⁴] Copyright 2015, Springer Nature Limited. D) PDMS embedded with laser‐patterned inclusions of EGaIn in the relaxed state (left) and stretched (right). Reproduced under the terms of the CC‐BY license.[⁵⁹] Copyright 2014, The Authors, published by Wiley‐VCH GmbH. E) Sample prints of soft electronic sensors with carbon black filled PDMS (cPDMS). Adapted with permission.[⁶³] Copyright 2017, IOP Publishing. F) Photographs of the stretchable LED circuitry made from a composite of PDMS@Ag and Ag flakes as the lead connection (red line in the inset) under stretching (left) and folding (right). Adapted with permission.[⁷²] Copyright 2019, American Chemical Society.

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A) Working principle of biphasic conductors. (i) Solid particles lose conductivity as they separate from each other. (ii) Bulk liquid metal tends to bead up to form spherical shapes that minimize surface energy when uncontained. (iii) When solid particles and liquid metal are combined, the LM works as a continuous phase that maintains conductivity between the solid fillers even under strain. B) Various approaches to biphasic conductors have been proposed including mixtures of (i) liquid metal and solid particles in which silver, gold, nickel, or low melting point metals can be used for the solid phase, (ii) micro/nanoscale droplets of liquid metal in a soft polymer matrix, and (iii) ternary combinations of liquid metal, solid particles, and soft polymers.

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Ashby graphic comparing published works on biphasic conductors in terms of their maximum conductivity and strain at their breaking point.

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A) TransM2ix‐made Chinese calligraphy samples written with a traditional Chinese brush on different substrate materials. B) Two graphic works painted with a painting brush on plastic substrate using TransM2ix. C) Lighted LED connected by conductive patterns painted on flexible transparent substrate. Scale bar: 20 mm. Adapted with permission.[⁹⁹] Copyright 2017, American Chemical Society. D) Paper clock made by painting GIN to form a circuit pattern on A4 printer paper. E) Demonstration of GIN‐based paper clock. Adapted with permission.[¹¹⁴] Copyright 2018, Wiley‐VCH GmbH. F) Fabrication of the Ni–EGaIn electronic tattoo. Adapted with permission.[¹¹⁵] Copyright 2019, Wiley‐VCH GmbH. G) Demonstration of biphasic GaInNi printed conductive with complex geometries for functional stretchable electronics. Bionic graphene‐like structure with metal conductive and stretchability. H) High density of serpentine structure with stretchability. I,J) High resolution of stretchable integrated conductive with stretchability and formability. Scale bar: 10 mm. Adapted with permission.[¹¹⁶] Copyright 2019, Wiley‐VCH GmbH. K) The fast and customizable fabrication process of Ni–GaIn circuits printed on Ecoflex substrate via a rolling brush. L) Three different shapes of flexible Ni–GaIn coils. Adapted with permission.[¹¹³] Copyright 2018, Wiley‐VCH GmbH. M,N) Stencil‐printed bGaIn circuits on VHB tape (M; scale bar: 5 mm) and paper (N; scale bar: 2 cm). O,P) Hand‐written bGaIn circuits on high‐porosity foam (O) and a latex balloon (P). Scale bar: 2 cm. Adapted with permission.[¹²⁴] Copyright 2021, Springer Nature.

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REVIEW

www.afm-journal.de

Gallium-Based Liquid–Solid Biphasic Conductors for Soft

Electronics

Manuel Reis Carneiro, Carmel Majidi,* and Mahmoud Tavakoli*

Soft and stretchable electronics have diverse applications in the ﬁelds of

compliant bioelectronics, textile-integrated wearables, novel forms of

mechanical sensors, electronics skins, and soft robotics. In recent years,

multiple material architectures have been proposed for highly deformable

circuits that can undergo large tensile strains without losing electronic

functionality. Among them, gallium-based liquid metals beneﬁt from ﬂuidic

deformability, high electrical conductivity, and self-healing property. However,

their deposition and patterning is challenging. Biphasic material architectures

are recently proposed as a method to address this problem, by combining

advantages of solid-phase materials and composites, with liquid deformability

and self-healing of liquid phase conductors, thus moving toward scalable

fabrication of reliable stretchable circuits. This article reviews recent biphasic

conductor architectures that combine gallium-based liquid-phase conductors,

with solid-phase particles and polymers, and their application in fabrication of

soft electronic systems. In particular, various material combinations for the

solid and liquid phases in the biphasic conductor, as well as methods

used to print and pattern biphasic conductive compounds, are discussed.

Finally, some applications that beneﬁt from biphasic architectures

are reviewed.

1. Introduction

Soft electronics is a rapidly growing discipline with a wide

range of potential applications across multiple ﬁelds such

M. Reis Carneiro, M. Tavakoli

Institute of Systems and Robotics

Department of Electrical and Computer Engineering

University of Coimbra

Coimbra 3030-290, Portugal

E-mail: mahmoud@isr.uc.pt

M. Reis Carneiro, C. Majidi

Soft Machines Lab

Department of Mechanical Engineering

Carnegie Mellon University

Pittsburgh, PA 15213, USA

E-mail: cmajidi@andrew.cmu.edu

The ORCID identiﬁcation number(s) for the author(s) of this article

can be found under https://doi.org/10.1002/adfm.202306453

Wiley-VCH GmbH. This is an open access article under the terms of the

Creative Commons Attribution-NonCommercial License, which permits

use, distribution and reproduction in any medium, provided the original

work is properly cited and is not used for commercial purposes.

DOI: 10.1002/adfm.202306453

as bioelectronics,[1–7 ] wearable comput-

ing,[8–11 ] and soft robotics.[12,13,22,14–21 ] In

recent years, multiple conductive materials

and fabrication approaches have been

proposed for implementation of highly

deformable circuits, some of which can

withstand large tensile strains without

losing electronic functionality.[13,23–26]

Stretchable functionality in electronic

conductors has been accomplished ei-

ther by engineering mechanically de-

formable structures (wrinkles,[27–33 ]

serpentines,[34–42 ] kirigami-inspired

structures[43–52 ]) based on rigid metallic

conductors (as depicted in Figure 1A–C)

or by employing conductive materials

that are intrinsically stretchable, for in-

stance, metallic alloys that are liquid at

room temperature[53–60 ] (Figure 1D) or

elastomers ﬁlled with conductive particles

such as carbon[59,61–67 ] or silver[61,68–74 ]

(Figure 1E,F). This last approach has the

advantage of enabling direct printing of

soft electronic circuits using low cost and

easily scalable methods that contrast with

the complex methods of patterning and

etching conventional rigid metals.[23,59,75 ]

Compared to conductive, particle-ﬁlled, elastomer composites,

liquid metal oﬀers higher electrical conductivity with reduced

electromechanical coupling (gauge factor), superior stretchabil-

ity, and deformability with negligible hysteresis, self-healing ca-

pabilities, and lower resistance to mechanical fatigue.[76] These

properties ensure eﬃcient signal transmission, reduced power

consumption, and increased durability in stretchable electronic

devices.

Common liquid metal-based circuit fabrication techniques

include injection moulding into preformed microﬂuidic

channels,[77–80 ] contact printing,[81–84 ] freeze casting[85,86 ] or

soft lithography.[87–89 ] However, these methods face challenges

related to the high surface tension of liquid metal making it

diﬃcult to pattern complex and intricate designs with high

resolution.[90–92 ] Moreover, these methods commonly involve

complex manual steps, and are therefore labor-intensive and

time-consuming.[75] The ﬂuidic behavior of pristine liquid metal

(LM), combined with its high surface tension, tendency to form

an oxide layer, and limited adhesion to most surfaces have

prevented reliable direct printing of pristine liquid metals, delay-

ing the development of highly conductive, and high-resolution

stretchable circuits.[80]

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Figure 1. A) Schematic of a stretchable circuit including and LED, based on wrinkled metallic sheets (top) and highly wrinkled stretchable copper

The Authors, published by Wiley-VCH GmbH. B ) Stretchability in noncoplanar electronics with serpentine bridge designs. FEM simulation before

(35% prestrain) and after (70% applied strain) stretching (top) and fabricated circuits in unstretched (90% prestrain) and stretched (140% tensile

C) Stress distribution of periodic kirigami cuts at 0% and 58% strains. Inset shows the fabricated sample under same strain. Adapted with permission.[44]

circuitry made from a composite of PDMS@Ag and Ag ﬂakes as the lead connection (red line in the inset) under stretching (left) and folding (right).

To address the challenges associated with fabrication of liquid

metal based stretchable electronics, researchers are exploring var-

ious material architectures and fabrication techniques. Among

them, biphasic material architectures have received increasing

attention during the last couple of years. These material systems

combine advantage of solid materials in terms of fabrication res-

olution and scalability, with self-healing and deformability of liq-

uid conductors.

In some cases, a biphasic architecture is achieved by ﬁrst

printing or patterning a solid-phase metallic conductor on a soft

substrate and then subsequently coating the conductor with liq-

uid metal. This typically requires deposition of metals like gold

or copper through vapor deposition or sputtering techniques.

Moreover, clean room lithography is typically used to pattern

the solid conductive ﬁlm. This is then followed by deposition of

liquid metal conductors that coat, bind, or alloy with the solid

conductor.[93–95]

In other cases, a biphasic ink is ﬁrst synthesized by mixing liq-

uid metal with solid materials and then deposited on a soft sub-

strate. These composites permit direct deposition over a desired

substrate and subsequent patterning.[96,97 ] In some cases, depo-

sition and patterning are simultaneous thanks to digital printing

techniques.[71,97,98 ]

Although both architectures are discussed in this review ar-

ticle, the primary focus is on the second of the two – i.e. solid–

liquid mixtures of gallium-based liquid metals, with a solid phase

matter that may include polymers, or particles, or a combina-

tion of both, as shown in Figure 2. The goal is to obtain a mate-

rial structure that is printable or can be easily deposited through

commonplace techniques similar to other solid phase conductive

composites, while also embed enough liquid phase conductor to

permit withstanding high mechanical strains, and stable behav-

ior over multiple cycles of mechanical strain.

In terms of the solid phase, the addition of particles such as

copper (Cu),[99] nickel (Ni)[100] or even quartz (SiO2)[101 ] to liq-

uid metal has been explored as a strategy to overcome the chal-

lenges associated with ﬂuidic behavior and high surface ten-

sion. By incorporating these particles without the use of poly-

mers, researchers could develop metal amalgams with consid-

erably higher viscosity than the LM, and from a printable paste.

However, this approach does not address problems related with

poor adhesion of LM to substrates and the smearing behavior of

LM traces when not encapsulated.

By mixing LM with a polymer, easy printability and adhe-

sion to various substrates was achieved yet these LM–polymer

compounds require mechanical or thermal sintering to achieve

conductivity, so that the polymer that surrounds the dispersed

LM droplets is ruptured and the LM can ﬂow into continuous

channels.[102–104 ] To overcome this limitation, ternary biphasic

compounds that include LM, polymers, and particles have been

proposed, allowing for sinter-free stretchable conductors that

are nonsmearing, easily printable and patternable through var-

ious methods, and can adhere to multiple substrates as shown

later.[105]

Furthermore, the use of liquid metals in biphasic conduc-

tors can also provide high thermal conductivity,[103,106] which is

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Figure 2. A) Working principle of biphasic conductors. (i) Solid particles lose conductivity as they separate from each other. (ii) Bulk liquid metal tends

to bead up to form spherical shapes that minimize surface energy when uncontained. (iii) When solid particles and liquid metal are combined, the LM

works as a continuous phase that maintains conductivity between the solid ﬁllers even under strain. B) Various approaches to biphasic conductors have

been proposed including mixtures of (i) liquid metal and solid particles in which silver, gold, nickel, or low melting point metals can be used for the

solid phase, (ii) micro/nanoscale droplets of liquid metal in a soft polymer matrix, and (iii) ternary combinations of liquid metal, solid particles,andsoft

polymers.

important for applications such as thermal management in

stretchable devices.[23,107 ] The use of biphasic material architec-

tures in stretchable conductors has also opened new possibilities

for the implementation of ﬂexible and stretchable electronic de-

vices with improved performance and reliability through novel

fabrication techniques such as digitally controlled direct ink writ-

ing or laser patterning.[23,107 ]

The purpose of this review paper is to provide an overview of

recent advances in metal-based biphasic conductor architectures

and their potential applications in various soft electronic systems.

In particular, we will focus on the various material combinations

for the solid and liquid phases in biphasic conductors as well

as the methods used for materials printing and circuit pattern-

ing. Moreover, we will highlight recent applications of the pre-

sented biphasic conductors across various ﬁelds. This review will

provide a comprehensive understanding of the current state-of-

the-art in gallium-based biphasic materials research and provide

some directions for future developments.

2. Liquid–Solid Material Combinations

Various material combinations have been proposed for the solid

and liquid phases used in biphasic conductors. These are pre-

sented in this section and discussed in terms of ﬁnal conductivity

of the biphasic compound and its stretchability.

For the solid phase, metals such as gold (Au), silver (Ag), cop-

per (Cu), iron (Fe), or nickel (Ni) have been widely used due to

their high conductivity and mechanical stability. These metals are

integrated in the biphasic formulations either in the form of mi-

cro and nanoparticles or as patterned thin ﬁlms. For the liquid

phase, alloys of gallium (Ga), indium (In), and other low melting

point metals have been used due to their combination of high

electrical conductivity, low viscosity, and ability to readily wet to

solid metals. One other strategy to prepare biphasic liquid–solid

compounds has been to use a noneutectic mixture of metals with

low melting point such as Ga–In and Ga–Sn, leading to the simul-

taneous coexistence of liquid and solid phases of the alloy and the

separate metals.

2.1. LM Wetted over Metallic Films

Li et al.[108 ] reported selective coating of Galinstan (a gallium–

indium–tin alloy with a −19 °C melting point) over lithography-

patterned Au thin ﬁlms. This method yielded a conductivity of

3.46 ×106Sm

−1and produced circuits that can be twisted more

than 180°or be stretched up to ≈60% for 6000 cycles. Taking ad-

vantage of the wettability properties of liquid metals was further

reported by Hirsch et al.[95] who used gallium to coat thin-ﬁlm Au

structures. Here the authors further explained that the solid and

liquid phases (Au and eutectic gallium–indium (EGaIn), respec-

tively) alloy together, creating an intermetallic phase composed

of AuGa2particles that improved the stretchability of the circuits

up to 400%. Similarly, Kim et al. in 2009[109 ] proposed the use of

eutectic gallium–indium alloy (EGaIn) as a way to reinforce de-

formable, and potentially breakable joints of patterned Au. The

EGaIn was shown to ﬁll the microcracks that occurred in the rigid

metal, preventing electrical failure at strains up to 30%. More re-

cently, by patterning a kirigami structure in an EGaIn-coated Au

ﬁlm, Choi et al.[110 ] presented elastic conductive electrodes that

can withstand 820% strain with an increase of only 33% in elec-

trical resistance.

In 2018, Pan et al.[111] proposed a method to pattern thin

Cu ﬁlms coated with EGaIn into elastic (strain limit >100%)

and optically transparent highly conductive circuits (resistivity =

1.77 ×10−6Ωm). Using selectively wetted EGaIn on Cu traces,

Ozutemiz et al. in 2018[94] introduced a scalable method for fabri-

cation of microelectronic circuits that allows for straightforward

integration and “soldering” of discrete components, leading to

integrated digital circuits with a stretchability of ≈85%.

Regardless of the choice of rigid metal, these architectures

are attractive because they are compatible with conventional

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fabrication techniques that are traditionally used for circuit

manufacturing, including lithographic and printing based tech-

niques. Moreover, leakage of LM is unlikely to happen during

deformation since the liquid metal wets and alloys with the un-

derlying solid traces or particles, thus increasing the durability

of soft circuits.[112 ] Nevertheless, the lithographic methods used

for patterning rigid metals are often complex and labor intensive.

For this reason, there has been growing interest in liquid–solid

biphasic LM compounds that can be directly deposited through

thin-ﬁlm coating, stencil or screen printing, rod coating, laser

patterning, and digital printing. Through the use of common-

place methods and equipment, and eliminating the need for com-

plex lithography techniques, these biphasic inks are desired for

low-cost and scalable fabrication.

2.2. Liquid Metal–Metal Particle Binary Amalgams

As an eﬀort toward addressing the challenges on deposition and

patterning of liquid metal, some groups demonstrated metallic

amalgams that include a mixture of a gallium-based liquid metal,

with microparticles of various metals.

In 2017, Tang et al.[99] proposed a mixture of EGaIn and Cu

particles that react to form a CuGa2intermetallic phase respon-

sible for reducing the ﬂuidity of pristine EGaIn. This increase in

stability of the liquid metal and improved surface wetting enables

direct manual brush-painting of features over various substrates

with an electrical conductivity of 6 ×106Sm

−1,aswellashigh

thermal conductivity of 50 W m−1K−1. To achieve this mixture,

electrical polarization in an alkaline solution (NaOH) is ﬁrst used

to remove the oxide layer from the liquid metal and copper par-

ticles. However, since this wet-processing stage introduces water

into the intermediate products, the resulting biphasic mix un-

dergoes vacuum-drying, leading to stable materials resembling

creams, or pastes.

Guo et al.[ 113] proposed a Ni–EGaIn amalgam with tuneable ad-

hesion to substrates depending on the amount of nickel particles

present in the mixture. This is simply achieved by introducing Ni

particles into EGaIn and manually stirring them until the metal

particles are coated with an oxidized layer of EGaIn. This mixing

process also accelerates the oxidation of EGaIn leading to a lower

surface tension. Thanks to the lower surface tension, this com-

pound could be stencil-printed and has a conductivity on the or-

der of 106Sm

−1and can be stretched up to 60%. Similarly, Chang

et al.[114 ] proposed a conductive Ni–EGaIn mixture (conductivity

of 1.75 ×106Sm

−1) with enhanced adhesion to paper thanks to

EGaIn oxide surrounding the nickel powder. This biphasic mix-

ture referred to as GIN is obtained by adding nickel powder to a

beaker containing EGaIn alloy. With stirring, the nickel powder

gradually agglomerates and partially immerses into the EGaIn

alloy and, after continuous stirring, two liquid layers form in

the beaker. The upper layer becomes stickier and lackluster over

time and is referred to as the GIN material. Daalkhaijav et al.[100 ]

similarly proposed a Ni–EGaIn paste that allows printing of 3D

mechanically stable freestanding vertical structures that show a

conductivity of 1.75 ×106Sm

−1and can undergo strains up to

350%. This compound is achieved by simply mixing and sonicat-

ing together Ni powders of various sizes and Galinstan. In 2019,

Guo et al.[ 115] showed another Ni–EGaIn mixture—based on the

same process presented by Guo et al.[113 ]–with conductivity of

1.61 ×106Sm

−1and soft enough to conform well to the human

skin, while Wu et al.[116] showed a similar material architecture

with conductivity in the range of 105–106Sm

−1and that main-

tained conductivity at strains as high as 300% that is achieved

by incorporating nickel (Ni) nanoparticles into liquid GaIn alloy.

The resulting GaIn–Ni mixture retains the ﬂow characteristics

of the liquid metal, enabling excellent liquid processing perfor-

mance. Upon heat treatment, intermetallic compounds (Ga4Ni3

and InNi3) form through a chemical reaction between Ni and

GaIn, resulting in the formation of the biphasic GaInNi mate-

rial. Lastly, Votzke et al.[ 117] mixed Ni particles with liquid Galin-

stan to form a compound that exhibited a conductivity of 2.97 ×

106Sm

−1and stretchability of up to 200 %. To fabricate this com-

pound, the eutectic gallium alloy is placed on top of the metal

powder, and sonication is used to mix them. The authors explain

that Cavitation bubbles formed during sonication disperse the

nickel particles into the liquid metal, while the implosion of the

bubbles leaves behind gallium oxide sheets.

David and Miki[118 ] demonstrated a method for the synthesis of

Au/AuGa2nanoparticle-based encapsulation of sub-micrometer

Galinstan droplets. Oxide-free galvanic replacement is employed

to encapsulate individual sub-micrometer LM droplets with AU.

The authors show that the biphasic multimetallic framework

could be synthesized directly onto substrates to form microscale

encapsulation-based patterns and demonstrated the possibility of

sintering the biphasic Au-LM droplets to obtain ﬂexible conduc-

tive circuits.

In 2018, Tavakoli et al.[119 ] proposed coating inkjet-printed

traces of silver nanoparticle (AgNP) ink with a thin layer of

EGaIn, thus increasing the electrical conductivity of printed

traces by six-orders of magnitude (up to 4.85 ×106Sm

−1)and

signiﬁcantly improving tolerance to tensile strains up to 80%.

The full process begins by depositing an ink of AgNPs onto a

polymeric ﬁlm using an inkjet printer. Subsequently, drops of

EGaIn (a gallium–indium liquid metal alloy) are deposited onto

the circuit and rubbed with a lint-free cloth to ensure full cov-

erage. Finally, any excess EGaIn is removed by applying a weak

aqueous solution of acetic acid. It was found that the liquid metal

is able to aggregate the AgNPs into larger clusters that lead to

an electrically conductive percolative network. This results in a

room-temperature “sintering” technique that turns nonconduc-

tive AgNP traces into conductive and stretchable lines. Using this

method, they showed ultrathin electronics over temporary tat-

too paper that could be transferred and conformed to complex

3D surfaces. Using a similar technique and synthesis method in

2020, Silva et al.[120 ] demonstrated high-resolution stretchable cir-

cuits with a trace width as low as 20 μm, through combination of

inkjet printing of silver nanoparticles and liquid metal deposi-

tion. The authors showed that the Galinstan coating selectively

wets to the printed AgNPs, resulting in highly conductive (6.65

×106Sm

−1) circuits that can withstand strains up to 100–200%.

2.3. Trinary Biphasic Composites

Although binary compositions of liquid metal and metal parti-

cles permitted a more stable deposition of the amalgam on the

substrate, these amalgams usually lack suﬃcient adhesion to a

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soft substrate. This results in circuits that can easily smear and

necessitates the use of polymer seals or other forms of circuit en-

capsulation. Moreover, fabrication of high-resolution circuits has

not been demonstrated using these amalgams.

For fabrication of reliable circuits using LM-based composites,

several factors should be considered. These include adhesion of

the ink to the substrate, being nonsmearing to touch, and being

able to hold liquid metal particles within the composite, when the

traces are subject to mechanical strain.

In 2021, Lopes et al.[98] implemented a trinary composite,

that included Ag microﬂakes, EGaIn, and matrix of styrene–

isoprene–styrene (SIS) block copolymer. This results in a nons-

mearing, highly adhesive, and elastic biphasic paste that could be

printed using extrusion-based methods. Circuits fabricated with

this biphasic “ink” were shown to be highly stretchable (with a

maximum strain of more than 600%), and to have high conduc-

tivity (7.02 ×105Sm

−1) with a modest gauge factor of 0.9. As

well, the authors demonstrated the important role of the liquid

metal in improving the stretchability (5×increase) and reducing

the gauge factor (≈27×decrease) of the printed conductor, when

compared to a similar formulation without EGaIn and only solid

Ag microﬂakes dispersed on a polymeric binder.

In 2022, Zu et al.[71] studied how the choice of Ag microﬂakes

aﬀects the electrical and electromechanical properties of the Ag–

EGaIn composite proposed by Lopes et al.[98] When comparing

the composite with diﬀerent silver ﬂakes, the authors demon-

strated that there is a 176% relative diﬀerence in conductivity,

>600 % diﬀerence in strain limit, and 277% relative diﬀerence

in electromechanical coupling for the fabricated biphasic com-

pound. Nevertheless, with speciﬁc Ag microﬂakes, Ag–EGaIn–

SIS inks that have conductivity as high as 6.38 ×105Sm

−1and a

strain limit of over 1000%, with low electromechanical coupling

could be synthesized. This same composite was further shown

to have even better stretchability as shown in Lopes et al.[121 ]

(conductivity of 8.21 ×105Sm

−1and maximum stretchability

of 1200%) and Tavakoli et al.[97] (conductivity of 2.10 ×105Sm

−1

and maximum stretchability of 1700%), and prints with resolu-

tion higher than 30 μm have been described. Moreover, in these

more recent works, methods for direct integration of rigid mi-

crochips and surface mount devices (SMD components) were

proposed allowing for implementation of complex stretchable cir-

cuits that can as well be decomposed into its constituent elements

and recycled. SMD chip integrated circuits were shown to have a

stretchability of over 600%, and both components and ink metals

could be recycled.[97]

Hajalilou et al.[105 ] presented a broad study on a series of

biphasic liquid metal composites for sinter-free writing of com-

plex stretchable circuits. Once again, the biphasic microparticle–

EGaIn mixture was dispersed in a block-co-polymer binder and

various solid particles were studied, including Ag ﬂakes, Ag-

coated-Ni, Ag-coated-Fe, Ni, ferrite, or TiC. The authors demon-

strated that a binary combination of Ag–EGaIn or EGaIn–SIS

does not result in the desired properties, and only a trinary com-

bination with conductive microparticles (μPs), preferably Ag, re-

sults in a printable, stretchable, and sinter-free composite. They

also showed that while Ag microparticles (AgμPs) lead to a non-

smearing highly conductive paste, the same did not happen with

Ag-coated-Ni, Ag-coated-Fe which, despite the lower cost and

similar conductivity, lead to more LM smearing out of printed

traces during straining. Moreover, tests with Fe–EGaIn–SIS and

Ni–EGaIn–SIS showed that while the compound was still print-

able and adhesive to the substrate, it was, however, susceptible to

smearing. Finally, by synthesizing a TiC–EGaIn–SIS compound,

using nonconductive TiC μPs, the authors demonstrated that the

solid particles in LM-μP biphasic compounds should be conduc-

tive, to create a percolating network between isolated LM droplets

within the polymeric matrix.

More recently, Reis Carneiro et al.[122] proposed the use of an

eco-friendly water-based polyurethane dispersion (WPU) in con-

junction with EGaIn and Ag microparticles to formulate a bipha-

sic conductive ink with low environmental impact that presents

a conductivity of 1.6 ×105Sm

−1and maximum stretchability

of 200%. Moreover, this composite is shown to adhere to both

polymeric thin-ﬁlm substrates for printing and to conventional

microchips and surface mount components, being used as a soft

solder to enable fully functional soft circuits.

2.4. Noneutectic and Near-Eutectic Alloys

In a noneutectic system, the mixture does not have a single com-

position with the lowest melting point but rather remains solid

over a range of temperatures. This results in the coexistence of

diﬀerent phases, such as solid particles dispersed within a liq-

uid matrix, as seen in noneutectic metallic alloys like Ga–In or

Ga–Sn. Taking advantage of this phenomenon, Liu et al.[ 123] in-

troduced temperature-driven reversible transition from biphasic

to monophasic state in In–Ga binary metallic alloys. The pro-

posed In–Ga composite becomes nonconductive when stretched

yet, when heated the insulating stretched ﬁlms become conduc-

tive. The composite’s resistance decreases with increasing tem-

perature (ranging from 1 to 108Ω), which can be explained by

diﬀerent regions of coexistence of In and Ga in either solid or

liquid phases.

Similarly, researchers from Yale University introduced the con-

cept of “biphasic gallium–indium” (bGaIn).[124 ] In this approach,

while the liquid phase of the biphasic conductive compound is

still EGaIn, the solid phase is composed of thermally treated

EGaIn nanoparticles that become crystalline solids. This biphasic

compound shows a high initial conductivity of 2.06 ×106Sm

−1

and near-constant resistance at strains over 1000%. Furthermore,

it presents high cyclic stability (consistent performance over 1500

cycles) and can also used as a reliable and robust interface with

rigid microelectronic components.

More recently, Timosina et al.[125] introduced a noneutectic

Ga–In alloy (50 wt% Ga and 50 wt% In, Ga50In50) as a shear-

thinning non-Newtonian ﬂuid that shows solid-like properties

when in a quasi-static state and can ﬂow like a liquid metal when

sheared, preventing leakage while allowing the eﬀective fabrica-

tion of biopotential recording electrodes using a stencil. This en-

ables fabrication of biopotential recording electrodes with better

performance and possibility of more complex morphology com-

pared to dry Cu and hydrogel electrodes.

2.5. LM Mixed with Magnetic Particles

By dispersing ferromagnetic Ni μPs in EGaIn through stirring,

Ma et al.[126 ] showed a method for direct patterning of EGaIn over

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[97]

[128]

[119]

[120]

[100]

[114]

[126]

[111] [113]

[117]

[108]

[71]

[116]

[98]

[124]

[121]

Ga In Au Cu Ag

[122]

200

400

600

800

1000

1200

1400

1600

1800

1E+5 1E+6 1E+7 1E+8

Maximum Strain (%)

Conductivity (S/m)

Figure 3. Ashby graphic comparing published works on biphasic conduc-

tors in terms of their maximum conductivity and strain at their breaking

point.

various substrates through a moving magnetic ﬁeld applied to the

LM droplet. The aggregated magnetically responsive μPs deform

the droplet to form a continuous line with conductivity of 3 ×

105Sm

−1and stretchability up to 400%. Zhang et al.[127 ] pro-

posed a similar approach using Fe particles dispersed in EGaIn.

This method was shown to enable fabrication of conductive cir-

cuits with maximum stretchability of 120%, that are easy to repair

by using a magnetic ﬁeld to guide droplets of the conductive mag-

netic biphasic compound. Guo et al.[128] also used Fe particles

in a liquid EGaIn medium, demonstrating the magnetic healing

properties of this compound and presented circuits with conduc-

tivities up to 1.53 ×106Sm

−1and the ability to withstand strains

up to 300%.

The use of magnetic particles as the solid phase in LM bipha-

sic mixtures opens doors to other relevant applications such as

the one as shown by Li et al.[129 ] Here the authors propose a

Galinstan–Fe biphasic mixture dispersed inside PDMS 3D struc-

tures. The use of the magnetic Fe particles enables them to be ori-

ented according to a magnetic ﬁeld, enabling the use of this com-

pound for electromagnetic interference shielding applications

with up to 20% stretchability. Similarly, Ren et al.[130 ] showed that

the viscosity and stiﬀness properties of a Galinstan–Fe biphasic

mixture can change when under the inﬂuence of magnetic ﬁelds

of various intensities.

In 2023, Hajalilou et al.[131 ] demonstrated the beneﬁt of us-

ing ferromagnetic particles i.e. nickel in trinary liquid metal–

polymer–particle composites. They showed that when it comes to

recycling, incorporation of ferromagnetic particles facilitates sep-

aration of microparticle ﬁllers from liquid metal through mag-

netic force, and therefore is a step toward facile recycling of met-

als from liquid metal based soft-matter electronics.

These studies are compared in terms of maximum initial elec-

trical conductivity and maximum stretchability before losing elec-

trical functionality in the plot in Figure 3. To put these char-

acteristics in perspective, the plot also shows the conductivity

for gallium (7.1 ×106Sm

−1), indium (1.2 ×107Sm

−1), gold

(4.11 ×107Sm

−1), copper (5.96 ×107Sm

−1), and silver (6.3 ×

107Sm

−1).

2.6. Biphasic LM Mixtures with Improved Thermal Conductivity

Gallium-based alloys beneﬁt from a high thermal conductivity.

For instance, thermal conductivity of Galinstan is in the range of

≈42–45 W m−1K−1, making it an attractive material for applica-

tions in the areas of electronics cooling, thermal management,

and energy generation and storage. Some researchers have in-

vestigated methods for further improving the thermal conductiv-

ity of liquid metals. Kong et al.[132 ] were able to further enhance

the thermal conductivity of Galinstan by mixing it with tung-

sten (W) particles, thus achieving a thermal conductivity of 57 ±

2.08 W m−1K−1for the W-LM biphasic compound. The authors

explained that W does not alloy with Ga but instead the formation

of a nanometer scale LM oxide layer enables highly nonwetting

W particles to mix into the LM, leading to a stable biphasic com-

pound.

Castro et al.[133] proposed a biphasic mixture of galinstan liq-

uid metal and gadolinium (Gd) solid particles that was observed

to present spontaneous magnetization and large magnetocaloric

characteristics. The LM-Gd ferroﬂuid is presented as a promis-

ing solution for magnetocaloric cooling applications both due to

its high thermal conductivity and the fact that it remains liquid

within the temperature window required for domestic refrigera-

tion purposes.

3. Printing and Patterning Techniques

In this section, various methods for printing and patterning

biphasic compounds are presented and compared in terms of fea-

ture resolution and scalability.

3.1. Brush Painting

With the brush painting method, a brush soaked with the con-

ductive ink is used to create a pattern on a soft substrate, allow-

ing for the creation of custom designs with minimal equipment.

Nevertheless, repeatable designs are diﬃcult to achieve, and the

quality and resolution of the ﬁnal print are highly dependent on

the skill of each user. This technique was used by Tang et al.[99] to

create intricate designs of Cu–EGaIn over various soft substrates

as shown in Figure 4A–C. Chang et al.[ 114] went a step further and

combined brush printing with a precut stencil to allow printing

more repeatable biphasic patterns using Ni–EGaIn ink over pa-

per substrate (Figure 4D,E). Still through manual printing, Guo

et al.[115 ] showed that it is possible to create complex Ni–EGaIn

patterns directly on the skin by ﬁrst drawing the designs using

PMA glue, to which the biphasic LM compound selectively wets

after being rolled over the painted features (Figure 4F).

3.2. Stencil Printing

For stencil printing, a template with the desired pattern is pre-

cut and applied over the substrate. The conductive ink is then

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Figure 4. A) TransM2ix-made Chinese calligraphy samples written with a traditional Chinese brush on diﬀerent substrate materials. B) Two graphic works

painted with a painting brush on plastic substrate using TransM2ix. C) Lighted LED connected by conductive patterns painted on ﬂexible transparent

printed conductive with complex geometries for functional stretchable electronics. Bionic graphene-like structure with metal conductive and stretchability.

H) High density of serpentine structure with stretchability. I,J) High resolution of stretchable integrated conductive with stretchability and formability.

VCH GmbH. M,N) Stencil-printed bGaIn circuits on VHB tape (M; scale bar: 5 mm) and paper (N; scale bar: 2 cm). O,P) Hand-written bGaIn circuits

spread over the stencil, leaving the desired pattern over the sub-

strate. The stencil, which acts as a mask only allowing the bipha-

sic material to be selectively deposited in the desired areas, can

sometimes be reused and allows for low cost and high manufac-

turing scalability. Wu et al.[ 116] proposed the use of a patterned

Kapton mask to create intricate conductive paths of Ni–EGaIn

over a polyacrylate substrate (Figure 4G–J), while Guo et al.[113]

used a similar Ni–EGaIn compound printed via a stainless-steel

mask and Ecoﬂex substrate, with 1 mm resolution as shown in

Figure 4K,L. More recently, Reis Carneiro et al.[61] showed sten-

cil printing of an Ag–EGaIn–elastomer compound directly over a

prefabricated textile spandex glove which, in combination with a

carbon-based ink, allowed for direct integration of pressure and

strain sensors in the textile glove. Liu et al.[124 ] used a stencil to

either transfer-print ﬁlms of bGaIn onto stretchable substrates

or alternatively by rubbing this biphasic EGaIn directly on the

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masked surf (Figure 4M–P). With this method, the authors were

able to achieve a printing resolution as low as 20 μm.

3.3. Photolithography

Photolithography, a well-established patterning technique in the

semiconductor industry, has also been used for fabrication of

high-resolution biphasic patterns. To achieve this, a metallic layer

is sputtered over an elastomeric carrier substrate. A photoresist

is then deposited over the metal and exposed to light through a

photomask, which contains the pattern that needs to be printed.

The light-exposed areas of the photoresist become either more or

less soluble (depending on the type of resist used), and the solu-

ble areas are removed. The now exposed metal areas are etched

and the metallic patterns are covered with the liquid conductive

phase. Li et al.[108 ] showed the use of this technique to imple-

ment Au–Galinstan features down to 10 μm width, as shown in

Figure 5A,B. Similarly, Ozutemiz et al.[93] used this technique

to show scalable fabrication of Cu–EGaIn stretchable circuits

(Figure 5C,D). In this work, after etching a Cu circuit on an

elastomer-coated wafer, the wafer was dip-coated in an EGaIn-

NaOH bath to selectively deposit the EGaIn onto the metal traces.

Hirsch et al.[95] had previously proposed a similar approach

where the photoresist was ﬁrst exposed and developed over a

PDMS substrate, followed by deposition of gold through sput-

tering. Afterward, gallium was deposited over Au through ther-

mal evaporation, forming a continuous thin solid–liquid ﬁlm. Fi-

nally, the hardened exposed resist was lift-oﬀ to reveal conduc-

tive features with high resolution down to 10 μm, as shown in

Figure 5E,F.

3.4. Laser Patterning

Another way of achieving very high resolutions is to use a UV

laser micromachining system for circuit patterning. Pan et al.[111]

employed this technique by using a UV laser to directly pattern

EGaIn–Cu–Cr ﬁlms over a PDMS substrate. This work achieved

outstanding resolution with minimum line widths of 4.5 μm,

which led to the implementation of visually imperceptible con-

ductive grids and circuits with light transmittance as high as

88.7%, as shown in Figure 5G,H. Another work from the same

group[94] (Figure 5I–K) showed laser patterning of Cu ﬁlms fol-

lowed by coating with EGaIn LM. This work demonstrates in-

tegration of rigid components through HCl treatment of the

EGaIn, enabling self-alignment of the SMDs to the printed pads

and reliable LM-SMD connection with improved conductivity.

As an alternative approach using the biphasic Ag–EGaIn ink,

Carneiro et al.[96] (Figure 5L,M), Lopes et al.[121 ] (Figure 5N,O),

and Tavakoli et al.[97] (Figure 5P,Q) have shown a low-cost and

scalable technique for laser patterning of complex circuits on ul-

trathin substrates. A resolution of 30 μm was shows to be achiev-

able using low-cost ﬁber lasers.[97]

3.5. Magnetic Printing

A magnetic-base printing technique has been employed in

Guo et al.[ 128] to print Fe–EGaIn. In this work the au-

thors attach a ball-point pen to a mechanical arm and di-

rectly write fructose patterns over a PVA substrate with 1

mm resolution. A magnet is then used to attract Fe par-

ticles which in turn carry the EGaIn that selectively ad-

heres to the fructose patterns, as shown in Figure 6A,B.

Ma et al.[126 ] also showed that magnetic printing could be em-

ployed for printing of a Ni–EGaIn biphasic compound over an

Ecoﬂex substrate, as shown in Figure 6C,D. This approach avoids

the need for a metallic wetting layer since the magnetic ﬁeld

signiﬁcantly enhances the contact between the liquid metal and

substrate. Moreover, the authors show that this printing method

brings advantages over other methods since it can be applied to

printing over nonplanar surfaces and narrow spaces as no contact

is between the manipulator (magnet) and the magnetic biphasic

compound. A similar method is used by Zhang et al.[127 ] to guide

Fe–EGaIn droplets into creating conductive patterns over smooth

and laser-structured polymeric substrates (Figure 6E–G).

3.6. Inkjet and Laser Printing

Inkjet printing has also been shown to have a role in printing

soft electronic systems based on biphasic compounds. Tavakoli

et al.[119 ] had shown a process by which AgNP ink is printed on

a5μm thick temporary tattoo paper using a conventional desk-

top inkjet printer, and then the printed ink is coated with a thin

layer of EGaIn. A biphasic compound is achieved thanks to the

interaction between Ag and EGaIn, resulting in circuits with a

resolution of 200 μm (Figure 6H–K). Building on this approach,

Silva et al.[120 ] demonstrated biphasic traces with 20 μmresolu-

tion by coating similar inkjet-printed AgNP traces with Galin-

stan. To achieve this high resolution, the substrates were ﬁrst

coated with PVA, which absorbs the AgNP ink but resists wet-

ting of the Galinstan, as shown in Figure 6L–N. This same re-

search team also employed conventional laser printers that were

employed in a similar fashion to the inkjet printer described pre-

viously. Lopes et al.[134] printed the circuit layout with toner over

temporary tattoo paper, as depicted in Figure 6O,P. The toner pat-

tern is then coated with silver epoxy (which does not adhere to the

nonprinted substrate), followed by wetting of EGaIn, which alloys

with the underlying silver. The fabricated circuits can then be hy-

drographically transferred to any 3D surface. This same method

had been used by Alberto et al.[135 ] and Leal et al.[136 ] for various

application that are reviewed in the next section.

3.7. 3D Printing and Direct Ink Writing

3D printing is another method that has been proposed for print-

ing biphasic conductive compounds. This method allows for the

creation of complex freestanding 3D structures as shown by

Daalkhaijav et al.[100 ] In this work the authors proposed incor-

poration of Ni particles in Galinstan to alter the physical proper-

ties of the LM, enabling printing of vertical structures that is not

possible with plain LM (Figure 7A–E). Similarly, Votzke et al.[ 117]

used a Ni–EGaIn compound to 3D print circuits with vertical

vias through a conventional FDM 3D printer adapted to extrude

pastes (Figure 7F–I).

Similar to 3D printing, direct ink writing (DIW) allows for

precise control over the deposition of pastes which are extruded

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Figure 5. A) Fabrication process of the liquid metal pattern embedded in PDMS. B) Complex patterns of liquid metal embedded in PDMS. (a–d)

E) Picture of a biphasic gold–gallium ﬁlm patterned by photolithography with critical dimension of 100 μmona40μm thick PDMS elastomer membrane.

Scale bar: 5 mm. Inset scale bar: 500 μm. F) Stretchable multilayered matrix of green surface mounted light emitting diodes interconnected and powered

through biphasic gold–gallium conductors. Scale bar: 15 mm. Inset: the LEDs are interconnected with two biphasic conductor planes. Scale bar: 2

4.5 μm wide EGaIn traces; the LM patterns are patterned by microscale laser ablation of the thin-ﬁlm architecture (LM/copper/chromium) on a PDMS

substrate. H) Representative circuit showing an exceptional combination of mechanical deformability, electrical functionality, and optical transparency.

circuit consisting of a digital IMU and a digital temperature sensor under deformation. The inset shows the top view of the component–LM interface

at the component pads. K) A photograph of a hybrid stretchable circuit consisting of a 3-axis analog accelerometer under stretch. The inset shows

electrodes through CO2laser-patterning for further transfer to and adhesive dielectric. M) DEA with intricate and detailed shape that can be fabricated

circuits using an accessible IR laser and single-step vapor-assisted soldering of miniaturized components. Q) Example circuit for multidirectional strain

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Figure 6. A) Preparation process of magnetic healing liquid meal electronic on PVA substrate. B) Schematic illustration of the healing process of

GmbH. C) Detailed operation steps for the patterning of LM using magnetic ﬁelds. D) Photographs of the patterned LM on diﬀerent substrate including

planar substrates (e.g., paper, PDMS, hydrogel) and curved surfaces of the eggshell and the inner wall of the glass vial. Scale bars: 10 mm. Adapted

of printing MLM on the smooth domain surrounded by laser-induced microstructures. G) LM “XJTU” printed patterns. Adapted with permission.[127 ]

human skin with an integrated LED on ﬁnger. J) Circuit placed on a toy lemon using hydrographic transfer. K) Functioning LEDs on a soft brain-shaped

Starting from the spin coating of PVA (i), inkjet printing (ii), alloy and cleaning process (iii–ix). M,N) Various examples of applications with diﬀerent line

printer, the circuit template is coated with subsequent layers of Ag paste and EGaIn. P) Process for hydroprinting of the thin-ﬁlm electronics composed

Society.

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Figure 7. A) Metal paste ﬁlament is leaving the needle at various times along a print. B) 3D printed star structure with an overhanging ﬁlament.

C) A ring 20 mm in diameter and 10 mm in height. D) A square 20 mm in width and 7 mm in height. E) A 3D printed circuit encapsulated in Ecoﬂex

liquid metal paste vertical interconnects in various heights. Columns of 2.5, 5, 7.5, 10, and 12.5 mm height with ≈1.25 mm diameter are shown here.

The 12.5 mm tall via proved unstable. Thinner columns are possible with increased vertical printer movement speeds. G) Printed liquid metal arches

demonstrating printed 3D structures. H) Assembled LED oscillator circuit, with printed via to second layer shown in top left. The circuit is now ready for

encapsulation in liquid silicone to the height of the next layer. I) Fully encapsulated LED oscillator circuit in stretched state. Circuit continues to function

forms a solid structure after coming out of the nozzle. L) A simple extrusion-based printer is used for the printing of stretchable inks. The inset shows

the possibility of extrusion of small freestanding geometries. M) Multilayer stretchable circuit made over an SIS substrate. Here, the SIS prepolymer is

applied with a desired thickness, using a thin-ﬁlm applicator, prior to the printing of each layer. N) Circuit printed over the Tegaderm medical adhesive

and transferred to the skin with LED powered on. O) Photo of 200 μm lines printed over an SIS substrate with varying spacing. P) Coils printed over a

Society.

through a nozzle that moves in predesigned paths. Lopes et al.[98]

(Figure 7J–Q) showed the use of DIW to extrude a polymer ﬁlled

with an Ag–EGaIn biphasic mixture that becomes highly stretch-

able and conductive after drying. This work achieved resolutions

up to 200 μm. Using the same Ag–EGaIn–SIS biphasic archi-

tecture, other works have shown the versatility of DIW to create

multiple soft systems for health monitoring[71,105,137 ] or energy

harvesting[138 ] and have also shown how DIW can be integrated

with other methods for implementation of chip-integrated soft

electronics[121 ] that can be recycled.[97]

3.8. Electrochemical Patterning

Li et al.[139 ] introduced a channel-less patterning method for

biphasic mixtures. Here the authors propose a Galinstan–Fe mix-

ture capable of suppressing instabilities during the electrochemi-

cal oxidation process of the compound, which allows for extreme

elongation of the LM core of the mixture to form a thin wire that

is tens of times of its original length when subjected to an elec-

tric ﬁeld. Moreover, it is shown that the elongated LM core can be

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manipulated freely on a 2D plane to form complex patterns and

can also climb a slope, paving the way to future use in 3D print-

ing of complex conductive geometries. In this work a minimum

trace width of 2 mm was achieved and the authors demonstrate

the application of this novel printing method by implementing a

ﬁnger bending sensor.

In summary, there are several printing and patterning tech-

niques available for the fabrication of biphasic compounds with

varying degrees of resolution and scalability. Each of these meth-

ods have their unique combination of advantages and disadvan-

tages. Brush printing, for example, is a straightforward and low-

cost technique that allows for the creation of custom designs but

has limited repeatability and accuracy. By contrast, photolithog-

raphy oﬀer high resolution and accuracy, but at a higher cost

and require specialized equipment. DIW oﬀers a good trade-oﬀ

between repeatability, scalability, and cost, although it does not

have the same ﬁne resolution of photolithography. Laser pattern-

ing, on the other hand, can reach to very high resolutions, while

keeping the cost of equipment and process low. The recently in-

troduced electrochemical patterning, despite the limited resolu-

tion and challenging scalability, shows promise to be further ex-

plored as it allows facile manipulation of LM droplets. Overall, the

choice of technique will depend on the speciﬁc application and

requirements of the biphasic compound. As the ﬁeld of soft elec-

tronics continues to develop, it is likely that new and improved

techniques will be developed, providing even greater opportuni-

ties for the design and fabrication of higher resolution easily cus-

tomizable soft electronic systems.

4. Applications

In this section, we explore the applications of biphasic liquid

metal conductors in greater detail. This includes categorizing the

applications based on their potential uses and functions, includ-

ing sensors and displays, soft actuators, wearables, and energy.

harvesting systems. We examine the latest research ﬁndings and

developments in each category and highlight the potential impact

that biphasic conductors can have across various ﬁelds.

One of the simplest demonstrations to showcase conductivity

of the biphasic compounds is to use them as conductive lines to

power up LEDs and create visual displays. Strain-sensitive trans-

ducers for sensing (usually for detection of strain or pressure)

have also been a common way of conveying the stretchability and

deformability of biphasic LM architectures. This has been shown

for instance by Zhang et al.,[127 ] where the authors implement

a simple LED array with Fe-LM conductive lines, as shown in

Figure 8A. Upon damage, this circuit can be repaired by mag-

netically guiding the biphasic compound to cover the damaged

portion of the conductor and restore electrical conductivity. More-

over, the fabricated circuits in this work have been shown to be

recyclable by ultrasounds that allow to separate all its compo-

nents to fabricate new circuits (Figure 8B). Also, as shown in

(Figure 8C), the authors demonstrate a simple tensile sensor

used to measure ﬁnger bending. Guo et al.[128] implemented a

transient 3 ×4 LED display based on Ge–EGaIn on a PVA sub-

strate that can be dissolved in water as shown in Figure 8D–G. A

similar demonstration was shown by Chang et al.,[114 ] where they

used Ni–EGaIn as a conductor to light up LEDs on a paper model

of a house (Figure 8H). In Figure 8I, Votzke et al.[ 117] demon-

strated a 3D printed resistive strain sensor with integrated read-

out circuitry that displays the level of strain by a color changing

LED. Similarly, Silva et al.[120] showed high-resolution printing of

a serpentine strain gauges as well as interdigitated capacitors and

coils (Figure 8J). Taking advantage of their high-resolution laser

patterning technique, Pan et al.[111] demonstrated an visually im-

perceptive proximity sensor (Figure 8K) and, a smart contact lens

to display air-quality information when placed over an eyeball

camera, as shown in Figure 8L,M. Ozutemiz et al.[94] demon-

strated a stretchable LM circuit with integrated IMU and tem-

perature sensor, as shown in Figure 8N–P. More recently, Reis

Carneiro et al.[122] proposed the use of a trinary biphasic Ag–

EGaIn–polyurethane (PU) water-based ink for fabricating smart

labels for monitoring of safe handling and storage practices of

perishable food items. These printed smart labels can be inte-

grated in existing food packages and can as well be fully recycled,

enabling the fabrication of complex soft electronic systems with

a fully circular life cycle.

Soft robotics and wearable systems represent additional ﬁelds

where biphasic conductors have great potential. These conduc-

tors can be used to create soft and ﬂexible actuators as well as

wearable sensors for controlling and monitoring movement of

robots. Reis Carneiro et al.[96] proposed the use of an Ag–EGaIn–

ﬁlled elastomer as soft compliant thin conductive electrodes for

dielectric elastomer actuators that are nonsmearing, as shown in

Figure 9A,D. Hirsch et al.[95] showed Au–Ga thin ﬁlms promis-

ing not only for implementation of stretchable optoelectronic

transducers (Figure 9E) but also as epidermal strain sensors that

can accurately monitor the bending angle of ﬁnger joints, as de-

picted in Figure 9F. In Figure 9G, a soft matrix of microheaters

wrapped on a human arm is shown, taking advantage of the

high-resolution patterning technique based on lithography that is

shown in this work. Finally, in Figure 9H, a soft cantilever dielec-

tric elastomer actuator is shown using the gold–gallium biphasic

thin ﬁlms as deformable electrodes. Reis Carneiro et al.[61] have

shown that an Ag–EGaIn–elastomer biphasic mixture can be di-

rectly printed on a textile glove to (in combination with a carbon-

based polymer) to create embedded strain sensor in a wearable

system that can detect up to 12 distinct hand gestures to control

the motion of a mobile robot (Figure 9I,J). Using a biphasic mix-

ture of indium and gallium, Liu et al.[124 ] presented a multilayer

signal conditioning circuit board integrated with a stretchable ca-

pacitive sensor that was attached to the surface of a user’s shirt

sleeve to measure the degree of bending of the user’s arm as de-

picted in Figure 9K–N.

One of the most promising ﬁelds that can beneﬁt from bipha-

sic conductors is wearable health monitoring. Taking advantage

of the intrinsic stretchability of biphasic architectures, these con-

ductors can be used to create stretchable and ﬂexible electronic

devices that can be directly interfaced to the human body and

measure a variety of health vitals and physiological signals. For

instance Ma et al.[126 ] showed a simple Ni–EGaIn strain sen-

sor applied over the users neck to accurately monitoring of the

carotid arterial pressure waveform and heart rate as shown in

Figure 10A,B.

Lopes at al.[121 ] proposed a straightforward chip integration

and soldering method for soft electronics using a biphasic Ag–

In–Ga mixture that enabled the fabrication of complex fully in-

tegrated soft-matter circuits, as shown in Figure 10C–E. Here,

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Figure 8. A) LED display being damaged and repaired by MLM droplet. B) Liquid metal circuit recycling. C) Preparation and characterization of a tensile

LED array based on PVA/fructose and Fe–EGaIn. E) Schematic illustration of the structure of the Fe–EGaIn lines on two layers. F) The dissolution and

recycle process of the double-layer LED array. G) The dissolution process of an LED circuit. Reproduced under the terms of the CC-BY license.[128]

digital communication. Yellow indicates minimal strain. Green, blue, purple, and red indicate roughly 50%, 100%, 130%, and 150% strain, respectively.

19 μm, p=500 μm) conductor lines. L) Block diagram and schematic of air quality monitoring system composed of a contact lens display based on an

LM circuit, an eyeball camera, an air quality sensor, and a microprocessor. M) Response sequence of air quality (CO2) measurement corresponding to

“SAFE”-green, “DANGEROUS”-red, “ALEART”-blue, and “SAFE”-green. LED coloring is set according to eCO2 thresholds set at <1000 ppm for “SAFE”

sensor circuit and analog accelerometer circuit based on Cu–EGaIn. O) Functioning IMU +temperature sensor circuit with real-time animated block.

Animated block’s orientation information comes from the IMU while the color of the block shows the temperature sensed from the sensor. P) IMU +

a compliant thermal sensor with an integrated LED display is

shown integrated in a facemask as well as worn directly over

the epidermis. Figure 10F shows a miniaturized soft thermal-

monitoring patch with integrated microchips and Bluetooth con-

nectivity for transferring data to a mobile phone. As shown in

Figure 10G, Hajalilou et al.[105] presented a simple biphasic strain

sensor printed over a medical wound dressing adhesive used

to monitor respiration and chest volume. Tavakoli et al.[97] have

shown multiple applications for monitoring of physiological pa-

rameters based on fully integrated soft patches using Ag–EGaIn

as the base for stretchable printed circuit boards. In Figure 10H,I

a multimodal epidermal sticker for mechanosensing is applied to

the user’s neck and is shown to monitor mild, normal, and deep

respiration, tapping, various neck and torso movements as well

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Figure 9. A,B) Ag–In–Ga ink-based radial DEA in the unactuated and actuated states, where radial expansion of the ink electrodes can be observed.

C) Ag–In–Ga DEA being rubbed with a ﬁnger. D) No smearing, smudging, or displacement of the Ag-LM electrode is observed afterward, nor is the

biphasic gold–gallium conductors as a function of applied uniaxial strain. Scale bar: 2 mm. F) Epidermal ﬂexion sensor skin monitoring the position of

the metacarpophalangeal (MCP) and proximal interphalangeal (PIP) joints of a ﬁnger. Joint angles as a function of time monitored by a soft biphasic

gold–gallium sensor (blue) and Vicon motion capture system (orange). G) Soft matrix of microheaters wrapped on a human arm. Temperature gradients

along the x-axis are indicated by the blue arrow. Infrared images of a 3 ×3 matrix of microheaters on the human forearm while sequentially heating one row

after the other. Scale bars: 10 mm except for zoomed microheater picture: 1 mm. H) Soft cantilever dielectric elastomer actuator prepared with biphasic

gold–gallium electrodes. The actuator is 14 mm long, 1 mm wide, and 120 μm thick. Displacement of the end point as a function of the applied voltage

interconnects and carbon-based strain sensors. J) The glove can control up to ten distinct gestures that control various gaits of a mobile robot. Adapted

attached to the surface of a user’s shirt sleeve. L) Top view of the signal conditioning circuit board. Scale bar: 5 mm. M) Normalized sensor reading

as a function of the capacitive sensor strain up 50% for 10 cycles at diﬀerent strains of the signal conditioning circuit board (0%, 50%, 100%, 150%,

200%). Each shaded region represents ±1 s.d. about the mean (solid line). N) A stretch sensor and signal conditioning circuit board were attached to

a spandex sleeve and stretched together during normal bending motions of a user’s arm. The insets show this sensory sleeve while the user’s arm is

as swallowing. Using the same circuit printing and microchip

integration techniques, this study demonstrated additional epi-

dermal stickers for temperature monitoring (Figure 10J) and ac-

quisition of blood oxygenation (SpO2) data from the user’s chest

(Figure 10K). Moreover, the authors showed a combination of

steps for decomposition of the soft circuits and separation of the

polymer, silver particles and EGaIn in the biphasic compound do

that these can be recycled into new functional circuits, as shown

in Figure 10L. Guo et al.[115] proposed Ni–EGaIn tattoos directly

printed on the epidermis that can be used for thermal moni-

toring (Figure 10M,N) and simple human–machine interfaces

(Figure 10O). Moreover, the authors show that these conductive

tattoos can be printed in very intricate shapes and anywhere in

the body, as depicted in Figure 10P.

The ﬁeld of bioelectronics and electrophysiology can also ben-

eﬁt from the stretchability and skin-conformability of bipha-

sic conductors, as shown by Lopes et al.[134 ] In this work, the

authors proposed digitally printed ≈5μm thick Ag–EGaIn tat-

toos that can be hydrotransferred to any surface, as shown in

Figure 11A. The tattoos are highly conformable to the epidermis,

making them excellent candidates for biopotential electrodes that

are used to measure electromyography signals used to control a

prosthetic hand as depicted in Figure 11B,D. When transferred

to the prosthetic hand itself, the tattoo-like conductors double as

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Figure 10. A) Schematic illustration of the procedure for fabrication of the LM-based strain sensor. B) ΔR/R0measured as a function of time for

integrated sensors, microprocessor, and LED display for temperature measurement. D) Same circuit was transferred to a textile mask and E) applied

to the skin. F) Untethered wireless temperature patch with integrated Bluetooth chip, temperature and humidity sensor, microprocessor, resistors,and

battery, produced through laser patterning and vapor exposure. The patch sends the data to the mobile phone app. Reproduced under the terms of the CC-

applied to the user’s neck. I) Measurement of neck and torso movements. J) Epidermal sticker for temperature monitoring and K) acquisition of blood

oxygenation (SpO2) data from the user’s chest. L) Proposed process for decomposition and recycling of soft circuits based on biphasic Ag-LM conductor.

temperature change of the hand back skin. O) Ni–EGaIn electronic tattoo used for human machine interaction. P) Ni–EGaIn electronic tattoo printed

touch sensors and interfaces for LEDs for straightforward recip-

rocal human–machine interaction.

Wu et al.[71] presented a fully integrated electrocardiography

sensor, shown in Figure 11E,F, where a biphasic Ag–EGaIn

compound is used not only for printed digital communication

lines but also used as the skin-interfacing electrodes. Using

a similar biphasic architecture, Reis Carneiro et al.[137] intro-

duced a combination of materials and manufacturing meth-

ods for implementation of multiple skin-interfacing electronic

patches that can conform to and deform with the human skin

as shown in Figure 11G. The proposed bioelectronic stick-

ers can measure various electrophysiological data including

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Figure 11. A) Process for application of an electronic tattoo over the forearm for EMG signal acquisition. B) The hand can be controlled by the hy-

droprinted circuit over the hands shell which contains C) tactile inputs and surface-mount LEDs for reciprocal human–machine interaction. D) The

able electrocardiography (ECG) system contains an LED that blinks at the same speed as the heart rate measured by the biopotential acquisition module

worn on the chest. Conductive lines and skin-interfacing electrodes are made of Ag–EGaIn. F) Representative signal of the recorded ECG and Detection

conformal nature of the e-patch: it keeps its conformability to the skin even under deformation. H–J) Single lead ECG patch and ECG wave. K) Multilead

ECG patch. L,M) multielectrode EMG patch adhered to the user’s forearm. N) EMG signal acquired from the user’s forearm when performing various

gestures p1–p6. O) Forehead-mounted electrophysiology patch for acquisition of EEG, EOG, and facial EMG. Reproduced under the terms of the CC-BY

Recorded signal segments for Q: ECG, R: EEG with open eyes and S: EEG with closed eyes. Reproduced under the terms of the CC-BY license.[125 ]

electrocardiography (ECG) (Figure 11H–K), electromyography

(EMG) (Figure 11L–N), electroencephalography (EEG), and elec-

trooculography (EOG) (Figure 11O). This work shows that the

biphasic conductor, due to the presence of EGaIn liquid metal,

can better conform to the skin’s microtexture, achieving better

signal quality than conventional electrodes used in clinical set-

ting. Moreover, the authors have shown that the biphasic con-

ductor used as biopotential skin interfacing electrodes maintains

high quality over longer periods that the traditional counterpart

even despite sweating or movement.

Timosina et al.[125] fabricated biopotential recording electrodes

using a biphasic noneutectic Ga–In alloy, oﬀering superior

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performance compared to commercial hydrogel electrodes, dry

electrodes, and conventional liquid metals for electrography

measurements. Moreover, these noneutectic Ga–In electrodes

(Figure 11P–S) are shown to be biocompatible and to maintain

low electrode-skin impedance over long periods of up to 3 days

of use for recording ECG, EMG, and EEG.

In addition to bioelectronics, wearables, and soft robotics,

biphasic conductors have potential applications in other ﬁelds

as well. For example, in the ﬁeld of energy harvesting, bipha-

sic conductors can be used to create stretchable and ﬂexible

antennas with complex geometries for wireless power transfer

to soft electronic systems. Guo et al.[113] presented a Ni–EGaIn

printed wearable healthcare monitor for pulse wave measure-

ment as well as a wearable LED array both powered wirelessly

via a printed energy harvesting coil, as shown in Figure 12A–G.

A similar approach was used by Alberto et al.[135 ] where the

authors demonstrate an untethered, battery-free and ultrathin

(≈5μm) passive “electronic tattoo” that interfaces with the hu-

man skin for acquisition and transmission of electrocardio-

graphy data with excellent signal quality. The device, shown

in Figure 12H–K, relies on a wireless power transfer (WPT)

system, using a printed stretchable Ag–In–Ga coil. The au-

thors also show that the soft biphasic WPT device can pro-

vide more than 300 mW of measured power if it is trans-

ferred over the skin and even works when implanted un-

der the skin, providing up to 100 mW of usable power.

Lopes et al.[98] used biphasic Ag–In–Ga mixtures to imple-

ment a printed NFC circuit that can be used in combina-

tion with a smartphone to wirelessly light up an LED on

the circuit or deploy a website on the smartphone’s web

browser (Figure 12L). With this method, a soft and stretch-

able biphasic RFID antenna can printed for lighting up an

LED through WPT (Figure 12M). Ozutemiz et al.[93] demon-

strated a soft LM UHF patch that works in a passive (battery-

free) mode via power harvesting from an RFID reader’s elec-

tromagnetic ﬁeld and consists of a temperature sensor and a

strain gauge as shown in Figure 12N–P. Figure 12Q shows

the patch being used for wireless detection of wrist ﬂexion.

Maranha et al.[138 ] demonstrated various untethered battery-

free patches powered through far-ﬁeld energy harvesting. Tak-

ing advantage of an Ag–EGaIn biphasic compound, customized

antennas are fabricated over various substrates. This includes

planar inverted-F antennas (PIFA) antennas (Figure 12R–T)

as well as a folded dipole antennas (Figure 12U–W) that

are evaluated and optimized for over-the-body usage in med-

ical environments to power an ECG-monitoring wearable belt

(Figure 12R,W).

Overall, biphasic conductors have the potential to enable

new capabilities in a wide range of applications and indus-

tries and their use has opened new possibilities for the de-

velopment of ﬂexible and stretchable electronic devices with

improved performance and reliability. In fact, as shown in

this section, the electronic circuits proposed for these Ga-

based biphasic architectures can be quite complex and used

for applications that translate across various ﬁelds. More-

over, these applications tend to take advantage of the com-

bined properties of biphasic conductors: high electrical con-

ductivity, stretchability, and adaptability to multiple fabrication

methods.

5. Discussion and Conclusion

In this review, we presented and discussed recent advancements

in biphasic conductor architectures that use gallium or gallium-

based LM alloys. These materials are highly deformable and

maintain high conductivity under extreme mechanical strains.

In this way, they have the potential to open up new possibilities

for the development of ﬂexible and stretchable electronic devices

with improved performance and reliability.

These biphasic architectures address long standing challenges

with handling, deposition, and patterning of liquid metal al-

loys. They include material systems that numerous research

groups have developed that combine advantages of solid conduc-

tors and liquid metals. In addition to discussing these emerg-

ing architectures, this review article covers methods used to

print and pattern biphasic conductive compounds as well as

the potential applications of biphasic conductors in various

ﬁelds such as wearables, energy harvesting or soft robotics and

sensors.

Table 1 summarizes the recent research eﬀorts on liquid–solid

biphasic conductors for stretchable electronics, the methods used

for printing and patterning, and the applications proposed by au-

thors across various ﬁelds.

It can be argued that biphasic architectures that are based on

coating liquid metal over patterned thin-ﬁlm metals, such as Au,

Cu, and Ag, beneﬁt from several advantages including the use of

established lithography techniques, and the possibility of high-

resolution patterning. However, relying on clean room lithogra-

phy and advanced deposition systems can be an obstacle for scal-

able and low-cost fabrication. To address this, a variety of biphasic

inks were synthesized and used for creation of stretchable circuits

via stencil printing, digital printing, contact printing, and laser

patterning. Rapid advances in this category have already resulted

in demonstration of inks that are digitally printable, sinter-free,

adhesive, and nonsmearing. These inks keep the promise for low-

cost and scalable fabrication of stretchable circuits, but the print-

ing resolution and long-term stability in industrial setting should

be yet demonstrated.

Some important factors that impact the usability of LM-based

compounds are the adhesion to substrates, smearing behav-

ior, and the ability to hold LM droplets within the compos-

ite. Nevertheless, these are not consistently and quantitatively

reported in literature. However, strong adhesion between the

substrate and the LM-compound typically relies on the pres-

ence of a polymeric binder (such as SIS or PU) and not the

metals themselves. When it comes to smearing behavior and

the ability to hold the LM within the compound, once more

most articles fail to objectively report these properties, which

can be tested by subjecting a non-encapsulated LM compos-

ite sample to mechanical strain and then performing a smear-

ing test. Yet nonsmearing behavior has usually been reported

as a consequence of higher aﬃnity and adhesion between liq-

uid and solid phases through the formation of intermetal-

lic components (IMCs).[105,131 ] The occurrence of IMCs pro-

motes a homogenous composite and is a barrier against phase

separation. As shown in Table 1, the formation of IMC is

limited to some combinations of solid and liquid phases as

is the case of silver and EGaIn forming AgIn2or gold and

gallium forming AuGa2. In the cases where the solid phase

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Figure 12. A–C) LED array ﬂexible Ni–EGaIn circuit with wireless power transfer. D,E) Wireless Ni–EGaIn pulse wave measurement circuit. F) Record

includes energy harvesting coil, two skin interfacing biopotential electrodes, and a reusable battery-free electronic circuit for data acquisition, processing,

and communication. I) Full ECG monitoring circuit transferred to the human body. J) Prototype transferred to the skin being used to transmit the ECG

information and K) the ECG signal received by the computer with the beats-per-minute (BPM) information. Reproduced under the terms of the CC-BY

to the forearm. Inset image: when a mobile phone approaches the circuit, it lights an LED on the circuit and opens the SPM-ISR website in the mobile

Society. N) Fabricated LM UHF patches containing a UHF chip, strain gauge, and antenna based on Cu–EGaIn. O) Tensile testing of a UHF patch. P)

LM UHF patch temperature sensor characterization plot between 21 and 75 °C. Q) Wireless detection of wrist ﬂexion, with corresponding inset showing

patient monitoring, using three PIFA antennas and a transmitter. S) PIFA antenna and demonstration of bending and twisting. T) Detail of the antennas

integrated into the WoW band (printed textile band for multiparameter vital monitoring) and the RF harvesters and supercapacitor integrated into printed

e-textile circuit. U) One dipole antenna on top of Tegaderm and foam powers an LED on the body. V) Demonstration of the functionality of the ECG

monitoring, against subject movements and punctual blocking of RF. W) Experimental ECG monitoring belt with Bluetooth communication, one dipole

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Tabl e 1 . Summary of recent developments in biphasic solid–liquid LM-based conductors.

Refs.

Conductivity

[S m−1]

Stretchability

[%]

Solid

phase

Liquid

phase

Proposed

applications

Fabrication method

Resolution

Polymeric

binder

Intermetallic

component

[61] – – Ag EGaIn Textile-printed strain sensors for gesture recognition

glove

Stencil printing – SIS AgIn2

[71] 6.38 ×1051000 Ag EGaIn ECG biosticker DIW – SIS AgIn2

[93] – 40 Cu EGaIn Soft passive LM UHF patch for strain and

temperature monitoring

Photolithography;

selective wetting

–– N.R.

[94] – 85 Cu EGaIn IMU and temperature sensor soft readout circuit Laser patterning;

selective wetting

–– N.R.

[95] (<1Ωsq−1) 400 Au Gallium Optoelectronic circuit; ﬁnger mounted strain sensor;

microheater; cantilever DEA

Photolithography;

selective wetting

100 μm– AuGa

[96] – – Ag EGaIn DEA electrodes Laser patterning – SIS AgIn2

[97] 2.10 ×1051700 Ag, Fe EGaIn Recyclability; thermal, mechanical, optical sensing Laser patterning, DIW 30 μm SIS AgIn2

[98] 7.02 ×105600 Ag EGaIn WPT and NFC DIW 200 μm SIS AgIn2

[99] 6.00 ×106– Cu EGaIn – Brush manual printing – – CuGa2

[100] 3.93 ×106350 Ni Galinstan – 3D printing – – N.R.

[105] – >100 Ag, Ni,

EGaIn Strain sensor for respiration monitoring DIW – SIS AgIn2

[108] 3.46 ×106100 Au Galinstan – Photolithography;

selective wetting

10 μm– N.R.

[111] 5.65 ×105100 Cu EGaIn Visually imperceptible touch sensor; smart

transparent contact lens

Laser patterning 4.5 μm– N.R.

[113] 1.00 ×10670 Ni EGaIn Wearable pulse monitor; wearable LED array Stencil printing – – N.R.

[114] 1.75 ×1060 Ni EGaIn LED displays on paper Brush manual printing;

stencil printing

–– N.R.

[115] 1.61 ×106– Ni EGaIn Epidermal tattoos for thermal monitoring and HMI Manual printing;

selective wetting

–– N.R.

[116] 1.00 ×106300 Ni EGaIn – Stencil printing – – Ga4Ni3InNi3

[117] 2.97 ×106200 Ni EGaIn Strain sensor with integrated circuit and LED 3D printing – – N.R.

[119] 4.85 ×10580 Ag EGaIn Temporary hydro transferrable tattoos with LEDs Inkjet printing; selective

wetting

200 μm– N.R.

[120] 6.65 ×106200 AgNP Galinstan Printed capacitor, strain gauge, and a coil Ag inkjet printing;

selective wetting

20 μm– AgIn

[121] 8.21 ×1051200 Ag EGaIn Fully integrated soft thermal monitoring and display

circuits

Laser patterning, DIW – SIS AgIn2

[122] 1.60 ×105200 Ag EGaIn Recyclable soft electronics; smart labels;

electrophysiology and thermal monitoring

biostickers

DIW; stencil 200 μmWPU AgIn

[124] 2.06 ×1061000 GaIn

oxide

GaIn Wearable capacitive strain sensor with integrated

circuit

Stencil printing; 25 μm– N.R.

[125] – – In Ga-In Biopotential recording electrodes Stencil printing – – N.R.

[126] 3.00 ×106400 Ni EGaIn HR strain sensor Magnetic printing – – N.R.

[127] – 120 Fe EGaIn Repairable and recyclable LED array Magnetic printing – – N.R.

[128] 1.53 ×106300 Fe EGaIn Water soluble transient LED array Selective wetting;

magnetic printing

–– N.R

[134] – – Ag EGaIn Hydrotransferrable tattoos for EMG and control of a

prosthetic hand

Toner printing; selective

wetting

–– N.R.

[135] – – Ag EGaIn ECG biosticker with wireless power transfer Toner printing; selective

wetting

–– N.R.

[137] – >30 Ag EGaIn Soft bioelectronic patches and biopotential electrodes DIW – SIS AgIn2

[138] – Ag EGaIn Energy harvesting in wearable bioelectronics DIW – SIS AgIn2

[139] – ≈1000 Fe Galinstan Led array and strain sensors Electrochemically

induced elongation

2mm – N.R.

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consists of copper, it may sometimes lead to the formation

of CuGa2yet oftentimes the IMC does not occur or is not re-

ported.

When comparing the performance between gallium-based

liquid–solid biphasic conductors and pure liquid metals, several

key factors come into play. For instance, gallium-based liquid–

solid biphasic conductors exhibit superior pattern resolution

compared to pure liquid metals. The solid phase present in the

biphasic conductors helps maintain the shape and stability of the

pattern, resulting in sharper edges, ﬁner features, and higher

aspect ratios. By contrast, pure liquid metals, due to their ﬂu-

idic nature, have less deﬁned patterns, making it challenging

to achieve intricate designs and precise fabrication processes.

Moreover, as shown throughout the review, liquid metals are

renowned for their exceptional electrical conductivity, making

them ideal for applications that require eﬃcient electrical path-

ways. Gallium-based liquid–solid biphasic conductors, although

possessing good conductivity, may exhibit slightly lower conduc-

tivity (in the range of 1.6 ×105to 6.65 ×106Sm

−1) compared

to pure liquid metals (gallium - 7.1 ×106Sm

−1, indium - 1.2

×107Sm

−1). However, the diﬀerence in conductivity is often

marginal, and the biphasic conductors still oﬀer suﬃcient elec-

trical performance for many applications. In terms of adhesion,

gallium-based liquid–solid biphasic conductors tend to exhibit

stronger adhesion properties compared to pure liquid metals.

The presence of the solid phase and oftentimes the inclusion

of a third polymeric phase, enhances the bonding strength and

improves the stability of connections with diﬀerent substrates.

Pure liquid metals, while having good surface wetting proper-

ties, may face challenges in achieving long-term stability and re-

liable adhesion. Depending on the speciﬁc requirements of a

given application, gallium-based liquid–solid biphasic conduc-

tors oﬀer a compelling alternative to pure liquid metals, particu-

larly when enhanced pattern resolution and improved adhesion

to substrates are desired. In fact, when it comes to patterning

resolution of biphasic compounds, this is highly dependent on

the process used to pattern the circuits. For example, when using

photolithography and laser patterning, resolutions in the range of

4.5–30 μm are possible, but this relies on the use of expensive mi-

crofabrication equipment. By contrast, methods like DIW, allow

for resolutions of ≈200 μm and can be achieved with lower cost

equipment along with more process ﬂexibility and the possibility

for faster iterative prototyping.

Progress toward commercialization and industrial-scale pro-

duction of liquid metal-based electronics requires further ef-

forts to address remaining challenges in materials composition,

synthesis, patterning, and electronics interfacing. This includes

demonstration of compatibility of these material architectures

with high volume manufacturing methods like screen print-

ing, gravure printing, and roll-to-roll printing. Moreover, one of

the main challenges that is often overlooked in the ﬁeld is re-

liable and scalable techniques for interfacing soft printed cir-

cuits with packaged CPUs and other silicon-based microelectron-

ics. While many groups have focused on problem of deposition

and patterning, further work is needed to have a more com-

plete understanding of techniques for reliable microchip inter-

facing. This is especially important since without the integration

of silicon-based microelectronics, soft-matter electronics are of

limited use.

In conclusion, biphasic conductors represent a promising ap-

proach for the development of stretchable electronics, provid-

ing a solution that enable stable electrical conductivity at high

strains while maintaining high levels of ﬂexibility and stretcha-

bility. Moreover, most biphasic compounds have been shown to

be compatible with multiple fabrication methods ranging from

stencil lithography to 3D printing. However, further research is

needed to optimize biphasic conductors for diﬀerent applications

and to fully realize their potential across various industries.

While biphasic composites seem to address some of the chal-

lenges in scalable fabrication of LM-based circuits, it is impor-

tant to note that these material systems are still in their infancy

and rigorous tests are necessary to evaluate their long-term stabil-

ity. Speciﬁcally, when the fabricated circuits are subject to repet-

itive cycle strain, additional interaction between liquid and solid

phase metals can occur. This may potentially result in alteration

of the composite over time through formation of IMCs, which

can aﬀect electrical conductivity and stretchability of the com-

posite. Therefore, one speciﬁc research direction should be long-

term evaluation of these composites with millions of strain cy-

cles. In this context, researchers should pay close attention to al-

loy compositions and reaction kinetics as these can directly im-

pact the softness and stretchability of the composite over time.

Also, addition of the binder material, as already present in some

of the composites[71,98,105,122 ] seems to be a possible method to

reduce excessive interaction between the metals. Moreover, elec-

tromechanical testing and microstructural analysis of the com-

posites should be systematically performed when developing new

biphasic LM-based conductors as these can help the decision pro-

cess when choosing the most suitable alloy composition and pro-

cessing parameters. In fact, one possible method for evaluating

long-term stability of the composite is performing microstruc-

tural analysis through electron microscopy and elemental analy-

sis after subjecting the sample to repetitive mechanical strain.

Lastly, further eﬀort is required to address environmental con-

cerns regarding e-waste accumulation. Many applications of soft-

matter electronics in bioelectronics and smart packaging are ex-

pected to rely on single-use disposable patches. It is therefore im-

portant to take into account the environmental concerns, and in-

vestigate methods for safe disposal protocols, and low-cost and

ecological recycling methods for biphasic conductors.

With the ongoing research in biphasic stretchable conductors,

and the rapid advances observed in the last decade, we can ex-

pect to signiﬁcant advancements in soft electronic systems with

improved performance and reliability for use in remote health-

care monitoring, soft robotics, and wearable computing.

Acknowledgements

This work was supported by the Fundação para a Ciência e a Tecnolo-

gia (Portuguese Foundation for Science and Technology) through the

Carnegie Mellon Portugal Program (Grant No. SFRH/BD/150691/2020),

and European Research Council, ERC project Liquid3D (Grant No.

101045072). Support also came from the CMU-Portugal project WoW

(45913), through European Regional Development Fund (ERDF) and the

Portuguese State through Portugal 2020 and COMPETE 2020.

Conﬂict of Interest

The authors declare no conﬂict of interest.

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Keywords

biphasic conductors, EGaIn, eutectic gallium-indium, gallium-based liquid

metals, liquid metals, soft electronics, stretchable electronics

Received: June 7, 2023

Revised: July 22, 2023

Published online: August 22, 2023

[1] G. Schiavone, F. Fallegger, X. Kang, B. Barra, N. Vachicouras, E.

Roussinova, I. Furfaro, S. Jiguet, I. Séañez, S. Borgognon, A. Rowald,

Q. Li, C. Qin, E. Bézard, J. Bloch, G. Courtine, M. Campogrosso, S.

P. Lac o u r, Adv. Mater. 2020,32,1.

[2] G.Schiavone,S.P.Lacour,Sci. Transl. Med. 2019,11,1.

[3] K. W. Cho, S.-H. Sunwoo, Y. J. Hong, J. H. Koo, J. H. Kim, S. Baik, T.

Hyeon, D.-H. Kim, Chem. Rev. 2022,122, 5068.

[4] D. Gao, K. Parida, P. S. Lee, Adv. Funct. Mater. 2020,30,1.

[5] Y. Liu, T. Yang, Y. Zhang, G. Qu, S. Wei, Z. Liu, T. Kong, Adv. Mater.

2019,31,1.

[6] C. Choi, Y. Lee, K. W. Cho, J. H. Koo, D.-H. Kim, Acc. Chem. Res. 2019,

52, 73.

[7] X.Tang,Y.He,J.Liu,Biophys. Rev. 2022,3, 011301.

[8] H. Kim, E. Kim, C. Choi, W.-H. Yeo, Micromachines 2022,13, 629.

[9] X. Zhao, Y. Zhou, J. Xu, G. Chen, Y. Fang, T. Tat, X. Xiao, Y. Song, S.

Li, J. Chen, Nat. Commun. 2021,12, 6755.

[10] J. Park, J. Kim, S. Kim, W. H. Cheong, J. Jang, Y. Park, K. Na, Y. Kim,

J. H. Heo, C. Y. Lee, J. H. Lee, F. Bien, J. Park, Sci. Adv. 2018,4,1.

[11] L.Dejace,N.Laubeuf,I.Furfaro,S.P.Lacour,Adv. Intell. Syst. 2019,

1, 1900079.

[12] M. D. Dickey, Adv. Mater. 2017,29, 1606425.

[13] W. Wu, Sci. Technol. Adv. Mater. 2019,20, 187.

[14] M. R. Vinciguerra, D. K. Patel, W. Zu, M. Tavakoli, C. Majidi, L. Yao,

ACS Appl. Mater. Interfaces 2023,15, 24777.

[15] S. Hong, S. Lee, D.-H. Kim, Proc. IEEE Inst. Electr. Electron. Eng. 2019,

2185, 107.

[16] R. Rocha, P.Lopes, A. T. De Almeida, M. Tavakoli, C. Majidi, IEEE Int.

Conf. on Intelligent Robots and Systems, IEEE, Piscataway, NJ 2017.

[17] J. Sgotti Veiga, M. Reis Carneiro, R. Molter, M. Vinciguerra, L. Yao,

C. Majidi, M. Tavakoli, Adv. Mater. Technol. 2023,8, 2300144.

[18] M. R. Carneiro, M. Tavakoli, IEEE Sens. J. 2021,21, 27374.

[19] J. Shu, D.-A. Ge, E. Wang, H. Ren, T. Cole, S.-Y. Tang, X. Li, X. Zhou,

R. Li, H. Jin, W. Li, M. D. Dickey, S. Zhang, Adv. Mater. 2021,33,1.

[20] E.Wang,J.Shu,H.Jin,Z.Tao,J.Xie,S.-Y.Tang,X.Li,W.Li,M.D.

Dickey, S. Zhang, iScience 2021,24, 101911.

[21] N. El-Atab, R. B. Mishra, F. Al-Modaf, L. Joharji, A. A. Alsharif, H.

Alamoudi, M. Diaz, N. Qaiser, M. M. Hussain, Adv. Intell. Syst. 2020,

2, 2000128.

[22] L. Hines, K. Petersen, G. Z. Lum, M. Sitti, Adv. Mater. 2017,29,

1603483.

[23] D. F. Fernandes, C. Majidi, M. Tavakoli, J. Mater. Chem. C 2019,7,

14035.

[24] M. Chen, Z. Wang, K. Li, X. Wang, L. Wei, Adv. Fiber Mater. 2021,3,

[25] K. G. Jordan, J. Clin. Neurophysiol. 2004,21, 341.

[26] Y. Zhao, Y. Ohm, J. Liao, Y. Luo, H.-Y. Cheng, P. Won, P. Roberts,

M. R. Carneiro, M. F. Islam, J. H. Ahn, L. M. Walker, C. Majidi, Nat.

Electron. 2023,6, 206.

[27] Y. Ma, Y. Xue, K. Jang, X. Feng, J. A. Rogers, Y. Huang, Proc. Math.

Phys. Eng. Sci. 2016,472.

[28] B. Wang, S. Bao, S. Vinnikova, P. Ghanta, S. Wang, npj Flexible Elec-

tron. 2017,1,5.

[29] J. Kim, S. Park, T. Nguyen, M. Chu, J. D. Pegan, M. Khine, Appl. Phys.

Lett. 2016,108, 061901.

[30] M. Drack, I. Graz, T. Sekitani, T. Someya, M. Kaltenbrunner, S. Bauer,

Adv. Mater. 2015,27, 34.

[31] Z. F. Liu, S. Fang, F. A. Moura, J. N. Ding, N. Jiang, J. Di, M. Zhang,

X. Lepró, D. S. Galvão, C. S. Haines, N. Y. Yuan, S. G. Yin, D. W. Lee,

R. Wang, H. Y. Wang, W. Lv, C. Dong, R. C. Zhang, M. J. Chen, Q.

Yin, Y. T. Chong, R. Zhang, X. Wang, M. D. Lima, R. Ovalle-Robles,

D. Qian, H. Lu, R. H. Baughman, Science 2015,349, 400.

[32] T.Chen,Y.Xue,A.K.Roy,L.Dai,ACS Nano 2014,8, 1039.

[33] D.-Y. Khang, J. A. Rogers, H. H. Lee, Adv. Funct. Mater. 2009,19,

1526.

[34] T. Pan, M. Pharr, Y. Ma, R. Ning, Z. Yan, R. Xu, X. Feng, Y. Huang, J.

A. Rogers, Adv. Funct. Mater. 2017,27, 1702589.

[35] Y. Zhang, S. Wang, X. Li, J. A. Fan, S. Xu, Y. M. Song, K. Choi, W. Yeo,

W. Lee, S. N. Nazaar, B. Lu, L. Yin, K. Hwang, J. A. Rogers, Y. Huang,

Adv. Funct. Mater. 2014,24, 2028.

[36] Z. Han, Y. Hong, X. Zhu, X. Gu, Y. Sun, J. Phys. Conf. Ser. 2022,2342,

012005.

[37] Y. Zhang, S. Xu, H. Fu, J. Lee, J. Su, K.-C. Hwang, J. A. Rogers, Y.

Huang, Soft Matter 2013,9, 8062.

[38] M. Nasreldin, R. Delattre, B. Marchiori, M. Ramuz, S. Maria, J. L. De

Bougrenet De La Tocnaye, T. Djenizian, APL Mater. 2019,7, 031507.

[39] Q. Ma, Y. Zhang, J. Appl. Mech. Trans. ASME 2016,83, 111008.

[40] N. Qaiser, A. N. Damdam, S. M. Khan, S. Bunaiyan, M. M. Hussain,

Adv. Funct. Mater. 2021,31, 2007445.

[41] M. Li, J. Xia, R. Li, Z. Kang, Y. Su, J. Mater. Sci. 2013,48, 8443.

[42] D.-H. Kim, J. Song, W. M. Choi, H.-S. Kim, R.-H. Kim, Z. Liu, Y. Y.

Huang, K.-C. Hwang, Y.-W. Zhang, J. A. Rogers, Proc. Natl. Acad.

Sci. USA 2008,105, 18675.

[43] P. Won, J. J. Park, T. Lee, I. Ha, S. Han, M. Choi, J. Lee, S. Hong, K.-J.

Cho, S. H. Ko, Nano Lett. 2019,19, 6087.

[44] T. C. Shyu, P. F. Damasceno, P. M. Dodd, A. Lamoureux, L. Xu, M.

Shlian, M. Shtein, S. C. Glotzer, N. A. Kotov, Nat. Mater. 2015,14,

785.

[45] H.Li,W.Wang,Y.Yang,Y.Wang,P.Li,J.Huang,J.Li,Y.Lu,Z.Li,Z.

Wang,B.Fan,J.Fang,W.Song,ACS Nano 2020,14, 1560.

[46] Z.Zhang,Y.Yu,Y.Tang,Y.-S.Guan,Y.Hu,J.Yin,K.Willets,S.Ren,

Adv. Electron. Mater. 2020,6, 1900929.

[47] K. Xu, Y. Lu, S. Honda, T. Arie, S. Akita, K. Takei, J. Mater. Chem. C

2019,7, 9609.

[48] Y. Morikawa, S. Yamagiwa, H. Sawahata, R. Numano, K. Koida, T.

Kawano, Adv. Healthcare Mater. 2019,8, 23.

[49] Z. Song, X. Wang, C. Lv, Y. An, M. Liang, T. Ma, D. He, Y. Zheng, S.

Huang, H. Yu, H. Jiang, T. Ma, D. He, Y. Zheng, S. Huang, H. Yu, H.

Jiang, Sci. Rep. 2015,5, 10988.

[50] H. Xu, M. Zhang, L. Chen, P. Zhang, Y. Jin, W. Wang, J. Microelec-

tromech. Syst. 2023,32, 82.

[51] R. Tang, H. Fu, J. Appl. Mech. Trans. ASME 2020,87,1.

[52] A. B. M. T. Haque, D. Hwang, M. D. Bartlett, Adv. Mater. Technol.

2022,7,1.

[53] R. K. Kramer, C. Majidi, R. J. Wood, Adv. Funct. Mater. 2013,23, 5292.

[54] A. Fassler, C. Majidi, Adv. Mater. 2015,27, 1928.

[55] D. Green Marques, P. Alhais Lopes, A. T. De Almeida, C. Majidi, M.

Tava k o li, Lab Chip 2019,19, 897.

[56] L. Majidi, D. Gritsenko, J. Xu, Front. Mech. Eng. 2017,3,9.

[57] Y.-G. Park, G.-Y. Lee, J. Jang, S. M. Yun, E. Kim, J.-U. Park, Adv.Health-

care Mater. 2021,10, 2002280.

[58] M. Li, Y. Wu, L. Zhang, H. Wo, S. Huang, W. Li, X. Zeng, Q. Ye,

T. Xu, J. Luo, S. Dong, Y. Li, H. Jin, X. Wang, Nanoscale 2019,11,

5441.

[59] T. Lu, L. Finkenauer, J. Wissman, C. Majidi, Adv. Funct. Mater. 2014,

24, 3351.

[60] Z. Ma, Q. Huang, Q. Xu, Q. Zhuang, X. Zhao, Y. Yang, H. Qiu, Z.

Yang, C. Wang, Y. Chai, Z. Zheng, Nat. Mater. 2021,20, 859.

www.advancedsciencenews.com www.afm-journal.de

[61] M. R. Carneiro, L. P. Rosa, A. T. De Almeida, M. Tavakoli, in 2022 IEEE

5th Int. Conf. on Soft Robotics (RoboSoft 2022), IEEE, Piscataway,

NJ 2022, pp. 722–728.

[62] H. J. Choi, M. S. Kim, D. Ahn, S. Y. Yeo, S. Lee, Sci. Rep. 2019,9,

6338.

[63] M. Tavakoli, R. Rocha, L. Osorio, M. Almeida, A. De Almeida, V.

Ramachandran, A. Tabatabai, T. Lu, C. Majidi, J. Micromech. Micro-

eng. 2017,27, 035010.

[64] R. P. Rocha, P. A. Lopes, A. T. De Almeida, M. Tavakoli, C. Majidi, J.

Micromech. Microeng. 2018,28, 034001.

[65] N. P. Kim, Polymers 2020,12, 1224.

[66] S. Gunde, Int. J. Adv. Res. Sci. Commun. Technol. 2023,3, 101.

[67] R. Mikkonen, P.Puistola, I. Jönkkäri, M. Mäntysalo, ACS Appl. Mater.

Interfaces 2020,12, 11990.

[68] H. Sun, Z. Han, N. Willenbacher, ACS Appl. Mater. Interfaces 2019,

11, 38092.

[69] M. R. Carneiro, A. T. De Almeida, M. Tavakoli, IEEE Sens. J. 2020,20,

15107.

[70] J.Park,D.Seong,Y.J.Park,S.H.Park,H.Jung,Y.Kim,H.W.Baac,

M. Shin, S. Lee, M. Lee, D. Son, Nat. Commun. 2022,131,1.

[71] W. Zu, Y. Ohm, M. Reis Carneiro, M. Vinciguerra, M. Tavakoli, C.

Majidi, Adv. Mater. Tech. 2022,7, 2200534.

[72] C. Pan, Y. Ohm, J. Wang, M. J. Ford, K. Kumar, S. Kumar, C. Majidi,

ACS Appl. Mater. Interfaces 2019,11, 42561.

[73] N. Matsuhisa, D. Inoue, P. Zalar, H. Jin, Y. Matsuba, A. Itoh,

T. Yokota, D. Hashizume, T. Someya, Nat. Mater. 2017,16,

834.

[74] M. Park, J. Im, M. Shin, Y. Min, J. Park, H. Cho, S. Park, M.-B. Shim, S.

Jeon, D.-Y. Chung, J. Bae, J. Park, U. Jeong, K. Kim, Nat. Nanotechnol.

2012,7, 803.

[75] I. D. Joshipura, H. R. Ayers, C. Majidi, M. D. Dickey, J. Mater. Chem.

C2015,3, 3834.

[76] Y. Lin, J. Genzer, M. D. Dickey, Adv. Sci. 2020,7, 2000192.

[77] A. Fassler, C. Majidi, Smart Mater. Struct. 2013,22, 055023.

[78] K. Khoshmanesh, S.-Y. Tang, J. Y. Zhu, S. Schaefer, A. Mitchell, K.

Kalantar-Zadeh, M. D. Dickey, Lab Chip 2017,17, 974.

[79] M. Kubo, X. Li, C. Kim, M. Hashimoto, B. J. Wiley, D. Ham, G. M.

Whitesides, Adv. Mater. 2010,22, 2749.

[80] M. D. Dickey, R. C. Chiechi, R. J. Larsen, E. A. Weiss, D. A. Weitz, G.

M. Whitesides, Adv. Funct. Mater. 2008,18, 1097.

[81] A. Tabatabai, A. Fassler, C. Usiak, C. Majidi, Langmuir 2013,29,

6194.

[82] B. A. Gozen, A. Tabatabai, O. B Ozdoganlar, C. Majidi, Adv. Mater.

2014,26, 5211.

[83] K. Doudrick, S. Liu, E. M. Mutunga, K. L. Klein, V. Damle, K. K.

Varanasi, K. Rykaczewski, Langmuir 2014,30, 6867.

[84] C. A. Gunawan, M. Ge, C. Zhao, Nat. Commun. 2014,5, 3744.

[85] A. Fassler, C. Majidi, Lab Chip 2013,13, 4442.

[86] A. Gannarapu, B. A. Gozen, Adv. Mater. Technol. 2016,1,1.

[87] Y. Zhang, H. Duan, G. Li, M. Peng, X. Ma, M. Li, S. Yan, J.

Nanobiotechnol. 2022,20,1.

[88] M. G. Kim, H. Alrowais, S. Pavlidis, O. Brand, Adv. Funct. Mater.

2017,27, 1604466.

[89] M.-G. Kim, C. Kim, H. Alrowais, O. Brand, Adv. Mater. Technol. 2018,

3,1.

[90] Y. Ding, M. Zeng, L. Fu, Matter 2020,3, 1477.

[91] T. Daeneke, K. Khoshmanesh, N. Mahmood, I. A. De Castro, D.

Esraﬁlzadeh, S. J. Barrow, M. D. Dickey, K. Kalantar-Zadeh, Chem.

Soc. Rev. 2018,47, 4073.

[92] L. Hu, L. Wang, Y. Ding, S. Zhan, J. Liu, Adv. Mater. 2016,28, 9210.

[93] K. B. Ozutemiz, C. Majidi, O. B. Ozdoganlar, Adv. Mater. Technol.

2022,7, 2200295.

[94] K. B. Ozutemiz, J. Wissman, O. B. Ozdoganlar, C. Majidi, Adv. Mater.

Interfaces 2018,5, 1701596.

[95] A. Hirsch, H. O. Michaud, A. P. Gerratt, S. de Mulatier, S. P. Lacour,

Adv. Mater. 2016,28, 4507.

[96] M. Reis Carneiro, C. Majidi, M. Tavakoli, Adv. Eng. Mater. 2021,24,

2100953.

[97] M.Tavakoli,P.A.Lopes,A.Hajalilou,A.F.Silva,M.ReisCarneiro,J.

Carvalheiro, J. Marques Pereira, A. T. de Almeida, Adv. Mater. 2022,

34, 2203266.

[98] P. A. Lopes, D. F. Fernandes, A. F. Silva, D. G. Marques, A. T. De

Almeida, C. Majidi, M. Tavakoli, ACS Appl. Mater. Interfaces 2021,

13, 14552.

[99] J. Tang, X. Zhao, J. Li, R. Guo, Y. Zhou, J. Liu, ACS Appl. Mater. Inter-

faces 2017,9, 35977.

[100] U. Daalkhaijav, O. D. Yirmibesoglu, S. Walker, Y. Mengüç, Adv.

Mater. Technol. 2018,3, 1700351.

[101] H. Chang, P. Zhang, R. Guo, Y. Cui, Y. Hou, Z. Sun, W. Rao, ACS

Appl. Mater. Interfaces 2020,12, 14125.

[102] S. Liu, S. Y. Kim, K. E. Henry, D. S. Shah, R. Kramer-Bottiglio, ACS

Appl. Mater. Interfaces 2021,13, 28729.

[103] P. Won, C. S. Valentine, M. Zadan, C. Pan, M. Vinciguerra, D. K.

Patel, S. H. Ko, L. M. Walker, C. Majidi, ACS Appl. Mater. Interfaces

2022,14, 55028.

[104] E. J. Markvicka, M. D. Bartlett, X. Huang, C. Majidi, Nat. Mater. 2018,

17, 618.

[105] A. Hajalilou, A. F. Silva, P.A. Lopes, E. Parvini, C. Majidi, M. Tavakoli,

Adv. Mater. Interfaces 2022,9, 2101913.

[106] M. D. Bartlett, N. Kazem, M. J. Powell-Palm, X. Huang, W. Sun,

J. A. Malen, C. Majidi, Proc. Natl. Acad. Sci. USA 2017,114,

2143.

[107] P. Won, S. Jeong, C. Majidi, S. H. Ko, iScience 2021,24, 102698.

[108] G. Li, X. Wu, D.-W. Lee, Sens. Actuators, B 2015,221,

1114.

[109] H. J. Kim, T. Maleki, P. Wei, B. Ziaie, J. Microelectromech. Syst. 2009,

18, 138.

[110] H. Choi, Y. Luo, G. Olson, P. Won, J. H. Shin, J. Ok, Y. J. Yang, T. Kim,

C. Majidi, Adv. Funct. Mater. 2023,33, 2301388.

[111] C. Pan, K. Kumar, J. Li, E. J. Markvicka, P. R. Herman, C. Majidi,Adv.

Mater. 2018,30, 1706937.

[112] M. H. Malakooti, M. R. Bockstaller, K. Matyjaszewski, C. Majidi,

Nanoscale Adv 2020,2, 2668.

[113] R. Guo, X. Wang, H. Chang, W. Yu, S. Liang, W. Rao, J. Liu, Adv. Eng.

Mater. 2018,20, 1800054.

[114] H. Chang, R. Guo, Z. Sun, H. Wang, Y. Hou, Q. Wang, W. Rao, J. Liu,

Adv. Mater. Interfaces 2018,5, 1800571.

[115] R. Guo, X. Sun, S. Yao,M. Duan, H. Wang, J. Liu, Z. Deng, Adv. Mater.

Technol. 2019,4, 1900183.

[116] Y.-H. Wu, Z.-F. Deng, Z.-F. Peng, R.-M. Zheng, S.-Q. Liu, S.-T. Xing,

J.-Y. Li, D.-Q. Huang, L. Liu, Adv. Funct. Mater. 2019,29, 1903840.

[117] C. Votzke, U. Daalkhaijav, Y. Menguc, M. L. Johnston, IEEE Sens.

2019,19, 3832.

[118] R. David, N. Miki, Nanoscale 2019,11, 21419.

[119] M. Tavakoli, M. H. Malakooti, H. Paisana, Y. Ohm, D. Green

Marques, P. A. Lopes, A. P. Piedade, A. T. Almeida, C. Majidi, Adv.

Mater. 2018,30, 1801852.

[120] A. F. Silva, H. Paisana, T. Fernandes, J. Góis, A. Serra, J. F. J. Coelho,

A. T. de Almeida, C. Majidi, M. Tavakoli, Adv. Mater. Technol 2020,5,

2000343.

[121] P. A. Lopes, B. C. Santos, A. T. De Almeida, M. Tavakoli, Nat. Com-

mun. 2021,12, 4666.

[122] M. Reis Carneiro, A. T. de Almeida, M. Tavakoli, C. Majidi, Adv. Sci.

2023, 2301673.

[123] H. Liu, Y Xin, H. K. Bisoyi, Y. Peng, J. Zhang, Q. Li, Adv. Mater. 2021,

33, 2104634.

[124] S. Liu, D. S. Shah, R. Kramer-Bottiglio, Nat. Mater. 2021,20,

851.

www.advancedsciencenews.com www.afm-journal.de

[125] V. Timosina, T. Cole, H. Lu, J. Shu, X. Zhou, C. Zhang, J. Guo, O.

Kavehei, S.-Y. Tang, Biosens. Bioelectron. 2023,235, 115414.

[126] B. Ma, C. Xu, J. Chi, J. Chen, C. Zhao, H. Liu, Adv. Funct. Mater. 2019,

29, 190137.

[127] C. Zhang, Q. Yang, J. Yong, C. Shan, J. Zhang, X. Hou, F. Chen, Int.

J. Extreme Manuf. 2021,3, 025102.

[128] R. Guo, X. Sun, B. Yuan, H. Wang, J. Liu, Adv. Sci. 2019,6,

1901478.

[129] J. Li, Y. Zhang, X. Li, C. Chen, H. Zou, P. Yi, X. Liu, R. Yu, Nano Res.

2022,16, 1764.

[130] L. Ren, S. Sun, G. Casillas-Garcia, M. Nancarrow, G. Peleckis, M.

Turdy, K. Du, X. Xu, W. Li, L. Jiang, S. X. Dou, Y. Du, Adv. Mater.

2018,30, 1802595.

[131] A. Hajalilou, E. Parvini, J. P. M. Pereira, P. A. Lopes, A. F. Silva, A. De

Almeida, M. Tavakoli, Adv. Mater. Technol. 2023,8, 2201621.

[132] W. Kong, Z. Wang, M. Wang, K. C. Manning, A. Uppal, M. D. Green,

R. Y. Wang, K. Rykaczewski, Adv. Mater. 2019,31, 1904309.

[133] I. A. De Castro, A. F. Chrimes, A. Zavabeti, K. J. Berean, B. J. Carey,

J. Zhuang, Y. Du, S. X. Dou, K. Suzuki, R. A. Shanks, R. Nixon-Luke,

G. Bryant, K. Khoshmanesh, K. Kalantar-Zadeh, T. Daeneke, Nano

Lett. 2017,17, 7831.

[134] P. Filipe, H. Paisana, A. T. de Almeida, C. Majidi, M. Tavakoli, ACS

Appl. Mater. Interfaces 2018,10, 38760.

[135] J. Alberto, C. Leal, C. Fernandes, P. A. Lopes, H. Paisana, A. T. De

Almeida, M. Tavakoli, Sci. Rep. 2020,10, 5539.

[136] C. Leal, P. A. Lopes, A. Serra, J. F. J. Coelho, A. T. De Almeida, M.

Tava k o li, ACS Appl. Mater. Interfaces 2020,12, 3407.

[137] M. R. Carneiro, C. Majidi, M. Tavakoli, Adv. Funct. Mater. 2022,32,

2205956.

[138] M. Maranha, A. F. Silva, P. A. Lopes, A. T. De Almeida, M. Tavakoli,

Adv. Sens. Res. 2023,2, 2200025.

[139] X. Li, L. Cao, B. Xiao, F. Li, J. Yang, J. Hu, T. Cole, Y. Zhang, M. Zhang,

J. Zheng, S. Zhang, W. Li, L. Sun, X. Chen, S.-Y. Tang, Adv. Sci. 2022,

9, 2105289.

Manuel Reis Carneiro received an M.Sc. degree in electrical engineering with a specialization in au-

tomation from the University of Coimbra, and he is currently a dual degree Ph.D. candidate in electrical

and computer engineering at the Carnegie Mellon University and in electrical engineering and intelli-

gent systems at the University of Coimbra. His research interests include soft and printed electronics,

bioelectronic systems, and human–machine interfaces for robotics, sensing, and medical applica-

tions. He is also interested in STEM education and scientiﬁc outreach.

Carmel Majidi is a professor of mechanical engineering at Carnegie Mellon University. He leads the

Soft Machines Lab, which develops novel materials architectures, modeling techniques, and fabrica-

tion methods for soft multifunctional composites and microﬂuidics. His research has applications

to the ﬁelds of soft bioinspired robotics, wearable computing, and mechanics of soft matter.Prior to

joining Carnegie Mellon University, he had postdoctoral fellowships at Princeton and Harvard. He

received his B.S. degree from Cornell University and M.S. and Ph.D. degrees from UC Berkeley.

Mahmoud Tavakoli received an M.Sc. in mechanical engineering from the Sharif University of Technol-

ogy,Iran, and a Ph.D. in electrical engineering from the University of Coimbra, Portugal. He is currently

director of the Soft and Printed Microelectronics Lab at the University of Coimbra, where he is also

Professor at the Department of Electrical Engineering. His research interests include soft-matter thin-

ﬁlm and stretchable electronics, and their applications in health, wearables, HMI, and robotics. More

speciﬁcally he is interested in gallium-based liquid metal composites and techniques for digital print-

ing and patterning of soft-matter hybrid electronics.

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The shift toward sustainable energy has increased the demand for efficient energy storage systems to complement renewable sources like solar and wind. While lithium‐ion batteries dominate the market, challenges such as safety concerns and limited energy density drive the search for new solutions. Liquid metals (LMs) have emerged as promising materials for advanced batteries due to their unique properties, including low melting points, high electrical conductivity, tunable surface tension, and strong alloying tendency. Enabled by the unique properties of LMs, four key scientific functions of LMs in batteries are highlighted: active materials, self‐healing, interface stabilization, and conductivity enhancement. These applications can improve battery performance, safety, and lifespan. This review also discusses current challenges and future opportunities for using LMs in next‐generation energy storage systems. image

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Jan 2025
ADV FUNCT MATER

Liquid metal nanoparticles (LMNPs) are being considered as promising candidates for printable electronics owing to their high conductivity and fluidity, as well as improved wettability comparable to that of bulk LMs. However, the high surface tension and weak adhesion of LMNPs pose formidable challenges to their facile patterning on various substrates for diverse applications. Herein, a new LMNP‐polymer composite is reported for multifunctional printable electronics, which is achieved by introducing α‐lipoic acid (LA)/choline chloride (ChCl) polymerizable deep eutectic solvents (PDESs) as ideal polymer carriers. Simply with the ring‐opening polymerization (ROP) of LA, highly conductive LMNP‐PDES composites can be patterned on the surface of various substrates using facile printing methods. Benefiting from the hydrogen bondings within PDES, the printed LMNP‐PDES traces feature enhance adhesion on substrates and consistent performances under different deformations. More importantly, they can be easily dissolved in ethanol via thermal treatment and reprinted for further use, showing excellent recyclable capability. Demonstrations of stretchable sensor, conformal electrode, and smart actuator validate the versatility and reliability of this LMNP‐PDES composite, ensuring its broad prospects on printable electronics.

Ubiquitous Liquid Metal 3D Printing: From Gas, Liquid to Rigid Media

Article

Full-text available

Nov 2024
ADV FUNCT MATER

Liquid metals (LMs) are opening large spaces for achieving functional 3D printing. However, previous fabrication strategies ever developed so far can only address a specific printing task that has yet to fulfill various needs. Conceiving that all fabrications and their output quality are dominated by the interactions between printing inks and their ambient environment, this review is dedicated to presenting a generalized framework toward the ubiquitous LM 3D printing and summarizing its fabrication category thus integrated. A panoramic view is provided that intentionally administrating the interfacial interactions among different media can guide ink modification and printing optimization. Further, the interactions between LM inks and various media are interpreted, ranging from gases, and liquids to soft matters, biological tissues, and even rigid materials, exploring key mechanisms such as oxidation facilitation, heat dissipation, structural support, modulus matching, strong wettability, and high reactivity. Following that, the LM inks, typical printing technologies, diverse working media, and enabled applications are reviewed. The elucidation of these interactions, particularly physical and chemical effects, can lead to the incubation of future LM 3D printing centers. It is expected that interactional LM 3D printing can mold additive manufacturing into ever‐powerful tools in the coming time.

Activation-independent biphasic liquid metal conductor enables multilayer stretchable electronics

Article

Jun 2025
MATER TODAY

Wear-less Mo coating achieved by the formation of Ga-Al lubricating film at current-carrying friction

Article

May 2025
APPL SURF SCI

“Heat‐Press‐N‐Go” Stretchable Interconnects Enabled by Liquid Metal Conductor with Supramolecular Confinement

Article

Full-text available

Jan 2025
ADV FUNCT MATER

The integration of soft, conformable components and rigid microelectronics or devices is a critical frontier in stretchable hybrid device development. However, engineering interconnects capable of tolerating high‐stress concentrations and preventing debonding failures remain a key challenge. Here a stretchable conductive interconnect derived from the liquid metal conductor with supramolecular confinement is reported, capable of reliably connecting soft and rigid parts through a simple “Heat‐Press‐N‐Go” method. Leveraging the dynamic bonding nature of supramolecular polymers, when confined within liquid metal compartments, not only effectively stabilizes the conductive path of the stretchable interconnect, but also offers high adhesion to diverse surfaces, reaching an exceptional electrical stretchability of up to 2800%. As proof of concept, this interconnect is used to assemble wearable devices including reconfigurable stretchable circuits, multifunctional sensors, and on‐skin electromyography, exhibiting high signal integrity and mechanical durability. The “Heat‐Press‐N‐Go” chip and circuit integration offers the boundless potential to enhance the adaptability, convenience, and versatility of on‐skin and wearable electronics across various applications.

Stretchable Shape Sensing and Computation for General Shape-Changing Robots

Article

Oct 2024

An ideal robot could autonomously complete diverse tasks such as ocean surveying, kitchen cleaning, and aerial environmental monitoring. However, robots optimized for each task typically have different shapes, posing a challenge in reconciling form and function. This challenge inspires the pursuit of general shape-changing robots (GSCRs). While soft materials and actuators are promising for GSCRs due to their ability to accommodate extreme deformations, there is a gap between the vision of GSCRs and the simple examples we see today. Two critical components are needed: robot-agnostic stretchable shape sensing and stretchable computing. Together, these components would enable closed-loop shape control and the first instantiations of GSCRs. This review aims to consolidate the literature on these components, encouraging researchers to bridge the gap between today's shape-changing robots and the envisioned GSCRs, ultimately advancing the field toward more versatile and adaptive robots.

Recyclable Thin‐Film Soft Electronics for Smart Packaging and E‐Skins

Article

Full-text available

Jul 2023

Despite advances in soft, sticker‐like electronics, few efforts have dealt with the challenge of electronic waste. Here, this is addressed by introducing an eco‐friendly conductive ink for thin‐film circuitry composed of silver flakes and a water‐based polyurethane dispersion. This ink uniquely combines high electrical conductivity (1.6 × 10⁵ S m⁻¹), high resolution digital printability, robust adhesion for microchip integration, mechanical resilience, and recyclability. Recycling is achieved with an ecologically‐friendly processing method to decompose the circuits into constituent elements and recover the conductive ink with a decrease of only 2.4% in conductivity. Moreover, adding liquid metal enables stretchability of up to 200% strain, although this introduces the need for more complex recycling steps. Finally, on‐skin electrophysiological monitoring biostickers along with a recyclable smart package with integrated sensors for monitoring safe storage of perishable foods are demonstrated.

Highly Stretchable and Strain‐Insensitive Liquid Metal based Elastic Kirigami Electrodes (LM‐eKE)

Article

Full-text available

Apr 2023
ADV FUNCT MATER

Kirigami, a traditional paper‐cutting art, is a promising method for creating mechanically robust circuitry for unconventional devices capable of extreme stretchability through structural deformation. In this study, this design approach is expanded upon by introducing Liquid Metal based Elastic Kirigami Electrodes (LM‐eKE) in which kirigami‐patterned soft elastomers are coated with eutectic gallium‐indium (EGaIn) alloy. Overcoming the mechanical and electrical limitations of previous efforts with paper‐like kirigami, the all soft LM‐eKE can be stretched to 820% strain while the electrical resistance only increases by 33%. This is enabled by the fluidic properties of the EGaIn coating, which maintains high electrical conductivity even as the elastic substrate undergoes extreme deformation. Applying the LM‐eKE to human knee joints and fingers, the resistance change during physical activities is under 1.7%, thereby allowing for stable electrical operation of wearable health monitoring devices for tracking electroencephalogram (EEG) signals and other physiological activity.

Toward Fully Printed Soft Actuators: UV‐Assisted Printing of Liquid Crystal Elastomers and Biphasic Liquid Metal Conductors

Article

Full-text available

Apr 2023

Development of soft and compliant actuators has attracted tremendous attention due to their use in soft robotics, wearables, haptics, and assistive devices. Despite decades of progress, the goal of entirely digitally‐printed actuators has yet to be fully demonstrated. Digital printing permits rapid customization of the actuator's geometry, size, and deformation profile, and is a step toward mass customization of user‐specific wearables and soft robotic systems. Herein, a set of materials and methods are demonstrated for rapid fabrication of 3D‐printed Liquid Crystal Elastomer actuators electrically stimulated via a printed joule heater composed of a Liquid Metal (LM)‐filled elastomer composite. Unlike other Ag‐based inks, this LM‐elastomer composite is sinter‐free, enabling room‐temperature printing, and is stretchable, allowing cyclic actuation without electrical or mechanical failure of the conductor. By optimizing printing parameters, and improving the photo‐polymerization setup, a printed actuator that bends to an angle of 320° is demonstrated, with lower power consumption than previous LCE actuators. We also demonstrate a customized UV‐polymerization setup that permits photo‐curing of the LCE actuator in ≈90s, i.e., >500x faster compared to previous works. The rapid photo‐polymerization enables progress toward 3D‐printing of multi‐layer actuators and is a step toward mass customization of fully digitally‐printed robotic and wearable devices.

Digitally Printable Magnetic Liquid Metal Composite for Recyclable Soft‐Matter Electronics

Article

Full-text available

Mar 2023

The increasing interest in epidermal patches for wearable monitoring makes it urgent to move toward “greener” and recyclable materials in printed electronics. However, combining scalable fabrication, stretchable function, and recyclable material architecture is challenging to achieve. Herein, this article introduces a printable biphasic liquid metal (LM)‐based composite that combines excellent electrical conductivity, high stretchability, low‐electromechanical gauge factor, printability, sinter‐free functionality, and compatibility with heat‐sensitive substrates, thanks to the sinter‐free formulation. The novel eutectic gallium indium LM‐nickel‐styrene‐isoprene (SIS) thermoplastic elastomer composite takes advantage of a “greener” solvent composed of a mixture of methyl acetate, cyclohexane, and 4‐chlorobenzotrifluoride compared to the toluene used in block copolymers (BCPs). Furthermore, it replaces the costly micron‐sized silver flake filler particles with nickel, a lower‐cost alternative that enables facile recyclability by using a magnetic force. The use of BCPs with non‐permanent physical cross‐links enables the possibility of recycling. A green route is shown for recycling the composite, using only bio‐friendly acids and daily used products such as sugar and Epsom salt. The application of these composites is shown through the digital printing of stretchable circuits and magnetic switches.

A self-healing electrically conductive organogel composite

Article

Full-text available

Mar 2023

Self-healing hydrogels use spontaneous intermolecular forces to recover from physical damage caused by extreme strain, pressure or tearing. Such materials are of potential use in soft robotics and tissue engineering, but they have relatively low electrical conductivity, which limits their application in stretchable and mechanically robust circuits. Here we report an organogel composite that is based on poly(vinyl alcohol)–sodium borate and has high electrical conductivity (7 × 10⁴ S m⁻¹), low stiffness (Young’s modulus of ~20 kPa), high stretchability (strain limit of >400%) and spontaneous mechanical and electrical self-healing. The organogel matrix is embedded with silver microflakes and gallium-based liquid metal microdroplets, which form a percolating network, leading to high electrical conductivity in the material. We also overcome the rapid drying problem of the hydrogel material system by replacing water with an organic solvent (ethylene glycol), which avoids dehydration and property changes for over 24 h in an ambient environment. We illustrate the capabilities of the self-healing organogel composite by using it in a soft robot, a soft circuit and a reconfigurable bioelectrode.

A Non-Newtonian liquid metal enabled enhanced electrography

Article

May 2023
BIOSENS BIOELECTRON

Biopotential signals, like electrocardiography (ECG), electromyography (EMG), and electroencephalography (EEG), can help diagnose cardiological, musculoskeletal and neurological disorders. Dry silver/silver chloride (Ag/AgCl) electrodes are commonly used to obtain these signals. While a conductive hydrogel can be added to Ag/AgCl electrodes to improve the contact and adhesion between the electrode and the skin, dry electrodes are prone to movement. Considering that the conductive hydrogel dries over time, the use of these electrodes often creates an imbalanced skin-electrode impedance and a number of sensing issues in the front-end analogue circuit. This issue can be extended to several other electrode types that are commonly in use, in particular, for applications with a need for long-term wearable monitoring such as ambulatory epilepsy monitoring. Liquid metal alloys, such as eutectic gallium indium (EGaIn), can address key critical requirements around consistency and reliability but present challenges on low viscosity and the risk of leakage. To solve these problems, here, we demonstrate the use of a non-eutectic Ga-In alloy as a shear-thinning non-Newtonian fluid to offer superior performance to commercial hydrogel electrodes, dry electrodes, and conventional liquid metals for electrography measurements. This material has high viscosity when still and can flow like a liquid metal when sheared, preventing leakage while allowing the effective fabrication of electrodes. Moreover, the Ga-In alloy not only has good biocompatibility but also offers an outstanding skin-electrode interface, allowing for the long-term acquisition of high-quality biosignals. The presented Ga-In alloy is a superior alternative to conventional electrode materials for real-world electrography or bioimpedance measurement.

Multimaterial Printing of Liquid Crystal Elastomers with Integrated Stretchable Electronics

Article

May 2023
ACS APPL MATER INTER

Liquid crystal elastomers (LCEs) have grown in popularity in recent years as a stimuli-responsive material for soft actuators and shape reconfigurable structures. To make these material systems electrically responsive, they must be integrated with soft conductive materials that match the compliance and deformability of the LCE. This study introduces a design and manufacturing methodology for combining direct ink write (DIW) 3D printing of soft, stretchable conductive inks with DIW-based "4D printing" of LCE to create fully integrated, electrically responsive, shape programmable matter. The conductive ink is composed of a soft thermoplastic elastomer, a liquid metal alloy (eutectic gallium indium, EGaIn), and silver flakes, exhibiting both high stretchability and conductivity (order of 105 S m-1). Empirical tuning of the LCE printing parameters gives rise to a smooth surface (<10 μm) for patterning the conductive ink with controlled trace dimensions. This multimaterial printing method is used to create shape reconfigurable LCE devices with on-demand circuit patterning that could otherwise not be easily fabricated through traditional means, such as an LCE bending actuator able to blink a Morse code signal and an LCE crawler with an on/off photoresistor controller. In contrast to existing fabrication methodologies, the inclusion of the conductive ink allows for stable power delivery to surface mount devices and Joule heating traces in a highly dynamic LCE system. This digital fabrication approach can be leveraged to push LCE actuators closer to becoming functional devices, such as shape programmable antennas and actuators with integrated sensing.

Carbon Nanotubes - Elastomer Actuator for Soft Prosthetics

Article

Jan 2023

Sakshi Gunde

Prosthetics play an important role as a substitute for any essential part of body. Soft prosthetic is an attempt at making these artificial devices more human-like. The main focus of this paper is the use of a carbon nanotube- elastomer based compound material as an actuator for prosthetic devices. One of the main limitations of Electroactive Polymer (EAP), which is high applied voltage, is overcome in this approach by using carbon nanotube (CNT) yarn permeated with a mixture of elastomer and methanol. The actuation is based on phase transition and this concept can be implemented for soft prosthetics.

Digitally Printed Liquid Metal Composite Antenna for Energy Harvesting: Toward Energy‐Autonomous Battery‐Free Wearable Bioelectronics

Article

Dec 2022

Rapid progress on wearable bioelectronics holds the promise for remote patient monitoring. However, there are concerns related with the use of batteries, including the need for frequent charging, user safety, and environmental pollution. The development of rapidly deployable battery‐free patches that harvest their energy from external sources is highly desired. Here, untethered battery‐free patches through far‐field energy harvesting are demonstrated. Taking advantage of a stretchable biphasic liquid metal‐based ink and digital fabrication techniques, that is, direct printing, and laser patterning, tailor‐made customized antennas are fabricated over various substrates, including polydimethylsiloxane, Kapton, and wound‐dressing medical‐grade adhesives. Folded dipole and planar inverted‐F antennas (PIFA) are evaluated and they are optimized for an over‐the‐body scenario. Compared to previous works that only show communication of data at <1 Hz, here, battery‐free acquisition and transmission of electrocardiogram (ECG) data at 100 Hz via Bluetooth is demonstrated. It is shown that using 3 parallel PIFA antennas, it is possible to harvest ≈10 mW at 30 cm from the transmitter, which is ≈7× the required power by the ECG circuit. This work proves the feasibility of next generation of battery‐free wearable biomonitoring patches/e‐textiles and demonstrates materials and methods for fabrication of optimized printed antennas. Using digital printing methods, the authors present two different approaches in antenna fabrication and optimization. Conductive ink with liquid metal and silver flakes permits the antennas to have record‐breaking stretchability and excellent electromechanical coupling upon repetitive strain movements. Finally, an electrocardiogram belt with 100 Hz communication using Bluetooth, is entirely powered with the antenna's harvested energy.

3D Printing of Liquid Metal Embedded Elastomers for Soft Thermal and Electrical Materials

Article

Dec 2022

Liquid metal embedded elastomers (LMEEs) are composed of a soft polymer matrix embedded with droplets of metal alloys that are liquid at room temperature. These soft matter composites exhibit exceptional combinations of elastic, electrical, and thermal properties that make them uniquely suited for applications in flexible electronics, soft robotics, and thermal management. However, the fabrication of LMEE structures has primarily relied on rudimentary techniques that limit patterning to simple planar geometries. Here, we introduce an approach for direct ink write (DIW) printing of a printable LMEE ink to create three-dimensional shapes with various designs. We use eutectic gallium-indium (EGaIn) as the liquid metal, which reacts with oxygen to form an electrically insulating oxide skin that acts as a surfactant and stabilizes the droplets for 3D printing. To rupture the oxide skin and achieve electrical conductivity, we encase the LMEE in a viscoelastic polymer and apply acoustic shock. For printed composites with a 80% LM volume fraction, this activation method allows for a volumetric electrical conductivity of 5 × 104 S cm-1 (80% LM volume)─significantly higher than what had been previously reported with mechanically sintered EGaIn-silicone composites. Moreover, we demonstrate the ability to print 3D LMEE interfaces that provide enhanced charge transfer for a triboelectric nanogenerator (TENG) and improved thermal conductivity within a thermoelectric device (TED). The 3D printed LMEE can be integrated with a highly soft TED that is wearable and capable of providing cooling/heating to the skin through electrical stimulation.

Gallium‐Based Liquid–Solid Biphasic Conductors for Soft Electronics

Abstract and Figures

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