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. Reversible and bidirectional self‐assembly of microstructures is realized by two‐photon‐lithography‐based 4D microprinting for reversible pattern/encapsulation, switchable wetting, and the dynamic actuation of microrobots/micromachines. In different evaporating solvents, the 4D‐printed microstructures can exhibit three reversible geometries: (I) curved state, (II) distance‐dependent coalesced clusters via traditional self‐assembly, and (III) distance‐independent arbitrary self‐assemblies (4D self‐assemblies) toward the curving directions.