Cornea, and the swelling of polyelectrolyte gels of biological interest

Abstract

Biological polyelectrolyte gels consist of insoluble aggregates of molecules which collectively form structural fibrils and these fibrils, or their chemically bound side chains, have a net electrical charge. These gels may be visualized as negatively charged fibrils immersed in aqueous solutions which include free diffusible ions (mainly sodium, potassium and chloride). All living cells and most of the extracellular spaces of the body are polyelectrolyte gels and they strive to swell by the absorption of additional fluid because of the Donnan potentials generated by their fixed charge. We review Donnan swelling using the cornea of the eye as prime material. Donnan swelling requires knowledge of only one parameter such as: (a) the electrical potential within the gel or (b) the distribution of any mobile ion inside and outside the gel or (c) measurement of the gel pressure or (d) the fixed charge density on the fibrils, in order to calculate all the other relevant factors. We describe the conditions (which usually exist in biological tissue) when the microscopic distribution of the fixed charge density within the gel is not important to the Donnan phenomena. Fixed charge density is generated by two sources: permanent negative charges in the structural fibrils and transient mobile ion binding to the fibrils. Ion binding to large molecules is reviewed. In the case of the cornea, transient mobile ion binding is the predominant factor in generating fixed charge density under physiological conditions. An irreversible thermodynamic treatment of gel swelling shows the intrinsic instability of polyelectrolyte gels and suggests new ways of approaching a microscopic model for osmosis. In order to stabilize the two forces (osmotic potential and chemical potential) which generate the polyelectrolyte gel instability we review the types of third forces which must be present in order to stabilize biological gels. These third forces include van der Waal's force, metabolically driven ion pumps or fibrillar cross-linking. In the case of the cornea, it is shown that the gel pressure is exploited in order to help make the tissue transparent to light.