Abstract
1. Introduction
Skin is the largest organ featuring soft mechanical properties and a complex sensory network that interfaces with the environment, which serves as an inspiration for the development of next-generation flexible electronics[
To this end, skin-like properties including mechanical stretchability, self-healing capability, and biodegradability have been incorporated into new classes of electronic materials[
In this review, we focus on recent progress of skin-inspired electronics including material selection, device design, and system improvement. We summarize advances, with particular emphasis on semiconductor materials and devices. Several strategies for biomimetic sensors, and advanced system-level functional improvement along with applications in robots/prostheses, human-machine interfaces, and healthcare monitoring are discussed. We conclude with the remaining challenges and potential future directions of skin-inspired electronics.
2. Semiconducting materials with skin-like properties
Skin-like properties, including stretchability, self-healing capability, biocompatibility, and biodegradability, have been demonstrated in a new class of semiconductor materials to enable their wide application in skin-inspired electronics. These skin-like features are primarily based on new material concepts or innovative engineering approaches.
2.1. Stretchability
Typical biological skin has a modulus of 140–600 kPa and a maximal stretchability up to 75% tensile strain[
Strain engineering is a facile approach to enable stretchability in conventionally rigid and brittle materials. Inspired by wrinkles and creases of natural skin, geometry designs like kirigami, serpentine, buckling, and microcracks are utilized to create stretchable semiconducting materials[
Intrinsically stretchable semiconducting materials can be stretched without degradation in their electronic performances, which is attractive for skin-inspired electronics to achieve stable and robust performances. Polymers are among the typical examples of stretchable semiconducting materials formed through standard solution processes[
Figure 1.(Color online) (a) Chemical structures of semiconducting polymers and mechanism for improved stretchability via dynamic bonding. (b) Schematic illustration of the embedded nanoscale polymer networks (top) and chemical structures of semiconducting polymer and elastomer (bottom). Panel (a) adapted with permission from Ref. [
Blending of functional nanomaterials into elastomers allows facile fabrication of stretchable composites with electrical conductivity and stable cycling[
2.2. Self-healing
Self-healing capability is a unique feature of natural skin to enable autonomous self-repair after damage[
Generally, self-healing capabilities are achieved through reversible bonds (e.g., hydrogen bonding or dynamic covalent bonding), dynamic interactions (e.g., host–guest interactions, π–π interactions, ionic interactions, and electrostatic interactions), and microcapsule-based composite with embedded self-healing agents[
Figure 2.(Color online) (a) Schematic illustration of the self-healing process of the polymer through reversible ion–dipole interactions. (b) Schematic illustration of the healing process of the treated polymer films (left) and transfer curves and mobility of the damaged and healed OTFTs (right). (c) Schematic of the self-healing composite of polymer networks and micro-nickel particles. Panel (a) adapted with permission from Ref. [
To realize fully self-healable devices, functional components with autonomous healing capabilities without any external stimuli are expected to be developed concurrently[
2.3. Biodegradability
Biodegradable materials are preferred in skin-inspired applications, which enable the devices to degrade into harmless components after their service life. It is essential for transdermal or implantable devices to avoid side effects for biomedical applications[
Biodegradable materials like metals (e.g., Mg, Zn, Mo, and Fe) and silicon-based technology is an interesting option for transient integrated devices with high hydrolysis rate but without any harmful products[
Figure 3.(Color online) (a) Schematic of silicon-based transient electronics on silk substrate (top) and dissolution process in water (bottom). (b) Flexible devices with disintegrable polymers as the active material and substrate (left) and images of a device at various stages of disintegration (right). Panel (a) adapted with permission from Ref. [
Decomposable conjugated polymers have emerged as another attractive choice for transient electronics[
Based on reversible imine chemistry, Lei et al. synthesized a biocompatible semiconducting polymer that is readily decomposable in mild acidic environments (Fig. 3(b))[
3. Skin-inspired devices and systems
Based on material selection and novel device design, artificial sensors imitating mechanisms and structures of natural skins, along with flexible circuits, are achieved. To go further beyond single-modal sensor unit, intelligent functionalities are incorporated to realize skin-like electronic devices and systems.
3.1. Sensors
Pressure sensing is a common and vital function to convert mechanical stimuli into electrical signals, which has been extensively studied. As an example, Lipomi et al. fabricated a transparent pressure and strain sensor based on elastic films of CNTs (Fig. 4(a))[
Figure 4.(Color online) (a) The working mechanism of the CNT-based pressure and strain sensor (left) and the array design (right). (b) Schematic and image (inset) of a hierarchically pyramidal-structured pressure sensor. (c) The fabrication step of pressure-sensitive polymer transistor with a microstructured PDMS dielectric layer. (d) Schematic of a chameleon-inspired e-skin. (e) Images of a Si nanomembrane diode sensor array with magnified views of a single sensor. Panel (a) adapted with permission from Ref. [
Schwartz et al. integrated microstructured PDMS dielectric with polymer transistor to amplify the capacitance responses and enable its operation in the subthreshold regime (Fig. 4(b))[
In natural skins, ionic mechanotransduction contributes to the conversion of mechanical stimuli into biochemical signals[
Temperature sensing is another vital function of the skin sensory system. Wearable skin thermography represents a complementary technology for traditional infrared imaging and point-contact sensors. Conventional measurement is typically based on temperature coefficient of resistance (TCR) that requires complicated readout circuits for amplification[
Considering that natural skin is responsive to various stimuli (e.g., pressure, strain, temperature, and humidity) simultaneously, multiplex sensors are desired for various wearable applications. Multimodal measurement can provide sufficient information to establish interconnections because some signals may be unavoidably affected by others[
Future efforts may be the integration of biochemical sensors into the skin system, which may target metabolites (e.g., glucose and lactate) and electrolytes (e.g., sodium and potassium ions)[
3.2. Transistors
Organic field-effect transistors (OFETs) represent the building blocks for complex circuit systems for skin-inspired electronics[
Figure 5.(Color online) (a) A photo (left) and structure (right) of a 300-nm-thick electronic skin, with an OFET and tactile sensor per pixel. (b) Schematic of a strain sensor array based on piezoelectric nanogenerators and coplanar-gate graphene transistors. (c) Images of intrinsically stretchable transistor array (left, scale bar: 1 mm), amplifier in its initial and stretched state (middle), and use of the amplifier for arterial pulse signal measurement. Panel (a) adapted with permission from Ref. [
A transistor-based matrix exhibits advantages in imitating skin that consists of thousands of sensory units due to advanced sensing performances, minimized scale, large-scale integration, and lower signal cross-talk as compared with simple resistive or capacitive sensors[
In addition to simply recording human vital signs, transistor-based sensing devices are also favorable for in-sensor signal processing, such as direct signal amplification and noise elimination[
Soft electronic devices have shown capabilities of acquiring high-quality biosignals due to the lowered interfacial impedance, reduced signal distortion, and high signal-to-noise ratio (SNR). Sugiyama et al. designed an ultraflexible organic differential amplifier for signal amplification and noise reduction, which allows the recording of weak electrocardiogram (ECG) signals with high signal integrity and sensitivity[
3.3. Circuit systems
For skin-inspired electronics, high-density functional circuits play an important part in signal collection, processing, and transmission, which pave the way for applications in implantable sensors, tissue engineering, and soft robotics[
The key challenges for the development of flexible circuit systems lie in thin-film interconnects and circuits compatible with flat encapsulations[
Figure 6.(Color online) (a) A multifunctional epidermal electronics (left) and its integration with tattoo. (b) Design of CNT TFT device (left) and photos of devices attached conformally to human skin (right). (c) Schematic illustration of a digital tactile system composed of flexible organic circuits. Panel (a) adapted with permission from Ref. [
In addition to the design of circuit layout, 3D integration is another efficient method to create stretchable electronic systems with complex functions[
In spite of the stretchability and flexibility for the overall system, the rigid island design has to deal with the complicated layout and fabrication process. The robustness of the circuit is often compromised by the weak interfaces between hard and soft components. Fully stretchable circuits based on compliant materials exhibit the inherent advantage in terms of the resilience[
3.4. Integrated skin-like functional systems
Skin-inspired electronics are attractive for various emerging fields such as wearable healthcare monitoring, soft robotics, and human–machine interface[
(1) Wireless data transmission
The trend for the next-generation electronic skin applications is developing wearable, on-demand, non-invasive, transdermal devices that detect physiological signals in real time[
Figure 7.(Color online) (a) Schematic illustration (left) and photo (right) of a bodyNET with on-skin sensors and flexible circuits on clothes. (b) A piezoresistive sensor array consisting of 548 sensors covering the entire hand. (c) A self-healable electronic skin system composed of sensors and display. Panel (a) adapted with permission from Ref. [
(2) Large-scale array
Electronic skin may serve as the enabler of tactile sensation for humanoid robots and prostheses[
Large-scale, high-resolution tactile datasets bring about abundant information for advanced signal processing tools such as machine learning[
(3) Closed-loops with feedback
For artificial intelligent electronic skin, sensory feedback is critical in building closed-loop control systems. For example, the effective and precise control of our hands largely rely on the sensory feedback and feedforward motor commands[
Another important step towards a smart electronic skin system is closed-loop system with instant smart drug delivery for point-of-care (PoC) healthcare applications[
4. Conclusion and outlook
In summary, we have reviewed recent progress in skin-inspired electronics from the aspect of semiconducting materials, sensor devices, and advanced systems. Inspired by the structures and mechanisms of living organisms, recent progress in soft materials and flexible electronics is beneficial for the achievement of skin-like materials and biomimetic sensors. An intelligent skin electronic system with advanced functions and capabilities like wireless data transmission, data collection of large-area arrays, and in-time feedback is explored to favor the real-world applications in fields such as wearable healthcare monitoring, human–machine interfaces, and smart robots/prostheses.
Several limitations still exist that require further studies including but not limited to the following aspects: (1) Enhancement of the functional characteristics. The flexibility, stretchability, biocompatibility, as well as sensing performances (including sensitivity, response speed, stability, anti-interference ability, and multimodal capability) need to be further improved for different application settings of wearable and implantable devices[
Acknowledgments
This work was supported by the National Natural Science Foundation of China under Grants 61825403, 61674078, and 61921005, the National Key Research and Development program of China under Grant 2017YFA0206302, and the PAPD program.
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