Stretchable self-healable semiconducting polymer film for active-matrix strain-sensing array

Investigation of stretchable strain sensor based on CNT/AgNW applied in smart wearable devices

Stretchable strain sensor, an important paradigm of wearable sensor which can be attached onto clothing or even human skin, is widely used in healthcare, human motion monitoring and human-machine interaction. Pattern-available and facile manufacturing process for strain sensor is pursued all the time. A carbon nanotube (CNT)/silver nanowire (AgNW)-based stretchable strain sensor fabricated by a facile process is reported here. The strain sensor exhibits a considerable Gauge factor of 6.7, long-term durability (>1000 stretching cycles), fast response and recovery (420 ms and 600 ms, respectively), hence the sensor can fulfill the measurement of finger movement. Accordingly, a smart glove comprising a sensor array and a flexible printed circuit board is assembled to detect the bending movement of five fingers simultaneously. Moreover, the glove is wireless and basically fully flexible, it can detect the finger bending of wearer and display the responses distinctly on an APP of a smart phone or a host computer. Our strain senor and smart glove will broaden the materials and applications of wearable sensors.

Molecular Design of Stretchable Polymer Semiconductors: Current Progress and Future Directions

Stretchable polymer semiconductors have advanced rapidly in the past decade as materials required to realize conformable and soft skin-like electronics become available. Through rational molecular-level design, stretchable polymer semiconductor films are now able to retain their electrical functionalities even when subjected to repeated mechanical deformations. Furthermore, their charge-carrier mobilities are on par with the best flexible polymer semiconductors, with some even exceeding that of amorphous silicon. The key advancements are molecular-design concepts that allow multiple strain energy-dissipation mechanisms, while maintaining efficient charge-transport pathways over multiple length scales. In this perspective article, we review recent approaches to confer stretchability to polymer semiconductors while maintaining high charge carrier mobilities, with emphasis on the control of both polymer-chain dynamics and thin-film morphology. Additionally, we present molecular design considerations toward intrinsically elastic semiconductors that are needed for reliable device operation under reversible and repeated deformation. A general approach involving inducing polymer semiconductor nanoconfinement allows for incorporation of several other desired functionalities, such as biodegradability, self-healing, and photopatternability, while enhancing the charge transport. Lastly, we point out future directions, including advancing the fundamental understanding of morphology evolution and its correlation with the change of charge transport under strain, and needs for strain-resilient polymer semiconductors with high mobility retention.

Recent Advances in Electronic Skins with Multiple-Stimuli-Responsive and Self-Healing Abilities

Wearable electronic skin (e-skin) has provided a revolutionized way to intelligently sense environmental stimuli, which shows prospective applications in health monitoring, artificial intelligence and prosthetics fields. Drawn inspiration from biological skins, developing e-skin with multiple stimuli perception and self-healing abilities not only enrich their bionic multifunctionality, but also greatly improve their sensory performance and functional stability. In this review, we highlight recent important developments in the material structure design strategy to imitate the fascinating functionalities of biological skins, including molecular synthesis, physical structure design, and special biomimicry engineering. Moreover, their specific structure-property relationships, multifunctional application, and existing challenges are also critically analyzed with representative examples. Furthermore, a summary and perspective on future directions and challenges of biomimetic electronic skins regarding function construction will be briefly discussed. We believe that this review will provide valuable guidance for readers to fabricate superior e-skin materials or devices with skin-like multifunctionalities and disparate characteristics.

Electrically Conducting Elastomeric Fibers with High Stretchability and Stability

Stretchable conducting materials are appealing for the design of unobtrusive wearable electronic devices. Conjugated polymers with oligoethylene glycol side chains are excellent candidate materials owing to their low elastic modulus and good compatibility with polar stretchable polymers. Here, electrically conducting elastomeric blend fibers with high stretchability, wet spun from a blend of a doped polar polythiophene with tetraethylene glycol side chains and a polyurethane are reported. The wet-spinning process is versatile, reproducible, scalable, and produces continuous filaments with a diameter ranging from 30 to 70 µm. The fibers are stretchable up to 480% even after chemical doping with iron(III) p-toluenesulfonate hexahydrate and exhibit an electrical conductivity of up to 7.4 S cm^-1 , which represents a record combination of properties for conjugated polymer-based fibers. The fibers remain conductive during elongation until fiber fracture and display excellent long-term stability at ambient conditions. Cyclic stretching up to 50% strain for at least 400 strain cycles reveals that the doped fibers exhibit high cyclic stability and retain their electrical conductivity. Finally, a directional strain sensing device, which makes use of the linear increase in resistance of the fibers up to 120% strain is demonstrated.

Molecular Rationale for the Design of Instantaneous, Strain-Tolerant Polymeric Adhesive in a Stretchable Underwater Human-Machine Interface

Strain-tolerant reversible adhesion under harsh mechanical deformation is important for realizing long-lasting polymeric adhesives. Despite recent advances, cohesive failure within adhesives remains a critical problem that must be solved to achieve adhesion that is robust against humidity, heat, and mechanical stress. Here, we report a molecular rationale for designing an instantaneous polymeric adhesive with high strain tolerance (termed as iPASTE) even in a stretchable human-machine interface. The iPASTE consists of two biocompatible and eco-friendly polymers, linearly oligomerized green tea extracts, and poly(ethylene glycol) for densely assembled networks via dynamic and reversible hydrogen bonds. Other than the typical approach containing nanoclay or branched adhesive precursors, the linear configuration and conformation of such polymer chains within iPASTE lead to strong and moisture-resistant cohesion/adhesion. Based on the strain-tolerant adhesion of iPASTE, it was demonstrated that a subaqueous interactive human-machine interface integrated with a robot arm and a gold nanomembrane strain-sensitive electronic skin can precisely capture a slithery artificial fish by using finger gesture recognition.

Recent Advances in Wearable Optical Sensor Automation Powered by Battery versus Skin-like Battery-Free Devices for Personal Healthcare-A Review

Currently, old-style personal Medicare techniques rely mostly on traditional methods, such as cumbersome tools and complicated processes, which can be time consuming and inconvenient in some circumstances. Furthermore, such old methods need the use of heavy equipment, blood draws, and traditional bench-top testing procedures. Invasive ways of acquiring test samples can potentially cause patient discomfort and anguish. Wearable sensors, on the other hand, may be attached to numerous body areas to capture diverse biochemical and physiological characteristics as a developing analytical tool. Physical, chemical, and biological data transferred via the skin are used to monitor health in various circumstances. Wearable sensors can assess the aberrant conditions of the physical or chemical components of the human body in real time, exposing the body state in time, thanks to unintrusive sampling and high accuracy. Most commercially available wearable gadgets are mechanically hard components attached to bands and worn on the wrist, with form factors ultimately constrained by the size and weight of the batteries required for the power supply. Basic physiological signals comprise a lot of health-related data. The estimation of critical physiological characteristics, such as pulse inconstancy or variability using photoplethysmography (PPG) and oxygen saturation in arterial blood using pulse oximetry, is possible by utilizing an analysis of the pulsatile component of the bloodstream. Wearable gadgets with “skin-like” qualities are a new type of automation that is only starting to make its way out of research labs and into pre-commercial prototypes. Flexible skin-like sensing devices have accomplished several functionalities previously inaccessible for typical sensing devices due to their deformability, lightness, portability, and flexibility. In this paper, we studied the recent advancement in battery-powered wearable sensors established on optical phenomena and skin-like battery-free sensors, which brings a breakthrough in wearable sensing automation.

Transparent, self-recoverable, highly tough, puncture and tear resistant polyurethane supramolecular elastomer with fast self-healing capacity via "hard-soft" hard domain design

The integration of superior mechanical properties and fast healing efficiency for self-healing polyurethane supramolecular elastomers is challenging due to the confliction between high chain mobility for healing and high chain rigidity for mechanical properties. Herein, a strategy to design a "hard-soft" hard domain by the cooperation of quadruple hydrogen bonds (HBs) in the mainchain as restriction units and single HBs in the side chain as diffusion units is reported. The resulting transparent supramolecular elastomer exhibited fast self-recoverability, good puncture resistance and superior mechanical properties with a tensile strength of 20.5 MPa, an extensibility of 2043.7%, a toughness of 146.1 MJ m-3 and a tear resistance of 13.8 kJ m-2. Moreover, the fast self-healing capacity (healing efficiency > 82% within 3 h under moderate condition) was realized due to the soft effects of weak HBs in the side chain on the strong HBs in the mainchain. Taking advantage of the merits of the supramolecular elastomer, a flexible sensor was simply fabricated, which showed good self-repairable and stable sensing properties. Thus, the elastomer has great potential in the field of flexible electronics and wearable devices.

A reconfigurable crosslinking system via an asymmetric metal–ligand coordination strategy

We report an asymmetric metal–ligand coordination strategy for reconfigurable elastomers. EXAFS is first introduced to monitor the structure change in M–L crosslinked polymers during stretching at the molecular level.

Elastomeric Nanodielectrics for Soft and Hysteresis-Free Electronics

Elastomeric dielectrics are crucial for reliably governing the carrier densities in semiconducting channels during deformation in soft/stretchable field-effect transistors (FETs). Uncontrolled stacking of polymeric chains renders elastomeric dielectrics poorly insulated at nanoscale thicknesses, thereby thick films are usually required, leading to high voltage or power consumption for on/off operations of FETs. Here, layer-by-layer assembly is exploited to build 15-nm-thick elastomeric nanodielectrics through alternative adsorption of oppositely charged polyurethanes (PUs) for soft and hysteresis-free FETs. After mild thermal annealing to heal pinholes, such PU multilayers offer high areal capacitances of 237 nF cm^-2 and low leakage current densities of 3.2 × 10^-8 A cm^-2 at 2 V. Owing to the intrinsic ductility of the elastomeric PUs, the nanofilms possess excellent dielectric properties at a strain of 5% or a bending radius of 1.5 mm, while the wrinkled counterparts show mechanical stability with negligible changes of leakage currents after repeated stretching to a strain of 50%. Besides, these nanodielectrics are immune to high humidity and conserve their properties when immersed into water, despite their assembly occurs aqueously. Furthermore, the PU dielectrics are implemented in carbon nanotube FETs, demonstrating low-voltage operations (< 1.5 V) and negligible hysteresis without any encapsulations.

Recent Advances in High-Mobility and High-Stretchability Organic Field-Effect Transistors: From Materials, Devices to Applications

Stretchable organic field-effect transistors (OFETs) are one of the essential building blocks for next-generation wearable electronics due to the high stretchability of OFET well matching with the large deformation of human skin. In recent years, some significant progress of stretchable OFETs have already been made via the strategies of stretchable molecular design and geometry engineering. However, the main opportunity and challenge of stretchable OFETs is still to simultaneously improve their stretchability and mobility. This review covers the recent advances in the research of stretchable OFETs with high mobility. First, the core stretchable materials are summarized, including organic semiconductors, electrodes, dielectrics, and substrates. Second, the materials and healing mechanism of self-healing OFET are summarized in detail. Subsequently, their different configurations and the potential applications are summarized. Finally, an outlook of future research directions and challenges in this area is presented.

Mechanically Strong, Autonomous Self-Healing, and Fully Recyclable Silicone Coordination Elastomers with Unique Photoluminescent Properties

The combination of excellent mechanical performances, high reprocess efficiency, and wide-range tunability for functional dynamic siloxane materials is a challenging subject. Herein, the fabrication of mechanically strong, autonomous self-healing, and fully recyclable silicone elastomers with unique photoluminescent properties by coordination of poly(dimethylsiloxane) (PDMS) containing coordination bonding motifs with Zn²⁺ ions is reported. Salicylaldimine groups, which are introduced into the polysiloxane backbone via mild Schiff-base reaction, coordinate with zinc ions to form elastomeric networks The obtained supramolecular elastomers have excellent mechanical properties, with the optimized tensile strength up to 10.0 MPa, which is unprecedented among the reported thermoplastic polysiloxane-based elastomers. Both mechanical properties and stress relaxation kinetics are tunable via adjusting the length of PDMS segments or the molar ratio of metal versus salicylaldimine. Furthermore, these elastomers can be conveniently healed and recycled to regain their original mechanical properties and integrity under mild conditions. In addition, this new kind of polysiloxane also exhibits coordination-enhanced fluorescence, showing great promise for preparing photoluminescent elastomers or coatings.

Transparent Omni-Directional Stretchable Circuit Lines Made by a Junction-Free Grid of Expandable Au Lines

Although various stretchable optoelectronic devices have been reported, omni-directionally stretchable transparent circuit lines have been a great challenge. Cracks are engineered and fabricated to be highly conductive patterned metal circuit lines in which gold (Au) grids are embedded. Au is deposited selectively in the cracks to form a grid without any junction between the grid lines. Since each grid line is expandable under stretching, the circuit lines are stretchable in all the directions. This study shows that a thin coating of aluminum on the oxide surface enables precise control of the cracks (crack density, crack depth) in the oxide layer. High optical transparency and high stretchability can be achieved simultaneously by controlling the grid density in the circuit line. Light-emitting diodes are integrated directly on the circuit lines and stable operation is demonstrated under 100% stretching.

Completely foldable electronics based on homojunction polymer transistors and logics

An increase in the demand for completely foldable electronics has motivated efforts for the development of conducting polymer electrodes having extraordinary mechanical stability. However, weak physical adhesion at intrinsic heterojunctions has been a challenge in foldable electronics. This paper reports the completely foldable polymer thin-film transistors (PTFTs) and logic gate arrays. Homojunction-based PTFTs were fabricated by selectively doping p-type diketopyrrolopyrrole-based semiconducting polymer films with FeCl₃ to form source/drain electrodes. The doping process caused a gradual work function change with depth, which promoted charge injection to semiconducting regions and provided a low contact resistance. In addition, the interfacial adhesion in the PTFTs was improved by interfacial cross-linking between adjacent component layers. The electrical performance of the resulting PTFTs was maintained without noticeable degradation even after extreme folding, suggesting that the proposed fabrication strategy can further be applied to various semiconducting polymers for the realization of foldable electronics.

Printed Stretchable Single-Nanofiber Interconnections for Individually-Addressable Highly-Integrated Transparent Stretchable Field Effect Transistor Array

Stretchable electronics have been spotlighted as promising next-generation electronics. In order to drive a specific unit device in an integrated stretchable device, the interconnection of the device should be placed in a desired position and addressed individually. In addition, practical stretchable interconnection requires reliable stretchability, high conductivity, optical transparency, high resolution, and fast and large-scale production. This study proposes an approach to meet these requirements. We print the single wavy polymer nanofibers (NFs) in a desired position and convert them into metal NF interconnections. The nanoscale diameter and the wavy cylindrical shape of the metal NFs are the main reasons for the reliable stretchability and the excellent transparency. Using the stretchable metal NFs and the stretchable organic semiconductor NFs, an array of all-stretchable transparent NF-field effect transistors (NF-FETs) is demonstrated. The highly integrated NF-FET array (10 FETs/mm²) shows uniform performance and good stability under repeated severe mechanical deformations.

Direct Observation of Confinement Effects of Semiconducting Polymers in Polymer Blend Electronic Systems

The advent of special types of polymeric semiconductors, known as "polymer blends," presents new opportunities for the development of next-generation electronics based on these semiconductors' versatile functionalities in device applications. Although these polymer blends contain semiconducting polymers (SPs) mixed with a considerably high content of insulating polymers, few of these blends unexpectedly yield much higher charge carrier mobilities than those of pure SPs. However, the origin of such an enhancement has remained unclear owing to a lack of cases exhibiting definite improvements in charge carrier mobility, and the limited knowledge concerning the underlying mechanism thereof. In this study, the morphological changes and internal nanostructures of polymer blends based on various SP types with different intermolecular interactions in an insulating polystyrene matrix are investigated. Through this investigation, the physical confinement of donor-acceptor type SP chains in a continuous nanoscale network structure surrounded by polystyrenes is shown to induce structural ordering with more straight edge-on stacked SP chains. Hereby, high-performance and transparent organic field-effect transistors with a hole mobility of ≈5.4 cm2 V-1 s-1 and an average transmittance exceeding 72% in the visible range are achieved.

Advanced Flexible Skin-Like Pressure and Strain Sensors for Human Health Monitoring

Recently, owing to their excellent flexibility and adaptability, skin-like pressure and strain sensors integrated with the human body have the potential for great prospects in healthcare. This review mainly focuses on the representative advances of the flexible pressure and strain sensors for health monitoring in recent years. The review consists of five sections. Firstly, we give a brief introduction of flexible skin-like sensors and their primary demands, and we comprehensively outline the two categories of design strategies for flexible sensors. Secondly, combining the typical sensor structures and their applications in human body monitoring, we summarize the recent development of flexible pressure sensors based on perceptual mechanism, the sensing component, elastic substrate, sensitivity and detection range. Thirdly, the main structure principles and performance characteristic parameters of noteworthy flexible strain sensors are summed up, namely the sensing mechanism, sensitive element, substrate, gauge factor, stretchability, and representative applications for human monitoring. Furthermore, the representations of flexible sensors with the favorable biocompatibility and self-driven properties are introduced. Finally, in conclusion, besides continuously researching how to enhance the flexibility and sensitivity of flexible sensors, their biocompatibility, versatility and durability should also be given sufficient attention, especially for implantable bioelectronics. In addition, the discussion emphasizes the challenges and opportunities of the above highlighted characteristics of novel flexible skin-like sensors.

A design strategy for high mobility stretchable polymer semiconductors

As a key component in stretchable electronics, semiconducting polymers have been widely studied. However, it remains challenging to achieve stretchable semiconducting polymers with high mobility and mechanical reversibility against repeated mechanical stress. Here, we report a simple and universal strategy to realize intrinsically stretchable semiconducting polymers with controlled multi-scale ordering to address this challenge. Specifically, incorporating two types of randomly distributed co-monomer units reduces overall crystallinity and longer-range orders while maintaining short-range ordered aggregates. The resulting polymers maintain high mobility while having much improved stretchability and mechanical reversibility compared with the regular polymer structure with only one type of co-monomer units. Interestingly, the crystalline microstructures are mostly retained even under strain, which may contribute to the improved robustness of our stretchable semiconductors. The proposed molecular design concept is observed to improve the mechanical properties of various p- and n-type conjugated polymers, thus showing the general applicability of our approach. Finally, fully stretchable transistors fabricated with our newly designed stretchable semiconductors exhibit the highest and most stable mobility retention capability under repeated strains of 1,000 cycles. Our general molecular engineering strategy offers a rapid way to develop high mobility stretchable semiconducting polymers.

Designing intrinsically stretchable semiconducting polymers with suitable charge transport and mechanical properties required for stretchable electronic devices remains a challenge. Here, the authors report terpolymer-based semiconductors with intrinsically high stretchability and mobility.

Solution-Processed Stretchable Ag2 S Semiconductor Thin Films for Wearable Self-Powered Nonvolatile Memory

Compared with the large plastic deformation observed in ductile metals and organic materials, inorganic semiconductors have limited plasticity (<0.2%) due to their intrinsic bonding characters, restricting their widespread applications in stretchable electronics. Herein, the solution-processed synthesis of ductile α-Ag₂ S thin films and fabrication of all-inorganic, self-powered, and stretchable memory devices, is reported. Molecular Ag₂ S complex solution is synthesized by chemical reduction of Ag₂ S powder, fabricating wafer-scale highly crystalline Ag₂ S thin films. The thin films show stretchability due to the intrinsic ductility, sustaining the structural integrity at a tensile strain of 14.9%. Moreover, the fabricated Ag₂ S-based resistive random access memory presents outstanding bipolar switching characteristics (I_on /I_off ratio of ≈10⁵ , operational endurance of 100 cycles, and retention time >10⁶ s) as well as excellent mechanical stretchability (no degradation of properties up to stretchability of 52%). Meanwhile, the device is highly durable under diverse chemical environments and temperatures from -196 to 300 °C, especially maintaining the properties for 168 h in 85% relative humidity and 85 °C. A self-powered memory combined with motion sensors for use as a wearable healthcare monitoring system is demonstrated, offering the potential for designing high-performance wearable electronics that are usable in daily life in a real-world setting.

Self-Assembled Monolayers: Versatile Uses in Electronic Devices from Gate Dielectrics, Dopants, and Biosensing Linkers

Self-assembled monolayers (SAMs), molecular structures consisting of assemblies formed in an ordered monolayer domain, are revisited to introduce their various functions in electronic devices. SAMs have been used as ultrathin gate dielectric layers in low-voltage transistors owing to their molecularly thin nature. In addition to the contribution of SAMs as gate dielectric layers, SAMs contribute to the transistor as a semiconducting active layer. Beyond the transistor components, SAMs have recently been applied in other electronic applications, including as remote doping materials and molecular linkers to anchor target biomarkers. This review comprehensively covers SAM-based electronic devices, focusing on the various applications that utilize the physical and chemical properties of SAMs.

A shapeable, ultra-stretchable rubber strain sensor based on carbon nanotubes and Ag flakes via melt-mixing process

Promoting the detection range, durability, and shapeable manufacturing of flexible strain sensors is essential to broaden their applications. Therefore, in this study, styrene ethylene butylene styrene (SEBS) rubber as a flexible material and a melt-mixing molding method are adopted to design an ultra-flexible strain sensor. Carbon nanotubes (CNTs) are added to form a conductive network, and the effect of Ag flakes on improving the sensor performance is studied. The experiment results exhibit good strain-resistance dependent characteristics of the obtained sensor, which demonstrates an excellent sensing range of about 540% with a gauge factor (GF) of 5.197. The good hydrophobicity (water contact angle ≈120.4°), repeatable characteristics at different rates, strain-dependence and long-term recycling of the sensor are demonstrated as well. Finally, the fabricated round bracelet sensor is applied to detect different cross-sections, the movement of human joints, balloon inflation, bottle cap sealing and numerous other aspects.

SMART transfer method to directly compare the mechanical response of water-supported and free-standing ultrathin polymeric films

Intrinsic mechanical properties of sub-100 nm thin films are markedly difficult to obtain, yet an ever-growing necessity for emerging fields such as soft organic electronics. To complicate matters, the interfacial contribution plays a major role in such thin films and is often unexplored despite supporting substrates being a main component in current metrologies. Here we present the shear motion assisted robust transfer technique for fabricating free-standing sub-100 nm films and measuring their inherent structural-mechanical properties. We compare these results to water-supported measurements, exploring two phenomena: 1) The influence of confinement on mechanics and 2) the role of water on the mechanical properties of hydrophobic films. Upon confinement, polystyrene films exhibit increased strain at failure, and reduced yield stress, while modulus is reduced only for the thinnest 19 nm film. Water measurements demonstrate subtle differences in mechanics which we elucidate using quartz crystal microbalance and neutron reflectometry.

Interface Design for Stretchable Electronic Devices

Stretchable electronics has emerged over the past decade and is now expected to bring form factor-free innovation in the next-generation electronic devices. Stretchable devices have evolved with the synthesis of new soft materials and new device architectures that require significant deformability while maintaining the high device performance of the conventional rigid devices. As the mismatch in the mechanical stiffness between materials, layers, and device units is the major challenge for stretchable electronics, interface control in varying scales determines the device characteristics and the level of stretchability. This article reviews the recent advances in interface control for stretchable electronic devices. It summarizes the design principles and covers the representative approaches for solving the technological issues related to interfaces at different scales: i) nano- and microscale interfaces between materials, ii) mesoscale interfaces between layers or microstructures, and iii) macroscale interfaces between unit devices, substrates, or electrical connections. The last section discusses the current issues and future challenges of the interfaces for stretchable devices.

A graphene-coated silk-spandex fabric strain sensor for human movement monitoring and recognition

Flexible and stretchable sensors are emerging and promising wearable devices for motion monitoring. Manufacturing a flexible and stretchable strain sensor with desirable electromechanical performance and excellent skin compatibility plays an essential role in building a smart wearable system. In this paper, a graphene-coated silk-spandex (GCSS) fabric strain sensor is prepared by reducing graphene oxide. The sensor functions as a result of conductive fiber extending and woven structure deforming. The conductive fabric can be stretched towards 60% with high sensitivity, and its performance remains constant after a 1000-cycle test. Based on its superior performance, the GCSS is successfully employed to detect full-range human movement and provide data for deep learning-based gesture recognition. This work offers a desirable method to fabricate low-cost strain sensors for industrial applications such as human movement detection and advanced information science.

Hollow-porous fibers for intrinsically thermally insulating textiles and wearable electronics with ultrahigh working sensitivity

Wearable smart devices should be flexible and functional to imitate the warmth and sensing functions of human skin or animal fur. Despite the recent great progress in wearable smart devices, it is still challenging to achieve the required multi-functionality. Here, stretchable hollow-porous fibers with self-warming ability are designed, and the properties of electrical heating, strain sensing, temperature sensing and pressure sensing are achieved. The hollow-porous TPU fiber possesses an ultra-high stretchability (1468%), and the textiles woven from the fibers present a splendid thermal insulation property (the absolute value difference in temperature |ΔT| = 68.5 and 44 °C at extreme temperatures of 115 and -40.0 °C). Importantly, after conductive filler decoration, the fiber-based strain sensor exhibits one of the highest reported gauge factor (2.3 × 10⁶) towards 100% strain in 7200 working stretch-release cycles. A low detection limit of 0.5% strain is also achieved. Besides, the fibers can be heated to 40 °C in 18 s at a small voltage of 2 V as an electrical heater. The assembled thermal sensors can monitor the temperature from 30 to 90 °C in real time, and the fiber-based capacitive type pressure sensor exhibits good sensing performance under force from 1 to 25 N. The hollow-porous fiber based all-in-one integrated wearable systems illustrate promising prospects for next generation electronic skins to detect human motions and body temperature with thermal therapy and inherent self-warming ability.

Electroactive material-based biosensors for detection and drug delivery

Electroactive materials are employed at the interface of biology and electronics due to their advantageous intrinsic properties as soft organic electronics. We examine the most recent literature of electroactive material-based biosensors and their emerging role as theranostic devices for the delivery of therapeutic agents. We consider electroactive materials through the lens of smart drug delivery systems as materials that enable the release of therapeutic cargo in response to specific physiological and external stimuli and discuss the way these mechanisms are integrated into medical devices with examples of the latest advances. Studies that harness features unique to conductive polymers are emphasized; lastly, we highlight new perspectives and future research direction for this emerging technology and the challenges that remain to overcome.

High-performance wearable thermoelectric generator with self-healing, recycling, and Lego-like reconfiguring capabilities

Thermoelectric generators (TEGs) are an excellent candidate for powering wearable electronics and the "Internet of Things," due to their capability of directly converting heat to electrical energy. Here, we report a high-performance wearable TEG with superior stretchability, self-healability, recyclability, and Lego-like reconfigurability, by combining modular thermoelectric chips, dynamic covalent polyimine, and flowable liquid-metal electrical wiring in a mechanical architecture design of "soft motherboard-rigid plugin modules." A record-high open-circuit voltage among flexible TEGs is achieved, reaching 1 V/cm² at a temperature difference of 95 K. Furthermore, this TEG is integrated with a wavelength-selective metamaterial film on the cold side, leading to greatly improved device performance under solar irradiation, which is critically important for wearable energy harvesting during outdoor activities. The optimal properties and design concepts of TEGs reported here can pave the way for delivering the next-generation high-performance, adaptable, customizable, durable, economical, and eco-friendly energy-harvesting devices with wide applications.

Mechano-responsive hydrogen-bonding array of thermoplastic polyurethane elastomer captures both strength and self-healing

Self-repairable materials strive to emulate curable and resilient biological tissue; however, their performance is currently insufficient for commercialization purposes because mending and toughening are mutually exclusive. Herein, we report a carbonate-type thermoplastic polyurethane elastomer that self-heals at 35 °C and exhibits a tensile strength of 43 MPa; this elastomer is as strong as the soles used in footwear. Distinctively, it has abundant carbonyl groups in soft-segments and is fully amorphous with negligible phase separation due to poor hard-segment stacking. It operates in dual mechano-responsive mode through a reversible disorder-to-order transition of its hydrogen-bonding array; it heals when static and toughens when dynamic. In static mode, non-crystalline hard segments promote the dynamic exchange of disordered carbonyl hydrogen-bonds for self-healing. The amorphous phase forms stiff crystals when stretched through a transition that orders inter-chain hydrogen bonding. The phase and strain fully return to the pre-stressed state after release to repeat the healing process.

Self-healing materials strive to emulate curable and resilient biological tissue but their performance is often insufficient for commercial applications because self-healing and toughening are mutually exclusive properties. Here, the authors report a tough and strong carbonate-type thermoplastic polyurethane elastomer that self-heals at ambient temperature.

The Effect of Alkyl Chain Length on Well-Defined Fluoro-Arylated Polythiophenes for Temperature-Dependent Morphological Transitions

Understanding the relationship between the molecular structure and morphological behaviors of well-defined semiconducting polymers is essential for developing novel conjugated building blocks and determining the origin of the functional characteristics of semiconducting polymers. Here, we provide insights into the significant temperature-dependent morphological transitions of novel well-defined polythiophene derivatives with m-alkoxy-substituted fluoro-aryl side units: poly(3-(4-fluoro-3-(hexyloxy)phenyl)thiophene) (PHFPT) and poly(3-(4-fluoro-3-(dodecyloxy)phenyl)thiophene) (PDFPT). We found that these unique morphological transitions depend on the alkyl chain length of the substituted fluoro-aryl side units. In PHFPT with short alkyl chains, the thermal treatment promotes a crowded interdigitated packing structure, resulting in narrow lamellar spacings in its crystalline structure. In contrast, the long alkyl chain of PDFPT acts as a physical spacer and disturbs the crowded interdigitation. In addition, the thermal treatment induces the backbone planarization and an ordered packing morphology in PDFPT. These demonstrations provide a critical milestone for the phase transitions of semiconducting polymers with conjugated side units.

Flexible/Stretchable Supercapacitors with Novel Functionality for Wearable Electronics

With the miniaturization of personal wearable electronics, considerable effort has been expended to develop high-performance flexible/stretchable energy storage devices for powering integrated active devices. Supercapacitors can fulfill this role owing to their simple structures, high power density, and cyclic stability. Moreover, a high electrochemical performance can be achieved with flexible/stretchable supercapacitors, whose applications can be expanded through the introduction of additional novel functionalities. Here, recent advances in and future prospects for flexible/stretchable supercapacitors with innate functionalities are covered, including biodegradability, self-healing, shape memory, energy harvesting, and electrochromic and temperature tolerance, which can contribute to reducing e-waste, ensuring device integrity and performance, enabling device self-charging following exposure to surrounding stimuli, displaying the charge status, and maintaining the performance under a wide range of temperatures. Finally, the challenges and perspectives of high-performance all-in-one wearable systems with integrated functional supercapacitors for future practical application are discussed.

Recent advances in bioelectronics chemistry

Research in bioelectronics is highly interdisciplinary, with many new developments being based on techniques from across the physical and life sciences. Advances in our understanding of the fundamental chemistry underlying the materials used in bioelectronic applications have been a crucial component of many recent discoveries. In this review, we highlight ways in which a chemistry-oriented perspective may facilitate novel and deep insights into both the fundamental scientific understanding and the design of materials, which can in turn tune the functionality and biocompatibility of bioelectronic devices. We provide an in-depth examination of several developments in the field, organized by the chemical properties of the materials. We conclude by surveying how some of the latest major topics of chemical research may be further integrated with bioelectronics.

Recent advances in bioelectronics chemistry

Research in bioelectronics is highly interdisciplinary, with many new developments being based on techniques from across the physical and life sciences. Advances in our understanding of the fundamental chemistry underlying the materials used in bioelectronic applications have been a crucial component of many recent discoveries. In this review, we highlight ways in which a chemistry-oriented perspective may facilitate novel and deep insights into both the fundamental scientific understanding and the design of materials, which can in turn tune the functionality and biocompatibility of bioelectronic devices. We provide an in-depth examination of several developments in the field, organized by the chemical properties of the materials. We conclude by surveying how some of the latest major topics of chemical research may be further integrated with bioelectronics.

Design of intrinsically stretchable and highly conductive polymers for fully stretchable electrochromic devices

Stretchable materials are essential for next generation wearable and stretchable electronic devices. Intrinsically stretchable and highly conductive polymers (termed ISHCP) are designed with semi interpenetrating polymer networks (semi-IPN) that enable polymers to be simultaneously applied to transparent electrodes and electrochromic materials. Through a facile method of acid-catalyzed polymer condensation reaction, optimized ISHCP films show the highest electrical conductivity, 1406 S/cm, at a 20% stretched state. Without the blending of any other elastomeric matrix, ISHCP maintains its initial electrical properties under a cyclic stretch-release of over 50% strain. A fully stretchable electrochromic device based on ISHCP is fabricated and shows a performance of 47.7% ∆T and high coloration efficiency of 434.1 cm2/C at 590 nm. The device remains at 45.2% ∆T after 50% strain stretching. A simple patterned electrolyte layer on a stretchable electrochromic device is also realized. The fabricated device, consisting of all-plastic, can be applied by a solution process for large scale production. The ISHCP reveals its potential application in stretchable electrochromic devices and satisfies the requirements for next-generation stretchable electronics.

Water Passivation of Perovskite Nanocrystals Enables Air-Stable Intrinsically Stretchable Color-Conversion Layers for Stretchable Displays

Conventional organic light-emitting devices without an encapsulation layer are susceptible to degradation when exposed to air, so realization of air-stable intrinsically-stretchable display is a great challenge because the protection of the devices against penetration of moisture and oxygen is even more difficult under stretching. An air-stable intrinsically-stretchable display that is composed of an intrinsically-stretchable electroluminescent device (SELD) integrated with a stretchable color-conversion layer (SCCL) that contains perovskite nanocrystals (PeNCs) is proposed. PeNCs normally decay when exposed to air, but they become resistant to this decay when dispersed in a stretchable elastomer matrix; this change is a result of a compatibility between capping ligands and the elastomer matrix. Counterintuitively, the moisture can efficiently passivate surface defects of PeNCs, to yield significant increases in both photoluminescence intensity and lifetime. A display that can be stretched up to 180% is demonstrated; it is composed of an air-stable SCCL that down-converts the SELD's blue emission and reemits it as green. The work elucidates the basis of moisture-assisted surface passivation of PeNCs and provides a promising strategy to improve the quantum efficiency of PeNCs with the aid of moisture, which allows PeNCs to be applied for air-stable stretchable displays.

Organic Bioelectronics: From Functional Materials to Next-Generation Devices and Power Sources

Conjugated polymers (CPs) possess a unique set of features setting them apart from other materials. These properties make them ideal when interfacing the biological world electronically. Their mixed electronic and ionic conductivity can be used to detect weak biological signals, deliver charged bioactive molecules, and mechanically or electrically stimulate tissues. CPs can be functionalized with various (bio)chemical moieties and blend with other functional materials, with the aim of modulating biological responses or endow specificity toward analytes of interest. They can absorb photons and generate electronic charges that are then used to stimulate cells or produce fuels. These polymers also have catalytic properties allowing them to harvest ambient energy and, along with their high capacitances, are promising materials for next-generation power sources integrated with bioelectronic devices. In this perspective, an overview of the key properties of CPs and examination of operational mechanism of electronic devices that leverage these properties for specific applications in bioelectronics is provided. In addition to discussing the chemical structure-functionality relationships of CPs applied at the biological interface, the development of new chemistries and form factors that would bring forth next-generation sensors, actuators, and their power sources, and, hence, advances in the field of organic bioelectronics is described.

Smart Ti3C2Tx MXene Fabric with Fast Humidity Response and Joule Heating for Healthcare and Medical Therapy Applications

An increasing utilization of flexible healthcare electronics and biomedicine-related therapeutic materials urges the development of multifunctional wearable/flexible smart fabrics for personal therapy and health management. However, it is currently a challenge to fabricate multifunctional and on-body healthcare electronic devices with reliable mechanical flexibility, excellent breathability, and self-controllable joule heating effects. Here, we fabricate a multifunctional MXene-based smart fabric by depositing 2D Ti₃C₂T_x nanosheets onto cellulose fiber nonwoven fabric via special MXene-cellulose fiber interactions. Such multifunctional fabrics exhibit sensitive and reversible humidity response upon H₂O-induced swelling/contraction of channels between the MXene interlayers, enabling wearable respiration monitoring application. Besides, it can also serve as a low-voltage thermotherapy platform due to its fast and stable electro-thermal response. Interestingly, water molecular extraction induces electrical response upon heating, i.e., functioning as a temperature alarm, which allows for real-time temperature monitoring for thermotherapy platform without low-temperature burn risk. Furthermore, metal-like conductivity of MXene renders the fabric an excellent Joule heating effect, which can moderately kill bacteria surrounding the wound in bacteria-infected wound healing therapy. This work introduces a multifunctional smart flexible fabric suitable for next-generation wearable electronic devices for mobile healthcare and personal medical therapy.

Advances in Materials for Soft Stretchable Conductors and Their Behavior under Mechanical Deformation

Soft stretchable sensors rely on polymers that not only withstand large deformations while retaining functionality but also allow for ease of application to couple with the body to capture subtle physiological signals. They have been applied towards motion detection and healthcare monitoring and can be integrated into multifunctional sensing platforms for enhanced human machine interface. Most advances in sensor development, however, have been aimed towards active materials where nearly all approaches rely on a silicone-based substrate for mechanical stability and stretchability. While silicone use has been advantageous in academic settings, conventional silicones cannot offer self-healing capability and can suffer from manufacturing limitations. This review aims to cover recent advances made in polymer materials for soft stretchable conductors. New developments in substrate materials that are compliant and stretchable but also contain self-healing properties and self-adhesive capabilities are desirable for the mechanical improvement of stretchable electronics. We focus on materials for stretchable conductors and explore how mechanical deformation impacts their performance, summarizing active and substrate materials, sensor performance criteria, and applications.

Tailoring the Self-Healing Properties of Conducting Polymer Films

The conducting polymer polyethylenedioxythiophene doped with polystyrene sulfonate (PEDOT:PSS) has received great attention in the field of wearable bioelectronics due to its tunable high electrical conductivity, air stability, ease of processability, biocompatibility, and recently discovered self-healing ability. It has been observed that blending additives with PEDOT:PSS or post-treatment permits the tailoring of intrinsic polymer properties, though their effects on the water-enabled self-healing property have not previously been established. Here, it is demonstrated that the water-enabled healing behavior of conducting polymers is decreased by crosslinkers or by acid post-treatment. Organic dopants of PEDOT have high water swelling ratios and lead to water-enabled healing, while inorganic dopants fail in the healing of PEDOT. The water-enabled healing of two isolated PEDOT:PSS squares with a 5 µm width gap and a thickness less than 1 µm is shown. This work will help pave the way for the further development of conducting polymer-based self-healable bioelectronics and flexible and stretchable electronics.

All-Soft Supercapacitors Based on Liquid Metal Electrodes with Integrated Functionalized Carbon Nanotubes

Soft energy storage devices, such as supercapacitors, are an essential component for powering integrated soft microsystems. However, conventional supercapacitors are mainly manufactured using hard/brittle materials that easily crack and eventually delaminate from the current collector by mechanical deformation. Therefore, to realize all-soft supercapacitors, the electrodes should be soft, stretchable, and highly conductive without compromising the electrochemical performance. This paper presents all-soft supercapacitors for integrated soft microsystems based on gallium-indium liquid metal (eutectic gallium-indium alloy, EGaIn) electrodes with integrated functionalized carbon nanotubes (CNTs). Oxygen functional groups on the surface of the CNTs ensure strong adhesion between the functionalized CNTs and the thin native oxide layer on the surface of EGaIn, which enables delamination-free soft and stretchable electrodes even under mechanical deformation. The electrochemical performances of fabricated all-soft supercapacitors in a parallel-plate arrangement were investigated without and with applied mechanical deformation. The fabricated supercapacitors exhibit areal capacitances as high as 12.4 mF cm^-2 and show nearly unchanged performance under 30% applied strain. They maintain >95% of their original capacitance after >4200 charging and discharging cycles with a periodic applied strain of 30%. Finally, fabricated supercapacitors have been successfully integrated with a commercial light-emitting diode to demonstrate an integrated soft microsystem.

Room-temperature autonomous self-healing glassy polymers with hyperbranched structure

Glassy polymers are extremely difficult to self-heal below their glass transition temperature (T _g) due to the frozen molecules. Here, we fabricate a series of randomly hyperbranched polymers (RHP) with high density of multiple hydrogen bonds, which show T _g up to 49 °C and storage modulus up to 2.7 GPa. We reveal that the hyperbranched structure not only allows the external branch units and terminals of the molecules to have a high degree of mobility in the glassy state, but also leads to the coexistence of "free" and associated complementary moieties of hydrogen bonds. The free complementary moieties can exchange with the associated hydrogen bonds, enabling network reconfiguration in the glassy polymer. As a result, the RHP shows amazing instantaneous self-healing with recovered tensile strength up to 5.5 MPa within 1 min, and the self-healing efficiency increases with contacting time at room temperature without the intervention of external stimuli.

A Skin-Conformal, Stretchable, and Breathable Fiducial Marker Patch for Surgical Navigation Systems

Augmented reality (AR) surgical navigation systems have attracted considerable attention as they assist medical professionals in visualizing the location of ailments within the human body that are not readily seen with the naked eye. Taking medical imaging with a parallel C-shaped arm (C-arm) as an example, surgical sites are typically targeted using an optical tracking device and a fiducial marker in real-time. These markers then guide operators who are using a multifunctional endoscope apparatus by signaling the direction or distance needed to reach the affected parts of the body. In this way, fiducial markers are used to accurately protect the vessels and nerves exposed during the surgical process. Although these systems have already shown potential for precision implantation, delamination of the fiducial marker, which is a critical component of the system, from human skin remains a challenge due to a mechanical mismatch between the marker and skin, causing registration problems that lead to poor position alignments and surgical degradation. To overcome this challenge, the mechanical modulus and stiffness of the marker patch should be lowered to approximately 150 kPa, which is comparable to that of the epidermis, while improving functionality. Herein, we present a skin-conformal, stretchable yet breathable fiducial marker for the application in AR-based surgical navigation systems. By adopting pore patterns, we were able to create a fiducial marker with a skin-like low modulus and breathability. When attached to the skin, the fiducial marker was easily identified using optical recognition equipment and showed skin-conformal adhesion when stretched and shrunk repeatedly. As such, we believe the marker would be a good fiducial marker candidate for patients under surgical navigation systems.

From design to applications of stimuli-responsive hydrogel strain sensors

Stimuli-responsive hydrogel strain sensors that synergize the advantages of both hydrogel and smart functional materials have attracted increasing interest from material design to emerging applications in health monitors and human–machine interfaces.