Elastomeric Ionic Hydrogel Sensor for Large Strains

An elastomeric hydrogel membrane containing dissolved salt is demonstrated to be a linear strain sensor for large strains up to 25%. AC sensing circuitry allows the ionic conductor to function as a sensor without polarization effects. The homogeneous gel is shown to be lacking a relaxation or hysteresis response found in the soft composite piezoresistors that are commonly used for large strain sensing. This allows operation of the sensor with minimal post-processing and increased accuracy during real-time operation. The gauge factor of the gel sensor is shown to be near two, consistent with the observation that the strain sensing of the material is solely derived from the deformation of the bulk. The very low Young's modulus of the gel (~1 kPa) allows deformation of a fabric substrate to be followed without significantly constraining the fabric. A prototype sleeve that uses these sensors for proprioceptive sensing of joint angle motion on a robotic arm is demonstrated as a potential application. Water loss from the hydrogel due to evaporation from the surface is slow. We show that the resulting trend in the impedance of the sensor can be easily corrected by simple signal processing.