Since the introduction of contact lenses to consumers in the 1950s, there has been an ongoing effort to make contact lenses safer and more comfortable.1–3 Consumer safety has been addressed primarily through attempts to increase the oxygen transmissibility (Dk) through lens materials. Subsequently, silicone hydrogel (SiHy) copolymers were introduced with higher oxygen transmissibility, as compared with traditional hydrogel contact lenses based on hydrophilic monomers, such as 2-hydroxy ethyl methacrylate, N-vinyl pyrrolidone, and polyvinyl alcohol. Improved ocular health has been addressed through multiple approaches, including efforts to increase the water content within the hydrogel and surface modifications to enhance the wettability and ocular biocompatibility of SiHy-based contact lenses. To this end, lens designs have used plasma oxidation of the hydrogel surface, creation of a compositionally distinct surface layer by plasma treatment, addition of internal wetting agents, backbone compositional changes, and synthetic attachment of high water content polymer networks to the outermost lens surface.1 Silicone hydrogel lenses constitute the latest generation of lens design in which advances in water content/oxygen permeability/modulus have been sought through the incorporation of silicone macromers within bulk hydrogel structures. Both the formulation of bulk lens materials and the presence of surface treatments can directly impact the composition of the outermost lens surface. In turn, the surface composition will influence the wettability of the lens and define the surface onto which components of the ocular environment may adsorb.
The contact lens industry reported a 6% growth in registered use of contact lenses in the United States in 2015; daily and weekly/monthly lenses accounted for 91% of the market share.4 During this period, SiHy lenses made up 68% of the lenses prescribed in this market.5 As discomfort still remains the number one reason for contact lens wear dropout,6 many manufacturers are seeking to modify current lens designs or to create new care and maintenance solutions to address health and comfort concerns. As new products are introduced, it is important to document the chemical nature of the lens surface to establish correlations with interfacial properties such as wettability, fouling, and wear. X-ray photoelectron spectroscopy (XPS) has proven many times to be a reliable and very sensitive tool to quantify the elemental composition of surfaces.7–10
The purpose of the work described herein is to report the compositional makeup of the outermost 10 nm of 11 commercially available silicone contact lenses with respect to the distribution of Si through the near-surface region. The focus of the study has been SiHy lenses; the scope of the study is summarized in Table 1.
METHODS AND MATERIALS
Eleven SiHy lenses were investigated in this study. They are detailed in Table 1 together with their USAN name, trade name, manufacturer, and wear schedule. As both Clariti 1 Day and Clariti Elite are somofilcon A, somofilcon A* is used to refer to Clariti Elite. Before analysis, each lens was removed from its blister pack and placed in 5 mL of phosphate buffered saline (PBS) (Mediatech Inc., Manassas, VA) for a minimum of 24 hr. This step served to remove the packaging solution from lenses, thus avoiding contributions of components of the packaging solution to the spectra.11 On removing lenses from solution, excess PBS was removed from the lenses by carefully touching the outermost edge to a piece of blotting paper. Subsequently, lenses were mounted to steel sample holders and dried under vacuum (∼10−6 to 10−7 Torr) for at least 8 hr. Results are presented for analysis of multiple areas across two separate lenses of each lens type listed in Table 1.
X-ray Photoelectron Spectroscopy Analysis
Surface chemical analysis was performed with an XPS system (ScientiaOmicron, Taunusstein, Germany) equipped with a monochromatic Al source (1,486.7 eV) and an EIS-Sphera hemispherical analyzer. Lenses were affixed to a stainless steel sample holder using tantalum strips with the anterior side facing upward before being dried under vacuum. Measurements were conducted under a vacuum environment of approximately 2×10−9 Torr. Charge neutralization was performed using low-energy electrons and argon ions to compensate for the surface charging of the nonconducting materials.12 Survey spectra for each sample were collected at a 50-eV pass energy, step size of 1 eV, and 0.2 sec dwell time. Core level spectra were collected at a pass energy of 20, 0.05 eV step size, and 0.2 sec dwell time.9 Under these settings, each measurement probed an area∼1.75×2.75 mm. The core level spectra of the elemental constituents were recorded as a function of take-off angle (TOA), at 55° and 75°, and used to evaluate the relative compositions as a function of depth within the near-surface region, sampling∼10 and∼3 nm in depth, respectively.13 All measurements were conducted at room temperature. The spectroscopic data were processed using CasaXPS software (Casa Software Ltd). No energy shift was performed when assigning elemental peak intensities. Spectra were analyzed after a linear background subtraction. Elemental percentages were determined through integration of the measured peak areas and normalization by published atomic sensitivity factors.14 Control experiments entailing the parallel and simultaneous transport and analysis of plasma cleaned gold samples confirmed no cross contamination of silicon-containing moieties.
After a 24-hr soak in PBS to remove lens packaging solution and overnight vacuum drying, the elemental composition of the surface of the 11 SiHy lenses was quantitatively determined through XPS analysis. Figure 1 displays typical survey spectra for all lenses.
These spectra contain silicon, carbon, nitrogen, and oxygen intensities at approximately 99, 285, 400, and 533 eV, respectively. The presence of silicon was further indicated through intensity at 151 eV, corresponding to photoemission from the Si 1s orbital. In addition, fluorine appears in the lotrafilcon A and B trace and comfilcon A spectra, consistent with the presence of fluorinated components of the silicon hydrogel polymers for these lens types. For each element present in the survey spectra, higher resolution core level scans, such as that displayed in Figure 2, were collected and integrated in determining the reported atomic percentages. The full elemental compositions of each lens are presented in Table 2, reported in atomic percent. For lotrafilcon A and B, the appearance of fluorine bordered on the detection limit of the system for the integration times used and is thus noted as being a trace amount (<0.5). This result differs from the known bulk fluorine composition because of the limited depth sensitivity of XPS and surface plasma treatment of the lenses producing an altered composition; the result is consistent with literature findings and results.15 The results indicate that the lens surfaces possess significant variations in composition, as reflected in Figure 1 through the relative differences in peak height observed for the different elements. The differences in composition are also clearly reflected in values of elemental composition reported in Table 2. Further evidence of the different surface compositions was detected through details of the core spectra. Deconvolution of the core spectra as illustrated in Figure 2 served to identify the contribution of atoms within the surface region found in different chemical states, thus resulting in multiple peaks within a given core region. For example, the deconvolution of the carbon core level spectra of comfilcon A led to the assignment of peaks at 285, 286.2, 287.9, and 293.1 eV as arising from C-C, C-Hx, and/or C-Si species, C-N and/or C-O species, C=O species, and C-F species, respectively. Relative contributions of these species varied among the different lenses, confirming the compositional variations seen on the surface spectra and elemental percentages; a complete analysis of the species contributing to each spectrum is beyond the scope of this study.
Further analysis of the surface composition was completed through the variation of TOA used to record each spectrum. Two TOAs were used in this study (55° and 75°). By increasing the TOA, the relative distance through the solid, which photoelectrons must travel in reaching the detector is increased. This effect increases surface sensitivity by means of selectively sampling species closer to the surface at higher TOAs. In practice, individual spectra reflect a scaled contribution of species residing at different depths. Based on the kinetic energy of the emitted photoelectrons and the inelastic mean free path in organic materials, it is estimated that 90% of the signal detected at a TOA of 55° arises from species in the outermost 10 nm, whereas that detected at 75° arises from the outermost 3 nm.
Trends in elemental composition with respect to increasing surface sensitivity can be seen across all lenses. This is illustrated in Table 3 and further reflected specifically for the case of Si in Figure 3. The variation in the relative integrated intensities derived from the carbon and oxygen core level spectra of each lens do not conform to a single trend across the lens types. Silicon is observed to increase in all lenses but delefilcon A as a function of decreasing sampling depth. A decrease in nitrogen is seen across all lens types except lotrafilcon B, for which no change was observed. The variation in increasing or decreasing oxygen and carbon intensities with increasing surface sensitivity across lens types likely results from the different chemical backbone compositions, the amount and size distribution of short chain oligomers present in each lens, and mobility of the oligomers through bulk material.
Figure 3 also presents the SD of measured silicon values through scale bars extending from the top of each data set; the total SD is twice the scale bar, centered on the top of each bar. In sum, the variation in data arose from three sources including the residual SD associated with the fitting of spectroscopic features, spot-to-spot variation across a lens, and lens-to-lens variation for a given lens type. The largest contribution to the observed variations was seen to originate from sample-to-sample differences for a given lens type and is believed to be associated with the vacuum drying of the lenses.
Both the silicon content and its relative distribution through the near-surface region (∼10 nm) of the lens materials varied substantially (Fig. 3). Although the dehydration of a hydrogel is known to drive hydrophobic moieties to the outermost surface,9 the differences in silicon content in the near-surface region nonetheless reflect differences in lens formulation and design,16,17 as will be discussed below. For all but one lens studied, increasing silicon concentration was observed with increasing surface sensitivity. The observed enrichment represents a gradient in elemental concentration and primarily reflects the thermodynamically driven mobility of silicon-based polymeric species in the near-surface region. Although the dehydrated conditions associated with the vacuum drying and analysis of the lenses can be considered extreme, physiological conditions within the eye such as tear film breakup may lead to similar mobility, presenting significant levels of silicon at surface of most lenses.
Balafilcon A, lotrafilcon A, lotrafilcon B, and samfilcon A are the only lenses tested in this study for which similar data have been previously reported in the literature.10,15,18 The elemental compositions reported here are in good agreement with previous results for oxygen, carbon, nitrogen, and silicon values, substantiating the validity of the approach. Additional elements previously reported, notably sodium, were considered contaminants associated with deposition from solution during the drying process.
In general, the reported elemental surface compositions reflect the bulk constituents of the SiHy lenses, influenced by molecular rearrangement during production processing or drying, as discussed above. In two known instances, the reported results instead reflect the composition of purposeful surface modifications introduced to modify interfacial properties. First, lotrafilcon A and B possess a 25-nm permanent surface treatment resulting from plasma-enhanced reaction precursors (trimethylsilane, methane, and oxygen) at the surface.19,20 A second example of detecting a surface modification entails delefilcon A, a water gradient lens. This lens consists of a SiHy core and an interpenetrating non-SiHy hydrophilic polymer network at the surface, composed of a cross-linked gel network of polyamidoamine and poly (acrylamide-acrylic) acid. The gel is reported to be approximately 4 to 5 µm in thickness (when hydrated) with a 1 to 2 µm transition zone from the core material to surface gel material.21 In both examples, these surface modifications are of sufficient thickness to shield the inner bulk composition from XPS detection. The presence of Si in the lotrafilcon A and B spectrum is consistent with the plasma deposition of trimethylsilane. The weak presence of Si in the delefilcon A spectrum is ascribed to the mobility of low–molecular weight oligomers within the bulk lens, drawn out by the vacuum drying process but not expected within the outermost hydrogel layer under hydrated conditions.
In all instances, the measured composition reflects the elemental nature of the species that will define the lens' interaction within the ocular environment. Not surprisingly, this class of SiHy lenses incorporating silicone macromers into the bulk material leads to the presence of silicon at the outermost surface. The results of this study make it clear that different lens formulas within this category indeed result in differing levels of silicone species located within nanometers of the lens surface. Also clear is the role of surface treatments and water gradient layers in modifying surface compositions, as in the case of delefilcon A and lotrafilcon A and B. The potential role of hydrophobic silicon–containing species in creating a less wettable contact lens surface is likely to negatively impact tear film stability, tear film evaporation rate, and tear lipid interaction with the lens1,2,22; together these can adversely affect the overall wearer experience.
Knowledge of lens surface composition is central to the design of technologies used to improve the wettability, comfort, and ocular surface compatibility of contact lenses. X-ray photoelectron spectroscopy represents a powerful probe of silicone hydrogel surface composition. Lenses differed most in their surface silicon concentration, with delefilcon A and lotrafilcon A and B exhibiting the lowest silicon contents within the outermost 10.0 nm of the lens surface. Silicon has hydrophobic properties which, when found at the surface, may influence the wettability of the contact lenses and their interaction with the tear film and ocular tissues.
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Keywords:© 2018 Contact Lens Association of Ophthalmologists, Inc.
Silicone hydrogel; Contact lenses; X-ray photoelectron spectroscopy; Surface analysis