Programming Shape Transformations of Gel-based Micro-helices

Nature provides abundant examples of helical structures over many length scales, which have inspired scientists and engineers for centuries. In particular, hydrogel-based helices are of great interest due to their similarity to biological tissue and because of their stimuli-responsive behaviors. The underlying principles of helix formation have been studied in such areas as soft devices, biosensors, and biomimetic materials. However, the precise control of helical shape and chirality in the micrometer regime can be challenging, particularly using a simple one-step maskless process.

Here we present a new approach to fabricating shape-shifting micro-helices from homopolymer films of poly(acrylic acid) (PAA) by electron-beam lithography. The spatial distribution of energy deposited by an incident electron beam drives PAA crosslinking and creates gradients in the crosslink density. Once hydrated, these gradients create bending, buckling, and twisting deformations that generate a helix from a straight-line precursor scribed by the electron beam. The helical properties such as radius and chirality can be programmed by controlling the internal stresses from the asymmetric swelling via the incident electron doses and the intentional patterning of swelling asymmetries. Since PAA is a weak polyelectrolyte, the helical properties are responsive to changes in pH. Finally, we demonstrate the fabrication of microgels with more complex geometries including tendril-like coils and double helices.