Diffusion of small molecular weight drugs in radiation-crosslinked poly(ethylene oxide) hydrogels

Poly(ethylene oxide) (PEG) hydrogels were formed by crosslinking of aqueous PEO solutions of (M) over bar(n) = 35 000, 900 000 and 5 000 000 in concentrations of 5-50% using gamma-irradiation with 3-10 Mrad. The hydrogels were characterized by equilibrium swelling experiments. The average interjunction molecular weight, (M) over bar(c), varied from 3500 to 32 000, whereas the mesh size, xi, varied from 120 to 550 Angstrom. The mesh size decreased with increased polymer molecular weight and solution concentration. Theophylline diffusion studies were performed through membranes equilibrium swollen in water at 37 +/- 1 degrees C using a side-by-side diffusion cell apparatus. The effective diffusion and partition coefficients of each membrane were determined. The partition coefficients decreased with increasing concentration of PEG. The effective diffusion coefficients of small molecular weight solutes did not depend on mesh size as the Lustig-Peppas theory would predict indicating that in PEO hydrogels, small molecular weight solute transport is not affected by the degree of crosslinking, but rather by the equilibrium swelling ratio.