Multi-segment Stretchable Strain Sensor using Time Domain Reflectometry

Strain sensors are critical for wearable electronics and soft robotics applications, where knowledge of mechanical stress and linear stretch are essential. In this work, we present a multi-segment strain sensor using liquid metal paste to create a single pair of conductors in a stretchable substrate made of silicone. The sensor is designed as a piecewise transmission line, where each segment has a different characteristic impedance. We use a time domain reflectometry measurement method as an alternative to typical resistive or capacitive strain sensors. The length of each segment is determined by introducing a voltage pulse at one end of the sensor and measuring the round-trip time-of-flight of resulting reflected voltages caused by the discrete impedance discontinuities. Active electronics are only needed at one end of the sensor. In addition, it is linear and highly stretchable. Here we present the theory behind the measurement technique, brief sensor fabrication methods, and measured results from tested 3-segment and 5-segment strain sensors. Long-term, this approach may enhance strain sensor readout for wearable electronics, including biomechanical measurements and gesture recognition, as well as incorporation into soft robotic systems.