Skin plays an important role in mediating our interactions with the world. Recreating the properties of skin using electronic devices could have profound implications for prosthetics and medicine. The pursuit of artificial skin has inspired innovations in materials to imitate skin's unique characteristics, including mechanical durability and stretchability, biodegradability, and the ability to measure a diversity of complex sensations over large areas. New materials and fabrication strategies are being developed to make mechanically compliant and multifunctional skin-like electronics, and improve brain/machine interfaces that enable transmission of the skin's signals into the body. This Review will cover materials and devices designed for mimicking the skin's ability to sense and generate biomimetic signals.

Pursuing prosthetic electronic skin.

The development of advanced flexible large-area electronics such as flexible displays and sensors will thrive on engineered functional ink formulations for printed electronics where the spontaneous arrangement of molecules aids the printing processes. Here we report a printable elastic conductor with a high initial conductivity of 738 S cm−1 and a record high conductivity of 182 S cm−1 when stretched to 215% strain. The elastic conductor ink is comprised of Ag flakes, a fluorine rubber and a fluorine surfactant. The fluorine surfactant constitutes a key component which directs the formation of surface-localized conductive networks in the printed elastic conductor, leading to a high conductivity and stretchability. We demonstrate the feasibility of our inks by fabricating a stretchable organic transistor active matrix on a rubbery stretchability-gradient substrate with unimpaired functionality when stretched to 110%, and a wearable electromyogram sensor printed onto a textile garment. Printable electronics is highly desirable for high throughput device manufacture. Here, Matsuhisa et al. report an electric ink, made of a self-assembled network of sliver flakes on the surface of a fluorine rubber matrix, which exhibits high conductivity and mechanical durability to achieve this goal.

/pdf/printable-elastic-conductors-with-a-high-conductivity-for-31w716v25b.pdf

Printable elastic conductors with a high conductivity for electronic textile applications

Humans rely increasingly on sensors to address grand challenges and to improve quality of life in the era of digitalization and big data. For ubiquitous sensing, flexible sensors are developed to overcome the limitations of conventional rigid counterparts. Despite rapid advancement in bench-side research over the last decade, the market adoption of flexible sensors remains limited. To ease and to expedite their deployment, here, we identify bottlenecks hindering the maturation of flexible sensors and propose promising solutions. We first analyze challenges in achieving satisfactory sensing performance for real-world applications and then summarize issues in compatible sensor-biology interfaces, followed by brief discussions on powering and connecting sensor networks. Issues en route to commercialization and for sustainable growth of the sector are also analyzed, highlighting environmental concerns and emphasizing nontechnical issues such as business, regulatory, and ethical considerations. Additionally, we look at future intelligent flexible sensors. In proposing a comprehensive roadmap, we hope to steer research efforts towards common goals and to guide coordinated development strategies from disparate communities. Through such collaborative efforts, scientific breakthroughs can be made sooner and capitalized for the betterment of humanity.

Technology Roadmap for Flexible Sensors.

An organic flexible temperature-sensor array exhibits great potential in health monitoring and other biomedical applications. The actively addressed 16 × 16 temperature sensor array reaches 100% yield rate and provides 2D temperature information of the objects placed in contact, even if the object has an irregular shape. The current device allows defect predictions of electronic devices, remote sensing of harsh environments, and e-skin applications.

A Low-Operating-Power and Flexible Active-Matrix Organic-Transistor Temperature-Sensor Array.

In vivo electronic monitoring systems are promising technology to obtain biosignals with high spatiotemporal resolution and sensitivity. Here we demonstrate the fabrication of a biocompatible highly conductive gel composite comprising multi-walled carbon nanotube-dispersed sheet with an aqueous hydrogel. This gel composite exhibits admittance of 100 mS cm(-2) and maintains high admittance even in a low-frequency range. On implantation into a living hypodermal tissue for 4 weeks, it showed a small foreign-body reaction compared with widely used metal electrodes. Capitalizing on the multi-functional gel composite, we fabricated an ultrathin and mechanically flexible organic active matrix amplifier on a 1.2-μm-thick polyethylene-naphthalate film to amplify (amplification factor: ∼200) weak biosignals. The composite was integrated to the amplifier to realize a direct lead epicardial electrocardiography that is easily spread over an uneven heart tissue.

/pdf/ultraflexible-organic-amplifier-with-biocompatible-gel-2ku9g4g5ew.pdf

Ultraflexible organic amplifier with biocompatible gel electrodes

We successfully fabricated a large-area flexible strain-sensing system based on a 2-D array of organic self-bias-feedback amplifier with a signal gain of 400. The amplifier system consists of three layers: a self-assembled monolayer (SAM) capacitor matrix, a 2-D array of organic pseudo-CMOS inverters with a floating-gate structure using SAM gate dielectric, and an active matrix of organic thin-film transistors. The amplifier sheet comprises 8 × 8 amplifier cells, with an effective size of 7 × 7 cm2. The organic transistors exhibit a mobility of 1.7 cm2/V·s in the saturation regime at an operation voltage of 2 V. A strain sensor is made of a polymeric piezoelectric [polyvinylidene difluoride (PVDF)] sheet. When a cell of the PVDF sheet is touched (that is, when mechanical pressure is applied), a small signal is generated by intermolecular polarization in the PVDF. These signals are amplified by the organic amplifier circuits from 10 to 150 mV.

Sheet-Type Flexible Organic Active Matrix Amplifier System Using Pseudo-CMOS Circuits With Floating-Gate Structure

Frequency-dependent carrier dynamics in dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT)-based organic field-effect transistors and metal-insulator-semiconductor capacitors were studied by using admittance measurements. Both the real (conductance) and imaginary (capacitance) components of admittance, which have a mutually complementary relationship, were analyzed in the same way. On the basis of an experimental study of devices with different contact geometries, peaks observed in the conductance spectra corresponding to slopes in the capacitance spectra were classified into two groups. The high-frequency peak was attributed to the carrier injection from the top contact to the DNTT/insulator interface just underneath the contact while the low-frequency peaks were attributed to the drift current spreading all over the interface. The model calculation of carrier diffusion reproduces the low-frequency peaks very well. It is shown that the field-effect mobility from the accumulation region to the subthreshold region can be estimated by the fitting.

Alternating current admittance of DNTT-based metal-insulator-semiconductor capacitors

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Sheet-Type Flexible Organic Active Matrix Amplifier System Using Pseudo-CMOS Circuits With Floating-Gate Structure

Alternating current admittance of DNTT-based metal-insulator-semiconductor capacitors