In Vitro Mechanical Wear of Contact Lenses Surface Gel Layers

Abstract

 

Purpose

 

Lubricity is recognized as a contributor of contact lens comfort (Brennan 2009). Out-of-pack lenses are traditionally used in experiments aimed at quantifying the lubricity of soft contact lenses (Dunn 2013). Previously, a scalable laboratory method to mechanically exercise contact lenses and simulate wear was established (Urueña 2013). Energy imparted to the lenses with our method can be tuned and its effects on lubricity measured as a function of simulated wear time and surface gel thickness.

 

Methods

 

A granular bed of polydisperse polyhydroxyethylmethacrylate (pHEMA) particles (65%) with a radius of 440 ± 210 um mechanically exercised the lenses. Contact lenses were placed in the granular bed and agitated at 600 and 800 rpm driving frequencies. Particles were tracked and the resulting shear field was quantified by image analysis. The surface gel thickness was measured by microscopy and the friction coefficient was measured using a microtribometer.

 

Results

 

Estimates of strain amplitude, strain rate, and stress within the granular bed suggest that two hours of agitation delivered mechanical energy equivalent to 3 to 5 weeks of blinking. At 600 and 800 rpm, approximately 5 µm of the surface gel is worn, with the lenses exercised at 800 rpm exhibiting a higher rate of degradation. The difference in degradation rate is accentuated in lubricity measurements which were compared with previously presented data (Urueña 2013). Lower driving frequencies resulted in no loss of lubricity, with a coefficient of friction, µ < 0.02. At 800 rpm the surface gel wore more aggressively, resulting in µ ~0.03.

 

Conclusions

 

mparting a controlled amount of mechanical energy changes the time-scale at which the lens loses lubricity. Results suggest that the surface gel is robust, consistent with a daily disposable modality but surface gel thickness decreases at longer time-scales and higher levels of energy input. References: - Brennan N. Contact lens based correlates of soft lens wearing comfort, Optom Vis Sci 2009; 86: E- abstract 90957 -Dunn, et al., Tribology Letters, 2013. -Urueña, Juan Manuel, et al., “Laboratory Model for Wear of Contact Lenses and Effects on Lens Lubricity of Surface Gel Layers”. ARVO 2013. Poster presentation.

 

 

Schematic of (A) granular bed and (B) gel thickness measurements.

 

 

(A) Surface gel thickness and (B) friction coefficient over time. Note that the 400 rpm experiment was performed in PBS as presented in Urueña et al.

 

Keywords: 477 contact lens