Flexible and Wearable Electronics in the Applications of Energy Harvesting and Sensing


Student thesis: Master's Thesis

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Award date29 Jul 2022


Over the past decades, flexible and wearable electronics have attracted worldwide attention due to their merits of high flexibility, stretchability, lightweight, and biocompatibility compared with traditional rigid electronics. They could be assembled with various interfaces conformally without scarifying their operating performance. These features allow wearable electronics to be applied in various potential applications in healthcare monitoring, bio-sensing, and human-machine interfaces. For most of electronics, a powering system is a necessity; however, traditional batteries are usually rigid and bulky, which hold the advantages of the flexible electronics back. To address this issue, the application of battery-free self-powered technology could be considered as a suitable breakthrough. Up to now, extensive self-powering systems have been explored based on various working principles, for example, electromagnetic generators, piezoelectric generators, and triboelectric nanogenerators (TENGs). Among them, TENGs are proven as a promising candidate in converting mechanical energy into electrical energy with their remarkable capability of various working modes, simple structures, and high-power generating performance, which allows them versatile ability to fulfil complex scenarios. In this work, the design, fabrication, and application of the flexible and wearable TENGs would be introduced in detail.

TENGs is an energy harvesting system with great potential, flexibility and simple operational mechanism. Since the different materials own various electronegativity, when they are brought close to each other, charges accumulate, and electrons transfer occurs once they are in contact, which generates a voltage output signal. In chapter 1, the details of the TENGs and flexible electronics would be introduced, including their working mechanism, design of the electrodes, material selection, potential challenges, the application of TENGs in flexible electronics, etc. In chapter 2, the design and fabrication of the flexible electronics and TENGs would be listed. With the ultrathin properties, the average thickness of the devices is at the micrometer level, and in order to turn some naturally rigid material into the stretchable format, some means are required, e.g. serpentine design. For ultrathin flexible electronics, photolithography is commonly applied in microfabrication to pattern parts on a thin film. With a thin layer of photoresist gel on the device, it is exposed in UV light with a cover of the desired pattern. Then, the photoresist gel, which is exposed under the UV light, would be dissolved, and the underneath material is exposed. After etching the exposed material, the device is then fabricated with the designed pattern, where the remaining photoresist gel would protect the underneath material from etching. Finally, the photoresist gel is dissolved, and an electrode with thin wires are fabricated.

Next, a series of my research works related to flexible and wearable TENGs would be introduced. In chapter 3, Graphene Coated Fabric Triboelectric Nanogenerator Based on Single-Electrode Mode for Bio-motion Energy Harvesting and Tactile Sensing is presented. It is a flexible, air-permeable graphene-coated fabric triboelectric nanogenerator (GF-TENG), which can be facilely sutured into clothes for mechanical energy harvesting. With its excellent electrical conductivity of graphene coated fabric, the GF-TENG demonstrates outstanding electrical characteristics that the open-circuit voltage and short-current outputs of ~213 V and ~3.12 μA, under a constant frequency and stress of 3 Hz and 6 kPa respectively. Combined with experimental and theoretical studies of the GF-TENG, it exhibits excellent mechanical tolerance towards bending and twisting, and it demonstrates a stable electrical output over hundreds of continuous working cycles.

In chapter 4, Tattoo-like Epidermal Electronics as Skin Sensors for Human Machine Interfaces is presented. It is an ultra-thin, skin-integrated designs of strain sensor with miniaturized dimensions, based on the piezoresistive effect, with excellent stability and robustness are introduced. Fractal curve-shaped Au electrode in a serpentine format, which is the dominant component of the strain sensor, is sensitive to the ambient strain variation and able to turn the mechanical motion into stable electrical signal output. With the advanced design of the metallic electrodes, the device presents good operational stability and excellent mechanical tolerance towards bending, stretching, and twisting. The stain sensor allows intimate mounting onto the human epidermal surface for detecting body motions. By adopting the liquid bandage (LB) as the encapsulation layers, the device owns an ultrathin thickness (6.2 µm), high sensitivity towards mechanical deformations, which is able to clearly detect motions such as walking, finger bending, and human pulse rate with identifiable electrical signals. Furthermore, the tattoo-like strain sensor is applied in robotic control by tracing finger bending motion and result in smooth control of a robotic hand nearly without any detention. The e-skin has exhibited a great potential in wearable electronics and the human-machine interfaces.

In chapter 5, Triboelectric Nanogenerator Tattoos Enabled by Epidermal Electronic Technologies, is presented. It is a series of ultrathin, soft, tattoo-like triboelectric nanogenerators (TL-TENGs) with well-designed aesthetic patterns, combining the advantages of both TENG and epidermal electronic technologies. With the ultrathin materials applied and state of art processing techniques in epidermal electronics, the TL-TENGs present an outstanding mechanical property of high robustness and thickness of tens of μm, which could be conformably interfaced with the skin surface like tattoos without additional adhesions. Besides, TL-TENGs own remarkable electrical characteristics, the open-circuit voltage and short circuit current can reach up to ~180 V and ~2.2 μA under constant tapping (~16 kPa), respectively. With the well structural mechanics designs, the TL-TENGs can be customized into various tattoo patterns, such as twelve Chinese zodiac signs.

To summarize, a series of self-developed TENGs is presented, the combination of flexible and wearable electronics and the TENGs demonstrates a great potential in various applications. They could be mounted on human skin surface for energy harvesting and motion sensing, indicating a superior potential in biomedical engineering and healthcare related industries.

    Research areas

  • Flexible and wearable electronics, triboelectric nanogenerators, E-skin, energy harvesters