Gelation characteristics and osteogenic differentiation of stromal cells in inert hydrolytically degradable micellar polyethylene glycol hydrogels

The use of poly(ethylene glycol) (PEG) hydrogels in tissue engineering is limited by their persistence in the site of regeneration. In an attempt to produce inert hydrolytically degradable PEG-based hydrogels, star (SPELA) poly(ethylene glycol-co-lactide) acrylate macromonomers with short lactide segments (<15 lactides per macromonomer) were synthesized. The SPELA hydrogel was characterized with respect to gelation time, modulus, water content, sol fraction, degradation, and osteogenic differentiation of encapsulated marrow stromal cells (MSCs). The properties of SPELA hydrogel were compared with those of the linear poly(ethylene glycol-co-lactide) acrylate (LPELA). The SPELA hydrogel had higher modulus, lower water content, and lower sol fraction than the LPELA. The shear modulus of SPELA hydrogel was 2.2 times higher than LPELA, whereas the sol fraction of SPELA hydrogel was 5 times lower than LPELA. The degradation of SPELA hydrogel depended strongly on the number of lactide monomers per macromonomer (nL) and showed a biphasic behavior. For example, as nL increased from 0 to 3.4, 6.4, 11.6, and 14.8, mass loss increased from 7 to 37, 80, 100% and then deceased to 87%, respectively, after 6 weeks of incubation. The addition of 3.4 lactides per macromonomer (<10 wt % dry macromonomer or <2 wt % swollen hydrogel) increased mass loss to 50% after 6 weeks. Molecular dynamic simulations demonstrated that the biphasic degradation behavior was related to aggregation and micelle formation of lactide monomers in the macromonomer in aqueous solution. MSCs encapsulated in SPELA hydrogel expressed osteogenic markers Dlx5, Runx2, osteopontin, and osteocalcin and formed a mineralized matrix. The expression of osteogenic markers and extent of mineralization was significantly higher when MSCs were encapsulated in SPELA hydrogel with the addition of bone morphogenetic protein-2 (BMP2). Results demonstrate that hydrolytically degradable PEG-based hydrogels are potentially useful as a delivery matrix for stem cells in regenerative medicine.