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J. Gonzalez–Meijome, A. Lopez–Alemany, Jr., J.B. Almeida, M.A. Parafita; Consistency of Surface Analysis of Silicone–Hydrogel Contact Lens Polymers with Atomic Force Microscopy . Invest. Ophthalmol. Vis. Sci. 2006;47(13):2387.
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© ARVO (1962-2015); The Authors (2016-present)
Atomic force microscopy (AFM) has been applied to the study of surface topography and mechanical properties of polymers used in contact lens manufacture. The purpose of this study was to investigate the repeatability of topographic measures of polymer topography on new silicone–hydrogel polymers
Samples of five different silicone–hydrogel contact lens polymers were assessed by atomic force microscopy. Repeated measures of surface roughness (Ra, Rms and Rmax) were taken at different places in the front and back surfaces. Measures were taken in non–contact scanning mode. After those measures, contact mode analysis was carried out in order to obtain repeated measures of force vs displacement curves which can be used for modulus and adhesion analysis.
Different materials showed different surface topography which seems to be characteristic of each contact lens. Also, different materials display different variability in topographic values of Ra –average roughness–, Rmax –maximum roughness– and Rms –root mean square roughness–. Qualitative analysis of repeated readings of force vs displacement curves were highly consistence for different samples of the same material (less than 10% variability among 5 samples taken at different locations within the same sample). Qualitative evaluation of those curves showed statistically significant differences in the slope of the curve (which is proportional to the modulus of the material) among different polymers as well as in the adhesion values . Less hydrated material showed significantly lower modulus (p<0,001), while more hydrated polymers showed higher values of adhesion between the cantilever and the polymer (p<0,05).
Atomic force microscopy allows to make reliable estimations of the mechanical properties of contact lens polymers, and is able to show nanometric differences in topographic profile of the surface as well as to estimate the modulus and adhesion of the polymeric surface of contact lenses in the hydrated state. This methodology will be applied in the future to investigate the spoliation of new silicone–hydrogel contact lens polymers. This could help to explain differences in the interaction of soft contact lens polymers with tears and bacteria with potential to cause serious disturbances to the ocular surface as a consequence of contact lens wear.
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