Fig. 1. The process of producing perforated nanopores in a freestanding polymer membrane via NIL and polymer reflowing.

a A silicon microneedle array was molded into a thin double layer of polymer resist. The insert is an SEM image of an Si microneedle. b The mold was released from the imprinted substrate. The insert is a cross-sectional SEM image of the imprinted double resist layer. c The reflowing process was conducted to further reduce the pore size. d The patterned resist was exposed to UV for curing, followed by the release of the membrane from the substrate by dissolving the sacrificial layer. NIL, nanoimprint lithography; SEM, scanning electron microscopy