Hydrophilization of hard contact lenses for improved comfort

Hard contact lenses stand out due to its high gas permeability which ensures an improved oxygen supply for the cornea. Particularly in the case of extended wearing time this is a great advantage compared to soft contact lenses.

However, in some cases hard contact lenses cause a lasting foreign body sensation in the eye caused by friction between the contact lens and the cornea or the eyelid. Therefore, a perfect, individually shaped contact lens is essential. In addition, the lachrymal fluid should moisten the entire surface of the contact lens as it constitutes a natural lubricating film.

Improved comfort due to surface modification

Fraunhofer IGB has developed a process in cooperation with Hecht Contactlinsen GmbH to modify the surface of contact lenses with the aim of achieving improved comfort. The surfaces are modified in such a way that an almost closed film of lachrymal fluid can form. At the same time the formation of protein-containing deposits which can cause clouding in lenses is reduced. However, it is essential that the positive properties of the bulk material of the contact lens, particularly the oxygen permeability and the optical properties are retained.

Hydrophilization of lenses by means of plasma technology

Due to the thermal instability of the material low-pressure plasma processes are particularly suited for surface modifications. Hydrophilic functions are systematically built into the surface of the lens by means of suitable process control. They improve the wettability of the surface and reduce the formation of protein deposits due to an increased adsorption of water in the boundary-layer.

A series of tests with different gases and gas mixtures, excitation frequencies (microwave / radio frequency), treatment times and power inputs was conducted to determine the optimal plasma parameters.

Adapted analysis methods produce findings on wettability and protein deposits under test conditions which are as realistic as possible. In order to evaluate the wettability, for instance, the modified contact lenses were stored in simulated tears and the wetting angle was determined by dosing an air bubble on the surface.

Results

The surface modifications developed resulted in a distinct improvement in wettability. It is currently being determined whether such treatment is sufficient to achieve a long-term effect. The widely available abrasive contact lens cleaning agents successively remove the top atom layers of a lens which could reduce the effect to zero in the long-term. To solve this problem the deposition of suitable biocompatible nanoscale layers via PECVD processes (plasma enhanced chemical vapor deposition) has been developed.

Further applications

It is generally possible to transfer this method to all applications requiring good wettability of plastic surfaces which come into contact with body fluids, as well as those requiring a reduction of protein deposits, such as stents and catheters.