The viscometer was cleaned by rinsing carefully with running tap water and then with acetone. The efflux time is proportional to the kinematic viscosity of the fluid. Where ν is the kinematic viscosity at the stated temperature, t is the observed efflux time and C is the viscometer constant based upon an absolute value of 1.0034 mm 2/s for the kinematic viscosity of pure water at 20 ☌ and normal atmospheric pressure. For any Newtonian fluid, the dynamic viscosity η defined by the equation: Ocular lubricants can be treated as Newtonian viscous liquids, whereby the rate of viscous flow is proportional to the shear stress. 8 Viscosity is the most commonly sought after rheological parameter. Rheology is the study of deformation and the flow of complex fluids such as polymers, emulsions, suspensions, and biological systems. 5, 6, 7 It is, therefore, essential that ocular lubricants are of sufficient viscosity to maximise bioavailability.
4 If the applied eye drops have a viscosity similar to that of tears, they are eliminated within a few minutes. Eye-drop instillation results in an increase in tear volume, and this slowly returns to its baseline level due to drainage through canaliculi, and because of fluid loss through other means such as transport across the ocular epithelia or evaporation. Various approaches have, therefore, been used to prolong the precorneal residence time such as occlusion of lacrimal puncta, 2 and slow release artificial tears, 3 but these have not had universal success.Īn alternative approach is to use lubricants with high viscosity. The main limitation of current ocular lubricants is the short duration of symptom control. The treatment of dry eye remains a significant clinical challenge, and topical lubricants form the mainstay of treatment.
1 Changes in ocular surface physiology and in the tear film occur in dry eye disease, but the mechanisms are not fully understood. Dry eye disease is one of the most frequently encountered problems in ophthalmology with estimates of prevalence ranging from 11 to 22%. Normal sight is dependent on a moist ocular surface and the human tear film has an essential role on the maintenance of optical clarity, as well as nourishment, lubrication, and protection.
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