All Eyes on This: Streaming Potential Analysis of Contact Lenses

Comfort is essential for contact lenses, because almost every user knows the feeling when the lens is no longer properly seated. Often users decide to discontinue the use due to discomfort. In order to improve hydration, oxygen permeability, wettability and wearing comfort, manufacturers can determine the flow potential of their contact lenses and, if necessary, make surface modifications to improve the properties of the lenses. In this application report we use the SurPASS 3 electro kinetic analyzer and a dedicated measuring cell for soft contact lenses to characterize the surface of different contact lenses.

Contact lenses have become indispensable in today's every-day life. People wear contact lenses for different reasons, such as aesthetics or functional reasons, as contact lenses provide better peripheral vision, which makes them suitable to wear during sports or other outdoor activities. But there are also medical reasons, such as keratoconus and aniseikonia, which are better treated with contact lenses compared to glasses.
Contact lenses are classified according to various criteria, such as their number of corrected distances (focal, bifocal), their shelf life (daily, monthly, yearly) or their dimensional stability. In terms of dimensional stability, a distinction is made between:

  • soft contact lenses (e.g. hydrogels, siliconehydrogels or hyaluronic acid)
  • hard contact lenses (e.g. silicone polymers)

One of the main requirements for contact lenses are not only a perfect fit on the eye, but they should not dehydrate to remain good visual clarity. Moreover, they must not absorb components of the tear liquid,such as protein, mucus or lipids, as this can impair eyesight and lead to discomfort. Soft contact lenses have to comply with different demands regarding oxygen permeability, wettability and wearing comfort. Biocompatible materials such as e.g. phosphorylcholinor polyvinylalcohol should furthermore diminish the formation of biofilms and bacterial adhesion on the surface, in order to reduce the risk of eye infections. Therefore, these substances are used as additives for soft contact lenses. All these properties above are strongly related to the lenses surface properties. 
They can be investigated by measuring the surface zeta potential, as zeta potential indicates the changes in the surface chemistry of contact lenses upon wearing. Surface charge measurements at the solid liquid interface of the hydrated contact lens provide an excellent method to probe the effects of surface ionicity, equilibrium water content and the effects of wear time.

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