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Dorota Tarnawska, Antoni Kocot, Maria Jastrzebska, Edward Wylegala, Alicja Ratuszna; Low-frequency dielectric spectroscopy of normal and keratoconus human cornea. Invest. Ophthalmol. Vis. Sci. 2013;54(15):5295.
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The analysis of changes in spatial molecular structure inside the keratoconus human cornea.
Low-frequency dielectric spectroscopy (1-107 Hz) in the temperature range of 140-300 K was used. Experimental results are interpreted in terms of ionic diffusion and space charge polarization according to Sawada’s theory [J. Appl. Phys. 38, 1418 (1999)]. Diffusion coefficient of the ions was estimated on the basis of a dielectric dispersion measurements for an aqueous NaCl solution with a well-known distance between the electrodes. Two cornea storage media were used: Eusol-C and phosphate buffered saline pH=7.4. The 100-400 μm specimens containing the middle layers of corneal stroma were examined. The 9 specimens of healthy, control corneas from Eye Bank were obtained with second microkeratom section during Ultra Thin-DSAEK. The 12 specimens of keratoconus corneas were manually prepared from corneas removed during penetrating keratoplasty.
The fitting procedure of a theoretical function to the experimental data allowed us to determine up to three relaxation regions. The narrow peak near 1MHz can be successfully traced by the Debye relaxation function. This peak is probably responsible for the dipolar polarization of bound water to collagen molecules as well as for the polarization of side polar groups in polypeptide chains. Two low frequency relaxation regions are related to diffusive processes with different structural distance parameters, i.e. describing the spatial arrangement of collagen fibrils in cornea. Peak amplitudes were increasing upon rising electric field applied from 0.05V to 1V. Comparison between the dielectric responses for normal and keratoconus corneas showed the significant shift of the ionic peaks towards lower frequencies ωn→ωk, with the ωn/ωk ~2. The observed shift in peak position may correspond to the increase in the average structural distance of different molecular compartments present in keratoconus cornea.
It has been shown that the method can be utilized to follow relative changes in the spatial molecular structure of keratoconus corneas with respect to the normal tissue. It is also considered that the method have a great potential to be used for structural analysis of biological systems in vivo, which are often characterized by high water content and the presence of multiple kinds of ions
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