The recognition and appreciation of soft contact lenses as simple, efficient and aesthetically gratifying vision-correction devices is ever growing, especially among younger population. Stable thin tear film uniformly spread over corrective lens surface is essential for acute vision, and also for comfortable and safe contact lens wear. The significant efforts have been invested by the contact lens industry to develop soft lens surface that is completely wet by tear aqueous in the ocular environment. Number of the publications dedicated to the wettability properties of the soft hydrogel lenses is on the steady rise. However, the clinical results show that no unambiguous correlation emerges when lens surface wettability in vitro is judged against tear film stability evaluated in vivo. This paper assesses and compares the modern techniques used for evaluation of soft contact lens surface wettability and reports some findings regarding relations between lens surface wettability in vitro and in vivo. © 2011 EDP Sciences and Springer.

With the advance of micro-technology, the feasibility of fabricating circuit-embedded contact lenses to improve visualacuity or monitor health is under active study. However, embedded microcircuits and concomitant batteries exhibitsignificantly lower oxygen transmissibility than that of available silicone-based lens materials. Because corneal cellsreceive oxygen primarily from the atmosphere, corneal hypoxia and concomitant swelling (i.e., edema) is a majorsafety concern. To address this concern, we predict cornea edema, which is the clinical symptom of hypoxia, inducedduring wear of multi-embedded contact lenses. Multi-embedded lens architectures were designed with Comsol Multiphysics 5.4 platform (Comsol Inc., Burlington MA),and incorporated into a 2D continuum diffusion-reaction computational model. To determine changes in metaboliteconcentrations of the cornea caused by the increased anaerobic metabolism during lens-induced hypoxia, theproposed model tracks the transport and kinetics of eight metabolites throughout the corneoscleral system. Changesin metabolite concentrations alter the chemical potential at the corneal endothelium. Via Kedem-Katchalsky membranetransport relations, the endothelium transports excess water from the aqueous humor into the cornea causing swelling.We calculate central-to-peripheral corneal edema with different types of embedded lenses based on oxygen transportacross the lens and post-lens tear film. We investigate lens oxygen permeability, number of embedded components,and location of the embedded components. Figure 1 provides an example encasement scleral lens containing two transparent axisymmetric embedments. Figure2 provides the oxygen tension contour throughout the example lens, post-lens tear, and the cornea when the oxygenpermeabilities ( Dk) are 0, 30, and 160 Barrers for peripheral embedment, central embedment, and lens encasementrespectively. Oxygen deprivation within the cornea is shown below the embedded components. Even though thegreatest oxygen deprivation occurs below the peripheral embedded component, maximum swelling occurs at theregion closer to the central cornea due to limbal metabolic supply reducing the swelling at the periphery. Figure 3provides the swelling profiles when changing the oxygen Dk of the central embedment while keeping other parametersconsistent to that of Figure 2. As the central embedment decreases in oxygen Dk , an increase in swelling is apparent.Figure 4 provides the difference in swelling by changing the location of the peripheral embedment while keeping otherparameters same as Figure 2. Due to limbal metabolic support, peripheral corneal edema significantly decreaseswhen the peripheral embedment is closer to the limbus. We have devised a computational design tool to evaluate the safety of embedded contact lens against corneal edema.Oxygen transmissibility of an embedded component plays a significant role on the localized swelling of the cornea. Asthe metabolic supply from the limbus reduces hypoxia near the lens periphery, any low oxygen-transmissibilityembedments should be placed near the lens periphery.

PURPOSE: A method to measure the tear film beneath a soft contact lens, referred to as post-lens tear thickness (PLTT), would have many applications to contact lens research. In this study a noninvasive technique for measuring the PLTT is presented. METHODS: The feasibility of measuring the tear layer by optical pachometry was first assessed using a model eye. The baseline corneal thickness (B) of both eyes of 21 subjects was measured, etafilcon-A ionic disposable soft contact lenses (58% water) were inserted, and the total thickness (T) of the cornea, contact lens, and PLTT were measured. After the pachometry readings the lenses were removed and their center thickness (C) determined. The PLTT was calculated using the equation: PLTT = T-(B+C). Two sets of measurements of T were performed at 15 and 25 minutes after lens insertion. The entire procedure was repeated at a second visit. RESULTS: The pachometry measurements of the small aqueous reservoir between the model eye and the lens closely matched those obtained by direct microscopic measurement. For human PLTT, the mean values (and 95% confidence intervals) for right eyes on visits 1 and 2 were 11 (8, 13) and 12 (10, 15) microm, respectively, and for left eyes were 12 (10, 15) and 11 microm (8, 14) microm, respectively. CONCLUSIONS: It is possible to measure the post-lens tear thickness using optical pachometry. The variability between repeated measurements suggests that with careful sample size planning, the technique is sufficiently precise to be useful in group assessments of PLTT.

A Schirmer tear test (STT) is commonly used to gauge human tear production, especially when dry-eye symptoms present. In an STT, the rounded tip of a standardized paper strip is inserted into the lower fornix of the eye, and the wetted length extending out from the lower lid is recorded after 5 min of eye closure. Longer wetted lengths suggest higher tear production rates. To date, however, there is no methodology to transform STT transient wetting lengths into basal tear- production rates. We develop a physical model to elucidate wetting kinetics in a Schirmer strip. Tear evaporation from the exposed portion of the strip and gravity are accounted for. Careful consideration of the initial depletion of tear in the closed-eye tear prism reveals an initial fast increase in wetted length followed by slower growth. Excellent agreement of the proposed model is achieved with experimental observation. When evaporation is negligible, the slow-growth regime exhibits a linear increase of wetted length in time. The linear-length-growth time regime permits simple calculation of quantitative tear-production rates. We suggest measuring several dynamic wetting lengths along a sheathed Schirmer strip and near the 5-min insertion duration followed by fitting to a straight line. The slope of the length-versus-time data gives the basal lacrimal-supply rate.

Purpose

Previous studies have shown that 1-hour closed-eye contact lens wear with a low-Dk lens causes a significant reduction in corneal pH and an increase in epithelial permeability. In the present study, we evaluated the effects of super-high-Dk/t soft lenses on corneal epithelial barrier function and stromal pH.

Methods

Corneal thickness was measured by optical pachometry, while epithelial permeability and stromal pH were measured by fluorophotometry. A paired-eye design was used in which one eye was randomly allocated to wear a high-oxygen-permeable soft lens (CIBAVision Focus/NIGHT & DAY (Dk/t= 175) while the other eye did not wear a lens.

Results

After 1-hour closed-eye lens wear, neither the difference in corneal swelling (P = 0.206) nor the permeability (P = 0.055) between both eyes was significantly different. The mean pH values under open-eye conditions were 7.66 vs. 7.57 for the treatment and control eyes, respectively (P = 0.082), dropping to 7.27 vs. 7.25 after 1-hour eye closure (P = 0.283).

Conclusions

Although our results are limited to a 1-hour observation period, they do provide evidence that high-Dk materials may eliminate corneal acidosis and reduced epithelial barrier function that accompany closed-eye contact lens wear with lower-Dk soft lens materials.

A novel in vivo flow evaporimeter is developed to measure human tear-evaporation rates. The flow evaporimeter relies on a well-defined flow field to the eye with known and adjustable flow rates and relative humidities, and quantitatively reproduces evaporation rates for pure water. Mass-transfer analysis of the evaporimeter data elucidates, for the first time, the resistance of the human tear-film lipid layer (TFLL) toward minimizing tear loss to the environment. A pilot study on human subjects validates the feasibility of the flow evaporimeter to obtain the tear-film evaporation rates in vivo. Resistance of the TFLL against tear evaporation is found subject specific. Our flow evaporimeter offers an accurate, safe, and convenient diagnostic tool for clinical evaluation of dry-eye-related maladies.