(721h) Structure, Properties, and Stromal Cell Response of Self-Assembled Micellar Star Poly(ethylene oxide-co-lactide) Hydrogels

Introduction: PEG hydrogels, due to their inert and non-fouling nature, provide enormous flexibility in designing and controlling the cell microenvironment. However, the use of PEG gels in tissue regeneration is limited by their persistence in the site of regeneration. Copolymerization of PEG with lactide has been used to impart degradability to PEG macromer. However, the covalently crosslinked copolymer networks, due to phase separation and entrapment of functional groups, had significantly lower stiffness than the PEG gels. The objective of this work was to investigate self-assembly, micelle structure formation of star acrylated poly(ethylene oxide-co-lactide) (SPELA) macromonomers in aqueous solution with lactide segment length, and the response of marrow stromal cells (MSCs) to the self-assembled hydrogels.

Methods: A two-step procedure was used to synthesize SPELA macromonomer. In the first step, star poly(ethylene glycol-co-lactide) macromers were synthesized by melt ring-opening polymerization with star PEG. In the second step, the terminal hydroxyl groups of SPEL macromers were reacted with acryloyl chloride to produce SPELA. Gelation kinetics of the macromonomer in aqueous solution and modulus of the hydrogel was measured by rheometry. Water content and degradation was measured in PBS as described. The effect of lactide segment length on micelle formation and micelle size was simulated by molecular dynamic simulation using the Mesocite module of the Materials Studio software (Accelrys). For evaluation of cell response, MSCs were mixed in the polymerizing precursor solution and UV crosslinked. The cell-laden gels were cultured in osteogenic media for 21 days. At each time point, homogenized samples were analyzed by DNA content, extent of mineralization, and mRNA analysis.

Results and Conclusions: As the number of lactide groups on each arm of the SPELA macromonomer was increased from 2 to 10, the diameter of the self-assembled micelles increased from 2 to 5 nm, respectively, but the micelle size was independent of SPELA concentration in aqueous solution. Interestingly, due to aggregation and micelle formation by the lactide segments, the inter-acrylate distance decreased, leading to increase in modulus with lactide segment length. For a given time, degradation of SPELA gel increased with lactide segment length up to segment length three, followed by a decrease in mass loss for higher values. The addition of only one lactide per macromonomer arm increased mass loss to 50% after 6 weeks. Lactide segment length did not have a significant effect on the viability of embedded MSCs. MSCs encapsulated in SPELA gels differentiated to osteogenic lineage when cultured in osteogenic media and formed a mineralized matrix. Results demonstrate that the inert non-fouling self-assembled SPELA gels are potentially useful as a degradable cell delivery matrix in tissue regeneration.

Acknowledgement: This work was supported by research grants to E. Jabbari from the National Science Foundation under grant Nos. CBET0756394, CBET0931998, and DMR1049381.

Reference: [1] Mercado, A.E. et al. J. Contr. Rel. 2009, 140:148.