Biophysics / Internet Invited Lecture
INTERFEROMETRIC ANALYSIS OF THE MATERIAL USED FOR PRODUCTION OF CONTACT LENSES
Michal S. Kaczmarek, Zenon Wozniak, Z. Blaszczak
Faculty of Physics, Division of Optics, Adam Mickiewicz University,
Umultowska 85, 61-614 Poznan, Poland
ABSTRACT
Correcting contact lenses are placed directly on the surface of the cornea which requires continuous wetting for its functioning. Therefore the contact lens material should be characterised not only by good optical properties but also by high permeability to gases and components of the tear fluid. The paper presents a method for determination of the permeability of contact lenses to the main ions of the tear fluid Na+ and Cl-, based on optical interference. A lens studied is mounted to separate two chambers of the measuring cell containing a 0.9% water solution of NaCl and distilled water. Changes in the refraction index dn of laser beams (632.8nm) passed through the chambers resulted in the appearance of interference lines in a Jamina refractometer. The number of interference lines and the rate of their passage in front of the photodiode were recorded by a computer with an analog-digital converter. The transportation times studied varied from t=1 to 6h. The Fick law (dN/dt=-DSdc/dx)describing the process of transportation links the number of moles dN flowing through the area S in time dt with the gradient of concentration of a given substance in solution dc/dx. The diffusion coefficient D describing the number of molecules or ions diffusing in 1 second through the area of 1 cm2, is related to the lens permeability of a given substance by P=D/dx . On the basis of the dependence of the number of the interference lines L on time t, the time changes in the refraction index dn were found. The dynamics of the molar concentration changes was described by plotting the light refraction index changes versus the molar concentration of the NaCl solution in the concentration range studied, to get the time dependence of the molar concentration of the NaCl solution in the chamber from which the transport occurred. Using the linearised equation (ln cII(t)/cII(0)=-PSt/V), describing the logarithmic change in the solvent concentration cII(t) after time t relative to its initial value cII(0), the lens permeability P was estimated by fitting the theoretical results to the experimental data. The experiment was performed for hydrogel lenses for day wearing within two weeks, of the optical power of –2.75 D and –3.75 D, new and used, made by Baush & Lomb. The method proposed in the paper is also suitable for testing the disinfecting fluids for lens conservation.
Representing Author:
Zenon Wozniak, Adam Mickiewicz University, (Poznan, Poland)
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