3D-Structured Stretchable Strain Sensors for Out-of-Plane Force Detection

Biomechanics on Ultra-Sensitivity of Venus Flytrap's Micronewton Trigger Hairs

Numerous plants evolve ingeniously microcantilever-based hairs to ultra-sensitively detect out-of-plane quasi-static tactile loads, providing a natural blueprint for upgrading the industrial static mode microcantilever sensors, but how do the biological sensory hairs work mechanically? Here, the action potential-producing trigger hairs of carnivorous Venus flytraps (Dionaea muscipula) are investigated in detail from biomechanical perspective. Under tiny mechanical stimulation, the deformable trigger hair, composed of distal stiff lever and proximal flexible podium, will lead to rapid trap closure and prey capture. The multiple features determining the sensitivity such as conical morphology, multi-scale functional structures, kidney-shaped sensory cells, and combined deformation under tiny mechanical stimulation are comprehensively researched. Based on materials mechanics, finite element simulation, and bio-inspired original artificial sensors, it is verified that the omnidirectional ultra-sensitivity of trigger hair is attributed to the stiff-flexible coupling of material, the double stress concentration, the circular distribution of sensory cells, and the positive local buckling. Also, the balance strategy of slender hair between sensitivity and structural stability (i.e., avoiding disastrous collapse) is detailed revealed. The unique basic biomechanical mechanism underlying trigger hairs is essential for significantly enhancing the performance of the traditional industrial static mode microcantilever sensors, and ensure the stability of arbitrary load perception.

A high-sensitivity flexible bionic tentacle sensor for multidimensional force sensing and autonomous obstacle avoidance applications

Bionic tentacle sensors are important in various fields, including obstacle avoidance, human‒machine interfaces, and soft robotics. However, most traditional tentacle sensors are based on rigid substrates, resulting in difficulty in detecting multidirectional forces originating from the external environment, which limits their application in complex environments. Herein, we proposed a high-sensitivity flexible bionic tentacle sensors (FBTSs). Specifically, the FBTS featured an ultrahigh sensitivity of 37.6 N-1 and an ultralow detection limit of 2.4 mN, which benefited from the design of a whisker-like signal amplifier and crossbeam architecture. Moreover, the FBTS exhibited favorable linearity (R2 = 0.98) and remarkable durability (more than 5000 cycles). This was determined according to the improvement in the uniformity of the sensing layer through a high-shear dispersion process. In addition, the FBTS could accurately distinguish the direction of external stimuli, resulting in the FBTS achieving roughness recognition, wind speed detection and autonomous obstacle avoidance. In particular, the ability of autonomous obstacle avoidance was suitably demonstrated by leading a bionic rat through a maze with the FBTS. Notably, the proposed FBTS could be widely applied in tactile sensing, orientation perception, and obstacle avoidance.

Fatigue crack-based strain sensors achieving flow detection and motion monitoring for reconnaissance robot applications

Crack-based flexible strain sensors with ultra-high sensitivity under tiny strain are highly desired for environmental perception and motion detection of novel flexible and miniature robots. However, previously reported methods for fabricating crack patterns have often sacrificed the cyclic stability of the sensor, leading to a trade-off relationship between the sensitivity and the cyclic stability. Here, a universal and simple strategy based on fatigue loading with an ultra-large cumulative strain of up to ∼1.2 × 107%, rather than the traditionally quasi-static pre-overloading methods, is proposed to introduce channel cracks in the sensing layer without sacrificing the cyclic stability. The developed flexible strain sensors exhibit high strain-sensitivity (gauge factor = 5798) under tiny strain (< 3%), high cyclic stability (15 000 cycles) and a low strain detecting limit (0.02%). Furthermore, a leaf-like mechanosensor is developed using the fatigue crack-based strain sensor for the realization of multifunctional applications in environment perception and micro-motion detection. Brilliant airflow sensing performance with a wide sensing range (0.93-11.93 m s-1) and a fast response time (0.28 s) for amphibious applications is demonstrated. This work provides a new strategy for overcoming limits of crack-based flexible strain sensors and the developed leaf-like mechanosensor shows great application potential in miniature and flexible reconnaissance robots.

Biomimetic Multimodal Receptors for Comprehensive Artificial Human Somatosensory System

Electronic skin (e-skin) capable of acquiring environmental and physiological information has attracted interest for healthcare, robotics, and human-machine interaction. However, traditional 2D e-skin only allows for in-plane force sensing, which limits access to comprehensive stimulus feedback due to the lack of out-of-plane signal detection caused by its 3D structure. Here, a dimension-switchable bioinspired receptor is reported to achieve multimodal perception by exploiting film kirigami. It offers the detection of in-plane (pressure and bending) and out-of-plane (force and airflow) signals by dynamically inducing the opening and reclosing of sensing unit. The receptor's hygroscopic and thermoelectric properties enable the sensing of humidity and temperature. Meanwhile, the thermoelectric receptor can differentiate mechanical stimuli from temperature by the voltage. The development enables a wide range of sensory capabilities of traditional e-skin and expands the applications in real life.

Skin-Inspired Multi-Modal Mechanoreceptors for Dynamic Haptic Exploration

Active sensing is a fundamental aspect of human and animal interactions with the environment, providing essential information about the hardness, texture, and tackiness of objects. This ability stems from the presence of diverse mechanoreceptors in the skin, capable of detecting a wide range of stimuli and from the sensorimotor control of biological mechanisms. In contrast, existing tactile sensors for robotic applications typically excel in identifying only limited types of information, lacking the versatility of biological mechanoreceptors and the requisite sensing strategies to extract tactile information proactively. Here, inspired by human haptic perception, a skin-inspired artificial 3D mechanoreceptor (SENS) capable of detecting multiple mechanical stimuli is developed to bridge sensing and action in a closed-loop sensorimotor system for dynamic haptic exploration. A tensor-based non-linear theoretical model is established to characterize the 3D deformation (e.g., tensile, compressive, and shear deformation) of SENS, providing guidance for the design and optimization of multimode sensing properties with high fidelity. Based on SENS, a closed-loop robotic system capable of recognizing objects with improved accuracy (≈96%) is further demonstrated. This dynamic haptic exploration approach shows promise for a wide range of applications such as autonomous learning, healthcare, and space and deep-sea exploration.

High-Performance Airflow Sensors Based on Suspended Ultralong Carbon Nanotube Crossed Networks

Airflow sensors are in huge demand in many fields such as the aerospace industry, weather forecasting, environmental monitoring, chemical and biological engineering, health monitoring, wearable smart devices, etc. However, traditional airflow sensors can hardly meet the requirements of these applications in the aspects of sensitivity, response speed, detection threshold, detection range, and power consumption. Herein, this work reports high-performance airflow sensors based on suspended ultralong carbon nanotube (CNT) crossed networks (SCNT-CNs). The unique topologies of SCNT-CNs with abundant X junctions can fully exhibit the extraordinary intrinsic properties of ultralong CNTs and significantly improve the sensing performance and robustness of SCNT-CNs-based airflow sensors, which simultaneously achieved high sensitivity, fast response speed, low detection threshold, and wide detection range. Moreover, the capability for encapsulation also guaranteed the practicality of SCNT-CNs, enabling their applications in respiratory monitoring, flow rate display and transient response analysis. Simulations were used to unveil the sensing mechanisms of SCNT-CNs, showing that the piezoresistive responses were mainly attributed to the variation of junction resistances. This work shows that SCNT-CNs have many superiorities in the fabrication of advanced airflow sensors as well as other related applications.

Achieving a Wide-Range Linear Piezoresistive Response in Electrowritten Soft-Hard Polymer Blends via Salami-Inspired Heterostructure Design

Flexible electronics capable of acquiring high-precision signals are in great demand for the development of the internet of things and intelligent artificial. However, it is currently a challenge to simultaneously achieve high signal linearity and sensitivity for stretchable resistive sensors over a wide strain range toward advanced application scenarios requiring high signal accuracy, e.g., sophisticated physiological signal discrimination and displacement measurement. Herein, a film strain sensor, which has an electrical and mechanical dual heterostructure, was fabricated via a direct near-field electrowriting and molecule-guided in situ growth of silver nanoparticles with different concentrations on high-modulus polystyrene domains and low-modulus styrene-butadiene copolymers with a salami-like morphology. Mechanism analyses from both theoretical and experimental investigations reveal that the salami-like heteromodulus microstructure regulates microcrack propagation routes, while the heteroconductivity changes the electron transport paths and amplifies the resistance increase during crack propagation. Therefore, the as-designed strain sensor shows a linear resistive response within ca. 70% strain with a gauge factor of 25, unveiling a simple and scalable strategy for trading off signal linearity and sensitivity over a wide strain range for the fabrication of high-performance linear strain sensors.

Composite Flexible Sensor Based on Bionic Microstructure to Simultaneously Monitor Pressure and Strain

To achieve the human sense of touch, a strain sensor needs to be coupled with a pressure sensor to identify the compliance of the contacted material. However, monitoring the pressure-strain signals simultaneously and ensuring no coupling effect between the two signals is the technical bottleneck for the flexible tactile sensor to. Herein, a composite flexible sensor based on microstructures of lotus leaf is designed and manufactured, which integrates the capacitive pressure sensor and the resistance strain sensor into one pixel to realize the simultaneous detection of pressure and strain. The electrode layer of the capacitance sensor also plays the role of the resistance strain sensor, which greatly simplifies the structure of the composite flexible sensor and obtains the compact size to integrate more easily. The device can simultaneously detect pressure and deformation, and more importantly, there is no coupling effect between the two kinds of signals. Here, the sensor has high pressure sensitivity (0.784 kPa-1 when pressure less than 100 kPa), high strain sensitivity (gauge factor = 4.03 for strain 0-40%), and can identify materials with different compliance, which indicates the tactile ability as the human skin performs.

Ultralow-Latency Textile Sensors for Wearable Interfaces with a Human-in-Loop Sensing Approach

The evolution of wearable technologies has led to the development of novel types of sensors customized for a wide range of applications. Wearable sensors need to possess a low form factor and be ergonomic, causing minimal impediment of the user's natural movement. Various principles have been explored to meet these requirements, ranging from optical, magnetic, resistive flex sensing to 3D printed sensors and liquid metals such as those using eutectic gallium-indium. However, manufacturing techniques for most current wearable sensors tend to be complex and difficult to scale. Challenges also exist in achieving high sensitivity with noise resistance and robustness to false detections, especially in capacitive sensors. In this research, a novel ultralow-latency soft tactile and pressure sensor developed using off-the-shelf e-textiles is proposed, which overcomes some of these limitations. The sensor does not use any specialized equipment or materials for manufacture. A human-in-loop (HIL) sensing technique is demonstrated, which provides high sensitivity, high sensing bandwidth, as well as ultralow latency, which makes it ideal as a wearable input device. In addition, the HIL method provides other advantages such as high noise rejection and resistance to accidental triggers that could be caused by other humans or environmental factors owing to its high signal to noise ratio. Finally, two applications-a wearable keyboard and gaming input device-were demonstrated using these sensors.

Decoupling Transmission and Transduction for Improved Durability of Highly Stretchable, Soft Strain Sensing: Applications in Human Health Monitoring

This work presents a modular approach to the development of strain sensors for large deformations. The proposed method separates the extension and signal transduction mechanisms using a soft, elastomeric transmission and a high-sensitivity microelectromechanical system (MEMS) transducer. By separating the transmission and transduction, they can be optimized independently for application-specific mechanical and electrical performance. This work investigates the potential of this approach for human health monitoring as an implantable cardiac strain sensor for measuring global longitudinal strain (GLS). The durability of the sensor was evaluated by conducting cyclic loading tests over one million cycles, and the results showed negligible drift. To account for hysteresis and frequency-dependent effects, a lumped-parameter model was developed to represent the viscoelastic behavior of the sensor. Multiple model orders were considered and compared using validation and test data sets that mimic physiologically relevant dynamics. Results support the choice of a second-order model, which reduces error by 73% compared to a linear calibration. In addition, we evaluated the suitability of this sensor for the proposed application by demonstrating its ability to operate on compliant, curved surfaces. The effects of friction and boundary conditions are also empirically assessed and discussed.

Artificial Whisker Sensor with Undulated Morphology and Self-Spread Piezoresistors for Diverse Flow Analyses

Harbor seal whiskers possess an undulated surface morphology that can effectively modify the vortex street behind the whiskers and suppress vortex-induced vibrations (VIVs). In this study, we propose a novel piezoresistive flow sensor that mimics the function of seal whiskers. The sensor consists of a bionic whisker with an undulated morphology and integrated out-of-plane piezoresistors. The piezoresistors are formed using a novel directional liquid spreading method to deliver a conductive nanocomposite ink into four Ω-shaped microchannels. Steady flow experiments indicate that the undulated morphology of the artificial whisker significantly reduces the drag forces and VIVs of the whisker at an angle of attack of 0°. Moreover, the whisker sensor can measure the oscillatory flow, which reaches a threshold detection limit of 8 mm/s. In addition, we demonstrate the function of the artificial whisker sensor to distinguish various wakes induced by upstream cylinders. Therefore, the facile fabrication and preliminary experiments of the artificial whisker sensor demonstrate its potential application in diverse flow analyses.

Carbon-Based Flexible Devices for Comprehensive Health Monitoring

Traditional public health systems suffer from incomprehensive, delayed, and inefficient medical services. Convenient and comprehensive health monitoring has been highly sought after recently. Flexible and wearable devices are attracting wide attention due to their potential applications in wearable human health monitoring and care systems. Using carbon materials with overall superiorities can facilitate the development of wearable and flexible devices with various functions and excellent performance, which can comprehensively and real-time monitor human health status and prevent diseases. Herein, the latest advances in the rational design and controlled fabrication of carbon materials for applications in health-related flexible and wearable electronics are reviewed. The fabrication strategies, working mechanism, performance, and applications in health monitoring of carbon-based flexible devices, including electromechanical sensors, temperature/humidity sensors, chemical sensors, and flexible conductive wires/electrodes, are reviewed. Furthermore, integrating multiple carbon-based devices into multifunctional wearable systems is discussed. Finally, the existing challenges and future opportunities in this field are also proposed.

Shape morphing of plastic films

Three-dimensional (3D) architectures have qualitatively expanded the functions of materials and flexible electronics. However, current fabrication techniques for devices constrain their substrates to 2D geometries and current post-shape transformation strategies are limited to heterogenous or responsive materials and are not amenable to free-standing inert plastic films such as polyethylene terephthalate (PET) and polyimide (PI), which are vital substrates for flexible electronics. Here, we realize the shape morphing of homogeneous plastic films for various free-standing 3D frameworks from their 2D precursors by introducing a general strategy based on programming the plastic strain in films under peeling. By modulating the peeling parameters, previously inaccessible free-standing 3D geometries ranging from millimeter to micrometer were predicted theoretically and obtained experimentally. This strategy is applicable to most materials capable of plastic deformation, including polymers, metals, and composite materials, and can even enable 4D transformation with responsive plastic films. Enhanced performance of 3D circuits and piezoelectric systems demonstrates the enormous potential of peeling-induced shape morphing for 3D devices.

Topological Gradients for Metal Film-Based Strain Sensors

Metal film-based stretchable strain sensors hold great promise for applications in various domains, which require superior sensitivity-stretchability-cyclic stability synergy. However, the sensitivity-stretchability trade-off has been a long-standing dilemma and the metal film-based strain sensors usually suffer from weak cyclic durability, both of which significantly limit their practical applications. Here, we propose an extremely facile, low-cost and spontaneous strategy that incorporates topological gradients in metal film-based strain sensors, composed of intrinsic (grain size and interface) and extrinsic (film thickness and wrinkle) microstructures. The topological gradient strain sensor exhibits an ultrawide stretchability of 100% while simultaneously maintaining a high sensitivity at an optimal topological gradient of 4.5, due to the topological gradients-induced multistage film cracking. Additionally, it possesses a decent cyclic stability for >10 000 cycles between 0 and 40% strain enabled by the gradient-mixed metal/elastomer interfaces. It can monitor the full-range human activities from subtle pulse signals to vigorous joint movements.

Assembly of complex 3D structures and electronics on curved surfaces

Electronic devices with engineered three-dimensional (3D) architectures are indispensable for frictional-force sensing, wide-field optical imaging, and flow velocity measurement. Recent advances in mechanically guided assembly established deterministic routes to 3D structures in high-performance materials, through controlled rolling/folding/buckling deformations. The resulting 3D structures are, however, mostly formed on planar substrates and cannot be transferred directly onto another curved substrate. Here, we introduce an ordered assembly strategy to allow transformation of 2D thin films into sophisticated 3D structures on diverse curved surfaces. The strategy leverages predefined mechanical loadings that deform curved elastomer substrates into flat/cylindrical configurations, followed by an additional uniaxial/biaxial prestretch to drive buckling-guided assembly. Release of predefined loadings results in an ordered assembly that can be accurately captured by mechanics modeling, as illustrated by dozens of complex 3D structures assembled on curved substrates. Demonstrated applications include tunable dipole antennas, flow sensors inside a tube, and integrated electronic systems capable of conformal integration with the heart.

Intelligent Nanomaterials for Wearable and Stretchable Strain Sensor Applications: The Science behind Diverse Mechanisms, Fabrication Methods, and Real-Time Healthcare

It has become a scientific obligation to unveil the underlying mechanisms and the fabrication methods behind wearable/stretchable strain sensors based on intelligent nanomaterials in order to explore their possible potential in the field of biomedical and healthcare applications. This report is based on an extensive literature survey of fabrication of stretchable strain sensors (SSS) based on nanomaterials in the fields of healthcare, sports, and entertainment. Although the evolution of wearable strain sensors (WSS) is rapidly progressing, it is still at a prototype phase and various challenges need to be addressed in the future in special regard to their fabrication protocols. The biocalamity of COVID-19 has brought a drastic change in humans' lifestyles and has negatively affected nations in all capacities. Social distancing has become a mandatory rule to practice in common places where humans interact with each other as a basic need. As social distancing cannot be ruled out as a measure to stop the spread of COVID-19 virus, wearable sensors could play a significant role in technologically impacting people's consciousness. This review article meticulously describes the role of wearable and strain sensors in achieving such objectives.

Ultrasensitive Airflow Sensors Based on Suspended Carbon Nanotube Networks

High-performance airflow sensors are in great demand in numerous fields but still face many challenges, such as slow response speed, low sensitivity, large detection threshold, and narrow sensing range. Carbon nanotubes (CNTs) exhibit many advantages in fabricating airflow sensors due to their nanoscale diameters, excellent mechanical and electrical properties, and so on. However, the intrinsic extraordinary properties of CNTs are not fully exhibited in previously reported CNT-based airflow sensors due to the mixed structures of macroscale CNT assemblies. Herein, this article presents suspended CNT networks (SCNTNs) as high-performance airflow sensors, which are self-assembled by ultralong CNTs and short CNTs in a one-step floating catalyst chemical vapor deposition process. The SCNTN-based airflow sensors achieved a record-breaking short response time of 0.021 s, a high sensitivity of 0.0124 s m^-1 , a small detection threshold of 0.11 m s^-1 , and a wide detection range of ≈0.11-5.51 m s^-1 , superior to most of the state-of-the-art airflow sensors. To reveal the sensing mechanism, an acoustic response testing system and a mathematical model are developed. It is found that the airflow-caused intertube stress change resulted in the resistance variation of SCNTNs.

Bio-Inspired Mechano-Sensor Based on the Deformation of Slit Wake

Internal mechano-sensors, as an indispensable part of the proprioceptive system of intelligent equipment, have attracted enormous research interest because of their extremely crucial role in monitoring machining processes, real-time diagnosis of equipment faults, adaptive motor control and so on. The mechano-sensory structure with signal-transduction function is an important factor in determining the sensing performance of a mechano-sensor. However, contrary to the wide application of the cantilever beam as the sensory structure of external mechano-sensors in order to guarantee their exteroceptive ability, there is still a lack of an effective and widely used sensory structure to significantly improve the sensing performance of internal mechano-sensors. Here, inspired by the scorpion using the specialized slit as the sensory structure of internal mechano-sensilla, the slit is ingeniously used in the design of the engineered internal mechano-sensor. In order to improve the deformability of the slit wake, the hollowed-out design around the slit tail of biological mechano-sensilla is researched. Meanwhile, to mimic the easily deformed flexible cuticular membrane covering the slit, the ultrathin, flexible, crack-based strain sensor is used as the sensing element to cover the controllable slit wake. Based on the coupling deformation of the slit wake, as well as the flexible strain sensor, the slit-based mechano-sensor shows excellent sensing performance to various mechanical signals such as displacement and vibration signals.

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.

Review of Recent Bio-Inspired Design and Manufacturing of Whisker Tactile Sensors

Whisker sensors are a class of tactile sensors that have recently attracted attention. Inspired by mammals' whiskers known as mystacial vibrissae, they have displayed tremendous potential in a variety of applications e.g., robotics, underwater vehicles, minimally invasive surgeries, and leak detection. This paper provides a supplement to the recent tactile sensing techniques' designs of whiskers that only sense at their base, as well as the materials employed, and manufacturing techniques. The article delves into the technical specifications of these sensors, such as the resolution, measurement range, sensitivity, durability, and recovery time, which determine their performance. The sensors' sensitivity varies depending on the measured physical quantity; for example, the pressure sensors had an intermediate sensitivity of 58%/Pa and a response time of around 90 ms, whereas the force sensors that function based on piezoelectric effects exhibited good linearity in the measurements with a resolution of 3 µN and sensitivity of 0.1682 mV/µN. Some sensors were used to perform spatial mapping and the identification of the geometry and roughness of objects with a reported resolution of 25 nm. The durability and recovery time showed a wide range of values, with the maximum durability being 10,000 cycles and the shortest recovery time being 5 ms. Furthermore, the paper examines the fabrication of whiskers at the micro- and nanoscales, as well as their contributions to mechanical and thermal behavior. The commonly used manufacturing techniques of 3D printing, PDMS casting, and screen printing were used in addition to several micro and nanofabrication techniques such as photolithography, etching, and chemical vapor deposition. Lastly, the paper discusses the main potential applications of these sensors and potential research gaps in this field. In particular, the operation of whisker sensors under high temperatures or high pressure requires further investigation, as does the design of sensors to explore larger topologies.

Anisotropic conductive networks for multidimensional sensing

In the past decade, flexible physical sensors have attracted great attention due to their wide applications in many emerging areas including health-monitoring, human-machine interfaces, smart robots, and entertainment. However, conventional sensors are typically designed to respond to a specific stimulus or a deformation along only one single axis, while directional tracking and accurate monitoring of complex multi-axis stimuli is more critical in practical applications. Multidimensional sensors with distinguishable signals for simultaneous detection of complex postures and movements in multiple directions are highly demanded for the development of wearable electronics. Recently, many efforts have been devoted to the design and fabrication of multidimensional sensors that are capable of distinguishing stimuli from different directions accurately. Benefiting from their unique decoupling mechanisms, anisotropic architectures have been proved to be promising structures for multidimensional sensing. This review summarizes the present state and advances of the design and preparation strategies for fabricating multidimensional sensors based on anisotropic conducting networks. The fabrication strategies of different anisotropic structures, the working mechanism of various types of multidimensional sensing and their corresponding unique applications are presented and discussed. The potential challenges faced by multidimensional sensors are revealed to provide an insightful outlook for the future development.

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.

Compact Biomimetic Hair Sensors Based on Single Silicon Nanowires for Ultrafast and Highly-Sensitive Airflow Detection

Wearable sensors that can mimic functionalities of human bodies have attracted intense recent attention. However, research on wearable airflow sensors is still lagging behind. Herein, we report a biomimetic hair sensor based on a single ultralong silicon nanowire (SiNW-BHS) for airflow detection. In our device, the SiNW can provide both mechanical and electrical responses in airflow, which enables a simple and compact design. The SiNW-BHSs can detect airflow with a low detection limit (<0.15 m/s) and a record-high response speed (response time <40 ms). The compact design of the SiNW-BHSs also enables easy integration of an array of devices onto a flexible substrate to mimic human skin to provide comprehensive airflow information including wind speed, incident position, incident angle, and so forth. This work provides novel-designed BHSs for ultrafast and highly sensitive airflow detection, showing great potential for applications such as e-skins, wearable electronics, and robotics.

New Frontiers in 3D Structural Sensing Robots

Advanced robotics is the result of various contributions from complex fields of science and engineering and has tremendous value in human society. Sensing robots are highly desirable in practical settings such as healthcare and manufacturing sectors through sensing activities from human-robot interaction. However, there are still ongoing research and technical challenges in the development of ideal sensing robot systems. The sensing robot should synergically merge sensors and robotics. Geometrical difficulty in the sensor positioning caused by the structural complexity of sensing robots and their corresponding processing have been the main challenges in the production of sensing robots. 3D electronics integrated into 3D objects prepared by the 3D printing process can be the potential solution for designing realistic sensing robot systems. 3D printing provides the advantage to manufacture complex 3D structures in electronics in a single setup, allowing the ease of design flexibility, and customized functions. Therefore, the platform of 3D sensing systems is investigated and their expansion into sensing robots is studied further. The progress toward sensing robots from 3D electronics integrated into 3D objects and the advanced material strategies, used to overcome the challenges, are discussed.

Stretchable Electronics Based on PDMS Substrates

Stretchable electronics, which can retain their functions under stretching, have attracted great interest in recent decades. Elastic substrates, which bear the applied strain and regulate the strain distribution in circuits, are indispensable components in stretchable electronics. Moreover, the self-healing property of the substrate is a premise to endow stretchable electronics with the same characteristics, so the device may recover from failure resulting from large and frequent deformations. Therefore, the properties of the elastic substrate are crucial to the overall performance of stretchable devices. Poly(dimethylsiloxane) (PDMS) is widely used as the substrate material for stretchable electronics, not only because of its advantages, which include stable chemical properties, good thermal stability, transparency, and biological compatibility, but also because of its capability of attaining designer functionalities via surface modification and bulk property tailoring. Herein, the strategies for fabricating stretchable electronics on PDMS substrates are summarized, and the influence of the physical and chemical properties of PDMS, including surface chemical status, physical modulus, geometric structures, and self-healing properties, on the performance of stretchable electronics is discussed. Finally, the challenges and future opportunities of stretchable electronics based on PDMS substrates are considered.

Designing Mechanical Metamaterials with Kirigami-Inspired, Hierarchical Constructions for Giant Positive and Negative Thermal Expansion

Advanced mechanical metamaterials with unusual thermal expansion properties represent an area of growing interest, due to their promising potential for use in a broad range of areas. In spite of previous work on metamaterials with large or ultralow coefficient of thermal expansion (CTE), achieving a broad range of CTE values with access to large thermally induced dimensional changes in structures with high filling ratios remains a key challenge. Here, design concepts and fabrication strategies for a kirigami-inspired class of 2D hierarchical metamaterials that can effectively convert the thermal mismatch between two closely packed constituent materials into giant levels of biaxial/uniaxial thermal expansion/shrinkage are presented. At large filling ratios (>50%), these systems offer not only unprecedented negative and positive biaxial CTE (i.e., -5950 and 10 710 ppm K^-1 ), but also large biaxial thermal expansion properties (e.g., > 21% for 20 K temperature increase). Theoretical modeling of thermal deformations provides a clear understanding of the microstructure-property relationships and serves as a basis for design choices for desired CTE values. An Ashby plot of the CTE versus density serves as a quantitative comparison of the hierarchical metamaterials presented here to previously reported systems, indicating the capability for substantially enlarging the accessible range of CTE.

Sensitivity Improvement of Stretchable Strain Sensors by the Internal and External Structural Designs for Strain Redistribution

Fiber strain sensors that are directly woven into smart textiles play an important role in wearable systems. These sensors require a high sensitivity to detect the subtle strain in practical applications. However, traditional fiber strain sensors with constant diameters undergo homogeneous strain distribution in the axial direction, thereby limiting the sensitivity improvement. Herein, a novel strategy of internal or external structural design is proposed to significantly improve the sensitivity of fiber strain sensors. The fibers are produced with directional increases in diameter (internal design) or polydimethylsiloxane (PDMS) microbeads attached to surfaces (external design) by combining hollow glass tubes used as templates with PDMS drops. The structural modification of the fiber significantly impacts the sensing performance. After optimizing structural parameters, the highest gauge factor reaches 123.1 in the internal-external structure design at 25% strain. A comprehensive analysis reveals that the desirable scheme is the internal structural design, which features a high sensitivity of 110 with a 100% improvement at ∼5-20% strain. Because of the sufficiently robust interface, even at the 800th cycle, fiber sensors still possessed an excellent stable performance. The morphology evolution mechanism indicates that the resistance increase is closely related with the increased peak width and distance, and the appearance of gaps. Based on the finite element modeling simulation, the quantified effective contributions of different strategies positively correlate with the improved sensitivity. The proposed fiber strain sensors, which are woven into the two-dimensional network structure, exhibit an excellent capability for displacement monitoring and facilitate the traffic control of crossroads.

Flexible Multiplexed In2O3 Nanoribbon Aptamer-Field-Effect Transistors for Biosensing

Flexible sensors are essential for advancing implantable and wearable bioelectronics toward monitoring chemical signals within and on the body. Developing biosensors for monitoring multiple neurotransmitters in real time represents a key in vivo application that will increase understanding of information encoded in brain neurochemical fluxes. Here, arrays of devices having multiple In₂O₃ nanoribbon field-effect transistors (FETs) were fabricated on 1.4-μm-thick polyethylene terephthalate (PET) substrates using shadow mask patterning techniques. Thin PET-FET devices withstood crumpling and bending such that stable transistor performance with high mobility was maintained over >100 bending cycles. Real-time detection of the small-molecule neurotransmitters serotonin and dopamine was achieved by immobilizing recently identified high-affinity nucleic-acid aptamers on individual In₂O₃ nanoribbon devices. Limits of detection were 10 fM for serotonin and dopamine with detection ranges spanning eight orders of magnitude. Simultaneous sensing of temperature, pH, serotonin, and dopamine enabled integration of physiological and neurochemical data from individual bioelectronic devices.

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We fabricated flexible In₂O₃ nanoribbon transistors using cleanroom-free processes

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Flexible In₂O₃ transistors withstood crumpling and bending with stable performance

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Flexible aptamer biosensors detect neurotransmitters in real time

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Multiplexed sensors monitor temperature, pH, serotonin, and dopamine simultaneously

An On-Skin Electrode with Anti-Epidermal-Surface-Lipid Function Based on a Zwitterionic Polymer Brush

On-skin flexible devices provide a noninvasive approach for continuous and real-time acquisition of biological signals from the skin, which is essential for future chronic disease diagnosis and smart health monitoring. Great progress has been achieved in flexible devices to resolve the mechanical mismatching between conventional rigid devices and human skin. However, common materials used for flexible devices including silicon-based elastomers and various metals exhibit no resistance to epidermal surface lipids (skin oil and grease), which restricts the long-term and household usability. Herein, an on-skin electrode with anti-epidermal-surface-lipid function is reported, which is based on the grafting of a zwitterionic poly(2-methacryl-oyloxyethyl, methacryloyl-oxyethyl, or meth-acryloyloxyethyl phosphorylcholine) (PMPC) brush on top of gold-coated poly(dimethylsiloxane) (Au/PDMS). Such an electrode allows the skin-lipids-fouled surface to be cleaned by simple water rinsing owing to the superhydrophilic zwitterionic groups. As a proof-of-concept, the PMPC-Au/PDMS electrodes are employed for both electrocardiography (ECG) and electromyography (EMG) recording. The electrodes are able to maintain stable skin-electrode impedance and good signal-to noise ratio (SNR) by water rinsing alone. This work provides a material-based solution to improve the long-term reusability of on-skin electronics and offers a unique prospective on developing next generation wearable healthcare devices.

Srivastava S. Wearable Skin Sensors and Their Challenges: A Review of Transdermal, Optical, and Mechanical Sensors

Wearable technology and mobile healthcare systems are both increasingly popular solutions to traditional healthcare due to their ease of implementation and cost-effectiveness for remote health monitoring. Recent advances in research, especially the miniaturization of sensors, have significantly contributed to commercializing the wearable technology. Most of the traditional commercially available sensors are either mechanical or optical, but nowadays transdermal microneedles are also being used for micro-sensing such as continuous glucose monitoring. However, there remain certain challenges that need to be addressed before the possibility of large-scale deployment. The biggest challenge faced by all these wearable sensors is our skin, which has an inherent property to resist and protect the body from the outside world. On the other hand, biosensing is not possible without overcoming this resistance. Consequently, understanding the skin structure and its response to different types of sensing is necessary to remove the scientific barriers that are hindering our ability to design more efficient and robust skin sensors. In this article, we review research reports related to three different biosensing modalities that are commonly used along with the challenges faced in their implementation for detection. We believe this review will be of significant use to researchers looking to solve existing problems within the ongoing research in wearable sensors.

A bioinspired stretchable membrane-based compliance sensor

Compliance sensation is a unique feature of the human skin that electronic devices could not mimic via compact and thin form-factor devices. Due to the complex nature of the sensing mechanism, up to now, only high-precision or bulky handheld devices have been used to measure compliance of materials. This also prevents the development of electronic skin that is fully capable of mimicking human skin. Here, we developed a thin sensor that consists of a strain sensor coupled to a pressure sensor and is capable of identifying compliance of touched materials. The sensor can be easily integrated into robotic systems due to its small form factor. Results showed that the sensor is capable of classifying compliance of materials with high sensitivity allowing materials with various compliance to be identified. We integrated the sensor to a robotic finger to demonstrate the capability of the sensor for robotics. Further, the arrayed sensor configuration allows a compliance mapping which can enable humanlike sensations to robotic systems when grasping objects composed of multiple materials of varying compliance. These highly tunable sensors enable robotic systems to handle more advanced and complicated tasks such as classifying touched materials.

Flexible pressure sensors with a highly pressure- and strain-sensitive layer based on nitroxyl radical-grafted hollow carbon spheres

The spherical structure of hollow carbon spheres (HCSs) makes their contact resistance and tunnel resistance extremely sensitive to the distance between them, which can be used as a conductive filler for high-sensitivity pressure sensors. Compared with one- and two-dimensional carbon-based materials, HCSs require a higher filling concentration for constructing an effective conductive network due to their average conductivity, which affects the mechanical properties of the sensor. In a single-electron system, electrons are transferred by hopping between the nitroxyl radical monomers and when the distance between the monomers is shortened, the electron transfer rate of nitroxyl radical compounds can be increased, thus further improving their conductivity. In this work, a composite of nitroxyl radical-modified hollow carbon spheres (HCS-g-NO˙) and polydimethylsiloxane (PDMS) polymer is introduced, and the resistivity of HCS-g-NO˙ is about one magnitude lower than that of HCSs at the same filling concentration. A flexible piezoresistive sensor with HCS-g-NO˙@PDMS as the sensitive layer coated on the PET electrode is presented, in which the spacing between HCS-g-NO˙ changes, causing changes in the contact and tunnel resistances in the sensitive layer when mechanical stresses are applied. The sensor achieved a piezoresistive response of -0.55 kPa-1 and the tensile response of 211 , and a sensor array of nine pixels was successfully demonstrated; thus, it can be used as a high sensitivity pressure and strain sensor.

Three-Dimensional Structured Dual-Mode Flexible Sensors for Highly Sensitive Tactile Perception and Noncontact Sensing

This work reports a three-dimensional (3D) structured multifunctional sensor by connecting a magnetowhisker with a superflexible patterned skin film. Composed of percolation networks of silver nanowires, the patterned skin film is integrated via a simple template manufacturing method without increasing the complexity and sacrificing the flexibility. The as-prepared 3D structured sensor can realize the multimodal detection of out-of-plane tactile stimuli and details of noncontact environmental obstacles in multiple directions. Here, the sensor's perception behaviors on compression, pulling, magnetic field, sound waves, airflow, water level, water flow, and backwash are presented. Furthermore, the 3D structured sensor obtains outstanding mechanical robustness and stability for 8000 cycles, excellent sensitivity (12 800% when the applied pulling displacement was 3.5 mm; 152% T^-1 when the magnetic flux density variation was 40.6 mT), ultrahigh response time, and ultrahigh recovery time (∼5 ms), which may meet the industrial sensing requirement for artificial tactile electronics. Facile manufacturing processes and outstanding multimodal sensing characteristics make the 3D structured sensor to possess great potential to be implemented in the next-generation intelligent bionic equipment or systems.

Inverse Design Strategies for 3D Surfaces Formed by Mechanically Guided Assembly

Deterministic transformations of 2D patterns of materials into well-controlled 3D mesostructures serve as the basis for manufacturing methods that can bypass limitations of conventional 3D micro/nanofabrication. Here, guided mechanical buckling processes provide access to a rich range of complex 3D mesostructures in high-performance materials, from inorganic and organic semiconductors, metals and dielectrics, to ceramics and even 2D materials (e.g., graphene, MoS₂ ). Previous studies demonstrate that iterative computational procedures can define design parameters for certain targeted 3D configurations, but without the ability to address complex shapes. A technical need is in efficient, generalized inverse design algorithms that directly yield sets of optimized parameters. Here, such schemes are introduced, where the distributions of thicknesses across arrays of separated or interconnected ribbons provide scalable routes to 3D surfaces with a broad range of targeted shapes. Specifically, discretizing desired shapes into 2D ribbon components allows for analytic solutions to the inverse design of centrally symmetric and even general surfaces, in an approximate manner. Combined theoretical, numerical, and experimental studies of ≈20 different 3D structures with characteristic sizes (e.g., ribbon width) ranging from ≈200 µm to ≈2 cm and with geometries that resemble hemispheres, fire balloons, flowers, concave lenses, saddle surfaces, waterdrops, and rodents, illustrate the essential ideas.

Material Design and Fabrication Strategies for Stretchable Metallic Nanocomposites

Stretchable conductive nanocomposites fabricated by integrating metallic nanomaterials with elastomers have become a vital component of human-friendly electronics, such as wearable and implantable devices, due to their unconventional electrical and mechanical characteristics. Understanding the detailed material design and fabrication strategies to improve the conductivity and stretchability of the nanocomposites is therefore important. This Review discusses the recent technological advances toward high performance stretchable metallic nanocomposites. First, the effect of the filler material design on the conductivity is briefly discussed, followed by various nanocomposite fabrication techniques to achieve high conductivity. Methods for maintaining the initial conductivity over a long period of time are also summarized. Then, strategies on controlled percolation of nanomaterials are highlighted, followed by a discussion regarding the effects of the morphology of the nanocomposite and postfabricated 3D structures on achieving high stretchability. Finally, representative examples of applications of such nanocomposites in biointegrated electronics are provided. A brief outlook concludes this Review.

Cyber-Physiochemical Interfaces

Living things rely on various physical, chemical, and biological interfaces, e.g., somatosensation, olfactory/gustatory perception, and nervous system response. They help organisms to perceive the world, adapt to their surroundings, and maintain internal and external balance. Interfacial information exchanges are complicated but efficient, delicate but precise, and multimodal but unisonous, which has driven researchers to study the science of such interfaces and develop techniques with potential applications in health monitoring, smart robotics, future wearable devices, and cyber physical/human systems. To understand better the issues in these interfaces, a cyber-physiochemical interface (CPI) that is capable of extracting biophysical and biochemical signals, and closely relating them to electronic, communication, and computing technology, to provide the core for aforementioned applications, is proposed. The scientific and technical progress in CPI is summarized, and the challenges to and strategies for building stable interfaces, including materials, sensor development, system integration, and data processing techniques are discussed. It is hoped that this will result in an unprecedented multi-disciplinary network of scientific collaboration in CPI to explore much uncharted territory for progress, providing technical inspiration-to the development of the next-generation personal healthcare technology, smart sports-technology, adaptive prosthetics and augmentation of human capability, etc.

An Artificial Somatic Reflex Arc

The emulation of human sensation, perception, and action processes has become a major challenge for bioinspired intelligent robotics, interactive human-machine interfacing, and advanced prosthetics. Reflex actions, enabled through reflex arcs, are important for human and higher animals to respond to stimuli from environment without the brain processing and survive the risks of nature. An artificial reflex arc system that emulates the functions of the reflex arc simplifies the complex circuit design needed for "central-control-only" processes and becomes a basic electronic component in an intelligent soft robotics system. An artificial somatic reflex arc that enables the actuation of electrochemical actuators in response to the stimulation of tactile pressures is reported. Only if the detected pressure by the pressure sensor is above the stimulus threshold, the metal-organic-framework-based threshold controlling unit (TCU) can be activated and triggers the electrochemical actuators to complete the motion. Such responding mechanism mimics the all-or-none law in the human nervous system. As a proof of concept, the artificial somatic reflex arc is successfully integrated into a robot to mimic the infant grasp reflex. This work provides a unique and simplifying strategy for developing intelligent soft robotics, next-generation human-machine interfaces, and neuroprosthetics.

Flexible Out-of-Plane Wind Sensors with a Self-Powered Feature Inspired by Fine Hairs of the Spider

The quest for out-of-plane and self-powered wind sensors has motivated the field of outdoor sports, exploration, space perception, and positioning. Fine hairs of spiders act as hundreds of individual wind sensors, allowing them to feel the nearby wind change caused by the predators or the prey. Inspired by this natural teacher, here, we demonstrate the fabrication of bioinspired self-powered out-of-plane wind sensors based on flexible magnetoelectric material systems. The shape of flexible sensors, by patterning silver nanoparticles on a thin polyethylene terephthalate film through a screen printing technique, mimics fine hairs of the spiders, allowing for out-of-plane tactile perceptual monitoring caused by the wind. Owing to the employment of flexible magnetoelectric materials, the sensors can distinguish forward/backward winds and are totally self-powered. The working mechanism for sensors has been explained by the Maxwell numerical simulation, allowing for further improvement of their performance by tuning diverse factors. Furthermore, the wind sensor can detect the wind with a velocity down to 1.2 m/s and distinguish multidegree wind by their arrays. It is expected that, in the near future, our design can provide new findings for out-of-plane wind sensors with superior self-powered properties toward new flexible electronics.

Dual-Core Capacitive Microfiber Sensor for Smart Textile Applications

Wearable sensors for smart textile applications have garnered tremendous interest in recent years and can have enormous potential for human machine interfaces and digital health monitoring. Here, we report a soft capacitive microfiber sensor that can be woven seamlessly into textiles for strain measurement. Comprising a dual-lumen elastomeric microtube and liquid metallic alloy, the microfiber sensor enables continual strain perception even after being completely severed. In addition, our microfiber sensor is highly stretchable and flexible and exhibits tunable sensitivity, excellent linearity, a fast response, and negligible hysteresis. More importantly, the microfiber sensor is minimally affected by train rate and compression during strain sensing. Even under drastic environmental changes, the microfiber sensor presents good electrical stability. By integrating the microfiber sensor imperceptibly with textiles, we devise smart textile wearables to interpret hand gestures, detect limb motion, and monitor respiration rate. We believe that this sensor presents enormous potential in unobtrusive continuous health monitoring.

Large area flexible pressure/strain sensors and arrays using nanomaterials and printing techniques

Sensors are becoming more demanding in all spheres of human activities for their advancement in terms of fabrication and cost. Several methods of fabrication and configurations exist which provide them myriad of applications. However, the advantage of fabrication for sensors lies with bulk fabrication and processing techniques. Exhaustive study for process advancement towards miniaturization from the advent of MEMS technology has been going on and progressing at high pace and has reached a highly advanced level wherein batch production and low cost alternatives provide a competitive performance. A look back to this advancement and thus understanding the route further is essential which is the core of this review in light of nanomaterials and printed technology based sensors. A subjective appraisal of these developments in sensor architecture from the advent of MEMS technology converging present date novel materials and process technologies through this article help us understand the path further.

Highly Stable and Stretchable Conductive Films through Thermal-Radiation-Assisted Metal Encapsulation

Stretchable conductors are the basic units of advanced flexible electronic devices, such as skin-like sensors, stretchable batteries and soft actuators. Current fabrication strategies are mainly focused on the stretchability of the conductor with less emphasis on the huge mismatch of the conductive material and polymeric substrate, which results in stability issues during long-term use. Thermal-radiation-assisted metal encapsulation is reported to construct an interlocking layer between polydimethylsiloxane (PDMS) and gold by employing a semipolymerized PDMS substrate to encapsulate the gold clusters/atoms during thermal deposition. The stability of the stretchable conductor is significantly enhanced based on the interlocking effect of metal and polymer, with high interfacial adhesion (>2 MPa) and cyclic stability (>10 000 cycles). Also, the conductor exhibits superior properties such as high stretchability (>130%) and large active surface area (>5:1 effective surface area/geometrical area). It is noted that this method can be easily used to fabricate such a stretchable conductor in a wafer-scale format through a one-step process. As a proof of concept, both long-term implantation in an animal model to monitor intramuscular electric signals and on human skin for detection of biosignals are demonstrated. This design approach brings about a new perspective on the exploration of stretchable conductors for biomedical applications.

3D Printing of Ultralight Biomimetic Hierarchical Graphene Materials with Exceptional Stiffness and Resilience

Biological materials with hierarchical architectures (e.g., a macroscopic hollow structure and a microscopic cellular structure) offer unique inspiration for designing and manufacturing advanced biomimetic materials with outstanding mechanical performance and low density. Most conventional biomimetic materials only benefit from bioinspired architecture at a single length scale (e.g., microscopic material structure), which largely limits the mechanical performance of the resulting materials. There exists great potential to maxime the mechanical performance of biomimetic materials by leveraging a bioinspired hierarchical structure. An ink-based three-dimensional (3D) printing strategy to manufacture an ultralight biomimetic hierarchical graphene material (BHGMs) with exceptionally high stiffness and resilience is demonstrated. By simultaneously engineering 3D-printed macroscopic hollow structures and constructing an ice-crystal-induced cellular microstructure, BHGMs can achieve ultrahigh elasticity and stability at compressive strains up to 95%. Multiscale finite element analyses indicate that the hierarchical structures of BHGMs effectively reduce the macroscopic strain and transform the microscopic compressive deformation into the rotation and bending of the interconnected graphene flakes. This 3D printing strategy demonstrates the great potential that exists for the assembly of other functional materials into hierarchical cellular structures for various applications where high stiffness and resilience at low density are simultaneously required.

Mechanocombinatorially Screening Sensitivity of Stretchable Strain Sensors

Stretchable strain sensors have aroused great interest for their application in human activity recognition, health monitoring, and soft robotics. For various scenarios involving the application of different strain ranges, specific sensitivities need to be developed, due to a trade-off between sensor sensitivity and stretchability. Traditional stretchable strain sensors are developed based on conductive sensing materials and still lack the function of customizable sensitivity. A novel strategy of mechanocombinatorics is proposed to screen the sensor sensitivity based on mechanically heterogeneous substrates. Strain redistribution over substrates is optimized by mechanics and structure parameters, which gives rise to customizable sensitivity. As a proof of concept, a local illumination method is used to fabricate heterogeneous substrates with customizable mechanics and structure parameters. A library of mechanocombinatorial strain sensors is created for extracting the specific sensitivity. Thus, not only is an effective strategy for screening of sensor sensitivity demonstrated, but a contribution to the mechanocombinatorial strategy for personalized stretchable electronics is also made.

Mechanically Flexible Conductors for Stretchable and Wearable E-Skin and E-Textile Devices

Considerable progress in materials development and device integration for mechanically bendable and stretchable optoelectronics will broaden the application of "Internet-of-Things" concepts to a myriad of new applications. When addressing the needs associated with the human body, such as the detection of mechanical functions, monitoring of health parameters, and integration with human tissues, optoelectronic devices, interconnects/circuits enabling their functions, and the core passive components from which the whole system is built must sustain different degrees of mechanical stresses. Herein, the basic characteristics and performance of several of these devices are reported, particularly focusing on the conducting element constituting them. Among these devices, strain sensors of different types, energy storage elements, and power/energy storage and generators are included. Specifically, the advances during the past 3 years are reported, wherein mechanically flexible conducting elements are fabricated from (0D, 1D, and 2D) conducting nanomaterials from metals (e.g., Au nanoparticles, Ag flakes, Cu nanowires), carbon nanotubes/nanofibers, 2D conductors (e.g., graphene, MoS₂ ), metal oxides (e.g., Zn nanorods), and conducting polymers (e.g., poly(3,4-ethylenedioxythiophene):poly(4-styrene sulfonate), polyaniline) in combination with passive fibrotic and elastomeric materials enabling, after integration, the so-called electronic skins and electronic textiles.

Bioinspired Hairy Skin Electronics for Detecting the Direction and Incident Angle of Airflow

The human skin has inspired multimodal detection using smart devices or systems in fields including biomedical engineering, robotics, and artificial intelligence. Hairs of a high aspect ratio (AR) connected to follicles, in particular, detect subtle structural displacements by airflow or ultralight touch above the skin. Here, hairy skin electronics assembled with an array of graphene sensors (16 pixels) and artificial microhairs for multimodal detection of tactile stimuli and details of airflows (e.g., intensity, direction, and incident angle) are presented. Composed of percolation networks of graphene nanoplatelet sheets, the sensor array can simultaneously detect pressure, temperature, and vibration, all of which correspond to the sensing range of human tactile perceptions with ultrahigh response time (<0.5 ms, 2 kHz) for restoration. The device covered with microhairs (50 μm diameter and 300 μm height, AR = 6, hexagonal layout, and ∼4400/cm²) exhibits mapping of electrical signals induced by noncontact airflow and identifying the direction, incident angle, and intensity of wind to the sensor. For potential applications, we implement the hairy electronics to a sailing robot and demonstrate changes in locomotion and speed by detecting the direction and intensity of airflow.

Materials, Structures, and Functions for Flexible and Stretchable Biomimetic Sensors

Currently, flexible and stretchable biomimetic sensing electronics have obtained a great deal of attention in various areas, such as human-machine interfaces, robotic smart skins, health care monitoring, and biointegrated devices. In contrast with the traditional rigid and fragile silicon-based electronics, flexible and stretchable sensing electronics can efficiently capture high-quality signals when integrated on curved surfaces due to their elastic and conformal characters, which are expected to play many important roles in the foreseeable age of intelligence. Its realization strongly relies on rapid advances in the development of high-performance and versatile flexible and stretchable sensors, and effective ways to achieve high performance are rational designs of the sensing materials and microstructural configurations. This Account showcases the recent progress in flexible and stretchable biomimetic sensors covering several critical aspects of materials, structures, and applications. Nature-inspired active matter and architectures, which have been well-tuned by evolution through millions of years of optimization, provide us the best learning choices to overcome the restrictions of current sensor techniques such as low sensitivity, instability, and delayed response time. Biomimetic sensing materials and microstructural patterns can efficiently acquire synthetic response abilities, endowing the new-type flexible sensors considered as "smart" electronic components on account of the counterparts to living organisms. Moreover, the developments of diverse functions and multifunctional applications become more and more important in the creation of novel flexible electronics beyond those existing technologies. For instance, flexible and stretchable sensors with the capability of mimicking various human behavioral patterns can be developed to boost the emergence of artificial robots, which can take the place of human beings in strenuous activities, enabling progress in social science, technology, and productivity to improve the quality of human life. For the above purpose, inspired by the in-depth understanding of working principles of living organisms how to operate their natural characteristics, sensing materials with stimuli response (light, humidity, mechanics, etc.) and multifunctionalities (superhydrophobicity, degradation, self-healing, etc.) provide distinctive and multiple detection features generally encountered in their traditional counterparts. In addition, artificial micro- to nanostructures derived from naturally existing high sensitivity structures (such as insect crack or leaves) and stretchable configurations (wrinkle, texture, mesostructures, etc.) offer additional feasible strategies for producing favorable sensitivity and stretchability. Flexible and stretchable biomimetic sensors with analogous senses to those of human beings (such as tactile and auditory senses) have attracted tremendous attention for their diverse applications for next generation smart electronics. The long-term progress of these novel sensors influencing the next generations of bioinspired intelligence systems and medical electronics are also envisioned.

Heterogeneous Strain Distribution of Elastomer Substrates To Enhance the Sensitivity of Stretchable Strain Sensors

Stretchable strain sensors, which convert mechanical stimuli into electrical signals, largely fuel the growth of wearable bioelectronics due to the ubiquitous, health-related strain in biological systems. In contrast to rigid conventional strain sensors, stretchable strain sensors present advantages of conformality and stretchability, solving the mechanical mismatch between electronics and the human body. However, the great challenge of stretchable strain sensors lies in achieving high sensitivity, which is required for both signal fidelity and cost considerations. Recent advances to solve this sensitivity challenge have focused on material optimization, in search of the optimum combination of conductive active materials and elastomer substrates among a myriad of artificial or natural materials. However, high sensitivity with a gauge factor larger than 50 remains a grand challenge, especially within large-strain regions. Here we present heterogeneous strain distribution of elastomer substrates as a powerful strategy to significantly enhance the sensitivity of stretchable strain sensors. The theoretical foundation of this strategy is mathematically proven on the basis of Ohm's law in electrics and mechanics of materials. First, the extent of the sensitivity enhancement is proved to be determined by the local strain in resistance-testing segments of heterogeneous strain sensors. Next, the local strain is proved to be quantitatively decided by material properties such as section area and Young's modulus. Thus, the necessary and sufficient condition to achieve high sensitivity in heterogeneous strain sensors is that the Young's modulus reciprocal or section area reciprocal in the resistance-testing segment is larger than the mean value. This provides a theoretical design guideline to achieve high sensitivity via heterogeneous strain distribution. On the basis of this guideline, we systematically summarize concrete instances of heterogeneity-induced sensitivity improvement in stretchable strain sensors, in sequence of increasing dimensionality. A typical example of a one-dimensional heterogeneous strain sensor is a structured fiber with microbeads, where the varied section area along the fiber axis results in heterogeneous strain and sensitivity improvement. Two-dimensional heterogeneous sensors in the form of thin films contain thickness gradient sensors and auxetic mechanical metamaterial sensors. The former exhibit heterogeneous section area via the self-pinning method, while the latter show heterogeneity in both the strain direction and amplitude, leading to a 24-fold improvement in sensitivity. Three-dimensional strain sensors include rationally structured sensors for out-of-plane force detection and asymmetric active materials in electronic whiskers. The resultant enhanced sensitivity in these heterogeneous strain sensors is beneficial for applications such as continuous health monitoring, biomedical diagnostics, and replacement prosthetics, taking advantage of augmented detection accuracy and declined device cost. Finally, we discuss possible future work in exploiting heterogeneous strain distributions, involving extended methodology to achieve heterogeneity, employing suppressed strain for stretchable electrodes, cyclic durability for long-term applications, and multifunctional system-level integration. We believe that this strategy of using heterogeneous strain distribution to enhance sensitivity can strongly promote the development of stretchable strain sensors for both practical and theoretical requirements.

A highly stretchable and transparent silver nanowire/thermoplastic polyurethane film strain sensor for human motion monitoring

Transparent film strain sensors based on silver nanowires and thermoplastic polyurethane are promising candidates for detecting various human motions and monitoring the mass of some kinetic objects.

Freestanding 3D Mesostructures, Functional Devices, and Shape-Programmable Systems Based on Mechanically Induced Assembly with Shape Memory Polymers

Capabilities for controlled formation of sophisticated 3D micro/nanostructures in advanced materials have foundational implications across a broad range of fields. Recently developed methods use stress release in prestrained elastomeric substrates as a driving force for assembling 3D structures and functional microdevices from 2D precursors. A limitation of this approach is that releasing these structures from their substrate returns them to their original 2D layouts due to the elastic recovery of the constituent materials. Here, a concept in which shape memory polymers serve as a means to achieve freestanding 3D architectures from the same basic approach is introduced, with demonstrated ability to realize lateral dimensions, characteristic feature sizes, and thicknesses as small as ≈500, 10, and 5 µm simultaneously, and the potential to scale to much larger or smaller dimensions. Wireless electronic devices illustrate the capacity to integrate other materials and functional components into these 3D frameworks. Quantitative mechanics modeling and experimental measurements illustrate not only shape fixation but also capabilities that allow for structure recovery and shape programmability, as a form of 4D structural control. These ideas provide opportunities in fields ranging from micro-electromechanical systems and microrobotics, to smart intravascular stents, tissue scaffolds, and many others.