Soft contact lenses represent a convenient and aesthetic option for many people. However, quality and wearing comfort depend on different factors, such as oxygen permeability, wettability, and the condition of the lens surface. And this is where the zeta potential comes in.
Poor hygienic conditions, overnight wear, or frequent handling of contact lenses can accelerate the loss of quality and the risks of eye infections such as the formation of a biofilm that consists of microorganisms sticking to the surface of the lens. Even though the likelihood of a biofilm forming decreases with low handling frequency, there is no protection from infection1. As a first step towards biofilm formation the contact with tear fluid initiates the deposition of proteins, such as albumin, lactoferrin, or lysozyme, and of lipids. Monitoring the rate of such deposition and the effect of surface modification to prevent biofilm formation helps to positively influence the contact lens surface.
So how does that relate to the surface zeta potential?
The adhesion of infectious bacterial strains can be analyzed in terms of their physico-chemical surface properties using different applications, such as atomic force microscopy, contact angle measurements, or X-ray photoelectron spectroscopy. Another highly reliable method is the measurement of the zeta potential as it provides information about the surface charge of macroscopic solid surfaces. Other than conventional surface characterization techniques that investigate dry samples, the zeta potential focuses on the solid-water interface. During the measurement, soft contact lenses are exposed to an aqueous solution or a model fluid representing tears. Besides the surface and interfacial charge the zeta potential informs about the surface functionality, the interaction between the surface and dissolved compounds, and liquid-on-solid surface adsorption processes. Therefore, by measuring the surface zeta potential we gain knowledge about the behavior of soft contact lenses in a biological fluid.
How can I visualize the measurement of the surface zeta potential?
The measurement of streaming potential is used to calculate the zeta potential of solid samples. The streaming potential is simply a DC voltage generated by the flow of water through a capillary. A requirement for the surface zeta potential analysis is the formation of this capillary, which is defined by the sample’s surface. It may be straightforward to imagine the formation of a rectangular slit channel through a parallel alignment of samples of a rigid polymer film. However, the convex shape of a soft contact lens and its hydrogel structure bring challenges for sample mounting and measurement. A novel design of a micro-channel surrounding the surface of a single soft contact lens enables the non-destructive measurement of the streaming potential and opens the path for different approaches of surface zeta potential analyses.
One of the most important parameters that influences the zeta potential is the pH value. When measuring the zeta potential at different pH we obtain the isoelectric point (IEP). At the isoelectric point the interfacial charge changes sign. Furthermore, it gives essential information about the outermost surface chemistry. Other solutes such as biological buffers or even proteins added to the aqueous solution may interact with the material surface and affect its zeta potential.
By applying these approaches to the zeta potential analysis of soft contact lenses, it is possible to characterize thin film coatings or the effect of changes in the hydrogel composition on the swelling behavior. Another utilization of the surface zeta potential is the monitoring of changes in the contact lens surface during wear. A comparison of the zeta potential of new and worn contact lenses in graph 1 clearly indicates the deposition of proteins by the shift in the isoelectric point. The clean contact lens shows an IEP at pH 4, which is characteristic for an acrylate-based hydrogel. After a wear period of one month the IEP shifts towards a pH range typical for the IEP of many proteins. At higher pH the zeta potential moves to more negative values, which is explained by the deposition of lipids and/or by a decreased swelling. Both of these effects render the soft contact lens less hydrophilic.
In conclusion, the zeta potential is used to examine the biocompatibility of biomaterial surfaces such as soft contact lenses. The SurPASS™ 3 electrokinetic analyzer from Anton Paar analyzes the outermost solid surface and visualizes the behavior of real-life samples under environmental conditions. Any sample can be measured in real-time without being dependent on model surfaces – its efficiency saves valuable time and provides a unique benefit for users of this method: The monitoring of trends in the zeta potential with pH, additive concentration, ionic strength, or – like in this example – simulated tear fluid gives you a complete insight into your surface properties depending on any treatments and environmental influences.
Keep your vision clear – with a method that tells you more than just a figure.
1Further information in: G. M. Bruinsma, H. C. van der Mei, H. J. Busscher, Biomaterials 22 (2001) 3217-3224