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A Comparative Study of Silver Microflakes in Digitally Printable Liquid Metal Embedded Elastomer Inks for Stretchable Electronics
Abstract
Printed soft conductive materials for stretchable electronics should have low electrical resistivity, high strain limit, and stable electrical properties when stretched. Previously, it has been shown that a bi‐phasic ink composed of silver (Ag) microflakes, eutectic gallium−indium (EGaIn) alloy, and styrene isoprene (SIS) block copolymer is a promising formulation for printed soft electronics and has the potential to satisfy the necessary criteria. In this study, further improvements to the ink formulation are explored, with a focus on how the choice of Ag microflakes affects the electrical and electromechanical properties of the composite. By using specific Ag microflakes, AgInGa‐SIS inks that have conductivity as high as 6.38 × 105 S m−1 and a strain limit of over 1000%, with low electromechanical coupling can be synthesized. More broadly, when comparing the composite with different silver flakes, there is a 176% relative difference in conductivity, >600% difference in strain limit, and 277% relative difference in electromechanical coupling. To demonstrate the applicability of these inks for various use cases such as wearable bioelectronics, interconnects are printed for connecting electronic breakout boards with microcontrollers that provide a stable electrical connection when stretched, and the interconnects and electrodes of a wearable electrocardiography system that monitors the heart pulses in real‐time. This work examines how the choice of silver filler particles influences the electrical and electromechanical properties of a digitally printable elastic conductor. In addition to silver, the conductor contains gallium‐based liquid metal alloy and a thermoplastic elastomer and can be extruded from a syringe to print stretchable circuits for soft digital electronics and bioelectronic stickers.
... To improve the ability to incorporate complex LM-based electronics into soft material systems, researchers have recently introduced a new class of highly conductive and stretchable inks composed of EGaIn, Ag flakes, and a polystyrene−isoprene− styrene (SIS) hyperelastic binder that can be printed using direct ink writing (DIW). 38,39 Because the material is initially diluted in a solvent, it exhibits shear thinning behavior needed for DIWbased deposition. Once the solvent evaporates, the conductive ink displays a low electromechanical coupling, exhibiting only a small change in its initial resistance over many loading cycles as compared to resistive silver inks and other materials that can degrade or delaminate. ...
... Once the solvent evaporates, the conductive ink displays a low electromechanical coupling, exhibiting only a small change in its initial resistance over many loading cycles as compared to resistive silver inks and other materials that can degrade or delaminate. 38,39 The material can strain >100% in many cases with little change to its resistance. The average conductivity of the EGaIn-Ag-SIS ink from prior studies 38,39 is on the order of 10 5 S m −1 ; the trade-off between the ink's conductivity when compared to that of its constituent materials (about 3.4 × 10 6 S m −1 for EGaIn, 40 6.3 × 10 7 S m −1 for silver 41 ) comes with the added benefit of the SIS matrix that allows the ink to bond well to various surfaces. ...
... The EGaIn-Ag-SIS ink is obtained by mixing SIS-block copolymer solution, silver flakes, and liquid metal in a ratio of 1:1.24:3 by weight. Electromechanical characterization was performed in the previous works 38, 39 and will not be focused on in this study. ...
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.
... For example, electronic ink, which comprises Ag flakes, PEDOT:PSS, poly(3-hexylthiophene-2,5-diyl) (P3HT)-nanofibrils, and ion gels, can be drawn on the skin with a ballpoint pen to create such an electrode [ Figure 6D] [160] . The drawn electrode demonstrated a low sheet resistance of 1.2 Ω·sq -1 under zero strain and 9.9 Ω·sq -1 under 30% strain and mechanical reliability under repeated stretching (1,000 cycles of 10% strain). ...
... Copyright 2018, American Chemical Society; (D) Stencil printing of the electronic ink (top) and the resulting electrode drawn directly on the skin (bottom); (E) Images of EP sensor (left) and recorded ECG signals (right) without and with applied strain. Reproduced with permission from ref [160] . Copyright 2020, The Author(s); (F) Analog voltage signals obtained from the Ag microflakes and EGaIn droplets-based ECG patch. ...
Soft conductive nanocomposites have introduced significant breakthroughs in bio-integrated electronics by mitigating the mechanical mismatch between the body and the device. Compared with conventional wearable sensors based on rigid electronic materials, the wearable sensors based on soft nanocomposites are advantageous to long-term and high-quality biosignal recordings. Materials used for the synthesis of the nanocomposites, especially nanofillers, are critical for determining the quality of recorded biosignals and the performance of the nanocomposites. In this review, we focus on recent advances in soft conductive nanocomposites, mainly on their electrical and mechanical properties according to the types of nanofillers, and present their applications to wearable biosignal recording devices. We have classified the nanofillers into four categories: carbon-based nanomaterials, conducting polymers, metal-based nanomaterials, and liquid metals. We then introduce the applications of nanocomposites as wearable sensors that record various biosignals, including electrophysiological, strain, pressure, and biochemical information. In conclusion, a brief outlook on the remaining challenges for future nanomaterial-based bioelectronics is provided.
... In contrast, silver conforms to various types of stretching surfaces including living tissues and/or a wide range of textiles 50 . Although the conductivity of the silver-based ink can be readily enhanced by increasing the concentration of silver flake, this results in lower flexibility and weaker stretchability properties 51,52 . Additionally, these inks are conventionally hindered by significant loss of conductivity under normal strain 53 . ...
The human body exhibits complex, spatially distributed chemo-electro-mechanical processes that must be properly captured for emerging applications in virtual/augmented reality, precision health, activity monitoring, bionics, and more. A key factor in enabling such applications involves the seamless integration of multipurpose wearable sensors across the human body in different environments, spanning from indoor settings to outdoor landscapes. Here, we report a versatile epidermal body area network ecosystem that enables wireless power and data transmission to and from battery-free wearable sensors with continuous functionality from dry to underwater settings. This is achieved through an artificial near field propagation across the chain of biocompatible, magneto-inductive metamaterials in the form of stretchable waterborne skin patches—these are fully compatible with pre-existing consumer electronics. Our approach offers uninterrupted, self-powered communication for human status monitoring in harsh environments where traditional wireless solutions (such as Bluetooth, Wi-Fi or cellular) are unable to communicate reliably.
... We attribute this to the relatively low volume fraction of conductive particles and the sparsity of the percolating network, although more study will be required to fully understand this difference in electromechanical behaviour. Interestingly, we observe a small decrease in resistance when stretching the sample with a small strain (0-5%), similar to what has been reported previously 48 . We postulate that as the composite is stretched, the oxide shell of EGaIn is ruptured. ...
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.
Liquid‐metal embedded elastomers (LMEEs) have been demonstrated to show a variety of excellent properties, including high toughness, dielectric constant, and thermal conductivity, with applications across soft electronics and robotics. However, within this scope of use cases, operation in extreme environments – such as high‐temperature conditions – may lead to material degradation. While prior works highlight the functionality of LMEEs, there is limited insight on the thermal stability of these soft materials and how the effects of liquid metal (LM) inclusions depend on temperature. Here, the effects on thermal stability, including mechanical and electrical properties, of LMEEs are introduced. Effects are characterized for both fluoroelastomer and other elastomer‐based composites at temperature exposures up to 325 °C, where it is shown that embedding LM can offer improvements in thermo‐mechanical stability. Compared to elastomer like silicone rubber that has been previously used for LMEEs, a fluoroelastomer matrix offers a higher dielectric constant and significant improvement in thermo‐mechanical stability without sacrificing room temperature properties, such as thermal conductivity and modulus. Fluoroelastomer‐LM composites offer a promising soft, multi‐functional material for high‐temperature applications, which is demonstrated here with a printed, soft heat sink and an endoscopic sensor capable of wireless sensing of high temperatures.
Room-temperature gallium-based liquid metals (RT-GaLMs) have garnered significant interest recently owing to their extraordinary combination of fluidity, conductivity, stretchability, self-healing performance, and biocompatibility. They are ideal materials for the manufacture of flexible electronics. By changing the composition and oxidation of RT-GaLMs, physicochemical characteristics of the liquid metal can be adjusted, especially the regulation of rheological, wetting, and adhesion properties. This review highlights the advancements in the liquid metals used in flexible electronics. Meanwhile related characteristics of RT-GaLMs and underlying principles governing their processing and applications for flexible electronics are elucidated. Finally, the diverse applications of RT-GaLMs in self-healing circuits, flexible sensors, energy harvesting devices, and epidermal electronics, are explored. Additionally, the challenges hindering the progress of RT-GaLMs are discussed, while proposing future research directions and potential applications in this emerging field. By presenting a concise and critical analysis, this paper contributes to the advancement of RT-GaLMs as an advanced material applicable for the new generation of flexible electronics.
Next generation on‐skin electrodes will require soft, flexible, and gentle materials to provide both high‐fidelity sensing and wearer comfort. However, many commercially available on‐skin electrodes lack these key properties due to their use of rigid hardware, harsh adhesives, uncomfortable support structures, and poor breathability. To address these challenges, this work presents a new device paradigm by joining biocompatible electrospun spider silk with printable liquid metal to yield an incredibly soft and scalable on‐skin electrode that is strain‐tolerant, conformable, and gentle on‐skin. These electrodes, termed Silky Liquid Metal (SLiM) electrodes, were found to be over 5 times more breathable than commercial wet electrodes, while the silk's intrisic adhesion mechanism allows SLiM electrodes to avoid the use of harsh artificial adhesives, potentially decreasing skin irritation and inflammation over long term use. Finally, the SLiM electrodes provide comparable impedances to traditional wet and other liquid metal electrodes, offering a high‐fidelity sensing alternative with increased wearer comfort. Human subject testing confirmed the SLiM electrodes ability to sense electrophysiological signals with high fidelity and minimal irritation to the skin. The unique properties of the reported SLiM electrodes offer a comfortable electrophysiological sensing solution especially for patients with pre‐existing skin conditions or surface wounds.
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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.
Stretchable electronic devices, with their excellent properties such as stretchability and conformal contact with complex surfaces, are widely used in health monitoring and human‐computer interactions. For stable operation of these devices, stretchable circuits are crucial, enabling integration of sensors, data collection, and transmission while under strain. To achieve stretchable system integration, addressing key issues such as elastic substrates, sensor devices, rigid components, stretchable electrodes, and packaging layers is necessary. The stretchable electrodes, which connect various functional devices, are crucial for achieving stretchability and are the main focus of this review. Firstly, this work explores the enhancement of electrode design to maintain good electrical conductivity even under stretching conditions, through studying stretchable conductive materials and the structural design of conventional materials. This work then presents studies on substrate interfaces and the design of elastic substrates, such as rigid islands, to enhance the stability of stretchable circuits during use. Finally, the article discusses the applications of stretchable circuits, the challenges they face, and provides new research directions and ways to develop stretchable circuits.
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.
Suspending microscale droplets of liquid metals like eutectic gallium-indium (EGaIn) in polydimethylsiloxane (PDMS) has been shown to dramatically enhance electrical permittivity without sacrificing the elasticity of the host PDMS matrix. However, increasing the dielectric constant of EGaIn-PDMS composites beyond previously reported values requires high EGaIn loading fractions (>50% by volume) that can result in substantial increases in density and loss of material integrity. In this work, we enhance permittivity without further increasing EGaIn loading by incorporating polydopamine (PDA)-coated graphene oxide (GO) and partially reduced GO. In particular, we show that the combination of EGaIn and PDA-GO within a PDMS matrix results in an elastomer composite with a high dielectric constant (∼10-57), a low dissipation factor (∼0.01), and rubber-like compliance and elasticity.
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. here, 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.
Liquid crystal elastomers (LCEs) are becoming increasingly popular as a shape memory material for soft robot actuators that operate in a contractile or flexural mode. There have been previously studies on the use of LCEs for reversible changes in surface topography. However, surface protrusions have typically been limited to the order of 1 μm or depend on light, heat, or electrical stimulation that are difficult to locally control or require relatively high voltage. This article presents a novel operation mode of LCE actuators based on the wrinkling behavior of an LCE-elastomer bilayer architecture. Embedding a liquid-metal-based conductive ink within the LCE enables electrical control of surface wrinkling through Joule heating. The actuator cells can generate wrinkles with amplitudes ranging from 17 to 45 μm within 30 s under an input power of 2 W and a voltage on the order of 1 V. As the bilayer is composed entirely of soft materials, it is highly deformable, flexible, and can be integrated into a multi-cell array capable of bending on curved surfaces.
Liquid metal (LM)-based elastomers have a demonstrated value in flexible electronics. Attempts in this area include the development of multifunctional LM-based elastomers with controllable morphology, superior mechanical performances, and great stability. Herein, inspired by the working principle of electric toothbrushes, we present a revolving microfluidic system for the generation of LM droplets and construction of desired elastomers. The system involves revolving modules assembled by a needles array and three-dimensional (3D) microfluidic channels. LM droplets can be generated with controllable size in a high-throughput manner due to the revolving motion-derived drag force. We have demonstrated that by employing a poly(dimethylsiloxane) (PDMS) matrix as the collection phase, the generated LM droplets can act as conductive fillers for the construction of flexible electronics directly. The resultant LM droplets-based elastomers exhibit high mechanical strength, stable electrical performance, as well as superior self-healing property benefiting from the dynamic exchangeable urea bond of the polymer matrix. Notably, due to the flexible programmable feature of the LM droplets embedded within the elastomers, various patterned LM droplets-based elastomers can be easily achieved. These results indicate that the proposed microfluidic LM droplets-based elastomers have a great potential for promoting the development of flexible electronics. This article is protected by copyright. All rights reserved.
The use of conductive materials to be printed/embedded onto/within a polymeric matrix has gained increasing attention in the fabrication of next-generation soft electronics. In this review, we provide a comprehensive overview of the materials and approaches for the fabrication of 2D and 3D conductive structures while focusing on the importance of the compatibility between the particles’ shape, the polymeric matrix type, and the deposition method, along with the target application. The review presents a summary of the main conductive materials and the type of polymeric materials which were utilized for fabricating soft electronic devices that are bendable/twistable and stretchable. It is divided into two sections: Introduction section, which presents briefly conductive materials, polymeric matrix, and fabrication methods, and Fabrication section, presenting the main 6 approaches of fabrication. Each of the fabrication subsections presents the main recent reports in a detailed table. The review is concluded with an Outlook section, describing the current challenges in this field. Despite the progress in the fabrication of 2D bendable/twistable electronic devices toward industrial integration, there is still a need for tailoring and improving the durability and robustness of 3D stretchable electronic devices.
Liquid metals (LMs) have emerged as key materials for soft and wearable electronics owing to their unique properties such as fluidity, deformability, low toxicity, high electrical and thermal conductivities, self-healing, and biocompatibility. Moreover, LMs can be mixed with other metal particles and soft materials to form two-phase structures. Wearable and stretchable electronic devices can utilize biphasic LM mixtures as a material with different characteristics from those of conventional LMs. Biphasic LM mixtures consist of solid and liquid phases which are defined, respectively, as LM with solid metal particles and LM inside a soft material. An LM with solid metal particles consists of an alloy of LM and a metal such as Ni or Cu. In contrast, an LM with a soft material is composed of LM and an elastomer such as polydimethylsiloxane (PDMS) or Ecoflex. Sensors and field-effect transistors (FETs) utilizing two-phase LM mixtures are discussed in this paper. Additionally, fabrication and patterning of two-phase LM mixtures are discussed.
A novel architecture of materials and fabrication techniques is proposed that serves as a universal method for implementation of thin‐film biostickers for high resolution electrophysiological monitoring. Unlike the existing wearable patches, the presented solution can be worn for several days, and is not affected by daily routines such as physical exercise or taking bath. A printable biphasic liquid metal silver composite is used, both as the electrical interconnects and the electrodes. This allows combining advantages of dry electrodes, i.e., printability and non‐smearing behavior, with benefits of wet electrodes, i.e., high‐quality signal. A human subject study showed that these biphasic printed electrodes benefit from a lower electrode‐skin impedance compared to clinical grade Ag/AgCl electrodes. Digital printing enables autonomous fabrication of biostickers that are taylor‐made for each user and each application. A universal miniaturized electronic system for biopotential acquisition and wireless communication is develpoed, and demonstrated multiple biopotential acquisition cases, including electrocardiography, electroencephalography, electromyography, and electrooculography.
Stretchable electronic circuits are critical for soft robots, wearable technologies and biomedical applications. Development of sophisticated stretchable circuits requires new materials with stable conductivity over large strains, and low-resistance interfaces between soft and conventional (rigid) electronic components. To address this need, we introduce biphasic Ga–In, a printable conductor with high conductivity (2.06 × 10⁶ S m⁻¹), extreme stretchability (>1,000%), negligible resistance change when strained, cyclic stability (consistent performance over 1,500 cycles) and a reliable interface with rigid electronics. We employ a scalable transfer-printing process to create various stretchable circuit board assemblies that maintain their performance when stretched, including a multilayer light-emitting diode display, an amplifier circuit and a signal conditioning board for wearable sensing applications. The compatibility of biphasic Ga–In with scalable manufacturing methods, robust interfaces with off-the-shelf electronic components and electrical/mechanical cyclic stability enable direct conversion of established circuit board assemblies to soft and stretchable forms.
Stretchable electronics find widespread uses in a variety of applications such as wearable electronics, on-skin electronics, soft robotics and bioelectronics. Stretchable electronic devices conventionally built with elastomeric thin films show a lack of permeability, which not only impedes wearing comfort and creates skin inflammation over long-term wearing but also limits the design form factors of device integration in the vertical direction. Here, we report a stretchable conductor that is fabricated by simply coating or printing liquid metal onto an electrospun elastomeric fibre mat. We call this stretchable conductor a liquid-metal fibre mat. Liquid metal hanging among the elastomeric fibres self-organizes into a laterally mesh-like and vertically buckled structure, which offers simultaneously high permeability, stretchability, conductivity and electrical stability. Furthermore, the liquid-metal fibre mat shows good biocompatibility and smart adaptiveness to omnidirectional stretching over 1,800% strain. We demonstrate the use of a liquid-metal fibre mat as a building block to realize highly permeable, multifunctional monolithic stretchable electronics.
Wearable electronics, conformable sensors, and soft/micro-robotics require conductive yet stretchable thin films. However, traditional free standing metallic thin films are often brittle, inextensible, and must be processed in strict environments. This limits implementation into soft technologies where high electrical conductivity must be achieved while maintaining high compliance and conformability. Here we show a liquid metal elastomeric thin film (LET) composite with elastomer-like compliance (modulus < 500 kPa) and stretchability (> 700%) with metallic conductivity (sheet resistance < 0.1 Ω/square). These 30-70 um thin films are highly conformable, free standing, and display a unique Janus microstructure, where a fully conductive activated side is accompanied with an opposite insulated face. LETs display exceptional electro-mechanical characteristics, with a highly linear strain-resistance relationship beyond 700% deformation while maintaining a low resistance. We demonstrate the multifunctionality of LETs for soft technologies by leveraging the unique combination of high compliance and electrical conductivity with transfer capabilities for strain sensing on soft materials, as compliant electrodes in dielectric elastomeric actuator (DEA), and as resistive heaters for liquid crystal elastomer (LCE).
The convergence of materials science, electronics, and biology, namely bioelectronic interfaces, leads novel and precise communication with biological tissue, particularly with the nervous system. However, the translation of lab‐based innovation toward clinical use calls for further advances in materials, manufacturing and characterization paradigms, and design rules. Herein, a translational framework engineered to accelerate the deployment of microfabricated interfaces for translational research is proposed and applied to the soft neurotechnology called electronic dura mater, e‐dura. Anatomy, implant function, and surgical procedure guide the system design. A high‐yield, silicone‐on‐silicon wafer process is developed to ensure reproducible characteristics of the electrodes. A biomimetic multimodal platform that replicates surgical insertion in an anatomy‐based model applies physiological movement, emulates therapeutic use of the electrodes, and enables advanced validation and rapid optimization in vitro of the implants. Functionality of scaled e‐dura is confirmed in nonhuman primates, where epidural neuromodulation of the spinal cord activates selective groups of muscles in the upper limbs with unmet precision. Performance stability is controlled over 6 weeks in vivo. The synergistic steps of design, fabrication, and biomimetic in vitro validation and in vivo evaluation in translational animal models are of general applicability and answer needs in multiple bioelectronic designs and medical technologies. Four synergistic steps of design, fabrication, and biomimetic in vitro validation and in vivo evaluation in translational animal models are proposed to advance soft implantable bioelectronics toward use in translational research. A biomimetic multimodal platform combines anatomy, mechanical loading, and electrochemical function to accelerate in vitro optimization of soft microtechnology.
Room‐temperature liquid metals, such as nontoxic gallium alloys, show enormous promise to revolutionize stretchable electronics for next‐generation soft robotic, e‐skin, and wearable technologies. Core–shell particles of liquid metal with surface‐bound acrylate ligands are synthesized and polymerized together to create cross‐linked particle networks comprising >99.9% liquid metal by weight. When stretched, particles within these polymerized liquid metal networks (Poly‐LMNs) rupture and release their liquid metal payload, resulting in a rapid 108‐fold increase in the network's conductivity. These networks autonomously form hierarchical structures that mitigate the deleterious effects of strain on electronic performance and give rise to emergent properties. Notable characteristics include nearly constant resistances over large strains, electronic strain memory, and increasing volumetric conductivity with strain to over 20 000 S cm−1 at >700% elongation. Furthermore, these Poly‐LMNs exhibit exceptional performance as stretchable heaters, retaining 96% of their areal power across relevant physiological strains. Remarkable electromechanical properties, responsive behaviors, and facile processing make Poly‐LMNs ideal for stretchable power delivery, sensing, and circuitry. Core–shell liquid metal particles functionalized with acrylate ligands are polymerized to create cross‐linked particle networks. When these polymerized liquid metal networks are stretched, their constituent particles rupture and the network transitions from insulating to conductive. These networks autonomously form hierarchical structures which help maintain stable electrical behavior under high strains and exhibit excellent performance as stretchable conductors and heaters.
Wearable electronic circuits based on films of Gallium and its alloys offer promising implementations in health monitoring and gaming sensing applications. However, the complex rheology of liquid metals makes it challenging to manufacture thin gallium‐based devices with reliable, reproducible, and stable properties over time. Here, we engineer the surface coating and topography of silicone substrate to enable precisely defined, micrometer‐thick liquid metal patterns over large (>10 cm2) surface areas and with high design versatility. Control over the film microstructure meets manufacturing conditions that enable gallium films with precise, repeatable and durable electromechanical performance. We demonstrate the robustness and applicability of this technology in a virtual reality scenario by implementing thin, soft and stretchable gallium‐based sensors to accurately monitor human hand kinematics. This article is protected by copyright. All rights reserved.
Recent progress in soft‐matter sensors has shown improved fabrication techniques, resolution, and range. However, scaling up these sensors into an information‐rich tactile skin remains largely limited by designs that require a corresponding increase in the number of wires to support each new sensing node. To address this, a soft tactile skin that can estimate force and localize contact over a continuous 15 mm2 area with a single integrated circuit and four output wires is introduced. The skin is composed of silicone elastomer loaded with randomly distributed magnetic microparticles. Upon deformation, the magnetic particles change position and orientation with respect to an embedded magnetometer, resulting in a change in the net measured magnetic field. Two experiments are reported to calibrate and estimate both location and force of surface contact. The classification algorithms can localize pressure with an accuracy of >98% on both grid and circle pattern. Regression algorithms can localize pressure to a 3 mm2 area on average. This proof‐of‐concept sensing skin addresses the increasing need for a simple‐to‐fabricate, quick‐to‐integrate, and information‐rich tactile surface for use in robotic manipulation, soft systems, and biomonitoring. Tactile information allows robots to understand more about their environment. Developing accurate, accessible, and robust tactile sensors is key to enable successful robotic technology in complex and dynamic environments. The soft‐sensing approach presented here has multiple advantages including scalability, robustness, and accuracy, in a compliant and conformal format.
3D printing of silicone elastomers with direct ink writing (DIW) process has demonstrated great potential in areas as diverse as flexible electronics, medical devices and soft robotics. However, most of current silicones are not printable due to their low viscosity and long curing time. The lack of systematic research on materials, devices and processes during printing makes it a huge challenge to apply DIW process more deeply and widely. In this report, aiming at the dilemmas in materials, devices and processes, we proposed a comprehensive guide for printing highly stretchable silicone. Specifically, to improve the printability of silicone elastomer, nano silica was added as rheology modifier without sacrificing any stretch ability. To effectively control print speed and accuracy, a theoretical model was built and verified. With this strategy, silicone elastomers with different mechanical properties can all be printed and can realize infinite time and high speed printing (>25 mm/s) while maintaining accuracy. Here, super-stretchable silicone that can be stretched to 2000% was printed for the first time and complex structures can be printed with high quality. For further demonstration, prosthetic nose, data glove capable of detecting fingers’ movement and artificial muscle that can lift objects were printed directly. We believe this work could provide a guide for further work using DIW process to print soft matters in a wide range of application scenarios.
Advances in next-generation soft electronic devices rely on the development of highly deformable, healable, and printable energy generators to power these electronics. Development of deformable or wearable energy generators that can simultaneously attain extreme stretchability with superior healability remains a daunting challenge. We address this issue by developing a highly conductive, extremely stretchable, and healable composite based on thermoplastic elastomer with liquid metal and silver flakes as the stretchable conductor for triboelectric nanogenerators. The elastomer is used both as the matrix for the conductor and as the triboelectric layer. The nanogenerator showed a stretchability of 2500% and it recovered its energy-harvesting performance after extreme mechanical damage, due to the supramolecular hydrogen bonding of the thermoplastic elastomer. The composite of the thermoplastic elastomer, liquid metal particles, and silver flakes exhibited an initial conductivity of 6250 S cm⁻¹ and recovered 96.0% of its conductivity after healing.
The necessity to place sensors far away from the processing unit in smart clothes or artificial skins for robots may require conductive wirings on stretchable materials at very low-cost. In this work, we present an easy method to produce wires using only commercially available materials. A consumer grade inkjet printer was used to print a wire of silver nanoparticles with a sheet resistance below 1 Ω/sq. on a non-pre-strained sheet of elastic silicone. This wire was stretched more than 10,000 times and was still conductive afterwards. The viscoelastic behavior of the substrate results in a temporarily increased resistance that decreases to almost the original value. After over-stretching, the wire is conductive within less than a second. We analyze the swelling of the silicone due to the ink’s solvent and the nanoparticle film on top by microscope and SEM images. Finally, a 60 mm long stretchable conductor was integrated onto wearables, and showed that it can bear strains of up to 300% and recover to a conductivity that allows the operation of an assembled LED assembled at only 1.8 V. These self-healing wires can serve as wiring and binary strain or pressure sensors in sportswear, compression underwear, and in robotic applications.
Large-area stretchable electronics are critical for progress in wearable computing, soft robotics and inflatable structures. Recent efforts have focused on engineering electronics from soft materials-elastomers, polyelectrolyte gels and liquid metal. While these materials enable elastic compliance and deformability, they are vulnerable to tearing, puncture and other mechanical damage modes that cause electrical failure. Here, we introduce a material architecture for soft and highly deformable circuit interconnects that are electromechanically stable under typical loading conditions, while exhibiting uncompromising resilience to mechanical damage. The material is composed of liquid metal droplets suspended in a soft elastomer; when damaged, the droplets rupture to form new connections with neighbours and re-route electrical signals without interruption. Since self-healing occurs spontaneously, these materials do not require manual repair or external heat. We demonstrate this unprecedented electronic robustness in a self-repairing digital counter and self-healing soft robotic quadruped that continue to function after significant damage.
Herein, we report an innovative application of doping conjugated-polypyrrole nanoparticles (PPy NPs) into electrically conductive adhesives (ECAs) to prepare low-electrical resistivity interconnecting materials. PPy NPs were synthesized via a facile one-step chemical oxidative polymerization method at room temperature with the average diameter as small as 86.8 nm. Particles' diameters and dispersity were manipulated under different polymerization conditions through the adjustment of weight percentages and molecular weights of polyvinylpyrrolidone (PVP) as the surfactants. Results showed that higher concentrations of PVP and the longer chains of PVPs resulted in smaller diameters of PPy NPs. We also found that a suitable portion of ethanol in the polymerization mixtures gives rise to a better dispersity than that observed in mixtures without ethanol. When a small amount of PPy NPs was added into traditional epoxy resin-based and silver-flakes-filled ECAs, the resistance measurements showed an enhancement in the electrical conductivity, or in other words, a reduction in resistivity significantly. For example, the electrical resistivity of 70 wt% silver-filled ECAs was reduced from 1.6 × 10⁻³ Ω cm to 9.4 × 10⁻⁵ Ω cm by using only 2.5 wt% PPy NPs as the dopants. Thus, our results confirmed new applications of PPy NPs in the field of ECAs for decreasing the resistance, reducing the dosage of silver in ECAs, and achieving flexible devices. Finally, flexible electrical patterns were printed on paper and polyimide substrates were used as conducting circuits to light LED devices.
Previous breakthroughs in stretchable electronics stem from strain engineering and nanocomposite approaches. Routes toward intrinsically stretchable molecular materials remain scarce but, if successful, will enable simpler fabrication processes, such as direct printing and coating, mechanically robust devices, and more intimate contact with objects. We report a highly stretchable conducting polymer, realized with a range of enhancers that serve a dual function: (i) they change morphology and (ii) they act as conductivity-enhancing dopants in poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). The polymer films exhibit conductivities comparable to the best reported values for PEDOT:PSS, with over 3100 S/cm under 0% strain and over 4100 S/cm under 100% strain—among the highest for reported stretchable conductors. It is highly durable under cyclic loading, with the conductivity maintained at 3600 S/cm even after 1000 cycles to 100% strain. The conductivity remained above 100 S/cm under 600% strain, with a fracture strain of 800%, which is superior to even the best silver nanowire– or carbon nanotube–based stretchable conductor films. The combination of excellent electrical and mechanical properties allowed it to serve as interconnects for field-effect transistor arrays with a device density that is five times higher than typical lithographically patterned wavy interconnects.
Make it stretch, make it glow
The skins of some cephalopods, such as the octopus, are highly flexible and contain color-changing cells. These cells are loaded with pigments that enable rapid and detailed camouflaging abilities. Larson et al. developed a stretchable electroluminescent actuator. The material could be highly stretched, could emit light, and could also sense internal and external pressure. A soft robot demonstrated these combined capabilities by stretching and emitting light as it moved.
Science , this issue p. 1071
Superelastic conducting fibers with improved properties and functionalities are needed for diverse applications. Here we report the fabrication of highly stretchable (up to 1320%) sheath-core conducting fibers created by wrapping carbon nanotube sheets oriented in the fiber direction on stretched rubber fiber cores. The resulting structure exhibited distinct short- and long-period sheath buckling that occurred reversibly out of phase in the axial and belt directions, enabling a resistance change of less than 5% for a 1000% stretch. By including other rubber and carbon nanotube sheath layers, we demonstrated strain sensors generating an 860% capacitance change and electrically powered torsional muscles operating reversibly by a coupled tension-to-torsion actuation mechanism. Using theory, we quantitatively explain the complementary effects of an increase in muscle length and a large positive Poisson's ratio on torsional actuation and electronic properties.
Copyright © 2015, American Association for the Advancement of Science.
The development of advanced flexible large-area electronics such as flexible displays and sensors will thrive on engineered functional ink formulations for printed electronics where the spontaneous arrangement of molecules aids the printing processes. Here we report a printable elastic conductor with a high initial conductivity of 738 S cm(-1) and a record high conductivity of 182 S cm(-1) when stretched to 215% strain. The elastic conductor ink is comprised of Ag flakes, a fluorine rubber and a fluorine surfactant. The fluorine surfactant constitutes a key component which directs the formation of surface-localized conductive networks in the printed elastic conductor, leading to a high conductivity and stretchability. We demonstrate the feasibility of our inks by fabricating a stretchable organic transistor active matrix on a rubbery stretchability-gradient substrate with unimpaired functionality when stretched to 110%, and a wearable electromyogram sensor printed onto a textile garment.
A hyperelastic pressure transducer is fabricated by embedding silicone rubber with microchannels of conductive liquid eutectic gallium-indium. Pressing the surface of the elastomer with pressures in the range of 0-100 kPa will deform the cross-section of underlying channels and change their electric resistance by as much as 50%. Microchannels with dimensions as small as 25 µm are obtained with a maskless, soft lithography process that utilizes direct laser exposure. Change in electrical resistance is measured as a function of the magnitude and area of the surface pressure as well as the cross-sectional geometry, depth and relative lateral position of the embedded channel. These experimentally measured values closely match closed-form theoretical predictions derived from plane strain elasticity and contact mechanics.
Conductive electrodes and electric circuits that can remain active and electrically stable under large mechanical deformations are highly desirable for applications such as flexible displays, field-effect transistors, energy-related devices, smart clothing and actuators. However, high conductivity and stretchability seem to be mutually exclusive parameters. The most promising solution to this problem has been to use one-dimensional nanostructures such as carbon nanotubes and metal nanowires coated on a stretchable fabric, metal stripes with a wavy geometry, composite elastomers embedding conductive fillers and interpenetrating networks of a liquid metal and rubber. At present, the conductivity values at large strains remain too low to satisfy requirements for practical applications. Moreover, the ability to make arbitrary patterns over large areas is also desirable. Here, we introduce a conductive composite mat of silver nanoparticles and rubber fibres that allows the formation of highly stretchable circuits through a fabrication process that is compatible with any substrate and scalable for large-area applications. A silver nanoparticle precursor is absorbed in electrospun poly (styrene-block-butadiene-block-styrene) (SBS) rubber fibres and then converted into silver nanoparticles directly in the fibre mat. Percolation of the silver nanoparticles inside the fibres leads to a high bulk conductivity, which is preserved at large deformations (σ ≈ 2,200 S cm(-1) at 100% strain for a 150-µm-thick mat). We design electric circuits directly on the electrospun fibre mat by nozzle printing, inkjet printing and spray printing of the precursor solution and fabricate a highly stretchable antenna, a strain sensor and a highly stretchable light-emitting diode as examples of applications.
Conducting polymer hydrogels represent a unique class of materials that synergizes the advantageous features of hydrogels and organic conductors and have been used in many applications such as bioelectronics and energy storage devices. They are often synthesized by polymerizing conductive polymer monomer within a nonconducting hydrogel matrix, resulting in deterioration of their electrical properties. Here, we report a scalable and versatile synthesis of multifunctional polyaniline (PAni) hydrogel with excellent electronic conductivity and electrochemical properties. With high surface area and three-dimensional porous nanostructures, the PAni hydrogels demonstrated potential as high-performance supercapacitor electrodes with high specific capacitance (~480 F·g(-1)), unprecedented rate capability, and cycling stability (~83% capacitance retention after 10,000 cycles). The PAni hydrogels can also function as the active component of glucose oxidase sensors with fast response time (~0.3 s) and superior sensitivity (~16.7 μA · mM(-1)). The scalable synthesis and excellent electrode performance of the PAni hydrogel make it an attractive candidate for bioelectronics and future-generation energy storage electrodes.
Graphene structures have never been found to play a role in accelerating fabrication of functional hydrogels. In this work, it is initially discovered that multifunctional hydrogels are fabricated via ultra-fast polymerization (∼minutes) by graphene oxide-adsorbed liquid metal nanodroplets ([email protected]) vs. by conventional approaches (∼hours/days). [email protected] are used to rapidly initiate and further cross-link polyacrylic acid (PAA) chains into a three-dimensional (3D) network without any extra molecular initiators, cross-linkers, heat source, and/or protective gas. The polymerization process with [email protected] is extremely faster than that without GO involved (20 seconds vs. 4 hours of prepolymer formation, and then 10 minutes vs. 3 days of crosslinking) for free radical polymerization of PAA hydrogels. The resulting hydrogel with 2 wt% reduced graphene oxide (rGO) exhibits 600% increase in tensile strength and 950% enhancement in conductivity, as well as excellent self-healing capabilities, in comparison with that of the pure PAA. The sensitivity studies show its great potential for the application of flexible sensors. Furthermore, the hydrogel possesses good dissolving properties, which is greatly beneficial for recyclability of the LM. This creative study not only broadens a novel application of graphene for making advanced multifunctional polymer materials, but also provides a brand-new route to realization of ultra-fast manufacturing technology that is significantly promising for industrial production in wearable devices.
Liquid metal embedded elastomers (LMEEs) have attracted interest from researchers on account of their combination of low elastic modulus, high flexibility, and tunable electrical and/or thermal conductivity. One interesting feature of LMEEs is that their electrical resistance remains constant when stretched, which is desirable in many real-world applications. This work aims to investigate this unique electromechanical coupling behavior of LMEEs with three-dimensional deterministic and statistical modeling. We first performed finite element analysis (FEA) of the electromechanical coupling response of a percolated liquid metal droplet repeating in the conductive network. Next, we modeled the internal conductive network of LMEE as a simple cubic structure and introduced stochasticity in representing the droplet–droplet connectivity. This combined approach allows us to calculate relative changes in resistance with stretch that are in reasonable agreement with experimental findings. Our analysis provides new theoretical insights that can guide efforts to tailor the electromechanical coupling behavior of LMEEs.
A bi-phasic ternary Ag-In-Ga ink that demonstrates high electrical conductivity, extreme stretchability, and low electromechanical gauge factor (GF) is introduced. Unlike popular liquid metal alloys such as eutectic gallium-indium (EGaIn), this ink is easily printable and nonsmearing and bonds strongly to a variety of substrates. Using this ink and a simple extrusion printer, the ability to perform direct writing of ultrathin, multi-layer circuits that are highly stretchable (max. strain >600%), have excellent conductivity (7.02 × 105 S m-1), and exhibit only a modest GF (0.9) related to the ratio of percent increase in trace resistance with mechanical strain is demonstrated. The ink is synthesized by mixing optimized quantities of EGaIn, Ag microflakes, and styrene-isoprene block copolymers, which functions as a hyperelastic binder. When compared to the same composite without EGaIn, the Ag-In-Ga ink shows over 1 order of magnitude larger conductivity, up to ∼27× lower GF, and ∼5× greater maximum stretchability. No significant change over the resistance of the ink was observed after 1000 strain cycles. Microscopic analysis shows that mixing EGaIn and Ag microflakes promotes the formation of AgIn2 microparticles, resulting in a cohesive bi-phasic ink. The ink can be sintered at room temperature, making it compatible with many heat-sensitive substrates. Additionally, utilizing a simple commercial extrusion based printer, the ability to perform stencil-free, digital printing of multi-layer stretchable circuits over various substrates, including medical wound-dressing adhesives, is demonstrated for the first time.
Elastomers embedded with micro- and nanoscale droplets of liquid metal (LM) alloys like eutectic gallium-indium (EGaIn) can exhibit unique combinations of elastic, thermal, and electrical properties that are difficult to achieve using rigid filler. For composites with sufficient concentrations of liquid metal, the LM droplets can form percolating networks that conduct electricity and deform with the surrounding elastomer as the composite is stretched. Surprisingly, experimental measurements performed on LM-embedded elastomers (LMEEs) show that the total electrical resistance of the composite increases only slightly even as the elastomer is stretched to several times its natural length. In contrast, Pouillet's Law would predict an exponential increase in resistance (Ω) with stretch (λ) due to the incompressibility of liquid metal and elastomer. In this manuscript, we perform a computational analysis to examine the unique electromechanical properties of conductive LMEE composites. Our analysis suggests that the gauge factor that quantifies electromechanical coupling (i.e. g = {ΔΩ/Ω0} / λ) decreases with increasing tortuosity of the conductive pathways formed by the connected LM droplets. A dimensionless parameter for path tortuosity can be used to estimate g for statistically homogeneous LMEE composites. These results rationalize experimental observations and provide insight into the influence of liquid metal droplet assembly on the functionality of the composite.
The stretchable conductor is a critical prerequisite to achieve various forms of stretchable electronics. In particular, directly printable stretchable conductors have gathered considerable attention with a recent growing interest in a variety of large-area, deformable electronics. In this study, we have developed a chemical pathway of incorporating a surfactant with a moderate hydrophilic-lipophilic balance in formulating composite pastes for printed stretchable conductors, with a possibility of a vertically stackable, three-dimensional printing process. We demonstrated that the addition of a non-ionic surfactant, sorbitane monooleate (commonly called SPAN 80) in Ag flake-based composite pastes allows a critical reduction in resistance variation under an external strain. The four-layer stacked, surfactant-added composite conductors showed a resistance variation of merely 1.6 at a strain of 0.6 and an excellent cycling durability over 1,000 times. The effectiveness of the methods suggested in this study was demonstrated with basic light-emitting diode circuits and the thermal heating characteristics of stretchable conductors.
Coating inkjet‐printed traces of silver nanoparticle (AgNP) ink with a thin layer of eutectic gallium indium (EGaIn) increases the electrical conductivity by six‐orders of magnitude and significantly improves tolerance to tensile strain. This enhancement is achieved through a room‐temperature “sintering” process in which the liquid‐phase EGaIn alloy binds the AgNP particles (≈100 nm diameter) to form a continuous conductive trace. Ultrathin and hydrographically transferrable electronics are produced by printing traces with a composition of AgNP‐Ga‐In on a 5 µm‐thick temporary tattoo paper. The printed circuit is flexible enough to remain functional when deformed and can support strains above 80% with modest electromechanical coupling (gauge factor ≈1). These mechanically robust thin‐film circuits are well suited for transfer to highly curved and nondevelopable 3D surfaces as well as skin and other soft deformable substrates. In contrast to other stretchable tattoo‐like electronics, the low‐cost processing steps introduced here eliminate the need for cleanroom fabrication and instead requires only a commercial desktop printer. Most significantly, it enables functionalities like “electronic tattoos” and 3D hydrographic transfer that have not been previously reported with EGaIn or EGaIn‐based biphasic electronics.
Eutectic gallium–indium (EGaIn) has attracted significant attention in recent years for its use in soft and stretchable electronics. However, advances in scalable fabrication approaches and effective electromechanical interfaces between liquid metal (LM) traces and microelectronics are still needed to create functional soft and stretchable electronics. In this study, EGaIn–metal interfacing for the effective integration of surface-mount microelectronics with LM interconnects is investigated. The electrical interconnects are produced by creating copper patterns on a soft-elastomer substrate, and subsequently exposing the substrate to EGaIn, which selectively wets the Cu traces. To create strong electromechanical connection between EGaIn and microelectronics, the terminals of the LM-coated traces are “soldered” to the metal pins of the packaged microelectronic circuits using a novel HCl-vapor treatment. In combination, the fabrication and microelectronics-interfacing approaches enable creating stretchable circuits composed of LM wiring and packaged microelectronics. It is found that the HCl-vapor treatment significantly improves electrical conductivity at the LM–pin interface while enhancing the strain limit of the soft circuits and the reproducibility of the interface. The applicability of this approach in creating soft-matter circuits is demonstrated through two illustrative examples—a circuit with a digital 9-axis inertial measurement unit and a temperature sensor; and a circuit with a 3-axis analog accelerometer.
Stretchable conductors are vital and indispensable components in soft electronic systems. The development for stretchable conductors has been highly motivated with different approaches established to address the dilemma in the conductivity and stretchability trade-offs to some extent. Here, a new strategy to achieve superelastic conductors with high conductivity and stable electrical performance under stretching is reported. It is demonstrated that by electrically anchoring conductive fillers with eutectic gallium indium particles (EGaInPs), significant improvement in stretchability and durability can be achieved in stretchable conductors. Different from the strategy of modulating the chemical interactions between the conductive fillers and host polymers, the EGaInPs provide dynamic and robust electrical anchors between the conductive fillers. A superelastic conductor which can achieve a high stretchability with 1000% strain at initial conductivity of 8331 S cm⁻¹ and excellent cycling durability with about eight times resistance change (compared to the initial resistance at 0% strain before stretching) after reversibly stretching to 800% strain for 10 000 times is demonstrated. Applications of the superelastic conductor in an interactive soft touch device and a stretchable light-emitting system are also demonstrated, featuring its promising applications in soft robotics or soft and interactive human–machine interfaces.
Printing is one of the easy and quick ways to make a stretchable wearable electronics. Conventional printing methods deposit conductive materials “on” or “inside” a rubber substrate. The conductors made by such printing methods cannot be used as device electrodes because of the large surface topology, poor stretchability, or weak adhesion between the substrate and the conducting material. Here, a method is presented by which conductive materials are printed in the way of being surface-embedded in the rubber substrate; hence, the conductors can be widely used as device electrodes and circuits. The printing process involves a direct printing of a metal precursor solution in a block-copolymer rubber substrate and chemical reduction of the precursor into metal nanoparticles. The electrical conductivity and sensitivity to the mechanical deformation can be controlled by adjusting the number of printing operations. The fabrication of highly sensitive vibration sensors is thus presented, which can detect weak pulses and sound waves. In addition, this work takes advantage of the viscoelasticity of the composite conductor to fabricate highly conductive stretchable circuits for complicated 3D structures. The printed electrodes are also used to fabricate a stretchable electrochemiluminescence display.
Printable elastic conductors promise large-area stretchable sensor/actuator networks for healthcare, wearables and robotics. Elastomers with metal nanoparticles are one of the best approaches to achieve high performance, but large-area utilization is limited by difficulties in their processability. Here we report a printable elastic conductor containing Ag nanoparticles that are formed in situ, solely by mixing micrometre-sized Ag flakes, fluorine rubbers, and surfactant. Our printable elastic composites exhibit conductivity higher than 4,000 S cm(-1) (highest value: 6,168 S cm(-1)) at 0% strain, and 935 S cm(-1) when stretched up to 400%. Ag nanoparticle formation is influenced by the surfactant, heating processes, and elastomer molecular weight, resulting in a drastic improvement of conductivity. Fully printed sensor networks for stretchable robots are demonstrated, sensing pressure and temperature accurately, even when stretched over 250%.
The use of liquid metals based on gallium for soft and stretchable electronics is discussed. This emerging class of electronics is motivated, in part, by the new opportunities that arise from devices that have mechanical properties similar to those encountered in the human experience, such as skin, tissue, textiles, and clothing. These types of electronics (e.g., wearable or implantable electronics, sensors for soft robotics, e-skin) must operate during deformation. Liquid metals are compelling materials for these applications because, in principle, they are infinitely deformable while retaining metallic conductivity. Liquid metals have been used for stretchable wires and interconnects, reconfigurable antennas, soft sensors, self-healing circuits, and conformal electrodes. In contrast to Hg, liquid metals based on gallium have low toxicity and essentially no vapor pressure and are therefore considered safe to handle. Whereas most liquids bead up to minimize surface energy, the presence of a surface oxide on these metals makes it possible to pattern them into useful shapes using a variety of techniques, including fluidic injection and 3D printing. In addition to forming excellent conductors, these metals can be used actively to form memory devices, sensors, and diodes that are completely built from soft materials. The properties of these materials, their applications within soft and stretchable electronics, and future opportunities and challenges are considered.
Stretchable hydrogel electronics and devices are designed by integrating stretchable conductors, functional chips, drug-delivery channels, and reservoirs into stretchable, robust, and biocompatible hydrogel matrices. Novel applications include a smart wound dressing capable of sensing the temperatures of various locations on the skin, delivering different drugs to these locations, and subsequently maintaining sustained releases of drugs.
Soft conductors are created by embedding liquid metal nanoparticles between two elastomeric sheets. Initially, the particles form an electrically insulating composite. Soft circuit boards can be handwritten by a stylus, which sinters the particles into conductive traces by applying localized mechanical pressure to the elastomeric sheets. Antennas with tunable frequencies are formed by sintering nanoparticles in microchannels.
Liquid metal nanoparticles that are mechanically sintered at and below room temperature are introduced. This material can be sintered globally on large areas of entire deposits or locally to create liquid traces within deposits. The metallic nanoparticles are fabricated by dispersing a liquid metal in a carrier solvent via sonication. The resulting dispersion is compatible with inkjet printing, a process not applicable to the bulk liquid metal in air.
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
A highly stretchable and conductive poly(dimethylsiloxane)-galinstan composite that exhibits unique electro-mechanical coupling is developed. Compression irreversibly induces conductivity in the virgin material, and allows for the ability to draw circuits into the composite via selectively applying force. Tensile loading causes an apparent increase in volumetric conductivity, resulting in only modest resistance change when the material is under strains as high as 100%.
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Block copolymer silver nanoparticle composite elastic conductors were fabricated through solution blow spinning and subsequent nanoparticle nucleation. The reported technique allows for conformal deposition onto non-planar substrates. We additionally demonstrated the ability to tune the strain dependence of the electrical properties by adjusting nanoparticle precursor concentration or localized nanoparticle nucleation. The stretchable fiber mats were able to display electrical conductivity values as high as 2000 ± 200 S/cm with only a 12% increase in resistance after 400 cycles of 150% strain. Stretchable elastic conductors with similar and higher bulk conductivity have not achieved comparable stability of electrical properties. These unique electromechanical characteristics are primarily the result of structural changes during mechanical deformation. The versatility of this approach was demonstrated by constructing a stretchable light emitting diode circuit and a strain sensor on planar and non-planar substrates.
Three-dimensional (3-D) Monte Carlo simulation has been performed for predicting the percolation threshold in electrical conductivity of polymer composites filled with randomly oriented conductive particles of various morphologies. The conductive fillers like graphite nanoplatelets have been simulated as interpenetrating thin disks and oblate ellipsoids of revolution. For large aspect ratios we find close agreement between disks and oblate ellipsoids of revolution for the average number of bonds per object BC ≈ 2.3, that is in agreement with previous calculations for other shapes of filler (e. g. oblate prisms). The vermicular particles of thermoexfoliated graphite (TEG) have been simulated as the chains of disks jointed in a special manner. The values of percolation threshold have been calculated for various configuration of graphite chains (fragments of vermicular TEG particles) using the Monte Carlo method. The percolation threshold was shown to be determined by the number of disks m and angle between connected disks. It was found that critical volume fraction strongly depends on disks' aspect ratio ε, their orientation within a chain and length of this chain: ϕc decreases with increase of m and ε. The comparison of numerical results with our previous experimental data showed a reasonably good agreement.
A method to produce soft and stretchable microelectronics composed of a liquid-phase Gallium-Indium alloy with micron-scale circuit features is introduced. Microchannels are molded onto the surface of a poly(dimethylsiloxane) (PDMS) elastomer and filled with EGaIn using a micro-transfer deposition step that exploits the unique wetting properties of EGaIn in air. The liquid-filled channels function as stretchable circuit wires or capacitor electrodes with a 2 μm linewidth and 1 μm spacing.
Research in stretchable conductors is fuelled by diverse technological needs. Flexible electronics, neuroprosthetic and cardiostimulating implants, soft robotics and other curvilinear systems require materials with high conductivity over a tensile strain of 100 per cent (refs 1, 2, 3). Furthermore, implantable devices or stretchable displays need materials with conductivities a thousand times higher while retaining a strain of 100 per cent. However, the molecular mechanisms that operate during material deformation and stiffening make stretchability and conductivity fundamentally difficult properties to combine. The macroscale stretching of solids elongates chemical bonds, leading to the reduced overlap and delocalization of electronic orbitals. This conductivity-stretchability dilemma can be exemplified by liquid metals, in which conduction pathways are retained on large deformation but weak interatomic bonds lead to compromised strength. The best-known stretchable conductors use polymer matrices containing percolated networks of high-aspect-ratio nanometre-scale tubes or nanowires to address this dilemma to some extent. Further improvements have been achieved by using fillers (the conductive component) with increased aspect ratio, of all-metallic composition, or with specific alignment (the way the fillers are arranged in the matrix). However, the synthesis and separation of high-aspect-ratio fillers is challenging, stiffness increases with the volume content of metallic filler, and anisotropy increases with alignment. Pre-strained substrates, buckled microwires and three-dimensional microfluidic polymer networks have also been explored. Here we demonstrate stretchable conductors of polyurethane containing spherical nanoparticles deposited by either layer-by-layer assembly or vacuum-assisted flocculation. High conductivity and stretchability were observed in both composites despite the minimal aspect ratio of the nanoparticles. These materials also demonstrate the electronic tunability of mechanical properties, which arise from the dynamic self-organization of the nanoparticles under stress. A modified percolation theory incorporating the self-assembly behaviour of nanoparticles gave an excellent match with the experimental data.