Contact Lens Spectrum
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c l s p e c t r u m . c o m 48 C O N T A C T L E N S S P E C T R U M S P E C I A L E D I T I O N 2 0 1 6 Atomic Force Microscopy (AFM) Atomic Force Microscopy (AFM) is a scanning probe microscopy that measures the force between a sharp- tipped probe and a substrate. This allows for both topo- graphical surface mapping and 3D reconstruction of the surface. A Dimension ICON Atomic Force Microscope (Bruker, Billerica, MA) was used to characterize the topography and morphology of the three contact lens surfaces in dry state. Images were captured using etched Si AFM tips (Bruker) and soft tapping mode. The root- mean-square (RMS) surface roughness was calculated as the standard deviation of all the height values within an image. Lenses incubated with pure HA solution were also dried and imaged to determine the effect of sorbed HA on the lens surfaces. Topographical images of dry lenses at two magnifications, as well as images after in- cubation of the lenses with 0.1% pure HA solution are shown in Figure 1. All three materials exhibit a smooth surface (RMS values of 5.4 ± 0.4 nm, 2.2 ± 0.3 nm, and 2.5 ± 0.4 nm for balafilcon A (Figures 1A and 1B), senofilcon A (Figures 1D and 1E), and samfilcon A (Figures 1G and 1H), averaged over 10x10µm image area). Nanometer-scale, pit-like features are observed on all polymers, with balafilcon A showing the larg- est features (25.6 ± 7.9 nm deep pits). In comparison, senofilcon A shows denser and shallower features (6.0 ± 1.5 nm), similar in depth to those of samfilcon A (4.5 ± 1.5 nm). The average pore diameter on senofil- con A (230.0 ± 54.7 nm) is similar to that on samfilcon A (189.2 ± 71.7 nm). After incubation with pure HA solu- tion, features on all three hydrogels are less pronounced as RMS surface roughness decreases to 1.9 ± 0.6 nm, 0.7 ± 0.4 nm, and 0.6 ± 0.1 nm for balafilcon A (C), senofilcon A (F), and samfilcon A (I), respectively. While the surface architecture of samfilcon A appears similar to that of senofilcon A, that of balafilcon A is much different. Balafilcon A has been described as having an "island-like" structure, and senofilcon A as having a "sponge-like" surface topography (Teichroeb et al, 2008). AFM section analysis of balafilcon A (Figure 2) confirms that the sorbed HA uniformly covers the lens and reduces pit depth (3.7 ± 1.4 nm) to obscure the islands and val- ley-like features observed on images of the control lens. Others similarly found decreased average lens pore size in scanning electron microcopy (SEM) photomicrographs of a different silicone hydrogel lens (Safilens, Staranzano, Figure 3. CSLM images of balafilcon A (A), senofilcon A (B), and samfilcon A (C) incubated overnight with 0.1% (w/v) pure HA solution, then stained with safranin; images of senofilcon A (D), and samfilcon A (E) after scratching the sorbed HA layer with forceps. HA on balafilcon A could not be imaged by this method because the stained lens did not sufficiently adhere to the glass slide. Images A-C were captured using a 20X microscope objective and 2X confocal magnification; images D-E were captured using a 4X microscope objective. HY ALURONAN IN LENS CARE