the ability of the OVD to wet and coat that material. Surface tension is a measure of the cohesion between molecules of the OVD, so a lower surface tension and higher adhesion and contact angle indicate a better ability to wet. A solution of sodium hyaluronate has a significantly higher surface tension and contact angle with the relevant tissues than a solution of chondroitin sulfate, HPMC, or a mixture of sodium hyaluronate and chondroitin sulfate, which indicates better coating by the latter materials. Cohesion and dispersion Cohesive type OVDs adhere to themselves. They have high viscosity and act like a gel. The molecules have long chains, with high molecular weight, and have high surface tension and pseudoplasticity. They are relatively easy to aspirate as they tend to stay in one piece as they are drawn towards the suction orifice. Dispersive tyes have lower surface tension and tend to spread and wet contact surfaces. They have lower viscosity, the molecules are not as strongly mutually attracted, they show low pseudoplasticity and have shorter molecular chains and lower molecular weight. Mechanism of action The OVD occupies the volume of the anterior chamber during surgery, maintains its volume and shape without requiring internal pressure, and does not flow out through open incisions when undisturbed. It allows the free passage of instruments, prevents low viscosity fluids from leaking out, and provides the surgeon with a clear view of the internal volume. The cohesive nature of some types facilitates rapid removal by aspiration at the end of the procedure. Manufacturing OVDs are sterile aqueous saline solutions of one or more viscoelatic compounds and buffers to control pH at 7 to 7.5. Viscoelastic compounds Sodium hyaluronate, is a linear polysaccharide molecule of sodium glycuronate and N-acetyl glucosamine. It is commercially extracted from rooster combs or produced by bacterial fermentation. Chondroitin sulfate extracted from shark fin cartilage. Hydroxypropyl methylcellulose (Hypromellose) is extracted from wood pulp. History Meyer and Palmer isolated hyaluronic acid from the vitreous cortex in 1934. Also in 1934, Endre Balazs extracted and purified hyaluronic acid from rooster combs and umbilical cord. In 1958, Balazs suggested the possibility of using hyaluronic acid as a sustitute for the vitreous substitute during surgery for retinal detachment, and in 1972 made the first injection of hyaluronic acid into the vitreous chamber. Ophthalmic viscosurgical devices were introduced in 1972. Various alternative formulations with varied physical characteristics have been developed