Progress of Proximity Sensors for Potential Applications in Electronic Skins

Recently,electronic skins and fl exible wearable devices have been developed for widespread applications in medical monitoring,artifi cial intelligence,human–machine interaction,and artifi cial prosthetics.Flexible proximity sensors can accurately perceive external objects without contact,introducing a new way to achieve an ultrasensitive perception of objects.This article reviews the progress of fl exible capacitive proximity sensors,fl exible triboelectric proximity sensors,and fl exible gate-enhanced proximity sensors,focusing on their applications in the electronic skin fi eld.Herein,their working mechanism,materials,preparation methods,and research progress are discussed in detail.Finally,we summarize the future challenges in developing fl exible proximity sensors.

Transparent Electronic Skin Device Based on Microstructured Silver Nanowire Electrode

Transparent, flexible electronic skin holds a wide range of applications in robotics, humanmachine interfaces, artificial intelligence, prosthetics, and health monitoring. Silver nanowire are mechanically flexible and robust, which exhibit great potential in transparent and electricconducting thin film. Herein, we report on a silver-nanowire spray-coating and electrodemicrostructure replicating strategy to construct a transparent, flexible, and sensitive electronic skin device. The electronic skin device shows highly sensitive piezo-capacitance response to pressure. It is found that micropatterning the surface of dielectric layer polyurethane elastomer by replicating from microstructures of natural-existing surfaces such as lotus leaf, silk, and frosted glass can greatly enhance the piezo-capacitance performance of the device. The microstructured pressure sensors based on silver nanowire exhibit good transparency, excellent flexibility, wide pressure detection range （0-150 kPa）, and high sensitivity （1.28 kPa-1）.

Bioinspired MXene-Based User-Interactive Electronic Skin for Digital and Visual Dual-Channel Sensing

User-interactive electronic skin(e-skin) that could convert mechanical stimuli into distinguishable outputs displays tremendous potential for wearable devices and health care applications. However, the existing devices have the disadvantages such as complex integration procedure and lack of the intuitive signal display function. Here, we present a bioinspired user-interactive e-skin, which is simple in structure and can synchronously achieve digital electrical response and optical visualization upon external mechanical stimulus. The e-skin comprises a conductive layer with a carbon nanotubes/cellulose nanofibers/MXene nanohybrid network featuring remarkable electromechanical behaviors, and a stretchable elastomer layer, which is composed of silicone rubber and thermochromic pigments. Furthermore, the conductive nanohybrid network with outstanding Joule heating performance can generate controllable thermal energy under voltage input and then achieve the dynamic coloration of silicone-based elastomer. Especially, such an innovative fusion strategy of digital data and visual images enables the e-skin to monitor human activities with evermore intuition and accuracy. The simple design philosophy and reliable operation of the demonstrated e-skin are expected to provide an ideal platform for next-generation flexible electronics.

Piezoresistive design for electronic skin: from fundamental to emerging applications

There is growing recognition that the developments in piezoresistive devices from personal healthcare to artificial intelli-gence,will emerge as de novo translational success in electronic skin.Here,we review the updates with regard to piezoresistive sensors including basic fundamentals,design and fabrication,and device performance.We also discuss the prosperous advances in piezoresistive sensor application,which offer perspectives for future electronic skin.

Bioinspired All‑Fibrous Directional Moisture‑Wicking Electronic Skins for Biomechanical Energy Harvesting and All‑Range Health Sensing

Electronic skins can monitor minute physiological signal variations in the human skins and represent the body’s state,showing an emerging trend for alternative medical diagnostics and human-machine interfaces.In this study,we designed a bioinspired directional moisture-wicking electronic skin(DMWES)based on the construction of heterogeneous fibrous membranes and the conductive MXene/CNTs electrospraying layer.Unidirectional moisture transfer was successfully realized by surface energy gradient and push-pull effect via the design of distinct hydrophobic-hydrophilic difference,which can spontaneously absorb sweat from the skin.The DMWES membrane showed excellent comprehensive pressure sensing performance,high sensitivity(maximum sensitivity of 548.09 kPa^(−1)),wide linear range,rapid response and recovery time.In addition,the single-electrode triboelectric nanogenerator based on the DMWES can deliver a high areal power density of 21.6μW m^(−2) and good cycling stability in high pressure energy harvesting.Moreover,the superior pressure sensing and triboelectric performance enabled the DMWES for all-range healthcare sensing,including accurate pulse monitoring,voice recognition,and gait recognition.This work will help to boost the development of the next-generation breathable electronic skins in the applications of AI,human-machine interaction,and soft robots.

Breathable Electronic Skins for Daily Physiological Signal Monitoring

With the aging of society and the increase in people’s concern for personal health,long-term physiological signal monitoring in daily life is in demand.In recent years,electronic skin(e-skin)for daily health monitoring applications has achieved rapid development due to its advantages in high-quality physiological signals monitoring and suitability for system integrations.Among them,the breathable e-skin has developed rapidly in recent years because it adapts to the long-term and high-comfort wear requirements of monitoring physiological signals in daily life.In this review,the recent achievements of breathable e-skins for daily physiological monitoring are systematically introduced and discussed.By dividing them into breathable e-skin electrodes,breathable e-skin sensors,and breathable e-skin systems,we sort out their design ideas,manufacturing processes,performances,and applications and show their advantages in long-term physiological signal monitoring in daily life.In addition,the development directions and challenges of the breathable e-skin are discussed and prospected.

Soft multifunctional neurological electronic skin through intrinsically stretchable synaptic transistor

Neurological electronic skin(E-skin)can process and transmit information in a distributed manner that achieves effective stimuli perception,holding great promise in neuroprosthetics and soft robotics.Neurological E-skin with multifunctional perception abilities can enable robots to precisely interact with the complex surrounding environment.However,current neurological E-skins that possess tactile,thermal,and visual perception abilities are usually prepared with rigid materials,bringing difficulties in realizing biologically synapse-like softness.Here,we report a soft multifunctional neurological E-skin(SMNE)comprised of a poly(3-hexylthiophene)(P3HT)nanofiber polymer semiconductor-based stretchable synaptic transistor and multiple soft artificial sensory receptors,which is capable of effectively perceiving force,thermal,and light stimuli.The stretchable synaptic transistor can convert electrical signals into transient channel currents analogous to the biological excitatory postsynaptic currents.And it also possesses both short-term and long-term synaptic plasticity that mimics the human memory system.By integrating a stretchable triboelectric nanogenerator,a soft thermoelectric device,and an elastic photodetector as artificial receptors,we further developed an SMNE that enables the robot to make precise actions in response to various surrounding stimuli.Compared with traditional neurological E-skin,our SMNE can maintain the softness and adaptability of biological synapses while perceiving multiple stimuli including force,temperature,and light.This SMNE could promote the advancement of E-skins for intelligent robot applications.

Wearable multifunctional organohydrogel-based electronic skin for sign language recognition under complex environments

Language barrier is the main cause of disagreement.Sign language,which is a common language in all the worldwide language families,is difficult to be entirely popularized due to the high cost of learning as well as the technical barrier in real-time translation.To solve these problems,here,we constructed a wearable organohydrogel-based electronic skin(e-skin)with fast self-healing,strong adhesion,extraor-dinary anti-freezing and moisturizing properties for sign language recognition under complex environ-ments.The e-skin was obtained by using an acrylic network as the main body,aluminum(III)and bay-berry tannin as the crosslinking agent,water/ethylene glycol as the solvent system,and a polyvinyl al-cohol network to optimize the network performance.Using this e-skin,a smart glove was further built,which could carry out the large-scale data collection of common gestures and sign languages.With the help of the deep learning method,specific recognition and translation for various gestures and sign lan-guages could be achieved.The accuracy was 93.5%,showing the ultra-high classification accuracy of a sign language interpreter.In short,by integrating multiple characteristics and combining deep learning technology with hydrogel materials,the e-skin achieved an important breakthrough in human-computer interaction and artificial intelligence,and provided a feasible strategy for solving the dilemma of mutual exclusion between flexible electronic devices and human bodies.

Intrinsically stretchable polymer semiconductor based electronic skin for multiple perceptions of force,temperature,and visible light

As a stretchable seamless device,electronic skin(E-skin)has drawn enormous interest due to its skin-like sensing capability.Besides the basic perception of force and temperature,multiple perception that is beyond existing functions of human skin is becoming an important direction for E-skin developments.However,the present E-skins for multiple perceptions mainly rely on different sensing materials and heterogeneous integration,resulting in a complex device structure.Additionally,their stretchability is usually achieved by the complicated microstructure design of rigid materials.Here,we report an intrinsically stretchable polymer semiconductor based E-skin with a simple structure for multiple perceptions of force,temperature,and visible light.The E-skin is on the basis of poly(3-hexylthiophene)(P3HT)nanofibers percolated polydimethylsiloxane(PDMS)composite polymer semiconductor,which is fabricated by a facile solution method.The E-skin shows reliable sensing capabilities when it is used to perceive strain,pressure,temperature,and visible light.Based on the E-skin,an intelligent robotic hand sensing and controlling system is further demonstrated.Compared with conventional E-skins for multiple perceptions,this E-skin only has a simple monolayer sensing membrane without the need of combining different sensing materials,heterogeneous integration,and complicated microstructure design.Such a strategy of utilizing intrinsically stretchable polymer semiconductor to create simple structured E-skin for multiple perceptions will promote the development of E-skins in a broad application scenario,such as artificial robotic skins,virtual reality,intelligent gloves,and biointegrated electronics.

Smart Fibers for Self‑Powered Electronic Skins

Smart fibers are considered as promising materials for the fabrication of wearable electronic skins owing to their features such as superior flexibility,light weight,high specific area,and ease of modification.Besides,piezoelectric or triboelectric electronic skins can respond to mechanical stimulation and directly convert the mechanical energy into electrical power for self-use,thereby providing an attractive method for tactile sensing and motion perception.The incorporation of sensing capabilities into smart fibers could be a powerful approach to the development of self-powered electronic skins.Herein,we review several aspects of the recent advancements in the development of self-powered electronic skins constructed with smart fibers.The summarized aspects include functional material selection,structural design,pressure sensing mechanism,and proof-to-concept demonstration to practical application.In particular,various fabrication strategies and a wide range of practical applications have been systematically introduced.Finally,a critical assessment of the challenges and promising perspectives for the development of fiber-based electronic skins has been presented.

Fully rubbery synaptic transistors made out of all-organic materials for elastic neurological electronic skin

Neurologic function implemented soft organic electronic skin holds promise for wide range of applications,such as skin prosthetics,neurorobot,bioelectronics,human-robotic interaction(HRI),etc.Here,we report the development of a fully rubbery synaptic transistor which consists of all-organic materials,which shows unique synaptic characteristics existing in biological synapses.These synaptic characteristics retained even under mechanical stretch by 30%.We further developed a neurological electronic skin in a fully rubbery format based on two mechanoreceptors(for synaptic potentiation or depression)of pressure-sensitive rubber and an all-organic synaptic transistor.By converting tactile signals into Morse Code,potentiation and depression of excitatory postsynaptic current(EPSC)signals allow the neurological electronic skin on a human forearm to communicate with a robotic hand.The collective studies on the materials,devices,and their characteristics revealed the fundamental aspects and applicability of the all-organic synaptic transistor and the neurological electronic skin.

Ultraconformable Integrated Wireless Charging Micro-Supercapacitor Skin

Conformable and wire-less charging energy storage devices play important roles in enabling the fast development of wearable,non-contact soft electronics.However,current wire-less charging power sources are still restricted by limited flexural angles and fragile connection of components,resulting in the failure expression of performance and constraining their fur-ther applications in health monitoring wearables and moveable artificial limbs.Herein,we present an ultracompatible skin-like integrated wireless charging micro-supercapacitor,which building blocks(including electrolyte,electrode and substrate)are all evaporated by liquid precursor.Owing to the infiltration and permeation of the liquid,each part of the integrated device attached firmly with each other,forming a compact and all-in-one configuration.In addition,benefitting from the controllable volume of electrode solution precursor,the electrode thickness is easily regulated varying from 11.7 to 112.5μm.This prepared thin IWC-MSC skin can fit well with curving human body,and could be wireless charged to store electricity into high capacitive micro-supercapacitors(11.39 F cm-3)of the integrated device.We believe this work will shed light on the construction of skin-attachable electronics and irregular sensing microrobots.

Advanced polymer materials-based electronic skins for tactile and non-contact sensing applications

Recently,polymer materials have been at the forefront of other materials in building high-performance flexible electronic skin(e-skin)devices due to con-spicuous advantages including excellent mechanical flexibility,good compatibil-ity,and high plasticity.However,most research works just paid considerable attention and effort to the design,construction,and possible application of e-skins that reproduce the tactile perception of the human skin sensory system.Compared with tactile sensing devices,e-skins that aim to imitate the non-contact sensing features in the sensory system of human skin tend to avoid undesired issues such as bacteria spreading and mechanical wear.To further promote the development of e-skins to the human skin sensory system where tactile perception and non-contact sensing complement each other,significant progress and advances have been achieved in the field of polymer materials enabled e-skins for both tactile perception and non-contact sensing applications.In this review,the latest progress in polymer material-based e-skins with regard to tactile,non-contact sensing capabilities and their practical applications are introduced.The fabrication strategies of polymer materials and their role in building high-performance e-skins for tactile and non-contact sensing are highlighted.Furthermore,we also review the research works that integrated the polymer-based tactile and non-contact e-skins into robots and prostheses,smart gloves,and VR/AR devices and addressed some representative problems to dem-onstrate their suitability in practical applications in human–machine interac-tions.Finally,the current challenges in the construction of high-performance tactile and non-contact e-skins are highlighted and promising properties in this direction,by taking advantage of the polymer materials,are outlined.

Thermally Conductive and UV-EMI Shielding Electronic Textiles for Unrestricted and Multifaceted Health Monitoring

Skin-attachable electronics have garnered considerable research attention in health monitoring and artificial intelligence domains,whereas susceptibility to elec-tromagnetic interference(EMI),heat accumulation issues,and ultraviolet(UV)-induced aging problems pose significant constraints on their potential applications.Here,an ultra-elas-tic,highly breathable,and thermal-comfortable epidermal sensor with exceptional UV-EMI shielding performance and remarkable thermal conductivity is developed for high-fidelity monitoring of multiple human electrophysiological signals.Via filling the elastomeric microfibers with thermally conductive boron nitride nanoparticles and bridging the insulating fiber interfaces by plating Ag nanoparticles(NPs),an interwoven thermal con-ducting fiber network(0.72 W m^(-1) K^(-1))is constructed benefiting from the seamless thermal interfaces,facilitating unimpeded heat dissipation for comfort skin wearing.More excitingly,the elastomeric fiber substrates simultaneously achieve outstanding UV protection(UPF=143.1)and EMI shielding(SET>65,X-band)capabilities owing to the high electrical conductivity and surface plasmon resonance of Ag NPs.Furthermore,an electronic textile prepared by printing liquid metal on the UV-EMI shielding and thermally conductive nonwoven textile is finally utilized as an advanced epidermal sensor,which succeeds in monitoring different electrophysiological signals under vigorous electromagnetic interference.This research paves the way for developing protective and environmentally adaptive epidermal electronics for next-generation health regulation.

Transparent,Ultra-Stretching,Tough,Adhesive Carboxyethyl Chitin/Polyacrylamide Hydrogel Toward High-Performance Soft Electronics

To date,hydrogels have gained increasing attentions as a flexible conductive material in fabricating soft electronics.However,it remains a big challenge to integrate multiple functions into one gel that can be used widely under various conditions.Herein,a kind of multifunc-tional hydrogel with a combination of desirable characteristics,including remarkable transparency,high conductivity,ultra-stretchability,tough-ness,good fatigue resistance,and strong adhesive ability is presented,which was facilely fabricated through multiple noncovalent crosslinking strategy.The resultant versatile sensors are able to detect both weak and large deformations,which owns a low detection limit of 0.1%strain,high stretchability up to 1586%,ultrahigh sensitivity with a gauge factor up to 18.54,as well as wide pressure sensing range(0-600 kPa).Meanwhile,the fabrication of conductive hydrogel-based sensors is demonstrated for various soft electronic devices,including a flexible human-machine interactive system,the soft tactile switch,an integrated electronic skin for unprecedented nonplanar visualized pressure sensing,and the stretchable triboelectric nanogenerators with excellent biomechanical energy harvesting ability.This work opens up a simple route for multifunctional hydrogel and promises the practical application of soft and self-powered wearable electronics in various complex scenes.

Dosimetric evaluation using the diode measurements for total skin electron therapy technique

Objective： The purpose of this study was to present the dosimetric study and evaluation the dose delivered to the skin tumor by using diode detector with total skin electron therapy （TSET）. Methods： The total skin electron irradiation （TSEI） technique was used to treat ten patients with histological confirmed mycosis fungoides according to the Stanford staging system at the Radiotherapy Department, National Cancer Institute, Cairo University, Egypt. High dose rate electron beams with low electron energy 5 MeV from a Siemens linear accelerator were used for treatment. Diodes were calibrated at TSET distance 300 cm and field size （35 × 35） cm^2. Results： The result of diodes measurements showed the dose to flat surface of the body was within ：1：10 % from the prescribed dose. Special areas of the body such as the perineum ＆ eyelid showed large deviation up to 30% variation from the prescription dose. Conclusion： The diode results of this study will be used as a quality assurance check for all new patients treated with TSET and to compare it to the prescribed dose delivered to the patients. It is recommends to evaluate the diodes measurements for all patients throughout the full treatment cycle and to identify individually the boost dose areas.

Highly Sensitive Ultrathin Flexible Thermoplastic Polyurethane/Carbon Black Fibrous Film Strain Sensor with Adjustable Scaffold Networks

In recently years,high-performance wearable strain sensors have attracted great attention in academic and industrial.Herein,a conductive polymer composite of electrospun thermoplastic polyurethane(TPU)fibrous film matrix-embedded carbon black(CB)particles with adjustable scaffold network was fabricated for high-sensitive strain sensor.This work indicated the influence of stereoscopic scaffold network structure built under various rotating speeds of collection device in electrospinning process on the electrical response of TPU/CB strain sensor.This structure makes the sensor exhibit combined characters of high sensitivity under stretching strain(gauge factor of 8962.7 at 155%strain),fast response time(60 ms),outstanding stability and durability(>10,000 cycles)and a widely workable stretching range(0–160%).This high-performance,wearable,flexible strain sensor has a broad vision of application such as intelligent terminals,electrical skins,voice measurement and human motion monitoring.Moreover,a theoretical approach was used to analyze mechanical property and a model based on tunneling theory was modified to describe the relative change of resistance upon the applied strain.Meanwhile,two equations based from this model were first proposed and offered an effective but simple approach to analyze the change of number of conductive paths and distance of adjacent conductive particles.

Skin-Inspired Electronics Enabled by Supramolecular Polymeric Materials

Next-generation electronics that intimately interact with the human body would play crucial roles in future health monitors and early disease diagnosis.Skin-inspired electronics have been rapidly growing in the past decade to emulate the remarkable sensory and responsive nature of the human skin tissue.

Advancing the pressure sensing performance of conductive CNT/PDMS composite film by constructing a hierarchical-structured surface

Flexible pressure sensors have attracted wide attention due to their applications to electronic skin,health monitoring,and human-machine interaction.However,the tradeoff between their high sensitivity and wide response range remains a challenge.Inspired by human skin,we select commercial silicon carbide sandpaper as a template to fabricate carbon nanotube(CNT)/polydimethylsiloxane(PDMS)composite film with a hierarchical structured surface(h-CNT/PDMS)through solution blending and blade coating and then assemble the h-CNT/PDMS composite film with interdigitated electrodes and polyurethane(PU)scotch tape to obtain an h-CNT/PDMS-based flexible pressure sensor.Based on in-situ optical images and finite element analysis,the significant compressive contact effect between the hierarchical structured surface of h-CNT/PDMS and the interdigitated electrode leads to enhanced pressure sensitivity and a wider response range(0.1661 kPa^(-1),0.4574 kPa^(-1)and 0.0989 kPa^(-1)in the pressure range of 0–18 kPa,18–133 kPa and 133–300 kPa)compared with planar CNT/PDMS composite film(0.0066 kPa^(-1)in the pressure range of 0–240 kPa).The prepared pressure sensor displays rapid response/recovery time,excellent stability,durability,and stable response to different loading modes(bending and torsion).In addition,our pressure sensor can be utilized to accurately monitor and discriminate various stimuli ranging from human motions to pressure magnitude and spatial distribution.This study supplies important guidance for the fabrication of flexible pressure sensors with superior sensing performance in next-generation wearable electronic devices.

Highly Sensitive Flexible Pressure Sensors based on Graphene/Graphene Scrolls Multilayer Hybrid Films

In recent years,flexible pressure sensors have attracted much attention owing to their potential applications in motion detection and wearable electronics.As a result,important innovations have been reported in both conductive materials and the underlying substrates,which are the two crucial components of a pressure sensor.1D materials like nanowires are being widely used as the conductive materials in flexible pressure sensors,but such sensors usually exhibit low performances mainly due to the lack of strong interfacial interactions between the substrates and 1D materials.In this paper,we report the use of graphene/graphene scrolls hybrid multilayers films as the conductive material and a microstructured polydimethylsiloxane substrate using Epipremnum aureum leaf as the template to fabricate highly sensitive pressure sensors.The 2D structure of graphene allows to strongly anchor the scrolls to ensure the improved adhesion between the highly conductive hybrid films and the patterned substrate.We attribute the increased sensitivity(3.5 k Pa^-1),fast response time(<50 ms),and the good reproducibility during 1000 loading-unloading cycles of the pressure sensor to the synergistic effect between the 1D scrolls and 2D graphene films.Test results demonstrate that these sensors are promising for electronic skins and motion detection applications.