3D Printing of Gradient Hydrogel Actuators via Christmas‐Tree‐Shaped Modular Printhead Design

ABSTRACT Hydrogel actuators with programmable internal gradients show great promise. However, the inability to program multi‐layered compositional gradients within a single three‐dimensional (3D)‐printable filament restricts their internal stress management and overall actuation performance. Here, we report a direct ink writing (DIW) strategy using a Christmas‐tree shaped modular printhead to manipulate two precursor streams, creating multi‐layered architectures with cross‐sectional gradients within single continuous filaments. Using poly(N‐isopropylacrylamide) (PNIPAM) as a model, we program controllable crosslinker gradients that systematically govern the swelling‐ratio profiles after ultraviolet (UV) curing. Silica nanoparticles are incorporated for rheology tuning and subsequently removed to generate interconnected pores, thereby enhancing the swelling capacity. The resulting gradient filaments exhibit predictable and reversible bidirectional thermo‐responsive bending. Crucially, compared to conventional bilayers, the multi‐layered architectures effectively mitigate interfacial stress, accelerate deformation, and improve cyclic stability. A 5‐layered filament actuator was successfully assembled into an underwater biomimetic finger for object grasping. By enabling controllable cross‐sectional gradient programming, this approach provides a versatile platform for constructing high‐performance soft actuators and 3D shape‐morphing hydrogel systems.