Mimicking Human and Biological Skins for Multifunctional Skin Electronics

Electronic skin (e‐skin) technology is an exciting frontier to drive the next generation of wearable electronics owing to its high level of wearability, enabling high accuracy to harvest information of users and their surroundings. Recently, biomimicry of human and biological skins has become a great inspiration for realizing novel wearable electronic systems with exceptional multifunctionality as well as advanced sensory functions. This review covers and highlights bioinspired e‐skins mimicking perceptive features of human and biological skins. In particular, five main components in tactile sensation processes of human skin are individually discussed with recent advances of e‐skins that mimic the unique sensing mechanisms of human skin. In addition, diverse functionalities in user‐interactive, skin‐attachable, and ultrasensitive e‐skins are introduced with the inspiration from unique architectures and functionalities, such as visual expression of stimuli, reversible adhesion, easy deformability, and camouflage, in biological skins of natural creatures. Furthermore, emerging wearable sensor systems using bioinspired e‐skins for body motion tracking, healthcare monitoring, and prosthesis are described. Finally, several challenges that should be considered for the realization of next‐generation skin electronics are discussed with recent outcomes for addressing these challenges.     

Laser Direct Writing of Ultrahigh Sensitive SiC‐Based Strain Sensor Arrays on Elastomer toward Electronic Skins

Electronic skins (e‐skins) have been widely investigated as important platforms for healthcare monitoring, human/machine interfaces, and soft robots. However, mask‐free formation of patterned active materials on elastomer substrates without involving high‐cost and complicate processes is still a grand challenge in developing e‐skins. Here, SiC‐based strain sensor arrays are fabricated on elastomer for e‐skins by a laser direct writing (LDW) technique, which is mask‐free, highly efficient, and scalable. The direct synthesis of active material on elastomer is ascribed to the LDW‐induced conversion of siloxanes to SiC. The SiC‐based devices own a highest sensitivity of ≈2.47 × 10⁵ achieved at a laser power of 0.8 W and a scanning velocity of 1.25 mm s^?1. Moreover, the LDW‐developed device provides a minimum strain detection limit of 0.05%, a small temperature drift, and a high mechanical durability for over 10 000 cycles. When it is mounted onto human skins, the SiC‐based device is able to monitor external stimuli and human health conditions, with the capability of wireless data transmission. Its potential application in e‐skins is further proved by an LDW‐fabricated device having 3 × 3 SiC sensor array for tactile sensing.     

White Light‐Emitting Diode From Sb‐Doped p‐ZnO Nanowire Arrays/n‐GaN Film

A whole interfacial transition of electrons from conduction bands of n‐type material to the acceptor levels of p‐type material makes the energy band engineering successful. It tunes intrinsic ZnO UV emission to UV‐free and warm white light‐emitting diode (W‐LED) emission with color coordinates around (0.418, 0.429) at the bias of 8–15.5 V. The W‐LED is fabricated based on antimony (Sb) doped p‐ZnO nanowire arrays/Si doped n‐GaN film heterojunction structure through one‐step chemical vapor deposition with quenching process. Element analysis shows that the doping concentration of Sb is ≈1.0%. The I–V test exhibits the formation of p‐type ZnO nanowires, and the temperature‐dependent photoluminescence measurement down to 4.65 K confirms the formation of deep levels and shallow acceptor levels after Sb‐doping. The intrinsic UV emission of ZnO at room temperature is cut off in electroluminescence emission at a bias of 4–15.5 V. The UV‐free and warm W‐LED have great potential application in green lights program, especially in eye‐protected lamp and display since television, computer, smart phone, and mobile digital equipment are widely and heavily used in modern human life, as more than 3000 h per year.     

Elaborately Aligning Bead‐Shaped Nanowire Arrays Generated by a Superhydrophobic Micropillar Guiding Strategy

Bead‐shaped 1D structures are of great interest due to their unique applications in mesoscopic optics/electronics and their specific ability to collect tiny droplets. Here, a novel method to fabricate aligning bead‐shaped nanowire arrays assisted by highly adhesive superhydrophobic surfaces based on a micropillar guiding strategy is presented. Different from previous fabrication techniques, bead‐shaped nanowires generated in this method are strictly oriented in a large scale. Rayleigh instability, which occurs at ultralow polymer concentration, can introduce bead‐shaped nanowires at the cost of structural strength. Thus, PS spheres are more suitable to serve as bead building blocks to generate firm bead‐shaped nanowire arrays. The bead number is tunable by tailoring the polystyrene‐sphere/polyvinyl‐formal ratio. Furthermore, as‐prepared bead‐shaped nanowires have the unique ability to directionally drive tiny drops and collect coalesced microdroplets when placed in mist. With an increase in humidity, the nanowires show a segmented swelling behavior in the “bead” parts whereas the “joint nanowire” parts remain the same. Because such bead‐shaped nanowires are formed regularly, collected microdroplets upon the beads would not interact with each other. The findings offer new insight into the alignment of bead‐shaped nanostructures and might provide promising opportunities in fundamental research and for industrial applications.     

Nickel Silicide Nanowire Arrays for Anti‐Reflective Electrodes in Photovoltaics

Conductive nanowires (NWs) provide several advantages as a template and electrode material for solar cells due to their favorable light scattering properties. While the majority of NW solar cell architectures studied are based on semiconductor materials, metallic NWs could provide equivalent anti‐reflection properties, while acting as a low‐resistance back contact for charge transport, and facilitate light scattering in thin layers of semiconductors coated on the surface. However, fabrication of single‐crystalline highly anti‐reflective NWs on low‐cost, flexible substrates remains a challenge to drive down the cost of NW solar cells. In this study, metallic Ni_xSi NW arrays are fabricated by a simple, bottom‐up, and low‐cost method based on the thermal decomposition of silane on the surface of flexible Ni foil substrates without the need for lithography, etching or catalysts. The optical properties of these NW arrays demonstrate broadband suppression of reflection to levels below 1% from 350 nm to 1100 nm, which is among the highest values reported for NWs. A simple route to control the diameter and density of the NWs is introduced based on variations in a carrier gas flow rate. A high‐resolution TEM, XRD and TEM‐EDS study of the NWs reveals that they are single crystalline, with the phase and composition varying between Ni₂Si and NiSi. The nanowire resistivity is measured to be 10^?4 Ω‐cm, suggesting their use as an efficient back electrode material for nanostructured solar cells with favorable light scattering properties.     

Hierarchical Toughening of Nacre‐Like Composites

Reinforced polymer‐based composites are attractive lightweight materials for aircrafts, automobiles, and turbine blades, but still show strength and fracture toughness lower than traditional metals. An interesting approach to address this issue is to fabricate composites with structural features that absorb part of the elastic energy stored in the material during fracture through extrinsic and intrinsic toughening mechanisms behind and ahead of the crack tip, respectively. Inspired by the nacreous layer of mollusk shells, the fracture behavior of multiscale composites that combine intrinsic toughness from a brick‐and‐mortar structure connected through nanoscale mineral bridges and extrinsic toughness arising from a brittle–ductile laminate architecture at the millimeter scale are fabricated and investigated. Such a hierarchical toughening approach increases the dissipated energy by more than 30‐fold during fracture with minimal loss in stiffness and strength. Using simple energy balance arguments and fracture mechanics concepts, guidelines are established for the design of nacre‐like composites with a remarkable combination of stiffness, strength, and toughness. This demonstrates the possibility to controllably introduce toughening mechanisms at different length scales and to thus fabricate hierarchical composites with high fracture resistance in spite of the brittle nature of their main inorganic constituents.     

Versatile Electronic Skins for Motion Detection of Joints Enabled by Aligned Few‐Walled Carbon Nanotubes in Flexible Polymer Composites

Here, novel multifunctional electronic skins (E‐skins) based on aligned few‐walled carbon nanotube (AFWCNT) polymer composites with a piezoresistive functioning mechanism different from the mostly investigated theory of “tunneling current channels” in randomly dispersed CNT polymer composites are demonstrated. The high performances of as‐prepared E‐skins originate from the anisotropic conductivity of AFWCNT array embedded in flexible composite and the distinct variation of “tube‐to‐tube” interfacial resistance responsive to bending or stretching. The polymer/AFWCNT‐based flexion‐sensitive E‐skins exhibit high precision and linearity, together with low power consumption (<10 µW) and good stability (no degradation after 15 000 bending–unbending cycles). Moreover, polymer/AFWCNT composites can also be used for the construction of tensile‐sensitive E‐skins, which exhibit high sensitivity toward tensile force. The polymer/AFWCNT‐based E‐skins show remarkable performances when applied to monitor the motions and postures of body joints (such as fingers), a capability that can find wide applications in wearable human–machine communication interfaces, portable motion detectors, and bionic robots.     

Entirely,Intrinsically, and Autonomously Self‐Healable,Highly Transparent,and Superstretchable Triboelectric Nanogenerator for Personal Power Sources and Self‐Powered Electronic Skins

Power and electronic components that are self‐healable, deformable, transparent, and self‐powered are highly desirable for next‐generation energy/electronic/robotic applications. Here, an energy‐harvesting triboelectric nanogenerator (TENG) that combines the above features is demonstrated, which can serve not only as a power source but also as self‐powered electronic skin. This is the first time that both of the triboelectric‐charged layer and electrode of the TENG are intrinsically and autonomously self‐healable at ambient conditions. Additionally, comparing with previous partially healable TENGs, its fast healing time (30 min, 100% efficiency at 900% strain), high transparency (88.6%), and inherent superstretchability (>900%) are much more favorable. It consists of a metal‐coordinated polymer as the triboelectrically charged layer and hydrogen‐bonded ionic gel as the electrode. Even after 500 cutting‐and‐healing cycles or under extreme 900%‐strain, the TENG retains its functionality. The generated electricity can be used directly or stored to power commercial electronics. The TENG is further used as self‐powered tactile‐sensing skin in diverse human–machine interfaces including smart glass, an epidermal controller, and phone panel. This TENG with merits including fast ambient‐condition self‐healing, high transparency, intrinsic stretchability, and energy‐extraction and actively‐sensing abilities, can meet wide application needs ranging from deformable/portable/transparent electronics, smart interfaces, to artificial skins.     

Design and Synthesis of Hierarchical Nanowire Composites for Electrochemical Energy Storage

Nanocomposites of interpenetrating carbon nanotubes and vanadium pentoxide (V₂O₅) nanowires networks are synthesized via a simple in situ hydrothermal process. These fibrous nanocomposites are hierarchically porous with high surface area and good electric conductivity, which makes them excellent material candidates for supercapacitors with high energy density and power density. Nanocomposites with a capacitance up to 440 and 200 F g^?1 are achieved at current densities of 0.25 and 10 A g^?1, respectively. Asymmetric devices based on these nanocomposites and aqueous electrolyte exhibit an excellent charge/discharge capability, and high energy densities of 16 W h kg^?1 at a power density of 75 W kg^?1 and 5.5 W h kg^?1 at a high power density of 3 750 W kg^?1. This performance is a significant improvement over current electrochemical capacitors and is highly competetive with Ni–MH batteries. This work provides a new platform for high‐density electrical‐energy storage for electric vehicles and other applications.     

Metal‐Catalyzed Etching of Vertically Aligned Polysilicon and Amorphous Silicon Nanowire Arrays by Etching Direction Confinement

A systematic study of metal‐catalyzed etching of (100), (110), and (111) silicon substrates using gold catalysts with three varying geometrical characteristics: isolated nanoparticles, metal meshes with small hole spacings, and metal meshes with large hole spacings is carried out. It is shown that for both isolated metal catalyst nanoparticles and meshes with small hole spacings, etching proceeds in the crystallographically preferred <100> direction. However, the etching is confined to the single direction normal to the substrate surface when a catalyst meshes with large hole spacings is used. We have also demonstrated that the metal catalyzed etching method when used with metal mesh with large hole spacings can be extended to create arrays of polycrystalline and amorphous vertically aligned silicon nanowire by confining the etching to proceed in the normal direction to the substrate surface. The ability to pattern wires from polycrystalline and amorphous silicon thin films opens the possibility of making silicon nanowire array‐based devices on a much wider range of substrates.     

A General Approach for Fabricating Arc‐Shaped Composite Nanowire Arrays by Pulsed Laser Deposition

Here, a new method is demonstrated that uses sideways pulsed laser deposition to deliberately bend nanowires into a desired shape after growth and fabricate arc‐shaped composite nanowire arrays of a wide range of nanomaterials. The starting nanowires can be ZnO, but the materials to be deposited can be metallic, semiconductor, or ceramic depending on the application. This method provides a general approach for rational fabrication of a wide range of side‐by‐side or “core–shell” nanowire arrays with controllable degree of bending and internal strain. Considering the ZnO is a piezoelectric and semiconductive material, its electrical properties change when deformed. This technique has potential applications in tunable electronics, optoelectronics, and piezotronics.     

High‐Performance Integrated ZnO Nanowire UV Sensors on Rigid and Flexible Substrates

Due to the large surface area‐to‐volume ratio and high quality crystal structure, single nanowire (NW)‐based UV sensors exhibit very high on/off ratios between photoresponse current and dark current. Practical applications require a large‐scale and low‐cost integration, compatibility to flexible electronics, as well as reasonably high photoresponse current that can be detected without high‐precision measurement systems. In this paper, NW‐based UV sensors were fabricated in large‐scale by integrating multiple NWs connected in parallel via the contact printing method. Linear scaling of the photoresponse current with the number of NWs is demonstrated. Integrated ZnO NW UV sensors were fabricated on rigid glass and flexible polyester (PET) substrates at the macroscopic scale. The flexible and rigid sensors performed comparably, exhibiting on/off current ratios approximately three orders of magnitude higher than sensors made from polycrystalline ZnO thin films. Under UV irradiance of 4.5 mW cm^?2 and 3 V bias, photoresponse currents and on/off current ratios for the rigid and flexible UV sensors reached 12.22 mA and 82 000, and 14.1 mA and 120 000, respectively. This result suggests that lateral integration of semiconductor NWs is an effective approach to large‐scale fabrication of flexible NW sensors that inherit the merits of single‐NW‐based systems with unaffected performance compared to using rigid substrate.     

Bioinspired Assembly of Hierarchical Light‐Harvesting Architectures for Improved Photophosphorylation

Molecular assembly offers a bottom‐up way to construct biomimetic architectures with unique structures and properties. Although artificial photophosphorylation systems have long been developed, their microstructures have yet to achieve the sophisticated order and hierarchy of natural organisms. Herein, by utilizing principles in the natural plant leaves, it is shown that a biomimetic system with hierarchically ordered and compartmentalized structures, combining photosystem II (PSII) and adenosine triphosphate (ATP) synthase, can be obtained through template‐directed layer‐by‐layer assembly. Under light illumination, PSII in such a highly ordered light‐harvesting array, splits water to produce protons and electrons. Furthermore, a remarkable proton gradient is created across the covering ATP synthase‐reconstituted lipid membrane. As a consequence, highly efficient photophosphorylation is achieved. Outstandingly, the rate of ATP production in this hierarchical light‐harvesting architecture is enhanced 14 times, compared to that in the nature. This study paves a new way to assemble bioinspired systems with enhanced solar‐to‐chemical energy conversion efficiency.     

Mass-Producible 3D Hair Structure-Editable Silk-Based Electronic Skin for Multiscenario Signal Monitoring and Emergency Alarming System

Structurally tunable electronic skin (e-skin) is beneficial for advancing wearable electronics, prosthetics, and human-machine interaction (HMI). However, the regulation of e-skin by traditional nanostructure technology is complex and expensive, moreover, the nanostructure's poor deformability leads to small detection range and low sensitivity. Herein, inspired by the structure of skin-hair and insect burr, a polypyrrole-silk/glycerol plasticized silk fibroin (P-silk/RG) e-skin fabricated by a simple 3D biomimetic structural strategy is reported. Benefitting from the editability (length, position) of this structure, P-silk/RG has a signal selectivity, long-cilia P-silk/RG demonstrates high sensitivity (respond to weak signal-airflow), while the short-cilia P-silk/RG exhibits wide pressure detection range (0.5–200 g) and high cycle stability (8000 compressions). Therefore, different forms of P-silk/RG are used in different scenarios (long-cilia for monitoring breathing and coughing for motion detection and disease diagnosis, short-cilia for pressure-sensitive Morse code). Besides, P-silk/RG exhibits good waterproof, editable conductive points and easy device integration, providing the basis for underwater information transmission, multibit coded command output, and early warning for emergency sports accidents and sedentary. Surprisingly, combining this structure with textile weaving can be mass-produced. Obviously, this 3D biomimetic structure strategy endows e-skin with editability and improved scene adaptability to provide a favorable way for mass production.     

Soft Electronic Skin for Multi‐Site Damage Detection and Localization

Soft‐matter technologies have a potentially central role in wearable computing, human–machine interaction, soft robotics, and other emerging applications that require highly compliant and elastic materials. However, these technologies are largely composed of soft materials that are susceptible to damage and loss of functionality when exposed to real‐world loading conditions. To address this critical challenge, we present a soft responsive material that, like natural nervous tissue, is able to identify, compute, and signal damage in real‐time. The soft composite material contains liquid metal droplets dispersed in an elastomer matrix that rupture when mechanical damage occurs (e.g., compression, fracture, or puncture), creating electrically conductive pathways. The resulting change in local conductivity can be actively sensed and coupled with actuation, communication, and computation in a manner that presents new opportunities to identify damage, calculate severity, and respond to prevent failure within soft material systems. When placed on the surface of a soft, humanoid‐like inflatable structure, the skin can detect puncture damage and control the operation of an embedded fan to prevent deflation.     

Extraordinarily Sensitive and Low‐Voltage Operational Cloth‐Based Electronic Skin for Wearable Sensing and Multifunctional Integration Uses: A Tactile‐Induced Insulating‐to‐Conducting Transition

Electronic skin sensing devices are an emerging technology and have substantial demand in vast practical fields including wearable sensing, robotics, and user‐interactive interfaces. In order to imitate or even outperform the capabilities of natural skin, the keen exploration of materials, device structures, and new functions is desired. However, the very high resistance and the inadequate current switching and sensitivity of reported electronic skins hinder to further develop and explore the promising uses of the emerging sensing devices. Here, a novel resistive cloth‐based skin‐like sensor device is reported that possesses unprecedented features including ultrahigh current‐switching behavior of ≈10⁷ and giant high sensitivity of 1.04 × 10⁴–6.57 × 10⁶ kPa^?1 in a low‐pressure region of <3 kPa. Notably, both superior features can be achieved by a very low working voltage of 0.1 V. Taking these remarkable traits, the device not only exhibits excellent sensing abilities to various mechanical forces, meeting various applications required for skin‐like sensors, but also demonstrates a unique competence to facile integration with other functional devices for various purposes with ultrasensitive capabilities. Therefore, the new methodologies presented here enable to greatly enlarge and advance the development of versatile electronic skin applications.     

Decoupling Diameter and Pitch in Silicon Nanowire Arrays Made by Metal‐Assisted Chemical Etching

The fabrication of well‐separated, narrow, and relatively smooth silicon nanowires with good periodicity is demonstrated, using non‐close‐packed arrays of nanospheres with precisely controlled diameters, pitch, and roughness. Controlled reactive ion etching in an inductively coupled plasma reduces the self‐assembled nanospheres to approximately a tenth of their original diameter, while retaining their surface smoothness and periodic placement. A titanium adhesion layer between the silicon substrate and gold film allows much thinner catalyst layers to be continuous, facilitating the film liftoff and formation of the perforated pattern without influencing catalyzed etching of silicon. Using these methods, a periodic array of silicon nanowires with a large pitch and small diameter (e.g., a 490 nm pitch and 55 nm diameter) is created, a combination not typically found in the open literature. This approach extends the types and quality of silicon nanostructures that can be fabricated using the combined nanosphere lithography and metal‐assisted chemical etching techniques.     

Charge Disproportionation and Voltage‐Induced Metal–Insulator Transitions Evidenced in β‐PbxV2O5 Nanowires

The roster of materials exhibiting metal–insulator transitions with sharply discontinuous switching of electrical conductivity close to room temperature remains rather sparse, despite the fundamental interest in the electronic instabilities manifested in such materials and the plethora of potential technological applications ranging from frequency‐agile metamaterials to electrochromic coatings and Mott field‐effect transistors. Here, unprecedented, pronounced metal‐insulator transitions induced by application of a voltage are demonstrated for nanowires of a vanadium oxide bronze with intercalated divalent cations, β‐Pb_xV₂O₅ (x ≈ 0.33). The induction of the phase transition through application of an electric field at room temperature makes this system particularly attractive and viable for technological applications. A mechanistic basis for the phase transition is proposed based on charge disproportionation evidenced at room temperature in near‐edge X‐ray absorption fine structure (NEXAFS) spectroscopy measurements, ab initio density functional theory calculations of the band structure, and electrical transport data, suggesting that transformation to the metallic state is induced by melting of specific charge localization and ordering motifs extant in these materials.     

Piezoelectric Nylon‐11 Nanowire Arrays Grown by Template Wetting for Vibrational Energy Harvesting Applications

Piezoelectric polymers, capable of converting mechanical vibrations into electrical energy, are attractive for use in vibrational energy harvesting due to their flexibility, robustness, ease, and low cost of fabrication. In particular, piezoelectric polymers nanostructures have been found to exhibit higher crystallinity, higher piezoelectric coefficients, and “self‐poling,” as compared to films or bulk. The research in this area has been mainly dominated by polyvinylidene fluoride and its copolymers, which while promising have a limited temperature range of operation due to their low Curie and/or melting temperatures. Here, the authors report the fabrication and properties of vertically aligned and “self‐poled” piezoelectric Nylon‐11 nanowires with a melting temperature of ≈200 °C, grown by a facile and scalable capillary wetting technique. It is shown that a simple nanogenerator comprising as‐grown Nylon‐11 nanowires, embedded in an anodized aluminium oxide (AAO) template, can produce an open‐circuit voltage of 1 V and short‐circuit current of 100 nA, when subjected to small‐amplitude, low‐frequency vibrations. Importantly, the resulting nanogenerator is shown to exhibit excellent fatigue performance and high temperature stability. The work thus offers the possibility of exploiting a previously unexplored low‐cost piezoelectric polymer for nanowire‐based energy harvesting, particularly at temperatures well above room temperature.