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Video Transcript for In-Vitro Methods for Characterising Eye Drops :
Eye drops are frequently prescribed to keep the eye moist and relieve discomfort. The surface and bulk deformation properties are key for optimal performance and can be investigated using rheological and surface analysis techniques.
The tear film found in our eyes is generally regarded as being made of three layers. A mucin-rich layer aids in the adhesion of the tear film to the surface of the eye, an aqueous layer which forms the bulk of the tear film and an oily layer which floats on top to minimize evaporation.
Eye drops are often delivered in microlitres, and at this scale, surface tension is a key determinant of droplet volume. Pendant drop analysis helps quantify the surface tension of a liquid droplet suspended from a needle. Surface tension is also frequently used to demonstrate equivalence to a reference pharmaceutical ophthalmics, or for claims support data when matching the surface tension of real tears. Contact angle measurement is used to quantify the wettability of a surface and has most often been used for assessing how well tears can wet soft contact lenses.
The ideal tear once dispensed, should adhere to the eye and form a thin protective film that does not drain away too quickly nor feel ‘draggy’ during blinking.
Viscosity profiling enables the characterization of materials under a well-defined range of shear conditions. Under low shear conditions or zero shears, a high viscosity will help maintain the persistence of the tear film. However, during high shear events such as during blinking, a high viscosity will be perceived as draggy and can also lead to blurry vision and “foreign body syndrome”.
Materials which reduce in viscosity as the shear rate is increased are described as non-Newtonian, or specifically shear thinning. Suspensions, gels and emulsions typically display shear-thinning behaviour. Shear thinning eye drops will maintain a high viscosity during low shear conditions, but will ideally reduce in viscosity during blinking.
Mucin present in natural tears helps provide desirable rheological properties and adhesion of the tear film directly to the mucosal layer of the eye. We can investigate the mucoadhesive properties of artificial tears with a variety of instrumental techniques.
Whether you are looking to compare the performance of various ingredients for the formulation of an eye drop, benchmark against competitor formulations or investigate other physical properties, please take a look at our capabilities and feel welcome to contact the team at [email protected]