(2fr) Microgel Surface Engineering to Enhance Cell Adhesion and Porosity of Injectable Granular Hydrogel Tissue Scaffolds

Granular hydrogels have been pursued with great interest for numerous biomedical applications, including as platforms for drug delivery and cell culture, scaffolds for accelerating tissue regeneration and wound healing, and inks for 3D bioprinting. Their utility arises from their multifunctionality, which includes injectability, compositional heterogeneity, and controlled porosity. Granular hydrogels assembled from micrometer-sized hydrogel particles (microgels). In this study, monodisperse polyethylene glycol diacrylate (PEGDA) microgels were prepared by photopolymerizing emulsion droplets of hydrogel forming solution within polydimethylsiloxane (PDMS) microchannels. PEG hydrogel surfaces alone cannot support cell adhesion due to their biologically inert nature. Consequently, the surfaces of PEGDA hydrogel particles were functionalized with an extracellular matrix protein, fibronectin, via coupling to a network-anchored linker (acrylate-PEG-biotin) to promote cell adhesion and proliferation. We have previously shown that acrylate-based hydrogel photopolymerization in oxygen permeable PDMS microfluidic devices is subject to oxygen-inhibition, which dramatically affects acrylate conversion and therefore the physical and chemical properties of hydrogel microparticle interfaces. In this paper we demonstrate how photopolymerization parameters quantitatively define hydrogel interfacial properties, and sequentially dictate cell-adhesive ligand density at the interface and, therefore cell adhesion. We show that hydrogel particles formed with lower macromer concentration must be polymerized at higher UV intensity and particles with higher macromer concentration should be polymerized with lower UV intensity to promote cell adhesion and proliferation on these particles. These results support the hypothesis that the effective extent of hydrogel surface functionalization is dictated by a competition between incorporated linker density and the availability of linker and cell-adhesive ligand, due to network density. Accordingly, macromer concentration must be matched to appropriate photopolymerization conditions to obtain a desirable linker density and the adhesion ligand density at the interface. By exploiting oxygen inhibition, adhesive ligand density at the interface can be decoupled from bulk hydrogel properties in a quantifiable and predictive manner. To demonstrate the importance of this for the assembly of granular tissue scaffolds, granular gels from microgels with different macromer concentrations were assembled by gravitational settling. Granular gels from microgels with lower macromer concentration showed higher void space fraction and more cell spreading while the gels from microgels possessing higher macromer concentration exhibiting lower void space fraction and less cell spreading. These findings reveal how PEG hydrogel surfaces can be engineered to promote cell adhesion and proliferation for a wide range of macromer concentrations via facile tuning of photopolymerization intensity, providing guidelines for designing complex PEG microgels and granular hydrogel cell scaffolds.