Exploring the Science and Technology of Contact Lens Comfort
CONTACT LENS COMFORT
Exploring the Science and Technology of Contact Lens Comfort
Is there more we can do to improve comfort and reduce dropouts?
By Howard A. Ketelson, PhD, Scott S. Perry, PhD, W. Gregory Sawyer, PhD & Jean T. Jacob, PhD
Lens discomfort is the most common complaint among soft contact lens wearers.1 Successful lens wearers may eventually encounter dryness and discontinue lens wear or become dissatisfied part-time wearers. Studies1 have shown that there is a high dropout rate that is linked to contact lens discomfort.2 Adding to this challenge is an increase in the number of aging contact lens wearers whose tear film changes and environmental stressors can play important roles in creating the pervasive dryness and end-of-day comfort problem. Is there more we can do to prolong comfort and decrease the lens dropout rates?
Lens Wetting and Lubrication
Exciting scientific developments have occurred in the past few years that address technologies and methodologies aimed at improving lens comfort, reducing the number of patient dropouts and allowing clinicians to better recommend a particular set of soft contact lenses and storage care solution to suit individual patient needs.
Two important elements contributing to the overall performance of contact lens wear are the wettability and lubrication properties of lens surfaces.3 These properties can have a significant impact on lens comfort.4 As the eyelid sweeps over the ocular surface—about 15 times a minute—lens wearers will be relatively unaware of the presence of the lens and they will be more comfortable if:
1) the lens has the ability to maintain moisture at the surface (wetting) and,
2) the lens remains lubricated so that the mechanical friction and the associated pressures from the eyelid do not impact the lens and cornea negatively.
The surface properties of silicone hydrogel (SiHy) lenses have undergone significant improvements in the last few years from material and manufacturing aspects.5-7 The wettability of SiHy materials exposed to air is poor due to the rapid rotation of siloxane bonds and the migration of these groups to the lens-air interface.8 This thermodynamically driven migration causes the lens surfaces to become hydrophobic and dewet. This may lead to incomplete tear film spreading and poor lubrication performance especially through the lens wear day. Advanced technologies, such as surface plasma treatment and inclusion of wetting agents in the lens matrix, have been used to overcome these challenges, yet patients continue to report dryness—especially toward the end of the day.
Many marketed contact lens disinfecting care solutions contain comfort agents but few are specifically designed to improve the wetting, moisture and lubrication properties of SiHy lenses. When designing a multi-purpose disinfecting care solution that optimizes wetting and moisture retention for silicone hydrogel lenses, the wetting agent ideally should be water soluble, have a high water-binding capacity and be compatible with the disinfection system. Opti-Free PureMoist MPDS incorporating a new wetting agent, HydraGlyde Moisture Matrix (EOBO-41), is the most recent attempt at designing a lens care product specifically for silicone hydrogel lenses.
In this article, we review novel surface measurement techniques that were developed and used to assess the wettability and lubrication properties of HydraGlyde Moisture Matrix (Figure 1) with a special emphasis on silicone hydrogel materials.
Figure 1. Molecular graphic of the HydraGlyde Moisture Matrix (EOBO-41).
Measuring & Improving Contact Lens Wetting
The science of wettability is based on contact angles, which measure the area between the non-adhering part of a droplet and the surface on which it sits. Contact angles can range from 0° (complete wetting) to 180° (non-wetting). The lower the angle, the more wettable the surface. For traditional soft hydrogel contact lenses, measurement of wettability in a lens material exposed to air is sufficient to characterize the surface properties.9 For silicone hydrogel materials, wettability is more difficult and challenging to measure because when silicone hydrogel materials are exposed to air during the measurements, there is rapid rotation of siloxane bonds and migration of these groups to the lens-air interface. For this reason, the wetting properties are affected by both the lens surface wettability and bulk moisture properties. To characterize these properties, different parameters must be measured in different environments, including contact angles in both air and aqueous environment and lipophilic dye diffusion characteristics in the lens matrix.
Methods to Measure Contact Angles
Laboratory-based techniques to assess contact angles include the sessile drop and the captive bubble techniques (Figure 2).
Figure 2. The sessile drop method measures the contact angle between a drop of water and a contact lens surface exposed to air. Dynamic captive bubble technique measures the contact angle between an air bubble and a contact lens surface in an aqueous environment.
The sessile drop method measures the contact angle between a drop of water and a contact lens surface exposed to air. Dynamic captive bubble technique measures the contact angle between an air bubble and a contact lens surface in an aqueous environment. There is a good deal of literature that explains the static and dynamic contact angle measuring techniques to discover and document the mechanisms for such wetting phenomena.10,11 Sessile drop contact angle tests, in particular, are troubled by the time-consuming air-exposed steps associated with specimen mounting, droplet application and video capture/calculation of the shortest-air-exposure contact angle images. There are also occasional other problems, such as syringe-contaminated test fluids and difficulties in maintaining purity of large volumes of fluid used in hard-to-clean apparatus for underwater contact angle measurements. Employing the “Captive Bubble” technique for monitoring the air-exposed, time-based changes in water wettabilities for in-vitro and ex-vivo lenses is an alternative technique that has a reduced number of experimental variables to obtain reproducible contact angles. Recently, specially designed lens-mounting cells have been used to reduce the experimental challenges and improve the reproducibility of the captive bubble method.10,12
Contact Angle Measurements with HydraGlyde Moisture Matrix (EOBO-41)
The contact lenses selected for each measurement represent a broad range of hydrophobic/hydrophilic surfaces and these lens materials include: Acuvue Advance (galyfilcon A); Biofinity (comfilcon A); PureVision (balafilcon A); O2 Optix (lotrafilcon B); Acuvue Oasys (senofilcon A). Results from the sessile drop advancing contact angle measurements showed statistically significant reduction (p<0.05, Student's T-test) in the angles when the lenses were soaked with the (EOBO-41) test care solution compared to the lens controls. All angles were less than 30°, indicating that all lens surfaces were transformed to hydrophilic surfaces following the HydraGlyde Moisture Matrix exposure (Figure 3).
Figure 3. Sessile drop advancing contact angles of water on SiHy lenses pre-soaked for 24 Hours in Unisol and HydraGlyde Moisture Matrix (EOBO-41).
Results from captive bubble advancing contact angle measurements (Figure 4) revealed statistically significant reduction (p<0.05, Student's T-test) in the advancing contact angles when the lenses were soaked with HydraGlyde Moisture Matrix compared to the lens controls. All lens surfaces were transformed to more hydrophilic surfaces. The low contact angles obtained using the sessile drop method follow a similar trend to those measured using the captive bubble method where significant reductions in the wetting angles are obtained except in the case of the O2 Optix lens material, where plasma treatment of the lens surface minimizes migration of the siloxane groups to the lens surface and helps maintain a wettable surface. This results in smaller differences between test groups relative to the other silicone hydrogel lenses.
Figure 4. In vitro captive bubble results of contact lenses exposed to HydraGlyde Moisture Matrix (EOBO-41).
Methods to Measure Lens Moisture Using Lipophilic Diffusion
Lipophilic dye staining visualizes the hydrophobic/hydrophilic surface and bulk regions of a contact lens that are available for lipid diffusion and attachment via absorption of a lipophilic dye (Sudan IV) onto the hydrophobic regions of the surface and in the bulk lens material. Lenses are placed in a concentrated dye solution (Sudan IV in silicone oil solution) for either 30 minutes or 16 hours (Figure 5). The length of time that the lenses are exposed to the hydrophobic dye environment is meant to simulate an exaggerated hydrophobic lens wear environment. The fewer hydrophobic regions in a lens, the less dye attachment occurs. Every lens behaves differently depending on how hydrophobic it is. The lower the amount of dye measured in a lens, the higher the moisture associated with the lens material.
Figure 5. Examples of photographs for Acuvue Oasys (senofilcon A) and PureVision before and after exposure to HydraGlyde Moisture Matrix, simulating 16 hours in a hydrophobic lens wear environment.
Results with HydraGlyde Moisture Matrix (EOBO-41)
The amount of dye on lenses was measured after exposure to Sudan IV for 16 hours. After 16 hours of Sudan IV exposure, significantly less dye staining was observed for all four lenses when presoaked in HydraGlyde Moisture Matrix compared to a Unisol 4 saline presoak (Figure 6).
Figure 6. Dye diffusion uptake at 16 hours before and after treatment with HydraGlyde Moisture Matrix.
These data illustrate that HydraGlyde Moisture Matrix decreases the extent of Sudan IV hydrophobic dye penetration in contact lenses. In effect, HydraGlyde Moisture Matrix transforms hydrophobic silicone hydrogel lenses into more hydrophilic moisture-loving lenses.
Measuring and Minimizing Surface Micro Friction Parameters
Micro-tribometry (Figure 7) is a technique that uses delicate flexures to resolve forces to levels approaching 10µN, which is approximately the force exerted by each mosquito's foot while standing. Such low force resolution is particularly important in measuring frictional forces on hydrogels because of their softness and the low contact pressures (3-5 kPa) that they experience when being worn. The experimental set-up can dynamically measure the normal and sliding forces across the contact lenses.13 Four commercially available soft contact lenses were tested: Acuvue Advance, Acuvue Oasys, O2 Optix, and PureVision. The lenses were prepared by cycling each lens 6 times through hydration and dehydration cycles and then mechanically secured onto a soft probe. The lens-coated probes were submerged in either a HydraGlyde Moisture Matrix solution or in a saline control solution, and lightly loaded (normal forces were less than 1.5 mN) against a highly polished SiO2 surface to average contact pressures below 5 kPa. The smooth SiO2 surface was then slid back-and-forth below the probe while dynamically measuring the frictional forces and normal forces.
Figure 7. Customized micro-tribometer instrument, used for contact lens friction analysis.
The calculation of a friction coefficient is based on the ratio of the frictional forces to the normal forces.
The micro-tribometry method used in this study was able to differentiate small changes in the friction coefficients of different silicone hydrogel lens materials after hydration and dehydration cycles. Lenses with a high coefficient of friction treated with HydraGlyde Moisture Matrix test solution showed dramatic changes in the friction coefficient. For example, an 84% reduction for Acuvue Advance was measured after lens treatment with HydraGlyde Moisture Matrix.
Figure 8. Average Friction coefficient of HydraGlyde Moisture Matrix Solution vs. Unisol 4 solution on 3 commercial lenses. The average contact pressure for these experiments was below 5kPa.
Measuring and Minimizing Surface Nanomolecular Friction Parameters
In assessing the frictional properties of an interface, the details of both the surface composition and the mechanical interaction across the interface will influence the measured coefficient of friction. Modern analytical methods now fully characterize the outermost few nanometers of a surface. Techniques such as X-ray photoelectron spectroscopy, atomic force microscopy and micro-tribometry have been used to investigate the surface properties of contact lens surfaces, before and after exposure to solutions containing the HydraGlyde Moisture Matrix.
The technique of X-ray photoelectron spectroscopy (XPS) uses X-rays to probe the outermost few nanometers of a material surface. As a result of the photoelectric effect, electrons are given off with energies reflective of the chemical elements present and their local bonding environment. Due to the need to measure electron energies in this technique, hydrogel lens samples must be analyzed in an ultra-high vacuum (UHV) environment. Although this leads to the dehydration of the lens, prior studies have used XPS to generate detailed pictures of the lens surface composition and relate these to in vivo performance characteristics.14,15 XPS has also been used to monitor the interaction of lens surfaces with the HydraGlyde Moisture Matrix.16 An initial analysis of three lens types (balafilcon A, senofilcon A, and lotrafilcon B) prior to exposure revealed specific differences in the concentration of C-O species in the near surface region, with greater concentrations observed for the lenses known to undergo plasma surface treatments at the duction. Subsequent measurements following 24-hour exposures of these lenses to saline solutions of HydraGlyde Moisture Matrix show a greater adsorption of the active copolymer on surfaces exhibiting a greater concentrations of C-O species, consistent with adsorption properties being determined by the polar character of the surface.
Atomic force microscopy (AFM) has become a mainstay in topographic analysis of material surfaces on the nanometer scale.
This technique works by detecting the deflection of a weak cantilever, which supports a probe tip, as function of the programmed motion across a sample surface. With this setup, it is also possible to quantitatively measure interfacial forces, such as friction forces acting within a shearing contact. For friction measurements on the soft lens samples, we have employed a 5-micron colloidal probe as the contacting surface to minimize molecular entanglements when drawing the microscopic probe tip across the surface. With respect to the lens analyzed with XPS, AFM measurements (Figure 9) detected frictional differences between each of the neat lens, as well as additional evidence for the uptake of the HydraGlyde Moisture Matrix through a reduction in friction following solution treatment. Specifically, the lens characterized by the higher C-O content exhibited the greatest reduction in friction, consistent with the higher uptake of the copolymer as measured by XPS.
Figure 9. The AFM instrument was used to measure interfacial friction on the nanometer scale under controlled solution environments.
The influence of HydraGlyde Moisture Matrix adsorption could also be detected by imaging the viscoelastic character of the lens surface. Phase images of a PureVision lens soaked in the HydraGlyde Moisture Matrix (Figure 10) exhibited a measurable difference in elasticity. The bright features appearing in the image can be correlated to microscopic pores and troughs appearing across the lens surface. Careful processing of the oscillating contact between the probe and the lens surface reveals a change in the mechanical properties of the surface upon treatment HydraGlyde Moisture Matrix.
Figure 10. Example of phase images of a PureVision lens soaked in the HydraGlyde Moisture Matrix illustrating viscoelastic character of the surface.
• HydraGlyde Moisture Matrix (EOBO-41) in Opti-Free PureMoist MPDS improves lens wettability and decreases friction
• HydraGlyde Moistrure Matrix masks the hydrophobic regions of silicone contact lenses, making contact lenses moist and lubricated.
• This mechanism of action, repeated during each overnight soak, is expected to provide a high level of lens moisture, wettability and lubrication throughout the daily contact lens wear period. CLS
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|Dr. Ketelson leads the contact lens and lens care performance and ocular surface research activities at Alcon. He is responsible for new technologies in the areas of contact lenses and dry eye. He has held several positions of increasing responsibility since joining Alcon Labs in 2000. His research interests include contact lens performance measurements, contact lens surface chemistry, ocular surface protection and tear film biophysics.|
Dr. Perry is professor of Materials Science and Engineering at the University of Florida. His research interests include contact lens surface chemistry, advanced solid lubricants, and the fundamentals of friction and wear.
Dr. Sawyer is a Distinguished Teaching Scholar and N.C. Ebaugh Professor of Engineering at the University of Florida. He leads the contact tribology efforts in the Mechanical and Aerospace Engineering Department at the University of Florida, and his research interests include soft biomaterials, space tribology, polymer nano-composites, and in situ techniques. Professor Sawyer has published over 100 journal papers, and has numerous patents in the area of tribological materials.
Dr. Jacob is professor of ophthalmology and neuro-science and director of Research at the Louisiana State University Eye Center. She is the chair of the Symposium on the Material Science and Chemistry of Contact Lenses series. She has received research funding from Alcon, Vistakon, Ciba Vision, CooperVision, and Bausch + Lomb.
Contact Lens Spectrum, Issue: September 2011