Clickable PEG-norbornene microgels support suspension bioprinting and microvascular assembly

The development of perfusable and multiscale vascular networks remains one of the largest challenges in tissue engineering. As such, there is a need for the creation of customizable and facile methods to produce robustly vascularized constructs. In this study, secondarily crosslinkable (clickable) poly(ethylene glycol)-norbornene (PEGNB) microbeads were produced and evaluated for their ability to sequentially support suspension bioprinting and microvascular self-assembly towards the aim of engineering hierarchical vasculature. The clickable PEGNB microbead slurry exhibited mechanical behavior suitable for suspension bioprinting of sacrificial bioinks, could be UV crosslinked into a granular construct post-print, and withstood evacuation of the bioink and subsequent perfusion of the patterned void space. Endothelial and stromal cells co-embedded within jammed RGD-modified PEGNB microbead slurries assembled into capillary-scale vasculature after secondary crosslinking of the beads into granular constructs, with endothelial tubules forming within the interstitial space between microbeads and supported by the perivascular association of the stromal cells. Microvascular self-assembly was not impacted by printing sacrificial bioinks into the cell-laden microbead support bath before UV crosslinking. Collectively, these results demonstrate that clickable PEGNB microbeads are a versatile substrate for both suspension printing and microvascular culture and may be the foundation for a promising methodology to engineer hierarchical vasculature. STATEMENT OF SIGNIFICANCE: In this study, we leveraged and combined advances in microgel biomaterials, granular hydrogels, suspension bioprinting, and vascular biology to create relatively large volume (>500 mm³) vascularized constructs. We fabricated secondarily crosslinkable (clickable) poly(ethylene glycol)-norbornene (PEGNB) microbeads and demonstrated their ability to sequentially support suspension bioprinting and microvascular self-assembly towards the aim of engineering hierarchical vasculature. To the best of our knowledge, this is the first study that uses PEG microgels as supportive materials for bioprinting, and one of the first papers to document microvascular self-assembly within granular constructs. The combination of top-down and bottom-up approaches within a single construct represents a significant and innovative contribution that we believe will be of broad interest to the biomaterials and regenerative medicine communities.