Contact Lens Update

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Stephanie Wong is a graduate student at the Centre for Contact Lens Research, in the University of Waterloo's School of Optometry and Vision Science.

Wettability refers to how easily a liquid spreads over the surface of a contact lens. Clinically, this can be observed by viewing the interaction between the tears and the lens surface. In a laboratory setting, in vitro wettability can be quantified by measuring the “contact angle.”

The contact angle is the angle formed between a drop of liquid and the surface of the lens. Small contact angles are associated with an increased ability of the tears to spread out over the surface of a contact lens and lead to a more stable tear film.^1-3 A completely wettable surface has a contact angle of 0˚,^{4, 5} greater than 0˚ to 90˚ is considered partially wettable, between 90˚ to 180˚ is partially nonwetting and greater than 180˚ is completely nonwetting.⁴

The three main laboratory techniques used to measure the contact angle of contact lenses are the sessile drop, captive bubble, and Wilhelmy plate methods, which are described in detail below. The Contact Lens Manufacturers Association uses the captive bubble method to test wettability, which is the ISO standard; however, this method was designed to be used with rigid gas-permeable contact lenses, and there is no current ISO standard for analysing soft lenses. As a result, researchers use a variety of techniques and data interpretation methods, making it difficult to compare data.^{2, 5} The following review summarises a 2013 article that describes different methodologies used to measure the contact angle of hydrogel contact lenses.

Campbell D, Carnell SM, Eden RJ. Applicability of contact Angle techniques used in the analysis of contact lenses, Part I: Comparative methodologies. Eye & Cont Lens 2013;39(3):254-262.

The sessile drop technique

The sessile drop technique involves using a syringe to put a drop of liquid on the surface of a contact lens.^{4, 5} The contact angle is the angle created between the surface of the contact lens and the droplet (Figure 1). Images or videos of the experiment are recorded and analysed with the instrument’s computer software to determine the contact angle.⁵

The sessile drop technique is fast and easy to perform. The sessile drop technique can also measure static and dynamic contact angles, which can help differentiate between various contact lens materials.

The sessile drop technique requires the lens surface to be blotted.^{4, 5} Data analysis can be subjective depending on the software analysis model that the instrument uses and some models require the user to estimate the angle, which can be difficult when small angles are analysed. The contact lens and probe liquid are also susceptible to dehydration during the measurement because the lens is exposed to air.^{5, 6}

The captive bubble technique

In the captive bubble technique, a contact lens is submerged in liquid (usually water). A syringe is used to place either a small air bubble or another liquid with a lower density on the surface of the lens.^{4, 5} The contact angle is the angle formed between the surface of the contact lens and the air bubble (Figure 2).

As with the sessile drop method, the captive bubble technique only requires a small amount of the probe liquid, minimal sample preparation and the data is collected by the instrument’s computer software. However, the sample is not prone to dehydration because it is suspended in liquid.⁶ Performing the captive bubble technique takes longer than the sessile drop technique because aligning the bubble with the lens surface is time consuming.⁵ Determining the contact angle is also dependent on the analytical model that the computer software was designed to use.

Wilhelmy Plate

The Wilhelmy plate technique involves inserting and removing a contact lens from a liquid. A rectangular sample of a contact lens is attached to a balance positioned above the liquid.³ The force of the sample is set to zero at the beginning of the experiment when the lens is almost in contact with the solution. The sample is then immersed into the solution, then withdrawn back to the initial position. This cycle is repeated a few times. The force that the sample undergoes when it is lowered and removed from the liquid is recorded and used to calculate the contact angle.^{4, 5}

The advancing contact angle is calculated as the sample is inserted in the solution and the receding contact angle is calculated when the lens is removed from the liquid.^{4, 5} The difference between the advancing and receding contact angle is known as the contact angle hysteresis.^{4, 5}

There are two main types of contact angle hysteresis: kinetic and thermodynamic. Kinetic hysteresis is observed with the Wilhelmy plate when the cyclic immersion and emersion curves change with time or frequency. Various reasons are believed to cause this, including sample swelling, deformation of the surface, the probe liquid penetrating the surface of the sample and reorientation of polymer chains at the surface of the lens.^{4, 5} Thermodynamic hysteresis is observed when the immersion and emersion curves can be replicated over multiple tests. This is thought to be due to lens surface roughness and heterogeneity.5

The Wilhelmy plate technique provides more data about the lens than the sessile drop or captive bubble methods. However, it requires a large volume of the probe liquid and obtaining a sample is difficult as it is easy to contaminate or tear the sample. Acquiring and analysing the data is also time consuming,^{5, 7} but the results obtained from this method are accurate and reproducible.⁷

Conclusion

As this review outlines, each technique for measuring contact angle has its own advantages and disadvantages (see Table 1).

This review highlights the difficulty of not having an ISO standard to measure the contact angle of soft contact lenses. The authors suggest that having a standardized method for each technique would be helpful to eliminate inter-laboratory variation, as would the addition of using more than one method to investigate each contact lens material.

REFERENCES:

1. Lin MC, Svitova TF. Contact lenses wettability in vitro: Effect of surface-active ingredients. Optom Vis Sci 2010;87: 440-7.
2. Maldonado-Codina C, Morgan PB. In vitro water wettability of silicone hydrogel contact lenses determined using the sessile drop and captive bubble techniques. J Biomed Mater Res A 2007;83: 496-502.
3. Tonge S, Jones L, Goodall S, Tighe B. The ex vivo wettability of soft contact lenses. Curr Eye Res 2001;23: 51-9.
4. French K. Contact lens material properties: Part 1 – Wettability. Optician 2005;230: 20-2, 4-6, 8.
5. Campbell D, Carnell SM, Eden RJ. Applicability of contact angle techniques used in the analysis of contact lenses, part 1: comparative methodologies. Eye Cont Lens 2013;39: 254-62.
6. Read ML, Morgan PB, Kelly JM, Maldonado-Codina C. Dynamic contact angle analysis of silicone hydrogel contact lenses. J Biomater Appl 2011;26: 85-99.
7. Uyama Y, Inoue H, Ito K, Kishida A, Ikada Y. Comparison of different methods for contact angle measurement. J Colloid Interface Sci 1991;141: 275-9.