Generation of mechanical and biofunctional gradients in PEG diacrylate hydrogels by perfusion-based frontal photopolymerization.

The spatial presentation of soluble growth factors, immobilized extracellular matrix molecules, as well as matrix rigidity, plays an important role in directed and guided cell migration. Synthetic hydrogel scaffolds offer the ability to systematically introduce gradients of these factors contributing to our understanding of how the 3D arrangement of biochemical and mechanical cues influence cell behavior. Using a novel photopolymerization technique, perfusion-based frontal photopolymerization (PBFP), we have engineered poly(ethylene glycol) diacrylate (PEGDA) hydrogel scaffolds with gradients of mechanical properties and immobilized biofunctionality. The controlled delivery of a buoyant photoinitiator, eosin Y, through a glass frit filter results in the formation and subsequent propagation of a polymer reaction front that is self-sustained and able to propagate through the monomeric mixture. Propagation of this front results in monomer depletion, leading to variations in cross-linking, as well as spatial gradients of elastic modulus and immobilized concentrations of the YRGDS cell adhesion ligand within PEGDA hydrogels. Furthermore, the magnitudes of the resulting gradients are controlled through alterations in polymerization conditions. Preliminary in vitro cell-culture studies demonstrate that the gradients generated stimulate directed 2D cell growth on the surface of PEGDA hydrogels. By day 14, fibroblast aggregates spread roughly twice as far in the direction parallel to the slope of the gradient as compared to the perpendicular direction. The presented technique has great potential in controlling gradients of mechanical properties and immobilized biofunctionality for directing and guiding 3D cell behavior within tissue-engineered scaffolds.