Introduction of the lowest printable (channel) characteristic length (LPCL) as a geometrical metric for the SLA 3D printing of embedded negative micro-structures

The use of photopolymer-based additive manufacturing (PAM) for the fabrication of microfluidics has recently increased as techniques like Stereolithography (SLA) are employed for the creation of channels by either the printing of a positive mold for casting, or of a monolithic object with embedded hollow structures. The latter is more advantageous; however, no standard metric is available to predict the printability of the embedded hollow structures as a function of their geometry. To address this gap, we selected a lab-on-disc design to confirm that the VAT photopolymerization of embedded hollow structures primarily fails because of channel clogging or collapse due to resin dragging. We introduce the concept of the lowest printable (channel) characteristic length (LPCL) as a threshold geometrical metric at which a minimum of ~ 80% of the hollow structures are successfully printed without collapse or blockage. At this threshold, only the printing failures that are not inherently related to the internal hollow structures remain. The clogging by resin dragging from the VAT is evidenced by the orientation-dependent printability and accuracy, where a 90° angle from the printing platform is ideal for both rectangular and cylindrical channels (LPCL ~ 600 µm). The resin-dependent LPCL increased from 600 to 900–1000 µm as the orientation of the channels decreased towards 0°. Concomitantly, the printing accuracy decreased (95% to 50–80%) linearly with the printing orientation. The LPCL was higher for cylindrical channels than for rectangular. This work contributes to the optimization of PAM settings for the direct fabrication of embedded negative structures inside monolithic objects.