(595b) Two-Phase Scaffolds with Nanoparticle Decorations for Growth Factor Delivery for Bone Regeneration

Controlled and time-dependent growth factor delivery in bone regenerating media remains a major challenge due to the difficulty in achieving distinct release profiles of multiple proteins, which is necessary to promote full bone healing. Hydrogels are an attractive choice for use in regenerative medicine due to their ability to provide biocompatible substrates for cell attachment and ability to deliver therapeutic molecules in a controlled manner. Encapsulation of growth factors within micro- or nano-carriers is a promising strategy that can provide greater control over their release kinetics by varying parameters such as particle size, crosslinking ratio, and hydrophilicity. In our present work, we have designed and characterized novel composite scaffolds as growth factor delivery vehicles for bone regeneration. These scaffolds consist of chitosan hydrogels decorated with polymeric nanoparticles. We explore strategies in which multiple crosslinked network structures can be used to tune the delivery of therapeutic agents, such as growth factors.

Chitosan hydrogels were prepared by casting 2% aqueous chitosan solutions into molds at constant temperatures where they developed physical crosslinks by chain rearrangement. The hydrogels were then lyophilized and controlled sublimation introduced an interconnected porosity as well as a directionality to the pores. Biopolymer nanoparticles were synthesized using an aqueous, one-pot UV-initiated emulsion polymerization. Various molar ratios of methyl methacrylate and methacrylic acid were studied to achieve hydrophilic/hydrophobic balance. Nanoparticle swelling studies were performed using dynamic light scattering and zeta potential measurements were analyzed using electrophoretic light scattering. Nanoparticles were covalently bound to the chitosan hydrogels using carbodiimide chemistry. The swelling capacities of the hydrogels, nanoparticles, and composite scaffolds were analyzed in controlled pH and ionic strength buffers. Bioactive agent delivery capacity was studied using α-chymotrypsin as a model protein for bone morphogenetic protein-2, a widely studied growth factor for bone regeneration.

It was found that nanoparticles with increased amounts of methacrylic acid showed increased swelling, as well as a pH responsive behavior, with increased swelling at higher pH values. Electrophoretic light scattering showed that all nanoparticle formulations exhibited a negative zeta potential at physiological pH, which allows for electrostatic interactions with high isoelectric point growth factors. Nanoparticles with increased methacrylic acid ratios were able to achieve higher loading capacities, up to 70 percent of protein loaded. In addition, increasing the methacrylic acid ratio resulted in decreased protein release within the first 24 hours. When protein was loaded directly to the scaffold, 50 percent was released within the first 24 hours. However, in the nanoparticle-decorated scaffolds, less than 25 percent of the loaded protein was released in the first 24 hours.

The release rates of a model growth factor were able to be modified by tuning the hydrophilicity of the nanoparticles and incorporating the particles within a bulk network. Thus, the current work shows promise for a dual network system to be used to tune the delivery of therapeutic agents for bone tissue engineering applications.

Acknowledgements: The work was supported in part by a grant from the National Institutes of Health (R01-EB-022025) and the Institute for the Cockrell Family Regents Chair. M.S. was supported in part by an NSF GRFP.