Skin-Integrated Stretchable Electronic Skin for Motion Detection and Mechanical Sensing

Abstract

With remarkable mechanical characteristics and electronical properties, wearable electronics have attracted substantial attentions over the recent few decades. Electronic skin, as one kind of wearable electronics, are known as flexible, stretchable electronics, which can mimic the functions of human skin or animal skin. Inspired by human skin or animal skin, electronic skin could be utilized for motion detection and health monitoring. Research on electronic skin has made great progress, while different working mechanisms and materials are integrated and fabricated. As an indispensable component, power source acts as a significant role in electronic skin. According to the power supply method, electronic skin can be divided into two categories: self-powered electronic skin and external power supplied electronic skin. To provide sufficient and continuous power to the state-of-art electronic skin, many wearable self-powered technologies have been developed, of which triboelectricity and piezoelectricity are two primary mechanisms. Triboelectric nanogenerators (TENGs) provide a prospective alternative option to efficiently transform mechanical energy during human daily movement into electricity, which can be utilized for motion capturing and energy harvesting. Similar to TENGs, by forming piezoelectric materials into flexible and stretchable formats and integrating with soft substrate, piezoelectric nanogenerator (PENGs) would be a considerable strategy to fabricate electronic skin. While limited by the materials and structure, most of the reported TENGs and PENGs are not stretchable, which make them unsuitable to be applied in the field of electronic skin. In this thesis, two stretchable and skin-integrated self-powered electronic skins are investigated, based on triboelectric effect and piezoelectric effect respectively. Apart from the self-power technologies, external power is now the common energy source of electronic devices. Relying on the external power, piezoresistive effect and capacitive coupling are two main mechanisms for designing electronic skin, of which piezoresistivity based electronic skin has been widely investigated for its simple structure and principle. Nevertheless, most reported piezoresistive sensors are not stretchable, which confine them to be utilized as electronic skin. In this thesis, by simply transducing the external pressure or stretching into resistor signal and integrated with flexible substrate and advanced functional sensing material, a skin-integrated, stretchable piezoresistivity based electronic skin is studied.

First, we have developed a thin, skin-integrated stretchable triboelectric nanogenerator based on the contact-separation mode through a low-cost fabrication process. By adopting the serpentine designed Cu electrodes, the device has exhibited excellent flexibility and stretchability. To separate the two triboelectric layers, a flexible pillar array is built in the middle air chamber by screening printing, realizing the thin format of the TENG. Due to mechanical design, the TENG exhibits a wide pressure sensing range from ~ 8.125 kPa to ~ 43.125 kPa, corresponding to the open-circuit voltages ranging from ~ 10 V to ~ 80 V, allowing sensing to various external pressures, such as finger touching, tapping, and punching. At the external pressure of 43.125 kPa, the power output of the TENG could reach up to 300 μW/cm². Under a constant tapping by fingers, the energy yielded by the device could light 40 LEDs. Furthermore, a 4 × 4 arrayed TENG-based pressure sensor was further fabricated and demonstrates its potential applications in human motion monitoring and tactile mapping.

Second, a skin-integrated rubbery electronic device that associates with a simple low-cost fabrication method for a ternary piezoelectric rubber composite of graphene, lead zirconate titanate (PZT), and polydimethylsiloxane (PDMS) is introduced. Comparing to the binary composite that blend with PZT and PDMS, the graphene-embedded ternary composite exhibits a significant enhancement of self-powered behavior, with a maximum power density of 972.43 μW cm⁻³ under human walking. Combined experimental and theoretical studies of the graphene-embedded PZT rubber allow the skin-integrated electronic device to exhibit excellent mechanical tolerance to bending, stretching, and twisting for thousands of cycles. Customized device geometries guided by optimized mechanical design enable a more comprehensive integration of the rubbery electronics with the human body. For instance, annulus-shape devices can perfectly mount on the joints and ensure great power output and stability under continuous and large deformations.

Third, we have developed a thin, skin-integrated, and flexible electronic skin based on piezoresistive working mode. Ultra-thin PDMS substrate integrated with serpentine-like Cu electrode could avoid mechanical failing of the device when it is stretched, bended and twisted. By adopting the advanced function material, MXene, graphene and Ecoflex composite, our electronic skin could sense not only a wide range of pressure from 20.8 kPa to 132 kPa but also stretching rate from 0 to 20%, allowing the potential application in human motion capturing. Furthermore, a 4 × 4 arrayed electronic skin was fabricated, and demonstrates the prospective application in pressure mapping.

In summary, we have developed three kinds of electronic skins, based on triboelectricity, piezoelectricity and piezoresistivity, respectively. By using two kinds of low-cost and biocompatible materials, Cu and PDMS, through facile and economical screen-printing method, our S-TENG is fabricated with a wide sensing range of external pressures. Besides, Our S-TENG can be utilized in human body motion monitoring and energy harvesting. Furthermore, the device can be integrated on the shoe pads frontal to measure the signal from body motion of walking, running, jumping, and climbing stairs. Generally, our S-TENG indicates a great potential as an electronic skin in various practical applications, including mechanical energy harvesting and human motion monitoring. Then, we introduced a thin, soft piezoelectric device capable of laminating on nearly every part of the body, as a skin-integrated system for energy conversion from mechanical activity to electricity and strain sensing. This platform combined the advances in materials engineering and mechanical designing, provided a new route for realizing self-powered electronic skins. Simply introducing graphene into the PZT, PDMS blend films could effectively enhance the piezoelectric output performance and improve the flexibility, thus significantly lower the fabrication cost, simply the processing steps. Last, we fabricated a skin-integrated, stretchable, piezoresistive electronic skin. Ultra-thin PDMS substrate integrated with serpentine-shaped Cu electrode and utilizing MXene and graphene mixed with super soft rubber, Ecoflex, ensure the excellent flexibility and stretchability of our electronic skin. The sensing material, MX/Gr/EF was one-step screen-printed onto the Cu electrode, which is facial and economical. Our E-skin could be utilized in sensing both stretching and pressure, with a wide scope of sensing range. Furthermore, by mounted on human skin, our E-skin could sense not only human motions like touching, tapping, punching and finger bending, but also slight human motion as eye blinking.

Date of Award	29 Jun 2023
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Wai Chiu King LAI (Supervisor)