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The cornea is the clear outer window of the eye, composed primarily of dense regular connective tissue called the stroma. The tissue is composed of 70% water and parallel collagen fibrils surrounded by proteoglycans containing high concentrations of acidic and neutral glycosaminoglycans (GAGs) and contains ∼2 – 10% cells by volume. When the surface membranes covering the stroma are damaged by injury or disease, the stroma swells due to the hydrophilic nature of the polysaccharides. At the same time, swelling spreads the collagen fibrils apart and the tissue loses transparency. We investigated the role of different salts in modifying swelling behavior and transparency loss. In saline (148 mM NaCl) solution, hydration increases as a multi exponential function of time: Initially the stroma gains hydration at a fast rate, then slows down. In the presence of divalent cations, swelling behavior significantly decreased, and the effect was weakly anion‐dependent. Under imposed stromal swelling pressures, Ca++ and Mg++ inhibited hydration equally, and their effects were additive, indicating that they work by similar mechanisms. They were also concentration‐dependent, with no apparent ceiling to the effect within normal biological ionic strengths. As stated above, Mg++ and Ca++ both dehydrate stromas and decrease swelling rates compared to saline, and as a stroma hydrates, it loses transparency: visible spectrophotometry of stromas bathed in NaCl lose transparency faster than those bathed in MgSO4, because MgSO4 solutions hydrate stromas slower. Stromas bathed in Ca++ hydrated slower than those bathed in NaCl, but interestingly, high (49 – 148 mM) concentrations of Ca++ caused transparency loss almost instantly. In hyaluronin (a collagen fibril‐free polyanionic biogel), when similar concentrations of Ca++ were added, transparency also decreased. Our conclusion is that the dehydrating effects of Mg++ and low concentrations of Ca++ are due to shielding of electrostatic repulsion between anionic GAGs. The higher concentrations of Ca++ dehydrate by a combination of covalent binding to (precipitating) GAGs and/or shielding their electrostatic repulsions, and scatter light by precipitating GAGs and disrupting collagen fibril order. These techniques and ionic probes are useful for studying the structure and function of polyionic hydrogels and may be useful for modifying the materials properties of biological and synthetic gels.

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