Hydrogel-Based Flexible Electronics Devices as Human Motion Sensors and Triboelectric Nanogenerators

Abstract

Flexible electronics is an emerging discipline in recent years because of its outstanding flexibility. It can be adapted to particular scenarios such as folded or bent, where conventional electronics are complex to apply. Flexible electronics are proliferating in areas, such as sensing, biomedicine, and energy harvesting. However, its development still needs to be improved due to weak biocompatibility, degradability and difficult-to-regulate mechanical properties. Flexible electronic devices need a new material that overcomes the above drawbacks to facilitate their development. Hydrogel is a kind of 3D cross-linked hydrated polymer network whose Young's modulus is close to the biological tissue. Its environmentally friendly properties make it a strong candidate for the next generation of flexible electronics. Currently, hydrogel-based flexible electronic devices are used as sensors for physicochemical and biological fields and energy harvesting and storage devices. This thesis focuses on applying hydrogel based flexible electronic materials in human motion sensing and as triboelectric nanogenerators to power electronic devices. In Chapter 2, flexible and mechanically strong wood-based hydrogels were successfully synthesized by combining cellulose backbone with PVA-Borax-TA hydrogel solution. It exhibits enhanced mechanical properties with a breaking strength of 19.8 MPa and it also has a good self-healing ability and excellent water retention properties, and its response time is as fast as 20ms when used as a flexible motion sensor. In Chapter 3, gelatin-based hydrogel was synthesized through a liquid nitrogen freezing process, after which the properties of the hydrogel were precisely controlled by adding different salt ions. A triboelectric nanogenerator (TENG) was then made by encapsulating the hydrogel in silicone rubber. The device has an open-circuit voltage of up to 160 V and a short-circuit current of 2.6 μA, as well as a tensile fracture strength up to 1.5 MPa. In addition, it is also highly biocompatible and harmless to the human body, and it is a multifunctional material.