Hydrogels are the material of choice for the manufacture of contact lens. They are a cross-linked hydrophilic polymer network filled with liquid that exhibits the behavior of both liquid and solid under applied stress. Hydrogels recover upon release of the mechanical stress as they support diffusion of the liquid through the pores in the solid meshwork . Typically, 30-70% of water is present in the polymeric matrix [1,2].
Figure 1. Various types of contact lenses are used by many people every day.
This article discusses the application of Anton Paar Nano Tribometer to perform a tribology test procedure for measurement of the coefficient of friction (CoF) of contact lenses. It also provides indications on the test conditions and their determination. It also illustrates a typical example with the CoF results and briefly discusses their evolution with the applied load. The test conditions, such as sliding speed and pressure, should be chosen if possible close to the in vivo conditions.
Blinking is typically considered to be the primary force and motion contribution for contact lens tribology research. The contact pressure exerted by the eyelid during the blinking was found to be 3.5 to 4.0 kPa and the estimated blinking average speed was 12 cm/s [3-5].
Earlier studies demonstrated that hydrogels have an elastic modulus ranging between 30 and 120 kPa, based on the water content and composition of the polymer matrices . The contact pressure can be calculated and the frictional contact can be simulated via appropriate experiments when these properties are known.
Here, the Anton Paar Nano Tribometer (NTR2) was used to test two types of commercially available contact lenses. Linear reciprocating mode was employed to simulate the eye blinking. Thanks to the dual beam cantilever and to the high resolution capacitive sensors, low contact pressures can be achieved and accurate measurements of very low forces (normal and tangential) can be ensured. A 3-mm-diameter hemispherical sapphire probe (ball) was employed as the counterbody. Sapphire is an inert material and does not alter its properties with time, making it an ideal reference material to compare the frictional behavior of various contact lenses and storage liquids.
The Nano Tribometer test setup shown in Figure 2 consisted of a specially designed contact lens holder and used the sapphire ball as a counterbody. Since the experiment involved the application and measurement of very low vertical (~mN) and lateral forces (~50 µN), the Nano Tribometer was placed on an active anti-vibration table to eliminate external perturbations.
Figure 2. Nano Tribometer setup with mounted contact lens holder.
Contact Lens Holder and Testing Procedure
The contact lenses were placed onto a semi-spherical sample holder with a plastic base matching the internal curvature of the lens. The lens was then clamped with the upper part to be secured in place. Three magnetic pegs were embedded in the clamping (upper) part and in the support (lower) part of the contact lens holder to hold the contact lens safely and firmly during the tribological testing (Figure 3).
Figure 3. Clamping parts of the sample holder: upper (left) and lower (support, right). Note three magnetic pegs for firm holding between the two parts.
All tests were performed in accordance with the following procedure:
- New contact lens package was opened
- The lens was removed from the package and transferred onto the curved sample holder using clean plastic tweezers
- Good clamping of the lens was verified before transferring the clamping part of the sample holder into the support part
- Drops of saline solution were added on the test surface of the lens
- The tribological test was run
Steps (1) to (4) were carried out within a minute.
The presence of liquid on the hydrogel surface is known to influence its frictional properties  significantly. Hence, to prevent dehydration of the lenses during the tests, storage solution from the lens package was always applied onto the lens surface before beginning the test. During the tests, the quantity of liquid was monitored so that it was added if required to ensure permanent hydration of the lens.
The contact pressure was estimated using Hertz solution for spherical contact:
F = Normal applied load
a = Contact radius
R = Radius of the hemispherical counterbody
E = Young’s modulus of the contact lens
Tribological Test Parameters
Earlier experiments conducted on other types of hydrogels showed a Young’s modulus range of 30-120 kPa for the contact lenses depending on the water content and the hydrogel composition type . In this study, the mechanical properties of the contact lenses were not known, but the Elastic’s modulus was assumed to be 50 kPa for all calculations.
Using hemispherical sapphire as a counterbody with a diameter of 3 mm during tests, the contact pressures reached at 1 mN and 10 mN loads are shown in Table 1. These values are in the same order of magnitude as the estimated in vivo conditions [3-5].
Table 1. Contact pressure variation with applied load
Table 2 lists the experimental conditions applied for all tests.
Table 2. Experimental conditions
|Normal applied load
||1 to 10 mN
|Sapphire ball diameter
The in vivo sliding speed showed to be challenging to reach due to the spherical shape of the lens, which makes it difficult to achieve proper control of the normal load at higher speeds. Moreover, the vibrations introduced by the requested speed could cause errors in CoF measurement. Therefore, the speed was set to 0.31 mm/s, which avoided most of the eventual vibrations and the ensuing errors in CoF measurement. This setup allowed in-depth study of the evolution of CoF during the measurement.
The raw friction data collected for one of the two types of tested contact lenses under the normal load of 5 mN is shown in Figure 4. The maxima and minima correspond to the measured CoF during the sliding period, whereas the vertical lines correspond to the changing direction (return) points.
Figure 4. Coefficient of friction as a function of number of cycles (normal load 5 mN).
A typical plot of friction coefficient as a function of linear position during one reciprocal cycle is shown in Figure 5.
Figure 5. Typical plot of CoF as a function of position during one reciprocal cycle.
The values collected in the central part of the trajectory of each cycle (red rectangle in Figure 6) were taken for the estimation of the averaged CoF. The CoF values measured during the change of direction (return points) were not considered in the estimation of the averaged CoF.
Figure 6. Ten superposed cycles of CoF versus linear position.
The average CoF values for different applied loads are shown in Figure 7. Each test was carried out in a different area of the contact lens.
Figure 7. Coefficient of friction as a function of number of cycles.
In all cases, the sliding speed set was 0.31 mm/s, while the applied load was varying between 1 and 10 mN. For this load configuration, the pressures were similar to the estimates of contact pressure experienced in the eye. For the tribological experiments, the in vivo conditions in terms of sliding speed were adjusted as such speeds could cause additional vibrations of the system. The resulting vibrations would cause errors in CoF measurements. Hence, it is possible that the CoF values might be slightly underestimated as demonstrated by other studies . However, considering the fact that the same measurement procedure was used, it is easier to compare the results.
Based on earlier experiments on the dependence of CoF on the load of different hydrogels,[1,8] a decrease in the CoF values was expected with load. However, such behavior was not observed in these tribological experiments: CoF was not decreasing with increasing load. Considering the complex nature of the frictional behavior of soft contact lenses and knowing that various parameters (applied load, material chemistry, sliding speed, liquid content, etc.,) can significantly affect the experiment results, explication of the results is not always straightforward.
To thoroughly understand all the parameters used in tribological testing of contact lenses, fundamental research is required as illustrated in, for example, . However, with the aforementioned parameters of the tribological tests and using a suitable instrument, repeatable tests of frictional behavior of contact lenses can be performed. This article can be used as a guideline by other research groups or company research centers to evaluate the frictional properties of the newly developed contact lenses. Nanoindentation and scratch instruments can also be employed for a complete characterization of mechanical and frictional properties of contact lenses. Using these instruments, additional information on mechanical properties of such soft materials such as scratch or creep resistance and elastic modulus can be obtained.
Figure 8. Table Top Nano Tribometer.
It is necessary to consider several factors for a successful characterization of the frictional properties of contact lenses:
- At first, it is necessary to have a dedicated sample holder matching the internal curvature of the lenses.
- Secondly, it is necessary to hydrate the contact lens at all times and carry out all the steps in the sample preparation phase as fast as possible to avoid possible dehydration. It is recommended to completely immerse the contact lens into storage (saline) solution. In fact, the CoF values significantly change when the test is carried out under dry conditions (increase by a factor of 10 or more). If there is an insufficient amount of saline solution, the lens surface needs to be hydrated frequently by adding drops of the available solution. The dehydration of the contact lens is accompanied by a considerable increase of CoF values, thus making the results invalid.
- The test parameters should be selected as closely as possible to the in vivo conditions without affecting the quality of the measured data.
- As discussed earlier, knowing the mechanical properties of the tested contact lens is important to better select the test conditions. For this purpose, dedicated nanoindentation instruments can be used.
- The normal applied load should be selected based on the elastic modulus of the lens and the selected counterbody to obtain contact pressure similar to the estimated contact pressure experienced in in vivo conditions.
- In case the counterbody used is much softer than sapphire (e.g. tissues or polymers), reduced modulus - a combination of the Young’s modulus of the lens and the counterbody- should be used for pressure calculations rather than the lens’ Young’s modulus.
- The in vivo sliding speed proved to be difficult to reach due to the increased vibrations of the frictional system. This parameter was therefore chosen arbitrarily as high as possible to avoid additional vibrations. One of the important parameters to be taken into account when setting the sliding speed is the reciprocating amplitude. It is important to choose linear speed mode (rather than sinusoidal mode) which will ensure realization of the entire reciprocating amplitude with constant speed.
- Stable CoF value can be achieved with 15 to 100 cycles. The initial run-in period usually takes only about two to five cycles.
- The reciprocating amplitude value is typically in the range of 0.2-1 mm based on the dimensions of the available testing area.
- The data recording frequency should be set in the range of 10-50 Hz to save enough data for proper cyclic analysis. (Figure 5 and Figure 6)
 J. Gong, Friction and lubrication of hydrogels, its richness and complexity, Soft Matter 2 (2006).
 D.K. Martin and B.A. Holden, Forces developed beneath hydrogel contact lenses due to squeeze pressure. Physics in Medicine and Biology 30 (1986).
 J.A. Nairn and T. Jiangizaire, Measurement of the friction and lubricity properties of contact lenses (1995), ANTEC ’95 3384.
 G. Hung, F. Hsu and L. Stark, Dynamics of the human eye blink, American Journal of Optometry and Physiological Optics 54 (1977).
 Koffas, T.S., OPdahl, A., Marmo, C., Somorjai, G.A., Effect of equilibrium bulk water content on the humidity-dependent surface mechanical properties of hydrophilic contact lenses studied by atomic force microscopy, Langmuir 19, 3453-3460 (2003).
 J. Nohava, Spherical Nanoindentation of Polyacrylamide Hydrogels by the Newly Developed Bioindenter (2015).
 B. Zhou, Y. Li, N.X.Randall, L. Li, A study of the frictional properties of senofilcon-A contact lenses (2011).
 A.C. Rennie, P.L. Dickrell, W.G. Sawyer, Friction coefficient of soft contact lenses: measurement and modeling. Tribology letters 4 (2005).
This information has been sourced, reviewed and adapted from materials provided by Anton Paar TriTec SA.
For more information on this source, please visit Anton Paar.