Hyaluronic acid (HA), also known as hyaluronan, is a naturally occurring substance found throughout the human body. This clear, gooey molecule is a major component of the connective, epithelial, and neural tissues. HA’s primary function is to retain water, making it a powerful natural moisturizer and structural support molecule in various biological systems.
Chemical Identity and Classification
Hyaluronic acid belongs to a class of compounds called glycosaminoglycans (GAGs), which are long, unbranched polysaccharides. Unlike other GAGs, HA is unique because it is not sulfated and does not typically attach to proteins to form proteoglycans. Its structure is a repeating chain made up of two alternating sugar derivatives: D-glucuronic acid and N-acetyl-D-glucosamine.
This specific chemical structure, featuring negatively charged carboxylate groups, is what allows HA to bind successfully to water. A single HA molecule has the remarkable capacity to attract and hold up to 1,000 times its own weight in water. This hydrophilic nature results in the molecule’s gel-like consistency and its ability to form thick, elastic solutions, a property known as viscoelasticity.
The size of HA molecules varies significantly, with molecular weights ranging from roughly 5,000 to 20,000,000 Daltons in the body. This size variation dictates its physiological role and location within the body’s extracellular matrix. HA is found in high concentrations in the skin, where it supports hydration and elasticity, and in the vitreous humor of the eye. It is also a significant component of the synovial fluid in joints and the soft connective tissues.
Essential Functions in Human Physiology
In the skin, approximately half of the body’s total HA is located, where it works to maintain moisture, contribute to tissue turgor, and support the overall firmness and elasticity of the dermis. This helps keep the skin smooth and supple.
In the joints, HA is a major component of synovial fluid, providing necessary lubrication and acting as a shock absorber. Its viscoelastic properties allow it to cushion the ends of bones and ensure smooth, low-friction movement between the articulating surfaces.
HA also plays a significant role in tissue repair and wound healing. When tissue damage occurs, the concentration of HA in the affected area increases, where it helps regulate inflammation and provides a temporary structural scaffold. This scaffold facilitates the migration and proliferation of cells necessary for regenerating the damaged tissue.
Furthermore, HA is an important element in maintaining eye health, where it is a component of the vitreous humor, the gel-like substance that fills the eyeball. The viscosity provided by HA helps maintain the eye’s shape and supports visual function.
Therapeutic and Cosmetic Uses
The unique physical and chemical properties of HA have made it highly valuable for various medical and cosmetic applications. One common therapeutic application is in treating osteoarthritis, a condition characterized by joint pain and stiffness. HA is administered directly into the affected joint through a procedure called viscosupplementation, which aims to supplement the diminished natural joint fluid, thereby restoring some of the lost lubrication and cushioning.
In cosmetic medicine, HA is widely used as a dermal filler. These injectable gels are used to restore facial volume, smooth deep wrinkles, and enhance contours by physically filling the tissue space beneath the skin. The HA within these fillers is often cross-linked, making it more stable and resistant to natural degradation, which prolongs the desired aesthetic effect.
For topical use, HA is a popular ingredient in skincare products, such as serums and moisturizers, to combat dry skin and the visible signs of aging. The efficacy of these products is often determined by the molecular weight of the HA used. High molecular weight HA (HMW-HA) is too large to penetrate the skin deeply, instead forming a protective, moisture-retaining film on the surface, which provides immediate plumping and barrier support.
Conversely, low molecular weight HA (LMW-HA) has been chemically or enzymatically fragmented into much smaller pieces, allowing it to penetrate deeper into the epidermis. Once absorbed, LMW-HA is thought to provide deeper hydration and may help stimulate the skin’s own production of collagen. Many advanced formulations combine both HMW-HA and LMW-HA to deliver both immediate surface hydration and longer-term anti-aging effects.