Gradient porous PNIPAM-based hydrogel actuators with rapid response and flexibly controllable deformation

Smart hydrogels play a vital role in fundamental research and industrial applications in the fields of biosensors, flexible devices and intelligent human–machine technologies; however, developing a simple, low-cost and large-scale method to obtain hydrogel actuators with rapid response and robust and steadily controllable motion remains a big challenge. In this work, a temperature-responsive, gradient structured hydrogel with quick bending and adjustable actuation was fabricated by the copolymerization of the N-isopropylacrylamide (NIPAM) monomer with dispersed montmorillonite (MMT) via a facile precipitation method. The introduction of MMT with good thermal conductivity made the volume phase transition of the PNIPAM-based hydrogel occur earlier, and the deformation degree and bending direction could be adjusted by changing the MMT content. The representative composite hydrogel with 0.2 g MMT in the precursor presented bidirectional bending characteristics with a bending amplitude of about 289° and an average bending speed of about 36.0° s−1, while the composite hydrogel with 0.3 g MMT in the precursor only possessed a unidirectional bending ability with a bending amplitude of about −259° and a bending velocity of about −28.8° s−1 due to the great difference in the shrinking capacity between the top and bottom sides of the composite hydrogel. In addition, flexibly controllable deformation was specially realized by a well-designed patterned hydrogel with a local component and a thickness difference. Our work provides a practical method for the ingenious design of a hydrogel for further development of programmable and versatile hydrogel-based smart actuators and soft robots.