The potential of naturally occurring substances as a source of biomedical materials is well-recognised and is being increasingly exploited. Silk fibroin membranes derived from Bombyx mori silk cocoons exemplify this, for example as substrata for the growth of ocular cells with the aim of generating biomaterial-cell constructs for tissue engineering. This study investigated the transport properties of selected silk fibroin membranes under conditions that allowed equilibrium hydration of the membranes to be maintained. The behaviour of natural fibroin membranes was compared with fibroin membranes that have been chemically modified with poly(ethylene glycol). The permeation of the smaller hydrated sodium ion was higher than that of the hydrated calcium ion for all three ethanol treated membranes investigated. The PEG and HRP-modified C membrane, which had the highest water content at 59.6 +/- 1.5% exhibited the highest permeation of the three membranes at 95.7 +/- 2.8 x 10(-8) cm(2) s(-1) compared with 17.9 +/- 0.9 x 10(-8) cm(2) s(-1) and 8.7 +/- 1.7 x 10(-8) cm(2) s(-1) for membranes A and B respectively for the NaCl permeant. Poly(ethylene glycol) was used to increase permeability while exploiting the crosslinking capabilities of horseradish peroxidase to increase the compressive strength of the membrane. Importantly, we have established that the permeation behaviour of water-soluble permeants with hydrated radii in the sub-nanometer range is analogous to that of conventional hydrogel polymers.