Investigation and comparison of resin materials in transparent DLP-printing for application in cell culture and organs-on-a-chip.

Organs-on-a-Chip (OOCs) have recently led to major discoveries and a better understanding of 3D cell organization, cell-cell interactions and tissue response to drugs and biological cues. However, their complexity and variability are still limited by the available fabrication technology. Transparent, cytocompatible and high-resolution 3D-printing could overcome these limitations, offering a flexible and low-cost alternative to soft lithography. Many advances have been made in stereolithography printing regarding resin formulation and the general printing process, but a systematic analysis of the printing process steps, employed resins and post-treatment procedures with a strong focus on the requirements in OOCs is missing. To fill this gap, this work provides an in-depth analysis of three different resin systems in comparison to polystyrene (PS) and poly(dimethylsiloxane) (PDMS), which can be considered the gold-standards in cell culture and microfluidics. The resins were characterized with respect to transparency, cytocompatibility and print resolution. These properties are not only governed by the resin composition, but additionally by the post-treatment procedure. The investigation of the mechanical (elastic modulus ∼2.2 GPa) and wetting properties (∼60° native / 20° plasma treated) showed a behavior very similar to PS. In addition, the absorbance of small molecules was two orders of magnitude lower in the applied resins (diffusion constant ∼0.01 µm2 s-1) than for PDMS (2.5 µm2 s-1), demonstrating the intrinsic suitability of these materials for OOCs. Raman spectroscopy and UV/VIS spectrophotometry revealed that post-treatment increased monomer conversion up to 2 times and removed photo initiator residues, leading to an increased transparency of up to 50% and up to 10-times higher cell viability. High magnification fluorescence imaging of HUVECs and L929 cells cultivated on printed dishes shows the high optical qualities of prints fabricated by the Digital Light Processing (DLP) printer. Finally, components of microfluidic chips such as high-aspect ratio pillars and holes with a diameter of 50 µm were printed. Concluding, the suitability of DLP-printing for OOCs was demonstrated by filling a printed chip with a cell-hydrogel mixture using a microvalve bioprinter, followed by the successful cultivation under perfusion. Our results highlight that DLP-printing has matured into a robust fabrication technology ready for application in extensive and versatile OOC research.