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BestThinking / Articles / Medicine / Wettability of Contact Lenses: A Recommended “Teflon Test” Control Technique (Article)
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Wettability by what? Why do we care? In the blinking or open eye, the competition between tear film components and the adjacent air for coverage of the lens surface is directly related to patient comfort and ocular health.
 
 
 

A Recommended “Teflon Test” Control Technique

image

Used only with express written permission

Water does not wet silicone-coated root canal file.

Wettability by what? Why do we care? In the blinking or open eye, the competition between tear film components and the adjacent air for coverage of the lens surface is directly related to patient comfort and ocular health. ALL liquids DO wet ALL solids to some extent, and we use the contact angle formalism of the venerable Thomas Young (1805) to make quantitative statements about the degree of surface wetting in terms of the equilibrium contact angles of test liquids with the surfaces in question. In such test systems, with many “unknowns”, it is critical that the air/liquid surface tension values of the wetting liquids be accurately determined AND maintained throughout—and beyond-- the lens/liquid interface interaction. For contact lenses, it has been universally accepted that their surface wetting by water will be the standard by which the comparisons should be made. Our institutional problem, leading to often-cited variability of reported contact angle values, is that most investigations use “dirty water” –that is, water NOT of the required initial highest purity with a measured high liquid/vapor surface tension of 72.8 dynes/cm at 20 degrees {at the moment of first contact with the lens surface and generally much lower after that}. It is especially difficult to maintain water’s required high surface tension in large-volume test systems where multiple contact surfaces are simultaneously present from containers, needles, syringes, and is a serious further error to actually add surfactants to the system to obtain investigator-sought “more reasonable” data.

image

Used only with express written permission

Water-wettable root canal file.

Liquids of starting surface tensions lower than that of pure water WILL exhibit lower equilibrium contact angles than will pure water on all surfaces, giving misleading indications of apparent hydrophilicities for those test materials. Pure water put onto a lens surface that “exports” surface-active ingredients to the water, even in minute amounts, substantially lowers the surface tension of that water and again gives a falsely lower apparent water contact angle value for that surface. Lens surfaces that imbibe water, no matter how pure it may be, do not allow the EQUILIBRIUM requirement of the Young equation to be met and so do not characterize the water wettability of those lens surfaces but rather their bulk uptake properties. For better comparisons, the advanced formalism of Zisman (1968) to determine Critical Surface Tension values from contact angle measurements with multiple pure liquids gives reliable, reproducible correlations with materials’ actual outermost atomic constitutions—even for hydogels where the water contact angle is not a stable value.

Here is a way to establish trustworthy analytical results. Take a single droplet of the proposed “water” test liquid, and apply it to a clean, smooth Teflon (PTFE) surface. According to Zisman and numerous following experts, that water-on-PTFE contact angle value should be greater than 110 degrees (and higher if there is surface roughness). If it is lower, you are NOT doing a water wettability test. If the initial water contact angle is 110 degrees or higher, proceed with the measurement on the lens surface AND then take water AFTER the test for re-application to the Teflon surface. If the contact angle is not STILL above 110 degrees, there has been surface-active contamination of the water and your measured values do NOT characterize lens materials’ water wettabilities. Simply touching a contact lens to a free-standing 110 degree water droplet on Teflon will cause that water’s contact angle to immediately slump to lower values in proportion to the surface-active components spontaneously eluted. When extensive prior elution has been done, and the pure water contact angle on an eluted lens surface is still low, THAT is a reliable signature of a material surface with sustained hydrophilicity. Simply adsorbing elutable water-loving substances onto lens surfaces, or compounding them with elutable bulk additives, can give potential short-term clinical benefits but this is not related to intrinsic material wettability.

References

T. Young, Phil. Trans. Roy. Soc. London 95, 65 (1805)

R.E. Baier, E.G. Shafrin, W.A. Zisman, Science 162, 1360 (1968)

 
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Robert E Baier
Professor Bob Baier, State University of New York at Buffalo, is a PhD Biophysicist and Registered Professional Engineer specializing in con

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