Bionic light-responsive hydrogel actuators with multiple-freedom motions in water environments

Soft actuators with biomimetic behavior have widely been utilized in intelligent robotics and artificial muscles. Herein, tannic acid (TA) non-covalently modified and gamma-methacrylic trimethoxysilane (MPS) covalently modified MXene (MPS-TA-MXene) are incorporated into a poly(n-isopropylacrylamide-polyacrylamide) (PNIPAM-PAM) copolymer hydrogel to yield a light-responsive hydrogel nanocomposite, denoted as MNA. TA effectively preserves the surface characteristics of MXene while the double bonds in MPS form covalent bonding between MXene and PNIPAM-PAM copolymer, ensuring uniform dispersion of MPS-TA-MXene in MNA hydrogel. Under exposure to ultraviolet light, the resulting MNA hydrogel actuators with nematode- or starfish-shaped exhibit not only bending capabilities in various directions and angles but also demonstrate remarkable multiple-freedom rolling behavior in water environments due to their volume shrinkage induced by the phase transition of MNA hydrogel. Overall, the proposed simply assembled MNA hydrogel actuators with multiple and continuous bionic motions, distinguishing them from conventional single soft actuators limited to bending in one specific direction or performing a simple singular action of rolling, look promising for use in underwater exploration.