Dynamic Mechanical Analysis (DMA) of human lenses
G. Eckert, C. Meyer, W. Pechhold, F. Soergel, H. Weeber
Introduction: For the development of lens substitute materials the knowledge of the dynamic mechanical properties of the human lens material and of the possible substitutes is important. Their frequency dependence is correlated with the accommodation time.
Purpose: Measurement of the dynamic mechanical properties of human lenses in dependence on age.
Methods: In Dynamic Mechanical Analysis (DMA), samples are subjected to a periodic shear deformation at different frequencies. Stress, deformation and their phase shift are measured. Then the dynamic shear compliance, can be calculated.
Results: 40 human lenses in the age range 18-90 years were investigated. The problems appearing during measurement of very soft human lenses have been solved by several modifications of a DMA device. Caused by the sample arrangement in the sample holder the measurement results are average values of the compliance over the whole lens. The results were obtained in the linear visco-elastic range. The shear compliance was strongly dependent on age. In the measured frequency range, 0.001-10 Hz, three relaxation processes (three different types of molecular motion) were found.
Discussion: The strength of all three relaxation processes decreased with age, whereas their relaxation frequencies, 0.001-0.01 Hz, 0.01-10 Hz and 10-1000 Hz, showed an unexpected increase. These changes are indicating an increase in cross linking density, like observed in rubber systems.
Institut für dynamische Materialprüfung, Helmholtzstr. 20, D-89081 Ulm