(257f) Incorporation of Polymerizable Surfactants in Hydroxyethyl Methacrylate Lenses for Improving Wettability and Lubricity

Authors: 
Chauhan, A., Colorado School of Mines
Hsu, K. H., University of Florida

Almost 50% of the 35 million contact lens wearers in the North America experience some dryness and discomfort, particularly towards the end of the day.  Discomfort is in fact the leading cause of contact lens dropouts.  While the exact mechanisms that cause the dryness and discomfort are complex and not completely understood, it is known that certain properties of the lenses impact both dryness and discomfort. For instance, contact lenses with poor wettability are considered to have an increased potential for causing dryness.  The detrimental effect of poor wettability can be attributed to the rapid breakup of the tear film on the lens surface leading to increased evaporation.  A good wettable lens surface will ensure a stable pre-lens tear film (PLTF) which is necessary to achieve good vision and a stable PLTF will provide a lubricating effect to ease the sliding between the eyelid and lens.  The contact lens discomfort is potentially caused by the interaction of the ocular epithelia with the lens and thus surface lubricity is also expected to be an important parameter for comfort.  Since lubricity and wettability are believed to be two of the main factors impacting dryness and discomfort, significant efforts have been made to improve both of these properties.  In this talk we will show that both lubricity and wettability can be significantly improved by incorporation of polymerizable surfactants in contact lenses.

 We have focused on incorporating polymerizable surfactants in hydroxyethyl methacrylate (HEMA) lenses to improve comfort, while minimizing the potential for surfactant release into the tears.  We used three types of commercially available surfactants with vinyl groups to facilitate incorporation into the HEMA gels.  The N10 and N30 surfactants belong to the Noigen series with values of 10 and 30 for n, respectively, while H30 belongs to the Hitenol series with n = 30.  The non-ionic N10 and N30 were chosen because these are expected to be nontoxic to cornea because Brij surfactants similar in structure are shown to be non-toxic.  The ionic H30 was chosen to study whether the charge at the interface is more effective in increasing wettability compared to the non-ionic surfactants. The surfactants were added to the polymerization mixture, followed by UV curing and extraction of leachables in hot water.  Wettability and lubricity were characterized by measuring the contact angle and coefficient of friction. Lenses were also characterized by measuring transmittance, loss and storage moduli and ion permeability. Incorporation of surfactants significantly reduced the advancing contact angle from 900 for p-HEMA gels to about 100 for 8% or larger surfactant loading (w/w) in hydrated gel. The contact angle was measured using a sessile drop and the angle was relatively independent of the spreading velocity.  The coefficient of rolling friction measured between a metal dumbbell and the surface also decreased from about 0.2 for HEMA gels to 0.05 for the gels with the 8% or larger surfactant loading. There was an excellent correlation between the coefficient of friction and the contact angle reduction, suggesting that surfactants from near the surface penetrate into the solutions to improve the wettability and lubricity.   FTIR studies demonstrated that the surface of the surfactant-incorporated gels binds significantly more water compared to the control gels.  The water content also increased and the modulus decreased, but an increase in the crosslinking density can bring both parameters to within the range for contact lenses.  The gels were clear and certain compositions also have the capability to block UVC and UVB radiation. 

 The surface energy of the surfactant covered surfaces was estimated by measuring the contact angles of water and diiodomethane and using the Fowkes’ method with the Owens and Wendt modifications, which relies on measurements of contact angles of two different liquids.  We used water as the polar liquid and diodomethane as the non-polar liquid.  Using this approach we obtained values 31.14 mJ/m2 for the control pHEMA gel and 67.5, 74.09, 75.02 and 75.72 for gels containing N10 surfactants at 0.01, 0.02, 0.035 and 0.055 mM, respectively.

 The results suggest that incorporation of polymerizable surfactants could be useful in improving surface properties without significantly impacting any bulk property.