Pressure-sensitive electronic skin composed of a hierarchical structural array exhibits outstanding linear and high sensitivity in the pressure range exerted by gentle touch. By virtue of monolayer graphene acting as electrode material, this device can be operated with low voltage. Especially, its high transparency enables an accurate placement of the device on the target position when it is used for health monitoring.

Human skin imperfectly discriminates between pressure and temperature stimuli under mixed stimulation, and exhibits nonlinear sensitivity to each stimulus. Despite great advances in the field of electronic skin (E-skin), the limitations of human skin have not previously been overcome. For the first time, the development of a stimulus-discriminating and linearly sensitive bimodal E-skin that can simultaneously detect and discriminate pressure and temperature stimuli in real time is reported. By introducing a novel device design and using a temperature-independent material, near-perfect stimulus discriminability is realized. In addition, the hierarchical contact behavior of the surface-wrinkled microstructure and the optimally reduced graphene oxide in the E-skin contribute to linear sensitivity to applied pressure/temperature stimuli over wide intensity range. The E-skin exhibits a linear and high pressure sensitivity of 0.7 kPa up to 25 kPa. Its operation is also robust and exhibits fast response to pressure stimulus within 50 ms. In the case of temperature stimulus, the E-skin shows a linear and reproducible temperature coefficient of resistance of 0.83% K in the temperature range 22-70 °C and fast response to temperature change within 100 ms. In addition, two types of stimuli are simultaneously detected and discriminated in real time by only impedance measurements.

Recently, some researchers have incorporated a sweat-collecting system into sweat sensors for sweat collection and transportation. [6] The sweat-absorbing layers that have been tested include paper [7] and rayon; [2a] however, these collection layers have no directionality in sweat transportation and are therefore not appropriate for continuous monitoring of freshly generated sweat in the sensing area. An alternative is a microfluidics channel system; [8] however, such a system can cause contamination of the sample by allowing old and new sweat to mix by diffusion within the channel. Such devices also have low sweatcollection efficiency because the channel must be filled to fill the sensing area. [9] In all cases, the directionality and the sweatcollecting efficiency are insufficient.In the present study, we demonstrate a sweat-collecting patch with directional sweat transportation and high sweat-collection efficiency, enabling fast and continuous monitoring of sweat by a sensor. The collecting component of the patch consists of channels with a narrow superhydrophilic wedge-shaped pattern within superhydrophobic bounds on a hierarchical microstructured/ nanostructured surface and a sweat reservoir that can be combined with a sweat sensor. The wedge-shaped wettability patterns were inspired by spines of cacti; they are narrow at the perimeter of the patch and gradually widen toward the junction with the sensing area at the center. The Laplace pressure is centripetal because of a combination of geometric structure and the difference between the surface energies of the superhydrophobic and superhydrophilic components. As a result, the sweat is transported spontaneously irrespective of gravity even when the substrate is aligned vertically. The patch transports the sweat almost without leaving it inside the channel and thereby concentrates the sweat from a large area of the skin onto the sensing area. The wedge-shaped wettability-patterned channel has greater sweat-collection efficiency than a conventional microfluidics channel and enables doubling of the speed of sweat collection. In an on-body test, the patch with a sensor responded to biochemicals within 5 min of its wearer beginning to exercise. By accelerating the circulation rate of sweat, the patch reduces the time that sweat remains in the sensing area and, by providing freshly generated sweat to it, enables the continuous sensor-based monitoring of changes in sweat biochemicals as blood changes.A sweat sensor is expected to be the most appropriate wearable device for noninvasive healthcare monitoring. However, the practical use of sweat sensors is impeded by irregular and low sweat secretion rates. Here, a sweatcollecting patch that can collect sweat efficiently for fast and continuous healthcare monitoring is demonstrated. The patch uses cactus-spine-inspired wedge-shaped wettability-patterned channels on a hierarchical microstructured/nanostructured surface. The channel shape, in combination with the superhydrophobic/superhydrophilic surface materials, ...

User‐interactive electronic skin (e‐skin) with a distinguishable output has enormous potential for human–machine interfaces and healthcare applications. Despite advances in user‐interactive e‐skins, advances in visual user‐interactive therapeutic e‐skins remain rare. Here, a user‐interactive thermotherapeutic device is reported that is fabricated by combining thermochromic composites and stretchable strain sensors consisting of strain‐responsive silver nanowire networks on surface energy‐patterned microwrinkles. Both the color and heat of the device are easily controlled through electrical resistance variation induced by applied mechanical strain. The resulting monolithic device exhibits substantial changes in optical reflectance and temperature with durability, rapid response, high stretchability, and linear sensitivity. The approach enables a low‐expertise route to fabricating dynamic interactive thermotherapeutic e‐skins that can be used to effectively rehabilitate injured connective tissues as well as to prevent skin burns by simultaneously accommodating stretching, providing heat, and exhibiting a color change.

Human skin plays a critical role in a person communicating with his or her environment through diverse activities such as touching or deforming an object. Various electronic skin (E‐skin) devices have been developed that show functional or geometrical superiority to human skin. However, research into stretchable E‐skin that can simultaneously distinguish materials and textures has not been established yet. Here, the first approach to achieving a stretchable multimodal device is reported, that operates on the basis of various electrical properties of piezoelectricity, triboelectricity, and piezoresistivity and that exceeds the capabilities of human tactile perception. The prepared E‐skin is composed of a wrinkle‐patterned silicon elastomer, hybrid nanomaterials of silver nanowires and zinc oxide nanowires, and a thin elastomeric dielectric layer covering the hybrid nanomaterials, where the dielectric layer exhibits high surface roughness mimicking human fingerprints. This versatile device can identify and distinguish not only mechanical stress from a single stimulus such as pressure, tensile strain, or vibration but also that from a combination of multiple stimuli. With simultaneous sensing and analysis of the integrated stimuli, the approach enables material discrimination and texture recognition for a biomimetic prosthesis when the multifunctional E‐skin is applied to a robotic hand.

The signals generated by these stimuli are transferred to the central neural network and brain to provide an appropriate response. In addition, biological skin possesses unique characteristics, such as stretchability, self-healing ability, and mechanical toughness. [1][2][3] It also provides an effective barrier against ambient elements such as chemicals, gases, radiation, and external biological agents.Electronic skin (e-skin) is an artificial smart skin composed of various electronic sensors distributed on a single uniform surface or stacked on multiple surfaces. It mimics some of the biological skin senses and provides a promising sensing platform for key application areas such as wearable sensors, robotics, and prosthetics (Figure 1). [1,[4][5][6][7][8][9][10][11] However, serious concerns have arisen regarding the production of electronic waste (e-waste), the influence of e-skin on human health, the safety of wearable electronic devices, costs, and environmental limitations. These concerns have encouraged researchers to develop biocompatible, renewable, sustainable, and biodegradable flexible wearable detection devices and biosensing platforms. [12][13][14][15][16] Biological skin regularly regenerates the old superficial layer of the epidermis with new skin cells during the healing process. Inspired by this natural degradation cycle, artificial biodegradable skin-based electronics should be designed for programmable degradation, in contrast to conventional electronic materials. To achieve this goal, the development of biocompatible, environmentally friendly electronic systems that can be used for the fabrication of biodegradable and sustainable e-skins is important.Various low-cost, nontoxic, renewable, and natural substances and polymer-based materials that can be applied to the development of sustainable, biodegradable e-skins and alleviate the environmental and health risks of e-waste materials are available. [17][18][19][20][21][22] E-waste disrupts the biological activity of microbial enzymes and their metabolisms, which decreases the resistance and the diversity of soil-based microbial communities. [23] In addition, in the human body, the accumulation of metals and metalloids used in the fabrication of conventional electronics can cause a range of biophysical malfunctions and diseases, such as liver damage by Cu, behavioral disorders by Pb, and lung cancer and kidney damage by Cd. [24][25][26][27] The rapid growth of the electronics industry and proliferation of electronic materials and telecommunications technologies has led to the release of a massive amount of untreated electronic waste (e-waste) into the environment. Consequently, catastrophic environmental damage at the microbiome level and serious human health diseases threaten the natural fate of the planet. Currently, the demand for wearable electronics for applications in personalized medicine, electronic skins (e-skins), and health monitoring is substantial and growing. Therefore, "green" characteristics such as biodegradability, self-healin...

Emerging plant diseases, caused by pathogens, pests, and climate change, are critical threats to not only the natural ecosystem but also human life. To mitigate crop loss due to various biotic and abiotic stresses, new sensor technologies to monitor plant health, predict, and track plant diseases in real time are desired. Wearable electronics have recently been developed for human health monitoring. However, the application of wearable electronics to agriculture and plant science is in its infancy. Wearable technologies mean that the sensors will be directly placed on the surfaces of plant organs such as leaves and stems. The sensors are designed to detect the status of plant health by profiling various trait biomarkers and microenvironmental parameters, transducing bio-signals to electric readout for data analytics. In this perspective, the recent progress in wearable plant sensors is summarized and they are categorized by the functionality, namely plant growth sensors, physiology, and microclimate sensors, chemical sensors, and multifunctional sensors. The design and mechanism of each type of wearable sensors are discussed and their applications to address the current challenges of precision agriculture are highlighted. Finally, challenges and perspectives for the future development of wearable plant sensors are presented.

Figure 11. Other fabricating methods to develop flexible or stretchable sensors. a) Left: Fabrication of a flexible sensor by thermal oxidation of copper foil to produce the wrinkled morphology. Right:As-prepared sensing layer with wrinkled structures on the surface and its pressure sensing performance (Reproduced with permission. [10] Copyright 2016, Wiley-VCH). b) Top: Wrinkling mechanism by evaporation of water droplets. Bottom: Atomic force microscopy images of the monolayer graphene with wrinkled geometry (Reproduced with permission. [87] Copyright 2020, Nature Publishing Group). c) Left: Schematic diagram of the wrinkling phenomenon by electrospinning. Right: Scanning electron microscopy (SEM) images of the wrinkled surfaces (Reproduced with permission. [88] Copyright 2020, Elsevier). d) Left: Preparation steps and schematic representation of hierarchical wrinkled conductors. Right: SEM images and illustrations of hierarchical wrinkles (Reproduced with permission. [89] Copyright 2016, Wiley-VCH).