Tailoring polymer material properties in microstereolithography

Abstract

We present a library of resin formulations for projection micro-stereolithography (PµSL) consisting of 4-hydroxybutyl acrylate (HBA), poly(ethylene glycol) methyl ether methacrylate (PEGMEMA) and poly(ethylene glycol) diacrylate (PEGDA), diluted with aqueous solutions of the photoinitiator lithium phenyl(2,4,6-trimethylbenzoyl)phosphinate (LAP) and the photoabsorber tartrazine. By varying the concentration and molecular weight of PEGMEMA and PEGDA, the swelling ratios of as-PµSL-printed hydrogel microstructures in water are well tunable with a reversible volume increase ranging from 13% to 86%. Furthermore, we illustrate the influence of exposure time per 3D-printed layer on the swelling ratio of the hydrogels, as well as the swelling time. The minimum feature size of rectangular void structures achieved with an exemplary resin from our material library is approx. 71 μm, while rectangular microchannels at the surface of a PµSLprinted hydrogel made from the same photopolymer formulation exhibit cross-sectional dimensions designed at 54 μm x 50 μm. Based on this initial characterization, microfluidic devices are fabricated to elucidate dimensional changes of microchannels under different swelling conditions (e.g., free swelling and confined swelling inside a chamber or microfluidic device). In addition, we PµSL-print microscopic parts with tailored geometries (cylindrical, pyramidal) that are capable of completely closing microfluidic chambers made from commercially available Perfactory R11 resin in a time-dependent fashion. Our resin library provides 3D-printed hydrogels with micron-scale feature size combined with tunable water uptake, rendering them suitable for designing functional microfluidic units such as membranes, valves and pumps.