Light-driven micron-scale 3D hydrogel actuator produced by two-photon polymerization microfabrication

Light-driven micro/nano-actuators are one of the most important topics in the biomedical micro-electromechanical systems (MEMS) field. Currently, their development is hampered due to the difficulties in designing and fabricating biocompatible light-driven microactuators with the dimensions less than one hundred micrometres and a response time in the order of seconds. In this work, gel photoresists were prepared by embedding photothermal surface-modified Fe 3 O 4 nanoparticles (NPs) into a mixture that included a photoinitiator, photosensitizer, monomers, crosslinkers and solvents. Macroscopic poly( N -isopropylacrylamide) (PNIPAM)/nano-Fe 3 O 4 hydrogels were prepared by ultraviolet photopolymerization of gel photoresists, which showed good temperature-responsive volume changes and light-triggered bending deformation. Then the two-photon polymerization (TPP) microfabrication properties of gel photoresists with 0, 0.48 and 0.95 wt% Fe 3 O 4 NPs were investigated in detail. Importantly, after the TPP microfabrication and subsequent solvent-exchange procedure, a double-armed near-infrared (NIR)-light-driven three-dimensional (3D) hydrogel microcantilever with a size of ∼26 μm was successfully fabricated. The hydrogel microactuator had a fast response time of ∼0.033 s in water under NIR radiation and showed good reversibility. Furthermore, the distance between the two arms of the hydrogel microcantilever could be manipulated by controlling the laser focus and incident laser power.