A Biocompatible and Self-Healable 3D-Printed Bidirectional Hydrogel Actuator with Needle Injectability

Multifunctional hydrogels are highly desirable for emerging material applications, particularly for biocompatible hydrogel actuators. However, integrating toughness, self-healing, and reversible bidirectional actuation into a biocompatible actuator remains challenging. Herein, a 3D-printable and biocompatible bilayer hydrogel actuator with reversible bidirectional actuation is developed using a new poly(N-isopropylacrylamide)-gelatin methacryloyl (PNIPAM-GelMA; "PNG") hydrogel as the active layer. The photo-cross-linked PNG hydrogel shows self-healing ability as well as good elasticity (storage modulus ∼13 kPa) and toughness (linear viscoelastic range up to 240% shear strain). Small-angle X-ray scattering analysis for the microstructure of PNG reveals the presence of dynamic PNIPAM clusters composed of interlocking PNIPAM side chains, accounting for the self-healing behavior of the PNG hydrogel. The 3D-printed bilayer actuator with PNG as the active layer and GelMA as the passive layer exhibits bidirectional actuation and fine needle injectability. Moreover, pairing the PNG active layer with a self-healable passive layer (e.g., polyurethane-GelMA composite hydrogel) gives rise to a self-healable actuator. This actuator, repaired upon cutting, retains significant bidirectional bending angles (∼380° at 37 °C; ∼-270° at 25 °C). The multifunctional PNG system effectively addresses key limitations of current biocompatible hydrogel actuators by integrating toughness, autonomous self-healing ability, and reversible bidirectional actuation, offering substantial progress in developing actuators for biomedical applications.