Hydrogels prepared by electron irradiation of poly(ethylene oxide) in water solution: unexpected dependence of cross-link density and protein diffusion coefficients on initial PEO molecular weight

Under ionizing radiation aqueous solutions of water-soluble polymers become cross-linked and form hydrogels, primarily by radiolysis of water-generating hydroxyl radicals which attack the polymer chains. The chain radicals thus formed create cross-links by coupling. In particular, hydrogels formed from poly(ethylene oxide) are of interest for biomedical applications, including those in which it is necessary to transport large molecules such as growth factors, in addition to nutrients, to cells attached to the surface. We sought a rapid and simple method for estimating diffusion coefficients by observing the diffusion of two red-coloured proteins: cytochrome C and hemoglobin of respective molecular weights 12,000 and 67,000. In the course of this we discovered a previously unreported effect of the primary molecular weight of the polymer, before cross-linking, on the cross-link density finally achieved and on the diffusion coefficient of the proteins.