Forming three-dimensional micro-objects using two-dimensional gradient printing

Controllable transformations from two-dimensional (2D) patterns to three-dimensional (3D) geometries independent of materials or external stimuli are being pursued in numerous fields. Here, we present an approach to forming various 3D structures through 2D printing using distributed stress inside a polymer. The key is to establish controlled stress fields by introducing composition and property gradients inside a photocurable polymer by femtosecond laser two-photon polymerization. Structural deformation induced by internal stress is a general bottleneck both in materials processing and 3D printing. In contrast to the significant efforts previously made to reduce stress and deformation, we use them to enable shape transformation to construct various 3D micro-objects through 2D printing with engineered stress fields inside. Multi-mode 2D-to-3D structural transformations, including bending, rolling, coiling, waving, spiraling, and out-of-plane distortions are realized in a shape- and location-specific fashion. This strategy promises a unique way to fabricate delicate 3D objects not feasible through conventional techniques and to circumvent the intrinsic stepping limitations in direct 3D printing using two-photon polymerization. When combined with the standard 2D patterning techniques such as nanoimprint and photolithography, such a 2D-to-3D transformation approach will lay a foundation for high-throughput and cost-effective production of complex 3D nanostructures