A study of the frictional properties of senofilcon-A contact lenses

Ultra-low friction in soft contact lenses: Analysis of dynamic free responses measured by the dynamic oscillating tribometer

Friction coefficient is considered as a measurement for clinical comfort of soft contact lenses. One of the main challenges in the tribology field is to evaluate the friction in soft materials such as soft contact lenses. In this work, we aim to evaluate the friction at the eyelid-lens contact using a new technique. This technique is based on a single-degree-of-freedom (SDOF) mass-spring system having a sliding contact. The experimental technique measures the free responses of the mechanical system. The friction is then evaluated from the damped free responses with a high accuracy. A variety of soft contact lenses are tested with some lubricants under many physiological conditions. The results are discussed and compared with those in the literature.

Impact of the physical properties of contact lens materials on the discomfort: role of the coefficient of friction

Contact Lens Discomfort (CLD) is the main cause in contact lens (CLs) discontinuation, referred in literature as drop-out phenomenon. Despite such evidence was reported in several clinical studies, a relationship between physico-chemical properties of CLs and CLD is not still totally understood. In this regard, the friction of CLs surfaces seems to be related to discomfort feeling events, probably due to an alteration of the lubricate function of the tear film after the CL placement inside the ocular environment. In the last years, many studies have been finalized to the friction measurements of CLs surface, finding conflicting data due to a lack in standardized protocol. The aim of this review is primarily to show evident relationships between CLs surface properties (i.e. wettability, tear evaporation, tear film quality, etc.) and the coefficient of friction (CoF), resulting therefore the most relevant physical quantity in the CLs characterization. In addition, we reported the most recent studies in CLs tribology, which highlight that the introduction of a standard protocol in CoF measurements is necessary to obtain reproducible results, considering the aim to evaluate in a more precise way the relationship between this material surface property and comfort in CLs users.

Superhydrophilic Poly(2-hydroxyethyl methacrylate) Hydrogel with Nanosilica Covalent Coating: A Promising Contact Lens Material for Resisting Tear Protein Deposition and Bacterial Adhesion

Tear protein deposition and bacterial adhesion are the main drawbacks of the hydrogel contact lens. In this study, we developed a novel superhydrophilic poly(2-hydroxyethyl methacrylate) (NSCC-pHEMA) hydrogel with nanosilica covalent coating by the combination of colloidal silica immersion and dehydration treatment. The infrared spectroscopy and energy dispersive X-ray spectroscopy analyses confirmed the successful formation of Si-O covalent bonding between nanosilica and pHEMA hydrogel. This coating was highly stable against powerful sonication or long-term shaking immersion treatment. Among various NSCC-pHEMA hydrogels with different colloidal silica concentrations, the 7%NSCC-pHEMA hydrogel generated a superhydrophilic micro wrinkle surface with a root-mean-square roughness of 43.10 nm, which dramatically reduced the deposition of lysozyme and bovine serum albumin by 65% and 57%, respectively, and decreased the adhesion of S. aureus and E. coli by 59% and 66%, respectively, in comparison to the pHEMA hydrogel. However, the nanosilica coating had little effect on the mechanical properties, light transmittance, oxygen permeability, and equilibrium water content of the pHEMA hydrogel. NSCC-pHEMA hydrogels were nontoxic to both mouse fibroblasts (L929) and human immortalized keratinocytes (HaCaT). Thus, the superhydrophilic NSCC-pHEMA hydrogel is a potential contact lens material for resisting tear protein deposition and bacterial adhesion.

Evaluation of the Lubricating Effect of Hyaluronic Acid on Contact Lenses Using a Pendulum-Type Friction Tester Under Mimicking Physiological Conditions

OBJECTIVE

To evaluate the lubricating effect of hyaluronic acid (HA) on soft contact lenses (SCLs) measured using a pendulum-type friction tester.

METHODS

We measured the coefficient of friction (CoF) of narafilcon A, delefilcon A, and etafilcon A with polyvinylpyrrolidone (PVP), daily disposable SCL material, using a modified pendulum-type friction tester. Sample SCLs were set on an acrylic plastic half-ball and placed into the polyethylene terephthalate hemisphere cup filled with 0.4 mL of test lubricants that included saline and 0.05%, 0.1%, 0.2%, 0.3%, 0.4%, and 0.5% (wt/vol) HA (molecular weight, 850 kDa). The viscosities of saline and HA were measured using an Ubbelohde viscometer.

RESULTS

The CoF of the SCL under a low concentration (0.05%) of HA was the lowest and significantly lower than saline in narafilcon A and delefilcon A (P<0.05, Steel multiple comparison test). Under higher HA concentrations (0.3%, 0.4%, and 0.5%), the CoF was significantly higher than that of saline (P<0.01, Steel' multiple comparison test) in all three SCLs. There were no significant differences of CoF among three SCLs in saline and all concentrations of HA. The HA viscosities increased exponentially with the concentration (Y=1.2829e9.286X).

CONCLUSION

The viscosity of a high concentration of HA may increase the friction of SCLs, which may have a deleterious effect on the ocular surface.

Pharmaceutical-loaded contact lenses as an ocular drug delivery system: A review of critical lens characterization methodologies with reference to ISO standards

Therapeutic contact lenses for ocular drug delivery have received considerable interest as they can potentially enhance ocular bioavailability, increase patient compliance, and reduce side effects. Along with the successful in vitro and in vivo studies on sustained drug delivery through contact lenses, lens critical properties such as water content, optical transparency and modulus have also been investigated. Aside from issues such as drug stability or burst release, the potential for the commercialization of pharmaceutical-loaded lenses can be limited by the alteration of lens physical and chemical properties upon the incorporation of therapeutic or non-therapeutic components. This review outlines advances in the use of pharmaceutical-loaded contact lenses and their relevant characterization methodologies as a potential ocular drug delivery system from 2010 to 2020, while summarizing current gaps and challenges in this field. A key reference point for this review is the relevant ISO standards on contact lenses, relating to the associated characterization methodologies. The content of this review is categorized based on the chemical, physical and mechanical properties of the loaded lens with the shortcomings of such analytical technologies examined.

Relationships between the material properties of silicone hydrogels: Desiccation, wettability and lubricity

Silicone hydrogels (SiHy), represent composite matrices composed of hydrophobic gas permeable silicone (Si) rich core and a surface enriched with hydrophilic polymer moieties. Their utilization in contact lens design requires number of SiHy properties (hydration, wettability, lubricity) to be optimized for the challenging conditions at the ocular surface. Typical limitations in literature are that (i) these properties are studied in isolation, monitoring only one parameter but not the rest of them, and (ii) measurements are performed with hydrated samples immediately after removal from storage solutions. Here we study the simultaneous evolution of critical material properties (evaporative loss of water, water contact angle, coefficient of friction) of different SiHy subjected to continuous blink-like desiccation/rehydration cycling. SiHy with wetting agents incorporated in their core (narafilcon A, senofilcon A) were particularly susceptible to extended desiccation. Stenfilcon A, a material with only 3% bulk Si content maintained its performance for 4 h of cycling, and delefilcon A (80% surface water content) resisted extended 8 h of desiccation/rehydration runs. Strong correlation exists between the evolution of SiHy wettability and lubricity at ≥4 h of blink-like cycling. Understanding the interplay between SiHy properties bears insights for knowledge based design of novel ophthalmic materials.

Comparison of the Lubricity and Surface Roughness of 5 Cosmetic Contact Lenses

OBJECTIVES

Cosmetic contact lenses are increasingly popular because of their eye enhancing cosmetic benefits. The pigment particles used in these lenses can impact lens surface characteristics. This article examines the surface characteristics and the differences between the clear and the pigmented regions among five limbal ring design lenses.

METHODS

Scanning electron microscopy was used to determine the location and depth of the pigment particles from the lens surface. The coefficient of friction (CoF) was determined with a Basalt-MUST microtribometer at clear and pigmented regions on either the front or the back surface. Atomic force microscopy was used to determine the surface roughness of each lens in root-mean-square (RMS) units at clear and pigmented regions. A linear mixed model for repeated measures was used for the analysis of the CoF and RMS roughness to compare all lenses.

RESULTS

Four lens types had pigments exposed on the surface and one lens type had pigment fully enclosed. The CoF difference between clear and pigmented regions were similar and not statistically significant (P=0.0124) for the lens type with pigments enclosed, whereas the CoF difference for the other four lens types showed statistically significant difference (P<0.0001).

CONCLUSIONS

Of the lenses tested here, cosmetic contact lenses with pigments enclosed in the lens matrix provided a more consistent surface between clear and pigmented regions compared with lenses that had exposed pigments.

The effect of albumin and cholesterol on the biotribological behavior of hydrogels for contact lenses

The irritation/discomfort associated with the use of contact lenses (CLs) is often related to the eyelid-lens friction. Although the use of such devices is widespread, the information about the influence of the lacrimal fluid biomolecules on the tribological behavior of the CLs hydrogels is scarce. In this work, we investigated the effect of the presence of albumin and cholesterol in the lubricant medium, on the frictional response of two model hydrogels for CLs: a hydroxyethylmethacrylate based hydrogel, HEMA/PVP, and a silicone based one, TRIS/NVP/HEMA. Tribological experiments were done in a nanotribometer, in water and in the presence of solutions of those biomolecules. It was observed a significant increase of the friction coefficient (μ) for HEMA/PVP when the lubricant contains cholesterol, and for TRIS/NVP/HEMA when it contains albumin. Solid-state NMR and DSC analysis revealed that HEMA/PVP hydrated in cholesterol solution has a lower amount of free and loosely bound water than the hydrogel hydrated in water. Therefore, a smaller amount of water shall be released into the contact region during the friction tests with cholesterol solution, leading to a thinner film in the contact zone, and consequently to a higher μ. Concerning TRIS/NVP/HEMA, QCM-D studies showed that this hydrogel adsorbs less albumin than HEMA/PVP and that the formed film is more rigid, which can explain the increase of μ. The obtained results contribute to understand the influence of lacrimal fluid composition on the tribological behavior of CLs materials, being relevant for the selection and optimization of these devices.

STATEMENT OF SIGNIFICANCE

Understanding the tribological behavior of contact lenses (CLs) materials in contact with the lacrimal fluid and the role of its components is of major importance to optimize the comfort and overall success of these devices. Nevertheless, the available information on this subject in the literature is scarce. In this work, the effect of albumin and cholesterol (two of the main components of the lacrimal fluid) on the frictional response of distinct types of hydrogels suitable for CLs, is compared, for the first time. Significant differences were observed with the two molecules, depending on the material, stressing the need to further study this issue to understand, predict and optimize the in vivo performance of CLs.