Optical approach to resin formulation for 3D printed microfluidics

Microfluidics imposes different requirements on 3D printing compared to many applications because the critical features for microfluidics consist of internal microvoids. Resins for general 3D printing applications, however, are not necessarily formulated to meet the requirements of microfluidics and minimize the size of fabricated voids. In this paper we use an optical approach to guide custom formulation of resins to minimize the cross sectional size of fabricated flow channels as exemplars of such voids. We focus on stereolithography (SL) 3D printing with Digital Light Processing (DLP) based on a micromirror array and use a commercially available 3D printer. We develop a mathematical model for the optical dose delivered through the thickness of a 3D printed part, including the effect of voids. We find that there is a fundamental trade-off between the homogeneity of the optical dose within individual layers and how far the critical dose penetrates into a flow channel during fabrication. We also experimentally investigate the practical limits of flow channel miniaturization given the optical properties of a resin and find that the minimum flow channel height is similar to 3.5-5.5 h(a) where h(a) is the optical penetration depth of the resin, and that the minimum width is 4 pixels in the build plane. We also show that the ratio of the build layer thickness to h(a) should be in the range 0.3-1.0 to obtain the minimum flow channel height for a given resin. The minimum flow channel size that we demonstrate for a custom resin is 60 mu m x 108 mu m for a 10 mm build layer thickness. This work lays the foundation for 3D printing of <100 mu m microfluidic features.