'Electrical viscosity' of piezoresistive sensors: Novel signal processing method, assessment of manufacturing quality, and proposal of an industrial standard.

Piezoresistive sensors become increasingly important in wearable devices, specifically sensors printed with piezoresistive inks. The electrical viscosity of these sensors causes phenomena such as stress relaxation, creep and hysteresis. This compromises sensor calibration and leads to inaccurate results. When subjecting a sensor to loading and unloading, the subsequent calculation of the fractional time derivative of the conductance leads to the elimination of the hysteresis. The order of the fractional derivative equals the magnitude of the sensor's viscosity. In our study, the viscosity of several pressure sensors was determined with a material testing machine and ranged from 0.55% to 22.35%. We found that shunt-mode sensors are less viscous than through-mode sensors. In addition, if a viscous material is placed on top of the sensor, then its viscosity increases. This is important as some calibration devices for pressure sensor arrays use rubber bladders which are viscous. From the perspective of manufacturing, the variation of the viscosity across a sensor array accounts for the quality of the pressure sensor mat and should be kept to a minimum. An industry standard is proposed that calculates and reports the sensor viscosity based on loading and unloading of the sensor and calculating the fractional time derivative of the conductance, whose fractional order reduces the pressure - conductance hysteresis to zero and thereby corresponds to the sensor's viscosity.