A Wearable and Unobstructive Qi-Compatible Wireless Charger

Abstract

Wearable wireless power transfer (WPT) systems offer untethered charging of wearable, implantable, and even mobile devices on the go, but face challenges in safety, efficiency, and wearability. We introduce a thin, stretchable, and safe hand band capable of watt-level wireless charging through the Qi protocol. The implementation of non-adhesive fabric encapsulation serves to protect the thin and spiral copper antenna from mechanical strain, ensuring an overall stretchability of 50%. We also created a stretchable “Ferrofabric”, characterized by a magnetic permeability of 11.3 and a tensile modulus of 75.3 kPa, that provides magnetic shielding for the antenna without compromising wearability. By addressing loss mechanisms such as the skin effect, proximity effect, core loss, and joule heating, we achieved a wireless charging efficiency of 72% and power delivery of 3.81 W in the kHz frequency range. Our WPT hand band is unobstructive to hand motion and can charge a handheld smartphone as fast as a desktop charger or power a battery-free chest-laminated e-tattoo, both with well-managed thermal and electromagnetic safety. Through a holistic electromagnetic, structural, and thermal design, our device culminated in a safe, rugged, and versatile solution for wearable WPT systems.

3D Woven Liquid Metals for High-Frequency Stretchable Circuits

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