Skin-Inspired Humidity and Pressure Sensor with a Wrinkle-on-Sponge Structure

Toward Stretchable Flexible Integrated Sensor Systems

Skin-like flexible sensors hold great potential as the next generation of intelligent electronic devices owing to their broad applications in environmental monitoring, human-machine interfaces, the Internet of Things, and artificial intelligence. Flexible electronics inspired by human skin play a vital role in continuous and real-time health monitoring. This review summarizes recent progress in skin-mountable electronics developed by designing flexible electrodes and substrates into different structures, including serpentine, microcrack, wrinkle, and kirigami. Furthermore, this review briefly discusses advances in wearable integrated sensor systems that mimic the flexibility of human skin, as well as multisensing functions. In the future, innovations in stretchable integrated sensor systems will be crucial to develop next-generation intelligent skin-based sensors for practical applications such as medical diagnosis, treatment, and environment monitoring.

A Decoupling Method for Multimode Flexible Capacitive Sensors to Decouple Spatial Forces and Dynamic Humidity

This paper focuses on a four-capacitor flexible sensor composed of two electrode materials; also, the decoupling method and sensing performance for multimodal sensing of spatial forces and dynamic humidity are described. In previous work, decoupling of multimode sensors is mostly done by monitoring the types of signals, numerical differences of the same signal, and stacking multiple parameter-sensitive materials. This paper mainly uses the different characteristics of the two electrode materials; in the simulation and experiment of humidity, the moisture-sensitive electrode quickly wets from the outside to the inside and expands, and the contact angle quickly decreases from 58.5 to 3.7° within 12.04 s, while the copper electrode has no obvious change; in the simulation and experiment of force, the capacitance value of the capacitor composed of the two electrodes changes steadily with the magnitude of the force. That is, the moisture-sensitive electrode can respond to both force and humidity, while the copper electrode responds only to force. So, we use the copper electrode to decouple the spatial force information and calculate the capacitance value of the moisture-sensitive electrode under the influence of only spatial force. The capacitance value of the moisture-sensitive electrode only affected by humidity can be obtained by the difference between the measured capacitance value and the capacitance value under the influence of only spatial force, and then, the humidity value can be obtained according to the material properties. When a single physical quantity changes, the built-in test platform of the experiment verifies that the decoupling accuracy of the force in the dual-mode sensor is as high as 0.95, and the decoupling accuracy of humidity is as high as 0.97. When the two physical quantities change synchronously, the decoupling accuracy of the force is relatively uniformly distributed within the range, and the decoupling accuracy of humidity can reach as high as 0.99 within the range of 31%RH-56%RH. As a humidity sensor, the sensitivity gradually decreases as the humidity increases. During the repeated changes from low humidity to high humidity, the dynamic characteristics, stability, and repeatability have very good performance. The repetition rate is 97.64%, the response time is 11.3 s, the recovery time is 6.8 s, and the capacitance value for 24 days remains basically unchanged. All of these provide some insight into the application of multimode sensors.

Recent Developments in Electrospun Nanofiber-Based Triboelectric Nanogenerators: Materials, Structure, and Applications

Triboelectric nanogenerators (TENGs) have garnered significant attention due to their high energy conversion efficiency and extensive application potential in energy harvesting and self-powered devices. Recent advancements in electrospun nanofibers, attributed to their outstanding mechanical properties and tailored surface characteristics, have meant that they can be used as a critical material for enhancing TENGs performance. This review provides a comprehensive overview of the developments in electrospun nanofiber-based TENGs. It begins with an exploration of the fundamental principles behind electrospinning and triboelectricity, followed by a detailed examination of the application and performance of various polymer materials, including poly (vinylidene fluoride) (PVDF), polyamide (PA), thermoplastic polyurethane (TPU), polyacrylonitrile (PAN), and other significant polymers. Furthermore, this review analyzes the influence of diverse structural designs-such as fiber architectures, bionic configurations, and multilayer structures-on the performance of TENGs. Applications across self-powered devices, environmental energy harvesting, and wearable technologies are discussed. The review concludes by highlighting current challenges and outlining future research directions, offering valuable insights for researchers and engineers in the field.

Flexible Pressure, Humidity, and Temperature Sensors for Human Health Monitoring

The rapid advancements in artificial intelligence, micro-nano manufacturing, and flexible electronics technology have unleashed unprecedented innovation and opportunities for applying flexible sensors in healthcare, wearable devices, and human-computer interaction. The human body's tactile perception involves physical parameters such as pressure, temperature, and humidity, all of which play an essential role in maintaining human health. Inspired by the sensory function of human skin, many bionic sensors have been developed to simulate human skin's perception to various stimuli and are widely applied in health monitoring. Given the urgent requirements for sensing performance and integration of flexible sensors in the field of wearable devices and health monitoring, here is a timely overview of recent advances in pressure, humidity, temperature, and multi-functional sensors for human health monitoring. It covers the fundamental components of flexible sensors and categorizes them based on different response mechanisms, including resistive, capacitive, voltage, and other types. Specifically, the application of these flexible tactile sensors in the area of human health monitoring is highlighted. Based on this, an extended overview of recent advances in dual/triple-mode flexible sensors integrating pressure, humidity, and temperature tactile sensing is presented. Finally, the challenges and opportunities of flexible sensors are discussed.

Fully Integrated Passive Wireless Sensor with Mechanical-Electrical Double-Gradient for Multifunctional Healthcare Monitoring

Accurate, effective, and continuous monitoring of pressure, moisture, and temperature is essential for routine health assessments and professional patient care. In this study, we present a fully integrated multiparameter passive wireless sensor (MWS) that employs a mechanical-electrical dual-gradient structure design. The unique gradient porous structure endows the MWS with significant advantages in terms of detection dimensions (pressure, moisture, and temperature), sensitivity, and stability. Compared to single mechanical gradient designs, the sensor demonstrates 2.6 times higher pressure sensitivity and a 5-tier moisture detection capability. By bridging the technology gap between high-precision multiparameter sensing, wireless communication, and energy management, the MWS is capable of measuring multiple physiological parameters, including breath, ballistocardiograph, moisture, and temperature at multiple points, providing real-time assessments of the physiological state of the subjects. This work offers valuable quantitative insights for caregivers and paves the way for significant advancements in personal healthcare management.

Wireless breathable face mask sensor for spatiotemporal 2D respiration profiling and respiratory diagnosis

Owing to air pollution and the pandemic outbreak, the need for quantitative pulmonary monitoring has greatly increased. The COVID-19 outbreak has aroused attention for comfortable wireless monitoring of respiratory profiles and more real-time diagnosis of respiratory diseases. Although respiration sensors have been investigated extensively with single-pixel sensors, 2D respiration profiling with a pixelated array sensor has not been demonstrated for both exhaling and inhaling. Since the pixelated array sensor allowed for simultaneous profiling of the nasal breathing and oral breathing, it provides essential respiratory information such as breathing patterns, respiration habit, breathing disorders. In this study, we introduced an air-permeable, stretchable, and a pixelated pressure sensor that can be integrated into a commercial face mask. The mask sensor showed a strain-independent pressure-sensing performance, providing 2D pressure profiles for exhalation and inhalation. Real-time 2D respiration profiles could monitor various respiratory behaviors, such as oral/nasal breathing, clogged nose, out-of-breath, and coughing. Furthermore, they could detect respiratory diseases, such as rhinitis, sleep apnea, and pneumonia. The 2D respiratory profiling mask sensor is expected to be employed for remote respiration monitoring and timely patient treatment.

In Situ Laser-Induced 3D Porous Graphene within Transparent Polymers for Encapsulation-Free and Tunable Ultrabroadband Terahertz Absorption

Three-dimensional (3D) porous carbon materials have great potential for fabricating flexible tunable broadband absorbers owing to their high electrical conductivity, strong dielectric loss, and unique microstructure. Herein, we introduce an innovative method for synthesizing 3D porous graphene that incorporates advanced tuning and encapsulation processes to augment its functional efficacy. Through the modulation of both thermal and nonthermal interactions between a femtosecond (fs) laser and a polydimethylsiloxane (PDMS) film, we have synergistically fine-tuned the surface morphology and lattice properties of 3D porous graphene. This approach enabled us to create a flexible terahertz (THz) absorber with customizable characteristics, boasting an impressive absorbance range of 80%-99% in the 0.4-1.0 THz spectrum, alongside a peak reflection loss (RL) of up to 35.6 dB. Furthermore, we have successfully demonstrated the production of photoinduced 3D porous graphene within a PDMS film, which serves as both a carbon precursor and protective layer. This simplifies the conventional packaging process. These devices exhibit a RL of up to 41.6 dB and an absorption bandwidth of 2.5 THz (0.6-3.1 THz). Our study presents a production methodology for high-performance, flexible THz absorbers, offering a straightforward and innovative solution for the rapid development of sophisticated, flexible THz absorbing materials.

Toward flexible piezoresistive strain sensors based on polymer nanocomposites: a review on fundamentals, performance, and applications

The fundamentals, performance, and applications of piezoresistive strain sensors based on polymer nanocomposites are summarized herein. The addition of conductive nanoparticles to a flexible polymer matrix has emerged as a possible alternative to conventional strain gauges, which have limitations in detecting small strain levels and adapting to different surfaces. The evaluation of the properties or performance parameters of strain sensors such as the elongation at break, sensitivity, linearity, hysteresis, transient response, stability, and durability are explained in this review. Moreover, these nanocomposites can be exposed to different environmental conditions throughout their lifetime, including different temperature, humidity or acidity/alkalinity levels, that can affect performance parameters. The development of flexible piezoresistive sensors based on nanocomposites has emerged in recent years for applications related to the biomedical field, smart robotics, and structural health monitoring. However, there are still challenges to overcome in designing high-performance flexible sensors for practical implementation. Overall, this paper provides a comprehensive overview of the current state of research on flexible piezoresistive strain sensors based on polymer nanocomposites, which can be a viable option to address some of the major technological challenges that the future holds.

Nanocarbon-based sensors for the structural health monitoring of smart biocomposites

Structural health monitoring (SHM) is a critical aspect of ensuring the safety and durability of smart biocomposite materials used as multifunctional materials. Smart biocomposites are composed of renewable or biodegradable materials and have emerged as eco-friendly alternatives of traditional non-biodegradable glass fiber-based composite materials. Although biocomposites exhibit fascinating properties and many desirable traits, real-time and early stage SHM is the most challenging issue to enable their long-term use. Smart biocomposites are integrated with sensors for in situ identification of the progress of damage and composite failure. The sensitivity of such smart biocomposites is a key functionality, which can be tuned by the introduction of an appropriate filler. In particular, nanocarbons hold promising potential to be incorporated in SHM applications of biocomposites. This review focused on the potential applications of nanocarbons in SHM of biocomposites. The aspects related to fabrication techniques and working mechanism of sensors are comprehensively discussed. Furthermore, their unique mechanical and electrical properties and sustainable nature ensure seamless integration into biocomposites, allowing for real-time monitoring without compromising the material's properties. These sensors offer multi-parameter sensing capabilities, such as strain, pressure, humidity, temperature, and chemical exposure, allowing a comprehensive assessment of biocomposite health. Additionally, their durability and longevity in harsh conditions, along with wireless connectivity options, provide cost-effective and sustainable SHM solutions. As research in this field advances, ongoing efforts seek to enhance the sensitivity and selectivity of these sensors, optimizing their performance for real-world applications. This review highlights the significant advances, ongoing efforts to enhance the sensitivity and selectivity, and performance optimization of nanocarbon-based sensors along with their working mechanism in the field of SHM for smart biocomposites. The key challenges and future research perspectives facing the conversion of nanocarbons to smart biocomposites are also displayed.

Facile and Cost-Effective Fabrication of Highly Sensitive, Fast-Response Flexible Humidity Sensors Enabled by Laser-Induced Graphene

The need to simplify fabrication processes and reduce costs for high-performance humidity sensors is increasingly vital, especially in fields such as healthcare and agriculture. This study introduces a simple and cost-effective approach using laser-induced graphene (LIG) on a polyimide film to create highly sensitive and fast-response flexible humidity sensors. The LIG acts as the electrode, while the porous polyimide between the interdigital LIG electrodes serves as the humidity sensing material, showing changes in electrical conductivity based on the humidity levels. The LIG humidity sensor, an ionic-conduction type, exhibits remarkable sensitivity, with a 28,231-fold increase in current as relative humidity changes from 26.1 to 90.2%. It also boasts of ultrashort response/recovery times (less than 0.5/7 s), providing significant advantages in detecting rapid and subtle humidity variations compared to a commercially available MEMS humidity sensor. We successfully demonstrated the LIG humidity sensor's capabilities in ultrafast breathing monitoring (≈174 times per minute), moisture detection of grains, and detection of sudden water pipe leakage. Due to its straightforward and cost-effective fabrication process, the LIG humidity sensor holds immense practical value for affordable, widespread use across various applications.

Multimodal Wearable Ionoskins Enabling Independent Recognition of External Stimuli Without Crosstalk

Wearable ionoskins are one of the representative examples of the many useful applications offered by deformable stimuli-responsive sensory platforms. Herein, ionotronic thermo-mechano-multimodal response sensors are proposed, which can independently detect changes in temperature and mechanical stimuli without crosstalk. For this purpose, mechanically robust, thermo-responsive ion gels composed of poly(styrene-ran-n-butyl methacrylate) (PS-r-PnBMA, copolymer gelator) and 1-butyl-3-methylimidazolium bis(trifluoromethyl sulfonyl)imide ([BMI][TFSI], ionic liquid) are prepared. The optical transmittance change arising from the lower critical solution temperature (LCST) phenomenon between PnBMA and [BMI][TFSI] is exploited to track the external temperature, creating a new concept of the temperature coefficient of transmittance (TCT). The TCT of this system (-11.5% °C-1 ) is observed to be more sensitive to temperature fluctuations than the conventional metric of temperature coefficient of resistance. The tailoring molecular characteristics of gelators selectively improved the mechanical robustness of the gel, providing an additional application opportunity for strain sensors. This functional sensory platform, which is attached to a robot finger, can successfully detect thermal and mechanical environmental changes through variations in the optical (transmittance) and electrical (resistance) properties of the ion gel, respectively, indicating the high practicality of on-skin multimodal wearable sensors.

Biomimetic Flexible Sensors and Their Applications in Human Health Detection

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.

Wrinkled Interfaces: Taking Advantage of Anisotropic Wrinkling to Periodically Pattern Polymer Surfaces

Periodically patterned surfaces can cause special surface properties and are employed as functional building blocks in many devices, yet remaining challenges in fabrication. Advancements in fabricating structured polymer surfaces for obtaining periodic patterns are accomplished by adopting "top-down" strategies based on self-assembly or physico-chemical growth of atoms, molecules, or particles or "bottom-up" strategies ranging from traditional micromolding (embossing) or micro/nanoimprinting to novel laser-induced periodic surface structure, soft lithography, or direct laser interference patterning among others. Thus, technological advances directly promote higher resolution capabilities. Contrasted with the above techniques requiring highly sophisticated tools, surface instabilities taking advantage of the intrinsic properties of polymers induce surface wrinkling in order to fabricate periodically oriented wrinkled patterns. Such abundant and elaborate patterns are obtained as a result of self-organizing processes that are rather difficult if not impossible to fabricate through conventional patterning techniques. Focusing on oriented wrinkles, this review thoroughly describes the formation mechanisms and fabrication approaches for oriented wrinkles, as well as their fine-tuning in the wavelength, amplitude, and orientation control. Finally, the major applications in which oriented wrinkled interfaces are already in use or may be prospective in the near future are overviewed.

The Soft-Strain Effect Enabled High-Performance Flexible Pressure Sensor and Its Application in Monitoring Pulse Waves

Flexible and wearable pressure sensors attached to human skin are effective and convenient in accurate and real-time tracking of various physiological signals for disease diagnosis and health assessment. Conventional flexible pressure sensors are constructed using compressible dielectric or conductive layers, which are electrically sensitive to external mechanical stimulation. However, saturated deformation under large compression significantly restrains the detection range and sensitivity of such sensors. Here, we report a novel type of flexible pressure sensor to overcome the compression saturation of the sensing layer by soft-strain effect, enabling an ultra-high sensitivity of ~636 kPa-1 and a wide detection range from 0.1 kPa to 56 kPa. In addition, the cyclic loading-unloading test reveals the excellent stability of the sensor, which maintains its signal detection after 10,000 cycles of 10 kPa compression. The sensor is capable of monitoring arterial pulse waves from both deep tissue and distal parts, such as digital arteries and dorsal pedal arteries, which can be used for blood pressure estimation by pulse transit time at the same artery branch.

Multifunctional Flexible Ionic Skin with Dual-Modal Output Based on Fibrous Structure

Flexible wearable electronic devices with multiple sensing functions that simulate human skin in all aspects have become a popular research topic. However, the current expensive and time-consuming means of integration and the complex decoupling process are hampering the further development of multifunctional sensors. Here, an ultraflexible ionic fiber membrane (IFM) prepared by a simple electrospinning technique is reported, which exhibits pressure and humidity sensing properties. With the help of different electrode structures, the IFM-based multifunctional sensor achieved pressure and humidity detection with different sensing mechanisms. Pressure sensing with high sensitivity (49.7 kPa^-1 at 0-30 kPa) and wide detection range (0-220 kPa) was indicated by the capacitive signal. Humidity sensing with high linearity (1.086% per percent relative humidity (RH)) in the range 15%-90% RH was indicated by the resistance signal. In particular, the multimodal output of capacitance/resistance corresponding to pressure/humidity in this study directly addresses the problem of accurately distinguishing the two stimuli. Furthermore, we have demonstrated that the impact between pressure and humidity is negligible when measured simultaneously and independently. Because of the excellent pressure/humidity sensing performance, we have fabricated a smart bracelet and mask for pulse, skin moisture, and breathe monitoring, which indicates the promising future of multifunctional flexible sensors based on IFM in the healthcare field.

Beyond Human Touch Perception: An Adaptive Robotic Skin Based on Gallium Microgranules for Pressure Sensory Augmentation

Robotic skin with human-skin-like sensing ability holds immense potential in various fields such as robotics, prosthetics, healthcare, and industries. To catch up with human skin, numerous studies are underway on pressure sensors integrated on robotic skin to improve the sensitivity and detection range. However, due to the trade-off between them, existing pressure sensors have achieved only a single aspect, either high sensitivity or wide bandwidth. Here, an adaptive robotic skin is proposed that has both high sensitivity and broad bandwidth with an augmented pressure sensing ability beyond the human skin. A key for the adaptive robotic skin is a tunable pressure sensor built with uniform gallium microgranules embedded in an elastomer, which provides large tuning of the sensitivity and the bandwidth, excellent sensor-to-sensor uniformity, and high reliability. Through the mode conversion based on the solid-liquid phase transition of gallium microgranules, the sensor provides 97% higher sensitivity (16.97 kPa^-1 ) in the soft mode and 262.5% wider bandwidth (≈1.45 MPa) in the rigid mode compared to the human skin. Successful demonstration of the adaptive robotic skin verifies its capabilities in sensing a wide spectrum of pressures ranging from subtle blood pulsation to body weight, suggesting broad use for various applications.

Surface Wrinkling for Flexible and Stretchable Sensors

Recent advances in nanolithography, miniaturization, and material science, along with developments in wearable electronics, are pushing the frontiers of sensor technology into the large-scale fabrication of highly sensitive, flexible, stretchable, and multimodal detection systems. Various strategies, including surface engineering, have been developed to control the electrical and mechanical characteristics of sensors. In particular, surface wrinkling provides an effective alternative for improving both the sensing performance and mechanical deformability of flexible and stretchable sensors by releasing interfacial stress, preventing electrical failure, and enlarging surface areas. In this study, recent developments in the fabrication strategies of wrinkling structures for sensor applications are discussed. The fundamental mechanics, geometry control strategies, and various fabricating methods for wrinkling patterns are summarized. Furthermore, the current state of wrinkling approaches and their impacts on the development of various types of sensors, including strain, pressure, temperature, chemical, photodetectors, and multimodal sensors, are reviewed. Finally, existing wrinkling approaches, designs, and sensing strategies are extrapolated into future applications.

Multimodal Sensors with Decoupled Sensing Mechanisms

Highly sensitive and multimodal sensors have recently emerged for a wide range of applications, including epidermal electronics, robotics, health-monitoring devices and human-machine interfaces. However, cross-sensitivity prevents accurate measurements of the target input signals when a multiple of them are simultaneously present. Therefore, the selection of the multifunctional materials and the design of the sensor structures play a significant role in multimodal sensors with decoupled sensing mechanisms. Hence, this review article introduces varying methods to decouple different input signals for realizing truly multimodal sensors. Early efforts explore different outputs to distinguish the corresponding input signals applied to the sensor in sequence. Next, this study discusses the methods for the suppression of the interference, signal correction, and various decoupling strategies based on different outputs to simultaneously detect multiple inputs. The recent insights into the materials' properties, structure effects, and sensing mechanisms in recognition of different input signals are highlighted. The presence of the various decoupling methods also helps avoid the use of complicated signal processing steps and allows multimodal sensors with high accuracy for applications in bioelectronics, robotics, and human-machine interfaces. Finally, current challenges and potential opportunities are discussed in order to motivate future technological breakthroughs.

Morphological Engineering of Sensing Materials for Flexible Pressure Sensors and Artificial Intelligence Applications

Various morphological structures in pressure sensors with the resulting advanced sensing properties are reviewed comprehensively. Relevant manufacturing techniques and intelligent applications of pressure sensors are summarized in a complete and interesting way. Future challenges and perspectives of flexible pressure sensors are critically discussed.

As an indispensable branch of wearable electronics, flexible pressure sensors are gaining tremendous attention due to their extensive applications in health monitoring, human –machine interaction, artificial intelligence, the internet of things, and other fields. In recent years, highly flexible and wearable pressure sensors have been developed using various materials/structures and transduction mechanisms. Morphological engineering of sensing materials at the nanometer and micrometer scales is crucial to obtaining superior sensor performance. This review focuses on the rapid development of morphological engineering technologies for flexible pressure sensors. We discuss different architectures and morphological designs of sensing materials to achieve high performance, including high sensitivity, broad working range, stable sensing, low hysteresis, high transparency, and directional or selective sensing. Additionally, the general fabrication techniques are summarized, including self-assembly, patterning, and auxiliary synthesis methods. Furthermore, we present the emerging applications of high-performing microengineered pressure sensors in healthcare, smart homes, digital sports, security monitoring, and machine learning-enabled computational sensing platform. Finally, the potential challenges and prospects for the future developments of pressure sensors are discussed comprehensively.

Tip-Induced In-Plane Ferroelectric Superstructure in Zigzag-Wrinkled BaTiO3 Thin Films

The complex micro-/nanoscale wrinkle morphology primarily fabricated by elastic polymers is usually designed to realize unique functionalities in physiological, biochemical, bioelectric, and optoelectronic systems. In this work, we fabricated inorganic freestanding BaTiO₃ ferroelectric thin films with zigzag wrinkle morphology and successfully modulated the ferroelectric domains to form an in-plane (IP) superstructure with periodic surface charge distribution. Our piezoresponse force microscopy (PFM) measurements and phase-field simulation demonstrate that the self-organized strain/stress field in the zigzag-wrinkled BaTiO₃ film generates a corresponding pristine domain structure. These domains can be switched by tip-induced strain gradient (flexoelectricity) and naturally form a robust and unique "braided" in-plane domain pattern, which enables us to offer an effective and convenient way to create a microscopic ferroelectric superstructure. The corresponding periodic surface potential distribution provides an extra degree of freedom in addition to the morphology that could regulate cells or polar molecules in physiological and bioelectric applications.

Macromolecule Relaxation Directed 3D Nanofiber Architecture in Stretchable Fibrous Mats for Wearable Multifunctional Sensors

Elastomer fiber mat sensors, which are capable of perceiving mechanical stimuli, temperature, and vapor of chemicals, are highly desirable for designing wearable electronics and human-robot interfacing devices due to good wearability, skin affinity, and durability, and so on. However, it is still challenging to fabricate multiresponsive flexible wearable sensors with three-dimensional (3D) architecture using simple material and structure design. Herein, we report an all-in-one multiresponsive thermoplastic polyurethane (TPU) nanofiber mat sensors composed of crimped elastomer fibers with deposited platinum nanoparticles (PtNPs) on the fiber surface. The 1D TPU nanofibers could be transferred to nanofibers with different 3D nanofiber architectures by controllable macromolecular chain relaxation of aligned elastomer polymers upon poor solvent annealing. The conductive networks of PtNPs on wavy TPU fibers enable the sensor susceptible to multiple stimuli like strain/pressure, humidity, and organic vapors. Besides, the 3D nanofiber architectures allow the strain sensor to detect wider tensile strain and pressure with higher sensitivity due to delicate fiber morphology and structure control. Therefore, this work provides new insights into the fabrication of multifunctional flexible sensors with 3D architecture in an easy way, advancing the establishment of a multiple signal monitoring platform for the health care and human-machine interfacing.

Flexible Electronics and Devices as Human-Machine Interfaces for Medical Robotics

Medical robots are invaluable players in non-pharmaceutical treatment of disabilities. Particularly, using prosthetic and rehabilitation devices with human-machine interfaces can greatly improve the quality of life for impaired patients. In recent years, flexible electronic interfaces and soft robotics have attracted tremendous attention in this field due to their high biocompatibility, functionality, conformability, and low-cost. Flexible human-machine interfaces on soft robotics will make a promising alternative to conventional rigid devices, which can potentially revolutionize the paradigm and future direction of medical robotics in terms of rehabilitation feedback and user experience. In this review, the fundamental components of the materials, structures, and mechanisms in flexible human-machine interfaces are summarized by recent and renowned applications in five primary areas: physical and chemical sensing, physiological recording, information processing and communication, soft robotic actuation, and feedback stimulation. This review further concludes by discussing the outlook and current challenges of these technologies as a human-machine interface in medical robotics.

Moisture-resistant MXene-sodium alginate sponges with sustained superhydrophobicity for monitoring human activities

Wearable mechanical sensors are easily influenced by moisture resulting in inaccuracy for monitoring human health and body motions. Though the superhydrophobic barrier has been extensively explored as passive water repel strategy on the sensor surface, the dense superhydrophobic surface not only limits the sensor working under large deformations but also inevitable degradation in high humidity or saturation water vapor environments. This work reports a superhydrophobic MXene-sodium alginate sponge (SMSS) pressure sensor with a low voltage Joule heating effect to provide sustain moisture-insensitive property for both sensing performance and superhydrophobicity by heating-driven water molecules away. Because of the positive temperature coefficient under pressure applied, the Joule heating can provides a stable temperature to the moisture-insensitivity property during the whole dynamic pressure cycled. Therefore, the pressure sensor with a simple spray-coating superhydrophobic coating on the outer layer demonstrates key capabilities even in extreme use scenarios with high humidity or water vapor and also provides stable and reliable bio-signal monitoring.

Pressure-Perceptive Actuators for Tactile Soft Robots and Visual Logic Devices

Soft actuators with sensing capabilities are important in intelligent robots and human-computer interactions. However, present perceptive actuating systems rely on the integration of multiple functional units with complex circuit design. Here, a new-type pressure-perceptive actuator is reported, which integrates functions of sensing, actuating, and decision making at material level without complex combination. The actuator is composed of an actuating unit and a pressure-sensing unit, both of which are fabricated by carbon nanotube (CNT), silk, and polymer composite. On the one hand, the actuating unit can be driven by low voltages (<13 V), owing to a Joule-heating effect. On the other hand, the current passing the pressure-sensing unit can be controlled by tactile pressure. In the integrated actuator, it is able to control the deformation amplitude of actuating unit by applying different pressures on the pressure-sensing unit. A portable tactile-activated gripper is fabricated to operate an object through pressure control, demonstrating its application in tactile soft robots. Finally, three visual logic gates (AND, OR, and NOT) are proposed, which convert "tactile" inputs into "visible" deformation outputs, using the CNT-silk-based material for sensing and actuating in the decision-making process. This study provides a new path for intelligent soft robots and new-generation logic devices.

Recent Advances in Multiresponsive Flexible Sensors towards E-skin: A Delicate Design for Versatile Sensing

Multiresponsive flexile sensors with strain, temperature, humidity, and other sensing abilities serving as real electronic skin (e-skin) have manifested great application potential in flexible electronics, artificial intelligence (AI), and Internet of Things (IoT). Although numerous flexible sensors with sole sensing function have already been reported since the concept of e-skin, that mimics the sensing features of human skin, was proposed about a decade ago, the ones with more sensing capacities as new emergences are urgently demanded. However, highly integrated and highly sensitive flexible sensors with multiresponsive functions are becoming a big thrust for the detection of human body motions, physiological signals (e.g., skin temperature, blood pressure, electrocardiograms (ECG), electromyograms (EMG), sweat, etc.) and environmental stimuli (e.g., light, magnetic field, volatile organic compounds (VOCs)), which are vital to real-time and all-round human health monitoring and management. Herein, this review summarizes the design, manufacturing, and application of multiresponsive flexible sensors and presents the future challenges of fabricating these sensors for the next-generation e-skin and wearable electronics.

Highly Sensitive Multifunctional Electronic Skin Based on Nanocellulose/MXene Composite Films with Good Electromagnetic Shielding Biocompatible Antibacterial Properties

Electronic skin has aroused extensive research interest due to high similarity with human skin. Realizing a multifunctional electronic skin that is highly consistent with skin functions and endowed with more other functions is now a more urgent need and important challenge. Here, we use 2,2,6,6-tetramethylpiperidinyl-1-oxyl (TEMPO)-oxidized cellulose nanofibril (TOCN) dispersion and highly conductive Ti₃C₂T_X dispersion to prepare TOCN/Ti₃C₂T_X composite film through vacuum-assisted filtration. The obtained composite film imitating the nacre-like lamellar structure of natural shells has good mechanical properties (124.6 MPa of tensile strength). Meanwhile, the composite film also showed excellent electromagnetic shielding performance (36 dB), biocompatibility, and antibacterial properties. In addition, the piezoresistive sensor assembled from the composite film exhibited a high sensitivity (11.6 kPa^-1), fast response and recovery time (≤10 ms), ultralow monitoring limit (0.2 Pa), and long-term stability (>10 000 cycles). It also could detect human daily activities such as finger bent, chewing, and so on.

Single-sided and integrated polyaniline/ poly(vinylidene fluoride) flexible membrane with micro/nanostructures as breathable, nontoxic and fast response wearable humidity sensor

Harmless and breathable flexible humidity sensor has important applications in continuous and real-time detection of human physiological activities. In this work, with hydrophobic poly (vinylidene fluoride) (PVDF) membrane as both the template and substrate and cetyltrimethylammonium bromide as a structure regulator, polyaniline (PANI) was unilaterally deposited on a PVDF microporous membrane to facilely fabricate a single-sided integrated flexible humidity sensor (IFHS). Such IFHS is featured with unique micro/nano structure and good air permeability. Moreover, it exhibits good humidity sensing properties at room temperature including fast response, small hysteresis and stable response even under bending deformation. The flexible sensor could realize non-contact monitoring of human respiration and speaking activities. Unilateral deposition of PANI and good breathability of IFHS avoids direct contact between PANI and human skin, thus averting harms to human and minimizing the deterioration of humidity sensing properties of PANI layer. The simple method is universal to the preparation of single-sided, integrated, breathable, nontoxic and fast response wearable humidity sensors based on PANI and hydrophobic microporous polymer membranes, offering useful references for the construction of advanced flexible sensors.

Wearable Sensors Based on Solid-Phase Molecular Self-Assembly: Moisture-Strain Dual Responsiveness Facilitated Extremely High and Damage-Resistant Sensitivity

Wearable sensing technologies have gained increasing interest in biomedical fields because they are convenient and could efficiently monitor health conditions by detecting various physiological signals in real time. However, common film sensors often neglect body moisture and enhance the sensitivity by enhancing the conductive dopants and self-healing ability. We report in this work a supramolecular film sensor based on solid-phase molecular self-assembly (SPMSA), which smartly utilizes the body moisture to enhance the sensitivity for human-machine interaction. The carbon nanotube (CNT)-doped SPMSA film is able to capture environmental moisture quickly. Upon contact to human skin, the moisture not only promotes the junction between CNTs but also contributes to the conductivity. As a result, the sensitivity can be enhanced 4 times. In this way, we are able to obtain the highest sensitivity of 700% with the lowest CNT doping rate of 0.5%. Furthermore, the current sensor displays damage-inert sensing performance. In the presence of a hole of up to 50% of the film area, the sensitivity remains unaffected due to the decreases in the absolute conductivity of the film sensor before and after a trigger to the same extent. In this way, we have developed a new principle in the design of a film sensor for human-machine interaction, which releases the sensor from focus on promoting conductivity and self-healing materials.

Nanomaterials and their applications on bio-inspired wearable electronics

Wearable electronics featuring conformal attachment, sensitive perception and intellectual signal processing have made significant progress in recent years. However, when compared with living organisms, artificial sensory devices showed undeniable bulky shape, poor adaptability, and large energy consumption. To make up for the deficiencies, biological examples provide inspirations of novel designs and practical applications. In the field of biomimetics, nanomaterials from nanoparticles to layered two-dimensional materials are actively involved due to their outstanding physicochemical properties and nanoscale configurability. This review focuses on nanomaterials related to wearable electronics through bioinspired approaches on three different levels, interfacial packaging, sensory structure, and signal processing, which comprehensively guided recent progress of wearable devices in leveraging both nanomaterial superiorities and biorealistic functionalities. In addition, opinions on potential development trend are proposed aiming at implementing bioinspired electronics in multifunctional portable sensors, health monitoring, and intelligent prosthetics.

A universal managing circuit with stabilized voltage for maintaining safe operation of self-powered electronics system

Harvesting mechanical energy via a triboelectric nanogenerator (TENG) is a promising strategy for solving energy problems. However, it is necessary to develop an effective and safe energy managing circuit for preventing high voltage breaking electronic devices. Here, a universal managing circuit is developed to optimize TENG's output performance, which for the first time allows the TENG to safely power various sensor systems with a safe and stable voltage. Based on the circuit, TENG's output can be transformed into a stable voltage with tunable amplitude, while an enhanced short-circuit current of 94 mA with an energy loss lower than 5% is achieved. For demonstrations, three different types of TENGs, respectively, targeting at ocean energy, wind energy, and walking energy have been prepared to reveal the capability of the circuit. This study offers a strategy to greatly enhance the output performance of TENGs to provide useful guidance for constructing self-powered and distributed sensor systems.

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UMC is designed for a TENG to maintain stable voltage with a lower resistance

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UMC provides a short-circuit current of 94 mA with an energy loss lower than 5%

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UMC can completely avoid the breakdown of electronic devices due to TENG's high voltage

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Three self-powered sensor systems have been successfully established

Electronic Skin from High-Throughput Fabrication of Intrinsically Stretchable Lead Zirconate Titanate Elastomer

Electronic skin made of thin, soft, stretchable devices that can mimic the human skin and reconstruct the tactile sensation and perception offers great opportunities for prosthesis sensing, robotics controlling, and human-machine interfaces. Advanced materials and mechanics engineering of thin film devices has proven to be an efficient route to enable and enhance flexibility and stretchability of various electronic skins; however, the density of devices is still low owing to the limitation in existing fabrication techniques. Here, we report a high-throughput one-step process to fabricate large tactile sensing arrays with a sensor density of 25 sensors/cm² for electronic skin, where the sensors are based on intrinsically stretchable piezoelectric lead zirconate titanate (PZT) elastomer. The PZT elastomer sensor arrays with great uniformity and passive-driven manner enable high-resolution tactile sensing, simplify the data acquisition process, and lower the manufacturing cost. The high-throughput fabrication process provides a general platform for integrating intrinsically stretchable materials into large area, high device density soft electronics for the next-generation electronic skin.

A facile and novel design of multifunctional electronic skin based on polydimethylsiloxane with micropillars for signal monitoring

Electronic skins (e-skins) with monitoring capabilities have attracted extensive attention and are being widely employed in wearable devices for medical diagnosis. In particular, e-skins based on strain sensors have been reported extensively due to their simple structure and efficient performance in collecting human physiological information. Flexible sensors with high sensitivity, simplified fabrication, and low-cost are highly desired for human signal monitoring; this work provides a novel strain-sensing e-skin with micro-structures, which is simply made of modified polydimethylsiloxane (PDMS) and silver nanowires (AgNWs). The fabricated e-skin has great sensitivity towards strain changes, and its mechanical properties and sensitivity could be regulated by varying the micro-structures. Furthermore, the e-skin demonstrated significant capacity for monitoring human body movements, temperature changes, and spatial resolution, highlighting its great potential in personalized medicine.

Stretchable respiration sensors: Advanced designs and multifunctional platforms for wearable physiological monitoring

Respiration signals are a vital sign of life. Monitoring human breath provides critical information for health assessment, diagnosis, and treatment for respiratory diseases such as asthma, chronic bronchitis, and emphysema. Stretchable and wearable respiration sensors have recently attracted considerable interest toward monitoring physiological signals in the era of real time and portable healthcare systems. This review provides a snapshot on the recent development of stretchable sensors and wearable technologies for respiration monitoring. The article offers the fundamental guideline on the sensing mechanisms and design concepts of stretchable sensors for detecting vital breath signals such as temperature, humidity, airflow, stress and strain. A highlight on the recent progress in the integration of variable sensing components outlines feasible pathways towards multifunctional and multimodal sensor platforms. Structural designs of nanomaterials and platforms for stretchable respiration sensors are reviewed.

4D Printing Strain Self-Sensing and Temperature Self-Sensing Integrated Sensor-Actuator with Bioinspired Gradient Gaps

Integrated sensor-actuators with exciting functionalities, such as action self-sensing, position self-sensing, posture self-sensing, or active sensing, are promising for applications in biomedical device, human-machine interaction, intelligent self-protection devices, and humanoid robots. Despite recent progress, it remains challenging to achieve a macroscopical integrated sensor-actuator in a material system with microstructures. To address this critical challenge, a 4D printing bioinspired microstructure strategy is reported to design a high-performance integrated sensor-actuator capable of simultaneous actuation and sensation. Decoupled thermal stimulation and strain sensation is achieved by combining nanocarbon black/polylactic acid composites with bioinspired gradient microgap structures. As a result, printed integrated sensor-actuators can actively touch objects triggered by thermal stimulation and self-sense the touching state through the resistance change. It is anticipated that the basic design principle underlying this behavior can be used to develop integrated sensor-actuators of various shapes and functionalities to meet desirable applications.

Body-Integrated, Enzyme-Triggered Degradable, Silk-Based Mechanical Sensors for Customized Health/Fitness Monitoring and In Situ Treatment

Mechanical signals such as pressure and strain reflect important psychological and physiological states of the human body. Body-integrated sensors, including skin-mounted and surgically implanted ones, allow personalized health monitoring for the general population as well as patients. However, the development of such measuring devices has been hindered by the strict requirements for human-biocompatible materials and the need for high performance sensors; most existing devices or sensors do not meet all the desired specifications. Here, a set of flexible, stretchable, wearable, implantable, and degradable mechanical sensors is reported with excellent mechanical robustness and compliance, outstanding biocompatibility, remotely-triggered degradation, and excellent sensing performance, using a conductive silk fibroin hydrogel (CSFH). They can detect multiple mechanical signals such as pressure, strain, and bending angles. Moreover, combined with a drug-loaded silk-based microneedle array, sensor-equipped devices are shown to be effective for real-time monitoring and in situ treatment of epilepsy in a rodent model. These sensors offer potential applications in custom health monitoring wearables, and in situ treatment of chronic clinical disorders.

Electron-Induced Perpendicular Graphene Sheets Embedded Porous Carbon Film for Flexible Touch Sensors

Graphene-based materials on wearable electronics and bendable displays have received considerable attention for the mechanical flexibility, superior electrical conductivity, and high surface area, which are proved to be one of the most promising candidates of stretching and wearable sensors. However, polarized electric charges need to overcome the barrier of graphene sheets to cross over flakes to penetrate into the electrode, as the graphene planes are usually parallel to the electrode surface. By introducing electron-induced perpendicular graphene (EIPG) electrodes incorporated with a stretchable dielectric layer, a flexible and stretchable touch sensor with "in-sheet-charges-transportation" is developed to lower the resistance of carrier movement. The electrode was fabricated with porous nanostructured architecture design to enable wider variety of dielectric constants of only 50-μm-thick Ecoflex layer, leading to fast response time of only 66 ms, as well as high sensitivities of 0.13 kPa-1 below 0.1 kPa and 4.41 MPa-1 above 10 kPa, respectively. Moreover, the capacitance-decrease phenomenon of capacitive sensor is explored to exhibit an object recognition function in one pixel without any other integrated sensor. This not only suggests promising applications of the EIPG electrode in flexible touch sensors but also provides a strategy for internet of things security functions.