Statement of Purpose: The use of mesenchymal stem cells (MSC) for bone regeneration requires the identification of osteoinductive scaffolds. Hydrophobic biomaterials such as poly(propylene fumarates) and polyanhydrides have shown promise as osteoinductive matrices, and recently, hydrophilic polyethylene glycol (PEG) hydrogels were employed as bone regeneration scaffolds. Thus, tuning scaffold hydrophobicity may be a potent method for optimizing the formulation of bone tissue engineering matrices. Furthermore, an extensive body of literature indicates that appropriately tailoring scaffold inorganic composition (e.g., bioactive glass) can enhance its osteoconductivity. In the present study, we therefore examined the impact of tailoring scaffold inorganic content and hydrophobicity through the photo-cure of star polydimethylsiloxane (PDMSstar) (hydrophobic inorganic polymer) and linear hydrophilic PEG on MSC differentiation. Results indicated a strong dependence of MSC fate on PDMSstar Mn at relatively high PDMSstar levels.
Methods: Materials. Pt-divinyltetramethyldisiloxane complex, tetrakis(dimethylsiloxy)silane (tetra-SiH), and octamethylcyclotetrasiloxane (D4) were obtained from Gelest. Other reagents were obtained from Sigma Aldrich. Synthesis of Macromers. PDMSstar-MA was prepared in two synthetic steps. First, PDMSstarSiH was synthesized by the acid-catalyzed equilibration of D4 with tetra-SiH. Mn was controlled by reaction stoichiometry. Allyl methacrylate was then added to each arm. PEG-DA was prepared by acrylation of corresponding PEG.4 Hydrogel Preparation. Gels containing 1 mM acrylate-derivatized RGDS were prepared by the photo¬polymerization of aqueous mixtures of PDMSstar-MA (2k, 7k) and PEG-DA (3.4 k) macromers. Aqueous precursor solutions were prepared at 10 wt% with the following wt% ratios of PDMSstar to PEG-DA: 0:100, 5:95, and 20:80. Hydrogel Characterization. Gel composition was characterized by XPS, and gel material properties were assessed by DMA, swelling, and serum protein adsorption analyses. Cell Characterization. Following three weeks of culture, the differentiation status of MSCs encapsulated within each hydrogel was examined using competitive ELISA.
Results: MSC differentiation toward osteoblast, adipocyte, and smooth muscle cell fates was evaluated by quantifying the levels of osteocalcin, A-FABP, and SM22α, respectively, across hydrogel formulations. Increasing the ratio of PDMSstar to PEG from 0:100 to 5:95 appeared to have limited impact on MSC differentiation for both 2k and 7k PDMSstar. In contrast, further increasing PDMSstar levels to 20:80 resulted in marked shifts in MSC differentiation for both PDMSstar Mn. However, the directionality of these shifts varied with PDMSstar Mn. Specifically. 20:80 PDMSstar (2k)-PEG gels were enriched in osteoblast-like cells, whereas MSC differentiation toward a smooth muscle cell phenotype was enhanced in 20:80 PDMSstar (7k)-PEG gels. To gain insight into material-based stimuli underlying observed differences in MSC differentiation, the modulus, swelling, and protein adsorption profiles (an indicator of hydrophobicity) of each hydrogel formulation was assessed. Comparison of gel mechanical properties demonstrated that the modulus of each PDMSstar (7k)-PEG gel was statistically indistinguishable from the PDMSstar (2k)-PEG gel of the corresponding weight ratio. In contrast, gel protein adsorption profiles, and therefore the biochemical stimuli presented to cells, varied markedly with PDMSstar Mn.
Conclusions: Results from the present study indicate that tuning scaffold inorganic content and hydrophobicity can serve as a powerful tool for directing MSC differentiation. Hydrogel characterization studies suggested that alterations in adsorbed serum proteins and/or calcium deposits with increasing PDMSstar levels and Mn may underlie the present differentiation results. Future studies will explore the influence of PDMSstar on MSC behavior over a broader Mn range toward identifying osteoinductive scaffold formulations.