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Soft Science

Published by OAE Publishing Inc.
Online ISSN: 2769-5441
Discipline: Materials Science
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Aims and scope

Soft Science is dedicated to rapidly reviewing and publishing high-quality research in the broad field of mechanically soft, deformable and deployable materials, devices and systems. Emphasis is laid on the impact, depth and originality of new concepts, observations, manufacturing, materials, structures, devices, and systems of the forefront of engineering and applied sciences.

The topical areas of interest include: Manufacturing, properties and applications of soft materials, structures, electronics and systems; Soft, flexible, stretchable, wearable electronics, optoelectronics, sensors, biomedicine and energy devices; Conformable and deployable electronics; Soft robotics, smart skins, and human machine interfaces; Soft materials, smart materials, and composites; Soft structures and architecture materials; Organic, inorganic and hybrid thin films, materials and devices; Additive manufacturing (e.g. 3D, 4D printing) of soft materials, structures and devices; Printed electronics and systems.

 

Editors

Recent publications
Article
The sintering of printed nanoparticle films is a necessary processing step for most nanoparticle inks to make the printed film functional. The sintering of nanoparticle is usually performed through thermal sintering, photonic sintering, induction sintering, etc. Intense pulsed light (IPL) sintering method is one of the most popular sintering methods for nanoparticle inks due to the fast and effective process, but it may yield mediocre performance if improper sintering parameters are used. In this work, we investigate the correlation between the two factors which are the print passes of aerosol jet printing and the sintering distance of the samples on the effect of the surface morphology and sheet resistance. A contradictory correlation between the two factors was observed and a multi-objective optimization was carried out using machine learning method to identify the most optimum conditions for both factors. We found that multi-objective optimization approach is effective in reducing the conflicting responses, thus the sintered thin film can have low sheet resistance and low surface roughness. This work provides an essential guide for achieving conductive films with electrical conductivity and low surface roughness using IPL sintering process for fast fabrication of multi-layered electronics such as electrochemical electrodes.
 
Article
Wound healing is one of the most complex processes in the human body, supported by many cellular events that are tightly coordinated to repair the wound efficiently. Chronic wounds have potentially life-threatening consequences. Traditional wound dressings come in direct contact with wounds to help them heal and avoid further complications. However, traditional wound dressings have some limitations. These dressings do not provide real-time information on wound conditions, leading clinicians to miss the best time for adjusting treatment. Moreover, the current diagnosis of wounds is relatively subjective. Wearable electronics have become a unique platform to potentially monitor wound conditions in a continuous manner accurately and even to serve as accelerated healing vehicles. In this review, we briefly discuss the wound status with some objective parameters/biomarkers influencing wound healing, followed by the presentation of various novel wearable devices used for monitoring wounds and accelerating wound healing. We further summarize the associated device working principles. This review concludes by highlighting some major challenges in wearable devices toward wound healing that need to be addressed by the research community.
 
PEDOT:PSS-based intrinsically soft and stretchable bioelectronics. (A) Chemical structure of PEDOT:PSS, the colloidal particle structure of PEDOT:PSS in water dispersion and the microstructure of a PEDOT:PSS film[2]. Reprinted with permission. Copyright 2016, Springer Nature. (B) Schematic illustration of PEDOT:PSS/IL composite, where IL serves as a enhancer of stretchability and electrical conductivity[5]. Reprinted with permission. Copyright 2017, AAAS. (C) Structure of stretchable PEDOT:PSS composite with a topological sliding network[6]. Reprinted with permission. Copyright 2022, AAAS. (D) Pure PEDOT:PSS hydrogel with a single network[8]. Reprinted with permission. Copyright 2019, Springer Nature. Three-dimensional printing of PEDOT:PSS into (E) circuit pattern or (F) neural probe (scale bars of 5 and 1 mm, respectively)[9]. Reprinted with permission. Copyright 2020, Springer Nature. (G) Schematic diagram of PEDOT:PSS/PVA double network hydrogels obtained from a mixed solution to form the PVA network and finally a double network[11]. Reprinted with permission. Copyright 2022, Wiley-VCH.
Article
Intrinsically soft and stretchable bioelectronics exhibit tissue-like mechanical behavior that enables the seamless integration of electronic devices with the human body to achieve high-quality biosignal recording and high-efficacy neural modulation. The conducting polymer poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) shows significant promise in this field because of its high conductivity, excellent biocompatibility and commercial availability. However, pristine PEDOT:PSS is brittle and rigid and thus cannot be used in soft and stretchable electronics. More effort is therefore required to engineer PEDOT:PSS into a stretchable conductor that meets the demands of bioelectronics. In this perspective, we review the recent progress and propose the possible future directions of PEDOT:PSS-based bioelectronics.
 
Article
Self-assembled monolayers (SAMs) have found use in diverse applications that range from corrosion prevention to biosensing. However, for all of these applications, stability remains a key challenge for the utilization of SAMs. Over the last decade, intermolecular crosslinking as a method to enhance the thermal and chemical stability of SAMs has attracted increased attention from scientists and engineers. As such, this review introduces a variety of crosslinked SAMs: (1) aromatic thiol-based SAMs; (2) olefinic- and acetylenic-based alkanethiols; (3) other aliphatic alkanethiols; (4) silane-based alkanethiols; (5) boronic acid-based alkanethiols; and (6) crosslinked SAMs realized by hydrogen bonding. By offering insight into the structure-application relationships of the aforementioned SAMs, this review seeks to inspire researchers toward the development of new classes of SAMs with enhanced stabilities and working lifetimes.
 
Article
Microfluidic technology has potential advantages in the complex manipulation of microfluidics on small-sized chips. However, it is difficult to integrate microvalves with complex flow channel structures, and this has limited the miniaturization of microfluidic systems and their portable applications. Light-responsive hydrogel (LRH) materials can rapidly change their volume under laser irradiation and can be used to prepare flexible microvalves to realize the integrated control of microfluidics. A simple fabrication method for an LRH microvalve on a microfluidic chip is proposed. The microspheres, as control elements of the microvalve based on an LRH modified with Laponite RD nanoclay and ferriferous oxide (Fe 3 O 4) nanoparticles, are prepared through a T-shaped flow channel. The microvalve is assembled on the microfluidic chip with a normally closed circulation channel. The open/close performance of the microvalve is represented by the color change of the photonic crystal material. The results show that the LRH microspheres shrink and the flow channel opens after laser irradiation for 2 s. After stopping the laser at 18 s, the valve core swells and the flow channel closes.
 
Self-assembly of all-DNA hydrogels through (A) hybridization of DNA sticky ends[28], (B) formation of i-motifs[29], (C) enzymatic ligation[27] and (D) physical entanglement[31]. (A, B) Reprinted with permission from Refs.[28,29]. Copyright 2009, 2011 Wiley-VCH. (C, D) Reprinted with permission from Refs.[27,31]. Copyright 2006, 2012 Springer Nature.
(A) Light-responsive hybrid DNA hydrogels based on azobenzene isomerization. Reprinted with permission from Ref.[33]. Copyright 2012, American Chemical Society. (B) pH-responsive hybrid DNA hydrogels based on cyclic switching of i-motifs. Reprinted with permission from Ref.[36]. Copyright 2015 Wiley-VCH. DNA hybrid hydrogels for (C) biomimetic protein release and signal transduction or (D) cell catch and release. (C) Reprinted with permission from Ref.[41]. Copyright 2017, Royal Society of Chemistry. (D) Reprinted with permission from Ref.[42]. Copyright 2012, American Chemical Society.
Drug delivery applications of (A) DNA hydrogels and (B) patterned hydrogel films. (A) Reprinted with permission from Ref.[48]. Copyright 2017, Royal Society of Chemistry. (B) Reprinted with permission from Ref.[52]. Copyright 2018, American Chemical Society. Biosensing applications of (C) bioimprinted aptamer hydrogels and (D) flexible electrochemical sensors. (C) Reprinted with permission from Ref.[61]. Copyright 2013, American Chemical Society. (D) Reprinted with permission from Ref.[67]. Copyright 2018, Wiley-VCH.
Article
Due to considerable progress in DNA nanotechnology, DNA is gaining significant attention as a programmable building block for the next generation of soft biomaterials. DNA has been used as either the only component to form all-DNA hydrogels or a cross-linker or functional entity to form hybrid DNA hydrogels through physical interactions or chemical reactions. The formed hydrogels exhibit adequate biocompatibility, convenient programmability, tunable multifunctionality, and capability of precise molecular recognition, making them an irreplaceable polymeric platform for interfacing biology. Responsive DNA hydrogels that are prepared through hybridization of DNA sticky ends, formation of i-motifs, enzymatic ligation, and enzymatic polymerization are commonly reported nowadays, which can undergo disassembly induced by various triggers, including alteration in ionic strength, pH, temperature, and biomolecules. These hydrogels are envisioned for applications of drug delivery and biosensing. This perspective aims to assess the most recent and important developments in this emerging class of biomedically useful DNA hydrogels.
 
Article
Additive manufacturing is an arising technology for soft materials and structures with improved complexity and functionality and has gradually become widespread in various fields, including soft robotics, flexible electronics and biomedical devices. Along with the development of material systems and fabrication techniques, mechanical design principles for additive manufactured soft materials have been greatly developed and evolved in recent years and some unique issues that are distinct from conventional manufacturing techniques have emerged. In this short review, we mainly focus on additive manufactured soft materials that are in significant need of mechanical models/simulations to provide design guidelines; therefore, topics such as soft robotics and electronics are not considered here. We first discuss the mechanical design methods for controlling shape distortions and interfacial strength, as they are directly related to the quality and reliability of additive manufactured soft materials. Design principles and manufacturing strategies for bioinspired composites, which represent a large part of current research on additive manufactured soft materials, are then summarized integrally with regards to three aspects. In addition, basic mechanical considerations for additive manufactured four-dimensional shape-changing structures are explained, together with a review of the recent theories and numerical approaches. Finally, suggestions and perspectives are given for future developments in soft material additive manufacturing.
 
Article
Mechanical energy conversion based on piezoelectric principle has received much attention due to its promising applications in sustainable power supply systems and sensor technology. Ferroelectric poly(vinylidene fluoride) (PVDF) combines the advantages of both good electromechanical coupling and easy processability, yet the low piezoelectric coefficient limits its output performances thus cannot meet the increasing requirements for power generation and sensing. Here, inorganic metal halide perovskite CsPbBr3 (CPB) nanoparticles have been incorporated into the PVDF fibers via electrospinning technique, where an in situ crystallization and growth process of CPB nanoparticles have been established. Meanwhile, both the CPB nanoparticles and PVDF fibers are poled by the electric field during electrospinning process, which promotes the formation of polar phase of PVDF and the distortion of CPB lattice, resulting in greatly enhanced piezoelectric performances of CPB/PVDF composites. The output performances under external force of the flexible generator developed from electrospun CPB/PVDF films are significantly enhanced compared with neat PVDF film, with the maximum Voc value 8.4 times higher; while the measurements on the microscopic piezoelectric responses unambiguously reveal that the increased polar phase mainly contributes to the enhanced electromechanical coupling. The functions of CPB/PVDF film as physiological signals monitoring sensor have been performed, demonstrating its potential applications as flexible piezoelectric generator and wearable health monitoring electronics.
 
Article
Fused deposition modeling (FDM) is an additive manufacturing technique with significant advantages, including cost effectiveness, applicability for a wide range of materials, user-friendliness and small equipment features. However, its poor resolution represents a hindrance for functional parts for commercial production. In this review, the key process parameters are presented with their factors and effects on the characteristics of FDM-printed polymeric products. Hence, better insights into the relationship between key parameters and three main printing characteristics, namely, surface roughness, mechanical strength and dimensional accuracy, in existing FDM research are provided. A conclusion that addresses the challenges and future research directions in this area is also presented.
 
Article
With the continuous development of soft conductive materials, polyelectrolyte-based conductive hydrogels have gradually become a major research hotspot because of their strong application potential. This review first considers the basic conductive theory of hydrogels, which can be divided into the hydrogel structure and zwitterionic enhancing conductivity theories. We then classify polyelectrolyte-based conductive hydrogels into different types, including double, ionic-hydrogen bond, hydrogen bond,and physically crosslinked networks. Furthermore, the mechanical, electrical, and self-healing properties and fatigue and temperature interference resistance of polyelectrolyte-based conductive hydrogels are described in detail. We then discuss their versatile applications in strain sensors, solid-state supercapacitors, visual displays, wound dressings, and drug delivery. Finally, we offer perspectives on future research trends for polyelectrolyte-based conductive hydrogels.
 
Article
Whereas piezoelectric pressure sensors (PPSs) have been applied in the monitoring of human body movement and physiological parameters, they show inherent limitations in wearable applications, including toxicity, degradation, and brittleness. In this study, we develop safe, stable, and mechanically flexible composite thin films consisting of polyvinylidene fluoride (PVDF), BaTiO3 nanoparticles (BTO-NPs), and textured aluminum nitride (AlN) thin film for the demonstration of wearable PPS with enhanced output performance and biocompatibility. The PPS made of BTO-NP-embedded-PVDF and AlN film on Cu foil is attached to different parts of human body to measure different output voltages depending on the physiological and physical stimulus. The simple bending (from breathing, chewing, and swallowing), joint motions (at wrist, elbow, and finger), and low- (from eyeball movement) and high-pressure applications (by squat, lunge, and walking) are measured. Our PVDF+BTO-NP/AlN-PPS (PBA-PPS) device has the potential for personal safety, healthcare, and activity monitoring applications with easy wearability.
 
Article
Interventional surgery has the advantages of small skin incision, little bleed loss, low postoperative infection and short recovery time, and thus has gradually become the preferred surgical approach over traditional open surgeries. Even though great achievements have been made towards clinical applications, limitations still exist, among which the loss of natural tactile perception of surgeons due to their indirect touch sense along the long catheter to the intervening human tissue is the crucial one. In recent years, researchers have dedicated great efforts in developing advanced medical catheters with smart tactile perception ability and made considerable progress. In this regard, we review the most recent development on the state-of-the-art miniature flexible and soft tactile sensors that are able to be integrated in the tip or on the side wall of medical catheters, with focus on the sensing mechanism, design requirement, device configuration and sensing performance of different types of sensors as well as their application demonstration in synthetic anatomical models and in-vivo animal experiment. After reviewing the representative research work, challenges that still exist are summarized and prospects toward future development are put forward.
 
Article
How the current industrial policy affects enterprise innovation is a core issue of concern by academia and policy makers. Unlike previous studies that focused on the economic "consequences" of industrial policies, the paper based on the 163 IT industrial policies issued by the country's departments at the ministerial level and above from year 2013 to 2019, empirically discusses the transmission mechanism between industrial policies and the quality of enterprise innovation, as well as the moderating role of the regional innovation environment on the mechanism. The study shows that: industrial policy has a significant positive effect on the level of science and technology (S&T) credit and innovation quality of firms; The level of S&T credit plays a partial intermediary role between industrial policy and enterprise innovation quality. Compared with areas with better innovation environment, industrial policies have a stronger incentive effect on enterprise innovation in areas with worse innovation environment; Further research also shows that industrial policies have different effects on heterogeneous enterprises. Compared with inefficient use of low-growth enterprises, high-growth enterprises can make full use of policy resources to carry out R&D activities and improve innovation quality.
 
Article
Portable flexible electronics based on petroleum-based polymers have stepped onto the stage of modern technology. Increasing environmental problems facilitate emerging technologies based on cellulose because of its abundant sources and the nature of CO 2 consumption and biodegradability. Bacterial cellulose (BC) stands out among all cellulose materials because of its unique features, including the abundant hydrogen bonds, small diameter, three-dimensional nano-networked structures, high purity and crystallinity, and the degree of polymerization. The adequate properties impart BC and its nano-nano composites with superior balance among ductility, strength, and porosity, which are crucial for wearables. The principles of this balance, the fabrication of the nano-nano composites, and the wearable electronic applications based on BC are discussed in detail in this review.
 
Article
Magnetism and magnetic monopoles are among the most classical issues in physics. Conventional magnets are generally composed of rigid materials and may face challenges in extreme situations. Here, as an alternative to rigid magnets, we propose, for the first time, the generation of fluidic endogenous magnetism and construct a magnetic monopole through tuning with a liquid metal machine. Based on theoretical interpretation and conceptual experimental observations, we illustrate that when liquid metals, such as gallium alloy, in a solution rotate under electrical actuation, they form an endogenous magnetic field inside. This explains the phenomenon where two such discrete metal droplets can easily fuse together, indicating their reciprocal attraction via the N and S poles. Furthermore, we reveal that a self-fueled liquid metal motor also runs as an endogenous fluidic magnet owing to the electromagnetic homology. When aluminum is added to liquid gallium in solution, it forms a spin motor and dynamically variable charge distribution that produces endogenous magnetism inside. This explains the common phenomena where reflective collision and attractive fusion between running liquid metal motors occur, which are partially caused by the dynamic adjustment of their N and S polarities, respectively. On this basis, more experimental approaches capable of generating dynamic electrical fields also work for the same target. Finally, we propose that such a fluidic endogenous magnet could lead to a magnetic monopole and four technical routes to realize this are suggested. The first involves matching the interior flow of liquid metal machines. The second is the superposition between an external electric effect and the magnetic field. The third route involves composite construction between magnetic particles and a liquid metal spin motor. Finally, chemical methods, such as via galvanic cell reactions, are proposed. Overall, the present theory and identified experimental evidence illustrate the role of a liquid metal machine as a fluidic endogenous magnet and highlight promising methods for the realization of magnetic monopoles. A group of unconventional magnetoelectric devices and applications could therefore be possible in the near future.
 
Article
Significant progress has been achieved for flexible polymer thermoelectric (TE) composites in the recent decade due to their potential application in wearable devices and sensors. In sharp contrast with the booming TE studies at room temperature, the TE performances of polymer TE composites received relatively less attention despite the significance for the application of TE composites in the high temperature environments. The TE and mechanical performances of flexible poly (3,4 ethylenedioxythiophene):poly(styrene sulfonate)/single-walled carbon nanotube (PEDOT:PSS/SWCNT) composite films with ionic liquid (IL) (refer to as “PEDOT:PSS/SWCNT-IL”) at high temperatures are studied in the present work. The resultant composite film shows the increasing TE performances with increasing temperatures and SWCNT contents. The maximum value of the power factor reaches 301.35 W m-1 K-2 at 470 K for the PEDOT:PSS/SWCNT-IL composite. Besides, the addition of IL can improve the elongation at break of composites compared to the IL-free composites. This work promotes the advance of flexible polymer TE composites and widens the potential applications at different temperature ranges.
 
Article
Frequency selective surfaces (FSSs) with reconfigurable resonant frequency show significant potential for engineering applications. In this study, we propose a flexible FSS with a buckling dipole prepared by releasing the substrate pre-strain to buckle the locally adhered two-dimensional precursors, which can withstand large mechanical tensile deformation and change their resonant frequency during deformation. When the FSS is subjected to uniaxial tensile deformation, the capacitive effect between the adjacent buckled metal unit cells is significantly reduced due to the increase in the gap between the unit cells and period. This significant change in the equivalent circuit parameters due to the geometry change is highly beneficial for actively tuning the resonant frequency of the FSS. Electromagnetic (EM) experiments and simulations and equivalent circuit calculations are used to explore the EM tuning mechanism of the FSS and consistent conclusions are obtained. The results show that the FSS exhibits band-stop EM wave transmission characteristics with a resonant frequency of 6.1 GHz in the unstretched state and 21% uniaxial stretching strain can introduce a ~1.1 GHz increase in the resonant frequency. The corresponding parameter analysis shows that when the gap of the buckling dipoles in the width direction is reduced, the change in the resonant frequency caused by uniaxial stretching can be significantly increased, even to 2.5 GHz, which may help the FSS adapt to complex practical applications by tailoring the geometry of the buckling dipole.
 
Article
The influence of increasing fused thiophene rings for the corresponding conjugated polymers [polythiophene (PT), poly(thieno[3,2-b]thiophene) (PTT) and poly(dithieno[3,2-b:2',3'-d]thiophene) (PDTT)] on their photophysical and electrochemical properties, morphology and electrochromic performance are investigated in detail in this study. PDTT is the easiest of the three polymers to prepare and has the lowest onset oxidation potential of 1.17 V because of its increased donor ability, lower than those of PTT (1.41 V) and PT (1.82 V). PDTT also exhibits the best electrochemical and thermal stability because of its extended conjugated skeleton. The PT, PTT and PDTT polymers present poor, good and moderate electrochromic properties, respectively, with increasing fused thiophene rings. PTT displays the highest ΔT of 35% in 700 nm, the fastest response time of 1.0 s and the maximum colouration efficiency (CE) of 94 cm 2 C-1 , which is attributed to its enhanced morphology, since the PTT film is conducive to the promotion of ions to dope and dedope. Flexible electrochromic devices are fabricated and Page 2 of Lin et al. Soft Sci 2021;1:12 https://dx.doi.org/10.20517/ss.2021.15 11 PTT exhibits the highest ΔT (60% in 480 nm and 16% in 660 nm), as well as excellent stability with less than a 5% ΔT reduction after successive cycling of 1000 s. All these findings indicate that the precise regulation of the fused thiophene is crucial in achieving high performance in electrochromism, which provides insight for the design of electrochromic conjugated polymers and flexible electrochromic devices.
 
Article
In the conventional scenario, it is believed that hydrogels typically consist of two-phase coexisting structures based on polymer structural networks filled with water droplets and that the polymer-water interfacial layer may not be a substantial component in determining their structure and functionality. Unfortunately, it is challenging to unveil the properties of the interfacial layer, if any, owing to the multiphase nature and structural complexity of hydrogels. In this work, the morphology and microstructures of the well-known non-covalent bonding dominant polyacrylonitrile-based hydrogels are characterized and it is confirmed that the as-prepared hydrogels do consist of polymer networks and filled water droplets. The dielectric relaxation behavior in the ice hydrogel state with different water/ice contents is investigated in detail by means of dielectric relaxation spectroscopy, in order to avoid the electrode polarization effect, which is non-negligible in liquid hydrogels, particularly in the low-frequency range. The dielectric relaxation spectroscopy data demonstrate the remarkable dielectric response contributed from the polymer-ice interfacial layer, which likely accommodates a high density of polar molecules/dipoles. The temperature-dependent dielectric relaxation behavior of the ice hydrogels with different water contents is discussed and the thermal activation energy for the interfacial polar structure may be likely extracted from the dielectric loss peak data. It is found that this energy is approximately consistent with the typical bonding energy of non-covalent bonding dominant hydrogels. This study represents a substantial step towards understanding the interfacial coupling in hydrogels, an issue that has not yet been thoroughly considered.
 
Crystal structures of a TiS2 single crystal and the hybrid inorganic-organic superlattices with different interlayer distances. (A) TiS2 single crystal. (B) HAADF-STEM image of TiS2/HA/H2O. (C) TiS2/HA/H2O. (D) TiS2/HA/DMSO corresponding to the 1D electron density map along the c-axis. (E) TiS2/HA/glycerin corresponding to the 1D electron density map along the c-axis.
Demonstration of the gradual two-dimensionalization of electrons with increasing interlayer distance. (A) Effective mass and mobility in the in-plane direction as a function of interlayer distance. (B) Temperature dependence of the cross-plane electrical conductivity of the hybrid organic-inorganic superlattices, in which (a-d) represent the TiS2 single crystal, TiS2/HA/DMSO, TiS2/HA/glycerin and TiS2/HA/H2O, respectively.
Electronic DOS of the conduction band as a function of the distance between TiS2 layers.
Demonstration of the gradual two-dimensionalization of electrons with increasing interlayer distance. The valence electron distribution of (A) bulk TiS2, (B) TiS2 with a van der Waals gap of 9.65 Å, (C) TiS2 with a van der Waals gap of 13.9 Å and (D) single layer TiS2.
In-plane thermoelectric properties of TiS2 and the hybrid inorganic-organic superlattices as a function of interlayer distance.
Article
The control of electron and phonon transport by manipulating dimensionality is essential for the performance of advanced electronic materials and devices, such as quantum electronics, thermoelectrics and superconductors, which may also lead to yet undiscovered, emergent electronic or thermal phenomena. In this study, we report a series of hybrid inorganic-organic superlattice structures, in which metallic TiS2 monolayers are spatially confined between soft and insulating organic molecules of varying thicknesses. By choosing different organic molecules that increase the interlayer distance, the electrons inside the TiS2 layers gradually become two-dimensional, with increasing density of states, as seen by their effective mass that increases from 5.3 to 8.6 m0 , where m0 is the mass of a bare electron. In addition, density functional theory calculations confirm a transition of the electron distribution from bulk to two-dimensional, due to the suppressed interlayer coupling. This result demonstrates that the thermoelectric transport of two-dimensional electrons can be realized in a three-dimensional inorganic-organic superlattice, thus enabling access to the interesting properties of individual two-dimensional materials in the bulk form, which may provide new opportunities in flexible thermoelectrics.
 
Flexible, ultrathin, skin-integrated and Au-based strain sensor. (A) Schematic of strain sensor. (B) Optical image of patterned Au/Cr electrode design. (C) Optical image of strain sensor mounted on the human epidermal surface. (D) Finite element analysis of strain distribution on strain sensor under stretching, twisting and bending. (E) Optical image of strain sensor attached on a pink PDMS surface with three mechanical deformations, including bending, stretching and twisting.
Electrical characteristics of strain sensor. (A) Electrical signals (∆R/R0) of strain sensor at different longitudinal tensions. (B) Electrical signals (∆R/R0) of strain sensor at different transverse tensions. (C, D) Electrical signals (∆R/R0) of strain sensor at different frequencies under 12.5% strain and its detailed signals at 6 Hz. (E, F) Electrical signals (∆R/R0) of strain sensor in a fatigue test for 150 cycles and its detailed signals from a selected range.
Electrical signals of flexible strain sensor under different external stimuli. (A) Optical image of strain sensor mounted on the back of a hand and its enlarged optical image of the strain sensor during deformation. (B) Electrical signals (∆R/R0) of strain sensor under different finger bending angles. (C) Optical image of strain sensor mounted on a wrist for human pulse rate detection and its electrical diagram. (D) Electrical signals (V) of strain sensor in the pulse rate test in (C). (E) Optical image of strain sensor mounted on a waist for ankle for walking motion detection. (F) Electrical signals (∆R/R0) of strain sensor under the walking motion test.
Robotic hand controlling performed by flexible strain sensors. (A) Schematic diagram of testing circuit for controlling robotic hand. (B) Optical image of six sensors mounted on the joints of a hand for robotic hand control. (C) Optical image of Arduino breadboard linked with the sensors and the robotic hand. (D) Optical image illustrating that the sensors control the robotic hand to make gestures of “C”, “like you” and “grasp” and their corresponding electrical signals (∆R/R0) of sensors at different gestures (E).
Article
Flexible electronic skin (e-skin) has been successfully utilized in diverse applications, including prosthesis sensing, body-motion monitoring and human-machine interfaces, due to its excellent mechanical properties and electrical characteristics. However, current e-skins are still relatively thick (> 10 µm) and uncomfortable for long-term usage on the human body. Herein, an ultrathin skin-integrated strain sensor with miniaturized dimensions, based on the piezoresistive effect, with excellent stability and robustness, is introduced. The fractal curve-shaped Au electrode in a serpentine format, which is the dominant component of the strain sensor, is sensitive to ambient strain variations and can turn the mechanical motion into a stable electrical signal output. With the advanced design of metallic electrodes, the device presents good operational stability and excellent mechanical tolerance towards bending, stretching and twisting. The stain sensor allows intimate mounting onto the human epidermal surface for the detection of body motion. By adopting a liquid bandage as an encapsulation layer, the device exhibits an ultrathin thickness (6.2 µm), high sensitivity towards mechanical deformations and capability for the clear Page 2 of Wong et al. Soft Sci 2021;1:10 https://dx.doi.org/10.20517/ss.2021.09 11 detection of motion, such as walking, finger bending and the human pulse rate with identifiable electrical signals. Furthermore, the tattoo-like strain sensor is applied in robotic control by tracing finger bending motion and results in the smooth control of a robotic hand nearly without any detention. This e-skin design exhibits excellent potential for wearable electronics and human-machine interfaces.
 
(A) Image of silicone-oil-dispersed sodium nanofluid. (B) Steps in fabricating a synthetic sandstone core. (C) Sand size distribution using the sieve mesh method.
(A) Schematic illustration of the core flooding equipment. (B) High-pressure core holder for synthetic sandstone core flooding. (C) Glass column for oil sand pack flow experiments.
Results from six static experiments in which oil sands were mixed with (A) brine, (B) silicone oil and brine, or (C) sodium nanofluid and brine. Individual photographs show initial results and those after soaking for at least 72 h at 22 °C and 50 °C.
(A) Mixture of 1 g heavy oil, 40 mg sodium nanoparticles dispersed in 0.2 mL silicone oil, and 1 mL brine containing 5.66 wt.% NaCl. (B) Optical image of the emulsion formed by the heavy oil, sodium nanofluid, and brine. Inset: photograph of the emulsion sample.
Composition of vials prepared for static experiments
Article
Nanomaterials exhibit unique chemical and physical properties in comparison with their bulk-phase counterparts, attracting significant attention from the oil and gas industry in the hope of solving challenging issues. Current heavy oil extraction methods are costly and have unsatisfactory efficiency, and facing environmental restrictions increasingly. Our recent introduction of sodium (Na) nanofluid provides a promising method for heavy oil extraction since it shows improved oil recovery without burning carbon-containing fuels. Here, we conducted core�flooding tests to further evaluate the effect of this Na nanofluid on recovering oil from different formations, which had not been previously demonstrated, as well as to deepen our understanding of the underlying mechanisms. The Na nanofluid exhibited excellent oil-extraction efficiency for both types of heavy oil tested. The recovery mechanisms were found to be complicated. We also found that post-injection soaking and using the proper solvent to disperse the sodium nanoparticles are important for further boosting oil recovery.
 
Article
Tactile sensors have received increasing research interest owing to the broad applications in areas of health monitoring, artificial intelligence, robotics, and prosthetics. The ability to understand and perceive touch and heat is of importance because it helps people to recognize objects, prevent injury, and provide heat information from grasped objects. However, bimodal tactile sensors often suffer from signal interference and complicated fabrication process. Numerous efforts have been undertaken to develop highly independent sensors based on different transduction principles as well as the device integration techniques. Here, strategies for improvement of main performance parameters such as sensitivity, sensing range, hysteresis, response/recovery time, and stability are discussed. A comprehensive overview of important progress in pressure and temperature tactile sensors in recent years is summarized. According to sensor units and transduction principles, temperature and pressure tactile sensors are categorized into two types: dual-parameter sensors and integrated bimodal sensors. Integration of tactile sensors from the viewpoint of power supply, wireless communication, and signal process circuit is given. Finally, challenges and outlook are provided and presented for pressure and temperature tactile sensors.
 
Article
Along with the rapid progress of wearable and portable electronic devices including electrical sensors, flexible displays, and health monitors, there is an ever-growing demand for wearable power sources. Supercapacitors, as a new kind of energy storage device, have received considerable attention for decades due to their high power density, excellent cycling stability, and easy fabrication. To fulfill the demand of wearable power sources, wearable supercapacitors are also further developed and studied. New electrode materials that play a significant role in determining both the wearability and electrochemical performance of wearable supercapacitors are also extensively explored. Herein, the recent progress on wearable soft electrode/electrolyte materials and the structure design strategies for developing wearable supercapacitors are summarized. Additionally, the existing challenges in current technologies and research are highlighted and discussed with the hope of inspiring future studies.
 
Top-view SEM images of pristine perovskite films (A). Top-view SEM images of perovskite films modified by PI solution with different concentrations: (B) 0.5 wt%; (C) 1.0 wt%; (D) 1.5 wt%; and (E) 2.0 wt%. The corresponding water contact angles based on the above perovskite films: (F) for (A); (G) for (B); (H) for (C); (I) for (D); and (J) for (E). SEM: Scanning electron microscopy. PI: polyimide.
(A) J-V curves and (B) the PCE statistical distributions of 20 devices with different concentrations of PI. (C) The steady-state photocurrent measurements at maximum power point. (D) EQE spectra for the devices with (w/) and without (w/o) PI. PCE: High power conversion efficiency; EQE: external quantum efficiency; PI: polyimide.
(A) PL spectra and (B) TRPL decay curves for the devices with (w/) and without (w/o) PI. (C) The dark current-voltage curves for hole-only devices with (w/) and without (w/o) PI. (D) Nyquist plots of the devices with (w/) and without (w/o) PI, measured in the dark. TRPL: Time-resolved photoluminescence; PI: polyimide; PL: photoluminescence.
(A) Variations of the UV-vis absorption and (B) XRD measurement of perovskite without and with PI under an RH of 75% at 25 °C in dark conditions with respect to the exposure duration. (C) Long-term stability of the six corresponding PSCs under an RH of 50% at 25 °C in dark conditions. The inserts are optical photographs. (D) Normalized average PCE of 10 PSCs as a function of stretching cycles under a curvature radius of 6.25 mm. The inset shows the photograph of the flexible device under curvature. XRD: X-ray diffraction RH: relative humidity; PSCs: perovskite solar cells; PI: polyimide.
Article
Perovskite solar cells (PSCs) have aroused tremendous attention due to the high power conversion efficiency (PCE) and flexibility of the organic-inorganic hybrid perovskite films. Whereas the commercialization of perovskite solar cells is still impeded due to the instability issue induced by moisture and mechanical stress. Herein, we introduce a soluble hydrophobic polyimide (PI) as an interfacial layer on top of perovskite film to block the infiltration of moisture into perovskite film. The MAPbI3 based solar cell with the insertion of PI layer exhibited an impressive stability, remaining 87% of initial PCE even after exposing to 50% relative humidity (RH) for 550 h, and a decent PCE of 21.22% due to its capability to extract holes and reduce trap-assisted recombination. Moreover, the high tolerance of PI to the mechanical stress gives a better flexible stability of the PSCs under constant bending.
 
Article
With the rapid development of artificial intelligence, human-machine interaction, and healthcare systems, flexible tactile sensors have huge market potentials and research needs, so that both fundamental research and application demonstrations are evolving rapidly to push the potential to reality. In this review, we briefly summarize the recent progress of the flexible tactile sensor system, including the common sensing mechanisms, the important performance evaluation parameters, the device design trend, and the main applications. Moreover, the current device design trend towards flexible tactile sensor systems is discussed, including novel structures for outstanding performance, sensor arrays for large-area information acquisition, multi-mode information acquisition, and integration of tactile sensors with transistors. Various emerging applications enabled with these sensors are also exemplified in this review to show the potentials of the tactile sensors. Finally, we also discuss the technical demands and the future perspectives of flexible tactile sensor systems.
 
(A) The preparation process of Bi2Se3+x/PVDF composite films; and (B) a digital photograph of the flexible Bi2Se3+x/PVDF films.
(A, B) XRD patterns of the Bi2Se3+x/PVDF composite films; (C) XPS spectra fitting curve for Bi2Se3.2 powder; and (D) Se/Bi molar ratio of ICP-MS measurement for Bi2Se3+x (x = 0, 0.2, 0.3, 0.4) powder.
(A, B) Surface and cross-section SEM image of Bi2Se3.2/PVDF composite films; and (C-E) in-plane TEM image, HRTEM image of the region in (C), and the SAED corresponding to the main phase.
Temperature-dependent thermoelectric properties of Bi2Se3+x/PVDF composite films: (A) Seebeck coefficient; (B) electrical conductivity; (C) power factor; and (D) Hall carrier concentration and mobility at room temperature.
Mechanical stability test of Bi2Se3.2/PVDF composite films.
Article
Bismuth selenide materials (Bi2Se3) have high performance around room temperature, demonstrating potential in thermoelectric applications. Presently, most vacuum preparation techniques used to fabricate the film materials, such as magnetron sputtering and molecular beam epitaxy, usually require complex and expensive equipment. This limits the practical applications of flexible thermoelectric films. Here, we prepared Bi2Se3+x nanoplate/polyvinylidene fluoride composite films with good flexibility using a facile chemical reaction method. Their thermoelectric performance and microstructures were systematically studied. The composite films exhibit a highly preferred orientation along (015). The carrier concentration and mobility were optimized by adding excessive element Se, eventually leading to an improvement in thermoelectric performance. The optimized power factor is 5.2 μW/K²m at 300 K. Furthermore, the performance remains stable after 2500 bending cycles at a radius of 1 cm, suggesting promising applications in wearable/portable electronics.
 
Article
The dielectric elastomer actuator (DEA) is one type of emerging soft actuator that has the attractive features of large actuation strains, high energy density, and inherent compliance, which is desirable for novel bio-inspired and soft robotic applications. Due to their inherent elasticity, when stimulated by an alternating current voltage with a frequency matching the natural frequency of the DEA system, the DEAs can exhibit resonant responses which maximize the oscillation amplitude. Silicone elastomers are widely utilized for resonant actuation applications for their reduced viscous damping hence better dynamic performance compared to VHB elastomers. However, the low pre-stretch ratios adopted by silicone elastomers could induce loss-of-tension of the membranes in high amplitude oscillations, yet its effects on the dynamic responses of a DEA are not fully understood. By using a numerical dynamic model, this work studies the effects of the loss-of-tension on the frequency response of the antagonistic pure-shear DEAs. A subharmonic frequency response curve isolated from the main response branch is uncovered for the first time in a parametrically forced DEA system, which causes a sudden jump in the oscillation amplitude and serves as a severe threat to the dynamic stability and controllability of the DEA system. By using a global analysis method, the evolution of the isolated response curve against the excitation components and system physical parameters is also investigated numerically. Page 2 Cao et al. Soft Sci 2021;1:1 I http://dx.
 
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Top-cited authors
Kuanming Yao
  • City University of Hong Kong
Jingkun Zhou
  • City University of Hong Kong
Daxing Huang
  • Chinese Academy of Sciences
Hongwei gu
  • 东北财经大学
Hongjing Shang
  • Chinese Academy of Sciences