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March 2007 |
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| ANSI Z80.20-2004 | ||||
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This past month we learned a lot more about
just one of the many applications for which our ramé-hart Contact Angle
Goniometers is being used. The application is the production and quality
control of contact lenses. Contacts are worn by over 100 million people
around the world. One quarter of contact wearers are in the US while
another 10% are in Japan.
The basic concept of putting a lens over the eye to modify vision actually dates back to the ideas of Leanardo da Vinci (1508). The modern soft contact lens is accredited to Dr. Otto Wichterle, a Czech chemist who studied hydrogels (hydrophilic gels) for biological use. During the 1960's the soft contact lens became prevalent and eventually more popular than the hard glass contact lens.
In the late 1990's the first silicone hydrogels were introduced. This new material allowed for even greater oxygen permeability compared with earlier polymers and allowed contact lens manufacturers to develop products that are both comfortable and can be left in overnight. The silicone lens is much more hydrophobic (i.e., less wettable) than earlier polymers. In order to overcome this issue -- which can lead to dryness and discomfort -- hydrogels are added; hence the name "silicone hydrogels" which is really a Polyacrylamide. During the manufacture of silicone contact lenses a process referred to as polyacrylamide gel electrophoresis is used to bond the hydrophilic polyacrylamide to hydrophobic silicone material. An electrical current is used to migrate the negatively charged proteins across the gel. Time and currency variables are used to control the process. Our ramé-hart Contact Angle Goniometer is used to measure the success of the electrophoresis and allow greater control over the quality of the process. In a more specific study (Bruinsma GM, van der Mei HC, Busscher HJ.; Department of Biomedical Engineering, University of Groningen, The Netherlands), the relative hydrophobicity of a contact lens surface serves to measure the composition of the absorbed tear film and bacterial adhesion. Contact angle can also serve to measure the effects of wear and overwear on adhesion over time. The American National Standards Institute (ANSI) has developed a standard for measuring hydrophobicity of contact angle lenses: ANSI Z80.20-2004. The standard specifically calls for measurements using a rame-hart Contact Angle Goniometer. It's somewhat unusual that a standard will mention a measuring tool by brand -- but we appreciate the endorsement. The standard calls for both sessile drop and captive bubble methods. When using the sessile drop method, the standard calls for a 2μL drop of saline on the lens material using a 32 gauge needle. The captive bubble method calls for a 10μL gas bubble on the underside of the polymer lens while submerged in saline at room temperature thus forming a contact angle at the gas-liquid-solid interface. The captive bubble method is facilitated by the use of the ramé-hart environmental fixture and requires an inverted needle -- as specified by the standard.
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