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

## TL;DR

A new biocompatible hydrogel actuator is developed that can self-heal, 3D-print, and actuate in two directions, making it suitable for biomedical applications.

## Contribution

A novel PNIPAM-GelMA hydrogel system is introduced that combines toughness, self-healing, and bidirectional actuation in a biocompatible actuator.

## Key findings

- The PNG hydrogel exhibits self-healing and high elasticity with a storage modulus of ∼13 kPa.
- The actuator achieves bidirectional bending angles of ∼380° at 37°C and ∼−270° at 25°C after self-healing.
- The system integrates toughness, self-healing, and bidirectional actuation, addressing key limitations in biocompatible actuators.

## Abstract

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.