Capillary-Force-Driven Self-Assembly of 4D-Printed Microstructures.

Capillary-force-driven self-assembly is emerging as a significant approach for the massive manufacture of advanced materials with novel wetting, adhesion, optical, mechanical, or electrical properties. However, academic value and practical applications of the self-assembly are greatly restricted because traditional micropillar self-assembly is always unidirectional. In this work, two-photon-lithography-based 4D microprinting is introduced to realize the reversible and bidirectional self-assembly of microstructures. With asymmetric crosslinking densities, the printed vertical microstructures can switch to a curved state with controlled thickness, curvature, and smooth morphology that are impossible to replicate by traditional 3D-printing technology. In different evaporating solvents, the 4D-printed microstructures can experience three states: (I) coalesce into clusters from original vertical states via traditional self-assembly, (II) remain curved, or (III) arbitrarily self-assemble (4D self-assembly) toward the curving directions. Compared to conventional approaches, this 4D self-assembly is distance-independent, which can generate varieties of assemblies with a yield as high as 100%. More importantly, the three states can be reversibly switched, allowing the development of many promising applications such as reversible micropatterns, switchable wetting, and dynamic actuation of microrobots, origami, and encapsulation.