Flexible electronics: Soft liquid metal vias; stretchable TENG; thermoelectric film.
Researchers from Virginia Tech and University of Pennsylvania found a way to create soft, flexible electric connections through circuit layers. The method could be used for soft robotics and wearable devices.
The technique uses liquid metal microdroplets to create a stair-like structure that forms soft vias and planar interconnects through and across circuit layers without the need for drilled holes. This makes it more compatible with flexible boards, where a punched hole could stretch open as it deforms.
“The process involves the directed stratification of liquid metal droplets within a photoresin. By leveraging irregularities that arise during ultraviolet exposure, the researchers create a stair-like structure that allows the droplets to controllably assemble in 3D,” explained Virginia Tech’s Alex Parrish in a press release. “This approach is highly versatile, and these liquid metal vias and interconnects can be implemented in several types of materials. They can go further and perform the fabrication approach multiple times and create more and more layers.”
“By integrating with both in-plane and thru-plane circuit layers, it’s possible to create soft, flexible circuits with complex, multilayered architectures,” said Michael Bartlett, associate professor in the Department of Mechanical Engineering at Virginia Tech, in a release. “This enables new forms of soft electronics, where multiple soft vias and interconnects are created in a parallel and spatially controlled manner. This is crucial for advancing the field.” [1]
Researchers from Dongguk University developed a stretchable gel polymer-based triboelectric nanogenerator that generates electrical signals from body movement to power electronics like LEDs and functions as a self-powered touch panel for user identification. The device can stretch up to 375% of its original size and withstand rigorous mechanical deformations.
The team used a gel mixture of polyethylene oxide (PEO) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) poured into an ecoflex mold, with a copper wire attached for electrical connection.
The durable, flexible, and semi-transparent device generates electrical signals when tapped or stretched, delivering a peak power of 0.36 W/m² at a load of 15 MΩ. In tests, the device stretched up to 375% of its original size without damage and could withstand two months of bending, twisting, folding, and stretching without any signs of delamination or loss of electrical performance. The researchers see possibilities for use in wearable devices that track joint activity for rehabilitation purposes or act as a biometric system in clothing. [2]
Researchers from Queensland University of Technology, University of Queensland, and University of Surrey created an ultra-thin, flexible thermoelectric film that could power wearable devices using body heat. Applications include powering heart rate, temperature, or movement monitors.
“Flexible thermoelectric devices can be worn comfortably on the skin where they effectively turn the temperature difference between the human body and surrounding air into electricity. They could also be applied in a tight space, such as inside a computer or mobile phone, to help cool chips and improve performance,” said Zhi-Gang Chen, a professor of energy materials in the School of Chemistry and Physics at QUT, in a statement. “We created a printable A4-sized film with record-high thermoelectric performance, exceptional flexibility, scalability and low cost, making it one of the best flexible thermoelectrics available.”
The approach to making the flexible thermoelectric films uses nanobinders, tiny crystals that form a consistent layer of bismuth telluride sheets. The nanocrystals are formed in a solvent under high temperature and pressure. This is combined with screen printing for large-scale film production and sintering to bond the particles together. The technique is also applicable to silver selenide-based thermoelectrics. [3]
[1] Ho, D.H., Hu, C., Li, L. et al. Soft electronic vias and interconnects through rapid three-dimensional assembly of liquid metal microdroplets. Nat Electron 7, 1015–1024 (2024). https://doi.org/10.1038/s41928-024-01268-z
[2] Pandey, P., Seo, M., Shin, K.H. et al. In-situ cured gel polymer/ecoflex hierarchical structure-based stretchable and robust TENG for intelligent touch perception and biometric recognition. Chemical Engineering Journal, Volume 499, 2024. https://doi.org/10.1016/j.cej.2024.156650
[3] Chen, W. et al. Nanobinders advance screen-printed flexible thermoelectrics. Science 386, 1265-1271 (2024). https://doi.org/10.1126/science.ads5868
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