An iron-on electronic circuit to create wearable tech

Iron-on patches can repair clothing or add personal flair to backpacks and hats. And now they could power wearable tech, too. Researchers reporting in ACS Applied Materials & Interfaces have combined liquid metal and a heat-activated adhesive to create an electrically conductive patch that bonds to fabric when heated with a hot iron. In demonstrations, circuits ironed onto a square of fabric lit up LEDs and attached an iron-on microphone to a button-up shirt.

“E-textiles and wearable electronics can enable diverse applications from health care and environmental monitoring to robotics and human-machine interfaces. Our work advances this exciting area by creating iron-on soft electronics that can be rapidly and robustly integrated into a wide range of fabrics,” says Michael D. Bartlett, a researcher at Virginia Tech and corresponding author on the study.

The flexible, soft nature of fabric makes it difficult to integrate electric circuitry into smart clothing and wearable technology. For example, printed-on conductive ink circuits can be damaged when exposed to the elements, and sandwiching the liquid metal circuits between polymer sheets provides protection but not stretchability. So, Bartlett and colleagues wanted to try a different approach, combining liquid metals and rubber to produce a stretchable film that can be ironed onto fabric and conduct electricity.

Bartlett and the team added microscopic droplets of a gallium-indium alloy into a polyurethane mixture and poured the composite into a thin layer. After air-drying for a day, it formed a soft, electrically conductive elastic sheet. Then they ironed small rectangular pieces of the sheets onto various fabrics (plain weave polyester, cotton twill, knit spandex and mesh jersey). The polymer in the conductive patches created strong bonds with the fibers, keeping the layers together once they cooled.

To demonstrate the iron-on circuit’s potential applications, the researchers created two proofs of concept:

• Their university’s logo, ironed onto a piece of fabric, with five LEDs. The flexible circuit kept the LEDs lit when repeatedly folded, twisted and stretched.
• A stretchable wire microphone inside a shirt, with the ironed-on circuit relaying power and audio signal underneath the garment and out to an external recording device. The ironed-on microphone recorded sound across the full human hearing range with performance comparable to a traditional microphone setup, and with less noticeable bulk.

This conductive patch technology isn’t just for attaching rigid components such as LEDs, however. The researchers say it could also be integrated into soft circuits, soft robots and other wearable electronic devices.

The authors acknowledge support from the Office of Naval Research; Virginia Tech’s Institute for Critical Technology and Applied Science; and Virginia Tech’s National Center for Earth and Environmental Nanotechnology Infrastructure, which is supported by the National Science Foundation.

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