What is Hyaluronic acid?
Hyaluronic acid is widely distributed in the human body. It plays an important role in connective tissue, skin, intervertebral discs, synovial fluid and nerve tissue and is responsible for the gel-like characteristics of the vitreous of the eye. Hyaluronic acid is able to bind very large amounts of water; in the vitreous of the eye, for example, 98% water is bound to only 2% hyaluronic acid. Hyaluronic acid is the main component of the synovial fluid, where it acts as a lubricant.
In permanent alternation hyaluronic acid is both synthesized and catabolized by the body. In the outer layers of the skin (epidermis) and the cornea (epithelium) per day half of the local hyaluronic acid is catabolized. In these avascular tissues it plays an important role in supplying nutrition and removal of metabolic waste products, as well as in cell differentiation, cell migration and wound healing.
Chemically, hyaluronic acid is a filament or chain of 500 to 100.000 sugar molecules. The length of these chains is described by the so called molar mass and the unit is megadalton (MDa).
In eye drops hyaluronic acid is used to increase the viscosity of the solution. If hyaluronic acid of low or intermediate molar mass is used, the concentration of the hyaluronic acid determines the rate of viscosity increase. However, if hyaluronic acid of very high molar mass is used, in the open eye the molecular chains can entangle. This is the reason why in the open eye the viscosity of eye drops with high molecular mass is higher than that of eye drops with low molecular mass.
During blinking the long hyaluronic acid molecules are arranged in parallel and show the same viscosity as hyaluronic acid of low molecular weight.
Only the hyaluronic acid of extremely high molar mass used in Comfort Shield® eye drops allows optimized effectiveness and compatibility.
Additional information on the rheology of commercially available hyaluronic eye drops and on the role of hyaluronic acid in the maintenance of the physiological homoeostasis of the eye can be found in the scientific publications listed below.