SOFT LENS ADVANCES
Advances in Soft Lens Materials and Designs
Ongoing research and development efforts strive to improve soft lens comfort and health.
By Thomas Dursch, PhD; Tatyana F. Svitova, PhD; & Meng C. Lin, OD, PhD
Contact lenses are biomaterials designed to exhibit a specific range of bulk and surface properties that are optimized for a dynamic ocular environment to be effective, safe, and comfortable to wear. Contact lenses have evolved significantly over the past several decades, from the earlier PMMA lenses to GP lenses and later to soft traditional hydrogel and silicone hydrogel (SiHy) lenses.
Yet even with these significant advances, only 15% to 25% of the eligible population in the United States wears contact lenses (Fonn, 2007; Pritchard et al, 1999). Moreover, up to one-third of new wearers discontinue within five years of initial prescription (Fonn, 2007; Pritchard et al, 1999). Lens-related wearing discomfort, especially late in the day, is the major impediment (Fonn, 2007; Pritchard et al, 1999; Dumbleton et al, 2013).
Historically, insufficient corneal oxygen supply has been identified as a key factor associated with worrisome adverse events (Guillon, 2013; Nicholson and Vogt, 2001; Stapleton et al, 2006). As a result, significant effort has been put forth over the years to develop SiHy lenses, which provide enhanced oxygen permeabilities but exhibit a less hydrophilic surface and a larger bulk modulus compared to that of traditional hydrogels (Guillon, 2013; Nicholson and Vogt, 2001; Stapleton et al, 2006). Today, SiHy lenses have reduced adverse events that are associated with contact lens-induced hypoxia; however, the frequency of microbial keratitis and corneal infiltrative events remains the same or higher compared with traditional soft contact lenses (Sweeney, 2013; Szczotka-Flynn and Diaz, 2007). Furthermore, there is no evidence of improved wearing comfort with these newly developed SiHy lenses (Guillon, 2013).
Because wearing comfort has become a major factor in gauging the success of lens wear, much effort has been expended toward elucidating the origin of contact lens discomfort (Fonn, 2007; Guillon, 2013; Nicholson and Vogt, 2001; Stapleton et al, 2006). To date, however, the etiology of lens discomfort remains elusive; lens-related discomfort is multifactorial, wearer dependent, and often varies diurnally. Potential causes fall primarily into two classes: contact lens factors (e.g., materials, designs, and care) and external factors (e.g., patient characteristics, ocular health, and environmental factors) (Nichols et al, 2013; Jones et al, 2013; Truong et al, 2014). Conventional wisdom indicates that to sustain comfort while on the eye, regardless of lens material, a soft lens should: 1) maintain a stable and continuous tear film for minimal aqueous tear film evaporation (Nichols and Sinnott, 2006; Thai et al, 2004) by providing a wettable and lubricous surface that resists dehydration and deposition of tear film components (Dunn, Urueña et al, 2013; Roba et al, 2011); 2) exhibit elasticity sufficient for centering (Chauhan and Radke, 2001); and 3) transport oxygen, water, and ions to preserve normal corneal metabolism and lens movement (Hoch et al, 2003; Guan et al, 2011).
A soft contact lens divides the 3µm to 5μm thick pre-corneal tear film into pre- and post-lens tear films, making the pre-lens tear film even thinner and more unstable compared with the pre-corneal tear film (King-Smith et al, 2004; Rohit et al, 2013). Therefore, maintaining a wettable and lubricous surface for the eyelid margin to repeatedly slide over presents a significant challenge.
As the superior eyelid traverses a soft lens more than 10,000 times during a single day of lens wear, lid-wiper sliding friction can result in damage to and irritation of the epithelial cells of the palpebral conjunctiva (Korb et al, 2005; Korb et al, 2002; Shiraishi et al, 2014). Lid wiper cell damage is called lid wiper epitheliopathy (LWE). According to Korb et al (2005, 2002), LWE has high sensitivity and specificity for diagnosing soft lens-induced dry eye. It is therefore reasonable to hypothesize that a wettable and lubricous contact lens surface is essential to minimize LWE during lens wear, thereby improving lens comfort. It is not surprising, then, that much attention in the contact lens industry is focused on improving soft lens surface chemistry.
Contact Lens Surface Wettability
Current thinking among contact lens researchers is that improved soft contact lens surface wettability provides better pre-lens tear film stability and, consequently, improves lens comfort (Lin and Svitova, 2010; Svitova and Lin, 2011; Keir and Jones, 2013; Blake, 1993; and others. Full list available at www.clspectrum.com/references.). The most widely used method to characterize the wettability of a hydrogel lens is to measure contact angles (CAs). However, CA measurement does not provide a true assessment of wettability in an in-vivo environment (i.e., a soft contact lens on eye) without taking into account the accompanying surface tension of the liquid film in contact with the lens surface (Lin and Svitova, 2010; Svitova and Lin, 2011; Keir and Jones, 2013).
To understand the role of lens surface wettability on pre-lens tear film stability, some basics regarding the physics and implications of wetting phenomena should be reviewed (Blake, 1993; Fatt, 1984; Campbell et al, 2013). A balance of interfacial forces acting along the contact line formed by a liquid droplet in air on a contact lens surface determines whether a liquid droplet can spread on a lens surface. For example, when a drop of pure water is placed on the surface of a surfactant-free homopolymer HEMA (hydroxyethyl methacrylate) hydrogel, it will have contact angles of ~90º or higher (i.e., measured through the liquid droplet). However, if the HEMA hydrogel lens is saturated with lens storage solution ingredients, some of which are surface-active and reduce surface tension to 42 mN/m, the CA will be as low as 10º to 12º (Holly and Refojo, 1975; Lin and Svitova, 2010; Svitova and Lin, 2011). Human tears have a surface tension of ~35 to 40 mN/m, which could reduce the CA on HEMA lenses to virtually 0º and form a completely spreadable thin film on the lens surface (Nagoyová and Tiffany 1999).
SiHy lens materials by nature have more hydrophobic surfaces compared to HEMA-based polymers, with initial CAs above 110º (Keir and Jones, 2013).To make these polymers more suitable as wearable materials (i.e., more hydrophilic), the surface is modified by either combining silicone monomers with more hydrophilic monomers or by treating the surface with plasma oxidation. Once surface modification is employed, the initial CAs of SiHy lenses fall below 90º. Due to lower surface tension, human tear aqueous is able to wet and spread on practically all solid surfaces with the initial CA measured for pure water that falls below 90º. Therefore, it is difficult to discern clinically any appreciable differences among different commercially available lenses once the lens surface is coated by healthy, natural tear film.
Nevertheless, surface properties differ between different classes of lens materials (HEMA versus SiHy). Therefore, it is important to have a systematic method to properly assess lens surface wettability by both CAs and the surface tension of the liquid film that coats the surface. Studies of traditional and SiHy contact lens wettability in vitro have shown that for most HEMA-based lenses, surface wettability is strongly dependent on the presence of surface-active ingredients added to blister packs and lens care solutions (Lin and Svitova, 2010). These components of lens packaging solutions prevent a lens from sticking to blister pack material and improve the initial wettability of the pristine (unused) lens surface (Lin and Svitova, 2010; Svitova and Lin, 2011).
The wettability of HEMA lenses was reduced dramatically after surface-active ingredients were removed from them (Lin and Svitova, 2010; Svitova and Lin, 2011). However, wettability of SiHy lens materials either worsens or shows no improvement after exposure to surface-active agents in the soaking solutions (Svitova and Lin, 2011; Lin and Svitova, 2010). In other words, additional exposure to a surfactant does not seem to improve the surface wettability of SiHy lenses.
Furthermore, when clinical studies were designed to permit isolating and highlighting the effects of lens surface wettability as assessed by CA and the accompanying surface tension on clinical performance and subjective comfort, there was no direct and unambiguous correlation between in-vitro and ex-vivo surface wettability and comfort (Svitova and Lin, 2011).
This raises the question of whether different formulations of soaking solutions are necessary for SiHy contact lenses or whether this exposes lens wearers to unnecessary surface-active ingredients. It is conceivable that patients can remove unnecessary ingredients by disinfecting and soaking SiHy lenses overnight in a surfactant-free lens care solution, which could be beneficial to patients who have sensitive eyes. Additionally, patients who have issues of hypersensitivity or intolerance to the “soapy” ingredients of the blister pack solutions might benefit from thoroughly rinsing the lenses with a surfactant-free saline solution before wearing SiHy lenses.
Contact Lens Lubricity
In addition to providing better pre-lens tear film stability, a strongly hydrophilic surface likely promotes high soft lens lubricity (i.e., low soft lens sliding friction). Over the past several years, significant effort has been directed toward quantifying and reducing lens-sliding friction in vitro (Dunn, Tichy et al, 2013; Peng et al, 2014; Tucker et al, 2012; Dunn, Urueña et al, 2013, and others). Table 1 summarizes the various methods of assessing lens-sliding friction as well as the pros and cons of each method, including 1) the inclined-plane method (Peng et al, 2014; Tucker et al, 2012; Radke et al, 2015; Subbaraman and Jones, 2013); 2) microtribology (Dunn, Urueña et al, 2013; Roba et al, 2011; Rennie et al, 2005; Subbaraman and Jones, 2013); and 3) atomic force microscopy (AFM) (Radke et al, 2015; Subbaraman and Jones, 2013).
|Inclined-Plane Method||Subjective||Cheap, simple||Low sensitivity, does not measure normal and lateral forces|
|Microtribology||Objective||Highly sensitive and controllable, averaged measurement of lateral/normal forces||Does not provide local force measurement, expensive, time consuming|
|Atomic Force Microscopy||Objective||Highly sensitive and controllable, local measurement of lateral/normal forces||Difficult calibration and cantilever preparation, expensive, time consuming|
Due to its simplicity, the inclined-plane method is the most commonly used for ascertaining lens-sliding friction (Peng et al, 2014; Tucker et al, 2012; Radke et al, 2015; Subbaraman and Jones, 2013). In this method, a glass plate is submerged in a phosphate-buffered saline bath at an angle that is adjusted until a lens (placed at the top of the glass plate) sustains movement over a fixed distance. The sliding-friction coefficient (CoF) is then calculated characteristic of the initial sliding motion as tan[sin-1(h/l)] where h is the height of a glass plate at sliding commencement and l is the length of the plate (Peng et al, 2014; Tucker et al, 2012; Radke et al, 2015; Subbaraman and Jones, 2013). While easy to perform, the inclined-plane method has several inherent disadvantages in that it is subjective, does not directly measure normal and lateral (i.e., friction) forces, and has a lower CoF limit of ~0.02, rendering this method unsuitable for low-friction contact lenses.
To overcome these shortcomings, microtribology and AFM, which are more sensitive and quantitative techniques, have been recently developed and applied to soft lenses (Dunn, Urueña et al, 2013; Roba et al, 2011; Radke, 2015; Rennie et al, 2005; and others). In these methods, the lateral force required to rotate or drag a counter-surface against a sample is measured at a fixed and known normal force and velocity. Because both normal and lateral forces are obtained, microtribology and AFM allow for investigation of all lubrication regimens (i.e., boundary, elasto-hydrodynamic, and hydrodynamic) experienced during a blink (Dunn, Tichy et al, 2013; Rennie et al, 2005; Subbaraman and Jones, 2013). An important distinction between microtribology and AFM is that microtribology provides a friction coefficient averaged over a relatively large counter surface (e.g., several millimeters) compared to that measured by AFM (e.g., nanometers to microns).
Regardless of the technique employed, friction measurements must be interpreted with caution because reported CoFs depend strongly on many parameters, including counter-surface material and shape, solution viscosity, normal force, and velocity (Dunn, Tichy et al, 2013; Peng et al, 2014; Tucker et al, 2012; Dunn, Urueña et al, 2013, and others).
To date, few clinical studies provide solid evidence supporting the relationship between lens-sliding friction and improved lens comfort. Recently, Coles and Brennan examined this relationship (Coles and Brennan, 2012; Brennan, 2009). After 30 days of wearing different soft lenses, patients rated “comfort” subjectively using a comfort questionnaire. All soft lenses were worn for 30 days, whereas friction was measured independently on clean lenses. Nevertheless, lower-friction materials were associated with better comfort (Coles and Brennan, 2012).
Exactly how CoF varies for values less than 0.01 is unknown because no commercial lenses exist in that region. Consequently, it is unclear what value of CoF is required to achieve a comfort rating of 100 and whether such a change would be clinically significant. Although the work of Coles and Brennan (2012) and Brennan (2009) suggests a relationship between CoF and lens comfort, further systematic investigation is needed to elucidate conclusively the role of lens lubricity on wearing comfort.
As previously mentioned, CoF measurements are complex. As many new lens materials and designs are being developed, we must make progress in several key areas to know whether lower sliding friction will actually lead to improved lens comfort. These areas include 1) establishing standardized and rigorous procedures to assess sliding-friction coefficients in vitro; 2) establishing a stronger connection among lens lubricity, lid-wiper health, and lens comfort; and 3) developing materials and designs to maintain high surface lubricity in the presence of tear film components. Recent work reveals that fouling by lipids and proteins strongly influences sliding friction (Peng et al, 2014). Thus, in addition to achieving low sliding friction when clean, soft contact lenses must maintain low sliding friction when exposed to tear and care solution components. Maintaining reduced sliding friction remains a significant challenge.
Materials and Designs for Improving Contact Lens Surface Behavior
Although the etiology of lens discomfort remains elusive, a soft, wettable, and highly lubricous lens surface that mimics corneal epithelial cells appears necessary for a contact lens to be mechanically undetectable. Over the years, considerable effort has been focused on improving soft lens surface behavior. Two basic avenues have been explored: 1) plasma processing to improve surface wettability (Coles and Brennan, 2012), and 2) incorporation of wetting agents (Brennan, 2009), such as aqueous-soluble polymers (e.g., polyvinyl alcohol and hydroxypropyl methylcellulose), that coat the lens’ anterior surface over a fixed release period. However, neither approach has achieved sufficient success to overcome lens discomfort, especially with the advent of SiHy lenses that are more prone to fouling and do not readily uptake wetting agents.
Several recent commercial SiHy lenses employ unique materials and designs for improved lens surface behavior. They aim to maintain high Dk and improve lens surface lubricity and wettability via a silicone-rich core and hydrophilic polymer-rich surfaces (for example, by laminating a traditional SiHy lens in thin hydrophilic surface-gel layers or decreasing lens surface silicone content through copolymerization of a hydrophilic surface active agent). While these newer SiHy lenses use distinct designs for improving lens surface behavior, a common approach is to reduce the silicone content of the anterior lens surface.
Soft lens discomfort remains the primary reason for lens wear discontinuation, despite decades of contact lens development and improvement. Today, new lens materials and designs focus primarily on improving the soft lens surface, as bulk-lens properties (e.g., oxygen permeability) have been thoroughly addressed over the years. It is unclear, however, whether such improvement in surface properties will actually improve lens comfort. Standardized in-vitro procedures for assessing surface properties, along with prospective, randomized, controlled clinical studies, are warranted to definitively correlate soft lens surface properties with soft lens wearing comfort. CLS
For references, please visit www.clspectrum.com/references and click on document #241.
Dr. Dursch joined Dr. Meng Lin’s research group as a post-doctoral scholar upon graduating from the University of California, Berkeley (UC Berkeley) Department of Chemical Engineering in 2014. He will continue his post-doctoral training at the Massachusetts Institute of Technology in 2016.
Dr. Svitova is an associate research specialist at UC Berkeley, Clinical Research Center. Since 2007, she has been collaborating with Dr. Lin on understanding mechanisms responsible for tear film stability.
Dr. Lin is an associate professor at UC Berkeley Optometry and Vision Science Graduate Group, director of the UC Berkeley Clinical Research Center, chief of the Ocular Surface Imaging Clinic, and co-chief of the Dry Eye Clinic. She is also a consultant or advisor to Google(x) and Alcon and has received research funding from CooperVision, Johnson & Johnson Vision Care, and Essilor.