October 2016
Volume 57, Issue 13
Open Access
Cornea  |   October 2016
Friction Measurements on Contact Lenses in a Physiologically Relevant Environment: Effect of Testing Conditions on Friction
Author Affiliations & Notes
  • Olof Sterner
    SuSoS AG, Dübendorf, Switzerland
  • Rudolf Aeschlimann
    SuSoS AG, Dübendorf, Switzerland
  • Stefan Zürcher
    SuSoS AG, Dübendorf, Switzerland
    Laboratory for Surface Science and Technology, Department of Materials, ETH Zurich, Zurich, Switzerland
  • Kathrine Osborn Lorenz
    Johnson & Johnson Vision Care, Inc., Jacksonville, Florida, United States
  • Joseph Kakkassery
    BetaLogics Venture, Janssen R&D LLC, Raritan, New Jersey, United States
  • Nicholas D. Spencer
    Laboratory for Surface Science and Technology, Department of Materials, ETH Zurich, Zurich, Switzerland
  • Samuele G. P. Tosatti
    SuSoS AG, Dübendorf, Switzerland
  • Correspondence: Samuele G. P. Tosatti, SuSoS AG, Lagerstrasse 14, CH-8006 Dübendorf, Switzerland; samuele.tosatti@susos.com
Investigative Ophthalmology & Visual Science October 2016, Vol.57, 5383-5392. doi:10.1167/iovs.16-19713
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      Olof Sterner, Rudolf Aeschlimann, Stefan Zürcher, Kathrine Osborn Lorenz, Joseph Kakkassery, Nicholas D. Spencer, Samuele G. P. Tosatti; Friction Measurements on Contact Lenses in a Physiologically Relevant Environment: Effect of Testing Conditions on Friction. Invest. Ophthalmol. Vis. Sci. 2016;57(13):5383-5392. doi: 10.1167/iovs.16-19713.

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      © ARVO (1962-2015); The Authors (2016-present)

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Abstract

Purpose: To characterize the effect of lubricant composition and in vitro ageing on the coefficient of friction (CoF) of a wide range of commercially available soft contact lenses (SCLs).

Methods: The CoF of SCLs was characterized by means of microtribometry against a mucin-coated glass disk. One reusable (RU) silicone-hydrogel (SiHy) lens, senofilcon A, and two daily disposable (DD) lenses, etafilcon A (hydrogel) and nelfilcon A (hydrogel), were tested under different lubricant solutions, including a tear-like fluid (TLF) containing proteins and lipids. Five RU (balafilcon A [SiHy], comfilcon A [SiHy], etafilcon A [hydrogel], lotrafilcon B [SiHy], senofilcon A [SiHy]) and five DD (delefilcon A [SiHy], etafilcon A [hydrogel; two lens types], narafilcon A [SiHy], nelfilcon A [hydrogel]) lenses were tested before and after exposure to an in vitro ageing process, consisting of continuous immersion and withdrawal from TLF for 18 hours. The CoF in TLF was further compared to previously published data collected in a different lubricant.

Results: After in vitro ageing, three RU (balafilcon A, etafilcon A, comfilcon A) and three DD (delefilcon A, etafilcon A, nelfilcon A) lenses displayed a significant increase in CoF (P < 0.05). Lenses that contained poly (vinyl pyrrolidone; PVP) showed unaltered CoF after ageing.

Conclusions: An in vitro methodology to simulate in vivo wearing of contact lenses has been proposed. The results suggest that certain lens materials show increased CoF after ageing, with potential clinical implications. The results indicate that the presence of a persistent wetting agent is of advantage to maintain a low CoF after prolonged wearing.

The coefficient of friction (CoF) of soft contact lenses (SCLs), as measured by Roba et al.,1 has been found to correlate with subjective comfort experienced in vivo,2 allowing the comfort that is associated with the lens material to be predicted from in vitro tribological data. Such in vitro tools are crucial to the development and screening of new SCL materials, as declining comfort of SCLs throughout the day is the most common reason for contact lens wearing discontinuation.3 Coefficient of friction is defined as the ratio of lateral to normal forces acting between two surfaces in relative motion. It is a function of not only the properties of the sliding partners, and lubricant, if present, but also is affected by the measurement conditions (speed, contact pressure, and so forth). As such, CoF is not an inherent material property. For the cornea-eyelid sliding system, Pult et al.4 proposed the existence of two separate lubrication regimes persisting at low and high sliding velocities. In the low sliding velocity regime, the CoF is a function of the two sliding partners as well as their interaction with the lubricant. The aim of this study was to investigate how the CoF of SCL materials, acquired at low sliding speeds, is affected by the buffer component of the lubricant (Part I), the organic composition of the lubricant by comparing data from Roba et al.1 with data acquired in a tear-like fluid (TLF; part II), and by extended exposure to TLF; that is, simulated ageing (Part III). 
To extrapolate information obtained from in vitro tribological experiments to the in-eye situation, the experimental conditions, such as contact pressure, sliding speed, as well as the chemical/biological composition of the lubricating fluid and sliding countersurface should closely resemble those existing between the eyelid and cornea. The contact pressure that the eyelid exerts on the cornea has been reported to be in the range of 1 to 7 kPa5 with sliding speeds up to approximately 10 cm/s during blinking.6 These conditions of low contact pressure, high speeds, and typically low frictional forces are challenging to measure using conventional tribometers. However, techniques based on microtribometry have been applied successfully to SCLs in either ball-on-lens7,8 or plate-on-lens1,9,10 configurations. In addition, methods have been developed to measure the friction properties of natural tissue in vivo, as reported by Dunn et al.11 on murine corneas, and between living corneal epithelial cells in culture and a contact lens.12 Another example has been presented by Samsom et al.,13 who measured the CoF between a human cornea and a contact lens in the presence of proteoglycan. 
The majority of SCL CoF measurements have been performed in simple buffers against surfaces, such as soda-lime glass or stainless steel.7,8,14 However, the countersurface, contact-pressure, and the lubricant all have a major influence on frictional force, as demonstrated by Roba et al.1 In this regard, the surface of the palpebral conjunctiva as well as the lid wiper,15 which are in contact with the lens during blinking, are covered by a mucous layer consisting of glycosylated proteins; that is, mucins.16 The carbohydrate chains linked to these proteins are known to form highly lubricious films, analogous to those formed by synthetic polymer brushes.17,18 To generate a biomimetic countersurface for the testing of SCLs, Roba et al.1 used a hydrophobized glass disk coated with physisorbed bovine submaxillary mucin.1 In the eye, the tear film lubricates the lid wiper during blinking, but acquiring enough tear fluid necessary for in vitro tribological experiments is not deemed to be feasible. Therefore, developing a tear substitute, containing lipids, proteins, and soluble mucin, is desirable. An important aspect of this is to find an appropriate tear solution buffer that does not alter the physical properties of the SCL, such as the elastic modulus, swelling ratio, or shape of the material. This would allow comparisons to be made between different SCL materials, and to interpret the measured frictional forces with regards to on-eye behavior. In Part I of the present study, different buffering components were first explored to elucidate the effect that the buffer has on the CoF of three different contact lens materials, using the same methodology as described by Roba et al.1 
Coefficient of friction measurements on SCLs typically have been performed in buffer solutions that were free of protein and lipid components.7,8,10 Ngai et al.9 exposed lenses to lysozyme before the measurement, although the friction experiment was performed in saline buffer. However, the deposition of biological material onto SCLs has been the focus of numerous studies—a topic recently reviewed by Luensmann and Jones,19 and it is well known that proteins and lipids do adsorb on the surface and absorb into the bulk of SCLs.2023 For example, negatively charged SCLs have been shown to have a high deposition of lysozyme and lactoferrin after exposure to an artificial tear solution,20,21 whereas nonionic, silicone hydrogel (SiHy) lenses had a higher degree of lipid deposition.23,24 Additionally, protein adsorption was reported to be influenced by the presence of lipids in the buffer, which points to a complex relationship between deposition, tear composition, and lens material.2325 To investigate the effect a lubricant with complex composition has on the CoF, a TLF was developed containing proteins, lipids, and mucin. In Part II of the current study, the CoF of multiple lens materials in TLF were compared to those reported by Roba et al.1 
The possibility of correlating the amount of deposits with end-of-day comfort and clinical symptoms has led to numerous studies in this field.26 However, no straightforward relationship between comfort and total amount of deposition has been found.2,27 Instead, it has been proposed that the conformation, rather than the quantity, of the deposited species is the determining factor associated with comfort.28 In this regard, Heuberger et al.29 found that friction of albumin-coated surfaces was not influenced by the total amount of adsorbed albumin, but the ratio of native to denatured proteins. Thus, the CoF of pristine lenses in lubricants with complex composition might not accurately represent the subjective perception of the lens after a day of wearing. For this reason, in Part III of the present study, the CoFs of a wide range of commercially available SCLs were measured before and after an in vitro ageing process. The ageing consisted of exposing the SCL to a continuous wetting and drying cycle aimed at mimicking the action of blinking, albeit without mechanical stress. A similar set-up has been presented previously (Copley KA, et al. IOVS 2006;47:ARVO E-Abstract 2407) to monitor changes in wettability of SCLs upon exposure to a protein and lipid solution. All ageing tests were performed in a TLF containing proteins and lipids to ensure a biologically relevant environment during cycling. 
Methods
Lubricants
The composition and the physical properties of the tear-mimicking solution prepared from contact lens packing solution (TMS-PS) and the TLF lubricant solutions are summarized in Table 1. A distinction is made between the lubricant nomenclatures TMS-PS as used by Roba et al.,1 and TLF, to highlight that the latter is fundamentally different in terms of complexity and lipid content. If not stated differently, lubricant solutions were prepared according to the protocols described in detail below. The resulting solutions were divided into 1 ml aliquots and stored at −20°C until further use. Immediately before tribological testing, the aliquots were thoroughly defrosted at room temperature. For each type of lubricant, the osmolality was determined using vapor pressure osmometry (Vapro 5520; Wescor, Inc., Logan, UT, USA). The osmometer was calibrated using standard solutions before the measurement. 
Table 1
 
Physical Properties of the TMS-PS and TLF Lubricants
Table 1
 
Physical Properties of the TMS-PS and TLF Lubricants
Variations of the TMS.
In general, variations of the TMS were prepared according to Roba et al.1 A total of 5 ml serum (Precinorm U; Roche, Basel, Switzerland) was diluted to 100 ml with the appropriate buffer (Table 2), and supplemented with 500 mg lysozyme (L6876; CAS No. 12650-88-3; Sigma-Aldrich Corp., St. Louis, MO, USA). In cases where the buffer consisted of individually mixed salts, the salts were dissolved in ultra-pure water (UPW; Milli-Q; EMD Millipore, Darmstadt, Germany). The pH (InLab Routine; Mettler, Toledo, Ohio, USA) of the final solutions were adjusted to 7.4 ± 0.05 with 2.0 N hydrochloric acid, and filtered with a 0.22-μm filter (Vacuum filtration system, 99150, TPP; Sigma-Aldrich Corp.). 
Table 2
 
Compositions of the Buffer Components Used in the Different Variations of the TMS
Table 2
 
Compositions of the Buffer Components Used in the Different Variations of the TMS
TLF Solution.
The TLF solution was prepared according to a protocol provided by Johnson and Johnson Vision Care, Inc. (JJVCI; Jacksonville, FL, USA) outlined below. Its full composition is shown in Table 3. The composition was chosen to closely resemble that listed in the literature for natural tears. Bright and Tighe30 reported on the concentration of the four major tear-specific proteins: lysozyme, lactoferrin, lipocalin, and immunoglobulin. The concentration of mucin, and the variety and concentration of lipids were obtained from the composition of the artificial tear solution published by Mirejovsky et al.31 Phosphate-buffered saline supplemented with glucose was chosen as the buffer for the TLF as suggested by Rebeix et al.32 Other proteins present in tear liquid, such as vitronectin, fibronectin, and albumin, were added in the form of diluted bovine serum to achieve appropriate concentrations in the ng/ml range.33,34 
Table 3
 
TLF Components and Concentrations
Table 3
 
TLF Components and Concentrations
The components listed under Buffer (Table 3) were mixed at room temperature under vigorous stirring until completely dissolved. Premade aliquots of the lipids (Table 3) were kept at −20°C until use. To prepare the protein solution, mucin was first added to a clean beaker, the lipid stock solution was added. The mucin was used as received, which may mean that some additional species, in particular albumin, also were present.35,36 A 10th of the buffer prepared above was added to the lipid and mucin mixture and the mixture was gently heated (to no more than 37°C) and stirred. Another 10th of the buffer was added after which the solution was required to be completely clear, otherwise the mixture was discarded and the preparation restarted. If clear, the rest of the buffer was added. The remaining proteins were added in the following order, ensuring full dissolution between additions: α1-acid glycoprotein, γ-globulins, β-lactoglobulin, fetal bovine serum, lysozyme, and lactoferrin. The solutions were left overnight at 4°C. The pH of the solution then was adjusted to 7.4 ± 0.05 with 2.0 N hydrochloric acid with a pH meter (InLab Routine; Mettler), and the solution filtered with a 0.22 μm filter (Vacuum filtration system, 99150, TPP; Sigma-Aldrich Corp.). Aliquots were prepared at either 1 ml (for tribological testing) or 15 ml (for the ageing process) and stored at −20°C until use. 
In Vitro Ageing Protocol
To simulate the wear that SCLs are exposed to in vivo during blinking, an ageing procedure was established. In the development of the ageing process, it was hypothesized that the driving parameter influencing the contact lens surface is the transition between closed (full immersion in tear solution) and open (only thin tear film present) eye. Therefore, SCLs were repeatedly immersed in TLF (equilibrated at 30°C in a heating bath) for 20 seconds (rapid immersion) and removed from the bath and exposed to air for 20 seconds (rapid removal; Fig. 1). During the cycling, the lenses were fixed inside a lens-sterilization vessel (Clear Care; Alcon Laboratories, Elkridge, MD, USA) attached to a linear motion drive (LTM-120; Owis GmbH, Staufen im Breisgau, Germany). Cycling was performed for 18 hours in a class II laminar flow box (VSA-180; Skan AG, Allschwil, Switzerland). After ageing, the SCLs were transferred to fresh TLF at room temperature for tribological investigation. 
Figure 1
 
(top) Photograph of the ageing device inside the laminar flood hood. (bottom) Schematic representation of the CL-ageing device. The lens was fixed in a lens holder and repeatedly immersed and withdrawn from a preheated (30°C) TLF solution.
Figure 1
 
(top) Photograph of the ageing device inside the laminar flood hood. (bottom) Schematic representation of the CL-ageing device. The lens was fixed in a lens holder and repeatedly immersed and withdrawn from a preheated (30°C) TLF solution.
Tribological Experiment
Instrument and Setup.
All friction tests were performed on a micro-tribometer (Basalt Must; Tetra, Melle, Germany), following the protocol published by Roba et al.1 (see Fig. 2). Calibrated cantilevers (Tetra) with normal spring constant between 14 and 16 N/m, and lateral spring constant 12 to 17 N/m were used (manufacturer's calibration). The Basalt Must system uses fiber-optics sensors to determine the deflection of the cantilever, which then is calculated into force from the known spring constants. The stroke length of 1 mm was chosen to ensure that the contact area remained below the glass pin, and not at the edge of the glass disk, which would lead to tilting of the cantilever. The speed was set to 0.1 mm/s for all experiments, as suggested by Roba et al.1 This ensures that the contact is in the boundary lubrication regime, and the CoF is representative of the surface properties of the lens.1,4 
Figure 2
 
Schematic representation of the tribological measurement setup.
Figure 2
 
Schematic representation of the tribological measurement setup.
Countersurface Preparation.
The countersurface consisted of a 5-mm diameter glass disk (coverslip thickness #1; Menzel-Gläser, Braunschweig, Germany), which had been exposed previously to oxygen plasma (Nano; Tetra) for 2 minutes followed by vapor silanization (hexamethyldisilazane; Fluka, Sigma-Aldrich Corp.) at reduced pressure (∼10 mbar) in a desiccator connected to a vacuum pump. The glass disk then was bonded to a glass rod by cyanoacrylate glue (Precision Nail Glue; Kiss Products, Inc., Port Washington, NY, USA). Immediately before friction testing, the glass disk was immersed in 1 mg/ml mucin from bovine submaxillary glands type I-S (Sigma-Aldrich Corp.) in PBS for 30 minutes, rinsed briefly in PBS, and mounted on the cantilever. 
Handling of the Contact Lenses.
The SCLs were removed from the packaging solution and rinsed three times with PBS before being installed on a sand-blasted rounded sample holder (cyclo olefin polymer; JJVCI), which had been wetted previously with PBS. The sample holder had the same radius of curvature as the lens. Trapping of solution between the SCL and the sample holder was avoided by moving the lens carefully on the holder in all directions. The SCL finally was centered on the sample holder, fixed with a silicone cover (polyvinylsiloxane; Provil Novo, Heraeus, Hanau, Germany), and covered with PBS as a mounting medium (Fig. 2). Right before the start of the measurement, the PBS was replaced by the lubricant solution. 
For lenses that had undergone the ageing procedure (Fig. 1), TLF was used as mounting medium instead of PBS. Using PBS as a mounting medium resulted in a change in CoF (data not shown), probably due to loosely deposited proteins and/or lipids being dispersed during PBS exposure. 
Measurement Protocol and Evaluation Procedure
For the determination of the CoF, a force ramp consisting of seven discrete normal loads: 0.25/0.5/1/1.5/2/3/4 mN was chosen, corresponding to a contact pressure range of 1 to 7 kPa,1 which has been reported to represent the pressure exerted on the cornea by the upper eyelid during blinking.5 The stroke length was 1 mm and the sliding speed was 0.1 mm/s. The protocol involved collecting the CoF after 0, 50, and 100 cycles of sliding at 2 mN normal load (simulated sliding wear). For data evaluation and statistical analysis, only the CoF data after 100 cycles were included. The data after 100 cycles was most reliable and incorporated the effect that a short-term simulated wearing has on the CoF.1 Before starting the force ramps, a measurement was performed to ensure correct alignment and position of the mucin-coated glass disk on the SCL. 
To evaluate the CoF, experimentally determined normal and lateral force values were calculated by averaging 20 data points in the middle of the stroke. This was done to avoid influence from the reversal points, which are not indicative of dynamic friction. With a measurement frequency of 1000 points/mm, an interval of 0.02 mm was included in the analysis. The CoF then was defined as the regression slope of a straight line fitted to the lateral versus normal force data (see Supplementary Fig. S1). 
Statistical Analysis
The applicability of parametric models was first evaluated with the Anderson-Darling test for normality on the largest sample group (n = 28). The test indicated insufficient evidence to reject the null hypothesis of normality (P = 0.3). Since all data were generated using the same method, normality was assumed for all lenses. However, equal variance was not assumed, considering that the standard deviation of reusable lens 1 (RU_1) was 0.114 versus 0.003 for RU_5. In Part I, the effect of lubricant composition, osmolality, and buffer on lens CoF was compared between the buffers after 100 cycles of sliding, using the Games-Howells37 method for multiple comparisons. In Parts II and III, Welch's t-test38 for pairwise comparison was applied between data acquired on lenses out of the blister (OoB) in TMS-PS and TLF , and between lenses OoB in TLF and aged lenses in TLF, respectively. Data were compared after 100 cycles sliding at a significance level of 0.05. 
Contact Lenses
All SCLs used are commercially available, of −1.00 diopter sphere (DS) and listed in Tables 4 and 5
Table 4
 
DD Lenses
Table 4
 
DD Lenses
Table 5
 
RU Lenses
Table 5
 
RU Lenses
Results
Part I: Effect of Lubricant Buffer on the CoF
To study the effect that lubricant composition had on the CoF, lens RU_5, disposable lens 3 (DD_3) and DD_5 were tested under 5 to 6 different sets of conditions. All lenses contained either an internal or a physisorbed wetting agent. 
The results are depicted in Figure 3 (see Supplementary Table S1 for tabulated values). For DD_3, the choice of lubricant solution had minimum and no significant effect on CoF; a difference of 0.011 was found between the highest and lowest measured CoF. Similarly, RU_5 showed a δ of 0.016, but with a significantly lower CoF in TLF compared to TMS-PS, TMS-PS+NaCl and TMS-HEPES+NaCl. In contrast, the equivalent difference for DD_5 was 0.224. Higher CoF values were obtained for DD_5 when the lubricant contained borate buffered saline (BBS) in comparison with PBS at high and low osmolality for the same protein mixture. For RU_5 and DD_5, a trend with lower CoF in TLF was seen. Besides the aforementioned difference between borate and borate-free buffers, no clear trend could be observed when comparing the CoF values in TMS-HEPES or TMS-PBS at low and high osmolality. 
Figure 3
 
Influence of the lubricant: CoF at 100 cycles for various lubricant solutions, from left to right: TMS-PS, TMS-Borate 300, TMS-PBS, TMS-PBS+NaCl, TMS-HEPES+NaCl, TLF, measured against mucin-coated hydrophobized glass. Error bars: 1 SD. *Data are from Roba et al.1 Statistical differences were tested between lubricant solutions on the same lens. Significance groups were indicated with letters on RU_5 and with numbers on DD_5. Lens DD_3 showed no significant differences between lubricant solutions.
Figure 3
 
Influence of the lubricant: CoF at 100 cycles for various lubricant solutions, from left to right: TMS-PS, TMS-Borate 300, TMS-PBS, TMS-PBS+NaCl, TMS-HEPES+NaCl, TLF, measured against mucin-coated hydrophobized glass. Error bars: 1 SD. *Data are from Roba et al.1 Statistical differences were tested between lubricant solutions on the same lens. Significance groups were indicated with letters on RU_5 and with numbers on DD_5. Lens DD_3 showed no significant differences between lubricant solutions.
Part II: Effect of Lubricant Organic Content on the CoF
Figures 4 and 5 show the CoF measured in TLF (in PBS) and, where available (data from Roba et al.1), for TMS-PS (in BBS), for DD and RU SCLs, respectively. Besides the case of DD_5, which showed a large increase in CoF in TMS-PS, it is possible to compare the CoF for four DD and four RU SCLs. For three of these eight lenses (DD_1, RU_2, and RU_3), the CoF in TLF was significantly lower than in TMS-PS (P < 0.05), while for the other lenses there was no significant difference between the two lubricants. 
Figure 4
 
Coefficient of friction for various commercially available DD contact lenses directly OoB tested against mucin-coated glass disc as a countersurface in TLF (columns 1–3) and comparison data measured in TMS-PS according to the method published by Roba et al.1 (columns 4–6). Error bars: 1 SD. *Data are from Roba et al.1 Statistical differences (P < 0.05) were tested on the 100 cycles data between TLF and TMS, and are indicated with black solid lines.
Figure 4
 
Coefficient of friction for various commercially available DD contact lenses directly OoB tested against mucin-coated glass disc as a countersurface in TLF (columns 1–3) and comparison data measured in TMS-PS according to the method published by Roba et al.1 (columns 4–6). Error bars: 1 SD. *Data are from Roba et al.1 Statistical differences (P < 0.05) were tested on the 100 cycles data between TLF and TMS, and are indicated with black solid lines.
Figure 5
 
Coefficient of friction for various commercially available RU contact lenses directly OoB tested against mucin-coated glass disc as a counter surface in TLF (columns 1–3) and comparison data measured in TMS-PS from Roba et al.1 (columns 4–6). Error bars: 1 SD. *Data are from Roba et al.1 Statistical differences (P < 0.05) were tested on the 100 cycles data between TLF and TMS, and are indicated with black solid lines.
Figure 5
 
Coefficient of friction for various commercially available RU contact lenses directly OoB tested against mucin-coated glass disc as a counter surface in TLF (columns 1–3) and comparison data measured in TMS-PS from Roba et al.1 (columns 4–6). Error bars: 1 SD. *Data are from Roba et al.1 Statistical differences (P < 0.05) were tested on the 100 cycles data between TLF and TMS, and are indicated with black solid lines.
Part III: Effect of In Vitro Ageing on the CoF
The CoF of 5 DD and 5 RU lenses OoB and after 18 hours ageing in TLF are shown in Figures 6 and 7, respectively (see Supplementary Tables S2 and S3 for the corresponding exact values). Among the 10 lenses tested, 6 lenses (DD_1, DD_2, DD_5, RU_1, RU_2, and RU_3) had a significantly higher CoF after 18 hours ageing and 100 cycles of sliding (P < 0.05). While the magnitude of the change in CoF was different between those SCLs, some general trends are discernable. For example, all hydrogel lenses without poly (vinyl pyrrolidone) (PVP) had an increase in CoF after ageing and 100 cycles of sliding. In particular, among the etafilcon A contact lenses, only DD_2 and RU_3, but not DD_3 had an increase in CoF. The same observation also holds for the SiHys, with the exception of RU_4. Both RU_4 and RU_1 are SiHys with a plasma-treated surface; however only RU_1 had a significant (P < 0.05) increase in CoF after ageing and 100 cycles of sliding. 
Figure 6
 
Coefficient of friction for various commercially available DD contact lenses tested against mucin-coated glass disc as a countersurface in TLF directly OoB (columns 1–3) and after cycling in TLF for 18 hours (columns 4–6). Error bars: 1 SD. Statistical differences (P < 0.05) were tested on the 100 cycles data between OoB and after 18 hours ageing in TLF, and are indicated with black solid lines.
Figure 6
 
Coefficient of friction for various commercially available DD contact lenses tested against mucin-coated glass disc as a countersurface in TLF directly OoB (columns 1–3) and after cycling in TLF for 18 hours (columns 4–6). Error bars: 1 SD. Statistical differences (P < 0.05) were tested on the 100 cycles data between OoB and after 18 hours ageing in TLF, and are indicated with black solid lines.
Figure 7
 
Coefficient of friction for various commercially available RU contact lenses tested against mucin-coated glass disc as a countersurface in TLF directly OoB (columns 1–3) and after cycling in TLF for 18 hours (columns 4–6). Error bars: 1 SD. Statistical differences (P < 0.05) were tested on the 100 cycles data between OoB and after 18 hours ageing in TLF, and are indicated with black solid lines.
Figure 7
 
Coefficient of friction for various commercially available RU contact lenses tested against mucin-coated glass disc as a countersurface in TLF directly OoB (columns 1–3) and after cycling in TLF for 18 hours (columns 4–6). Error bars: 1 SD. Statistical differences (P < 0.05) were tested on the 100 cycles data between OoB and after 18 hours ageing in TLF, and are indicated with black solid lines.
Discussion
The lubricity of certain SCL materials were affected by the nature of the lubricant buffer, and by exposure to a newly developed in vitro ageing process. The purpose of characterizing SCL materials in terms of a CoF is to gain an in vitro accessible parameter that can be used to predict potential subjective comfort performance of the lens in vivo. The results of this study implied that SCL materials may have changes in CoF as a consequence of prolonged interaction with the components of the tear film. These changes in CoF may have to be considered when attempting to correlate end-of-day comfort with the lubricity of a lens material. 
To evaluate how the choice of buffer of the lubricant may affect the CoF measurement, several inorganic and organic buffers were first used as the base for the TMS. Among the three lenses tested in different variations of TMS, only DD_5 was markedly affected by the buffer; it also was the only SCL to contain PVA. Poly (vinyl pyrrolidone) is expected to form a brush-like layer on top of the lens when immersed in a good solvent (the notation “good” refers here to a solvent in which the monomer units of the polymer interact more favorably with the solvent than with itself and, thus, swells). End-grafted dense polymer films in a good solvent; that is, “polymer brushes,” can drastically reduce the CoF between two surfaces in boundary contact.3943 The mechanism behind this phenomenon is thought to be the ability of the brush to minimize adhesion forces, providing an elastic resistance to compression, and having a fluid-like interface that can be easily sheared.44 Indeed, DD_5 had a low CoF in TLF, and in all the TMS formulations lacking borate. In this regard, Schultz et al.45 showed that the modulus of a PVA gel could be tuned by incorporation of different ratios of borate ions. Furthermore, borate also has been reported to act as a cross-linker for PVA.46 Based on these observations, it is likely that the physical properties of the surface of the lens are directly influenced by the borate ions. Thus, the increase in friction of DD_5 in borate-containing buffers could be a consequence of a collapsed brush layer, resulting in an interface with reduced ability to relax shear strain during sliding. As such, it is believed that the increase in CoF on DD_5 in TMS-PS is due to a borate-induced alteration of the material. 
Roba et al.1 investigated several experimental parameters (e.g., sliding speed and counter surface) with regards to how they influenced the CoF of SCL materials. However, the influence of lubricant (TMS-PS) was not studied. To evaluate how a lubricant with a composition more similar to tears than TMS-PS may affect the CoF of SCL materials, results obtained in TLF were compared to those of Roba et al.1 The CoF in TLF generally was lower compared to that in TMS-PS, and significantly lower (P < 0.05) for three of the lenses (i.e., DD_1, RU_2, and RU_3; excluding DD_5 discussed above). Lubricant TMS-PS has a slightly higher protein concentration than TLF, which could lead to an increased protein deposition on the lenses, increasing the CoF. However, not only the protein concentration, but also the nature of the buffer, the proteins, and the presence of lipids have been shown to influence deposit formation. For example, Ng et al.25 reported that lysozyme adsorption on RU_5 and RU_4 was reduced when the buffer also contained lipids. The explanation for this behavior was competitive adsorption of lipids onto the lens surface, blocking subsequent protein adsorption. In addition, lipids also have been shown to act as boundary lubricants, which may provide another explanation for the lower CoF observed in TLF.47,48 Thus, the higher lipid content of TLF compared to TMS-PS could influence the nature and the quantity of deposition of the contact lenses, generally leading to a lower CoF. A lubricant that closely mimics the composition of tear liquid is required to simulate the effects that the interplay between lipid and protein adsorption (expected in vivo) has on the CoF of SCL materials. 
Etafilcon A, which is an anionic hydrogel mixture of PHEMA and poly (methacrylic acid; (PMAA), has a reportedly high adsorption of lysozyme and lactoferrin compared to SiHys and uncharged conventional hydrogels.21,22 In addition, adsorption has been reported not to be restricted to the surface, but to extend throughout the lens.20 It is interesting to compare the etafilcon A DD_2 and RU_3 lenses, neither of which have an internal/embedded wetting agent. Lens RU_3 had a decrease of CoF in TLF compared to TMS-PS, whereas the CoF of DD_2 was unchanged. Further analysis of the nature of the deposits is needed to understand this observation. The other two lenses that had a reduction in CoF in TLF, RU_2 and DD_1, are both SiHys that are either copolymerized with a hydrophilic monomer (RU_2) or have a hydrophilic surface coating (DD_1).49 Lenses containing PVP as a wetting agent (DD_3, DD_4, RU_5) did not show any difference in CoF between TMS-PS and TLF (Fig. 8), which may be a consequence of the presence of a polymer brush that prevents deposits from accumulating at the interface.50 
Figure 8
 
Comparison between the CoF before and after 18 hours of ageing. For better readability, data for RU_1 are not shown. Among the lenses that have PVP as an embedded wetting agent, none showed an increase in CoF after ageing. Error bars: 1 SD.
Figure 8
 
Comparison between the CoF before and after 18 hours of ageing. For better readability, data for RU_1 are not shown. Among the lenses that have PVP as an embedded wetting agent, none showed an increase in CoF after ageing. Error bars: 1 SD.
A potential clinical aspect of the lubricity of the SCL is how it may change upon prolonged wear. To simulate this, the SCLs were cycled in and out of a tempered TLF solution for 18 hours. The cycling is believed to mimic the disturbance of the stratified tear-film structure that occurs during a blink,51 and its effects on surface deposits. Interestingly, the bulk lens material did not seem to influence, nor be predictive of changes occurring in CoF upon ageing. Instead, the presence of a wetting agent appears crucial in retaining a low CoF after prolonged exposure to TLF. For example, among all the lenses that contained PVP, none showed an increase in CoF after ageing, regardless of whether the lens was made from a SiHy or hydrogel material (Fig. 8). Lens DD_5, which contains PVA, poly (ethylene glycol) (PEG) and hydroxypropylmethylcellulose (HPMC) as wetting agents, showed an increase in CoF after ageing. This could be related to a depletion of these wetting agents over time, which is inherent in the design of this particular lens.52 Among the lenses that have a surface coating, RU_4 was the only lens that did not show any increase in CoF upon ageing. This can be compared to the large increase seen for RU_1 and may be said to be an unexpected result as the two lenses are both silicone hydrogels with a plasma-treated surface. However, the plasma treatment of RU_4 has been reported to result in a hydrophilic layer of approximately 25 nm that prevents protein adsorption in the bulk.20 In contrast, RU_1 has an incomplete plasma coating consisting of hydrophobic islands that could lead to increased deposits with a higher degree of denatured proteins and, as a result, a higher CoF.29 In fact, RU_4 displayed a slight decrease in CoF upon ageing—a difference that was removed as the CoF measurement proceeded (0, 50, and 100 cycles). This may indicate the presence of a thin film of loosely adsorbed species with inherently lower CoF compared to the pristine lens surface, as also observed by Ngai et al.9 after lysozyme adsorption. Lens DD_1 has a hydrogel coating at the interface.49 The CoF of this lens has been reported previously to be approximately 0.02 in PBS against a silica sphere,53 in agreement with our results before ageing. Dursch et al.49 characterized DD_1 with regards to solute-partitioning, and demonstrated the hydrogel characteristics of the lens surface, as well as showing that the gel layer has a negative charge at neutral pH. The increase in CoF after ageing could be a consequence of electrostatic adsorption of proteins, or an increased crosslink density of the hydrogel due to association of divalent cations present in the lubricant – similar to what was observed by Dunér et al.54 for poly (acrylic acid) polymer brushes in the presence of calcium. Certain lenses had an increase in CoF after prolonged exposure to TLF. This may be of significance when attempting to correlate CoF with SCL comfort after wear. 
All CoF measurements performed in this study have been performed using a mucin-coated glass disk as countersurface. Although the interface may be expected to have similarities with the glycocalyx of the conjunctiva, the modulus is several orders of magnitude higher. As recently shown by Dunn et al.,55 the friction response between hard-soft and soft-soft contact may be significantly different. This may lead to other frictional mechanisms coming into play than those captured in this study, and to that experienced by the eyelid. Future work will be devoted to developing a countersurface where the surface and the bulk have properties similar to those of the inner eyelid. Additionally, commercially purified mucin has been reported to contain some additional species, mainly in the form of albumin. Nikogeorgos et al.35 reported that purified mucin had improved boundary lubricating function compared to the as-received material. However, if some contaminants also were adsorbed on the countersurface, all lenses would be expected to be equally affected. Although the absolute CoF values might be different between nonpurified and purified mucin, the relative order in terms of CoF is likely to be maintained. 
A simulated wearing procedure has been presented and it has been shown that some SCLs display an increased CoF after continuous immersion and removal from TLF. The most likely cause of this increase is protein and lipid deposits forming on the lens surface—a process that also can be expected in vivo. Therefore, it is suggested that in attempting to correlate end-of-day comfort of SCLs with CoF, effects of ageing should be taken into account. A key aspect of the ageing process was the intermittent drying of the lubricant film, simulating the effect that the opened phase of the blink cycle has on deposit formation. However, a drawback was the absence of mechanical stress, which is necessary to more closely mimic the wear associated with blinking. The development of a method that includes this component will be the focus of future studies. 
Conclusions
An in vitro ageing process has been presented, in which SCLs were continuously immersed into and removed from a TLF over a period of up to 18 hours. The effects of ageing were probed through changes in the frictional properties between the pristine and aged SCL. Results indicated that a persistent wetting agent is beneficial in maintaining a low CoF after prolonged simulated wearing. Increases in CoF after prolonged exposure to TLF may have clinical implications related to changes in the eyelid–contact lens interaction over the course of a day's wearing. Additionally, the composition of the lubricant has been shown to affect the CoF values for many SCL materials. 
Acknowledgments
Supported by Johnson & Johnson Vision Care, Inc., Jacksonville, FL, USA. 
Disclosure: O. Sterner, JJVCI (F), SuSoS AG (E); R. Aeschlimann, Vistakon (F), SuSoS AG (E); S. Zürcher, JJVCI (F), SuSoS AG (E); K. Osborn Lorenz, JJVCI (E); J. Kakkassery, Janssen (E); N.D. Spencer, None; S.G.P. Tosatti, JJVCI (F) SuSoS AG (E) 
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Figure 1
 
(top) Photograph of the ageing device inside the laminar flood hood. (bottom) Schematic representation of the CL-ageing device. The lens was fixed in a lens holder and repeatedly immersed and withdrawn from a preheated (30°C) TLF solution.
Figure 1
 
(top) Photograph of the ageing device inside the laminar flood hood. (bottom) Schematic representation of the CL-ageing device. The lens was fixed in a lens holder and repeatedly immersed and withdrawn from a preheated (30°C) TLF solution.
Figure 2
 
Schematic representation of the tribological measurement setup.
Figure 2
 
Schematic representation of the tribological measurement setup.
Figure 3
 
Influence of the lubricant: CoF at 100 cycles for various lubricant solutions, from left to right: TMS-PS, TMS-Borate 300, TMS-PBS, TMS-PBS+NaCl, TMS-HEPES+NaCl, TLF, measured against mucin-coated hydrophobized glass. Error bars: 1 SD. *Data are from Roba et al.1 Statistical differences were tested between lubricant solutions on the same lens. Significance groups were indicated with letters on RU_5 and with numbers on DD_5. Lens DD_3 showed no significant differences between lubricant solutions.
Figure 3
 
Influence of the lubricant: CoF at 100 cycles for various lubricant solutions, from left to right: TMS-PS, TMS-Borate 300, TMS-PBS, TMS-PBS+NaCl, TMS-HEPES+NaCl, TLF, measured against mucin-coated hydrophobized glass. Error bars: 1 SD. *Data are from Roba et al.1 Statistical differences were tested between lubricant solutions on the same lens. Significance groups were indicated with letters on RU_5 and with numbers on DD_5. Lens DD_3 showed no significant differences between lubricant solutions.
Figure 4
 
Coefficient of friction for various commercially available DD contact lenses directly OoB tested against mucin-coated glass disc as a countersurface in TLF (columns 1–3) and comparison data measured in TMS-PS according to the method published by Roba et al.1 (columns 4–6). Error bars: 1 SD. *Data are from Roba et al.1 Statistical differences (P < 0.05) were tested on the 100 cycles data between TLF and TMS, and are indicated with black solid lines.
Figure 4
 
Coefficient of friction for various commercially available DD contact lenses directly OoB tested against mucin-coated glass disc as a countersurface in TLF (columns 1–3) and comparison data measured in TMS-PS according to the method published by Roba et al.1 (columns 4–6). Error bars: 1 SD. *Data are from Roba et al.1 Statistical differences (P < 0.05) were tested on the 100 cycles data between TLF and TMS, and are indicated with black solid lines.
Figure 5
 
Coefficient of friction for various commercially available RU contact lenses directly OoB tested against mucin-coated glass disc as a counter surface in TLF (columns 1–3) and comparison data measured in TMS-PS from Roba et al.1 (columns 4–6). Error bars: 1 SD. *Data are from Roba et al.1 Statistical differences (P < 0.05) were tested on the 100 cycles data between TLF and TMS, and are indicated with black solid lines.
Figure 5
 
Coefficient of friction for various commercially available RU contact lenses directly OoB tested against mucin-coated glass disc as a counter surface in TLF (columns 1–3) and comparison data measured in TMS-PS from Roba et al.1 (columns 4–6). Error bars: 1 SD. *Data are from Roba et al.1 Statistical differences (P < 0.05) were tested on the 100 cycles data between TLF and TMS, and are indicated with black solid lines.
Figure 6
 
Coefficient of friction for various commercially available DD contact lenses tested against mucin-coated glass disc as a countersurface in TLF directly OoB (columns 1–3) and after cycling in TLF for 18 hours (columns 4–6). Error bars: 1 SD. Statistical differences (P < 0.05) were tested on the 100 cycles data between OoB and after 18 hours ageing in TLF, and are indicated with black solid lines.
Figure 6
 
Coefficient of friction for various commercially available DD contact lenses tested against mucin-coated glass disc as a countersurface in TLF directly OoB (columns 1–3) and after cycling in TLF for 18 hours (columns 4–6). Error bars: 1 SD. Statistical differences (P < 0.05) were tested on the 100 cycles data between OoB and after 18 hours ageing in TLF, and are indicated with black solid lines.
Figure 7
 
Coefficient of friction for various commercially available RU contact lenses tested against mucin-coated glass disc as a countersurface in TLF directly OoB (columns 1–3) and after cycling in TLF for 18 hours (columns 4–6). Error bars: 1 SD. Statistical differences (P < 0.05) were tested on the 100 cycles data between OoB and after 18 hours ageing in TLF, and are indicated with black solid lines.
Figure 7
 
Coefficient of friction for various commercially available RU contact lenses tested against mucin-coated glass disc as a countersurface in TLF directly OoB (columns 1–3) and after cycling in TLF for 18 hours (columns 4–6). Error bars: 1 SD. Statistical differences (P < 0.05) were tested on the 100 cycles data between OoB and after 18 hours ageing in TLF, and are indicated with black solid lines.
Figure 8
 
Comparison between the CoF before and after 18 hours of ageing. For better readability, data for RU_1 are not shown. Among the lenses that have PVP as an embedded wetting agent, none showed an increase in CoF after ageing. Error bars: 1 SD.
Figure 8
 
Comparison between the CoF before and after 18 hours of ageing. For better readability, data for RU_1 are not shown. Among the lenses that have PVP as an embedded wetting agent, none showed an increase in CoF after ageing. Error bars: 1 SD.
Table 1
 
Physical Properties of the TMS-PS and TLF Lubricants
Table 1
 
Physical Properties of the TMS-PS and TLF Lubricants
Table 2
 
Compositions of the Buffer Components Used in the Different Variations of the TMS
Table 2
 
Compositions of the Buffer Components Used in the Different Variations of the TMS
Table 3
 
TLF Components and Concentrations
Table 3
 
TLF Components and Concentrations
Table 4
 
DD Lenses
Table 4
 
DD Lenses
Table 5
 
RU Lenses
Table 5
 
RU Lenses
Supplement 1
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