Digital light processing (DLP) bioprinting is an emerging technology for 3D bioprinting (3DBP) owing to its high printing fidelity, fast fabrication speed, and higher resolution. Low viscosity bioink such as poly(ethylene glycol) diacrylate (PEGDA) or gelatin methacryloyl (GelMA) are routinely used for DLP-based 3D printing. However, the crosslinking of PEGDA and GelMA proceeds via chain-growth photopolymerization that displays strong oxygen inhibition, leading to delayed gelation and heterogeneity in crosslinking density. As such, high concentrations of macromers (e.g., 20 wt% to 40 wt%) are typically needed for DLP 3D printing. In contrast, step-growth thiol-norbornene photopolymerization is not oxygen inhibited, requires low photoinitiator concentration, and produces hydrogels with an ideal network structure. The high cytocompatibility and rapid gelation of thiol-norbornene photopolymerization has lend itself to the crosslinking of cell-laden hydrogels but has not been extensively used for DLP bioprinting. In this study, we explored 8-arm PEG-norbornene (PEG8NB) and norbornene-functionalized gelatin (GelNB) as photo-click bioinks for visible light initiated DLP-based 3DBP. Using relatively low concentration of norbornene-based DLP resins (5 wt%), we showed high printing fidelity and cytocompatibility. In addition, we demonstrated the versatility of norbornene-based bioinks by printing solid structures as cell culture devices, hollow channels for endothelialization, and microwells for generating cell spheroids. We also exploited the unique post-gelation modification of norbornene-based orthogonal click chemistry for dynamic modification of biological properties of the printed structures. This work not only expands the selection of bioinks for DLP-based 3DBP, but also provides a platform for dynamic modification of the bioprinted constructs.

Norbornene-based photo-click bioinks for digital light processing 3D bioprinting

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