Hyaluronic acid (HA) is a large molecule that consists of repeated sugar units and may be formed from several different cells. HA is found naturally in all organs of the body, in joints, in the eyes and in the skin. A male who weighs 70 kg has approx. 15 g hyaluronic acid in his body, about a third of which is metabolised every day. In the knee and joints it has a lubricating and shock absorbing function, and in softer tissue it gives structure and elasticity. Its unique ability to bind moisture gives it many important tasks in the various transport systems of the body.
HA is an enormously large molecule with a unique ability to bind water. The molecule forms a large volume, about 1,000 times its own volume, in which water molecules can move freely in and out of the “branches”.
The structure of HA can be compared in simple terms to the crown of a real tree in which the birds can move in and out. When the molecule is at rest it builds a large, tangled nest and is almost jelly-like, but when it is exposed to forces, for example when a joint is set in motion, the nest is combed out and the layers of fluid can flow over each other.
The property that hyaluronic acid gives fluids is known as shear attenuating (pseudoplastic). Shear attenuating means that fluids become less viscous when they are exposed to shear forces. For example, synovial fluid owes its properties to hyaluronic acid and is therefore shock absorbing, yet lubricating in repeated movements. As hyaluronic acid is found naturally in the body its biocompatibility is very high and it is therefore suitable for medical uses.