Dehydration and vitrification of corneal gel

Abstract

Temporal changes in the weight and elastic properties during the dehydration of pig corneal gel were investigated. The moisture from the corneal gel evaporated in two stages. At the crossover point between these two stages, the elastic modulus of the corneal gel showed an anomaly: the loss tangent (tan δ) showed an anomalous peak with the jump of the dynamic elastic modulus, E′. Two fractions of water exist in the corneal gel: one strongly bound to the relaxation centers, and the other almost freely diffusible as liquid bulk water. A quantitative analysis shows that the bound fraction of the corneal water under physiological hydration makes up about 3% of the total water. The abrupt increase in non-freezing water content, and the distinct change in the bound water fraction η of the cornea were also observed near the crossover point of weight reduction. From these observations, water exerts a plasticizing effect on the corneal gel, thereby reducing the elastic stiffness below that of the dry cornea. The absolute value of the elastic stiffness of the corneal gel is smaller than that for non-crystalline polymeric glasses by about a few orders of magnitude. It is plausible to explain that the monolayer coverage of bound water on the lattice of the cornea prevents direct contact of the polymer chains of the corneal gel.