Ocular Tribology

The word “ocular” refers to “related to eyes or vision”. Ocular tribology can be regarded as one of the application areas of biotribology where tribology is applied to solve tribological issues (friction, wear and lubrication) related to eyes. Let’s talk about two main areas related to ocular tribology, where most of the today’s research is concentrated. [1][2][3][4][5][6][7]

Dry eye syndrome: Also known as keratoconjunctivitis sicca, is a medical condition related to eyes, in which proper production, secretion and distribution of tears get hampered. It’s a very common disease – 10 million cases per year (India), 10%-20% adults in America. Dry eye syndrome can range from mild (irritation in eyes, heavy eyes, red eyes) to moderate (blurred vision, inability to do daily tasks) and in some cases even severe (loss of vision). [1][2][3][4][5][6][7]

There are two kinds of dry eye syndrome – one in which production of tears from tear glands deplete and other in which unbalanced composition of tears leads to quick evaporation. Treatment of this condition normally includes putting artificial tear solutions in eyes, blocking tear ducts to restrict draining of tears and increasing tear productions via dietary supplements and medicines. [1][2][3][4][5][6][7]

From a tribological point of view, our focus will be on, artificial tears solutions and eye lubricating solutions. Tears have a complex composition comprising of lipid, aqueous, and mucin components. Tears act as a lubricant for eyelids, the conjunctiva and the cornea and replicating their composition is difficult task. Tribology helps in development and examination of different artificial tear solutions and eye lubricating ointments for friction, wear and lubricating effect on eyes. Rheological studies further help in studying properties of tears and eye lubricants like viscosity. Tribological testing would be mainly concentrated on soft contacts. [1][2][3][4][5][6][7]

Contact Lens: It’s a thin lens placed directly on the surface of the eye to correct vision. These lenses are generally made of hydrophilic plastics called hydrogels. These lenses are generally made of hydrophilic plastics called hydrogels. The water content of various hydrogel contact lenses can range from approximately 38% to 75%. Higher water content of lenses allows the lens to transmit oxygen through them and keep the cornea healthy. [8][9][10][11]

Today more and more people having glasses are switching to contact lenses but still a significant number of people are unable to wear them due to extensive eye itchiness. Friction is considered as a main reason behind this irritation.The properties of these hydrogels are affected by sliding speed (12 cm/s), low contact pressures (3.5 to 4.0 KPa), humidity and water content. Micro-tribometer are generally used to test hydrogels because of their delicacy and low contact pressure conditions. It has to be also noted that frictional properties of contact lenses should be measured in biological relevant conditions to avoid potentially different and misleading results. [8][9][10][11]

Silicone hydrogel (SiH) contact lenses are rapidly replacing convention hydrogel contact lenses. SiH contact lenses offer high oxygen permeability which leads to more comfort and lesser complications. Though superior, yet some people complain of discomfort when wearing these lenses. Tribological studies are being carried out to solve friction, wear and to study tear film characteristics, surface tension in combination with SiH lenses. [8][9][10][11]

References:

  1. Maskin, Steven L., and Pamela Thomas. “Chapter 1: What Is Dry Eye Syndrome and Who Gets It.” Reversing Dry Eye Syndrome: Practical Ways to Improve Your Comfort, Vision, and Appearance. New Haven: Yale UP, 2007. 1-15. Google books preview only
  2. Menezes, pradeep. “Section 3.1: ocular tribology.” tribology for scientists and engineers. Springer-verlag New York, 2016 Google books preview only
  3. Todres, Zory V. “Section 6.3.6: Ocular Tribology.” Organic Mechanochemistry and Its Practical Applications. Boca Ratón, Florida: CRC, 2006. Google books preview only
  4. Ocular tribology PPT by Sajeed Mahaboob                https://www.slideshare.net/SajeedMahaboob/ocular-tribology?from_action=save
  5. Understanding Dry Eye Syndrome, Reviewed by Richard Adler, MD, allaboutvision.com http://www.allaboutvision.com/conditions/dryeye-syndrome.htm
  6. Dry Eye, American Optometric Association                                                      http://www.aoa.org/patients-and-public/eye-and-vision-problems/glossary-of-eye-and-vision-conditions/dry-eye?sso=y
  7. Dry eye, PubMed Health                            https://www.ncbi.nlm.nih.gov/pubmedhealth/PMHT0023579/
  8. Rennie, A. C., P. L. Dickrell, and W. G. Sawyer. “Friction Coefficient of Soft Contact Lenses: Measurements and Modeling.” Tribology Letters 18.4 (2005): 499-504. Full paper available online
  9. Ngai, V., J.b. Medley, L. Jones, J. Forrest, and J. Teiehroeb. “Friction of Contact Lenses: Silicone Hydrogel versus Conventional Hydrogel.” Life Cycle Tribology Tribology and Interface Engineering Series (2005): 371-79 Abstract only
  10. Roba, M., E. G. Duncan, G. A. Hill, N. D. Spencer, and S. G. P. Tosatti. “Friction Measurements on Contact Lenses in Their Operating Environment.” Tribology Letters 44.3 (2011): 387-97 Full paper available online
  11. Santodomingo-Rubido, Jacinto, James S. Wolffsohn, and Bernard Gilmartin. “Changes in Ocular Physiology, Tear Film Characteristics, and Symptomatology With 18 Months Silicone Hydrogel Contact Lens Wear.” Optometry and Vision Science 83.2 (2006): 73-81. Full paper available online
  12. All images used are free for commercial use 

HARSHVARDHAN SINGH
About HARSHVARDHAN SINGH 19 Articles
Harshvardhan Singh is an Automotive Engineer and has good experience in lubrication science and experimental tribology. He loves to write about tribology and related fields such as coating technology, surface engineering and others.

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