Extreme Size and Irradiance Dependence in High-Resolution Vat Photopolymerization of Hydrogels

Vat photopolymerization is a high-resolution and high-throughput technology used in many biomedical applications. However, achieving geometric precision in printed devices with features spanning orders of magnitude in length scale is non-trivial. Here, a new characterization tool combining fast, high-resolution optical coherence tomography imaging with a high-powered digital light processing projector enables real time measurements of photopolymer curing. The direct, quantitative measurement of hydrogel working curves (the relationship between cure depth and light exposure) shows that the critical energy for gelation (E_c) exhibits extreme size dependence, demanding a rethinking of gray-scaling light intensity for achieving predictable voxel formation at high resolutions. This in situ method also enables measurement of size-dependent working curves and dead zone thicknesses using oxygen permeable window materials, which is impossible via ex situ methods. Generally, size sensitivity is amplified at low irradiance, high dye-loading, and in the presence of oxygen permeable windows. Despite the extreme size sensitivity, calibrating the light exposure to the size dependent E_c allows a 3x improvement in layer-growth uniformity compared to a naïve approach. Overall, these results highlight the challenges in high-resolution printing of hydrogels and provide a framework to measure and account for size dependence.