Programming stretchable planar coils as strain-invariant inductors and ultrasensitive wearable sensors

Stretchable planar coils play increasingly important roles in flexible electronics, from wireless antennas, electrical components, to inductive sensors. Understanding the governing law that affects the inductance-strain behavior is of critical importance. In this paper, we identify that aspect ratio (AR) is the only crucial design parameter, with rigorous numerical analysis and experimental validation. The inductance response of the stretchable planar coil can be controlled during the strain process, and the strain sensitivity can be tailored across 3 orders of amplitude. A strain-invariant stretchable coil is designed with a maximum inductance change less than 1 % when stretched by 50 %, and is demonstrated for wireless power transfer and wireless communication. In addition, high aspect ratio coils are designed as inductive strain sensors with hysteresis free, temperature and pressure invariant, linear response to strain over 100 %, and a detection limit down to 0.01 % strain. We demonstrate that the inductive strain sensors worn on the forearm enable imperceptible monitoring of fine finger movements, muscle fatigue, response time, grasping force and size, and hand gestures.