Optometry & Vision Science:
Soft Contact Lens Surface Profile by Atomic Force Microscopy
Giraldez, Maria J.*; Serra, Carmen†; Lira, Madalena†; Real Oliveira, M. Elisabete C. D.†; Yebra-Pimentel, Eva‡
Department of Applied Physics (Optometry Group), University of Santiago de Compostela, Santiago de Compostela, Spain (MJG, EY-P), Nanotechnology and Surface Analysis Service, C.A.C.T.I., University of Vigo, Vigo, Spain (CS), and Department of Physics (Optometry), University of Minho, Braga, Portugal (ML, ME, RO).
This work was presented at the American Academy of Optometry Meeting, Anaheim, CA, October 22 to 25, 2008.
Received June 11, 2009; accepted February 15, 2010.
Purpose. This study was designed to qualitatively and quantitatively characterize the surface morphology of four unworn conventional hydrogel contact lenses (omafilcon, hioxifilcon, nelfilcon A, and ocufilcon B) and two silicone-hydrogel contact lenses (senofilcon A and comfilcon A) without surface treatment.
Methods. Surface roughness was assessed using atomic force microscopy in Tapping ModeTM to determine the representative roughness parameters mean surface roughness (Ra), mean square roughness (Rms), kurtosis (Rku), and skewness (Rsk). To date, these last two parameters have not been used to characterize contact lens surfaces. Rku provides information on the distribution of spikes above and below the mean line, and Rsk provides information about the asymmetric roughness of surfaces. The surface topography of the lenses was also mapped in detail.
Results. In all the lenses, higher Ra and Rms values were obtained when larger surface areas were examined. The daily replacement contact lenses (nelfilcon A and ocufilcon B) showed the highest Ra and Rms values but according to their Rku scores, their surface profiles were less spiky than the remaining lenses. On the contrary, the lowest Ra and Rms values were recorded for comfilcon A and omafilcon A, which also exhibited the spikiest surface profiles. All the materials except the hioxifilcon showed a predominance of peaks (Rsk >0) over troughs.
Conclusions. The shape parameters Rku and Rsk are useful for characterizing contact lens surfaces, because they provide different yet complementary information to that offered by Ra and Rms. Precise knowledge of the shape profile of a contact lens surface will give an idea of its susceptibility to deposit formation or colonization by microorganisms.
Contact lenses are a safe and effective mode of vision correction and today's industry offers wearers the choice of continuous wear, overnight orthokeratology, frequent-replacement or daily-disposable lenses among others. However, despite these options, including different care and maintenance systems, there are still features of contact lenses that could be improved such as reducing microbial contamination.1 The susceptibility of a contact lens to microbial contamination is determined by factors such as its surface roughness because imperfections of the lens surface is where deposits are likely to form.2 Some authors have demonstrated that a greater surface roughness will increase the risk of a biofilm forming on the lens surface3 and that bacterial transfer from a contact lens is determined by the roughness and hydrophobicity of the surface receiving the bacteria.4 Further, a smooth surface is essential for the optical quality of a contact lens because reduced scattered light improves the performance of an optical system.5 Developments in soft contact lens materials continue to be an important issue, because the performance and comfort of a contact lens will depend on the material, its surface architecture, and the quality of the lens manufacturing process.6–8 In addition, the performance of contact lenses does not remain constant over time and lens surface changes induced by wear will also affect their performance and determine a need to replace the lens.
Atomic force microscopy (AFM) provides detailed information on the surface characteristics of contact lenses3,9–17 and is a powerful tool for the high-resolution examination of the structure of the hydrated contact lens surface. The method has the advantages that it avoids artifacts due to dehydration and coating9,18 and allows for non-destructive surface topography and roughness measurements. AFM consists of a microscale cantilever with a sharp tip (probe) that is used to scan the specimen surface. The cantilever is typically made of silicon or silicon nitride with a tip radius of curvature of the order of nanometers. When the tip is brought into the proximity of a sample surface, forces between the tip and the sample cause the cantilever to deflect according to Hooke law.12 The advantage of AFM over conventional microscopy or scanning electron microscopy (SEM) is the high level of resolution offered in three dimensions and that topographic information can be obtained in aqueous, non-aqueous or dry conditions, eliminating the need for sample preparation (e.g., dehydration, freezing, or coating). In effect, AFM has proved useful for characterizing tear deposits on worn soft contact lens surfaces3,19 or characterizing the rigid gas permeable contact lens surface.11
The parameters generally used to quantify roughness include height parameters such as average roughness (Ra) and mean square roughness (Rms),9,10,12–15,20 whose mathematical definitions are as follows:
where, M and N are the number of data points in X and Y, and Z is the surface height relative to the mean plane. Ra is the average deviation or arithmetic mean of the profile from the mean line; it is universally accepted and is the most used international parameter of roughness. Rms is the standard deviation from the mean surface plane. Although Ra and Rms seem to be the most informative and consistent parameters used to define the surface topography of contact lenses,14 they both show a dependency on sample length.20–22 Moreover, these parameters make no distinction between spikes and troughs, and thus provide no information about the spatial structure of a surface. The practical consequence is that two surfaces that are indistinguishable according to their Ra or Rms values could show different spatial features that may only be identified by a parameter that is sensitive to these characteristics. The real geometry of a surface is so complex that only by increasing the number of parameters used can a more accurate description be obtained.23 Two statistical parameters of roughness, not generally used to analyze contact lens surfaces, are kurtosis (Rku) and skewness (Rsk):
where, Z is the surface height relative to the mean plane. Rku is a measure of the sharpness of the profile about the mean line that provides information on the distribution of spikes above and below the mean line. Thus, spiky surfaces will have a high kurtosis value (Rku >3) and bumpy surfaces a low value (Rku <3).23 Rsk is a measure of the symmetry of the profile about the mean line, giving information on asymmetrical profiles for surfaces with the same values of Ra and Rms. Negative values of Rsk indicate a predominance of troughs, whereas positive ones are observed for surfaces with peaks. The use of both shape parameters, Rku and Rsk, which serve to distinguish between two profiles with the same Ra and/or Rms,23 has been reported in several biomedical fields.24–31 Fig. 1 shows the shape profiles of two surfaces with a similar Ra but different values of Rsk or Rku.23,32
The clinical applications of Rku and Rsk in the contact lens field could be to provide an idea of the susceptibility of a contact lens surface to deposit formation or colonization by microorganisms. Different shapes could determine a greater specific surface area, and thus more available active sites for thermodynamic adherence reactions. As two surfaces with similar Ra or Rms could differ in shape (Fig. 1), they may also differ in their performance.
The aim of this study was to qualitatively and quantitatively characterize using AFM the surfaces of four unworn conventional hydrogel contact lenses and two unworn silicone-hydrogel lenses free of surface treatment. For this purpose, we determined the roughness parameters Ra and Rms and shape parameters Rku and Rsk. As far as we are aware, although some of the contact lenses examined here have been analyzed by other authors using AFM,15,16 these studies did not consider shape parameters.
Six commercially available contact lenses were examined: four conventional hydrogel and two silicone-hydrogel lenses. The characteristics of the six contact lenses are provided in Table 1. They all were manufactured by cast-molding and had no surface treatment. Although all the lenses are suitable for daily wear, manufacturers recommend a different replacement frequency (Table 1). Senofilcon A and comfilcon A are silicone-hydrogel contact lenses, whereas hioxifilcon, omafilcon A, and ocufilcon B are hydroxyethylmethacrylate (HEMA) copolymers and nelfilcon A is a polyvinyl alcohol-based material. Osmo 2, the hioxifilcon-based lens used here, contains the main monomers of hioxifilcon (2-HEMA glycerylmethacrylate) plus methacrylic acid (MA).
Contact lens surfaces roughness and topography were determined by AFM (Veeco, multimode-nanoscope V) in Tapping Mode™. To image the samples, lens surface areas of 5 μm × 5 μm and 14 μm × 14 μm were scanned. Although the method used is the same as for dry conditions, we designed a special cell so that measurements could be made on the lenses in their original shipping fluid. After removing the lens from its container using sterile silicone-protected tweezers, a small piece of each contact lens was obtained and fixed with double-sided adhesive carbon tape to a glass cover slip, which had been previously glued to the magnetic stainless steel sample puck, housed in the cell, using a solvent-free epoxy resin. Finally, the cell was filled with the same saline solution used to store the soft contact lens to keep it hydrated during microscopy observation. All procedures and examinations were conducted in the same room kept at 21°C and approximately 50% relative humidity. The images were processed using the Vision®32 and Nanoscope v7.20 software packages. Roughness parameters Ra and Rms were calculated for areas of 25 and 196 μm2 and Rku and Rsk for a 25 μm2 area. Given their statistical nature, roughness parameters are calculated by the instrument from multiple measures in each area. Measurements were repeated three times per contact lens material and surface area.
Statistical analysis was performed using the SPSS Professional Statistics 17 program (Statistical Package for the Social Sciences). Independent sample t-tests were used for comparisons among different areas within the same material whereas the different materials were compared by one-way ANOVA. In both cases, the Levene test was used to assess equality of variances and, depending on the outcome, the Tukey or Games-Howell method was used.
The mean roughness parameters Ra and Rms recorded for each of the contact lens materials are shown in Table 2. Although higher roughness parameters were obtained for larger areas, the difference was only significant for the hioxifilcon-based and nelfilcon A contact lenses (paired t-test, p < 0.05).
Surface topographies, Ra and Rms varied according to the lens material. Thus, across a 25 μm2 area, comfilcon A showed the smoothest and flattest surface (Ra = 1.56 nm), followed by omafilcon A (Ra = 1.90 nm).
The daily replacement contact lenses exhibited significantly higher roughness scores for both areas examined than the rest of the lenses (Tukey test, p < 0.05) and in turn rendered similar roughness values among themselves (Tukey test, p > 0.05) (Ra = 11.25 nm and 12.99 nm for nelfilcon A, and Ra = 11.45 nm and 11.01 nm for ocufilcon B, respectively, for the areas 25 μm2 and 196 μm2). The difference in roughness recorded for the different areas was significant in the case of nelfilcon A (t-test, p < 0.05).
Fig. 2 illustrates the surface topographies of the lenses with the lowest (comfilcon A and omafilcon A) and highest (nelfilcon A and ocufilcon B) roughness scores determined for the 25 μm2 area as 3-D images.
Differences in roughness values for the same area were found between the other two high-water content lenses, hioxifilcon and omafilcon A (Tukey test, p < 0.05); the hioxifilcon-based contact lens showing a rougher surface (Ra values of 4.31 and 5.91 nm for the areas 25 μm2 and 196 μm2, respectively) than omafilcon A.
When comparing the silicone hydrogel contact lenses, although higher roughness scores were recorded for senofilcon A (Ra values of 3.33 and 3.76 nm for 25 μm2 and 196 μm2, respectively), differences with respect to the comfilcon A contact lens were not significant (Tukey test, p > 0.05). However, senofilcon A showed a lower variation in Ra values when the area measured was increased. The corresponding 3-D image of the senofilcon A lens surface over a 25 μm2 area is shown in Fig. 3. There seems to be no standard difference in roughness between silicone hydrogel and conventional hydrogel contact lenses, because fairly similar values were obtained for the senofilcon A and both the hioxifilcon-based and omafilcon A contact lenses (Tukey test, p > 0.05). Similarly, differences in roughness between comfilcon A and omafilcon A were not significant (Tukey test, p > 0.05).
Table 3 provides Rku and Rsk data measured across a 25 μm2 area of each lens. Thus, the Rku values recorded for comfilcon A and omafilcon A failed to vary (Games-Howell test, p > 0.05) and indicate a larger number of peaks than the other lenses; their positive Rsk values reflect a predominance of peaks on their surface. The remaining lenses were less spiky indicated by a significantly lower Rku than observed for comfilcon A (Games-Howell test, p < 0.05). The negative Rsk value obtained for hioxifilcon (Rsk = −0.22) reveals a predominance of troughs on the lens surface, which can be observed in the 3D image of a 25 μm2 area of this lens (Fig. 4). However, no significant differences were observed between the hioxifilcon-based, ocufilcon B and senofilcon A lenses (Tukey test, p > 0.05), which rendered different skewness values to the other lenses examined (Tukey test, p < 0.05).
Despite the possibility of using disposable soft contact lenses to avoid complications associated with long time wear, improving hydrogel contact lens materials is one of the main focuses of research in this field.19 The surface properties of contact lenses and interface interactions between the lens and eye surface may induce biological deposits, corneal damage, and infection.33,34 To improve tolerance and reduce the adsorption of deposits, more research needs to be targeted at modifying surfaces and developing new polymer materials.
A different surface roughness in a new lens can be the result of the manufacturing method and the material's properties. The spin casting method generates contact lenses with the smoothest surfaces, followed by cast-molding and then lathe-cut lenses.13,35 All the lenses tested in our study were cast-molded, and their roughness parameters were similar to the ranges reported for other non-surface-treated cast-molded lenses.13 Thus, the roughness differences between lenses observed here cannot be attributed to the manufacturing procedure. Besides the mode of elaboration, other authors have linked the presence of MA10 or a reduced water content4,13 to a greater lens surface roughness.
In this study, highest roughness values were recorded for the daily replacement hydrophilic contact lenses (nelfilcon A and ocufilcon B), which showed similar roughness scores for both surface areas analyzed. In contrast, comfilcon A showed the smoothest, or flattest surface (Ra = 1.56 nm), followed closely by omafilcon A (Ra = 1.90 nm). Similar roughness values were observed for the hioxifilcon-based material and senofilcon A, yet their surface appearance was different (Figs. 3 and 4). Although the hioxifilcon-based contact lens contains MA, which should determine a greater surface roughness, its similar Ra to senofilcon A could be attributed to its high water content. As may be observed in Fig. 3, senofilcon A shows a granulated surface structure, which is similar to that previously reported for the AFM observation of senofilcon A16, of galyfilcon A12 and for the cryogenic SEM visualization of the latter.36 Galyfilcon A is a non-surface-treated silicone hydrogel contact lens that contains polyvinylpirrolidone (PVP) as an internal wetting agent.
Silicone-hydrogel contact lenses exhibit different surface characteristics depending on their chemical composition and surface treatments.37 Surface treatments are targeted at obtaining wettable surfaces38 although the surfaces of the silicone-hydrogel contact lenses examined here were untreated. Thus, senofilcon A incorporates an internal wetting agent (polyvinyl pyrrolidone) that apparently leaches to the lens surface, and the Aquaform™ technology used in comfilcon A minimizes lens dehydration by forming hydrogen bonds with water molecules, creating a naturally hydrophilic contact lens that retains water inside the lens.39,40
The roughness parameters obtained for these lenses were similar to those observed previously in silicone-hydrogel contact lenses lacking surface treatment, such as galyfilcon A and comfilcon A,12,15 but lower than those reported for surface-treated lenses of this type.13,14 Although not significant, the differences observed in the present study between senofilcon A and comfilcon A could be related to the effect of water content on surface roughness.13 Despite the similar surface appearance of our silicone hydrogels and those examined by others,15,16 Teichroeb et al. observed higher roughness parameters for senofilcon A than comfilcon A when measuring a 25 μm2 area. These differences could be related to the fact that the lenses were analyzed after drying in ambient conditions for 15 min.
Our results indicate that roughness varies with magnification. Ra is the arithmetic mean of the departures of the profile from the mean line.20 Thus, it should not vary with magnification for a surface with homogeneously distributed irregularities, regardless of how smooth or rough the surface is. However, the irregularities of most surfaces are not perfectly evenly distributed and effective differences in contact lens surface roughness values have been observed at different magnifications, with higher roughness scores obtained for larger areas.14 Hence, the amount of variation could reflect how homogeneous a surface is. According to our results, senofilcon A has the most homogeneous surface, because it shows the least variation in Ra measured over 25 μm2 and 196 μm2. Among the daily replacement contact lenses, which rendered the highest roughness scores in this study, the Ra values for the two surface areas obtained for nelfilcon varied significantly as did those recorded for the hioxifilcon-based contact lens. This could mean that the two lenses, despite a similar roughness distribution over the lens surface, show a different extent of roughness. However, this assumption would only hold true if the small areas analyzed by AFM were representative of the entire lens surface. Because roughness varies with magnification, the size of the measured area must be considered when comparing the results of different studies.21,22
Occasionally, a contact lens wearer will suffer an adverse response to a lens. These problems are frequently caused by bacterial contamination of the contact lens surface, and keratitis is one of the most feared complications.41,42 Contact lenses absorb tear film proteins and lipids, and this induces lens contamination and deterioration. Moreover, the build-up of tear film components on contact lenses can cause discomfort and inflammatory complications such as giant papillary conjunctivitis,43,44 and this may occur with any type of daily or extended wear lenses.45 This adsorption depends mainly on the contact lens material and varies according to the tear secretion rate and certain pathological conditions. Research on conventional poly-HEMA-based lens materials has shown that the deposition of lysozyme and albumin depends on the polymer's composition,46 charge,47,48 and water content.49 Silicone-hydrogel materials give rise to different deposition profiles to those associated with the use of conventional poly-HEMA hydrogel lenses in that they induce less protein deposition and more lipid deposition.50–52 Surface roughness also needs to be considered, because deposits are more likely to form on imperfections of the lens surface.2 Our findings would seem to suggest that daily replacement contact lenses are more prone to deposit formation during wear because these lenses showed the highest roughness scores. Accordingly, a strict replacement regime would be recommended for the use of nelfilcon A and ocufilcon B contact lenses.
As mentioned above, when measuring larger areas, a higher variability of roughness parameters values is observed on a non-homogeneous surface. Because the present study is the first to apply the parameters Rku and Rsk to contact lenses, and to reduce the possible effect of variations in their values, we only determined these shape parameters across a single surface area (25 μm2). Measuring Rku and Rsk over larger areas could nevertheless be the aim of future studies. The Rku and Rsk values obtained for the nelfilcon A and ocufilcon B lenses reveal a low peak distribution profile such that despite a predominance of peaks, the surfaces of these lenses have few high peaks and deep troughs. For the lenses displaying the lowest roughness parameters (comfilcon B and omafilcon A), Rku values indicated the highest peaks and deepest troughs with more peaks than troughs because Rsk was positive. The clinical implications of high peaks and/or deep troughs over a contact lens surface could be enhanced deposit formation, even for a surface with a low Ra. Thus, the present observations indicate that including statistical parameters such as Rku and Rsk when characterizing a contact lens surface will complete the information on the finished quality of the surfaces. Further work is needed to establish whether a surface with low Ra and Rms is really less prone to deposits if its Rku is high and Rsk is negative. An improved understanding of the surface roughness of different types of contact lenses will better prepare practitioners to prescribe the most suitable lens for a given patient, and interpret the clinical performance of the lenses they prescribe in terms of patient symptoms and ocular surface signs.
We thank Prof. Cesar Sanchez Sellero (Department of Statistics and Operations Research, University of Santiago de Compostela, Spain) for his help with the statistical analysis.
M. Jesús Giraldez
Escuela de Óptica y Optometria;
Universidad de Santiago de Compostela
Santiago de Compostela, Coruña
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Polymer Engineering and ScienceCharacterization of Surface Roughness of Unworn Hydrogel Contact Lenses at a Nanometric Scale Using Methods of Modern MetrologyPolymer Engineering and Science
atomic force microscopy; contact lens; hydrogel; surface analysis
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