The incorporation and sustained release of bioactive insulin from a bead-formed macroporous hydrogel matrix

Freeze-thaw photopolymerization of a mixed solution of monomers and bovine insulin around frozen ice crystals has been used to generate a bead-formed macroporous hydrophilic matrix of p-HEMA. The largest proportion of beads was 500-1000 microns in size (distribution < 106-1700 microns) with a mean EBC of 71.7 +/- 0.92%. Insulin release was monitored using RIA and insulin bioactivity determined using the rate of insulin stimulated D-[U14C] glucose oxidation to 14CO2. The cumulative insulin release profile was characterized by an initial lag phase followed by an almost linear increase in insulin release for up to 30 days. Insulin release at 4 degrees C was significantly greater than release at 37 degrees C both in the presence and absence of 2.5% thiomersalate as preservative. The latter served to extend the time period over which significant insulin release could be detected. Increasing the monomer concentration decreased the mean equilibrium buffer content (EBC), the total mean cumulative release of insulin, and the proportion of the incorporated insulin load subsequently released at both 4 degrees C and 37 degrees C in the presence of preservative. Insulin determination using RIA and bioassay confirmed that insulin released from beads was bioactive and that immunoreactivity was a reasonably reliable indicator of bioactivity.