Surface charge characterization of hydrogel and silicone hydrogel contact lenses

Transcription

1 Surface charge characterization of hydrogel and silicone hydrogel contact lenses Liu, Lina (Department of Chemical Engineering, McMaster University) Guidi, Giuliano (Department of Chemical Engineering, McMaster University) Sheardown, Heather (Department of Chemical Engineering, McMaster University) Introduction Contact lens surfaces may be ionic or electrically neutral. Base on how different lenses interact with various lens care solutions and how they are affected by interaction of different proteins or lipids from tear film, FDA categorised soft contact lenses into 4 distinct groups including two ionic contact lens Group III and group IV. Although little information can be found from literatures[1], surface charge characteristic is very important, as it affects protein and lipid deposition on contact lens surfaces, solution compatibility and water content of contact lenses. Ionic contact lenses normally possess negative charge resulted from lens material components containing carboxylic acid group. The ionic surfaces tend to attract positively charged substance from tear film, such as the most abundant protein lysozyme. Due to strong

2 interaction with materials, the protein deposition may go deeply into matrix and is not easily removed. Furthermore denatured proteins can cause clouding of contact lenses, inflammatory response and patient s discomfort. In addition, negatively charged surface is more sensitive to changes in ph and osmolarity. In this work, zeta potential titration and streaming current method which was used in paper pulp analysis [2] was used to quantify surface charge density of various commercially available contact lenses. Materials and Methods Different contact lens types from 4 FDA groups were studied. Zeta potential titration was carried out using MuTek PCD03 BTG Particle Charge Detector. The method to determine contact lens surface charge amount includes two steps, neutralization and zeta potential titration. Briefly, after soaking in 2.5mM NaCl solution for 4 hours, the equilibrated lens was transferred into excess of positively charged electrolyte solution, 10ml of 1mM Poly(diallyldimethylammonium chloride) (polydadmac) in 2.5mM NaCl solution to neutralize

3 the negative surface charge. The negative charges on lens surface were neutralized for certain time period (0.5 hour to 24 hours) under gentle agitation. After neutralization, the lens was removed and the remaining solution was back titrated with negatively charged electrolyte, 1 mm of potassium poly(vinyl sulfate) (PVSK). The volume of PVSK used can be converted to charge amount on the contact lens surfaces. To investigate positive charge on the surface of contact lenses, the same method was used except switching use of polydadmac and PVSK reagents. The effect of such factors as molecular weight of polydadmac, neutralization time, PVSK concentration, material water content were studied and the conditions were optimized. Results As expected, Acuvue 2 and Focus Monthly used a lower amount of PVSK for titration of polydadmac remaining in their lens neutralizing solutions than those for the other non-ionic lenses, since these two lenses consumed some polydadmac in neutralization due to their negatively charged surface (Fig. 1). The result shown in Fig. 2

4 suggested that using low concentration of PVSK for titration can increase sensitivity of the method. Fig.1: Surface charge determination of 4 different contact lenses using 3 different molecular weight polydadmac. Fig. 2: PVSK concentration effect on sensitivity of the surface charge determination method Discussion and Conclusion The result of surface charge determination is consistent with material charge indication from the FDA grouping, validating the method. It was also found that molecular weight of polyelectrolyte used in neutralization has a major impact on surface charge measurement. The molecular weight of polyeletrolyte influenced the extent of penetration into the matrix of contact lens materials. A low molecular mass polyelectrolyte has higher accessibility to charges in a deeper layer of the lens than a high molecular mass polyelectrolyte. Fortunately, this effect was minimized in testing of silicone hydrogel lens types and low water content contact lens types. It is believed polydadmac neutralized contact lens surface charge hyper

5 stoichiometrically, likely due to stronger interactions between polydadmac, which is a very strong poly electrolyte, and surface charges on contact lens in low concentration sodium chloride medium. Wetting agents demonstrated masking effect, so that surface charge density was slightly lower than the same contact lens without wetting agent modification. Comparing with 1mM PVSK, using 0.2mM concentration of PVSK to do the titration increased sensitivity of the measurement significantly. Surface charge density of contact lenses from 4 different FDA groups was successfully measured. The method was also evaluated and the conditions were optimized. The result suggested that streaming current titration is a reliable method for contact lens surface charge determination and it is sufficiently sensitive to detect differences between lens materials.

6 Acknowledgements

7 Funding from 20/20 NSERC Ophthalmic Materials Network is gratefully acknowledged. References 1. Stanislawa Szarska and Katarzyna Platek, Surface Charge Change in Contact Lens Materials, Advances in Electrical Engineering & Electrical Machines, Vol 134, pp , D. Stumm, R. Grenz, W. Dietz, Surface Charge Determination for Balancing Charge Quantities and Modelling the Reactivity of Furnish for the Efficient Application of Wet Strength Agents, Papermaking Research Symposium, Kuopio, Finland, June 1-4, 2009