BCLA CLEAR - Contact lens wettability, cleaning, disinfection and interactions with tears

A common method for assessing wettability of a liquid deposited on a contact lens, either in vitro or ex vivo, is the measurement of the contact angle []. The contact angle is the angle between the substrate surface and the tangent drawn at the triple point between the three phases where the liquid–vapor interface meets the solid-liquid interface (θ in Fig. 1) []. The contact angle provides an inverse measure of wettability: higher angles correspond to lower wettability and vice versa.

In the ideal case of a pure liquid spreading on a smooth inert solid, Young's equation gives the equilibrium contact angle in terms of interfacial tensions existing at the three-phase interface [,]:

where γ is the interface tension between two phases at the interface which can be the solid (S), the liquid (L), or the vapor (V) phase, as indicated by the subscripts in Eq. (1). When the vapor phase is air, the interface tension ${\gamma }_{SV}$ is the surface free energy of the solid and the interface tension ${\gamma }_{LV}$ is the surface tension of the liquid. The difference (${\gamma }_{SV}-{\gamma }_{SL})$ is the adhesion tension, which describes the liquid attraction towards the solid. As can be deduced from Eq. (1), the degree of wetting results from the balance between the intermolecular adhesive interactions represented by the numerator (liquid to surface interactions, causing a liquid drop to spread out on the surface) and the cohesive interactions represented by the denominator (liquid to liquid, causing the drop to ‘ball up’ to avoid the contact with the surface) []. The θ angle, which is between 0° and 180°, is less than 90° (wettable surface) if the adhesive interaction at the numerator is positive because, in this case, the cosine of the angle is positive. On the contrary, when the cosine of the angle is negative (because the adhesive interaction at the numerator is negative), the cosine of the angle is negative and, thus, the surface is non-wettable (θ > 90°). Wettability is maximum when the adhesive interaction at the numerator of Eq. (1) tends to the denominator value. In this case, the cosine of the angle tends to 1 and, therefore, the θ angle tends to 0°.

An ideal surface is flat, rigid, smooth, and chemically homogeneous. In this ideal case, a relatively simple measurement of the static contact angle can be made. Table 2 summarises the techniques adopted in the literature to measure contact angles and wettability of contact lenses.

The main static methods to measure wettability are:

• •
  Sessile drop goniometry (Fig. 2). A droplet of liquid is deposited by a syringe on a solid surface. A digital camera and a goniometer are used to capture the profile and measure the contact angle. In the case of polymers, especially for hydrated samples, the measurements must be performed in a temperature- and humidity-controlled environment and in a relatively short time, since the drop permeation into the matrix of the material can lead to variability of the measured angle or the sample itself can dehydrate.

• •
  Captive bubble method (Fig. 2). A sample of the solid of interest is immersed in a liquid. An air bubble or a bubble of another immiscible fluid is injected under the solid surface, i.e. attached from below to the solid surface. The shape of the bubble is then evaluated. This method is often used with hydrogels such as contact lenses, as the sample does not dehydrate.

Contact angles measured by sessile drop are typically less repeatable than when using the captive bubble method, especially for SiHy contact lenses, probably due to surface dehydration and the blotting required to remove excess liquid [].

In practice, real surfaces often are not flat, rigid, smooth, or chemically homogeneous, and the measured contact angle may vary from point to point on the surface. Wettability is very sensitive to surface contamination, non-homogeneity and the roughness of the surface. In these cases, the contact angle measurement through a static method is affected by the local slope of the surface and the local chemical composition []. In the case of soft contact lenses, sample swelling and deformation of the surface can also influence static measurements of the contact angle. In these cases, measurements of the hysteresis, defined as the difference between the so-called ‘advancing’ and ‘receding’ contact angles, can be useful. Static measurements on non-ideal samples yield values between the advancing and receding contact angles. The hysteresis can also be estimated by comparing results from two methodologies, with the sessile drop method producing the advancing-type angle and the captive bubble method producing the receding-type angle [].

Dynamic methods to measure wettability [,,,] are:

• 1
  Dynamic sessile drop on inclined substrates. A drop is deposited on a substrate and the substrate is inclined. At a given angle of tilt, the advancing and receding contact angles are reached on the two sides of the drop respectively, just before the drop starts moving on the surface.
• 2
  Dynamic sessile drop goniometry (also known as the extension and contraction technique (Fig. 3)). The advancing contact angle is measured by adding liquid to gradually increase the sessile drop in volume. The increase in volume causes an increase of the contact angle without increasing the solid–liquid interfacial area until a maximum angle is reached, and the contact line begins to move across the surface (advancing phase). The receding angle is measured by removing liquid from the drop. The decrease in volume causes a decrease of the observed contact angle without increasing the solid–liquid interfacial area until a stable value of the angle is reached, and the contact line begins to move (receding phase).

• 3
  Dynamic captive bubble method (an extension and contraction technique). The advancing and receding contact angles are measured by gradually expanding and contracting an air bubble in contact with the solid surface (see the static captive bubble method), similarly to the extension and contraction technique based on the sessile drop discussed above and illustrated in Fig. 3.
• 4
  Wilhelmy plate. The solid sample is oriented perpendicularly to the air/liquid interface and it is attached to a balance positioned above the liquid. The wetting force on the solid is measured as the solid is immersed into the liquid (advancing contact angle) or withdrawn back to the initial position (receding contact angle). The measured force is used to calculate the two angles.

In the case of dynamic contact angle measurements on contact lenses, the advancing contact angle is considered to be the parameter most closely describing the initial spreading of the pre-lens tear film (PLTF) on the contact lens surface, whereas the receding contact angle is considered as an indicator of the PLTF stability when the eyelids open and the PLTF starts to retract []. Reducing hysteresis, the difference between the advancing and receding contact angles, has been one of the goals in contact lens development in order to produce a more wettable surface [,].

Other techniques for assessing contact lens wettability in vitro include the observation and characterization of images taken of the tear film over the contact lens. Taking inspiration from some in vivo investigation techniques, a thin film interferometer has been used to capture images of the pre-lens liquid film of contact lenses to describe their drying properties []. Images are obtained from the lens surface when it is wet until it becomes dry and the following parameters can be deduced: time of the first break-up (onset latency), duration of lens surface drying (drying duration), maximum speed of increase in the drying area (maximum speed), and the time to reach maximum drying speed (peak latency). A slightly different technique, non-invasive keratograph dry-up time, has shown that dewetting of SiHy contact lenses was non-uniform across the contact lens surface []. Videokeratoscopy has been used to compare time-dependent contact lens wettability on a corneal model []. A corneal topographer has been used to assess the pre-lens non-invasive tear break-up time (NIBUT) in an in vitro model []. In general, a more complete characterization of the surface energy may be necessary to better model the behaviour of contact lenses in vivo, rather than just relying on the assessment of the contact angles under specific conditions [,,].

Regarding wetting measurements on contact lenses, one aspect to consider is that soft contact lenses are distributed in blister packs filled with a liquid which may contain buffered sodium chloride solution, preserving agents, disinfecting agents and additives introduced to improve wettability. These components can penetrate inside the porous lens matrix and be released when the lens is exposed to another liquid. The latter can also occur during wettability measurements if the lens is not properly rinsed. Often wettability measurements are carried out in vitro using water or saline. However, a contact lens is wet by tears during wear, and water and saline do not mimic the tear film. In normal participants, tears have a surface tension which is approximately two-thirds that of water or saline (42−46 10⁻³ J/m² in SI units corresponding to 42−46 dynes/cm in the centimetre–gram–second system) [,]. The main components responsible for the surface tension of human tears has been reported to be a complex of tear lipocalin with polar lipids []. The use of saline or water that do not contain these components are unlikely to adequately mimic human tears. In vitro NIBUT measurements of SiHy contact lenses taken immediately out of their blister packs or exposed to an artificial tear solution containing various lipids (i.e. oleic acid methyl ester, cholesterol, triolein, phosphatidylcholine, cholesteryl oleate, and oleic acid), salts, urea, glucose, proteins (lysozyme and hen egg albumin), and mucin (with concentrations based on those in normal human tears) demonstrated that these artificial tears tended to reduce differences in surface wettability [].

Furthermore, tear film components adsorb onto and absorb into contact lenses [,] and this affects contact lens wettability [,]. Using a captive-bubble technique, the addition of lysozyme and/or mucin eliminates hysteresis and improves wettability of unworn contact lenses []. Decorating the surface of a SiHy lens with simple model lipids such as dipalmitoylphosphatidylcholine and cholesterol increased its hydrophilicity and inhibited dewetting, whereas addition of whole meibum reduced hydrophilicity and wetting [].

Although this section deals with wettability, it should be noted that materials, manufacturing processes, surface coatings to increase wettability, and wetting agents (internal or released) also contribute to the tribological characteristics of contact lenses, which are expected to play an important role during wear (including affecting comfort of lenses – see section 2.4) [, , ]. Tribology is the study of the interaction between surfaces in relative motion. In the case of contact lenses, the surfaces in relative motion are those of the corneal surface, the tear film, the contact lens and the eyelid. Friction contributes to the tribological properties of this system and is the force resisting the relative motion of these components sliding against each other. Lubricants help in reducing friction and wetting agents are also lubricants.

When a contact lens is placed onto the ocular surface, factors in the ocular environment such as the temperature, osmolarity and composition of the tears can impact the chemistry of the material, changing its surface properties and in turn wettability []. Some attempts to adapt in vitro techniques (Table 2) to assess in vivo wettability have been sporadically proposed in the literature. A measure of contact angle in vivo has been proposed both for rigid corneal lenses [] and soft contact lenses []. A measure of the rate of liquid spreading on the contact lens surface in vivo has also been proposed for soft contact lenses []. However, these methods need validation before they can be translated into clinical practice.

Indirectly, contact lens wettability can be evaluated by assessing features of the PLTF that provide useful information about the level of tears spreading on the lens. Tear coverage can be assessed by examining any PLTF deficiency on the contact lens surface through slit lamp observation by examining the quality of a specular reflection at high magnification, using interferometry observation techniques and with the use of grading scales [,, , , , , , , , ]. Another possibility is to evaluate the optical quality of the contact lens on the eye in terms of higher-order aberrations, since the surface wetting of a contact lens influences optical quality []. Finally, examining the loss of superficial optical quality by measuring the time elapsed between cessation of blinking and blur-out of a threshold letter on the acuity chart has been advanced as another measure of wettability [].

However, NIBUT (sometimes called non-invasive surface drying time when used with contact lenses) is the most utilised in vivo method for assessing contact lens wettability. This is performed using an image (keratoscopy, videokeratoscopy, grid, etc.) projected onto the contact lens and evaluating the quality of the reflection from the PLTF [,, , , , , , , , , ]. NIBUT of the PLTF can also provide contact lens practitioners with an indirect assessment of the lubricity and thus potentially on-eye friction of the contact lens, which is impossible to measure directly in the eye []. NIBUT has several advantages such as accessibility for clinicians, coverage of a large portion of the contact lens surface, and minimum influence of eye movements []. However, the measurements are often not very reproducible and not transferable from one instrument to another [,]. NIBUT has been extensively reviewed in the TFOS (Tear Film and Ocular Surface) International Workshop on Contact Lens Discomfort: Report of the Contact Lens Interactions With the Tear Film Subcommittee and TFOS DEWS II Tear Film Reports [,].

HEMA-based hydrogel lens materials often incorporate more hydrophilic monomers such as methacrylic acid, GMA (glyceryl methacrylate), NVP, PVA and phosphorylcholine to increase the material water content, thereby increasing material Dk [, , , ]. In addition to increasing the equilibrium water content in the bulk, these monomers also influence the wettability of the surface [,]. For example, when HEMA-based hydrogels are modified with NVP, MA or GMA comonomers, the contact angle decreases as the concentration of the crosslinker in the hydrogels increases []. Incorporation of surfactants in the manufacture of contact lens materials reduces the contact angle and can reduce the coefficient of friction of hydrogel lenses []. A correlation between the contact angle and coefficient of friction might be attributed to the stretching of the surfactant tails near the surface into the aqueous phase, and not only to the general increase in water content.

Photo-crosslinkable methacrylated HA has been incorporated into hydrogel contact lenses as an internal wetting agent [] and this improved hydrophilicity. HA of lower molecular weight and degree of methacrylation has been associated with an increased mobility and improved hydrophilicity than the less mobile HA []. Crosslinked HA, despite being only present in very small amounts, lowered the contact angle of hydrogel contact lenses [].

Silicone hydrogel materials can be categorised as first, second, or next generation contact lenses based on the presence and role of wetting agents []. Hydrophilic copolymers such as HEMA and/or DMA or NVP are used as internal agents to enhance wettability of SiHy lenses [,,]. This one-phase process can, in some cases, make SiHy contact lenses more hydrophilic than conventional HEMA-based hydrogel lenses []. Hydrophilic functionalised silicone-based macromers have also been used with hydrophilic monomers in materials such as comfilcon A, enfilcon A and somofilcon A [,]. For example, samfilcon A is a SiHy and, in this case, PVP was incorporated through a semi-interpenetrating network []. Increasing PEG methacrylate in a silicone-based hydrogel formed from poly(dimethylsiloxane) dialkanol (PDMS), isophorone diisocyanate and HEMA led to a lower water contact angle and higher water content [].

Fluorosilicone acrylate rigid corneal lenses have become popular in recent years due to their relatively high oxygen permeability []. These materials require surface modification to increase their wettability using chemical, mechanical, laser, plasma, electron beam irradiation or micro-structuring of the lens surface (a lotus leaf eff ; ;ect) [, , , , , , ]. For example, low temperature nitrogen or argon plasmas have been used to modify lens surfaces [,]. The incorporation of nitrogen or oxygen-containing groups on the surface and the transformation of silicone into hydrophilic silicate after plasma treatment are the main reasons for the improvement in surface hydrophilicity [,]. Oxygen plasma has also been employed to improve surface hydrophilicity through the incorporation of oxygen and the transformation of Si CH3 into hydrophilic Si O [].

The surface of HEMA-based soft lenses can be modified with the use of wetting agents to improve the hydrophilicity of the lens surface. The first commercial lens material to utilise this concept related to the incorporation of non-releasable PVP into the etafilcon A hydrogel material, which resulted in improved wettability and comfort compared with the base etafilcon A material [, , ]. A thin surfactant layer on the surface of HEMA lenses reduced the contact angle from 91 degrees to less than 10 degrees []. Functionalizing the surface of HEMA-based contact lenses, using various methods including "click" chemistry, with HA increases surface wettability and water retention [] and decreases water-based contact angle and the rate of lens dehydration []. Click chemistry refers to a group of reactions that are fast and simple to use, which give physiologically stable products with high yields [,]. A common reaction employed in click chemistry is copper-catalyzed azide-alkyne cycloaddition which tolerates a broad range of temperatures, works in aqueous conditions and over a very broad (4–12) pH range []. This reaction can proceed by functionalising a surface with an azide coating (or similar) and adding a compound that has been functionalised to contain an alkyne; copper catalyses the formation of a covalent linkage via a 1,3 cycloaddition. HEMA materials treated with sulfonated poly(ethylene glycol) via crosslinking reduced dynamic contact angles and protein adsorption with the increasing concentration of sulfonated poly(ethylene glycol) []. However, it must be noted that protein adsorption was assessed using fibrinogen rather than with the proteins commonly found in tears [].

The first commercially available SiHys, balafilcon A and lotrafilcon A, were plasma treated to improve wettability. The surface of balafilcon A was oxidised by plasma [], whereas lotrafilcon A (and the later lotrafilcon B) were treated with a plasma surface coating []. In both cases, surface modification led to more wettable surfaces []. However, clinical studies have shown reduced surface wettability of lotrafilcon A contact lenses compared to the later commercialised (but no longer available) galyfilcon A SiHy material, which was not surface treated, but contained PVP as an internal wetting agent [,]. This concept of incorporating PVP into a SiHy material was also used in the later development of the currently available senofilcon A [,,].

A more recent development is the commercialisation of a SiHy contact lens with a water-gradient structure (delefilcon A). This lens is made of a SiHy core and a surrounding hydrogel surface layer a few micrometres thick, with a water content of approximately 80 % [, , , ]. The advancing and receding water contact angles of narafilcon A and senofilcon A (having no surface coating), stenfilcon A (with just 4.4 % bulk silicone content), and delefilcon A (>80 % water at its surface) have been compared using the advancing contact angle []. The low silicone content at the lens surface and the high surface hydration were the major determinants of SiHy wettability []. The clinical performance of water gradient delefilcon A lenses has also been shown to be superior in a cross-over, randomised, masked study in comparison to two other daily disposable SiHy lenses (somofilcon A and narafilcon A) []. Also, delefilcon A lenses had a longer NIBUT, comparatively greater inferior tear meniscus height and less corneal staining after 16 h of lens wear than narafilcon A and somofilcon A lenses, although no significant differences were found between lens types in subjective comfort, visual acuity, or quality of vision [].

Phosphorylcholine has been coated onto SiHy materials to improve their wettability [,]. Novel amphiphilic (both hydrophilic and lipophilic) networks based on polyallyl-methacrylate and PEG with l-cysteine conjugated to the surface have been produced, which could be potentially used to improve lens wettability []. The co-networks had relatively low water contact angles (less than 80°, down to 25° for some of these networks) and the l-cysteine improved the surface wettability of these systems []. Surface polyelectrolyte multilayers (PEMs) have also been proposed to enhance lens wettability. A SiHy composed of PDMS and PEG methacrylate with a surface PEM of chitosan and HA (as positive- and negative-charged agents, respectively) showed decreased contact angles as the number of PEM grafting layers increased [].

Proteoglycan 4 (PRG4 or lubricin) is a lubricant mucin-like glycoprotein that minimises friction between articular cartilages [, , , , ] that can be found on the ocular surface []. It has also been used as a surface wetting agent in commercially available soft contact lenses []. Addition of PRG4 to hydrogel or SiHy lenses enhances their wettability and lubricity [,] as well as the kinetic coefficient of friction []. Incorporation of both HA and PRG4 to form an HA/PRG4 complex produces synergistic properties that reduces friction [,,]. A different surface modification strategy has also been proposed, whereby a HA-binding peptide is bound to the surface of contact lenses. Bound HA, via the HA-binding peptide, significantly reduced water loss from the modified contact lens [,].

A dimethyl acrylamide hydrogel layer (2.1 ± 1.4 μm thick) has been coated onto narafilcon A SiHy contact lenses. This resulted in a significant reduction in contact angle from ∼95 degrees for untreated lenses to ∼15 degrees []. A limitation, however, was that treated lenses tended to warp due to increased thickness [].