Date of Award

12-15-2014

Document Type

Open Access Dissertation

Department

Chemical Engineering

First Advisor

Esmaiel Jabbari

Abstract

A feasible approach to the restoration of injured bone tissue is the use of tissue engineering strategies to deliver mesenchymal stem cells (MSCs) to the site of regenration in degradable, in-situ gelling, bio-active hydrogels. Although inert Polyethylene glycol (PEG) hydrogels provide flexibility in controlling the cell microenvironment, their use for in vivo applications in tissue regeneration is limited by their persistence (nondegradability) in the site of regeneration. In an attempt to produce inert hydrolytically degradable matrices for encapsulation of MSCs, hydrogels based on star PEG macromonomers chain-extended with short hydroxy acid (HA) segments (SPEXA) were synthesized and characterized with respect to gelation kinetics, water content, degradation rate and mechanical properties. HA monomers included glycolide, lactide, p-dioxanone and ε-caprolactone. The degradation rate of SPEXA hydrogels was strongly dependent on HA type and number of HA repeat units. SPEXA gels chain-extended with the least hydrophobic glycolide completely degraded within days, lactide within weeks, and p-dioxanone and ε-caprolactone degraded within months. Further, there was a biphasic relationship between HA segment length and gel degradation. There was a significant decrease in gelation time of SPEXA macromonomers with HA chain-extension for all HA types due to micelle formation. Meso-scale simulations revealed formation of micellar structures within the SPEXA precursor solutions for all HA types. The micellar SPEXA hydrogels supported osteogenic differentiation, collagen production, and mineralization of MSCs. We further investigated the effect of concentration and hydrophobicity a BMP-2 protein derived peptide on osteogenic differentiation of mesenchymal stem cells (MSCs) encapsulated in a PEG based hydrogel. The dose-osteogenic response curve of the BMP-2 peptide was in the 0.0005-0.005 mM range, and osteoinductive potential of the BMP-2 peptide was significantly less than that of BMP-2 protein even at 1000-fold higher concentrations. There was a higher osteogenic differentiation of encapsulated hMSCs when the BMP-2 peptide was dissolved in the hydrogel matrix as compared to the peptide conjugated to the hydrogel network. The BMP-2 peptide with a positive index of hydrophobicity had a critical micelle concentration (CMC) and formed aggregates in aqueous solution. Results revealed that osteoinductive potential of the BMP-2 peptide is correlated with its CMC and the free peptide concentration in aqueous medium and not the total concentration.

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