4D gradient printing of light-responsive hydrogel-based composite actuators with programmable deformation

Developing soft actuators that mimic biological motion remains a key objective in robotics. However, the conventional multi-material 4D printing method is often susceptible to interfacial delamination and exhibits limited actuation performance. This study introduces a novel 4D gradient printing approach using direct ink writing (DIW) to fabricate composite actuators. A DIW-printable multi-walled carbon nanotube (MWCNT)/poly(N-isopropylacrylamide) (PNIPAm) composite ink was formulated by sodium alginate (SA) for rheological modification and MWCNTs for photothermal actuation and mechanical reinforcements. The bilayer four-armed light-responsive composite actuators were printed by programming the crosslinker concentration layer-by-layer. Upon near-infrared (NIR) irradiation, the gradient actuators exhibited rapid and large-amplitude bending deformations. By increasing the crosslinking gradient (the difference in the crosslinker content) from 0.5 wt% to 1.5 wt%, the bending curvature increased from 0.152 mm−1 to 0.508 mm−1 while achieving rapid actuation in 7 s. Furthermore, programmable shape morphing was demonstrated through two distinct ways: (i) localised NIR irradiation of selective arms, and (ii) global exposure of the entire hybrid actuator comprising gradient and non-gradient arms. This work establishes a versatile gradient printing approach for fabricating functional 4D-printed structures, paving the way for advanced applications in soft robotics and intelligent systems.