Article Date: 4/1/2010

Solution Optimization and Physical Properties of Healthy Human Tears
HUMAN TEARS AND SOLUTION OPTIMIZATION

Solution Optimization and Physical Properties of Healthy Human Tears

A multipurpose solution that exhibits the properties of of normal human tears would be ideal.

By Ami Abel Epstein, OD



Dr. Abel Epstein has a private practice in Birmingham, Ala., and is a long-time member of the American Optometric Association. She is an advisor to Bausch + Lomb.

Multipurpose solution (MPS) products for contact lens care have come a long way over lens care regimens requiring two or more products or steps. This can be attributed to various factors including convenience, simplicity, cost efficiency, safety and efficacy.1 The main functions of a contact lens care solution are to clean the lens by removing proteins and lipids deposited by the tear film, to disinfect the lens, and to make lens wear as comfortable as possible. These functions are interrelated, as proper lens cleaning and disinfection are vital to comfort, which is a key factor in continued lens use and satisfaction.2 Lens solutions differ in their physical properties, and it has been suggested that several of these differences may influence patient comfort and preference for one product over another.3

A relatively new discipline known as biomimicry uses lessons from nature to enhance existing technology and solve problems.4 The study of natural mechanisms and processes sometimes leads to scientific innovation. Examples of recent research in biomimicry include the replacement of damaged joints with biologically inspired materials,5 as well as the development of cell-sized lipid-based containers6 and proteins attached to bio-mimetic nanocrystals7 that may one day be used for drug delivery. In the care of contact lenses, an MPS product that exhibits the properties of normal human tears is considered an optimal target formula.3 Such properties include pH, osmolality, viscosity and surface tension. If any of these properties differ significantly from those of normal tears, patient discomfort may result.3 An important consideration for bio-inspired products is that their development is firmly rooted in science. It is not enough for contact lens care solutions and formulas to simply have properties that mimic tears. For example, disinfection efficacy should never be compromised; as new contact lens solutions are developed, eyecare practitioners should look for better disinfection efficacy given what is known about patient compliance and the development of “super-bugs” like methicillin-resistant Staphylococcus aureus.

Importance of the Tear Film

The tear film consists of a lipid component, a mucinous component and an aqueous component.8 Each component is important in the physiology of the ocular surface. The lipid component contains wax esters, sterol esters, fatty acids, and fatty alcohols.9 The mucinous component is comprised of mucins that are constituted largely of sugars.9,10 The aqueous component, which constitutes the bulk of the tear film, is composed of 98% water but also contains salts, mucins, and proteins including hyaluronan, lysozyme, lactoferrin, lipocalin, and secretory immunoglobulin A.9,11-13

The composition of the tear film is maintained within narrow limits, as any perturbations can interfere with health and function.14,15 Abnormalities of the tear film are the main cause of conditions such as dry eye syndrome.16 Disruption of the homeostasis of the tear film results in ocular surface inflammation, which may lead to cell damage. Abnormalities of any tear component can result in tear film instability and hyperosmolarity.17 The formation and stability of the tear film in both health and disease are dependent upon the physical properties of tears.18,19

Characteristics of Human Tears

A solution's acidity or alkalinity is measured in pH units.3 In general, the pH of healthy tears is reported to range from 7.3 to 7.7,20 with a mean value ± standard deviation of 7.5 ± 0.23.21 pH is influenced by dissolved substances, especially by the bicarbonate–carbon dioxide buffer system.22 Tear pH is lowest upon wakening due to acid byproducts associated with prolonged eyelid closure.23 When the eyelids are open, pH increases rapidly due to carbon dioxide loss.22

The pH of a contact lens solution is affected by the buffering agents used in the solution.3 Patient comfort is a function of the buffering capacity of a solution.24,25 Solutions instilled with a pH below 6.6 and above 7.8 can cause irritation, rapid blinking, and reflex tears.22,23 Stinging and ocular discomfort have also been reported with solutions that have a pH beyond the ocular comfort zone of 6.6 to 7.8.3,25,26 Osmolality is defined as the total concentration of dissolved particles in a solution irrespective of size, density, configuration, or electrical charge.23 Normal tear osmolality is approximately 305 mOsm/kg.3,27 The osmolality of healthy tears varies from 244 to 344 mOsm/kg, depending on the method and the location in the eye from which the tear sample was collected.28,29 Osmolality decreases at night due to decreased tear film evaporation. When the eyes are open, osmolality rises due to increased evaporation of the tear film.22,23,27 Osmolality increases in diseases such as dry eye, signifying an increased concentration of electrolytes.23

Osmolality levels play a vital role in the discomfort experienced by patients with dry eye, as higher osmolality levels result in higher patient discomfort.30-32 Therefore, it has been suggested that patients with dry eye could be more sensitive to the osmolality of contact lens solutions compared with normal subjects.3 Early contact lens wear causes decreased osmolality, but osmolality increases following lens adaptation due to increased evaporation from the disrupted lipid component of the tear film.23

Viscosity is a measure of the resistance of a fluid to flow.33,34 Human tears have a viscosity in the range of 1 to 10 cP, and viscosity decreases with an increasing rate of shear stress (this property is known as “shear thinning”).35,36 In the open eye, higher viscosity resists damage and tear film break up. Blinking, conversely, requires low tear viscosity to avoid damage to epithelial surfaces.37 Viscosity may affect patient discomfort due to interactions between the solution, the lens and the tear film.3

Surface tension is the inward attraction of molecules at the surface of a solution. The surface tension of water is approximately 72 millinewtons per meter (mN/m).3 The combination of proteins, lipids, mucins, and electrolytes, among other substances found in human tears3 results in a lower surface tension (42 to 46 mN/m).38 Surface tension influences the formation and stability of the preocular tear film. Patients with conditions such as dry eye have a higher mean surface tension than healthy individuals.18,39 Contact lenses remain on the cornea because of the surface tension of the tear film.40 Surfactants, a common component of contact lens care solutions, tend to reduce surface tension3 and therefore, may add to contact lens movement and discomfort to the user. Surface tension outside a normal range can lead to an imbalance in the composition and instability in the tear film, which could result in patient discomfort.

Optimizing a Formula

The above characteristics of human tears are important for ocular health; therefore, a contact lens care solution with a pH, osmolality, viscosity, and surface tension within the ranges observed in normal human tears would be considered an optimal formula. Such a product would be expected to provide superior comfort for daily lens wear.3

Within the field of biomimicry, the development of an optimal formula for an MPS product should take into account all of the above physical properties, as well as superior disinfection efficacy. Each property should be adjusted, if possible, to fall within the reported range of that property for normal human tears to ensure maximal health and ocular harmony for contact lens wearers. In the crowded and competitive contact lens care market, a biologically inspired MPS product engineered to resemble human tears with unsurpassed disinfection efficacy would stand out as a significant development. CLS

References

  1. Morgan PB, Efron N. A decade of contact lens prescribing trends in the United Kingdom (1996-2005). Cont Lens Anterior Eye. 2006;29:59-68.
  2. Nichols JJ. Available lens care options. Adv Stud Ophthalmol. 2008;5:76-78.
  3. Dalton K, Subbaraman LN, Rogers R, Jones L. Physical properties of soft contact lens solutions. Optom Vis Sci. 2008;85:122-128.
  4. Biomimicry Institute. What is biomimicry? Available at: biomimicryinstitute.org/index2.php?option=com_content&do_ pdf=l&id=63
  5. Kon E, Delcogliano M, Filardo G, Altadonna G, Marcacci M. Novel nano-composite multi-layered biomaterial for the treatment of multifocal degenerative cartilage lesions. Knee Surg Sports Traumatol Arthrose. 2009;17:1312-1315.
  6. Tresset G, Takeuchi S. Utilization of cell-sized lipid containers for nanostructure and macromolecule handling in microfabricated devices. Anal Chem. 2005;77:2795-2801.
  7. Iafisco M, Palazzo B, Falini G, et al. Adsorption and conformational change of myoglobin on biomimetic hydroxyapatite nanocrystals functionalized with alendronate. Langmuir. 2008;24:4924-4930.
  8. Foster CS, Azar DT, Dohlman CH. Smolin and Thoft's The Cornea: Scientific Foundations and Clinical Practice. Philadelphia, PA: Lippincott Williams & Wilkins; 2004.
  9. Efron N. Contact Lens Practice. Oxford, UK: Butterworth-Heinemann; 2001.
  10. Maskin SL. Reversing Dry Eye Syndrome: Practical Ways to Improve Your Comfort, Vision, and Appearance. New Haven, CT: Yale University Press; 2007.
  11. Mudgil P, Torres M, Millar TJ. Adsorption of lysozyme to phospholipid and meibomian lipid monolayer films. Colloids Surf B Biointerfaces. 2006;48:128-137.
  12. Hoang-Xuan T, Baudouin C, Creuzot-Garcher C. Inflammatory Diseases of the Conjunctivae. Stuttgart, Germany: Georg Thieme Verlag; 2001.
  13. Frescura M, Berry M, Corfield A, Carrington S, Easty DL. Evidence of hyaluronan in human tears and secretions of conjunctival cultures. Biochem Soc Trans. 1994;22:228S.
  14. Ohashi Y, Dogru M, Tsubota K. Laboratory findings in tear fluid analysis. Clin Chim Acta. 2006;369:17-28.
  15. Pavan-Langsten D. Manual of Ocular Diagnosis and Therapy. Philadelphia, PA: Lippincott Williams & Wilkins; 2007.
  16. Johnson ME, Murphy PJ. Changes in the tear film and ocular surface from dry eye syndrome. Prog Retin Eye Res. 2004;23:449-474.
  17. Asbell PA, Lemp MA. Dry Eye Disease: The Clinician's Guide to Diagnosis and Treatment. New York, NY: Thieme; 2006.
  18. Tiffany JM. Surface tension in tears. Arch Soc Esp Oftalmol. 2006;81:363-366.
  19. Sullivan DA, Dartt DA, Meneray MA. Lacrimal Gland, Tear Film, and Dry Eye Syndromes 2: Basic Science and Clinical Relevance (Advances in Experimental Medicine and Biology). New York, NY: Plenum Press; 1998.
  20. Moses RA, Hart WMJ. Adler's Physiology of the Eye, Clinical Application. St. Louis, MO: C.V. Mosby; 1981.
  21. Yamada M, Mochizuki H, Kawai M, Yoshino M, Mashima Y. Fluorophotometric measurement of pH of human tears in vivo. Curr Eye Res. 1997;16:482-486.
  22. Washington N, Washington C, Wilson C. Physiological Pharmaceutics: Barriers to Drug Absorption. London, England: Taylor and Francis Inc.; 2000.
  23. Korb D, Smith G, Tomlinson A, Craig J, Doughty MJ. The Tear Film: Structure, Function and Clinical Examination. Amsterdam, Netherlands: Elsevier Health Sciences; 2002.
  24. Carney LG, Fullard RJ. Ocular irritation and environmental pH. Aust J Optom. 1979;62:335-336.
  25. Carney LG, Hill RM. Human tear pH. Diurnal variations. Arch Ophthalmol. 1976;94:821-824.
  26. Tang I, Wong DM, Yee DJ, Harris MG. The pH of multi-purpose soft contact lens solutions. Optom Vis Sci. 1996;73:746-749.
  27. Agarwal A. Dry Eye: A Practical Guide to Ocular Surface Disorders and Stem Cell Surgery. Thorofare, NJ: Slack Incorporated; 2006.
  28. Benjamin WJ, Hill RM. Tonicity of human tear fluid sampled from the cul-de-sac. Br J Ophthalmol. 1989;73:624-627.
  29. White KM, Benjamin WJ, Hill RM. Human basic tear fluid osmolality. Importance of sample collection strategy. Acta Ophthalmol (Copenh). 1993;71:524-529.
  30. Farris RL, Gilbard JP, Stuchell RN, Mandel ID. Diagnostic tests in keratoconjunctivitis sicca. CLAO J. 1983;9:23-28.
  31. Gilbard JP, Rossi SR, Gray KL. A new rabbit model for keratoconjunctivitis sicca. Invest Ophthalmol Vis Sci. 1987;28:225-228.
  32. Tomlinson A, Khanal S. Assessment of tear film dynamics: quantification approach. Ocul Surf. 2005;3:81-95.
  33. Toledo RT. Fundamentals of Food Process Engineering. New York, NY: Springer; 2001.
  34. Watkins J. An Introduction to Biomechanics of Sport and Exercise. New York, NY: Churchill Livingstone; 2007.
  35. Tiffany JM. Tears in health and disease. Eye (Lond.). 2003;17:923-926.
  36. Pandit JC, Nagyova B, Bron AJ, Tiffany JM. Physical properties of stimulated and unstimulated tears. Exp Eye Res. 1999;68:247-253.
  37. Tiffany JM. The viscosity of human tears. Int Ophthalmol. 1991;15:371-376.
  38. Nagyova B, Tiffany JM. Components responsible for the surface tension of human tears. Curr Eye Res. 1999;19:4-11.
  39. Tiffany JM, Winter N, Bliss G. Tear film stability and tear surface tension. Curr Eye Res. 1989;8:507-515.
  40. Hom MM, Bruce AS. Manual of Contact Lens Prescribing and Fitting with CD-ROM. Oxford, UK: Butterworth-Heinemann; 2006.


Contact Lens Spectrum, Issue: April 2010