(173g) Affinity Hydrogels: Tailored Protein Delivery from Permissive Tissue Engineering Matrices

Over the past few decades, advances in hydrogel technologies have spurred development in many biomedical applications including controlled delivery of biomacromolecules. Of particular interest to the field of tissue engineering is a novel class of hydrogels with specific affinities for protein therapeutics developed in the current work. These gels mimic the extracellular matrix by sequestering large depots of protein growth factors until they are needed by encapsulated cells or surrounding tissue. Previously, Sakiyama-Elbert and Hubbell developed fibrin-based hydrogels for controlled release of growth factors based on heparin binding1 while Hoffman and colleagues incorporated heparin in PEG-based hydrogels to control the release of vascular endothelial growth factor2. These systems as well as a number of others based on crosslinked polyelectrolytes rely upon non-specific electrostatic interactions between encapsulated proteins and the charged network to control protein release rates. However, non-specific binding of polyelectrolytes can be unstable and produce protein release profiles that are difficult to predict and/or control in complex biological environments. Here, we describe an affinity hydrogel formulation for achieving sustained protein release based on specific binding between a target protein and a metal-ion chelator. A synthetic protein-receptor (methacrylated iminodiacetic acid, GMIDA) is copolymerized within crosslinked poly(ethylene glycol) (PEG) hydrogel networks to act as immobile protein-binding sites and sustain release of histidine-tagged proteins through protein-specific affinity binding. As will be shown, protein binding within PEG-co-GMIDA affinity hydrogels overcomes the drawbacks of current controlled-release devices and is advantageous to the design of tissue engineering matrices formed in situ such as photopolymerized hydrogels. First, protein-receptor binding within these gels prevents undesirable side-reactions that can denature the protein during in situ matrix fabrication and increases the overall delivery efficiency. Second, the coupled reaction-diffusion release mechanism that governs protein release from these novel gels prevents the initial burst commonly seen with matrix-based devices and allows a sustained release profile to be readily tailored based on the amount and chemistry of the protein receptor incorporated into the gel. All of this is accomplished without the need for high gel crosslinking densities or degradable PLGA microspheres that can severely denature the protein, limit viability of encapsulated cells, or lead to a host immune response. Finally, our results also demonstrate that these gels can be used for the simultaneous delivery of multiple biomacromolecules at independently controlled rates. This ability is critical to the design of advanced tissue engineering scaffolds. The versatility, tunability, and predictability of this release system indicate its strong potential to advance the design of combination products aimed at sustained biotherapeutic delivery and tissue regeneration.