In the past 30 years, contact lens wear has increased from 10 million to over 125 million wearers.1 Over the past 5 years, the greatest revolution in contact lenses has been the development of highly oxygen permeable contact lens materials, based upon the combination of siloxane groups with conventional hydrogel (CH) monomers.2–4 These silicone hydrogel (SH) materials transmit significantly greater amounts of oxygen than conventional poly(2-hydroxyethyl methacrylate)-based materials4,5 and their physiological performance, both on an overnight and daily wear basis, is substantially better than that seen with older materials.6–12 However, one major disadvantage with these materials is that the incorporation of siloxane groups makes the materials more hydrophobic. This is demonstrated by their reduced wettability [as determined by contact angle (CA) assessment]13–16 and increased level of lipid deposition17 compared with CHs. To make the surfaces of SH lens materials hydrophilic and more wettable, techniques incorporating plasma into the surface processing of the lens have been developed.3,4,18,19 More recent techniques have involved incorporating hydrophilic monomers into the lens material that “migrate” to the surface of the lens and aid wettability.20–21 The purpose of these surface treatments is to mask the hydrophobic silicone from the tear film, increasing the surface wettability of the materials and reducing lipid deposition.22
Wettability is an important property of contact lens materials, potentially governing the interaction between the ocular surface and eyelid with the contact lens surface. Studies investigating the impact of frequent replacement of hydrogels lenses have shown that as wettability reduces over time, so does lens comfort.23 This may be because of the surface drying between blinks, resulting in hydrophobic areas that irritate the lid as it moves over the lens surface. Wettability may be assessed in a number of ways, using both in vivo techniques and laboratory-based in vitro techniques, as previously reviewed by French.24 Analysis has shown that the treatment of the surfaces of the “first generation” SH materials (Bausch & Lomb PureVision and CIBA Vision Night & Day) have been only partly effective at masking the silicone, with the lenses having significantly more silicone exposed at or near the surface and a more hydrophobic surface (as evidenced by the presence of higher advancing water CAs),25,26 as opposed to conventional lenses.13–16,25,26
In vitro wettability may be assessed using a variety of methods.24 These include sessile drop,13,15,27 captive bubble,14 and Wilhelmy plate,28 all of which have been used to investigate hydrogel wettability. The sessile drop method measures the advancing CA and has been recently reported for SH and CH materials.27,29,30 In the sessile drop method, a poorly wetting surface would demonstrate a high CA, typically of >70°, as seen in Fig. 1. In contrast, a sessile drop image demonstrating a contact lens material that is deemed completely wettable will have a CA close to 0°, and the water droplet will be virtually indistinguishable from the contact lens surface, as shown in Fig. 2.
To date, no published work has been reported on the impact of the deposition of lipids on SH surface wettability. The purpose of this study was to determine if lipid deposition would influence material wettability, determined by in vitro CA assessment, in both conventional and silicone-based hydrogels.
MATERIALS AND METHODS
In this experiment, nine different contact lens materials were utilized, consisting of five SHs (Table 1) and four CH materials (Table 2). Each of these contact lens materials were artificially incubated with two different lipid tear solutions (LTS) containing five lipids, the composition of which is described below. After an incubation period of 2 or 5 days, the wettability of the lens materials was determined using a sessile drop method. These two time periods were chosen to investigate whether differences in incubation periods would influence the wettability during the early periods of lens wear.
Lipid Stock Preparation.
The first step in this experiment was to make a lipid stock solution that contained high concentrations of the desired lipids in ratios similar to that found in a normal healthy tear film. The lipids used were cholesterol, oleic acid, oleic acid methyl ester, cholesteryl oleate, and triolein (Sigma, St. Louis, MO). These lipids are representative of the typical lipid types found in the human tear film.31–35 Information pertaining to these lipids and their concentrations can be found in Table 3.
Initially, to make the lipid stock solution, pure lipids were analytically measured by weight or volume and placed in an amber vial and dissolved in 50:50 hexane and ether. The vial was then wrapped in aluminum foil and stored at −20°C until required.
The second step was to make a LTS that could be used to incubate the contact lenses. The LTS was prepared by removing the lipid stock solution from the freezer and allowing it to thaw in a dark place at room temperature. Phosphate buffered saline (PBS) was heated up to 37°C in a sterile culture tube. Once the PBS had reached its desired temperature, lipid stock was pipetted into the culture tube under a culture fume hood, to maintain sterility of the solutions. The lid of the culture tube was left off to allow the hexane or ether to evaporate off. Once the solution was cooled, the culture tube was reheated to 37°C. The evaporation and reheating stage were completed three times to ensure maximal evaporation of the solvent. Once all the hexane and ether had evaporated, the solution was stored at −4°C for a maximum of 1 week. Two concentrations of the lipids in the LTS were chosen and labeled “low” and “high.” These concentrations were chosen in an attempt to simulate the variable lipid availability during in-eyewear. The low concentration was taken as being arbitrarily close to that seen in a subject with a “normal” tear film. The“high” concentration solution contained a higher concentration of each lipid. This was used in an attempt to mimic the constant replenishment of the tear film, which would occur everyday throughout the duration of lens wear, which was not possible with our in vitro protocol. This concentration was also used in an attempt to mimic more severe in vivo conditions, such as that encountered in subjects with very oily tear films, blepharitis, overnight wear, and in subjects who may not comply with their replacement frequency. The actual concentrations of each lipid used for both LTS concentrations can be seen in Table 3.
There are only a handful of researchers examining the various lipids that are found within the tear film,35–37 and it is well known that lipid concentrations between individuals or even in the same individual can vary from day to day.23,38 Therefore, only ranges or approximate percentages of the lipid types are published, and these are usually based on meibomian gland secretions39–42 and not tear film concentrations. Furthermore, very little data is known about the concentrations of individual lipids found in a healthy “normal” tear film; therefore, the concentrations used here are representative of this possible range.
Cholesterol and cholesteryl oleate were not increased in concentration as much as the other three lipids for the high concentration solution as they do not easily dissolve in aqueous solutions.43 Instead of dissolving within the solution, cholesterols tend to form micelles, which are a mass of cholesterol molecules that aggregate together in aqueous solutions so that the hydrophobic tail is “hidden” in the center of the mass.43 As a result, we were forced to limit the amount that these lipids could be increased in our high concentration LTS.
Contact Lens Incubation.
To incubate the lenses, the LTS was sonicated for 30 min at 37°C to ensure the solution was homogenous. While in the culture fume hood, 1.5 ml aliquots of LTS were placed in amber vials and one contact lens was placed in each vial via forceps, as per Mirejovsky et al.32 The vials were then incubated at 37°C with constant shaking. Lenses were soaked in the two LTS types for 2 or 5 days and compared with lenses soaked in PBS only. During incubation, lenses were kept incubated at 37°C with constant shaking. This experiment was completed in triplicate.
After incubation, the contact lenses were removed from the sample vials using silicone-tipped forceps (taking care to remove the lens by the lens edge and ensuring that the center of the lens remained untouched) and placed anterior side down on a piece of clean lens paper for a few seconds, to remove any excess nonabsorbed or adsorbed LTS. The lens was then removed from the lens paper, using the silicone forceps, and was placed posterior side down on a custom convex mantle that mimics the lens curvature. The mantle was then placed on the Optical Contact Analyzer (Dataphysics Instruments GmbH, Filderstadt, Germany) directly underneath a syringe. A high-speed camera was focused upon both the lens and the syringe and a 5-μl drop of high performance liquid chromatography grade water was dispensed from the syringe under computer control. The drop was allowed to stabilize and then the mantle was slowly and manually raised until contact was made with the contact lens. After the water drop had settled on the contact lens surface for 2 to 3 seconds, an image of the lens and water interface was taken and saved to the computer. Because of the curved surface of the contact lens, a curved baseline profile-detection fitting algorithm software program was used to determine the angle that formed between the drop and the lens surface (SCA 20 software, Version 2.04, Build 4). The CA on the right and left of the image was determined and the mean recorded as the CA for that material. The experiment was repeated with each of the three lenses and the mean of these three (±SD) reported (Table 4). All data were considered in the statistical model (see Data Analysis).
All data are reported as mean ± standard deviation and range, unless otherwise indicated. The data was investigated using a three-way repeated measures analysis of variance (ANOVA), with incubation time, lipid concentration, and lens material as the factors and the three replicates being the repeated term. Tukey post hoc analysis was completed. In all cases, significance level was taken as p < 0.05.
The average ± SD of CAs for all nine lens materials and under all concentration and incubation conditions are reported in Table 4 and the ANOVA summary table is reported in Table 5.
The highest level interaction term of the ANOVA is significant (Table 5, time × concentration × lens_type, p < 0.001), indicating that the individual comparisons between samples is where the differences lie. Fig. 3 provides a graphical interpretation of the interactions between the variables. To investigate specific relationships, Tukey post hoc tests were conducted between all pairs of samples in the dataset.
The results from the graph in Fig. 3 clearly show three distinct groups, based on contact lens materials and their changes in wettability when comparing PBS against the high concentration LTS, via sessile drop CA measurements. Therefore, all Tukey post hoc results for incubation time and concentration will be examined within these groups of lens materials.
Group 1: SH Wettability
Three of the five SH lenses tested [balafilcon (BA), galyfilcon (GA), and senofilcon (SE)] had unchanging CA values of >95° throughout the entire experiment. According to the post hoc testing, no statistical difference was found between 2 and 5 days of incubation or between any concentrations of LTS (all p > 0.05). When these three individual lenses were compared with all other lens materials, for the 5 days incubation in PBS and the 5 days incubation in the high concentration LTS, no statistical difference was found between BA and GA lenses (both instances have p = 1.00). All other combinations of lens materials under these conditions were statistically significant (all p < 0.02). In other words, this SH group (BA, GA, and SE) was significantly different than all CHs and plasma-treated SHs, being unaffected by the length of incubation or the concentration of solution.
Group 2: CH Wettability
The CH materials all exhibited lower CAs after soaking in the high concentration LTS, with values typically decreasing to approximately 35°, which was significantly lower than that seen with PBS [p < 0.01, except etafilcon (ET), which was not significant]. This result can be seen graphically in Fig. 3, when comparing conventional lenses incubated in PBS and the high concentration LTS over 5 days. Although ET did not show a statistically significant difference between PBS and the high concentration LTS (p = 0.095), there was a significant difference between the low and high LTS for this lens at the 5-day point (p = 0.026), with a lower CA for the high concentration LTS.
Incubation Time Effects.
Next, the relationship between CA and time of incubation was examined for the conventional lenses. From these tests it was found that in a low and high concentration LTS, alphafilcon (AL) lenses had a significant decrease in CA over time (both p = 0.002). Omafilcon (OM) and polymacon (PO) showed no significant change in CA for the low concentration LTS over time (p = 1.00 and p = 0.886, respectively), but did for the high concentration (p = 0.0002 and p = 0.005, respectively). ET showed no significant decrease in CA over time for either solution (p > 0.05).
When the concentration effect was examined within a time period (2 or 5 days), it is found that AL has significant lowering of CAs between 2 days PBS/high, 2 days low/high, and all combinations of the 5 days incubation concentrations (all p ≤ 0.004). For the 2 days incubation of OM, the only significant lowering of CAs was seen between the PBS and high concentration condition (p = 0.02). With 5 days of incubation, OM had lower CAs between PBS/high and low/high incubation solutions (both p = 0.0002). ET only showed significant lowering of CAs for 5 days low/high conditions (p = 0.026) and at all other incubation times and concentrations, the CA did not change (p ≥ 0.09). Finally, PO had lowered CAs for 5 days of incubation for PBS/high and low/high (p = 0.0002) only.
Comparisons Within and Between Lens Material Groups.
At 5 days of incubation in PBS, the conventional lenses were all significantly different from each other (all p ≤ 0.017), except when looking at OM/ET (p = 0.97). The conventional lenses, when compared with the untreated and plasma-treated SH lenses showed no specific trend, as some lenses had significantly different CAs, whereas others did not. When the 5-day high concentration LTS results were examined, there was no difference in CA between any of the conventional lenses (p > 0.2), but all the conventional lenses were significantly different than all SH lenses, including both treated and nontreated (all p = 0.0002).
In summary, conventional lenses were affected by the length of incubation and the concentration of incubation. The CAs for the four conventional lenses grouped into pairs with CA of approximately 95° (AL, OM) and CA of approximately 40° (ET, PO). Incubation in PBS between 2 and 5 days gave little change in CA for either pair. However, in low concentration LTS, the CAs of the pairs converged to approximately 50° at 5 days of incubation and, more markedly, converged to <40° in high concentration LTS at 5 days. This was a general trend and it is emphasized that not every condition showed significant lowering of CAs.
Group 3: Plasma-Coated SH Wettability
The plasma-coated SH materials [lotrafilcon A (LOA), lotrafilcon B (LOB)] both exhibited markedly reduced CAs after lipid exposure, particularly after 5 days incubation with the high concentration LTS, reducing the CA to <5° (p < 0.01). The wettability of LOB in PBS is illustrated in Fig. 4 and in the high concentration of LTS in Fig. 2. The dramatic difference in CA is obvious. Individual comparisons for all combinations are seen in Fig. 3.
Incubation Time Effects.
On examining the relationship between CA and the duration of incubation (2 or 5 days) for the plasma-treated SH materials, it was found that there was no significant difference between CAs over time for either lens when incubated in PBS (p = 1.0). However, CAs did reduce over time in the low concentration LTS for LOA (p = 0.002), but not for LOB (p = 0.99). For the high concentration LTS both showed significant lowering of CAs over time (p = 0.0002).
When the concentration effect was examined within a time period (2 or 5 days), it is found that no difference in CAs is seen for 2 days incubation between any concentration of incubation solution (all p > 0.36). For 5 days of incubation, no difference in CAs was seen for either lens between PBS and low concentration LTS. However, for comparisons between the PBS/high and low/high there were significantly lower CAs for the high concentration LTS (all p = 0.0002).
Comparisons Within and Between Lens Material Groups.
With 5 days incubation in PBS, there was no difference in CA for LOA and LOB. In addition to this, these two materials were not different from two of the conventional lenses, PO and ET (p > 0.75), but were statistically different than OM and AL (p = 0.0002). All of the treated SHs had lower CAs when compared with the untreated SH lenses (all p = 0.0002). After 5 days of incubation in the high concentration LTS, LOA and LOB had no difference in CAs (p = 1.0) but had CAs that were significantly lower than all other lenses (all p = 0.0002).
Overall, the plasma-treated SH lenses were influenced by the length and concentration of incubation, but not for every combination of variables. This group of lenses behaved significantly different than the conventional and the untreated SHs.
The lenses in this experiment seemed to behave according to their broad lens material classifications, irrespective of their relative affinity to lipid deposition. For example, it is known that group II CH lenses deposit lipid at higher quantities when compared with group I or IV lenses.23,44–47 Despite this, all CHs, no matter what the initial CAs, all had a CA of approximately 35° when they were subjected to a high concentration LTS for 5 days. All CHs had similar results for wettability, despite their individual differences in material. This decrease in CA seen for Acuvue 2 lenses, when lipid was deposited, was also seen in a previous report by Copley et al.,48 who used the captive bubble technique.
The SH contact lenses are divided into two distinct groups: those that are plasma treated and those that are not. The Bausch & Lomb balafilcon material is surface treated through a plasma oxidation process that leaves hydrophobic glassy islands.2,4,18,49,50 The CIBA Vision materials (lotrafilcon A and B) have a high refractive index thin plasma coating that gives the contact lens a highly homogenous surface.3,4,19,49,50 The two Johnson & Johnson materials (GA and SE) are TRIS-based materials, but are nonsurface treated in that they incorporate an internal wetting agent based upon polyvinyl pyrrolidone that migrates to the lens surface and acts to enhance wettability of the lens materials.20,21,51,52 The data from this experiment show that the two CIBA Vision materials behaved uniquely when compared with the other materials, regardless of whether they were siloxane based or not. This grouping of “surface-treated” and “nonsurface-treated” materials for siloxane-based lenses has also been reported in wettability research by Maldonado-Codina and Morgan.29 The two plasma-coated CIBA Vision materials began with a reasonably low CA and became completely wettable with lipid deposition. Similar increases in wettability—or decreases in CAs—were found using the captive bubble method by Copley when in vitro lipid deposited Night & Day lenses were analyzed.48
Throughout this study, it is assumed that the lipid from the LTS deposits on to the surface of the contact lens. Previous in vitro experiments in our laboratory have shown that lipid from the LTS does indeed deposit.53 The only question that we cannot answer is whether this lipid adsorbs on the surface or absorbs into the contact lens matrix.
It is hypothesized that the degree of wettability of these contact lenses, when lipid is involved, is highly dependent on the degree of penetration of lipid into the matrix. The contact lens materials that encourage lipid to penetrate deep into the matrix do not interrupt the surface chemistry of the lenses and therefore, there is no change in wettability. This may help to explain the results seen for the nonplasma-coated SH materials. The materials where lipid only slightly penetrates into the matrix, with some lipid also remaining at the surface, produce a moderate improvement in surface wettability, which may help to explain the results with the CH lens materials. We hypothesize that the plasma-coated SH lenses do not allow any (or at most very little) lipid to penetrate into the matrix, and therefore, the lipid is forced to remain on the surface of the lens, causing a significant increase in wettability. In this case, the lipid deposition must alter the surface chemistry in such a way that decreases occur in the surface tension between the tear film and the contact lens, therefore making it easier for the tear film to spread over the contact lens surface. The exact mechanism and cause for the changes in wettability remain unknown.
This was an introductory study aimed to see if lipid incubation would indeed affect the wettability of a contact lens. Now that this has been confirmed, further studies will be conducted to see which lipid species are responsible for the increased wettability. More research is required to look at the degree of penetration of specific lipids into the matrix to support our hypotheses.
The results from this experiment may help to explain why some contact lens wearers report an increase in comfort with their SH lenses in the first few hours or days of wear.54 We believe that this may occur because of certain lipids being surface deposited on certain SHs, which enhances their wettability. Eventually, the build up of lipid deposition stops being advantageous to contact lens wear and becomes deleterious, resulting in reductions in surface wettability, comfort, and vision.23
In conclusion, our data would appear to suggest that lipid deposition may play a significant beneficial role in the overall wettability and, therefore, comfort of a contact lens, for certain lens materials during the initial wearing period. Specifically, lipid deposition tends to increase wettability, as evidenced by a decrease in CA measurement, for both CH lens materials and plasma-coated SH lens materials. This may help to explain why certain SH materials improve in comfort after the first few hours or days of wear.
It is clear that more research needs to be completed to determine when lipid deposition becomes deleterious, the degree to which lipid penetrates the contact lens material, and what results when a more complex artificial tear solution is used containing proteins and mucins. Of course, in vitro experiments are not always directly representative of what is occurring in vivo, therefore, follow-up in vivo experiments should be conducted to verify and substantiate the results.
We thank Dr. Natalie Hutchings for assistance with statistical analysis of the data.
Centre for Contact Lens Research
School of Optometry, University of Waterloo
200, University Avenue West, Waterloo
Ontario, Canada N2L 3G1
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