Measuring Friction and Lubricity of Soft Contact Lenses: A Review
Comfort may be tied to friction and lubricity.
By Lakshman N. Subbaraman, PhD, BSOptom, MSc, FAAO, & Lyndon W. Jones, PhD, FCOptom, FAAO
|Dr. Subbaraman is Head of Biological Sciences at the Centre of Contact Lens Research, School of Optometry, University of Waterloo. He is a consultant/advisor to Alcon and Johnson & Johnson. He has received grants from Alcon, Allergan, Bausch + Lomb, CooperVision, Johnson & Johnson, TearScience, Essilor and Visioneering.|
Dr. Jones is a Professor at the School of Optometry and Vision Science and Director of the Centre for Contact Lens Research at the University of Waterloo. He is a consultant/advisor to Alcon and Johnson & Johnson. He has received grants from Alcon, Allergan, Bausch + Lomb, CooperVision, Johnson & Johnson, TearScience, Essilor and Visioneering.
Over the last 35 years, the number of contact lens wearers worldwide has increased from 10 million to 140 million, with the vast majority (over 90%) being fitted with soft lenses.1 Recent reports suggest that the contact lens industry is healthy and the worldwide annual soft contact lens market is estimated at $5.3 billion, with the U.S. market estimated at $1.9 billion.2 However, despite this apparently buoyant position, many wearers continue to be dissatisfied with their lenses and approximately 35% of lens wearers discontinue wear, with the majority reporting the major reasons being discomfort and dryness, particularly at the end of the day.3 Contact lens-related discomfort and dryness is influenced by several factors, and likely include both the interaction of the posterior surface of the lens with the corneal surface and the anterior surface of the contact lens with the posterior surface of the eyelid during the blink.
Hydrogel lenses rapidly attract various components from the tear film, particularly proteins and lipids, following their insertion.4 These can result in alterations to the surface of the contact lens that can change the frictional forces that exist during blinking. Furthermore, dehydration of lenses can result in increased lid-lens interaction due to a reduction in lens front-surface wettability and lubricity, and the development of corneal epithelial staining due to pervaporation and subsequent desiccation.5 Studies have suggested that the frictional properties of contact lenses may also be associated with certain clinically observable phenomena, notably lid wiper epitheliopathy and lid parallel conjunctival folds.6,7 Finally, increased friction may lead to contact lens associated papillary conjunctivitis due to the mechanical interaction of the palpebral conjunctiva with the contact lens surface.8
Issues such as these have made it clear that the frictional properties of lenses are an important design consideration in the fabrication and manufacture of soft lenses. Furthermore, understanding the frictional forces that occur at the lens surface will provide insight into the relationship between the lens material surface properties and biological responses such as protein deposition and bacterial adhesion.9 However, to-date, very few studies report on the frictional characteristics of soft lenses10-16 and contact lens practitioners may be unaware of the relevance of this factor to everyday practice. The scope of this review article is to provide an overview of various methods that have been employed to determine the friction of hydrogel lenses.
Material scientists and researchers who work on the surface properties of lens materials commonly use terms such as “tribology”, “friction” and “lubricity.” However, eye care practitioners may not be familiar with these terms as they do not use them on a day-to-day basis and a description of these terms is valuable.
Tribology comes from the Greek word “tribos,” which means “to rub.” Tribology is generally defined as the study of three areas — friction, lubrication and wear. These three areas are highly inter-related; however, the relationship between friction and wear is not well understood. Generally, friction is produced when two sliding surfaces come into contact, resulting in wear. Wear can be prevented by lubrication, and the separation of the surfaces by a lubricant will result in a reduction in friction. The purpose of tribological research is to minimize friction and wear, therefore, tribology plays a major role in the effective treatment of some of the common medical conditions involving bodily implants and joint diseases.
Biotribology is a relatively new term introduced in the early 1970s to describe a group of sciences that focus on one single topic — the study of friction, wear and lubrication within biology. This is a multidisciplinary subject covering the areas of engineering, material science, biological science, physical science and medicine.
Friction can be defined as the force that acts at the surface of two solid surfaces to resist sliding over one another. The force that prevents one surface sliding over the other is quantified by a simple index called the coefficient of friction (CoF). In order to determine CoF, two measurements are needed: (a) the force required to initiate and/or sustain sliding and (b) the normal force holding the two surfaces together. CoF can then be calculated by dividing the initiating/sustaining force by the normal force. A lubricant can be used to reduce friction between two surfaces. Since a lubricant reduces friction, CoF is an easy measure of quantifying the lubricating ability of any system. Generally, a comparison between CoF generated by an instrument under identical conditions is acceptable. However, comparing CoF values from different instruments under different conditions should be interpreted with caution.
Lubrication is defined as any means capable of controlling friction and wear of interactive surfaces in relative motion. There are two types of lubrication; one where there is a solid-to-solid contact of the sliding surfaces is known as “boundary lubrication” and the other where there is a thin layer of fluid that is present in the intervening space (hydrostatic lubrication) or the motion produces a layer of fluid on which the moving surface planes over the counter face (hydrodynamic lubrication).
Methods of Measuring Friction and Lubricity
The most common method for determining the CoF of soft contact lenses is through the use of a “microtribometer.” Some researchers have used a method based on atomic force microscopy15,16 and others have developed novel proprietary techniques17 to determine the lubricity of contact lenses and the following section describes these various methods.
A microtribometer is an instrument that directly measures the frictional force and the normal force in order to calculate the coefficient of friction. Many different parameters must be controlled to generate repeatable measurements and the following section provides an outline of the parameters that influence the CoF values obtained using a microtribometer.
Sliding speed: Test speeds have a large effect on the measured COF and speeds as low as 0.01 mm/sec13 have been used to measure “boundary lubrication”, which occurs when the lens is in direct contact with the ocular tissues. Higher speeds approximating the blink speed of 12 cm/sec have been used10 to measure “mixed” or “hydrodynamic lubrication”, where a thin fluid film separates the contact lens from the ocular tissues. Studies have used a wide range of sliding speeds (63 to 6280 μm/sec,11 0.01 to 0.5cm/sec,12 or 10 to 600 μm/sec18) to determine the effect of sliding speeds on the reported CoF values.
Figure 1. Inclined plane method
Normal Force Pressure: In-vivo eyelid pressures are typically estimated at 1-7 kPa and achieving accurate force measurements at these low pressures can be challenging. A recent study has determined frictional values at very low contact pressures, approaching 1kPa.18
Substrate / Counter Surface: All friction measurements involve two surfaces and the choice of substrate against which to determine the CoF can affect the reported results. Substrates utilized include glass,11,18 stainless steel,12 or mucin-coated silanized glass.13
Lubricating fluid: Nairn and colleagues10 used various commercial ophthalmic solutions including B+L ReNu, Allergan Complete, Alcon Opti-Free and B+L saline and showed that differing lubricants resulted in differences in the reported CoF values. Artificial tear fluids and solutions containing a variety of tear proteins that mimic human tears have also been used as lubricating fluids.13,14
Type of movement: Microtribometers typically use either a flat plate or a curved probe that is moved across the lens surface. The flat plate method maintains contact with only a single point on the contact lens surface during testing,13 while the probe method is able to expose fresh lens surface to the probe as the probe is moved across the lens surface.18
Lens sample preparation: To-date, all published CoF data has been conducted only on unworn lenses. The effects of lens wear and contact lens care solutions have not been adequately tested in order to determine whether the coefficient of friction is likely to change with time after exposure to tear film components and further work is warranted in this area.
Finger Lubricity Method
Recently, a qualitative “finger rubbing” method has been described to determine contact lens lubricity.17 In this method, contact lenses were rinsed overnight in a phosphate buffered saline (PBS) solution to remove any packaging solution, and the investigator rubbed the lens between their thumb and index fingers and the lubricity was rated on a 0 (most lubricious) to 4 (least lubricious) scale. The advantage of this method is that it is a simple, quick method that does not require any sophisticated instrumentation. The authors reported that this method was highly repeatable, but only when used by an experienced investigator. 17 A major disadvantage of this method is that it is a limited qualitative scale and not all lens types can be differentiated with this technique.
Inclined Plane Method (Figure 1)
This is a novel, quantitative method of determining contact lens friction using a PBS solution.17 In this method, a clean glass plate is adjusted to a desired angle in a PBS bath. The contact lens under test is placed at the top of the glass plate and a 0.8 gram stainless steel ferrule (0.88 kPa) is placed on the lens to initiate movement. A minimum critical angle is determined, which maintains the movement of the lens over a distance of approximately 100 mm. The tangent of the critical angle is a measure of the kinetic coefficient of friction. Thus, in this method, the tangent of the largest angle where the lens is unable to maintain movement will determine the kinetic coefficient of friction.17
(Image courtesy University of Florida)
Figure 2. Beam Deflection AFM
Atomic Force Microscopy (Figure 2)
Atomic force microscopy (AFM) is a standard technique used to study the surface topography of conventional and silicone hydrogel contact lens materials.19 AFM is a very powerful tool for high-resolution examination of the contact lens surface and also permits the analysis of surface topography and roughness by means of a non-destructive methodology. AFM consists of a microscale cantilever with a sharp tip, which scans the surface of the lens. The cantilever is typically made of silicon or silicon nitride with a tip radius of curvature in the order of nanometers. The AFM can be operated in a number of modes, including static (also called contact) mode and a variety of dynamic (non-contact or tapping) modes, where the cantilever is vibrated. The advantage of AFM over traditional microscopic techniques is the high-resolution, three-dimensional images and also that topographic information can be obtained in several conditions (such as aqueous, non-aqueous or dry), thereby eliminating the need for sample preparation.
Kim and colleagues15,16 used a contact-mode AFM to determine the surface properties of pHEMA-based soft contact lenses. AFM images of the lens surface were taken in two conditions: (a) with the lens surface exposed to air (“surface-dehydrated”) and (b) in the presence of saline solution on the lens surface. After successfully optimizing a method to quantify friction using AFM, the authors concluded that the friction force imaging of the “surfacedehydrated” soft contact lenses made of cross-linked pHEMA showed low friction at the surfaces. In saline, the surface friction was significantly reduced compared to those measured for the “surface-dehydrated” lens.15 In a recent study, Rudy and colleagues20 determined the surface mechanical and tribological properties of silicone hydrogels using AFM. In this study, they measured the elastic modulus by indenting a probe into the surface of the hydrogel in a controlled manner and obtained a modulus value by fitting the characteristic force versus indentation behaviour. Their results showed that pHEMA-based etafilcon A lenses have a modulus between 100 and 130 kPa, whereas balafilcon A lenses were an order of magnitude higher in value. The frictional properties followed a similar trend, with plasma surface-treated lenses (such as balafilcon A) exhibiting CoF values five times those of delefilcon A water gradient silicone hydrogels. These studies show that the elastic modulus and frictional properties of different hydrogel and silicone hydrogel lenses can be evaluated at a nanoscopic level using AFM.
Determining the frictional properties of soft lenses is complicated and a review of the literature reveals that there are several methods of determining the lubricity of contact lens materials. Due to the vast difference in techniques and methodology for determining friction, there are noticeable differences in the coefficient of friction values published to-date. Though significant differences do seem to exist among lens materials, it remains challenging to make comparisons until standard methods of testing are agreed upon. Material chemistry, water content, test media, applied load and the sliding velocity all have an impact on the results, and may impact different materials to varying degrees. Deposition of tear-derived components can also impact friction forces for both silicone and conventional hydrogels and may be a difficult condition to reproduce in an in vitro testing model and future studies should include testing under conditions closer to that obtained in-eye.
A recent analysis that attempted to correlate several contact lens material-related properties (Dk/t, modulus, water content and lubricity) with end-of-day wearing comfort found coefficient of friction to be the principal physical property associated with comfort.21 In this work, end-of-day comfort data obtained from over 700 separate 1-month wearing trials were derived using a sensitive and sophisticated method. The friction data showed consistently high correlation with comfort, implicating that coefficient of friction could be a major driving factor in subjective comfort. However, data from more carefully conducted clinical trials are needed to determine the relationship between subjective symptoms of discomfort and dryness and the lubricity of various commercial contact lens materials. The influence of contact lens care regimens and how they influence the friction values of contact lenses should also be investigated. CLS
The authors would like to acknowledge Alcon Laboratories Ltd, USA for their support in developing this article through an unrestricted grant.
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