Strain-Rate Response of Self-Powered Triboelectric Tactile Sensors

Triboelectric transduction of applied pressures offers an attractive approach to force sensing that can enable next-generation self-powered sensors. Self-powered sensing approaches are ideal for wearable and robotic “e-skin” applications, where the form factor restricts the ability to integrate batteries or other energy storage devices. For triboelectric sensing, an applied force causes either a change in contact surface area or induces a parallel sliding between two materials. Both phenomena can induce static charge transfer which can then be measurAlsoed as an open-circuit voltage. These phenomena have been explored previously, but without direct control of the rate of the applied force. The rate of deformation in the materials within the triboelectric series, however, plays a crucial role in the measured charge transfer and should be explored directly to accurately measure forces with varied temporal characteristics.<br/><br/>In this work, we have designed a triboelectric sensor which consists of interdigitated copper electrodes encapsulated in polyimide. On top of that sensor, we place a lightly lubricated PDMS layer. As a load is applied to the incompressible PDMS, it induces a lateral sliding effect between the PDMS and the underlying electrodes. This sliding motion causes a differential static charge between the two materials which can be measured as an open-circuit voltage. Specifically, we apply a normal force to strain the PDMS to 75% compressive strain at two strain rates, 50% s^-1 and 100% s^-1. In both scenarios, the 75% strain occurred at a stress value of approximately 5.5 – 6 MPa.<br/><br/>In all scenarios, during the loading phase the open-circuit voltage increases symmetrically on each electrode within the dual-electrode IDE and subsequently decreases during unloading with the maximum open-circuit voltage achieved at the peak strain. To increase the signal to noise ratio, we sum the voltages from each electrode within the IDE. While many studies then correlate the maximum open-circuit voltage directly to pressure, we find that this is highly dependent on strain-rate. For example, the maximum summed voltage at 75% strain (5.7 MPa) is 226 mV for the 50% s^-1 strain rate, while the maximum summed voltage at 75% strain (6 MPa) is 368 mV for the 100% s^-1strain rate. To overcome the strain-rate dependence, one must observe the temporally resolved voltage signal. We find that through integrating the open-circuit voltage over time we are able to correlate the electrical signal directly to the applied force irrespective of strain rate. The resulting voltage-integral with respect to applied pressure results in a quadratic trend with an average sensitivity (derived from the instantaneous derivative of the fitting quadratic function) of 19.27 mV-ms kPa^-1.<br/><br/>Ultimately, our work shows that through temporal analysis of the open-circuit voltage, a triboelectric sensor that directly senses applied pressures can be derived with minimal strain-rate dependence, ultimately allowing for self-powered transduction of applied forces. The elimination of strain-rate dependence, coupled with the simple fabrication of the device, can enable fully flexible and self-powered sensors for wearable and robotic sensing applications.