Characterization of Hydrogel Curing Methods for Manufacturability

In the field of soft robotics, hydrogels possess material properties that allow them to function as both soft strain sensors and electrodes for dielectric elastomer actuators. However, there is still much that needs to be understood about the curing process of hydrogels and the resulting material characteristics before devices can be reliably manufactured. In this study, we investigated the effect of curing temperature on the as-cured material properties of acrylamide-based hydrogels hydrated with lithium chloride and magnesium chloride salt solutions. Samples were cured at room temperature, 60◦C and 100◦C, and the resulting changes in mechanical stiffness, rehydration rate, and electrical conductivity were measured. We found that stiffness decreases with increasing curing temperature, and the rehydration rate and electrical conductivity had an erratic relationship with curing temperature. These hydrogels were also shown to function as strain gauges. To determine under what conditions hydrogel would spontaneously cure at room temperature, uncured hydrogel was deposited in drops between 1 L and 0.5mL. We found that uncured hydrogel did not spontaneously cure when placed in volumes smaller than 20 L at room temperature. These samples did, however, cure at elevated temperatures. These experiments show that hydrogels have material properties and curing times that can be tuned depending on the needs of the manufacturing process, and that conductive hydrogels retain their required functionality after prolonged use at ambient temperatures.