Electronic Skin Research Articles

Janus organohydrogels to realize strain sensitivity, tough adhesion, and environmental stability

Conductive hydrogels (CHs) have been successfully applied in wearable sensors, health monitoring, and smart electronic skins. However, conventional CHs are unstable for long-term use, mainly because of the dehydration in dry environments, freezing at low temperatures, and lack of self-adhesion properties. Herein, a Janus organohydrogel was reported to address this issue, which had a porous matrix and a dense layer to maximize their functionalities. The bottom was made from polyvinyl alcohol (PVA), water, and glycerol, to fabricate an environmental stable and robust matrix; while the top was consisted of grape seed protein (GSP), tannic acid (TA), carbon nanotubes (CNTs), and water to form the conductive yet adhesive layer. Due to the strong hydrogen bonding between glycerol and water, the evaporation and freezing issues were restrained, resulting in a resilient, conductive, adhesive, and stable organohydrogel. For example, the weight loss was 14 % after storing at ambient conditions for 7 days, and the tensile strength and elastic modulus remained 790 and 360 kPa after cooling at −20 °C. Additionally, the catechol groups from TA and amino acids from GSP endowed robust adhesion to wood, cardboard, iron, rubber, plastics, glass, and pigskin (>7 days). Moreover, the introduction of CNTs within Janus structures promoted electron transportation, resulting in high sensitivity (GF = 1.4) at low content (0.02 wt%). Bearing several merits, the present strategy is expected to greatly enrich the functions and expand the applications of conductive hydrogels.

Skin-inspired, sensory robots for electronic implants

Drawing inspiration from cohesive integration of skeletal muscles and sensory skins in vertebrate animals, we present a design strategy of soft robots, primarily consisting of an electronic skin (e-skin) and an artificial muscle. These robots integrate multifunctional sensing and on-demand actuation into a biocompatible platform using an in-situ solution-based method. They feature biomimetic designs that enable adaptive motions and stress-free contact with tissues, supported by a battery-free wireless module for untethered operation. Demonstrations range from a robotic cuff for detecting blood pressure, to a robotic gripper for tracking bladder volume, an ingestible robot for pH sensing and on-site drug delivery, and a robotic patch for quantifying cardiac function and delivering electrotherapy, highlighting the application versatilities and potentials of the bio-inspired soft robots. Our designs establish a universal strategy with a broad range of sensing and responsive materials, to form integrated soft robots for medical technology and beyond.

Ultrafast Polymerization of a Self-Adhesive and Strain Sensitive Hydrogel-Based Flexible Sensor for Human Motion Monitoring and Handwriting Recognition.

Hydrogel-based flexible electronic devices have great potential in human motion monitoring, electronic skins, and human-computer interaction applications; hence, the efficient preparation of highly sensitive hydrogel-based flexible sensors is important. In the present work, the ultrafast polymerization of a hydrogel (1-3 min) was achieved by constructing a tannic acid (TA)-Fe3+ dynamic redox system, which endowed the hydrogel with good adhesion performance (the adhesion strength in wood was 17.646 kPa). In addition, the uniform dispersal ensured by incorporating polydopamine-decorated polypyrrole (PPy@PDA) into the hydrogel matrix significantly improved the hydrogel's stretching ability (575.082%). The as-prepared PAM/CS/PPy@PDA/TA hydrogel-based flexible sensor had a high-fidelity low detection limit (strain = 1%), high sensitivity at small strains (GF = 5.311 at strain = 0-8%), and fast response time (0.33 s) and recovery time (0.25 s), and it was reliably applied to accurate human motion monitoring and handwriting recognition. The PAM/CS/PPy@PDA/TA hydrogel opens new horizons for wearable electronic devices, electronic skins, and human-computer interaction applications.

Design criteria for conformal integration of flexible electronics on advanced aircraft surfaces

This paper introduces a set of design criteria that aim to integrate flexible electronics onto three-dimensional surfaces in a mechanically adaptive manner. Previous studies have been limited to conformal integration on specific shapes (Such as spherical surfaces and corrugated surfaces) of rigid surfaces or soft substrates, there is a lack of a universal design strategy that takes into account the complexity of substrate shapes and varying hardness. To address this gap, the paper outlines a mechanical model that utilizes Hertzian contact theory to describe the interaction between the thin film and the substrate, and the conformal surface morphology and stress state were derived from minimizing the total energy of the island-substrate structure. The proposed formulations are verified through finite element methods and experiments, analyzing the effects of Young's modulus, in-plane size, and thickness of the “island” on its conformability. Therefore, this paper proposes a generic on-demand conformal design strategy that takes into account the complexity and hardness of the substrate. This study's findings will contribute to the development of conformal electronics that can be used in various fields, such as epidermal electronics, flexible robots, robot electronic skin, and aircraft smart skin.

Ionic Liquid Enabled Transparent Heterogeneous Elastomers for Soft Electronics

AbstractDielectric elastomer (DE), as an electrically active soft material, has enabled stretchable electronics, electronic skins, artificial muscles, biomimetic soft robots, and non‐magnetic motors. High permittivity is urgently desired for increasing the sensitivity of DE sensors and lowering the driving voltage of DE actuators. However, existing efforts for significantly enhanced permittivity is usually achieved at the expense of transparency of the DE by compositing external fillers, which limits the applications in optoelectronics, such as displays, touch screens, artificial eyes, etc. Here, by utilizing of transparent functional filler, ionic liquid micro‐droplet, and precise matching the refractive index with polydimethylsiloxane (PDMS) elastomer matrix, heterogeneous composites with high transparency and high permittivity are obtained simultaneously. The composites are transparent in the visible range (transmittance, 80%–95%). The permittivity reaches up to 22, which is more than seven times higher than the original value of the elastomer matrix. Moreover, the arbitrary deformability of ionic liquid filler endows the composites with better stretchability (300%) and much lower elastic modulus (0.5 MPa, about a quarter the value of PDMS), which is beneficial for soft electronics. Based on the as‐prepared high‐performance composites, high sensitivity and transparent soft sensors and touch panels have been realized, as well as wireless transmission of sensing signals.

Semi-liquid metal-based highly permeable and adhesive electronic skin inspired by spider web

Soft and stretchable electronics have garnered significant attention in various fields, such as wearable electronics, electronic skins, and soft robotics. However, current wearable electronics made from materials like conductive elastomers, hydrogels, and liquid metals face limitations, including low permeability, poor adhesion, inadequate conductivity, and limited stretchability. These issues hinder their effectiveness in long-term healthcare monitoring and exercise monitoring. To address these challenges, we introduce a novel design of web-droplet-like electronics featuring a semi-liquid metal coating for wearable applications. This innovative design offers high permeability, excellent stretchability, strong adhesion, and good conductivity for the electronic skin. The unique structure, inspired by the architecture of a spider web, significantly enhances air permeability compared to commercial breathable patches. Furthermore, the distribution of polyborosiloxane mimics the adhesive properties of spider web mucus, while the use of semi-liquid metals in this design results in remarkable conductivity (9 × 106 S/m) and tensile performance (up to 850% strain). This advanced electronic skin technology enables long-term monitoring of various physiological parameters and supports machine learning recognition functions with unparalleled advantages. This web-droplet structure design strategy holds great promise for commercial applications in medical health monitoring and disease diagnosis.

The Design and numerical simulation of a 3D-printed flexible capacitive pressure sensor with a composite structure

Abstract Flexible pressure sensors are crucial for many areas, such as electronic skins, human motion, robot monitoring, and medical treatment. However, conventional manufacturing methods limit the speed and accuracy of manufacturing sensor dielectric layers. In this paper, a unique capacitive pressure sensor with a composite structure is introduced that can be fabricated by 3D printing technology. The structure, which has 3D hierarchical pyramids microstructures and porous substrate, can be directly fabricated by a DLP 3D printer. This allows flexible dielectric layers to be fabricated quickly while maintaining their superior performance. The dimension of the produced sensor is 10×10×4.5 mm3. The sensor’s design process, numerical simulation, and performance are provided and elucidated. The performance of the sensor is measured between 0 and 1 MPa. It has low hysteresis and high sensitivity (6.83 MPa−1 within the scope of 0 to 0.5 MPa and 2.56 MPa−1 within the scope of 0.5 to 1 MPa).

Ultrathin, solvent-resistant dielectric for monolithic fabrication of low-power, intrinsically stretchable active-matrix electronic skin

Ultrathin, solvent-resistant dielectric for monolithic fabrication of low-power, intrinsically stretchable active-matrix electronic skin

A bioinspired neuromuscular system enabled by flexible electro-optical N2200 nanowire synaptic transistor

Mimicking the functional traits of the muscle system evolves the development of the neuromorphic prosthetic limbs. Herein, a bioinspired neuromuscular system was constructed by connecting an information processor that uses a flexible electro-optical synaptic transistor (FNST) to an effector that uses artificial muscle fibers. In this system, the response of artificial muscle fibers, which imitate the movement of biological muscle fibers, is manipulated by neuromorphic synaptic devices. The FNST is regulated by light pulses and electrical spikes to emulate biological synaptic functions, and thereby applied in secure communication. The feasibility of n-type organic nanowires acting as the channels for neuromorphic devices was demonstrated. Attributing to the flexibility of the n-type organic semiconductor N2200 nanowires, the current of the FNST retains >85% of its initial value after the 5000 bending cycles to radius = 1 cm. The tolerance of bending of the FNST implies its potential applications in wearable electronics. This work offers an approach to potentially advancing electronic skin, neuro-controlled robots, and neuromorphic prosthetic limbs.

NdFeB-based magnetic triboelectric nanogenerator for enhanced bioenergy harvesting and tactile perception

High performance and versatility may be the key for the development of flexible electronic skins that are based on triboelectric nanogenerators (TENGs), and multifunctional components at the triboelectric interface is essential for empowering these devices to mimic or even surpass the sensing properties of human skin, and enabling their response to both environmental and intrinsic conditions. In this context, we present an efficient, low-cost strategy for constructing magnetically responsive TENGs by introducing NdFeB powders into a polydimethylsiloxane film. Besides superior stability, resiliency, and flexibility, the as-designed TENG presents an enhanced output of open circuit voltage of 150 V, short circuit current of 16.7 mA/m2, and Qsc of 35 nC, which is almost 3.5 times of the pristine one. Interestingly, the output performance of MR-TENG shows dependence on both the concentration of NdFeB and the direction and intensity of magnetization. Such a correlation in turn provides the linear relationship of the output with the magnetic field (∼80 V/T) and the applied dynamic force (∼0.64 V/N), which helps seamlessly integrating energy harvesting with tactile sensing for self-powered human activity monitoring and other intelligent sensing applications.

Satisfactory Tensile Strength and Strain of Recyclable Polyurethane with a Trimaleimide Structure for Thermal Self-Healing and Anticorrosive Coatings.

Bismaleimide (BMI) is often used as the cross-linking reagent in Diels-Alder (D-A)-type intrinsic self-healing materials (DISMs) to promote the connectivity of damaged surfaces based on reversible D-A bond formation on the molecular scale. Until now, although DISMs have exhibited great potential in the applications of various sensors, electronic skin, and artificial muscles, it is still difficult to prepare DISMs with satisfactory self-healing abilities and high tensile strengths and strains at the same time, thus largely limiting their applications in self-healing anticorrosive coatings. Herein, symmetrical trimaleimide (TMI) was successfully synthesized, and trimaleimide-structured D-A self-healing polyurethane (TMI-DA-PU) was prepared via the reversible D-A reaction (cycloaddition of furan and maleimide). As a DISM, TMI-DA-PU exhibits apparently higher self-healing efficiency (98.7%), tensile strength (25.4 MPa), and strain (1378%) compared to bismaleimide-structured D-A self-healing polyurethane (BMI-DA-PU) (self-healing efficiency, 90.2%; tensile strength, 19.3 MPa; strain, 1174%). In addition, TMI-DA-PU shows a high recycling efficiency (>95%) after 4 cycles of recycling. A series of characterizations indicate that TMI provides more monoene rings as the self-healing sites, forms denser cross-linked structures compared to BMI, and is, thus, more appropriate to be used for DISM applications. Moreover, the barrier abilities of coatings can be semi-quantitatively expressed by the impedance value at 0.01 Hz (|Z|0.01Hz). The |Z|0.01 Hz value of the TMI-DA-PU coating is 3.93 × 109 Ω cm2 on day 0, which is significantly higher than that of the BMI-DA-PU coating (6.76 × 108 Ω cm2 on day 0), indicating that the denser rigid cross-linked structure of TMI results in the small porosity in the TMI-DA-PU coating, thus effectively improving the anticorrosion performance. The construction of DISMs with the structure of TMI demonstrates immense potential in self-healing anticorrosive coatings.

A three-dimensionally architected electronic skin mimicking human mechanosensation.

Human skin sensing of mechanical stimuli originates from transduction of mechanoreceptors that converts external forces into electrical signals. Although imitating the spatial distribution of those mechanoreceptors can enable developments of electronic skins capable of decoupled sensing of normal/shear forces and strains, it remains elusive. We report a three-dimensionally (3D) architected electronic skin (denoted as 3DAE-Skin) with force and strain sensing components arranged in a 3D layout that mimics that of Merkel cells and Ruffini endings in human skin. This 3DAE-Skin shows excellent decoupled sensing performances of normal force, shear force, and strain and enables development of a tactile system for simultaneous modulus/curvature measurements of an object through touch. Demonstrations include rapid modulus measurements of fruits, bread, and cake with various shapes and degrees of freshness.

Smart versatile hydrogels tailored by metal-phenolic coordinating carbon and polypyrrole for soft actuation, strain sensing and writing recognition

Biomimetic intelligent conductive hydrogels integrating flexibility, signal sensing, and contactless actuation has become ideal candidates for the production of electronic skins, wearable devices, and soft robotics. However, the development of multifunctional smart hydrogels capable of both sensing and actuating simultaneously to enhance intelligent interaction interfaces remains a significant challenge. Herein, a multifunctional hydrogel composite with integrated sensing and actuating capabilities is proposed through the incorporation of Fe3+-tannic acid (TA) coordination compound-coated carbon nanotubes (Fe-TA@SWNT) and the in-situ formation of conductive polymer polypyrrole (PPy) inside the hydrogel networks composed of carboxymethylcellulose (CMC) and a copolymer of acrylic acid (AA) and [2-(methacryloyloxy) ethyl]dimethyl-(3-sulfopropyl) ammonium hydroxide (SBMA). Benefiting from multiple effective dynamic interactions within the internal matrix, the prepared hydrogel exhibits excellent stretchability (1283.42 % elongation) and toughness (1.11 MJ/m3). Additionally, the catechol groups carried by Fe-TA@SWNT, along with the zwitterionic monomer SBMA, provide the hydrogel with repeatable self-adhesion performance. Based on the outstanding photothermal effect of Fe-TA@SWNT and PPy, the hydrogel demonstrates reversible and stable photothermal responsiveness, exhibiting exceptional actuation performance under near-infrared (NIR) radiation. As a strain sensor, the hydrogel shows excellent sensitivity (GF = 3.32), rapid response time, and reliable stability, enabling precise monitoring of various physiological signals in the human body, including joint movements, facial expressions, and heartbeats. Therefore, this work significantly expands the application prospects of multifunctional smart hydrogels in fields of wearable electronics, soft robotics, and remote light-controlled devices.

Bioinspired electronic-skin for proximity and pressure detection in robot active sensing

Active sensing technology plays an essential role in environment–robot interactions. Inspired by the proximity sensing approach of weakly electric fish, which relies on distributed electroreceptors capable of detecting electric fields, we propose a flexible electronic skin (e-skin) for proximity and pressure detection. Conductive thermoplastic polyurethane and dielectric polyurethane are employed for fabricating flexible electrodes and substrates, respectively. An Ecoflex-based elastic layer enables proximity and pressure information to be decoupled from the electric field. The proposed e-skin can detect objects up to 160 mm away while performing real-time proximity and pressure sensing. Finally, we demonstrate that robots equipped with the e-skin can easily explore their surroundings and perform specific tasks such as recognition, avoidance, and grasping. Because of its proximity and pressure sensing capabilities and low-cost fabrication process, the e-skin has broad application potential for robot active sensing.

Directional Moisture-Wicking Triboelectric Materials Enabled by Laplace Pressure Differences.

Wearable sensors are experiencing vibrant growth in the fields of health monitoring systems and human motion detection, with comfort becoming a significant research direction for wearable sensing devices. However, the weak moisture-wicking capability of sensor materials leads to liquid retention, severely restricting the comfort of the wearable sensors. This study employs a pattern-guided alignment strategy to construct microhill arrays, endowing triboelectric materials with directional moisture-wicking capability. Within 2.25 s, triboelectric materials can quickly and directionally remove the droplets, driven by the Laplace pressure differences and the wettability gradient. The directional moisture-wicking triboelectric materials exhibit excellent pressure sensing performance, enabling rapid response/recovery (29.1/37.0 ms), thereby achieving real-time online monitoring of human respiration and movement states. This work addresses the long-standing challenge of insufficient moisture-wicking driving force in flexible electronic sensing materials, holding significant implications for enhancing the comfort and application potential of electronic skin and wearable electronic devices.

Multifunctional roles of TEMPO-oxidized cellulose nanofibrils on the enhancement of mechanical and conductive properties of acrylic-based hydrogels for temperature response and human motion sensing

Simultaneously having competitive mechanical properties, fatigue-resistant stability, temperature adaptability, excellent conductivity and sensitivity has been a challenge in the design of acrylic-based conductive hydrogels if they are meant for application in electronic skin mimics capable of sensory and stimulus responses. In this work, an acrylic-based conductive hydrogel with both excellent mechanical and electrical properties was prepared based on an integrated strategy employing the multifunctionality of TEMPO-oxidized CNFs (TOCNFs) and metal ion interaction. The TOCNFs with their abundant –COOH groups played a key role in the hydrogel by a) well-dispersing polyacrylic acid (PAA) chains in the hydrogel to form a homogeneous and porous multi-network, b) furnishing more –COOH to interact with Fe3+, c) forming more hydrogen bonds with PAA and glycerol, and d) offering high modulus to the final hydrogel. The obtained optimal hydrogel showed competitive mechanical properties (0.88 MPa at 70 % compressive strain; 0.24 MPa at 873 % tensile strain) and fatigue-resistant properties (56.5 % strength retention after 500 cycles of 50 % compression; 51.7 % strength retention after 20 cycles of 200 % tensile), high electrical conductivity (2.45 S m−1) and sensitivity (GF up to 2.62 for tensile strains over 100 %), while still maintaining a high electrical conductivity (1.67 S m−1) at −25 °C. Accordingly, the hydrogel prepared in this work was able to act as an electronic skin that responds to temperature variations and detect human body movement of pressing, writing, stretching and bending with highly sensitive conductive signals, which endows it great potential for applications in humanoid robotics and multi-scenario strain sensing.

Multifunctional and high‐stability RGO/PANI wrapped polyurethane foam for flexible piezoresistive sensor applications

AbstractDue to its excellent durability and steady mechanical qualities, three‐dimensional porous polyurethane foam (PUF) presents a wide range of possibilities for flexible piezoresistive sensors. Its extreme flammability, poor electrical conductivity, and susceptibility to outside factors present serious difficulties, though. In this study, a waterborne polyurethane‐coated flexible PUF sensor that incorporates reduced graphene oxide (RGO) and polyaniline (PANI) through a methodical, step‐by‐step dip‐coating methodology is successfully developed. The final product has a water contact angle of 133°, which improves its ability to adapt to a variety of environmental circumstances. Flexible graphene sheets are incorporated to improve heat resistance and flame retardancy, and PANI and RGO provide strong bonding to the PUF framework, ensuring outstanding structural stability even after 1500 cycles. The flexible foam sensor shows promise for use in flexible piezoresistive sensors and electronic skin due to its remarkable strain monitoring range of up to 70%, quick response time of 0.39 s in sensitivity experiments, and adaptability to different physical activities like walking and gesturing.

Carbon nanotube/nonwoven fabric-based electronic skins for smart clothing and electronic glove

In recent years, flexible, easy-to-fabricate, and low-cost electronic skins (E-skins) have attracted considerable attention, due to their enormous demand in wearable device applications. Based on a carbon nanotube (CNT)/nonwoven fabric, three types of E-skins were fabricated using a low-cost and scalable approach, and detected various signals, like pressure, stretch, flexion, temperature, and humidity. Due to the wrinkled and porous microstructure of CNT/nonwoven fabric, the pressure-sensitive E-skin demonstrates a high gauge factor (GF) value of 19.12 kPa-1 and 5.38 kPa-1 with a pressure range of 15–6125 Pa and 6125–12005 Pa, respectively. Unlike traditional paper-based E-skins, the stretch/flexion/temperature-sensitive E-skin could detect the stretch strain, and its GF values were 3.81, 0.51 rad-1, and 1.4×10-3 ℃-1 for the detection of stretch, flexion and temperature. For the humidity-sensitive E-skin, its GF value was calculated to be 2.8×10-2 and 0.20 with the relative humidity range of 55–75% and 75–95%. The favorable performance of the E-skins allows their integration into smart clothing, where they successfully monitored human motion, physiological and external environmental signals in real time. Furthermore, the prepared E-skins have also been integrated into an electronic glove, where they simultaneously detected the bending and pressure signals of human fingers, demonstrating application potential in the field of telediagnosis.

Coral-Inspired Terahertz-Infrared Bi-Stealth Electronic Skin.

The development of electronic skin with dual stealth functionality is crucial for enabling devices to operate effectively in dynamic electromagnetic environments, thereby facilitating intelligent electromagnetic protection for autonomous perception. However, achieving compatibility between terahertz (THz) and infrared (IR) stealth technologies remains largely unexplored due to their inherent contradictions. Herein, inspired by natural corals, a novel coral-like multi-scale composite foam (CMSF) was proposed that ingeniously reconciles these contradictions. The design capitalizes on the conductive network and heat insulation properties of the foam skeleton, the loss effects and low infrared emission of metal particles, and the infrared transparency of magneto-optical materials. This approach leads to the realization of a THz-IR bi-stealth electronic skin concept. The CMSF exhibits a maximum reflection loss of 84.8 dB in the terahertz band, while its infrared stealth capability ensures environmental adaptability under varying temperatures. Furthermore, the electronic skin exhibits exceptional sensitivity and reliability as a wearable device for perceiving environmental changes. This advanced material, combining multispectral stealth with sensing capabilities, holds immense potential for applications ranging from camouflage technology to smart wearables.

Coral‐Inspired Terahertz‐Infrared Bi‐Stealth Electronic Skin

AbstractThe development of electronic skin with dual stealth functionality is crucial for enabling devices to operate effectively in dynamic electromagnetic environments, thereby facilitating intelligent electromagnetic protection for autonomous perception. However, achieving compatibility between terahertz (THz) and infrared (IR) stealth technologies remains largely unexplored due to their inherent contradictions. Herein, inspired by natural corals, a novel coral‐like multi‐scale composite foam (CMSF) was proposed that ingeniously reconciles these contradictions. The design capitalizes on the conductive network and heat insulation properties of the foam skeleton, the loss effects and low infrared emission of metal particles, and the infrared transparency of magneto‐optical materials. This approach leads to the realization of a THz‐IR bi‐stealth electronic skin concept. The CMSF exhibits a maximum reflection loss of 84.8 dB in the terahertz band, while its infrared stealth capability ensures environmental adaptability under varying temperatures. Furthermore, the electronic skin exhibits exceptional sensitivity and reliability as a wearable device for perceiving environmental changes. This advanced material, combining multispectral stealth with sensing capabilities, holds immense potential for applications ranging from camouflage technology to smart wearables.