The instilled fluid dynamics and surface chemistry of polymers in the preocular tear film

Using slit lamp fluorophotometry it was demonstrated that the rate of drainage of a vehicle placed in the eye increased with increasing volume and that polymer solutions increased the thickness of the precorneal tear film (PTF). By increasing the viscosity of the delivery vehicle, (e.g., a hydroxypropylmethylcellulose polymer solutions), the PTF retention of fluorescein could be increased. The increased retention was shown to be due to an increase in the tear reservoir volume provided by the more viscous solutions. The PTF retention of fluorescein in a polyvinyl alcohol (PVA) vehicle was not as viscosity dependent, although PVA did seem to produce greater initial PTF fluorescence. This suggested that PVA initially produced a thicker PTF. The PTF retention of fluorescein by five commercial solutions did not have any relation to their wetting properties. The only good correlation with fluorescein retention in the PTF measured, seemed to be the ability of different polymer solutions to stabilize a thick layer of water as measured by the spontaneous spreading of polymer molecules at the air/liquid interface on wet glass surfaces. This model was designed to simulate tear film spreading in vivo. The results suggest that different polymer solutions may produce thicker PTF's than normal by virtue of their ability to drag water with them as they spread over the ocular surface with each blink. Mechanisms by which polymer solutions may increase the thickness of the PTF are discussed.