Directed Capillary Self-Assembly of Mono- and Multi-Material Micropillar Arrays Fabricated by Two-Photon Polymerization

Abstract

Surfaces coated with microfilaments, such as micropillars, are of interest thanks to their large surface area and surface topography distinct from the underlying substrate. Micropillars are prone to capillary self-assembly during the development process. It is possible to direct these forces to obtain a highly controllable hierarchical microstructure. Previous works presented directed capillary assembly of micropillars for periodic pillar arrays obtained by molding or two-photon polymerization (TPP). However, these arrays were manufactured using a single power and are therefore only showing a uniform assembly. In this work, inspired by the concept of grayscale lithography, the fabrication of hierarchical architectures is achieved by combining TPP with controlled capillary force assembly by solvent evaporation. This also allows obtaining multi material pillar arrays by sequential fabrication. Various assemblies of polymeric micropillars with different geometric configurations can be achieved by adjusting the height or arrangement of micropillars, as well as by changing the printing parameters, such as laser power and printing speed to guide capillary force. This mechanical tunable self-assembly of microstructures can be used for surface modification applications, micro-optics, trapping of microparticles and microfluidics.