Performance characterization of ionic-hydrogel based strain sensors

Ionic hydrogels, owing to the advantages of stretchability, conductivity and transparency, have attracted more attention for developing new soft sensors and artificial skins. Existing works on ionic-hydrogel based sensors mostly focus on material synthesis, structure design and functional integration, while few studies investigate the characterization of their sensing performances. In this paper, we present a method to characterize the performance (e.g., sensitivity, linearity and repeatability) of a kind of ionic-hydrogel based strain sensors by varying the testing frequencies and the sensors’ geometry (e.g., length-width (L/W) ratio). To this end, we first develop an experimental testing platform and fabricate a series of strain sensors made of the polyacrylamide (PAAm) hydrogel containing ionic conductive medium. We establish an equivalent electrical model to represent the ionic-hydrogel based strain sensors, indicating the influence of the testing frequencies and L/W ratio of the ionic hydrogels on their sensing performances. These theoretical predictions are in agreement with results obtained through experimental mea-surements. We further demonstrate that (1) the sensitivity of the strain sensors, characterized by the gauge factor (GF), increases with the rise of testing frequencies but tends to be stable over the frequency of 5 kHz; (2) the sensitivity GF has a nonlinear relation with the L/W ratio of the strain sensors, but with a certain maximum value under the same testing frequency when the L/W ratio equals to 4.80. We verify the above experimental observations with two commonly used electrolytes, including lithium chloride and sodium chloride. With the optimum testing frequency and L/W ratio, we finally conduct various experiments to demonstrate the low hysteresis and good repeatability of our ionic-hydrogel based strain sensors. This work provides an approach to characterize the performance of the ionic-hydrogel based strain sensors, which may be an important step forward in further applications of ionic hydrogels in soft robotics.