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Gelatinous underwater invertebrates such as jellyfish have organs that are transparent, stretchable, touch-sensitive and self-healing, which allow the creatures to navigate, camouflage themselves and, indeed, survive in aquatic environments. Artificial skins that emulate such functionality could be used to develop applications such as aquatic soft robots and water-resistant human–machine interfaces. Here we report a bio-inspired skin-like material that is transparent, electrically conductive and can autonomously self-heal in both dry and wet conditions. The material, which is composed of a fluorocarbon elastomer and a fluorine-rich ionic liquid, has an ionic conductivity that can be tuned to as high as 10−3 S cm−1 and can withstand strains as high as 2,000%. Owing to ion–dipole interactions, it offers fast and repeatable electro-mechanical self-healing in wet, acidic and alkali environments. To illustrate the potential applications of the approach, we used our electronic skins to create touch, pressure and strain sensors. We also show that the material can be printed into soft and pliable ionic circuit boards. A transparent electronic skin, composed of an elastomer and an ionic liquid, can autonomously self-heal in both dry and wet conditions due to ion–dipole interactions.

Human skin is a self-healing mechanosensory system that detects various mechanical contact forces efficiently through three-dimensional innervations. Here, we propose a biomimetic artificially innervated foam by embedding three-dimensional electrodes within a new low-modulus self-healing foam material. The foam material is synthesized from a one-step self-foaming process. By tuning the concentration of conductive metal particles in the foam at near-percolation, we demonstrate that it can operate as a piezo-impedance sensor in both piezoresistive and piezocapacitive sensing modes without the need for an encapsulation layer. The sensor is sensitive to an object’s contact force directions as well as to human proximity. Moreover, the foam material self-heals autonomously with immediate function restoration despite mechanical damage. It further recovers from mechanical bifurcations with gentle heating (70 °C). We anticipate that this material will be useful as damage robust human-machine interfaces., Designing mechanosensory system that detects mechanical contact forces like human skin remains a challenge. Here, the authors present artificially innervated self-healing foams by embedding 3D electrodes for piezo-impedance sensors that can operate in both piezoresistive and piezocapacitive sensing modes to address various proximity and mechanical interactions efficiently.

The present review will introduce the basic concepts of silk-based electronics/optoelectronics including the latest technological advances on the use of silk fibroin in combination with other functional components, with an emphasis on improving the performance of next-generation silk-based materials. It also highlights the patterning of silk fibroin to produce micro/nano-scale features, as well as the functionalization of silk fibroin to impart antimicrobial (i.e. antibacterial) properties. Silk-based bioelectronics have great potential for advanced or futuristic bio-applications including e-skins, e-bandages, biosensors, wearable displays, implantable devices, artificial muscles, etc. Notably, silk-based organic field-effect transistors have highly promising applications in e-skins and biosensors; silk-based electrodes/antennas are used for in vivo bioanalysis or sensing purpose (e.g., measurement of neurotransmitter such as dopamine) in addition to their use as food sensors; silk-based diodes can be applied as light sources for wound healing or tissue engineering, e.g., in cutaneous wound closure or induction of photothrombosis of corneal neovascularization; silk-based actuators have promising applications as artificial muscles; whereas silk-based memristors have exciting applications as logic or synaptic network for realizing e-skins or bionic brains.

Tuning the threshold voltage of a transistor is crucial for realizing robust digital circuits. For silicon transistors, the threshold voltage can be accurately controlled by doping. However, it remains challenging to tune the threshold voltage of single-wall nanotube (SWNT) thin-film transistors. Here, we report a facile method to controllably n-dope SWNTs using 1H-benzoimidazole derivatives processed via either solution coating or vacuum deposition. The threshold voltages of our polythiophene-sorted SWNT thin-film transistors can be tuned accurately and continuously over a wide range. Photoelectron spectroscopy measurements confirmed that the SWNT Fermi level shifted to the conduction band edge with increasing doping concentration. Using this doping approach, we proceeded to fabricate SWNT complementary inverters by inkjet printing of the dopants. We observed an unprecedented noise margin of 28 V at V(DD) = 80 V (70% of 1/2V(DD)) and a gain of 85. Additionally, robust SWNT complementary metal-oxide-semiconductor inverter (noise margin 72% of 1/2VDD) and logic gates with rail-to-rail output voltage swing and subnanowatt power consumption were fabricated onto a highly flexible substrate.

Solution coating of organic semiconductors offers great potential for achieving low-cost manufacturing of large-area and flexible electronics. However, the rapid coating speed needed for industrial-scale production poses challenges to the control of thin-film morphology. Here, we report an approach—termed fluid-enhanced crystal engineering (FLUENCE)—that allows for a high degree of morphological control of solution-printed thin films. We designed a micropillar-patterned printing blade to induce recirculation in the ink for enhancing crystal growth, and engineered the curvature of the ink meniscus to control crystal nucleation. Using FLUENCE, we demonstrate the fast coating and patterning of millimetre-wide, centimetre-long, highly aligned single-crystalline organic semiconductor thin films. In particular, we fabricated thin films of 6,13-bis(triisopropylsilylethynyl) pentacene having non-equilibrium single-crystalline domains and an unprecedented average and maximum mobilities of 8.1±1.2 cm2 V−1 s−1 and 11 cm2 V−1 s−1. FLUENCE of organic semiconductors with non-equilibrium single-crystalline domains may find use in the fabrication of high-performance, large-area printed electronics. Solution printing of organic semiconductors could in principle be scaled to industrial needs, yet attaining aligned single-crystals directly with this method has been challenging. By using a micropillar-patterned printing blade designed to enhance the control of crystal nucleation and growth, thin films of macroscopic, highly aligned single crystals of organic semiconductors can now be fabricated.

Skin is the body's largest organ and is responsible for the transduction of a vast amount of information. This conformable material simultaneously collects signals from external stimuli that translate into information such as pressure, pain, and temperature. The development of an electronic material, inspired by the complexity of this organ is a tremendous, unrealized engineering challenge. However, the advent of carbon-based electronics may offer a potential solution to this long-standing problem. In this Account, we describe the use of an organic field-effect transistor (OFET) architecture to transduce mechanical and chemical stimuli into electrical signals. In developing this mimic of human skin, we thought of the sensory elements of the OFET as analogous to the various layers and constituents of skin. In this fashion, each layer of the OFET can be optimized to carry out a specific recognition function. The separation of multimodal sensing among the components of the OFET may be considered a "divide and conquer" approach, where the electronic skin (e-skin) can take advantage of the optimized chemistry and materials properties of each layer. This design of a novel microstructured gate dielectric has led to unprecedented sensitivity for tactile pressure events. Typically, pressure-sensitive components within electronic configurations have suffered from a lack of sensitivity or long mechanical relaxation times often associated with elastomeric materials. Within our method, these components are directly compatible with OFETs and have achieved the highest reported sensitivity to date. Moreover, the tactile sensors operate on a time scale comparable with human skin, making them ideal candidates for integration as synthetic skin devices. The methodology is compatible with large-scale fabrication and employs simple, commercially available elastomers. The design of materials within the semiconductor layer has led to the incorporation of selectivity and sensitivity within gas-sensing devices and has enabled stable sensor operation within aqueous media. Furthermore, careful tuning of the chemical composition of the dielectric layer has provided a means to operate the sensor in real time within an aqueous environment and without the need for encapsulation layers. The integration of such devices as electronic mimics of skin will require the incorporation of biocompatible or biodegradable components. Toward this goal, OFETs may be fabricated with99% biodegradable components by weight, and the devices are robust and stable, even in aqueous environments. Collectively, progress to date suggests that OFETs may be integrated within a single substrate to function as an electronic mimic of human skin, which could enable a large range of sensing-related applications from novel prosthetics to robotic surgery.

The development of an electronic skin is critical to the realization of artificial intelligence that comes into direct contact with humans, and to biomedical applications such as prosthetic skin. To mimic the tactile sensing properties of natural skin, large arrays of pixel pressure sensors on a flexible and stretchable substrate are required. We demonstrate flexible, capacitive pressure sensors with unprecedented sensitivity and very short response times that can be inexpensively fabricated over large areas by microstructuring of thin films of the biocompatible elastomer polydimethylsiloxane. The pressure sensitivity of the microstructured films far surpassed that exhibited by unstructured elastomeric films of similar thickness, and is tunable by using different microstructures. The microstructured films were integrated into organic field-effect transistors as the dielectric layer, forming a new type of active sensor device with similarly excellent sensitivity and response times.

Flexible/stretchable electronic devices and systems are attracting great attention because they can have important applications in many areas, such as artificial intelligent (AI) robotics, brain-machine interfaces, medical devices, structural and environmental monitoring, and healthcare. In addition to the electronic performance, the electronic devices and systems should be mechanically flexible or even stretchable. Traditional electronic materials including metals and semiconductors usually have poor mechanical flexibility and very limited elasticity. Three main strategies are adopted for the development of flexible/stretchable electronic materials. One is to use organic or polymeric materials. These materials are flexible, and their elasticity can be improved through chemical modification or composition formation with plasticizers or elastomers. Another strategy is to exploit nanometer-scale materials. Many inorganic materials in nanometer sizes can have high flexibility. They can be stretchable through the composition formation with elastomers. Ionogels can be considered as the third type of materials because they can be stretchable and ionically conductive. This article provides the recent progress of soft functional materials development including intrinsically conductive polymers for flexible/stretchable electrodes, and thermoelectric conversion and polymer composites for large area, flexible stretchable electrodes, and tactile sensors.

Morphology control of solution coated solar cell materials presents a key challenge limiting their device performance and commercial viability. Here we present a new concept for controlling phase separation during solution printing using an all-polymer bulk heterojunction solar cell as a model system. The key aspect of our method lies in the design of fluid flow using a microstructured printing blade, on the basis of the hypothesis of flow-induced polymer crystallization. Our flow design resulted in a ∼90% increase in the donor thin film crystallinity and reduced microphase separated donor and acceptor domain sizes. The improved morphology enhanced all metrics of solar cell device performance across various printing conditions, specifically leading to higher short-circuit current, fill factor, open circuit voltage and significantly reduced device-to-device variation. We expect our design concept to have broad applications beyond all-polymer solar cells because of its simplicity and versatility., Solution printing is a desirable route for manufacturing organic solar cells, whilst the major challenge lies with morphology control. Here, Diao et al. use a microstructured blade to guide the solution flow during printing, which improves polymer crystallization and the resulting device performance.

Human skin is a remarkable organ. It consists of an integrated, stretchable network of sensors that relay information about tactile and thermal stimuli to the brain, allowing us to maneuver within our environment safely and effectively. Interest in large-area networks of electronic devices inspired by human skin is motivated by the promise of creating autonomous intelligent robots and biomimetic prosthetics, among other applications. The development of electronic networks comprised of flexible, stretchable, and robust devices that are compatible with large-area implementation and integrated with multiple functionalities is a testament to the progress in developing an electronic skin (e-skin) akin to human skin. E-skins are already capable of providing augmented performance over their organic counterpart, both in superior spatial resolution and thermal sensitivity. They could be further improved through the incorporation of additional functionalities (e.g., chemical and biological sensing) and desired properties (e.g., biodegradability and self-powering). Continued rapid progress in this area is promising for the development of a fully integrated e-skin in the near future.

Pressure sensitivity and mechanical self-healing are two vital functions of the human skin. A flexible and electrically conducting material that can sense mechanical forces and yet be able to self-heal repeatably can be of use in emerging fields such as soft robotics and biomimetic prostheses, but combining all these properties together remains a challenging task. Here, we describe a composite material composed of a supramolecular organic polymer with embedded nickel nanostructured microparticles, which shows mechanical and electrical self-healing properties at ambient conditions. We also show that our material is pressure- and flexion-sensitive, and therefore suitable for electronic skin applications. The electrical conductivity can be tuned by varying the amount of nickel particles and can reach values as high as 40 S cm−1. On rupture, the initial conductivity is repeatably restored with ∼90% efficiency after 15 s healing time, and the mechanical properties are completely restored after ∼10 min. The composite resistance varies inversely with applied flexion and tactile forces. These results demonstrate that natural skin's repeatable self-healing capability can be mimicked in conductive and piezoresistive materials, thus potentially expanding the scope of applications of current electronic skin systems. A supramolecular polymer with embedded nanostructured Ni particles shows mechanical and electrical self-healing capabilities as well as piezoresistive properties, making it a good candidate for electronic skin applications.

Conducting polymer hydrogels represent a unique class of materials that synergizes the advantageous features of hydrogels and organic conductors and have been used in many applications such as bioelectronics and energy storage devices. They are often synthesized by polymerizing conductive polymer monomer within a nonconducting hydrogel matrix, resulting in deterioration of their electrical properties. Here, we report a scalable and versatile synthesis of multifunctional polyaniline (PAni) hydrogel with excellent electronic conductivity and electrochemical properties. With high surface area and three-dimensional porous nanostructures, the PAni hydrogels demonstrated potential as high-performance supercapacitor electrodes with high specific capacitance (∼480 F·g -1 ), unprecedented rate capability, and cycling stability (∼83% capacitance retention after 10,000 cycles). The PAni hydrogels can also function as the active component of glucose oxidase sensors with fast response time (∼0.3 s) and superior sensitivity (∼16.7 μA·mM -1 ). The scalable synthesis and excellent electrode performance of the PAni hydrogel make it an attractive candidate for bioelectronics and future-generation energy storage electrodes.

Transparent, elastic conductors are essential components of electronic and optoelectronic devices that facilitate human interaction and biofeedback, such as interactive electronics, implantable medical devices and robotic systems with human-like sensing capabilities. The availability of conducting thin films with these properties could lead to the development of skin-like sensors that stretch reversibly, sense pressure (not just touch), bend into hairpin turns, integrate with collapsible, stretchable and mechanically robust displays and solar cells, and also wrap around non-planar and biological surfaces such as skin and organs, without wrinkling. We report transparent, conducting spray-deposited films of single-walled carbon nanotubes that can be rendered stretchable by applying strain along each axis, and then releasing this strain. This process produces spring-like structures in the nanotubes that accommodate strains of up to 150% and demonstrate conductivities as high as 2,200 S cm(-1) in the stretched state. We also use the nanotube films as electrodes in arrays of transparent, stretchable capacitors, which behave as pressure and strain sensors.

Single crystals of C60 have been widely prepared previously. However, their electronic properties are much less frequently studied, although C60 is known as an outstanding electronic material. Also, the reported electron mobility values (∼10−2 cm2 V−1 s−1) of C60 single-crystals are unexpectedly low possibly due to the difficulties in the fabrication of single-crystal devices. We have recently reported a droplet receding method for the solution-grown C60 single-crystals with mobilities above 1 cm2 V−1 s−1. In this work, we systematically investigate the effects of solvent and surface properties of the substrate on the growth of C60 single-crystals. Well-aligned C60 needle-like and ribbon-like single-crystals were grown from suitable solvents (m-xylene or a mixed solvent of m-xylene and carbon tetrachloride) conformally on the field-effect transistor (FET) substrates that were wet well by the receding droplet. Based on the ribbon-like single-crystals, an average electron mobility of 2.0 ± 0.61 cm2 V−1 s−1, Ion/Ioff > 106, and a VT between 36 and 85 V were achieved from 60 field-effect transistors. Insights provided by this work may help accelerate the development of solution-grown single-crystals of organic semiconductors.