Kulim Kht Contact Angle/Goniometer

Captive bubble method in contact lens studies

Contact lenses are widely used and hydrogel contact lenses are the most

prescribed type of contact lenses around the world. Hydrogel lenses are made from

hydrophilic polymers, such as polyhydroxyethyl. Contact lenses should have certain

characteristics to avoid discomfort during wear. These include an adequate oxygen

permeability, lens motion and surface wettability. A fairly recently discovered

material for contact lens manufacturing is the silicone hydrogel, which is suitable for

long term use as it has improved oxygen delivery on the ocular surface. However,

this material has been reported to have reduced wettability compared to the other

materials used in hydrogel contact lenses.

The wettability of a lens is measured on the abilities of the lens to support a

continuous and rupture-resistant tear film on its surface. A lens with a limited

wettability is uncomfortable and has a reduced visual performance. Therefore it has

been a major investment for the contact lens industry to develop a soft lens surface

that is highly wettable in the ocular environment.

How to study the wettability of contact lenses?

Contact angle measurement is the most common method used to characterize

wettability of solid surfaces. There are a variety of contact angle techniques that can

be used for this purpose: the sessile drop method and the captive bubble method.

Sessile drop and captive bubble methods can also be used simultaneously to

measure the different contact angles of a lens. Sessile drop method can be used to

examine the advancing contact angle and captive bubble method to obtain the

receding contact angle. Both of these contact angles can also be obtained only by

captive bubble method. Captive bubble method is very useful as it is possible to

maintain fully hydrated conditions on the lens surface during the experiment. This is

essential as the drying of the lens causes the setup to be far from the ideal

conditions. Figure 1a illustrates the sessile drop method, where a drop of water is

replaced on a glass slide. The contact angle obtained with this method was 140°. In

Figure 1b the contact angle of an air bubble in water is obtained with the captive

bubble method and it is calculated to be 40°. Contact angle hysteresis is the

difference between the advancing and receding contact angles. It can be used to

characterize the surface roughness, chemical heterogeneity, re-arrangement of

molecules at the surface, solid swelling and mobility. In highly water wetting lenses

the contact angle hysteresis is eliminated and both the advancing and receding

contact angles approach zero.

Captive bubble technique

In the captive bubble method the contact angle is measured by placing a bubble of

air onto the lens surface with a capillary while the lens is immersed in a liquid.

Preferably the lens should be soaked in liquid for a couple days before the

measurement takes place in order to get the lens fully wet. The conditions in this

experiment should mimic the in vivo conditions as good as possible, meaning that

the liquid used in the contact angle experiments should be close to the real nature

of a tear film in the eye. The tear film is composed of surface active components

such as mucin and lysozyme.

Receding and advancing contact angles are both important measures of contact

lens wettability. A receding angle forms as the water phase moves away from the

capillary. This corresponds to the situation when the eyelid is open and the tear film

starts rupturing on the lens. The advancing contact angle develops as the water

phase moves towards the capillary. This is the case when the eyelid closes and the

tear film on the lens starts to recover.

A solution for finding improved wettability?

With captive bubble method contact angles of lenses can be measured in different

solutions to obtain the most ideal solution that wets the lens best. Some have

reported that significant change can’t be seen in whether the solution is pure water

or a liquid containing surfactants (like the contact lens solutions on the market).

Also studies of different coatings to improve the lens wettability have been made.

Protein coatings have found to be successful in improving the wettability of contact

lenses. Protein coatings resist the tear protein adsorption and deposition into the

lens.