Contact Lens Spectrum
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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 c l s p e c t r u m . c o m 46 C ontact lens wettability and sur- face hydration are among many factors that affect the comfort and satisfaction of lens wearers. A wettable contact lens supports spreading of a stable tear film over the lens. Practitioners and manufacturers alike consider a wettable surface neces- sary for optimal lens function (Maldonado-Codina and Morgan, 2007; Jones et al, 2006; Weikart et al, 2001). Most conventional hydrogels based on poly(2-hydroxy- ethyl methacrylate) (HEMA) are inherently wettable, while many silicone hydrogels are relatively less so. First-generation silicone hydrogel lenses like balafil- con A and lotrafilcon A are plasma-treated to increase their surface wettability (Gromacki, 2005) while main- taining the high oxygen permeability of the bulk lens (Lopez-Alemany et al, 2002; Jones et al, 2006). Second and third generation silicone hydrogels like senofilcon A (Maiden et al, 2002) and samfilcon A (Wygladacz et al, 2014), respectively include polyvinylpyrrolidone (PVP) as a wetting agent within the lens itself, rendering both the lens bulk and lens surface inherently wettable. While this may increase lens comfort on insertion, wet- tability can be lost during wear by two mechanisms, lens dehydration (Jones et al, 2002) and deposition of tear film lipids (Bhamla et al, 2015). Interactions between lens material and lens care solutions also affect lens wettability. Lens care solu- tions including packaging solution, multipurpose solution (MPS), and lens rewetting drops often in- clude surfactants that reduce solution surface tension (Lin and Svitova, 2010) and promote complete and uniform spreading of a hydrated film across the lens. Non-ionic surfactants, such as Tetronic 1107 (Tonge et al, 2001) and 1304 (Ketelson et al, 2005), in lens care solutions have proven effective at maintaining etafilcon A lens wettability. Some care solutions also include a wetting agent with affinity for lens materials. One such agent is the humectant hyaluronan (HA), a bio- polymer present in human ocular tissues and tears (Aragona, 2004). HYALURONAN IN CONTACT LENS CARE PRODUCTS As an effective wetting agent, HA possesses desirable physical and chemical properties, such as viscoelasticity (Necas et al, 2008), hydrophilicity (Brown et al, 1999), and lubricity (Johnston, 2000). As the eye's natural lubricant (Aragona, 2004), it binds up to 1,000 times its weight in water (Laurent et al, 1995), and retains mois- ture at the corneal surface. HA is included as a com- ponent in some contact lens care products, including lens packaging solutions (K121201), MPS (K083757), and lens lubricating eye drops (K032030). The reported benefits of tear HA supplementation include increased tear fluid stability (Lerner et al, 1998), increased cor- neal wettability (Wysenbeek et al, 2004), increased tear film break-up time (Johnson et al, 2006; Mengher et al, 1986), decreased tear evaporation rate (McCann et al, 2012), decreased corneal staining (Condon et al, 1999), and alleviation of the symptoms of dry eye (Johnson et al, 2006; Condon et al, 1999). Uptake of HA via bulk absorption and surface adsorption onto lenses is referred to as HA sorption and can be chemical or physico-chemical in nature. It can be effected by chemical attraction between appropri- ate functional groups of HA and functional groups on/ in a silicone hydrogel. Suitable (e.g., porous or rough) surface morphology may also provide a mechanism for the HA biopolymer to anchor onto the polymeric mate- rial. Senofilcon A and samfilcon A materials are both A c o n fl u e n t h y a l u r o n a n n e t w o r k c oa t s s i l i c o n e h y d r o g e l l e n s e s a f t e r a n o v e r n i g h t s oa k i n B i o tr u e m u l t i - p u r po s e s o l u t i o n REFRESHING THE CONTACT LENS SURFACE K ATA R Z Y N A W YG L A DAC Z, M S, P H D, & DA N I E L H O O K , B S, M S, P H D HY ALURONAN IN LENS CARE