Ultrafast and Programmable Shape Memory Hydrogel of Gelatin Soaked in Tannic Acid Solution.

Shape memory hydrogels have been paid plenty of attention as a kind of intelligent soft material. However, complicated preparation, slow and uncontrollable shape change have hindered their applications in smart actuators. In this work, a temperature responsive strong hydrogel was prepared by a facial soaking method without any chemical reactions, i.e., soaking gelatin hydrogel in aqueous tannic acid solution. The hydrogel was constructed by hydrogen bonding between gelatin and tannic acid besides the triple helix of gelatin without any chemical crosslinkers. The hydrogel showed ultrafast shape memory and body-temperature response. The hydrogel can be fixed in temporary shape in only 1 s at 25 °C and recover to the original shape in also 1 s at 37 °C, superior to the reported shape memory hydrogels. Furthermore, the hydrogel shape change can be programmed by fixing temperature, and the designed shape is achieved stepwise by adjusting recovery temperature. In addition, the hydrogel is stable in water without further swelling. These excellent features will initiate a new prosperity of the shape memory hydrogel in biomedical technology, underwater actuators, and soft robots.