Stretchable thin film inductors for wireless sensing in wearable electronic devices

The unique soft and elastic nature of stretchable electronics has potential to advance wearable devices as human-machine interfaces. The integration of wireless power and data communication technology into stretchable electronics, which could be realised by inductive coupling and oscillator circuits, is key to achieve continuous monitoring of body signals with minimally invasive devices. As one of the main components for inductive coupling and oscillator circuits, the development of stretchable inductors is therefore compelling. The most common strategy to fabricate stretchable inductors is to add periodic waves to a spiral conductor, which provides mechanical robustness but inevitably increases resistance. In this work, we introduce a method to fabricate stretchable inductors, which relies on creating a wrinkled thin film inductor on a polystyrene substrate, functionalizing the inductor surface with an adhesive layer, and then transferring the structure onto a polydimethylsiloxane (PDMS) elastomer. Contrary to inductors created through the addition of periodic wave patterns, the wrinkled inductor features low resistance while providing high stretchability. The wrinkled inductors fabricated using this approach exhibited 30% decrease in resistance compared to their flat counterparts of the same size and geometry. Resistance and inductance under uniaxial stretching remained unchanged up to 45% strain, revealing exceptional electrical and mechanical stability. The strong chemical bonding between the functionalized wrinkled inductor and the PDMS elastomer contributes to the robustness and long-term stability of the device. This method provides an added advantage of miniaturization of the stretchable inductor, as it is shrunk to 16% of its original size during the wrinkling process. This technology has potential for building high performance stretchable inductors for stretchable wireless electronic devices and can eventually benefit the design of electronics for implants, health care monitoring and wearable communication.