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Stretchable electronics that can elongate elastically as well as flex are crucial to a wide range of emerging technologies, such as wearable medical devices, electronic skin, and soft robotics. Critical to stretchable electronics is their ability to withstand large mechanical strain without failure while retaining their electrical conduction properties, a feat significantly beyond traditional metals and silicon-based semiconductors. Herein, we present a review of the recent advances in stretchable conductive polymer nanocomposites with exceptional stretchability and electrical properties, which have the potential to transform a wide range of applications, including wearable sensors for biophysical signals, stretchable conductors and electrodes, and deformable energy-harvesting and -storage devices. Critical to achieving these stretching properties are the judicious selection and hybridization of nanomaterials, novel microstructure designs, and facile fabrication processes, which are the focus of this Review. To highlight the potentials of conductive nanocomposites, a summary of some recent important applications is presented, including COVID-19 remote monitoring, connected health, electronic skin for augmented intelligence, and soft robotics. Finally, perspectives on future challenges and new research opportunities are also presented and discussed.

We present an electronic skin with proximity sensing capabilities. Through a simple and cheap fabrication process, we integrate carbon nanotube forests, which serve as the sensing element, into soft, flexible, and transparent artificial skin. Numerical simulations demonstrate the operational principle of the electronic skin proximity sensors. Experimental characterization reveals excellent repeatability and performance of the electronic skin proximity sensors, which are unaffected by the applied strain. Mounting the electronic skin on experimenter’s finger further demonstrates its high performance, variability, and potential to interface with humans. Thus, the cheap and simple fabrication process of this electronic skin, together with its straightforward operation, high reliability, and excellent performance under large deformations and different working conditions, set the stage for the development of a new generation of high-end flexible sensory electronic skin, which is attractive in a wide range of engineering fields, such as autonomous soft robotics, biomedicine, and the Internet of Things.

Wet-resistant flexible electronics have acquired increasing attention on applications in wet environments, such as sweaty skin, rainy weather, biological fluids, and underwater. However, it remains challenging to achieve non-swelling and underwater self-healing hydrogel sensors for the mechanical perception in aqueous solutions. Herein, a self-healing and non-swellable hydrogel is successfully fabricated, which presents an automatically healing behavior in various aquatic environments, including deionized water, seawater, sweat, alkali and acidic aqueous solutions. Moreover, the hydrogel demonstrates high stretchability and stable electromechanical sensing properties in water. Furthermore, an electronic skin is designed with the features of fast responsiveness, reliability, and high sensitivity for detecting breathing, speaking, coughing, and diverse body movements. The self-healing hydrogel sensors enable a brilliant mechanical sensibility for detecting a series of dynamic stimuli in air and underwater, even after the healing of fracture interface in water. The underwater self-healing and anti-swelling hydrogel would provide enticing potential on various stable electronic devices for aquatic environments, such as implantable electrodes, triboelectric nanogenerators, and underwater soft robotics.

Stretchable strain sensors have received intensive attention recently due to their widespread potentials in the fields of healthcare monitoring, soft robotics, and electronic skin. Besides high stretchability and excellent sensing properties, the transparency of the sensor is another important parameter when designing multi-functional sensors in reality. In this review paper, we mainly focused on the different strategies to realize the transparency of stretchable strain sensor. Some typical advances in different types of stretchable and transparent strain sensors as well as their mechanisms, sensing properties, and applications are summarized. Finally, the open question and possible solution in this field have also been showcased.

Flexible hydrogel strain sensors have received increased attention due to their potential applications in human–machine interfaces, soft robotics, and electronic skin. However, it still remains a c...

Highly sensitive and cost-effective flexible piezoresistive sensors are pursued as essential components of electronic skin (e-skin) for a variety of applications such as soft robotics and body prosthesis. A common strategy for achieving high sensitivity of these sensors is to fabricate their electrode material with complex 3-D microstructures, but manufacturing cost-effective micro-structured sensors with high reliability and reproducibility is a challenge. Herein, a robust approach for producing high-reproductive microstructured polydimethylsiloxane (PDMS) films composing the sensors is proposed, based on silicon molds with a honeycomb-like architecture fabricated rapidly and cost-effectively at a standoff distance by femtosecond laser pulses in the self-channeling regime. Single-walled carbon nanotubes (SWNTs) are embedded in the micro-structured PDMS films composing sandwich-structural flexible piezoresistive sensors. The sensing devices exhibit good performance with high sensitivity, fast response time, and excellent stability and are used with success for the detection of hand motion pressure and blood pressure at the wrist, thus showing a great potential as components of e-skin for detection of various human motions.

Flexible and conductive hydrogels have been widely used in the field of flexible wearable sensors. However, the traditional carbon-based composite hydrogel sensors tend to present weak adhesion and mechanical strength, limiting the sensitivity and reliability of practical applications. Here, a gum arabic-tackified reduced graphene oxide composite hydrogel is successfully fabricated. The introduction of gum arabic endows the hydrogel with high mechanical strength, and fatigue resistance, as well as robust adhesion on metal, plastic, glass, rubber, and skin. Furthermore, the reduced graphene oxide would improve the conductivity of hydrogels. As a result, the skin-attached hydrogel sensors featuring high sensitivity, fast response, and wide detection range were demonstrated for monitoring various mechanical stimuli and human motions. It is anticipated that the gum arabic-tackified skin-attached hydrogel sensors hold prospective applications in electronic skin, human–machine interfaces, and soft robotics.

Skin-mimicking electronics have aroused extensive interest in personalized health monitoring, human movement detections, and biomedical implants. However, the existing flexible electronics are still unable to compare with the advanced performance of human skin, and it remains challenging to achieve a high skin-mimicking capability. Herein, we design a DNA-inspired hydrogel mechanoreceptor with skin-like perception and mechanical behavior, as well as excellent biocompatibility. The hydrogels are constructed using a DNA-inspired adenosine monophosphate crosslinked quaternized chitosan network and a NaCl-containing polyacrylamide network. The hydrogels possess a low modulus, high toughness, self-stiffness, and fast self-recovery. As a result, the hydrogel mechanoreceptors exhibit high sensitivity toward strain and pressure, and present negligible electromechanical hysteresis even at a big deformation (strain of 500%). Impressively, the hydrogels can function as reliable wearable sensors for detecting whole-body movements and physiological signals, including various joint motions, facial emotions, vocalization, and respiration. It is anticipated that the DNA-inspired hydrogel mechanoreceptors would fuel the development of new generation of flexible electronics, such as electronic skin, medical implants, soft robotics, and flexible touchpads.

Electronic skin (E-skin) capable of detecting various physical stimuli is required for monitoring external environments accurately. Here, we report an all-soft multiaxial force sensor based on liquid metal microchannel array for electronic skin applications. The proposed sensor is composed of stretchable elastomer and Galinstan, a eutectic gallium-indium alloy, providing a high mechanical flexibility and electro-mechanical durability. Liquid metal microchannel arrays are fabricated in multilayer and positioned along a dome structure to detect multi-directional forces, supported by numerical simulation results. By adjusting the height of the dome, we could control the response of the multiaxial sensor with respect to the deflection. As a demonstration of multiaxial force sensing, we were able to monitor the direction of multidirectional forces using a finger by the response of liquid metal microchannel arrays. This research could be applied to various fields including soft robotics, wearable devices, and smart prosthetics for artificial intelligent skin applications.

Recent advancement in wearable and robot-assisted healthcare technology gives rise to the demand for smart interfaces that allow more efficient human-machine interaction. In this paper, a hydrogel-based soft sensor for subtle touch detection is proposed. Adopting the working principle of a biomedical imaging technology known as electrical impedance tomography (EIT), the sensor produces images that display the electrical conductivity distribution of its sensitive region to enable touch detection. The sensor was made from a natural gelatin hydrogel whose electrical conductivity is considerably less than that of human skin. The low conductivity of the sensor enabled a touch-detection mechanism based on a novel short-circuiting approach, which resulted in the reconstructed images being predominantly affected by the electrical contact between the sensor and fingertips, rather than the conventionally used piezoresistive response of the sensing material. The experimental results indicated that the proposed sensor was promising for detecting subtle contacts without the necessity of exerting a noticeable force on the sensor.

Flexible pressure sensors have attracted significant attention in field of electronic skin, medical equipment, and soft robotics. However, the poor sensing performance and high working voltage have hindered their practical applications. Here, a low-voltage wearable pressure sensor with high sensitivity in a large useable range is demonstrated. The pressure sensor has been designed with a random distributed sphenoid surface and highly conductive silver films, exhibiting an excellent sensitivity of 5.9 kPa−1 with a wide linearity of 0–15 kPa and ultralow operating voltage of 20 mV. Meanwhile, the flexible pressure sensor could be used for sensitively monitoring human physiological signals, such as wrist pulse, voice recognition, and human activities. Furthermore, a resistive pressure sensor array has been applied for detecting the spatial distribution of external pressure, indicating huge potential as wearable detectors for self-powered real-time monitoring human healthcare.

State monitoring of the complex system needs a large number of sensors. Especially, studies in soft electronics aim to attain complete measurement of the body, mapping various stimulations like temperature, electrophysiological signals, and mechanical strains. However, conventional approach requires many sensor networks that cover the entire curvilinear surfaces of the target area. We introduce a new measuring system, a novel electronic skin integrated with a deep neural network that captures dynamic motions from a distance without creating a sensor network. The device detects minute deformations from the unique laser-induced crack structures. A single skin sensor decodes the complex motion of five finger motions in real-time, and the rapid situation learning (RSL) ensures stable operation regardless of its position on the wrist. The sensor is also capable of extracting gait motions from pelvis. This technology is expected to provide a turning point in health-monitoring, motion tracking, and soft robotics., Real-time monitoring human motions normally demands connecting a large number of sensors in a complicated network. To make it simpler, Kim et al. decode the motion of fingers using a flexible sensor attached on wrist that measures skin deformation with the help of a deep-learning architecture.

Electronic skins (e-skins) have attracted increasing research interest in recent years, owing to their tremendous potential application in artificial intelligence. Nevertheless, the scarce functionality and complex manufacturing process still severely hinder their application. Meanwhile, how to integrate various sensing capabilities into a single sensor is still an urgent problem. In this study, a novel ionogel e-skin was fabricated by a very facile strategy, polymerizing acrylic acid (AA) in ionic liquid 1-ethyl-3-methylimidazolium chloride ([EMIM]Cl). The obtained e-skin exhibited super functions, including excellent transparency, superior stretchability, extreme temperature tolerance and outstanding self-adhesion. The e-skin demonstrated reasonably high sensitivity to various external stimuli, including strain and temperature. It is believed that this study provides a novel and facile approach to fabricate high-performance bimodal e-skin for wearable electronics, biomedical devices, and soft robotics.

The human skin is an essential organ of the human body for sensing various changes in the external environment, including pressure, temperature, and humidity, as well as for interacting with the outside world. Therefore, in the field of soft electronics, the development of the electronic skin (e-skin) that mimics the functions and properties of the human skin is an important research topic. The potential applications of the e-skin technology include wearable electronics, soft robotics, human–machine interfaces (HMIs), and prosthetic devices. Toward such research goals, various technologies are required. Among these, the stretchable electrode technology is a key component for achieving soft, stretchable e-skin and its applications. Therefore, various materials and design strategies for stretchable electrodes have been developed by many research groups. The successful development of stretchable electrodes has resulted in the rapid progress of high-performance stretchable device components such as stretchable sensors and actuators. Moreover, the integration of these device components with stretchable interconnects has yielded an integrated e-skin system, through which interactive human–machine interfacing can be achieved. This paper presents an overview of state-of-the-art technological advances in materials and design strategies for the development of stretchable electrodes. In addition, e-skin systems and their HMI applications based on such stretchable electrodes are briefly reviewed.

Conductive self-healing hydrogels (CSHs) that match the mechanical properties of biological tissues are highly desired for emerging wearable electronics. However, it is still a fundamental challenge to balance the trade-offs among the mechanical, electronic, and self-healing properties in CSHs. In this study, we presented supramolecular double-network (DN) CSHs by pre-infiltrating conductive polyaniline (PANI) precursor into the self-healable hydrophobic association poly(acrylic acid) (HAPAA) hydrogel matrix. The dynamic interfacial interactions between the HAPAA and PANI networks efficiently enhanced the mechanical performances of the HAPAA/PANI (PAAN) hydrogel and could compensate for the negative effect of the enhanced mechanical strength on self-healing. In addition, the interconnected PANI network endowed the PAAN hydrogel with high conductivity and excellent sensory performances. As such, the mechanical and electronic properties of the PAAN hydrogel were simultaneously enhanced significantly without compromising the self-healing performance of the HAPAA matrix, achieving balanced mechanical, electronic, and self-healing properties in the PAAN hydrogel. Lastly, proof-of-concept applications like human physiological monitoring electronics, flexible touch screens, and artificial electronic skin are successfully demonstrated using the PAAN hydrogel with the capability of restoring their electronic performances after the healing process. It is anticipated that such hydrogel network design can be extended into next-generation hydrogel electronics for human–machine-interfaces and soft robotics.

Skin-like electronics that can provide comprehensively tactile sensing is required for applications such as soft robotics, health monitoring, medical treatment, and human-machine interfaces. In particular, the capacity to monitor the contact parameters such as the magnitude, direction, and contact location of external forces is crucial for skin-like tactile sensing devices. Herein, a flexible electronic skin which can measure and discriminate the contact parameters in real time is designed. It is fabricated by integrating the three-dimensional (3D) hollow MXene spheres/Ag NW hybrid nanocomposite-based embedded stretchable electrodes and T-ZnOw/PDMS film-based capacitive pressure sensors. To the best of our knowledge, it is the first stretchable electrode to utilize the 3D hollow MXene spheres with the essential characteristic, which can effectively avoid the drawbacks of stress concentration and shedding of the conductive layer. The strain-resistance module and the pressure-capacitance module show the excellent sensing performance in stability and response time, respectively. Moreover, a 6 × 6 sensor array is used as a demonstration to prove that it can realize the multiplex detection of random external force stimuli without mutual interference, illustrating its potential applications in biomimetic soft wearable devices, object recognition, and robotic manipulation.

Self-healing materials can prolong the lifetime of structures and products by enabling the repairing of damage. However, detecting the damage and the progress of the healing process remains an important issue. In this study, self-healing, piezoresistive strain sensor fibers (ShSFs) are used for detecting strain deformation and damage in a self-healing elastomeric matrix. The ShSFs were embedded in the self-healing matrix for the development of self-healing sensor fiber composites (ShSFC) with elongation at break values of up to 100%. A quadruple hydrogen-bonded supramolecular elastomer was used as a matrix material. The ShSFCs exhibited a reproducible and monotonic response. The ShSFCs were investigated for use as sensorized electronic skin on 3D-printed soft robotic modules, such as bending actuators. Depending on the bending actuator module, the electronic skin was loaded under either compression (pneumatic-based module) or tension (tendon-based module). In both configurations, the ShSFs could be successfully used as deformation sensors, and in addition, detect the presence of damage based on the sensor signal drift. The sensor under tension showed better recovery of the signal after healing, and smaller signal relaxation. Even with the complete severing of the fiber, the piezoresistive properties returned after the healing, but in that case, thermal heat treatment was required. With their resilient response and self-healing properties, the supramolecular fiber composites can be used for the next generation of soft robotic modules.

Self-healing materials can prolong the lifetime of structures and products by enabling the repairing of damage. However, detecting the damage and the progress of the healing process remains an important issue. In this study, self-healing, piezoresistive strain sensor fibers (ShSFs) are used for detecting strain deformation and damage in a self-healing elastomeric matrix. The ShSFs were embedded in the self-healing matrix for the development of self-healing sensor fiber composites (ShSFC) with elongation at break values of up to 100%. A quadruple hydrogen-bonded supramolecular elastomer was used as a matrix material. The ShSFCs exhibited a reproducible and monotonic response. The ShSFCs were investigated for use as sensorized electronic skin on 3D-printed soft robotic modules, such as bending actuators. Depending on the bending actuator module, the electronic skin was loaded under either compression (pneumatic-based module) or tension (tendon-based module). In both configurations, the ShSFs could be successfully used as deformation sensors, and in addition, detect the presence of damage based on the sensor signal drift. The sensor under tension showed better recovery of the signal after healing, and smaller signal relaxation. Even with the complete severing of the fiber, the piezoresistive properties returned after the healing, but in that case, thermal heat treatment was required. With their resilient response and self-healing properties, the supramolecular fiber composites can be used for the next generation of soft robotic modules.

The development of a multimodal sensing platform with multiple layers for electronic skin (e-skin) sensing of temperature and pressure has attracted considerable interest to practical applications in soft robotics, human-machine interfaces, and wearable health monitoring. In this work, we demonstrated a new platform technology with multiple sandwiched layers of highly oriented carbon nanotube membrane and polyacrylonitrile for the integration of pressure and temperature sensory functionalities into a single platform that is thin, ultra-lightweight, flexible, and wearable. The key technology of in situ deposition of sensor platform on objects or in robot interface makes this a unique method for the development of e-skins for robotic applications, offering a new approach to wearable electronics and portable health care.

Conductive self-healing polymer hydrogel and related soft sensor devices are receiving considerable attention from academia to industry because of their impacts on the lifetime and ergonomic design of soft robotics, prosthesis, and health monitoring systems. However, the development of such a material has thus far been limited considering performances and accessibility. Herein, robustness, self-healing, and conductivity for soft electronic skin are realized by an interpenetrating polymer network (IPN) system based on chemical/ionic cross-linked poly(acrylic acid) containing ferric ions, intercalated with physically cross-linked poly(vinyl alcohol). This IPN hydrogel successfully satisfies all three aforementioned capabilities; elongation at break greater than 1400%; recovery to original mechanical properties greater than 80% after 24 h; and 0.14 S m−1 of ionic conductivity, which is electrically healable. Such ionic conductivity of hydrogels enables multimodal sensing capabilities, i.e., for strain, pressure, and temperature. Particularly, a uniquely designed dual sensor attached to a finger simultaneously detects mechanical folding and pressure changes independently and can undergo large deformation 1000 times repeated and heating up to 90 °C.

Intelligent skinlike materials have recently attracted tremendous research interests for employing in electronic skin, soft robotics, and wearable devices. Because the traditional soft matters are ...

Capacitive sensors have been studied for various purposes, including detecting touch, pressure, and physical motions. However, a complicated process involving numerous electrode pattern arrays is necessary to detect touch positions. Here, a simply configured capacitive sensor with a single-layer ion gel (SIG) layer and copper electrodes without electrode arrays is reported. The touch sensor with a SIG is capable of precise touch sensing and touch positioning. The SIG touch sensor can detect the touch position by variation of the electric field in the ion gel layer and the change of the capacitance created by the touching object. Such a capacitive touch system has fast and accurate touch sensing capabilities and is independent of touch pressure. In addition, the touch sensor enables dynamic moving touch detection even when attached to the back of the human hand. Furthermore, an epidermal touch keypad is demonstrated by fabricating an array of three SIG touch sensors. The proposed capacitive touch sensor based on a SIG can enable the development of future electronic devices such as wearable electronics, soft robotics, electronic skin, and human–machine interaction systems.

As the use of soft robotics and soft actuators becomes more popular, there is also an increased need for soft sensors that can perform robustly under various extreme conditions. While some applications find ultrathin flexible sensors sufficient, like electronic skin and wearable devices, there is still a critical need for robust sensing under extreme deformations. Previous work in highly stretchable sensing modalities commonly uses liquid conductors that couple a change in measured electrical resistance with applied stress. However, liquid conductors are expensive and difficult to encapsulate, leading to leakages that can damage machinery. As an alternative, this letter proposes an elastomeric waveguide coated with a thin reflective metallic layer that reflects internally propagating light. Upon deformation, microcracks in the reflective layer modulate the intensity of the light at the photodetector, leading to decrease in optical power. This letter describes the design, fabrication, and characterization of the proposed optical soft sensor. A prototype was created and characterized for pressure and strain up to 110 kPa and 30% strain, respectively.

Contains fulltext : 238926.pdf (Publisher’s version ) (Closed access) Soft and electrically active materials are currently being utilized for intelligent systems, including electronic skin, cybernetics, soft robotics, and wearable devices. However, fabricating materials that fulfill the complex requirements of such advanced applications remains a challenge. These attributes include electronic, adhesive, self-healing, flexible, moldable, printable, and strong mechanical properties. Inspired by the recent interest in transforming monofunctional materials into multifunctional ones through nanoreinforcement and mussel-inspired chemistry, we have designed a simple two-step methodology based on halloysite nanotube (HNT) and polydopamine (PDA) to address the grand challenges in the field. In brief, HNTs were coated with PDA and embedded within a poly(vinyl alcohol) (PVA)-based polymeric matrix in combination with ferric ions (Fe(3+)). The final composite displayed a 3-fold increase in electrical conductivity, a 20-fold increase in mechanical stiffness, and a 7-fold increase in energy dissipation in comparison to their nonfunctional counterparts, which arose from a combination of nanotube alignment and mussel-inspired chemistry. Moreover, the developed composite could elongate up to 30000% of its original length, maintain its electrical properties after 600% strain, self-heal within seconds (both electrically and mechanically), and display strain-sensitivity. Finally, it was 3D-printable and thus amenable for engineering of customized wearable electronics.

Electronic skin (e-skin) with skin-like flexibility and tactile sensation will promote the great advancements in the fields of wearable equipment. Thus, the multifunction and high robustness are two important requirements for sensing capability of the e-skin. Here, a fully organic self-powered e-skin (FOSE-skin) based on the triboelectric nanogenerator (TENG) is developed. FOSE-skin based on TENG can be fully self-healed within 10 hours after being sheared by employing the self-healing polymer as a triboelectric layer and ionic liquid with the temperature sensitivity as an electrode. FOSE-skin based on TENG has the multifunctional and highly robust sensing capability and can sense the pressure and temperature simultaneously. The sensing capability of the FOSE-skin based on TENG can be highly robust with no changes after self-healing. FOSE-skin based on TENG can be employed to detect the arm swing, the temperature change of flowing water, and the motion trajectory. This work provides a new idea for solving the issues of monofunctional and low robust sensing capability for FOSE-skin based on TENG, which can further promote the application of wearable electronics in soft robotics and bionic prosthetics.

Electronic materials and devices that can mimic biological systems featured with elasticity, toughness, self-healing, degradability, and environmental friendliness drive the technological developments in fields spanning from bioelectronics, biomedical diagnosis and therapy, electronic skin, and soft robotics to Internet-of-Things with "green" electronics. Among them, ionic devices based on gel electrolytes have emerged as attractive candidates for biomimetic systems. Herein, we presented a straightforward approach to demonstrate soft ionic microdevices based on a versatile organohydrogel platform acting as both a free-standing, stretchable, adhesive, healable, and entirely degradable support and a highly conductive, dehydration- and freezing-tolerant electrolyte. This is achieved by forming a gelatin/ferric-ion-cross-linked polyacrylic acid (GEL/PAA) dual dynamic supramolecular network followed by soaking into a NaCl glycerol/water solution to further toughen the gelatin network via solvent displacement, thus obtaining a high toughness of 1.34 MJ·cm-3 and a high ionic conductivity (>7 mS·cm-1). Highly stretchable and multifunctional ionic microdevices are then fabricated based on the organohydrogel electrolytes by simple transfer printing of carbon-based microelectrodes onto the prestretched gel surface. Proof-of-concept microdevices including resistive strain sensors and microsupercapacitors are demonstrated, which displayed outstanding stretchability to 300% strain, resistance to dehydration for >6 months, autonomous self-healing, and rapid room-temperature degradation within hours. The present material design and fabrication approach for the organohydrogel-based ionic microdevices will provide promising scope for life-like and sustainable electronic systems.

Thin and imperceptible soft skins that can detect internal deformations as well as external forces, can go a long way to address perception and control challenges in soft robots. However, decoupling proprioceptive and exteroceptive stimuli is a challenging task. In this paper, we present a silicone-based, capacitive E-skin for exteroception and proprioception (SCEEP). This soft and stretchable sensor can perceive stretch as along with touch at 100 different points via its 100 tactels. In this paper, we present a novel algorithm that decouples global strain from local indentations due to external forces. The soft skin is 10.1cm in length and 10cm in width and can be used to accurately measure the global strain of up to 25% with an error of under 3%; while at the same time, can determine the amplitude and position of local indentations. This is a step towards a fully soft electronic skin that can act as a proprioceptive sensor to measure internal states while measuring external forces.

Researchers have made a lot of effort for the lightweight and high flexibility of wearable electronic devices, which also requires the associated energy harvesting equipment to have ultra-thin thickness and high stretchability. Therefore, a piezoelectric-triboelectric hybrid self-powered sensor (PTHS) has been proposed which can be used as the second layer of the human body. This elastic PTHS can even work on a person's fingers without disturbing the body's movements. The open circuit voltage (OCV) and short circuit current (SCC) of devices with a projected area of 30 mm × 25 mm can reach 1.2 V and 30 nA, respectively. Two piezoelectrically-triboelectrically sensors with machine learning optimized identification strategies were experimentally proven as the potential applications of the PTHS. The PTHS's ultra-thin thickness, high stretchability and superior geometry control features are promising in electronic skin, artificial muscle and soft robotics. The novelty of this work is that a smart mask integrated with PTHS can generate signal of the hybrid sensor for the biomechanical motion classifier. After suitable training, an overall accuracy of 87.9% using long short-term memory (LSTM) can be achieved.

There is an urgent demand for flexible multifunctional sensitive electronic devices in several potential applications, including personalized health monitoring, human motion detection, human–machine interfaces, soft robotics, and so on. Although exciting progress has been witnessed in recent decades, the excessive dependence on inorganic sensors and flexible substrates makes it difficult to attain multi-function sensing and excellent flexibility simultaneously, as well as their complicated fabrication, technical barriers and high cost. Herein, we report on a family of flexible multi-functional sensitive nanocomposites consisting of multi-walled carbon nanotubes (MWCNTs) uniformly distributed in a high elastic form-stable phase change polymer (HEPCP) that exhibit dramatic response to infrared light (IR), temperature and tensile stress in air, together with outstanding flexibility and stretchability. Optimum sensitivity to on/off IR, temperature and tensile stress is demonstrated with electrical conductivity ratios of 103.7, 11.8 and 1084.0 times at room temperature, respectively. The excellent performance of the MWCNT/HEPCP nanocomposites was largely attributed to the cyclic and reversible changes of their MWCNTs conductive network owing to the reversible form-stable phase transitions and high elasticity of the HEPCP substrate, which subsequently affected the thickness of the interfacial HEPCP between adjacent conductive MWCNTs, and thereby the electron tunneling efficiency between the MWCNTs. The novel MWCNT/HEPCP nanocomposites open a new window for multi-function sensing of electronic skin.

Soft and stretchable electronic devices play an important role in the growing fields of “soft robotics”, electronic skin, prosthetics, etc. As one of the fundamental units for stretchable electronics, strain sensors have been widely investigated in previous studies, although large strain detection is still a well-known challenge. Intrinsically stretchable sensor could undergo large strain depending on the stretchable matrix, and have attracted lots of attentions in recent years. In this study, we introduce liquid metal into the volumetric matrix of the flexible substrates to construct a novel intrinsically strain sensor, which could undergo large strain of the flexible substrates, avoiding the conventional fracture failures at the interfaces of the sensor and the substrate. Due to the advantages of the extremely stretchable ability, intrinsically strain sensors, i.e., elongation sensor and bending sensor, are further developed. It shows that, up to 178% strain could be measured by these stretchable sensors, and 74 m−1 of curvature for the bending sensor. The proposed sensors also show excellent responses, less than 120 ms for steady, and without delay and flushness. The proposed intrinsically strain sensor provides an alternative way to construct new types of intelligent flexible electronics and artificial skins.

The development of flexible and sensitive three-axis pressure sensors is of great interest for various wearable applications, such as electronic skin, soft robotics, and health monitoring. This study aims to propose a highly sensitive and flexible pressure sensor that can measure three-axis pressure based on conductive microstructured carbon nanotubes (CNTs) and polydimethylsiloxane (PDMS) elastomer with a simple and large-scale fabrication method that uses a polymer wet etching process. The randomly exposed CNTs on the surface using PDMS wet etching improve the sensitivity of the pressure sensors. The fabricated sensors exhibit a high sensitivity of 1.39 kPa−1 for a pressure range of 250 Pa, excellent limit of detection capability of 1.26 Pa, fast response time of within 52 ms, and high reliability of over 10,000 times of pressure loading/unloading. As an application, the pressure sensors are demonstrated using electronic skin on human fingers for the detection of the pressure levels and pattern recognition of distributed pressure. Moreover, to measure the three-axis pressure, pressurization tests are conducted using artificial fingers, demonstrating that the fabricated sensor can successfully measure the pressures applied along the three axes.

The latest generation of wearable devices features materials that are flexible, conductive, and stretchable, thus meeting the requirements of stability and reliability. However, the metal conductors that are currently used in various equipments cannot achieve these high performance expectations. Hence, a mussel-inspired conductive hydrogel (HAC-B-PAM) is prepared with a facile approach by employing polyacrylamide (PAM), dopamine-functionalized hyaluronic acid (HAC), borax as a dynamic cross-linker agent, and Li+ and Na+ as conductive ions. HAC-B-PAM hydrogels demonstrate an excellent stretchability (up to 2800%), high tensile toughness (42.4 kPa), self-adhesive properties (adhesion strength to porcine skin of 49.6 kPa), and good self-healing properties without any stimuli at room temperature. Furthermore, the fabricated hydrogel-based strain sensor is sensitive to deformation and can detect human body motion. Multifunctional hydrogels can be assembled into flexible wearable devices with potential applications in the field of electronic skin and soft robotics.

For comfortable and convenient human life, human-interactive and communicative robot has attractive attention from industrial to home-use applications. One of human-friendly robotic structures is pneumatic balloon type soft robots using mainly silicone rubber operated by compressed air. In this study, we develop a tactile pressure sensor array integrated with pneumatic soft robotic hand to monitor tactile pressure like a human skin. The important advance of this study is to integrate flexible sensors embedded in soft robotic hand without sacrificing softness and flexibility.

Polymeric elastomers integrated with high mechanical toughness and excellent self-healing ability can find attractive applications in electronic skin, soft robotics, and electrical devices. However, simultaneously enhancing the mechanical and self-healing properties of elastomers is still a great challenge because it is difficult to balance the effects between strong and weak cross-linking bonds. Here, a novel self-healing elastomer is developed via a one-pot polycondensation reaction between bis(3-aminopropyl)-terminated poly(dimethylsiloxane) and 2,4'-tolylene diisocyanate, followed by being coordinated with Al(III) ions by metal-coordination bonds. In this elastomer system, the quadruple hydrogen bonds not only are able to achieve rapid reformation after fracture but also can dissipate strain energy as a weak dynamic bond, endowing the elastomer with excellent self-healing ability and high stretchability, while the treble Al-coordination bonds acting as a strong dynamic bond contribute to the robust molecular networks, leading to the significantly improved robustness and elasticity of the self-healing elastomer. Owing to the accuracy design, the synthesized elastomer exhibits all the desired properties, including high tensile stress (2.6 MPa), exceptional toughness (∼14.7 MJ m-3), high stretchability (∼1700%), and excellent self-healing ability (90%). The robust self-healing elastomer enables the easy fabrication of flexible electronic skin, which will open a new avenue for next-generation electrical devices.

Biological tissues are multiresponsive and functional, and similar properties might be possible in synthetic systems by merging responsive polymers with hierarchical soft architectures. For example, mechanochromic polymers have applications in force-responsive colorimetric sensors and soft robotics, but their integration into sensitive, multifunctional devices remains challenging. Herein, a hierarchical nanoparticle-in-micropore (NP-MP) architecture in porous mechanochromic polymers, which enhances the mechanosensitivity and stretchability of mechanochromic electronic skins (e-skins), is reported. The hierarchical NP-MP structure results in stress-concentration-induced mechanochemical activation of mechanophores, significantly improving the mechanochromic sensitivity to both tensile strain and normal force (critical tensile strain: 50% and normal force: 1 N). Furthermore, the porous mechanochromic composites exhibit a reversible mechanochromism under a strain of 250%. This architecture enables a dual-mode mechanochromic e-skin for detecting static/dynamic forces via mechanochromism and triboelectricity. The hierarchical NP-MP architecture provides a general platform to develop mechanochromic composites with high sensitivity and stretchability.

Flexible sensors have shown great potential in remote health monitoring, body movements track, electronic skin, human-machine interfaces, and soft robotics. Hydrogels possess exceptional stretchability, flexibility and biocompatibility that render them appealing candidates for wearable flexible sensors. Among them, considerable efforts have been devoted to developing conductive hydrogels to achieve multifunctional wearable sensing through using functional groups/additives/nanofillers to modify the hydrogel network in recent years. This review summarizes recent advances of applications of hydrogels in flexible wearable sensors, such as sweat sampling and flexible electrodes, strain/pressure sensors and touch panels, focuses on the multifunctional conductive hydrogels-based flexible wearable sensors with self-healing, self-adhesion, or anti-freezing capabilities. A brief introduction to representative synthesis methods and strategies of conductive hydrogels is also presented. In the end, we also provide a personal perspective on the future development, and address the remaining challenges in the commercialization of conductive hydrogels-based multifunctional flexible wearable sensors.

Mimicry of the somatosensory system in the human skin via electronic devices exhibits broad applications in intelligent robotics and wearable electronics. Here, we report a novel biomimetic flexible dual-mode pressure sensor that is based on the interlocked piezoelectric and piezoresistive films with pyramid microstructures. The sensitivity of the sensor is significantly enhanced because of the induced larger stain variation along the thickness direction of the former piezoelectric film and increased contact area in the latter piezoresistive film. The synergistic effect of the piezoelectric and piezoresistive responses to stimuli also allows the dual-mode sensor to detect over broad pressure and frequency ranges. The analysis of these signals can deconvolute multiple aspects of the complex stimuli loading processes, including their loading direction, rate, magnitude, and duration. As a proof-of-concept demonstration, the dual-mode pressure sensor is successfully integrated with manipulators and human bodies to decode the complex and delicate picking processes and human motions, respectively. When combined with the other sensing modalities, the multifunctional dual-mode pressure sensor delivers new application opportunities in intelligent soft robotics and human-machine interfaces.

Engineered silk protein hydrogel that resembles skin tissue is a promising material for artificial electronic skin; it can be interfaced with real biological tissues seamlessly and used as an artificial tissue in soft robotics. Herein, we report a soft, biocompatible, and skin-adhesive silk hydrogel incorporating ZnO nanorods (ZnONRs) for a tribo- and piezo-electric energy-generating skin (EG-skin) that can harvest biomechanical energy and sense biomechanical motions. Incorporation of ZnONRs mediates an eight-fold enhancement of piezoelectricity compared to pristine silk hydrogel. An additional two-fold increase in the electrical response is possible when it is encapsulated in silk protein layers because of the hybrid effect of tribo- and piezo-electricity. The high power generated (~1 mW/cm2) is sufficient to activate low-power electrical devices, such as LEDs, oximeters, and stopwatches. Additionally, the EG-skin can be used as a tactile identifier for finger movements with quantized real-time electrical signals. The softness and skin-adhesive properties provide conformal interfaces with human skin and biological tissues, and we can harvest energies of approximately 6.2 and 0.9 μW/cm2, respectively, from their mechanical stimulation. The silk-protein-based artificial EG-skin can be effectively utilized in human–machine interfaces, tactile sensors, soft robotics, and biomedical implants.