Tensile Testing of Contact Lenses in a Temperature-Controlled Bath
Millions of people use contact lenses on a daily basis, making them one of the most widely used medical devices in the world. Preferred over eyeglasses, they are used for a variety of eye correction prescription and are manufactured in an array of forms ranging from hard to soft lenses. When testing lenses, loading the specimen into the grips can pose many problems: the soft lens material is slippery and delicate, and fractures at very low forces; and the test specimens are extremely small, providing a limited amount of gripping surface. Contact lenses must also be tested in an environment that best simulates physiological conditions; ambient testing results in the drying out and cracking of the material. The most common method for characterizing contact lenses is a basic tensile test to failure.
For this test, we used a single column electromechanical test frame configured with a 10 N load cell and 250 N submersible pneumatic grips with extra long surfalloy faces for this test. Additionally, we used the BioPuls™ Submersible Pneumatic Grips and Temperature-Controlled Bath to keep the contact lenses hydrated and at body temperature. We cut the specimens into strips in order to obtain accurate data for measuring strength and tensile strain; and used a test speed of 5 mm/min. Bluehill® 3 software was used to evaluate maximum load, tensile strain at the maximum load and the modulus for two contact lens specimens.
Our results show that the system configuration was successful in testing the contact lenses, and we were able to accurately demonstrate the low forces required to measure specimen failure. In conclusion, contact lenses can easily be tested using the previously described configuration. We recommend that the faces used for this testing allow for zero grip separation in order to accommodate for the small overall length of the contact lens specimen.
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