Search

Copyright of Journal of Materials Engineering / Cailiao Gongcheng is the property of Journal of Materials Engineering Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Electronic skin is a novel type of flexible and wearable sensor that mimics human skin perception， exhibiting characteristics such as lightweight， softness， and flexibility. It has the capability to convert external stimuli into diverse output signals， showing substantial potential in health monitoring and human-computer interaction. This article provides a comprehensive review from the perspective of intelligent materials for constructing electronic skin， focusing on commonly used substrates， conductive fillers， and their geometric structures. It discusses the requirements for biocompatibility， adhesion， self-healing， and self-powering performance of electronic skin based on the complex environmental conditions it faces in applications. It points out that in the research process， there are still issues such as poor comprehensive perception of human skin， complex and expensive fabrication processes， and delayed response to sensory signals. By optimizing materials and structures to enhance the basic performance of electronic skin， the development trend is to build outstanding performance， multifunctionality， and simultaneous detection of various external stimuli. Electronic skin shows great potential in medical diagnosis， soft robotics， smart prosthetics， and human-machine interaction.

This study presents a novel transparent, sensitive electronic skin (S-TENG) using a frictional nanogenerator for energy harvesting and physiological motion monitoring. Featuring thermally treated PVDF fiber membranes and silver nanowire electrodes, the e-skin boasts 80% transparency, high sensitivity (detecting 0.13 g touch), stability, and breathability. It generates 301 V and 2.7 µA under 8N force, achieving 306 mW/m2 power density. S-TENG holds promise for self-powered wearables and health training applications, representing a significant step in e-skin innovation. [Display omitted] Against the backdrop of advancements in modern multifunctional wearable electronics, there is a growing demand for simple, sustainable, and portable electronic skin (e-skin), posing significant challenges. This study aims to delineate the development of a straightforward, transparent, highly sensitive, and high power-density electronic skin based on a triboelectric nanogenerator(S-TENG), designed for harvesting human body energy and real-time monitoring of the physiological motion status. Our e-skin incorporates thermally treated polyvinylidene fluoride (PVDF) fiber membranes as the contact layer and a film of silver nanowires as the conductive electrodes. The resulting contact-separation type e-skin exhibits an impressive transparency of 80 %, along with a nice sensitivity value, capable of detecting a light touch from a 0.13 g sponge and demonstrating good working stability and breathability. Leveraging the triboelectric effect, our e-skin generates an open-circuit voltage of 301 V and a short-circuit current of 2.7 μA under an extrinsic force of 8 N over an interaction area of 4 × 4 cm2, achieving a power density up to 306 mW/m2. With its signal processing circuitry, the integrated S-TENG showcases nice energy harvesting and signal transmission capabilities. Accordingly, we contend that S-TENG has potential applications in energy capture and real-time human motion state monitoring. This research is anticipated to blaze a novel and practical trail for self-powered wearable devices and personalized health rehabilitation training regimens. [ABSTRACT FROM AUTHOR]

The field of electronic skin has received considerable attention due to its extensive potential applications in areas including tactile sensing and health monitoring. With the development of electronic skin devices, electronic skin can be attached to the surface of human skin for long-term health monitoring, which makes comfort an essential factor that cannot be ignored in the design of electronic skin. Therefore, this paper proposes an assessment method for evaluating the comfort of electronic skin based on neurodynamic analysis. The holistic analysis framework encompasses the mechanical model of the skin, the modified Hodgkin–Huxley model for the transduction of stimuli, and the gate control theory for the modulation and perception of pain sensation. The complete process, from mechanical stimulus to the generation of pain perception, is demonstrated. Furthermore, the influence of different factors on pain perception is investigated. Sensation and comfort diagrams are provided to assess the mechanical comfort of electronic skin. The comfort assessment method proposed in this paper provides a theoretical basis when assessing the comfort of electronic skin. [ABSTRACT FROM AUTHOR]

In the context of an aging population and escalating work pressures, cardiovascular diseases pose increasing health risks. Electrocardiogram (ECG) monitoring presents a preventive tool, but conventional devices often compromise comfort. This study proposes an approach using Ag NW/TPU composites for flexible and breathable epidermal electronics. In this new structure, TPU fibers are used to support Ag NWs/TPU nanocomposites. The TPU fiber-reinforced Ag NW/TPU (TFRAT) nanocomposites exhibit excellent conductivity, stretchability, and electromechanical durability. The composite ensures high steam permeability, maintaining stable electrical performance after washing cycles. Employing this technology, a flexible ECG detection system is developed, augmented with a convolutional neural network (CNN) for automated signal analysis. The experimental results demonstrate the system's reliability in capturing physiological signals. Additionally, a CNN model trained on ECG data achieves over 99% accuracy in diagnosing arrhythmias. This study presents TFRAT as a promising solution for wearable electronics, offering both comfort and functionality in long-term epidermal applications, with implications for healthcare and beyond. [ABSTRACT FROM AUTHOR]

Real-time monitoring of human health can be significantly improved by designing novel electronic skin (E-skin) platforms that mimic the characteristics and sensitivity of human skin. A high-quality E-skin platform that can simultaneously monitor multiple physiological and metabolic biomarkers without introducing skin discomfort or irritation is an unmet medical need. Conventional E-skins are either monofunctional or made from elastomeric films that do not include key synergistic features of natural skin, such as multi-sensing, breathability, and thermal management capabilities in a single patch. Herein, a biocompatible and biodegradable E-skin patch based on flexible gelatin methacryloyl aerogel (FGA) for non-invasive and continuous monitoring of multiple biomarkers of interest is engineered and demonstrated. Taking advantage of cryogenic temperature treatment and slow polymerization, FGA is fabricated with a highly interconnected porous structure that displays good flexibility, passive-cooling capabilities, and ultra-lightweight properties that make it comfortable to wear for long periods of time. It also provides numerous permeable capillary channels for thermal-moisture transfer, ensuring its excellent breathability. Therefore, the engineered FGA-based E-skin can simultaneously monitor body temperature, hydration, and biopotentials via electrophysiological sensors and detect glucose, lactate, and alcohol levels via electrochemical sensors. This work offers a previously unexplored materials strategy for next-generation E-skin platforms with superior practicality.

Highlights An ultraconformable skin-like integrated wireless charging micro-supercapacitor (IWC-MSC) could be wireless charged to store electricity into high capacitive micro-supercapacitors (11.39 F cm−3), and fits well with human surface. Building blocks of IWC-MSC skin are all evaporated by liquid precursor, and each part of the device attached firmly benefitting from the liquid permeation, forming a compact and all-in-one configuration. The electrode thickness easily regulated varying from 11.7 to 112.5 μm by controlling the volume of electrode solution precursor.

Advances in biomimetic triboelectric nanogenerators (TENGs) have significant implications for electronic skin (e-skin) and human–machine interaction (HMI). Emphasizing the need to mimic complex functionalities of natural systems, particularly human skin, TENGs leverage triboelectricity and electrostatic induction to bridge the gap in traditional electronic devices’ responsiveness and adaptability. The exploration begins with an overview of TENGs’ operational principles and modes, transitioning into structural and material biomimicry inspired by plant and animal models, proteins, fibers, and hydrogels. Key applications in tactile sensing, motion sensing, and intelligent control within e-skins and HMI systems are highlighted, showcasing TENGs’ potential in revolutionizing wearable technologies and robotic systems. This review also addresses the challenges in performance enhancement, scalability, and system integration of TENGs. It points to future research directions, including optimizing energy conversion efficiency, discovering new materials, and employing micro-nanostructuring techniques for enhanced triboelectric charges and energy conversion. The scalability and cost-effectiveness of TENG production, pivotal for mainstream application, are discussed along with the need for versatile integration with various electronic systems. The review underlines the significance of making bioinspired TENGs more accessible and applicable in everyday technology, focusing on compatibility, user comfort, and durability. Conclusively, it underscores the role of bioinspired TENGs in advancing wearable technology and interactive systems, indicating a bright future for these innovations in practical applications.

Advances in biomimetic triboelectric nanogenerators (TENGs) have significant implications for electronic skin (e-skin) and human–machine interaction (HMI). Emphasizing the need to mimic complex functionalities of natural systems, particularly human skin, TENGs leverage triboelectricity and electrostatic induction to bridge the gap in traditional electronic devices' responsiveness and adaptability. The exploration begins with an overview of TENGs' operational principles and modes, transitioning into structural and material biomimicry inspired by plant and animal models, proteins, fibers, and hydrogels. Key applications in tactile sensing, motion sensing, and intelligent control within e-skins and HMI systems are highlighted, showcasing TENGs' potential in revolutionizing wearable technologies and robotic systems. This review also addresses the challenges in performance enhancement, scalability, and system integration of TENGs. It points to future research directions, including optimizing energy conversion efficiency, discovering new materials, and employing micro-nanostructuring techniques for enhanced triboelectric charges and energy conversion. The scalability and cost-effectiveness of TENG production, pivotal for mainstream application, are discussed along with the need for versatile integration with various electronic systems. The review underlines the significance of making bioinspired TENGs more accessible and applicable in everyday technology, focusing on compatibility, user comfort, and durability. Conclusively, it underscores the role of bioinspired TENGs in advancing wearable technology and interactive systems, indicating a bright future for these innovations in practical applications. [ABSTRACT FROM AUTHOR]

Polyvinyl alcohol (PVA) hydrogel is favored by researchers due to its good biocompatibility, high mechanical strength, low friction coefficient, and suitable water content. The widely distributed hydroxyl side chains on the PVA molecule allow the hydrogels to be branched with various functional groups. By improving the synthesis method and changing the hydrogel structure, PVA-based hydrogels can obtain excellent cytocompatibility, flexibility, electrical conductivity, viscoelasticity, and antimicrobial properties, representing a good candidate for articular cartilage restoration, electronic skin, wound dressing, and other fields. This review introduces various preparation methods of PVA-based hydrogels and their wide applications in the biomedical field. [ABSTRACT FROM AUTHOR]

Highlights: An ultraconformable skin-like integrated wireless charging micro-supercapacitor (IWC-MSC) could be wireless charged to store electricity into high capacitive micro-supercapacitors (11.39 F cm−3), and fits well with human surface. Building blocks of IWC-MSC skin are all evaporated by liquid precursor, and each part of the device attached firmly benefitting from the liquid permeation, forming a compact and all-in-one configuration. The electrode thickness easily regulated varying from 11.7 to 112.5 μm by controlling the volume of electrode solution precursor. Conformable and wireless charging energy storage devices play important roles in enabling the fast development of wearable, non-contact soft electronics. However, current wireless 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 further 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 112.5 μm. 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. [ABSTRACT FROM AUTHOR]

Human skin comprises multiple hierarchical layers that perform various functions such as protection, sensing, and structural support. Developing electronic skin (E‐skin) with similar properties has broad implications in health monitoring, prosthetics, and soft robotics. While previous efforts have predominantly concentrated on sensory capabilities, this study introduces a hierarchical polymer system that not only structurally resembles the epidermis‐dermis bilayer structure of skin but also encompasses sensing functions. The system comprises a polymeric hydrogel, representing the "dermis", and a superimposed nanoporous polymer film, forming the "epidermis". Within the film, interconnected nanoparticles mimic the arrangement of interlocked corneocytes within the epidermis. The fabrication process employs a robust in situ interfacial precipitation polymerization of specific water‐soluble monomers that become insoluble during polymerization. This process yields a hybrid layer establishing a durable interface between the film and hydrogel. Beyond the structural mimicry, this hierarchical structure offers functionalities resembling human skin, which includes (1) water loss protection of hydrogel by tailoring the hydrophobicity of the upper polymer film; (2) tactile sensing capability via self‐powered triboelectric nanogenerators; (3) built‐in gold nanowire‐based resistive sensor toward temperature and pressure sensing. This hierarchical polymeric approach represents a potent strategy to replicate both the structure and functions of human skin in synthetic designs. [ABSTRACT FROM AUTHOR]

Flexible tactile sensors with multifunctional sensing functions have attracted much attention due to their wide applications in artificial limbs, intelligent robots, human‐machine interfaces, and health monitoring devices. Here, a multifunctional flexible tactile sensor based on resistive effect for simultaneous sensing of pressure and temperature is reported. The sensor features a simple design with patterned metal film on a soft substrate with cavities and protrusions. The decoupling of pressure and temperature sensing is achieved by the reasonable arrangement of metal layers in the patterned metal film. Systematically experimental and numerical studies are carried out to reveal the multifunctional sensing mechanism and show that the proposed sensor exhibits good linearity, fast response, high stability, good mechanical flexibility, and good microfabrication compatibility. Demonstrations of the multifunctional flexible tactile sensor to monitor touch, breathing, pulse and objects grabbing/releasing in various application scenarios involving coupled temperature/pressure stimuli illustrate its excellent capability of measuring pressure and temperature simultaneously. These results offer an effective tool for multifunctional sensing of pressure and temperature and create engineering opportunities for applications of wearable health monitoring and human‐machine interfaces. [ABSTRACT FROM AUTHOR]

The emerging field of flexible tactile sensing systems, equipped with multi-physical tactile sensing capabilities, holds vast potential across diverse domains such as medical monitoring, robotics, and human–computer interaction. In response to the prevailing challenges associated with the limited integration and sensitivity of flexible tactile sensors, this paper introduces a versatile tactile sensing system capable of concurrently monitoring temperature and pressure. The temperature sensor employs carbon nanotube/graphene conductive paste as its sensitive material, while the pressure sensor integrates an ionic gel containing boron nitride as its sensitive layer. Through the application of cost-effective screen printing technology, we have successfully manufactured a flexible dual-mode sensor with exceptional performance, featuring high sensitivity (804.27 kPa − 1 ), a broad response range (50 kPa), rapid response time (17 ms), and relaxation time (34 ms), alongside exceptional durability over 5000 cycles. Furthermore, the resistance temperature coefficient of the sensor within the temperature range of 12.5 °C to 93.7 °C is −0.17% °C−1. The designed flexible dual-mode tactile sensing system enables the real-time detection of pressure and temperature information, presenting an innovative approach to electronic skin with multi-physical tactile sensing capabilities. [ABSTRACT FROM AUTHOR]

Developing electronic skins (e‐skins) that are comparable to or even beyond human tactile perception holds significant importance in advancing the process of intellectualization. In this context, a machine‐learning‐motivated micropyramid array bimodal (MAB) e‐skin based on capacitive sensing is reported, which enables spatial mapping applications based on bimodal sensing (proximity and pressure) implemented via fringing and iontronic effects, such as contactless measurement of 3D objects and contact recognition of Braille letters. Benefiting from the iontronic effect and single‐micropyramid structure, the MAB e‐skin in pressure mode yields impressive features: a maximum sensitivity of 655.3 kPa−1 (below 0.5 kPa), a linear sensitivity of 327.9 kPa−1 (0.5–15 kPa), and an ultralow limit of detection of 0.2 Pa. With the assistance of multilayer perceptron and convolutional neural network, the MAB e‐skin can accurately perceive 6 materials and 10 surface shapes based on the training and learning using the collected datasets from proximity and pressure modes, thus allowing it to achieve the precise perception of different objects within one proximity‐pressure cycle. The development of this MAB e‐skin opens a new avenue for robotic skin and the expansion of advanced applications. [ABSTRACT FROM AUTHOR]

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.

Background: Devices that mimic the functions of human skin are known as “electronic skin,” and they must have characteristics like high sensitivity, a wide dynamic range, high spatial homogeneity, cheap cost, wide area easy processing, and the ability to distinguish between diverse external inputs. Methods: This study introduces a novel approach, termed microfluidic droplet-based emulsion self-assembly (DMESA), for fabricating 3D microstructured elastomer layers using polydimethylsiloxane (PDMS). The method aims to produce accurate capacitive pressure sensors suitable for electronic skin (e-skin) applications. The DMESA method facilitates the creation of uniform-sized spherical micropores dispersed across a significant area without requiring a template, ensuring excellent spatial homogeneity. Results: Micropore size adjustment, ranging from 100 to 600 μm, allows for customization of pressure sensor sensitivity. The active layer of the capacitive pressure sensor is formed by the three-dimensional elastomer itself. Experimental results demonstrate the outstanding performance of the DMESA approach. It offers simplicity in processing, the ability to adjust performance parameters, excellent spatial homogeneity, and the capability to differentiate varied inputs. Capacitive pressure sensors fabricated using this method exhibit high sensitivity and dynamic amplitude, making them promising candidates for various e-skin applications. Conclusion: The DMESA method presents a highly promising solution for fabricating 3D microstructured elastomer layers for capacitive pressure sensors in e-skin technology. Its simplicity, performance adjustability, spatial homogeneity, and sensitivity to different inputs make it suitable for a wide range of electronic skin applications.

This paper presents an in-depth discussion of the development of electronic skin (e-skin) devices for transdermal drug delivery (TDD) for skin health management. E-skin devices, grounded in flexible conductive polymers, demonstrate immense potential as a versatile platform for TDD due to the adaptable properties factors. The integration of sensors and electronic components into e-skin devices seamlessly allows for real-time monitoring of skin health parameters, such as temperature, hydration, and pH levels. Additionally, e-skin devices can also realize the prospect of targeted and controlled drug delivery through the utilization of iontophoresis. This paper explores the current state-of-the-art in e-skin technology, emphasizing its applications in TDD and skin disease management. Furthermore, the paper outlines the prospective directions and prevailing challenges in this rapidly evolving domain. [ABSTRACT FROM AUTHOR]

The conformal integration of the electronic skin on the non‐developable surface is in great demand for the comprehensive tactile sensing of robotics and prosthetics. However, the current techniques still encounter obstacles in achieving conformal integration of film‐like electronic skin on non‐developable surfaces with substantial curvatures for contact pressure detection and tactile mapping. In this paper, by utilizing the 3D printing technology to prepare the 3D electrode array in the structural component following its surface curvature, and covering it with a molded functional shell to form the pressure sensitive iontronic interface, a device is proposed to achieve high‐sensitive pressure detection and high‐fidelity tactile mapping on a complicated non‐developable surface, called structural electronic skin (SES). The SES is prepared in a 3D printed fingertip with 46 tactile sensing units distributed on its curved surface, achieving the integration of both structural and tactile functions in a single component. By integrating the smart fingertip into a dexterous hand, a series of demonstrations are presented to show the dead‐zone free pressure detection and tactile mapping with high sensitivity, for instance, 2D pulse wave monitoring and robotic injection in a medical robot, object recognition and compliant control in a smart prosthesis. [ABSTRACT FROM AUTHOR]

As key interfaces for the disabled, optimal prosthetics should elicit natural sensations of skin touch or proprioception, by unambiguously delivering the multimodal signals acquired by the prosthetics to the nervous system, which still remains challenging. Here, a bioinspired temperature‐pressure electronic skin with decoupling capability (TPD e‐skin), inspired by the high‐low modulus hierarchical structure of human skin, is developed to restore such functionality. Due to the bionic dual‐state amplifying microstructure and contact resistance modulation, the MXene TPD e‐skin exhibits high sensitivity over a wide pressure range and excellent temperature insensitivity (91.2% reduction). Additionally, the high‐low modulus structural configuration enables the pressure insensitivity of the thermistor. Furthermore, a neural model is proposed to neutrally code the temperature‐pressure signals into three types of nerve‐acceptable frequency signals, corresponding to thermoreceptors, slow‐adapting receptors, and fast‐adapting receptors. Four operational states in the time domain are also distinguished after the neural coding in the frequency domain. Besides, a brain‐like machine learning‐based fusion process for frequency signals is also constructed to analyze the frequency pattern and achieve object recognition with a high accuracy of 98.7%. The TPD neural system offers promising potential to enable advanced prosthetic devices with the capability of multimodality‐decoupling sensing and deep neural integration. [ABSTRACT FROM AUTHOR]

Wearable electronics have received extensive attention in human–machine interactions, robotics, and health monitoring. The use of multifunctional sensors that are capable of measuring a variety of mechanical or environmental stimuli can provide new functionalities for wearable electronics. Advancements in material science and system integration technologies have contributed to the development of high-performance flexible multifunctional sensors. This review presents the main approaches, based on functional materials and device structures, to improve sensing parameters, including linearity, detection range, and sensitivity to various stimuli. The details of electrical, biocompatible, and mechanical properties of self-powered sensors and wearable wireless systems are systematically elaborated. Finally, the current challenges and future developmental directions are discussed to offer a guide to fabricate advanced multifunctional sensors. [ABSTRACT FROM AUTHOR]

Abstract Human skin comprises multiple hierarchical layers that perform various functions such as protection, sensing, and structural support. Developing electronic skin (E‐skin) with similar properties has broad implications in health monitoring, prosthetics, and soft robotics. While previous efforts have predominantly concentrated on sensory capabilities, this study introduces a hierarchical polymer system that not only structurally resembles the epidermis‐dermis bilayer structure of skin but also encompasses sensing functions. The system comprises a polymeric hydrogel, representing the “dermis”, and a superimposed nanoporous polymer film, forming the “epidermis”. Within the film, interconnected nanoparticles mimic the arrangement of interlocked corneocytes within the epidermis. The fabrication process employs a robust in situ interfacial precipitation polymerization of specific water‐soluble monomers that become insoluble during polymerization. This process yields a hybrid layer establishing a durable interface between the film and hydrogel. Beyond the structural mimicry, this hierarchical structure offers functionalities resembling human skin, which includes (1) water loss protection of hydrogel by tailoring the hydrophobicity of the upper polymer film; (2) tactile sensing capability via self‐powered triboelectric nanogenerators; (3) built‐in gold nanowire‐based resistive sensor toward temperature and pressure sensing. This hierarchical polymeric approach represents a potent strategy to replicate both the structure and functions of human skin in synthetic designs.

Abstract Flexible tactile sensors with multifunctional sensing functions have attracted much attention due to their wide applications in artificial limbs, intelligent robots, human‐machine interfaces, and health monitoring devices. Here, a multifunctional flexible tactile sensor based on resistive effect for simultaneous sensing of pressure and temperature is reported. The sensor features a simple design with patterned metal film on a soft substrate with cavities and protrusions. The decoupling of pressure and temperature sensing is achieved by the reasonable arrangement of metal layers in the patterned metal film. Systematically experimental and numerical studies are carried out to reveal the multifunctional sensing mechanism and show that the proposed sensor exhibits good linearity, fast response, high stability, good mechanical flexibility, and good microfabrication compatibility. Demonstrations of the multifunctional flexible tactile sensor to monitor touch, breathing, pulse and objects grabbing/releasing in various application scenarios involving coupled temperature/pressure stimuli illustrate its excellent capability of measuring pressure and temperature simultaneously. These results offer an effective tool for multifunctional sensing of pressure and temperature and create engineering opportunities for applications of wearable health monitoring and human‐machine interfaces.

Abstract Developing electronic skins (e‐skins) that are comparable to or even beyond human tactile perception holds significant importance in advancing the process of intellectualization. In this context, a machine‐learning‐motivated micropyramid array bimodal (MAB) e‐skin based on capacitive sensing is reported, which enables spatial mapping applications based on bimodal sensing (proximity and pressure) implemented via fringing and iontronic effects, such as contactless measurement of 3D objects and contact recognition of Braille letters. Benefiting from the iontronic effect and single‐micropyramid structure, the MAB e‐skin in pressure mode yields impressive features: a maximum sensitivity of 655.3 kPa−1 (below 0.5 kPa), a linear sensitivity of 327.9 kPa−1 (0.5–15 kPa), and an ultralow limit of detection of 0.2 Pa. With the assistance of multilayer perceptron and convolutional neural network, the MAB e‐skin can accurately perceive 6 materials and 10 surface shapes based on the training and learning using the collected datasets from proximity and pressure modes, thus allowing it to achieve the precise perception of different objects within one proximity‐pressure cycle. The development of this MAB e‐skin opens a new avenue for robotic skin and the expansion of advanced applications.

Abstract Flexible and wearable electronics have been widely investigated for the extensive applications in real life. Piezoresistive based sensor is one of the common flexible electronics that could be utilized as electronic skin. By simply transducing the external pressure or stretching into resistor signal and integrated with flexible substrate and advanced functional sensing material, piezoresistive based sensors have been applied as the electronic skin. Here, a thin, skin‐integrated, and flexible electronic skin based on piezoresistive working mode is developed. Ultra‐thin polydimethylsiloxane substrate integrated with serpentine‐like Cu electrode could avoid mechanical failing of the device when it is stretched, bended, or twisted. By adopting the advanced function material, MXene, graphene and Ecoflex mixed composite, the electronic skin could sense not only a wide range of pressure from 20.8 to 132 kPa, but also stretching rate from 0% to 20%, allowing the potential application in human motion capturing. Furthermore, a 4 × 4 arrayed electronic skin is fabricated, and demonstrates the prospective application in pressure mapping.

Wearable piezoresistive sensors are being developed as electronic skins (E-skin) for broad applications in human physiological monitoring and soft robotics. Tactile sensors with sufficient sensitivities, durability, and large dynamic ranges are required to replicate this critical component of the somatosensory system. Multiple micro/nanostructures, materials, and sensing modalities have been reported to address this need. However, a trade-off arises between device performance and device complexity. Inspired by the microstructure of the spinosum at the dermo epidermal junction in skin, a low-cost, scalable, and high-performance piezoresistive sensor is developed with high sensitivity (0.144 kPa-1 ), extensive sensing range ( 0.1-15 kPa), fast response time (less than 150 ms), and excellent long-term stability (over 1000 cycles). Furthermore, the piezoresistive functionality of the device is realized via a flexible transparent electrode (FTE) using a highly stable reduced graphene oxide self-wrapped copper nanowire network. The developed nanowire-based spinosum microstructured FTEs are amenable to wearable electronics applications.

Electronic skin and flexible wearable devices have attracted tremendous attention in the fields of human-machine interaction, energy storage, and intelligent robots. As a prevailing flexible pressure sensor with high performance, the piezoresistive sensor is believed to be one of the fundamental components of intelligent tactile skin. Furthermore, graphene can be used as a building block for highly flexible and wearable piezoresistive sensors owing to its light weight, high electrical conductivity, and excellent mechanical. This review provides a comprehensive summary of recent advances in graphene-based piezoresistive sensors, which we systematically classify as various configurations including one-dimensional fiber, two-dimensional thin film, and three-dimensional foam geometries, followed by examples of practical applications for health monitoring, human motion sensing, multifunctional sensing, and system integration. We also present the sensing mechanisms and evaluation parameters of piezoresistive sensors. This review delivers broad insights on existing graphene-based piezoresistive sensors and challenges for the future generation of high-performance, multifunctional sensors in various applications.

Abstract Human-machine interface has been considered as a prominent technology for numerous smart applications due to their direct communication between humans and machines. In particular, wearable electronic skins with a free form factor have received a lot of attention due to their excellent adherence to rough and wrinkled surfaces such as human skin and internal organs. However, most of the e-skins reported to date have some disadvantages in terms of mechanical instability and accumulation of by-products at the interface between the human skin and the device. Here, we report a mechanically stable e-skin via a newly designed pattern named the “eyes.” The ingeniously designed pattern of the eyes allowed mechanical stress and strain to be dissipated more effectively than other previously reported patterns. E-skin permeability of by-product was experimentally confirmed through sweat removal tests, showing superior sweat permeability compared to conventional e-skins. Finally, the real-time monitoring of the body temperature was carried out using our resistive-type thermometer in the e-skin.

The field of electronic skin has received considerable attention due to its extensive potential applications in areas including tactile sensing and health monitoring. With the development of electronic skin devices, electronic skin can be attached to the surface of human skin for long-term health monitoring, which makes comfort an essential factor that cannot be ignored in the design of electronic skin. Therefore, this paper proposes an assessment method for evaluating the comfort of electronic skin based on neurodynamic analysis. The holistic analysis framework encompasses the mechanical model of the skin, the modified Hodgkin–Huxley model for the transduction of stimuli, and the gate control theory for the modulation and perception of pain sensation. The complete process, from mechanical stimulus to the generation of pain perception, is demonstrated. Furthermore, the influence of different factors on pain perception is investigated. Sensation and comfort diagrams are provided to assess the mechanical comfort of electronic skin. The comfort assessment method proposed in this paper provides a theoretical basis when assessing the comfort of electronic skin.

In the context of an aging population and escalating work pressures, cardiovascular diseases pose increasing health risks. Electrocardiogram (ECG) monitoring presents a preventive tool, but conventional devices often compromise comfort. This study proposes an approach using Ag NW/TPU composites for flexible and breathable epidermal electronics. In this new structure, TPU fibers are used to support Ag NWs/TPU nanocomposites. The TPU fiber-reinforced Ag NW/TPU (TFRAT) nanocomposites exhibit excellent conductivity, stretchability, and electromechanical durability. The composite ensures high steam permeability, maintaining stable electrical performance after washing cycles. Employing this technology, a flexible ECG detection system is developed, augmented with a convolutional neural network (CNN) for automated signal analysis. The experimental results demonstrate the system’s reliability in capturing physiological signals. Additionally, a CNN model trained on ECG data achieves over 99% accuracy in diagnosing arrhythmias. This study presents TFRAT as a promising solution for wearable electronics, offering both comfort and functionality in long-term epidermal applications, with implications for healthcare and beyond.

Abstract As key interfaces for the disabled, optimal prosthetics should elicit natural sensations of skin touch or proprioception, by unambiguously delivering the multimodal signals acquired by the prosthetics to the nervous system, which still remains challenging. Here, a bioinspired temperature‐pressure electronic skin with decoupling capability (TPD e‐skin), inspired by the high‐low modulus hierarchical structure of human skin, is developed to restore such functionality. Due to the bionic dual‐state amplifying microstructure and contact resistance modulation, the MXene TPD e‐skin exhibits high sensitivity over a wide pressure range and excellent temperature insensitivity (91.2% reduction). Additionally, the high‐low modulus structural configuration enables the pressure insensitivity of the thermistor. Furthermore, a neural model is proposed to neutrally code the temperature‐pressure signals into three types of nerve‐acceptable frequency signals, corresponding to thermoreceptors, slow‐adapting receptors, and fast‐adapting receptors. Four operational states in the time domain are also distinguished after the neural coding in the frequency domain. Besides, a brain‐like machine learning‐based fusion process for frequency signals is also constructed to analyze the frequency pattern and achieve object recognition with a high accuracy of 98.7%. The TPD neural system offers promising potential to enable advanced prosthetic devices with the capability of multimodality‐decoupling sensing and deep neural integration.

Abstract The conformal integration of the electronic skin on the non‐developable surface is in great demand for the comprehensive tactile sensing of robotics and prosthetics. However, the current techniques still encounter obstacles in achieving conformal integration of film‐like electronic skin on non‐developable surfaces with substantial curvatures for contact pressure detection and tactile mapping. In this paper, by utilizing the 3D printing technology to prepare the 3D electrode array in the structural component following its surface curvature, and covering it with a molded functional shell to form the pressure sensitive iontronic interface, a device is proposed to achieve high‐sensitive pressure detection and high‐fidelity tactile mapping on a complicated non‐developable surface, called structural electronic skin (SES). The SES is prepared in a 3D printed fingertip with 46 tactile sensing units distributed on its curved surface, achieving the integration of both structural and tactile functions in a single component. By integrating the smart fingertip into a dexterous hand, a series of demonstrations are presented to show the dead‐zone free pressure detection and tactile mapping with high sensitivity, for instance, 2D pulse wave monitoring and robotic injection in a medical robot, object recognition and compliant control in a smart prosthesis.

Human-machine interface has been considered as a prominent technology for numerous smart applications due to their direct communication between humans and machines. In particular, wearable electronic skins with a free form factor have received a lot of attention due to their excellent adherence to rough and wrinkled surfaces such as human skin and internal organs. However, most of the e-skins reported to date have some disadvantages in terms of mechanical instability and accumulation of by-products at the interface between the human skin and the device. Here, we report a mechanically stable e-skin via a newly designed pattern named the "eyes." The ingeniously designed pattern of the eyes allowed mechanical stress and strain to be dissipated more effectively than other previously reported patterns. E-skin permeability of by-product was experimentally confirmed through sweat removal tests, showing superior sweat permeability compared to conventional e-skins. Finally, the real-time monitoring of the body temperature was carried out using our resistive-type thermometer in the e-skin. [ABSTRACT FROM AUTHOR]

Wearable hydrogel sensors provide a user-friendly option for wearable electronics and align well with the existing manufacturing strategy for connecting and communicating with large numbers of Internet of Things devices. This is attributed to their components and structures, which exhibit exceptional adaptability, scalability, bio-compatibility, and self-healing properties, reminiscent of human skin. This review focuses on the recent research on principal structural elements of wearable hydrogels: toughening networks and conductive networks, highlighting the strategies for enhancing mechanical and electrical properties. Wearable hydrogel sensors are categorized for an extensive exploration of their composition, mechanism, and design approach. This review provides a comprehensive understanding of wearable hydrogels and offers guidance for the design of components and structures in order to develop high-performance wearable hydrogel sensors. [ABSTRACT FROM AUTHOR]

Bionic flexible sensors are a new type of biosensor with high sensitivity, selectivity, stability, and reliability to achieve detection in complex natural and physiological environments. They provide efficient, energy-saving and convenient applications in medical monitoring and diagnosis, environmental monitoring, and detection and identification. Combining sensor devices with flexible substrates to imitate flexible structures in living organisms, thus enabling the detection of various physiological signals, has become a hot topic of interest. In the field of human health detection, the application of bionic flexible sensors is flourishing and will evolve into patient-centric diagnosis and treatment in the future of healthcare. In this review, we provide an up-to-date overview of bionic flexible devices for human health detection applications and a comprehensive summary of the research progress and potential of flexible sensors. First, we evaluate the working mechanisms of different classes of bionic flexible sensors, describing the selection and fabrication of bionic flexible materials and their excellent electrochemical properties; then, we introduce some interesting applications for monitoring physical, electrophysiological, chemical, and biological signals according to more segmented health fields (e.g., medical diagnosis, rehabilitation assistance, and sports monitoring). We conclude with a summary of the advantages of current results and the challenges and possible future developments. [ABSTRACT FROM AUTHOR]

The past decades have seen the rapid development of tactile sensors in material, fabrication, and mechanical structure design. The advancement of tactile sensors has heightened the expectation of sensor functions, and thus put forward a higher demand for data processing. However, conventional analysis techniques have not kept pace with the tactile sensor development and still suffer from some severe drawbacks, like cumbersome models, poor efficiency, and expensive costs. Machine learning, with its prominent ability for big data analysis and fast processing speed, can offer many possibilities for tactile data analysis. Herein, the machine learning techniques employed for processing tactile signals are reviewed. Supervised learning and unsupervised learning for analog signals are covered, and processing spike signals with machine learning are summarized. Furthermore, the applications in robotic tactile perception and human activity monitoring are presented. Finally, the current challenges and future prospects in sensors, data, algorithms, and benchmarks are discussed. [ABSTRACT FROM AUTHOR]

Highlights Bioinspired directional moisture-wicking electronic skin (DMWES) was successfully realized by surface energy gradient and push–pull effect via the design of distinct hydrophobic-hydrophilic difference. The DMWES membrane showed excellent comprehensive pressure sensing performance with high sensitivity and good single-electrode triboelectric nanogenerator performance The superior pressure sensing and triboelectric performance enabled the DMWES for all-range healthcare sensing, including accurate pulse monitoring, voice recognition, and gait recognition. Abstract 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 21.6 µW 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.

The emerging field of flexible tactile sensing systems, equipped with multi-physical tactile sensing capabilities, holds vast potential across diverse domains such as medical monitoring, robotics, and human–computer interaction. In response to the prevailing challenges associated with the limited integration and sensitivity of flexible tactile sensors, this paper introduces a versatile tactile sensing system capable of concurrently monitoring temperature and pressure. The temperature sensor employs carbon nanotube/graphene conductive paste as its sensitive material, while the pressure sensor integrates an ionic gel containing boron nitride as its sensitive layer. Through the application of cost-effective screen printing technology, we have successfully manufactured a flexible dual-mode sensor with exceptional performance, featuring high sensitivity (804.27 kPa−1), a broad response range (50 kPa), rapid response time (17 ms), and relaxation time (34 ms), alongside exceptional durability over 5000 cycles. Furthermore, the resistance temperature coefficient of the sensor within the temperature range of 12.5 °C to 93.7 °C is −0.17% °C−1. The designed flexible dual-mode tactile sensing system enables the real-time detection of pressure and temperature information, presenting an innovative approach to electronic skin with multi-physical tactile sensing capabilities.

As the main core component of wearable devices, flexible strain sensors have broad application prospects in health monitoring, motion monitoring, human-machine interface, rehabilitation, entertainment technology and other fields. In this paper, a rectangular sandwich resistive pressure sensor is constructed with porous conductive sponge, and its working mechanism is analyzed. The linearity of the sensor is improved and the stress range is increased by gel modification. Through experimental tests, it can withstand more than 80% compressive strain, and shows a sensitivity of 0.398 kPa ^−1 in the range of 6 ∼ 11 kPa; the maximum range is close to 40 kPa, and the minimum detection limit is 20 Pa; under constant loading/releasing speed, the response/recovery time is about 133/150 ms; it also shows good linearity and stability. With the help of a single sensor entity, Morse code can be sent, and some human activity signals can be measured, such as speech recognition, weighing measurement, limb movement; and 8 sensors create an interesting smart insole for gait recognition. The results show that piezoresistive sensors with porous composite materials have broad application prospects in motion monitoring and human-computer interaction.

The past decades have seen the rapid development of tactile sensors in material, fabrication, and mechanical structure design. The advancement of tactile sensors has heightened the expectation of sensor functions, and thus put forward a higher demand for data processing. However, conventional analysis techniques have not kept pace with the tactile sensor development and still suffer from some severe drawbacks, like cumbersome models, poor efficiency, and expensive costs. Machine learning, with its prominent ability for big data analysis and fast processing speed, can offer many possibilities for tactile data analysis. Herein, the machine learning techniques employed for processing tactile signals are reviewed. Supervised learning and unsupervised learning for analog signals are covered, and processing spike signals with machine learning are summarized. Furthermore, the applications in robotic tactile perception and human activity monitoring are presented. Finally, the current challenges and future prospects in sensors, data, algorithms, and benchmarks are discussed.

In recent years, extensive research has been conducted on the development of high-performance flexible tactile sensors, pursuing the next generation of highly intelligent electronics with diverse potential applications in self-powered wearable sensors, human–machine interactions, electronic skin, and soft robotics. Among the most promising materials that have emerged in this context are functional polymer composites (FPCs), which exhibit exceptional mechanical and electrical properties, enabling them to be excellent candidates for tactile sensors. Herein, this review provides a comprehensive overview of recent advances in FPCs-based tactile sensors, including the fundamental principle, the necessary property parameter, the unique device structure, and the fabrication process of different types of tactile sensors. Examples of FPCs are elaborated with a focus on miniaturization, self-healing, self-cleaning, integration, biodegradation, and neural control. Furthermore, the applications of FPC-based tactile sensors in tactile perception, human–machine interaction, and healthcare are further described. Finally, the existing limitations and technical challenges for FPCs-based tactile sensors are briefly discussed, offering potential avenues for the development of electronic products. [ABSTRACT FROM AUTHOR]

Herein, a multimodal tactile sensing module that can improve the dexterous manipulation capabilities of robots with parallel grippers is reported. The tactile sensor consists of a 4 × 4 matrix 3‐axis force sensor array (with spacings of 2 mm) and a single‐temperature sensor. The tactile sensor uses inorganic silicon and gold as materials for the detection of strain and temperature and a polymer‐based elastomer to encapsulate the sensing layer. The sensing module is equipped with a readout circuitry for signal processing. Within the measurable force range (≈1.5 N) of the sensor cell, a prototype module exhibits a repeatability error within 2%, a hysteresis error within 3%, and a resolution as small as 10 mN. Furthermore, each sensor cell independently measures 3‐axis forces with a cross‐talk error of approximately 3%. The temperature sensor exhibits linear output properties in the approximate range of 5–75 °C. Experiments are performed by mounting the module on a parallel gripper to grasp a paper cup and a reflex hammer toy. According to the experimental results, the sensor can accurately detect the state of contact with an object by analyzing three tactile modalities (i.e., 3‐axis force distribution, vibration, and temperature). [ABSTRACT FROM AUTHOR]

Pressure sensors show significant potential applications in health monitoring, bio-sensing, electronic skin, and tactile perception. Consequently, tremendous research interest has been devoted to the development of high-performance pressure sensors. In this paper, recent progress on the polymer composite-based flexible pressure sensor is reviewed. The parameters of pressure sensors, including sensitivity, linear response range, detection limit, response speed, and reliability, are first introduced. Secondly, representative types of pressure sensors and relevant working principles are introduced and discussed. After that, the applications in human physiology monitoring, health monitoring, artificial skin, and self-powered smart system are listed and discussed in detail. Finally, the remaining challenges and outlook of polymer composite-based flexible sensors are summarized at the end of this review paper. This work should have some impact on the development of high-performance flexible pressure sensors. [ABSTRACT FROM AUTHOR]

This paper reviews current progress on flexible and stretchable transistors and sensors for the next-generation multiplex electronics (commonly referred to as 'electronics skin' or 'e-skin') that is capable of simultaneous detection of multiple information from a variety of surfaces including human skin. Flexible chemical sensors for sweat analysis as well as physical sensors for detecting tactile force, bending, and temperature will be discussed, with emphasis on materials, detection mechanisms, and device demonstration to realize multiplex human-interactive devices. Next, system integration enabling the real-time monitoring of health conditions is also demonstrated as a proof of concept. Finally, perspectives on e-skin for moving toward realizing practical wearable electronics in the market are discussed. This paper targets the translation of the nano- and flexible technologies from academic innovations to industrial practical applications with high-impact breakthroughs.