| Apr 19, 2022 |
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(Nanowerk News) The field of epidermal electronics, or e-tattoos, covers a wide range of flexible and stretchable monitoring gadgets that are wearable directly on the skin. We have covered this area in multiple Nanowerk Spotlights, for instance stick-on epidermal electronics tattoo to measure UV exposure or tattoo-type biosensors based on graphene; and we also have posted a primer on electronic skin.
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Taking the concept of e-tattoos a step further, integrating them with triboelectric nanogenerators (TENGs), for instance for health monitoring, could lead to next generation wearable nanogenerators and Internet-of-things devices worn directly on and powered by the skin.
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In work reported in Advanced Functional Materials (“Triboelectric Nanogenerator Tattoos Enabled by Epidermal Electronic Technologies”), researchers report a tattoo-like TENG (TL-TENG) design with a thickness of tens of micrometers, that can interface with skin without additional adhesive layers, and be used for energy harvesting from daily activities.
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The researchers report a maximum output voltage and current of TL-TENG are ∼180 V and ∼2.2 µA, respectively, and say that 30 LEDs could be lit up by gently tapping the TL-TENG.
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Furthermore, the advanced structural designed mechanics and adhesive nature of the device ensured excellent stability and mechanical properties that TL-TENG is able to endure different deformations, including stretching, twisting, and bending.
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As illustrated in the figure below, the TL-TENG’s layer-by-layer structure consists of polydimethylsiloxane (PDMS), polyimide (PI), copper (Cu), PI, and liquid bandage (LB) from up to down. The wire like appearance of the device comes from the thin serpentine copper wires (thickness, 200 nm; width, 200 µm). This serpentine design of the Cu layer provides space for deformation and increases its flexibility.
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| Figure 1: TL-TENG. a) Schematic illustration of the TL-TENG. b) Optical image of the TL-TENG mounted on a human skin surface (front view). c) Optical image of the TL-TENG mounted on a human skin surface (side view). d) Scanning electron microscope (SEM) image of the TL-TENG. e) Working mechanism of the TL-TENG. f) Optical images of the TL-TENG under stretching, twisting, and bending and their corresponding finite element analysis. (Reprinted with permission by Wiley-VCH Verlag)
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Figure 1e) illustrates the working principle the TL-TENG as it acts as an energy harvester for collecting mechanical energy from daily human body movements and turning into electrical energy.
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The Chinese authors note that the design of wearable tattoo-like electronics, apart from the mechanical and energy harvesting performance, should also take into account the visual appearance – if the device is visually attractive, users could be more willing to wear it. As an example, they design 12 versions of their TL-TENG, each in the animal shape of a zodiac sign.
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| Figure 2: TL-TENGs of the 12 Chinese zodiac animals. a) Optical images of the TL-TENG in a series of the Chinese zodiac with their corresponding area percentage, and finite element analysis respectively. The numbers at the corner represent their pattern coverage on a 75 mm x 50 mm PDMS. (Reprinted with permission by Wiley-VCH Verlag)
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As a demonstration of the TL-TENGs suitability as a human-machine interface control device, the researchers designed an arrow-like TL-TENG to serve as the remote-control of a small electric car with four arrows representing moving forward, backward, turning left, and right, respectively.
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The TL-TENG is attached on a volunteer’s forearm and the user could tap the corresponding arrows to control the remote-control car. In this set-up, Arduino is applied as the micro-controller (Arduino nano), sending commands via Bluetooth (WH-BLE103) so that the user could wirelessly control the remote-control car.
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| Figure 3: Top left) Optical image of the arrow-like TL-TENG for a remote-control car control. Top middle) Optical image of the arrow-like TL-TENG mounted onto a human forearm as the control system. Top right) Circuit diagram of the control panel. Middle and bottom row) Optical images of illustrating the controlling process and the optical images of the remote-control car, where finger tapping on different positions corresponds to “forward,” “backward,” “left,” “right,” and “stop”. (Reprinted with permission by Wiley-VCH Verlag)
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These applications indicate a promising potentiality of this TL-TENG design in human– machine interface applications.
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