Gradient wood-derived hydrogel actuators constructed by an isotropic-anisotropic structure strategy with rapid thermal-response, high strength and programmable deformation

Stimulus-responsive hydrogels are considered to be promising platforms for soft actuators. However, simultaneous implementation of thermal-responsive hydrogel actuators with high mechanical strength, rapid response, controllable deformation and transparency remains an enormous challenge. Herein, a gradient wood hydrogel actuator with an isotropic thermal-responsive TEMPO-oxidized cellulose nanofiber/poly(N-isopropylacrylamide) (TOCN/PNIPAM) hydrogel embedded by an anisotropic delignified wood (DW) is fabricated through a facial one-pot polymerization. TOCNs enhance the mechanical properties of PNIPAM hydrogels and the interfacial bonding between hydrogels and DW. DW further improves the mechanical strength of hydrogels to 1.8 MPa. Compared with traditional PNIPAM hydrogel actuator with bilayer isotropy-isotropy structure, the wood hydrogel actuator with gradient isotropy-anisotropy structure can produce controllable and programmable deformations by cutting at different angles from the wood texture. Due to the simply embedded natural anisotropic wood, the thermal-responsive wood hydrogel actuator exhibits fast response speed (200 ° s −1 ) because of the entire hydrogel being active layer, allowing for a range of biomimetic behaviors. The transparent hydrogel enables information encryption under thermal stimulation, as well as rapid and repeatable information recording using ethanol writing. This programmable hydrogel provides a promising option for the materials of future intelligent biomimetic actuators, smart wearable devices and soft robots.