Conductive Thermoplastic Elastomer Composite Capacitive Strain Sensors and Their Application in a Wearable Device for Quantitative Joint Angle Prediction

Abstract

Wearable devices can provide valuable information for the user when completing rehabilitation or attempting to improve athletic performance. Seamlessly incorporated soft strain sensors enable the correlation of strain to more complex metrics such as quantitative multiaxis joint angle motion tracking that is useful for analysis of activities such as walking, running, or cycling. Using a dip-coating technique, a capacitive filament was produced that achieved a relatively high performance with simple materials and fabrication processes. The design of the capacitor produced a sensor with a high baseline capacitance with low noise through the use of a combination of dielectric layers and interfaces between electrodes, good mechanical response through the use of a pristine core filament to support the capacitive sensor components, and a highly stable and reliable signal without requiring exotic or expensive conductive components aided by using specific electronics. The sensor was highly linear within the sensing region required in our proof-of-concept knee brace device, which could predict knee angles to within 1.79° with a simple and easy calibration protocol.