Tribological Model System Testing for Ophthalmic Applications
Eye drops are used to treat eye diseases. Tribology can be a helpful tool for understanding and improving the performance of eye drops. Within this study, tribological model system testing was used to mimic the eyelid-cornea contact. Complementary rheological measurements allow for a more comprehensive understanding of the lubrication mechanisms of eye drops.
Tears cover the cornea and conjunctiva in human eyes. They serve different functions such as lubrication, prevention of drying of the eye surfaces, smoothing the surface for refraction of light and also providing antimicrobial properties. In most individuals, the normal tear volume ranges from 3.4 µL to 10.7 µL. The tear fluid film typically consists of three layers:
- Outer layer (anterior) with lipid components
- Aqueous intermediate layer
- Bottom layer with mucin
Each of the layers serves a specific purpose. The outer lipid layer prevents evaporation. The aqueous intermediate layer consists of ions, soluble mucins, and proteins (including enzymes). These enzymes can be important for the antimicrobial function of the tear fluid. The mucin in the bottom layer enables an improved wetting of the tear film on the hydrophobic ocular surface. The performance of tears can be limited due to diseases such as dry eye syndrome or an insufficient quantity or incorrect composition of tears. In such cases, eye drops provide a potential solution. They can be formulated to either bolster particular deficient aspects of the tears or to mimic their entire composition. Another application of eye drops is to lubricate the eye, or improve the wetting behavior, with or without the use of contact lenses. Eye drops can be used to augment the natural bodily fluids and thereby facilitate the improvement of wellbeing and health for the patient. One of the abovementioned aspects - lubrication - can be investigated through tribology. The following tribosystem can be considered relevant for the human eye: Eyelid – fluid – cornea. More precisely, the conjunctiva palpebrarum, which is the inner surface of the eyelid, moves against the cornea. The cornea epithelium itself is hydrophobic but the mucin layer on the cornea makes this surface hydrophilic.
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 Scherz et al., Tear volume in normal eyes and keratoconjunctivits sicca, Albrecht von Grafes Archiv für klinische und experimentelle Ophtalmologie, 1974.
 Kwon et al., High-speed camera characterization of voluntary eye blinking kinematics, Journal of the Royal Society Interface, 2013.
 Reinshagen, “Trockenes Auge” Grundlagen, Analyse, Behandlunsansätze, Klinik Pallas.
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