Fabricating Microstructures on Freeform Surfaces via Flexible Hydrogel Micromolds

Fabricating microstructures on curved surfaces is of great importance in several engineering fields, and it is an ongoing challenge to develop technologies that do not require complicated devices and cumbersome procedures. A novel approach is presented for fabricating microstructures with different topographies on curved surfaces using flexible microstructured hydrogels as molds. The hydrogel film is first microstructured via spatially controlled ultraviolet illumination utilizing its photo-responsive property. The microstructured hydrogel film is then transferred to a desired surface. Due to its low modulus, the hydrogel film can be adapted to the curved surfaces including semi-cylindrical and semi-spherical surfaces as well as freeform surfaces with both convex and concave features. The final freeform surfaces with microstructures are obtained by polydimethylsiloxane (PDMS) casting. The resulting microstructures show high uniformity and high surface smoothness with an average roughness (Ra) ≈2.7 nm and a root mean square roughness (Rq) ≈4.1 nm. The ability to fabricate microstructures on large-area curved surfaces of ≈4.5 cm by 6.5 cm is demonstrated. Finally, it is shown that the fabricated microstructures allow good imaging performance. This method utilizes the hydrogel's photo-responsive ability to construct flexible molds and flexibility to adapt to freeform surfaces, thereby allowing significantly simplified fabrication processes.