A Stretchable Inductor With Integrated Strain Sensing and Wireless Signal Transfer

Objective: A soft and stretchable inductive sensor is presented with the ability to wirelessly transmit large strain measurements without the need for directly connected circuitry. Methods: The sensor is fabricated using a scaffold-removal technique to create microfluidic channels within an elastomer sheet (0.07 MPa modulus), which are injected with a non-toxic liquid metal alloy electrode. An analytical model is derived to predict the static inductance of the sensor while being uniaxially stretched and experimental characterisation of the sensor's dynamic response to axial and biaxial deformation is conducted, as well as wireless measurements within ex-vivo porcine tissue. Results: Experimental validation showed that the sensor's inductance change is linear with uniaxial and biaxial strains up to twice its original length. Strain-rate dependent hysteresis was negligible during slow deformation ( < {5} {s}^{-{1}} ) and below 10% during more rapid changes in length ( 15\,\,{s}^{-{1}} ). For wireless strain sensing, the variation of inductance in the sensor induced an increase in the transmitting voltage from 3.1 V to 5.7 V for strains up to 83%. While implanted within different layers of porcine tissue, the transmitter voltage linearly increased by 750 mV on average under bending deformation. Conclusion: Large stretching (83%) and bending ( 8\,\,{m}^{-{1}} ) deformations in the inherently soft sensor can be wirelessly captured, including through biological tissue. Significance: The wireless sensor's simplified structure, where the antenna and sensor are integrated into a single soft spiral electrode with modulus comparable to biological tissue, means that this novel design shows significant potential for implantable and wearable biomedical devices to monitor dynamic deformation.