Surface deposits on low water content hydrogel contact lenses: Comparison of SEM and protein assay techniques

Study of silicone hydrogel contact lenses' surface reflection characteristics using confocal microscopy

The physical and chemical properties of contact lenses (CLs) differ significantly from one another. This is already covered by the FDA classification, which divides soft lenses into groups and subgroups for additional characteristics. The differences relate to both the interior and surface of the lens. Several differences in the surface characteristics of individual contact lenses have been studied and demonstrated to date. However, one of their fundamental physical properties, that is light reflection or, quantitatively, reflectance has not been compared. This paper describes the surface differences of a range of silicone-hydrogel (SiHy) lenses using reflectance confocal microscopy. It shows the relationship between the amount of light reflected from the lens surface and the material parameters. Common SiHy lens materials were used in the study, including two lenses with surface modifications. Light incident at the interface between two media (phosphate-buffered saline and lens) with different refractive indices is partially reflected. The normalized results show significant differences between the reflection signals (1 vs 0.07), and that they are not correlated with the refractive index (R2 = 0.5536). For the water content (%H2O), a general trend was observed that the higher the %H2O, the lower the reflection signal is (R2 = 0.8105). The reflection signal and surface modulus show the best correlation. (R2 = 0.9883). The proposed CLs analysis method, using reflectance confocal microscopy, provides data to differentiate between lenses with and without surface modifications.

Deposition of Fluorescently Tagged Lysozyme on Contact Lenses in a Physiological Blink Model

PURPOSE

To visualize the deposition of fluorescein isothiocyanate (FITC) lysozyme on daily disposable contact lenses (CLs) using a novel blink model.

METHODS

Three daily disposable conventional hydrogel CLs (etafilcon A, omafilcon A, and nelfilcon A) and three silicone hydrogel CLs (delefilcon A, senofilcon A, and somofilcon A) were evaluated in the study. The CLs were mounted onto a novel blink model and exposed to an artificial tear solution containing FITC lysozyme for 2 and 10 hr. The flow rate and blink speed were set to 1 μL/min and 6 blinks/min, respectively. After the incubation period, a 5-mm-diameter disc was punched out from the center of the lens and mounted on a microscope slide. The slides were imaged using the Zeiss 510 Meta confocal laser scanning microscope, which scanned the lens from the front to the back surface at 5-μm increments.

RESULTS

There was an increase in deposition of FITC lysozyme for all lens types with increasing incubation time (P<0.05), with the exception of somofilcon A, which did not show statistical significance between 2 and 10 hr (P>0.05). The conventional hydrogel CLs deposited higher amounts of FITC lysozyme than the silicone hydrogel CLs (P<0.001), with etafilcon A depositing the highest at all time points (P<0.05). Interestingly, at the 2-hr incubation time, most CLs showed a higher amount of deposition at the front surface than the back surface of the lens. In particular, etafilcon A showed preferred deposition at the front surface at all time points.

CONCLUSION

The results suggest that there is differential deposition at the front surface of the CL, which is exposed to the prelens tear film, compared with the back surface of the CL, which is exposed to the postlens tear film. Therefore, it may be beneficial to design CL materials with differing surface properties for the front and back surfaces of the CL to enhance interactions with the tear film and ocular surface.

A Review of Techniques to Measure Protein Sorption to Soft Contact Lenses

PURPOSE

To compare and critically evaluate a variety of techniques to measure the quantity and biological activity of protein sorption to contact lenses over short time periods.

METHODS

A literature review was undertaken investigating the major techniques to measure protein sorption to soft contact lens materials, with specific reference to measuring protein directly on lenses using in situ, ex situ, protein structural, and biological activity techniques.

RESULTS

The use of in situ techniques to measure protein quantity provides excellent sensitivity, but many are not directly applicable to contact lenses. Many ex situ techniques struggle to measure all sorbed proteins, and these measurements can have significant signal interference from the lens materials themselves. Techniques measuring the secondary and tertiary structures of sorbed proteins have exhibited only limited success.

CONCLUSIONS

There are a wide variety of techniques to measure both the amount of protein and the biological activity of protein sorbed to soft contact lens materials. To measure the mass of protein sorbed to soft contact lenses (not just thin films) over short time periods, the method of choice should be I radiolabeling. This technique is sensitive enough to measure small amounts of deposited protein, provided steps are taken to limit and measure any interaction of the iodine tracer with the materials. To measure the protein activity over short time periods, the method of choice should be to measure the biological function of sorbed proteins. This may require new methods or adaptations of existing ones.

Quantification of protein deposits on silicone hydrogel materials using stable-isotopic labeling and multiple reaction monitoring

This study was designed to use multiple reaction monitoring (MRM) for accurate quantification of contact lens protein deposits. Worn lenses used with a multipurpose disinfecting solution were collected after wear. Individual contact lenses were extracted and then digested with trypsin. MRM in conjunction with stable-isotope-labeled peptide standards was used for protein quantification. The results show that lysozyme was the major protein detected from both lens types. The amount of protein extracted from contact lenses was affected by the lens material. Except for keratin-1 (0.83 ± 0.61 vs 0.77 ± 0.20, p = 0.81) or proline rich protein-4 (0.11 ± 0.04 vs 0.15 ± 0.12, p = 0.97), the amounts of lysozyme, lactoferrin, or lipocalin-1 extracted from balafilcon A lenses (12.9 ± 9.01, 0.84 ± 0.50 or 2.06 ± 1.6, respectively) were significantly higher than that extracted from senofilcon A lenses (0.88 ± 0.13, 0.50 ± 0.10 or 0.27 ± 0.23, respectively) (p < 0.05). The amount of protein extracted from contact lenses was dependent on both the individual wearer and the contact lens material. This may have implications for the development of clinical responses during lens wear for different people and with different types of contact lenses. The use of MRM-MS is a powerful analytical tool for the quantification of specific proteins from single contact lenses after wear.