Wearable electronics is a new frontier of research to bridge the gap between rigid computational units and soft tissues. Unprecedented human-machine interfaces with new functions including physical and physiological sensors and personal cooling devices are developed based on novel soft materials such as conformal polymers, semiconducting nanomaterials, and electrocaloric polymers. However, there is still distance between a new functional material and a practical device in terms of reliability, cost, encapsulation, and lifetime. This dissertation focused on design, fabrication and verification of wearable electronic devices for human-machine interfaces enabled by new functional materials. Four selected topics are presented from chapter 2 through chapter 5.

Chapter 2 concerns tissue-like bioelectronics which offer an ideal means of interfacing with the body, but their implementation typically requires rigid shuttle devices that can cause additional scarring and tissue damage during implantation. To address this issue, we developed a self-softening polymer-based neural device that can record both electrochemical and neurochemical signals in vivo. The mechanically adaptive polymer is stiff at room temperature, boasting a Young's modulus of approximately 100MPa, and thus can be implanted without the need for a shuttle. Once implanted, the device becomes soft with a Young's modulus of roughly 10 kPa. A multi-modal device was created using this self-softening polymer, which combines electrophysiological recording with neurotransmitter biosensors. The device enables simultaneous recording of both electrophysiological signals and serotonin concentrations in vivo.

In the field of skin-attachable electronics, debonding-on-demand (DoD) adhesives are highly sought after, as they allow for repeated usage without damaging the skin. Chapter 3 developed a simple and versatile method for fabricating biocompatible bonding/debonding bistable adhesive polymers (BAPs) that exhibit conformal adhesion at skin temperature and easy detachment at room temperature. Additionally, the potential application of BAPs in a mechanosensitive communication system is explored. The BAPs are designed by incorporating stearyl acrylate (SA) and tetradecyl acrylate (TA) into a chemically cross-linked elastomer, which undergoes a semicrystalline-to-amorphous transition between 26 �C and 32 �C, leading to high adhesive flowability and significant energy dissipation. An optically transparent and mechanically compliant debonding-on-demand triboelectric nanogenerator (DoD-TENG) was also fabricated using the BAP as the DoD substrate, a polydimethylsiloxane (PDMS) elastomer as the electrification layer, and an ion-conductive elastomer as the electrode. This device can serve as a human-machine interface for a self-powered drone navigation system. This work is published and cited asGao, M.1, Wu, H.1, Plamthottam, R., Xie, Z., Liu, Y., Hu, J., ... & Pei, Q. (2021). Skin temperature-triggered, debonding-on-demand sticker for a self-powered mechanosensitive communication system. Matter, 4(6), 1962-1974.

Flexible and conformable transistors that incorporate semiconductive single-walled carbon nanotubes (SWNTs) have been extensively studied for biosensing applications. However, their sensing capabilities are often hampered by high electrolytic leakage currents, which negatively impact their detection abilities. While data processing can help to amplify the signals, it will also sacrifice sampling rates and leave the sensors vulnerable to fluctuations in the electrolyte solutions. To address these issues, chapter 4 introduces SWNT-based twin-transistors, where one transistor acts as a sensor and the other as a reference. Both transistors share gate and source electrodes, and all source/drain electrodes are sealed by a parylene layer to minimize electrolytic leakage. A common-source amplifier circuit generates voltage signal readouts from the sensor and reference transistors, and differential outputs enhance the signal-to-noise ratios by 92%. The arrays of twin-transistors were fabricated using microfabrication techniques, including photolithography and solution-based deposition of SWNTs, followed by transfer to a polyurethane substrate. To demonstrate glucose biosensing, glucose oxidase was immobilized onto the SWNTs in the sensor channels. This resulted in a sensor that can deliver real-time detection of glucose in human serum, exhibiting a 100% increase in normalized responses per decade of glucose concentrations between 100 μM to 100 mM. The response is proportional to the cubic root of glucose concentration, indicating that the redox electrons conducted by the nanotubes in the channel length direction contribute to the sensor response. Finally, the study demonstrated a portable glucose sensing system utilizing the flexible twin-transistors.

The demand for compact and flexible cooling technology has increased significantly in the thermal management of wearable electronics and personal comfort. Electrocaloric (EC) cooling holds great potential as a solution, but its low adiabatic temperature change has impeded its progress. However, chapter 5 developed a cascade EC cooling device that overcomes this bottleneck by increasing the temperature change while enhancing cooling power and efficiency. The device integrates multiple units of EC polymer elements and an electrostatic actuation mechanism that work in synergy. Each pair of adjacent EC elements function in antiphase, allowing for continuous heat flow from the heat source to the heat sink. This antiphase operation also facilitates internal charge recycling, which improves energy efficiency. By operating at the EC electric field with a 3.0 K adiabatic temperature change, a four-layer cascade device can achieve a maximum temperature lift of 8.7 K under no-load conditions. The coefficient of performance is estimated to be 9.0 at a temperature lift of 2.7 K and 10.4 at zero temperature lift. This work is published and cited asMeng, Y., Zhang, Z., Wu, H., Wu, R., Wu, J., Wang, H., & Pei, Q. (2020). A cascade electrocaloric cooling device for large temperature lift. Nature Energy, 5(12), 996-1002.