Biocompatible Multifunctional E-Skins with Excellent Self-Healing Ability Enabled by Clean and Scalable Fabrication

Electronic skins(e-skins) with an excellent sensing performance have been widely developed over the last few decades.However,wearability,biocompatibility,environmental friendliness and scalability have become new limitations. Self-healing ability can improve the long-term robustness and reliability of e-skins. However,self-healing ability and integration are hardly balanced in classical structures of self-healable devices. Here,cellulose nanofiber/poly(vinyl alcohol)(CNF/PVA),a biocompatible moisture-inspired self-healable composite,was applied both as the binder in functional layers and the substrate. Various functional layers comprising particular carbon materials and CNF/PVA were patterned on the substrate. A planar structure was beneficial for integration,and the active self-healing ability of the functional layers endowed self-healed e-skins with a higher toughness. Water served as both the only solvent throughout the fabrication process and the trigger of the self-healing process,which avoids the pollution and bioincompatibility caused by the application of noxious additives. Our e-skins could achieve real-time monitoring of whole-body physiological signals and environmental temperature and humidity. Cross-interference between di erent external stimuli was suppressed through reasonable material selection and structural design. Combined with conventional electronics,data could be transmitted to a nearby smartphone for post-processing. This work provides a previously unexplored strategy for multifunctional e-skins with an excellent practicality.

Design of AI-Enhanced and Hardware-Supported Multimodal E-Skin for Environmental Object Recognition and Wireless Toxic Gas Alarm

Post-earthquake rescue missions are full of challenges due to the unstable structure of ruins and successive aftershocks.Most of the current rescue robots lack the ability to interact with environments,leading to low rescue efficiency.The multimodal electronic skin(e-skin)proposed not only reproduces the pressure,temperature,and humidity sensing capabilities of natural skin but also develops sensing functions beyond it—perceiving object proximity and NO2 gas.Its multilayer stacked structure based on Ecoflex and organohydrogel endows the e-skin with mechanical properties similar to natural skin.Rescue robots integrated with multimodal e-skin and artificial intelligence(AI)algorithms show strong environmental perception capabilities and can accurately distinguish objects and identify human limbs through grasping,laying the foundation for automated post-earthquake rescue.Besides,the combination of e-skin and NO2 wireless alarm circuits allows robots to sense toxic gases in the environment in real time,thereby adopting appropriate measures to protect trapped people from the toxic environment.Multimodal e-skin powered by AI algorithms and hardware circuits exhibits powerful environmental perception and information processing capabilities,which,as an interface for interaction with the physical world,dramatically expands intelligent robots’application scenarios.

Recent advances and innovations in the design and fabrication of wearable flexible biosensors and human health monitoring systems based on conjugated polymers

Wearable biosensors have received great interest as patient-friendly diagnostic technologies because of their high flexibility and conformability.The growing research and utilization of novel materials in designing wearable biosensors have accelerated the development of point-of-care sensing platforms and implantable biomedical devices in human health care.Among numerous potential materials,conjugated polymers(CPs)are emerging as ideal choices for constructing high-performance wearable biosensors because of their outstanding conductive and mechanical properties.Recently,CPs have been extensively incorporated into various wearable biosensors to monitor a range of target biomolecules.However,fabricating highly reliable CP-based wearable biosensors for practical applications remains a significant challenge,necessitating novel developmental strategies for enhancing the viability of such biosensors.Accordingly,this review aims to provide consolidated scientific evidence by summarizing and evaluating recent studies focused on designing and fabricating CP-based wearable biosensors,thereby facilitating future research.Emphasizing the superior properties and benefits of CPs,this review aims to clarify their potential applicability within this field.Furthermore,the fundamentals and main components of CP-based wearable biosensors and their sensing mechanisms are discussed in detail.The recent advancements in CP nanostructures and hybridizations for improved sensing performance,along with recent innovations in next-generation wearable biosensors are highlighted.CPbased wearable biosensors have been—and will continue to be—an ideal platform for developing effective and user-friendly diagnostic technologies for human health monitoring.

Highly Stretchable and Transparent Hydrogel as a Strain Sensor

The rapid developments of artificial intelligence have attracted attention in designing electronic skin(e-skin)to realize the mechanical and sensory properties of human skin.To better imitate the tactile sensing properties of human skin,a stretchable and transparent hydrogel is produced.Thus,an elastic and capacitive strain sensor was successfully produced through the as-prepared hydrogel.The sensor was elastic with a high conductive stability and could detect the strain changes in different states,which had very short response time that could be applied into the detection of large and small deformations and would shed light on its application in e-skin.

Tactile and temperature sensors based on organic transistors:Towards e-skin fabrication

Tactile and temperature sensors are the key components for e-skin fabrication.Organic transistors,a kind of intrinsic logic devices with diverse internal configurations,offer a wide range of options for sensor design and have played a vital role in the fabrication of e-skin-oriented tactile and temperature sensors.This research field has attained tremendous advancements,both in terms of materials design and device architecture,thereby leading to excellent performance of resulting tactile/temperature sensors.Herein,a systematic review of organic transistor-based tactile and temperature sensors is presented to summarize the latest progress in these devices.Particularly,we focus on spotlighting various device structures,underlying mechanisms and their performance.Lastly,an outlook for the future development of these devices is briefly discussed.We anticipate that this review will provide a quick overview of such a rapidly emerging research direction and attract more dedicated efforts for the development of next-generation sensing devices towards e-skin fabrication.

Recent advances in ultrathin materials and their applications in e-skin

Intelligent technologies based on artificial intelligence and big data hold great potential for health monitoring and human–machine capability enhancement.However,electronics must be connected to the human body to realize this vision.Thus,tissue or skin-like electronics with high stretchability and low stiffness mechanical properties are highly desirable.Ultrathin materials have attracted significant attention from the research community and the industry because of their high performance and flexibility.Over the past few years,considerable progress has been made in flexible ultrathin sensors and devices based on ultrathin materials.Here,we review the developments in this area and examine representative research progress in ultrathin materials fabrication and device construction.Strategies for the fabrication of stretchable ultrathin materials and devices are considered.The relationship between the thin-film structure and performance is emphasized and highlighted.Finally,the current capabilities and limitations of ultrathin devices were explored.

Ultra-robust stretchable electrode for e-skin:In situ assembly using a nanofiber scaffold and liquid metal to mimic water-to-net interaction

The development of stretchable electronics could enhance novel interface structures to solve the stretchability-conductivity dilemma,which remains a major challenge.Herein,we report a nano-liquid metal(LM)-based highly robust stretchable electrode(NHSE)with a self-adaptable interface that mimics water-tonet interaction.Based on the in situ assembly of electrospun elastic nanofiber scaffolds and electrosprayed LM nanoparticles,the NHSE exhibits an extremely low sheet resistance of 52 mΩsq^(-1).It is not only insensitive to a large degree of mechanical stretching(i.e.,350%electrical resistance change upon 570%elongation)but also immune to cyclic deformation(i.e.,5%electrical resistance increases after 330000 stretching cycles with 100%elongation).These key properties are far superior to those of the state-of-the-art reports.Its robustness and stability are verified under diverse circumstances,including long-term exposure to air(420 days),cyclic submersion(30000 times),and resilience against mechanical damages.The combination of conductivity,stretchability,and durability makes the NHSE a promising conductor/electrode solution for flexible/stretchable electronics for applications such as wearable on-body physiological signal detection,human-machine interaction,and heating e-skin.

Robust conductive skin hydrogel e-skin constructed by top-down strategy for motion-monitoring

The construction of biomass-based conductive hydrogel e-skins with high mechanical properties is the research hotspot and difficulty in the field of biomass materials.Traditional collagen-based conductive hydrogels,constructed by the typical"bottom-up"strategy,normally have the incompatible problem between high mechanical property and high collagen content,and the extraction of collagen is often necessary.To solve these problems,inspired by the high mechanical properties and high collagen content of animal skins,this work proposed a"top-down"construction strategy,in which the extraction of collagen was unnecessary and the skin collagen skeleton(SCS)with the 3D network structure woven by natural collagen fibers in goatskin was preserved and used as the basic framework of hydrogel.Following a four-step route,namely,pretreatment→soaking in AgNPs(silver nanoparticles)solution→soaking in the mixed solution containing HEA(2-hydroxyethyl methacrylate)and AlCl_(3)→polymerization,this work successfully achieved the fabrication of a new skin-based conductive hydrogel e-skin with high mechanical properties(tensile strength of 2.97 MPa,toughness of 6.23 MJ·m^(-3)and breaking elongation of 428%)by using goatskin as raw material.The developed skin hydrogel(called PH@Ag)possessed a unique structure with the collagen fibers encapsulated by PHEA,and exhibited satisfactory adhesion,considerable antibacterial property,cytocompatibility,conductivity(3.06 S·m^(-1))and sensing sensitivity(the maximum gauge factor of 5.51).The PH@Ag e-skin could serve as strain sensors to accurately monitor and recognize all kinds of human motions such as swallowing,frowning,walking,and so on,and thus is anticipated to have considerable application prospect in many fields including flexible wearable electronic devices,health and motion monitoring.

基于金属织物和自然摩擦带电的电子皮肤对人体运动的智能识别

目前已开发出各种基于电子或光学信号的技术来感知身体运动,这在医疗保健、康复和人机交互等领域至关重要.然而,这些信号都是从身体外部获取的.本研究中,我们制备了一种电子皮肤(e-skin)人体运动传感器,它利用有机聚合物和金属织物的组合,通过人体的自然电荷感应(EI)来检测运动.该电子皮肤可获得高达450 V的人体电势信号.此外,该信号可通过最先进的深度学习技术自动提取和训练.该传感器能准确识别睡眠活动,准确率约为96.55%.这种可穿戴运动传感器可以与物联网技术无缝集成,实现多功能应用,展示了其在人类活动识别和人工智能方面的潜在用途.

Self-powered sensor based on compressible ionic gel electrolyte for simultaneous determination of temperature and pressure

The simultaneous detection of multiple stimuli,such as pressure and temperature,has long been a persistent challenge for developing electronic skin(eskin)to emulate the functionality of human skin.Meanwhile,the demand for integrated power supply units is an additional pressing concern to achieve its lightweightness and flexibility.Herein,we propose a self-powered dual temperature–pressure(SPDM)sensor,which utilizes a compressible ionic gel electrolyte driven by the potential difference between MXene and Al electrodes.The SPDM sensor exhibits a rapid and timely response to changes in pressure-induced deformation,while exhibiting a slow and hysteretic response to temperature variations.These distinct response characteristics enable the differentiation of current signals generated by different stimuli through machine learning,resulting in an impressive accuracy rate of 99.1%.Furthermore,the developed SPDM sensor exhibits a wide pressure detection range of 0–800 kPa and a broad temperature detection range of 5–75C,encompassing the environmental conditions encountered in daily human life.The dual-mode coupled strategy by machine learning provides an effective approach for temperature and pressure detection and discrimination,showcasing its potential applications in wearable electronics,intelligent robots,human–machine interactions,and so on.

非平衡压缩实现高灵敏度、线性响应协同的离电压力传感器

Flexible pressure sensors with high sensitivity and linearity are highly desirable for robot sensing and human physiological signal detection.However,the current strategies for stabilizing axial microstructures(e.g.,micro-pyramids)are mainly susceptible to structural stiffening during compression,thereby limiting the realization of high sensitivity and linearity.Here,we report a bending-induced nonequilibrium compression process that effectively enhances the compressibility of microstructures,thereby crucially improving the efficiency of interfacial area growth of electric double layer(EDL).Based on this principle,we fabricate an iontronic flexible pressure sensor with vertical graphene(VG)array electrodes.Ultra-high sensitivity(185.09 kPa^(-1))and linearity(R^(2)=0.9999)are realized over a wide pressure range(0.49 Pa–66.67 k Pa).It also exhibits remarkable mechanical stability during compression and bending.The sensor is successfully employed in a robotic gripping task to recognize the targets of different materials and shapes based on a multilayer perception(MLP)neural network.It opens the door to realizing haptic sensing capabilities for robotic hands and prosthetic limbs.

High-sensitivity self-powered temperature/pressure sensor based on flexible Bi-Te thermoelectric film and porous microconed elastomer

Electronic skins are artificial skin-type multifunctional sensors,which hold great potentials in intelligent robotics,limb prostheses and human health monitoring.However,it is a great challenge to independently and accurately read various physical signals without power supplies.Here,a self-powered flexible temperature-pressure bimodal sensor based on high-performance thermoelectric films and porous microconed conductive elastic materials is presented.Through introducing flexible heat-sink design and harvesting body heat energy,the thin-film thermoelectric device could not only precisely sense temperature signal but also drive the pressure sensor for detecting external tactile stimulus.The integration of Bi-Te based thermoelectric film with high stability in wide temperature range enables the sensor to sense the ambient temperature with high resolution(<0.1 K)as well as excellent sensitivity(3.77 mV K^(-1)).Meanwhile,the porous microconed elastomer responds to pressure variation with low-pressure detection(16 Pa)and a high sensitivity of 37 kPa^(-1).Furthermore,the bimodal sensor could accurately and simultaneously monitor human wrist pulse and body temperature in real time,which demonstrates promising applications in self-powered electronic skins for human health monitoring systems.

Ultradurable,freeze-resistant,and healable MXene-based ionic gels for multi-functional electronic skin

Hydrogel is a potential matrix material of electronic-skins(E-skins)because of its excellent ductility,tunability,and biocompatibility.However,hydrogel-based E-Skins will inevitably lose their sensing performance in practical applications for water loss,physical damage,and ambient interferences.It remains a challenge to manufacture highly durable gel-based E-skins.Herein,an E-Skin is fabricated by introducing ionic liquids(ILs)into MXene-composited binary polymer network.The obtained ionic gel shows excellent mechanical properties,strong adhesion,and superior tolerance to harsh environments.The E-skin exhibits high sensitivity to both strain and pressure in a wide range of deformations,which enables a monitoring function for various human motions and physiological activities.Importantly,the E-skin shows consistent electrical response after being stored in the open air for 30 days and can be quickly healed by irradiation with 808 nm near-infrared light,originating from the photo-thermal effect induced self-healing acceleration.It is noteworthy that the E-skin also reveals a highly sensitive perception of temperature and near-infrared light,displaying the promising potential applications in the multifunctional flexible sensor.

Flexible and transparent capacitive pressure sensor with patterned microstructured composite rubber dielectric for wearable touch keyboard application

The development of pressure sensors with highly sensitivity, fast response and facile fabrication technique is desirable for wearable electronics. Here, we successfully fabricated a flexible transparent capacitive pressure sensor based on patterned microstructured silver nanowires(AgNWs)/polydimethylsiloxane(PDMS) composite dielectrics. Compared with the pure PDMS dielectric layer with planar structures, the patterned microstructured sensor exhibits a higher sensitivity(0.831 kPa^-1, <0.5 kPa), a lower detection limit,good stability and durability. The enhanced sensing mechanism about the conductive filler content and the patterned microstructures has also been discussed. A 5×5 sensor array was then fabricated to be used as flexible and transparent wearable touch keyboards systems. The fabricated pressure sensor has great potential in the future electronic skin area.

Recent advances of wearable and flexible piezoresistivity pressure sensor devices and its future prospects

The human skin inspired soft electronic devices have attracted broadly research attention in the past decades as the promising potential applications in health monitoring and diagnosis,robotics,and prosthetics.The soft wearable piezoresistivity pressure sensor is one of the most attractive candidates for the development of advanced electronic skin for its simple mechanism,compact structure,low cost and power energy consumption and ease of signal acquisition and transforms advantages.In this review,we will explore the recent progress and achievements in the field of piezoresistivity pressure sensor,focusing on the fundamentals of the piezoresistivity pressure sensor and the materials related to the devices,including active materials,substrate materials,and electrode materials.Subsequently,the challenges and outlook are discussed.We list several current challenges perspectives on the development of pressure sensors.Several critical topics for the optimization of the sensitivity and working range of sensing devices toward practical applications are discussed.Finally,perspectives on the slip and force vectors sensors,the developing technologies for multi-function and high-resolution sensor systems and signals process technologies are examined to highlight the near future development tendency in piezoresistivity pressure sensor research field.

A highly conductive and stretchable wearable liquid metal electronic skin for long-term conformable health monitoring

Conventional rigid electronics are usually unconformable with soft skins and tend to fail in accurate physiological monitoring and precise treatment. Electronic skins(e-Skins) made by conductive and stretchable materials offer mechanical compliance for fabricating flexible and conformable wearables. Compared to common organic or inorganic conductive materials, gallium-based liquid metals alone own superior conductivity and compliance. Here, we demonstrate a highly conductive and stretchable electronic skin with liquid metal circuits(LMCs) embedded in silicone rubber film, which are functionalized for physiological signals monitoring. Through the designs of serpentine structure, LMCs maintained good electrical conductivity and functionality under over 100% strain. Also, a wearable electrocardiogram(ECG) recording device was fabricated and tested. The device was able to acquire steady signals during real-time measurement of physical activities. The proposed liquid metal e-Skin can be further extended to conformable bio-integrated healthcare devices and intelligent health monitoring networks.

Facile preparation and high performance of wearable strain sensors based on ionically cross-linked composite hydrogels

Flexible sensors that can respond to multiple mechanical excitation modes and have high sensitivity are of great significance in the fields of electronic skin and health monitoring.Simulating multiple signal responses to skin such as strain and temperature remains an important challenge.Therefore,new multifunctional ion-crosslinked hydrogels with toughness and conductivity were designed and prepared in this work.A chemical gel with high mechanical strength was prepared by cross-linking acrylamide with N,N’-methylenebisacrylamide and ammonium persulfate.In addition,in order to enhance the conductive properties of the hydrogel,Ca^(2+),Mg^(2+)and Al^(3+)ions were added to the hydrogel during crosslinking.The double-layer network makes this ionic hydrogel show excellent mechanical properties.Moreover,the composite hydrogel containing Ca^(2+)can reach a maximum stretch of 1100%and exhibits ultra-high sensitivity(Sp=10.690 MPa^(-1)).The obtained hydrogels can successfully prepare wearable strain sensors,as well as track and monitor human motion.The present prepared multifunctional hydrogels are expected to be further expanded to intelligent health sensor materials.

Theoretical and experimental study of 2D conformability of stretchable electronics laminated onto skin

Smoothly attaching the stretchable epidermal electronic devices(EEDs) onto the skin surface is highly desired to improve the measurement accuracy of electrophysiological signal.The paper presents an analytical approach to study interfacial mechanics of the 2D planar EEDs on the checkerboard buckling patterns of human skin.Energy variation method is proposed to determine a criterion whether EEDs laminate conformally onto the skin surface under undeformed and stretched cases.EEDs with low bending stiffness(thin,soft devices/backing layer),smooth and soft skin,and strong adhesion promote conformal contact.Furthermore,the adhesion energy at the EED/skin interface is measured by the homemade peeling experiment platform with different substrate thicknesses and areal coverages.The upper limit of the areal coverage for EED conformal contact with the skin is proposed with given EED/skin properties.Conformability of EEDs are validated by experiments with different substrate thickness,areal coverage and external loadings.It provides a design guideline for EED to conformally contact with the skin surface for more accurate biological signal monitoring.

Stretchable human-machine interface based on skin-conformal sEMG electrodes with self-similar geometry

Current stretchable surface electrodes have attracted increasing attention owing to their potential applications in biological signal monitoring, wearable human-machine interfaces（HMIs） and the Internet of Things. The paper proposed a stretchable HMI based on a surface electromyography（sEMG） electrode with a self-similar serpentine configuration. The sEMG electrode was transfer-printed onto the skin surface conformally to monitor biological signals, followed by signal classification and controlling of a mobile robot. Such electrodes can bear rather large deformation（such as 〉30%） under an appropriate areal coverage. The sEMG electrodes have been used to record electrophysiological signals from different parts of the body with sharp curvature, such as the index finger,back of the neck and face, and they exhibit great potential for HMI in the fields of robotics and healthcare. The electrodes placed onto the two wrists would generate two different signals with the fist clenched and loosened. It is classified to four kinds of signals with a combination of the gestures from the two wrists, that is, four control modes. Experiments demonstrated that the electrodes were successfully used as an HMI to control the motion of a mobile robot remotely.