Humidity-responsive actuation of programmable hydrogel microstructures based on 3D printing

•Programmable poly(ethylene glycol) diacrylate hydrogel microstructures are fabricated by 3D printing of two-photon photopolymerization.•The controllable humidity-driven swelling ability is achieved by adjusting the crosslinking density of voxels in the microstructure.•Binary codes micropillar array for information encryption storage has been presented by a two-steps fabricating process. The design and fabrication of devices that based on adaptive soft matter with the autonomous transduction of environmental and field signals is an interesting area of material science and device engineering. Additive manufacturing, also known as 3D printing, has gained great attention as it allows the creation of complex 3D geometries with precisely prescribed microarchitectures, which enable new functionalities or improved performance. Here, we report on poly(ethylene glycol) diacrylate hydrogel microstructures with excellent humidity responsiveness by 3D printing of two-photon photopolymerization. The voxels of fabricated hydrogel microstructures have controllable crosslinking density because adjusting fabrication parameters, therefore controllable humidity-driven swelling ability can be achieved. Using the proper parameters, we present an array of microstructures which can realize the function of nano-interconnected network and a hydrogel microstructure with pores to mimic the open and close of the stomata of plants. Based on a flexible two-steps fabrication method and the combination of active and inert materials, binary encoding micropillar arrays and joint-like cantilever microstructure have been easily fabricated. The humidity-responsive actuation of hydrogel microstructures is repeatable and stable over 10000 cycles. This kind of composite hydrogel microstructures may lead to great promise for the diverse applications such as sensors, actuators or construction of soft robots.