This paper describes the development and application of scaling laws relating bulkdegradation behavior of poly(lactic acid)-poly(ethylene glycol)-poly(lactic acid) (PLA-b-PEG-b-PLA) copolymer hydrogels to the ever-changing transport properties during network degradation. Specifically, the relationships developed were used to extend previous work and to predict the effects of cross-linking density and hydrolytic degradation rate on the controlled release of high molecular weight proteins from these hydrogels. Theoretical drug release profiles, obtained using scaling laws, agree qualitatively with experimental observations and show a decrease in the protein release rate with an increase in protein size, extent of macromer functionalization, and macromer concentration. Quantitative predictions of release behavior were possible under certain experimental conditions.

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Continuing advances in islet cell transplantation have been promising; however, several limitations, including severe shortage of transplantable islets, hinder the widespread use of this therapy. Pancreatic precursor cells are one alternative to cadaveric donor islets. These cells found in the developing pancreatic buds are capable of self-renewal and also have the innate ability to become insulin-producing beta-cells. For this work, bioinert polyethylene glycol (PEG) hydrogels were chosen as the supportive three-dimensional matrix for encapsulation of dissociated pancreatic precursor cells obtained from the dorsal pancreatic bud of day-15 rat embryos. This culture system was selected in order to eliminate cell-extracellular matrix and cell-cell signal heterogeneity present when intact pancreatic buds are embedded in protein-based gels, the typical in vitro culture conditions used to study this cell population. In this study it was found that (1) dissociated precursor cells maintain a robust viability for 7 days in PEG hydrogel culture, (2) encapsulated cells selectively differentiate into insulin-expressing beta-cells, and (3) differentiated beta-cells have releasable insulin stores, but are not achieving a mature, glucose responsive phenotype. These findings suggest that encapsulating dissociated pancreatic precursor cells in an environment designed to minimize the heterogeneous signaling cues present during development or in standard culture conditions generates a population highly enriched in pancreatic beta-cells; however, future efforts must focus on achieving glucose responsiveness in this cell population. Further, these results indicate that differentiation down a beta-cell lineage may be the default pathway in pancreatic development.