Spectral analysis of viscoelasticity of the human lens

PURPOSE: In the aging lens, two different structural changes become progres sively apparent: loss of accommodation and formation of cataract. These cha nges are due to different alterations in lens structure taking place on a m olecular scale. METHODS: For the characterization of liquids, gels, elastic bodies and soli ds, dynamic mechanical analysis (DMA) represents a non-destructive macrosco pic analysis method with considerable sensitivity to microscopic structural changes. Spectra of the shear compliance (reciprocal sheer modulus) of hum an lenses at different ages were measured in a wide frequency range (1 mHz to 1 kHz), By means of computer analysis, the parameters of the involved re laxation processes and the viscous flow were determined. RESULTS: The increase in cross-linking density was correlated with the disa ppearance of viscous flow in the measured spectra, The accumulation of high -molecular-weight aggregates was held responsible for the shift of the lowe r frequency relaxation process to lower frequencies with increasing stiffne ss, which is likely to be correlated with a prolonged accommodation time. CONCLUSION: Dynamic mechanical analysis represents a useful method for the spectroscopic characterization and quantification of the changes in the mac roscopic viscoelastic properties caused by molecular changes in cataract fo rmation and loss of accommodation.