Focus on stretchable electronics

Semiconducting single-walled carbon nanotube (SWNT) networks are promising for use as channel materials in field-effect transistors (FETs) in next-generation soft electronics, owing to their high intrinsic carrier mobility, mechanical flexibility, potential for low-cost production, and good processability. In this article, we review the recent progress related to carbon nanotube (CNT) devices in soft electronics by describing the materials and devices, processing methods, and example applications in soft electronic systems. First, solution-processed semiconducting SWNT deposition methods along with doping techniques used to achieve stable complementary metal-oxide-semiconductor devices are discussed. Various strategies for developing high-performance SWNT-based FETs, such as the proper material choices for the gates, dielectrics, and sources/drains of FETs, and methods of improving FET performance, such as hysteresis repression in SWNT-based FETs, are described next. These SWNT-based FETs have been used in flexible, stretchable, and wearable electronic devices to realize functionalities that could not be achieved using conventional silicon-based devices. We conclude this review by discussing the challenges faced by and outlook for CNT-based soft electronics.

Medical training simulations that utilize 3D-printed, patient-specific tissue models improve practitioner and patient understanding of individualized procedures and capacitate pre-operative, patient-specific rehearsals. The impact of these novel constructs in medical training and pre-procedure rehearsals has been limited, however, by the lack of effectively embedded sensors that detect the location, direction, and amplitude of strains applied by the practitioner on the simulated structures. The monolithic fabrication of strain sensors embedded into lifelike tissue models with customizable orientation and placement could address this limitation. The demonstration of 3D printing of an ionogel as a stretchable, piezoresistive strain sensor embedded in an elastomer is presented as a proof-of-concept of this integrated fabrication for the first time. The significant hysteresis and drift inherent to solid-phase piezoresistive composites and the dimensional instability of low-hysteresis piezoresistive liquids inspired the adoption of a 3D-printable piezoresistive ionogel composed of reduced graphene oxide and an ionic liquid. The shear-thinning rheology of the ionogel obviates the need to fabricate additional structures that define or contain the geometry of the sensing channel. Sensors are printed on and subsequently encapsulated in polydimethylsiloxane (PDMS), a thermoset elastomer commonly used for analog tissue models, to demonstrate seamless fabrication. Strain sensors demonstrate geometry- and strain-dependent gauge factors of 0.54–2.41, a high dynamic strain range of 350% that surpasses the failure strain of most dermal and viscus tissue, low hysteresis (<3.5% degree of hysteresis up to 300% strain) and baseline drift, a single-value response, and excellent fatigue stability (5000 stretching cycles). In addition, we fabricate sensors with stencil-printed silver/PDMS electrodes in place of wires to highlight the potential of seamless integration with printed electrodes. The compositional tunability of ionic liquid/graphene-based composites and the shear-thinning rheology of this class of conductive gels endows an expansive combination of customized sensor geometry and performance that can be tailored to patient-specific, high-fidelity, monolithically fabricated tissue models.

In this work, the successful integration of a-Si:H thin-film transistors (TFTs) and high-efficiency μ-iLEDs on large-area flexible substrates has been demonstrated. A conventional low-temperature a-Si:H TFT fabrication process combined with a laser lift-off transfer procedure was used to integrate μ-iLEDs with flexible TFT pixel circuits. Electrical and optical characterization showed the current-voltage and electroluminescence characteristics of the TFTs and LEDs did not change after integration onto the flexible platforms. This approach provides a potential methodology for creating flexible optoelectronic systems for wearable and large-area display applications.

The fabrication of multifunctional epidermal electronic devices capable of efficiently reading electrophysiological signals and converting low-amplitude mechanical signals into electric outputs promises to pave the way towards the development of self-powered wearable sensors, smart consumer electronics, and human-machine interfaces. This article describes the scalable and cost-effective fabrication of epidermal, nanotexturized, triboelectronic devices (EnTDs). EnTDs can be conformably worn on the skin and efficiently monitor electrophysiological signals, temperature, and hydration levels. EnTDs, while measuring electrophysiological signals, can also convert imperceptible time-variant body motions into electrical signals using a nanotexturized triboelectric layer, enabling the self-powered monitoring of respiration, swallowing, and arterial pulse. These results suggest the potential of EnTDs as a new class of multifunctional skin-like sensors for biomedical monitoring and self-powered sensing applications.

Copper nanowires (Cu NWs) are suitable material as an electrode for flexible, stretchable and wearable devices due to their excellent mechanical properties, high transparency, good conductivity, and low cost, but oxidation problem limits their practical use and application. In order to use Cu NWs as an electrode for advanced flexible, stretchable and wearable devices attached directly to the skin, the influence of the body temperature on the oxidation of Cu NWs needs to be investigated. In this paper, the oxidation behavior of Cu NWs at high temperature (more than 80 °C) as well as body temperature is studied which has been remained largely questionable to date, and an effective encapsulation method is proposed to prevent the oxidation of Cu NWs electrode in the range of body temperatures.