Overview of Hyaluronic Acid and its Biomedical Applications

Hyaluronic Acid

Hyaluronic acid (HA), or hyaluronan, or sodium hyaluronate, is a non-sulfated glycosaminoglycan and a biopolymer, which is distributed widely throughout connective, epithelial, and neural tissues. It is one of the chief components of the extracellular matrix, contributes significantly to cell proliferation and migration, and may also be involved in the progression of some malignant tumours. The average 70 kg man has roughly 15 grams of HA in his body, one third of which is turned over (degraded and synthesised) daily.

Structure and Synthesis of Hyaluronic Acid

The structure of HA is well characterised; it is composed of repeated units of disaccharide of D-glucuronic acid and D-N-acetylglucosamine, linked together via alternating ß-1,4 and ß-1,3 glycosidic bonds. HA can be 25,000 disaccharide repeats in length. The molecular weight of HA can range from 5,000 to 20,000,000 in vivo. The average molecular weight in human synovial fluid is 3-4 million and HA purified from human umbilical cord is 3,140,000.

HA is synthesized by a class of integral membrane proteins called HA synthases, of which three types occur in vertebrates: HAS1, HAS2, and HAS3. These enzymes lengthen HA by repeatedly adding glucuronic acid and N-acetylglucosamine to the nascent polysaccharide as it is extruded through the cell membrane into the extracellular space.

Commercially, HA can be produced from animal sources, for example, from rooster combs, umbilical cords, and cartilage of joints or can be produced by fermentation. The average molecular weight of HA varies according to sources and generally falls within the range 60,000 to 14,000,000.

The Role of Hyaluronic Acid in Joints

HA is a major component of synovial fluid where it increases the viscosity and together with lubricin, is one of the main lubricating components. HA is also an important component of articular cartilage, where it is present as a coat around each cell (chondrocyte). When aggrecan monomers bind to HA in the presence of a link protein, large highly negatively charged aggregates form. These aggregates imbibe water and are responsible for the resilience of cartilage (its resistance to compression). The molecular weight (size) of HA in cartilage decreases with age, but the amount increases.

The Role of Hyaluronic Acid in Skin / Tissue Biology

HA is an important component of skin, where it is involved in tissue repair. HA acts as a free radical scavenger, absorbing and degrading them. When skin is excessively exposed to UVB rays and the skin becomes inflamed (sunburn), the cells in the dermis stop producing as much HA and increase the rate of its degradation. HA degradation products also accumulate in the skin after UV exposure.

Whilst HA is abundant in extracellular matrices, it also contributes to tissue hydrodynamics, movement and proliferation of cells, and participates in a number of cell surface receptor interactions, notably those including its primary receptor, CD44. Upregulation of CD44 itself is widely accepted as a marker of cell activation in lymphocytes. HA's contribution to tumour growth may be due to its interaction with CD44. CD44 participates in the cell adhesion interactions required by tumour cells. Although HA binds to CD44, there is evidence to support that HA degradation products transduce their inflammatory signal through the Toll-like receptor 2 (TLR2), TLR4 or both TLR2 and TLR4 in macrophages and dendritic cells. TLR and HA play a role in innate immunity.

HA is nontoxic, non-immunogenic and biodegradable. HA is degraded by a family of enzymes called hyaluronidases. In humans, there are at least seven types of hyaluronidase-like enzymes, several of which are tumour suppressors. The degradation products of HA, the oligosaccharides and very low molecular weight HA, exhibit pro-angiogenic properties. In addition, recent studies showed that HA fragments, not native high molecular mass HA, can induce inflammatory responses in macrophages and dendritic cells in tissue injury and in skin transplant rejection.

Biocompatibility and Medical Applications of Hyaluronic Acid

HA is naturally found in many tissues of the body such as skin, cartilage, and the vitreous humor. It is therefore well suited to biomedical applications targeting these tissues. The first HA biomedical product, Healon, was developed in the 1970s and 1980s and is approved for use in eye surgery (i.e. corneal transplantation, cataract surgery, glaucoma surgery and surgery to repair retinal detachment). Other biomedical companies also produce brands of HA for ophthalmic surgery. In recent developments I, BBL's partner, world leader biopolymer manufacturer,Novozyme has engineered a stronger HA molecule derived from recombant (engineered) strains of Streptococcus equi to make pyrogen free and purer HA.

HA is also used to treat osteoarthritis of the knee. Such treatments, called viscosupplementation, are administered as a course of injections into the knee joint and are believed to supplement the viscosity of the joint fluid thereby lubricating the joint, cushioning the joint and producing an analgesic effect. It has also been suggested that HA has positive biochemical effects on cartilage cells. However, some placebo controlled studies have cast doubt on the efficacy of HA injections, and HA is recommended primarily as a last alternative to surgery. Oral use of HA has been lately suggested although effectiveness needs to be demonstrated. Some preliminary clinical studies exist that suggest that oral administration of HA has a positive effect on osteoarthritis.

Due to its high biocompatibility and its natural presence in the extracellular matrix of tissues, HA is gaining popularity as a biomaterial scaffold in tissue engineering research.

In some cancers, HA levels correlate well with malignancy and poor prognosis. HA is thus often used as a tumor marker for prostate and breast cancer. It may also be used to monitor the progression of the disease.

HA may also be used postoperatively to induce tissue healing, notably after cataract surgery. Current models of wound healing propose that larger polymers of HA appear in the early stages of healing to physically make room for the white blood cells, that mediate the immune response.

Cosmetic Application of Hyaluronic Acid

The skin needs an optimum proportion of water to retain its softness and suppleness. Hydro regulative ingredients incorporated into cosmetic emulsions provide the skin with moisture. Hyaluronic acid is a gel-like, water-holding molecule that is the space filler and cushioning agent for skin. The remarkable ability of hyaluronic acid to hold moisture ensures soft, smooth, hydrated and elastic skin with the desired sensory effect. Many cosmetic products contain HA as a moisturiser and claim to have anti-ageing and anti-wrinkle effect via topical applications. Due to the molecular size of the HA used in topical creams it is unlikely and unproven that any penetration of the dermis occurs.

In cosmetic application of dermal fillers however HA is injected using a syringe/needle into the epithelial tissue. This allows the HA to be placed into the prescribed area for tissue augmentation. The HA essentially fills the tissue and slowly degrades over time once in the body. There are several products containing HA and crosslinked HA in the market. The fillers with crosslinked HA typically last for up to 6 months. BBL has established a crosslinking technology platform, which enables a gentle modification of HA to create a biomatrix for tissue repairing in a natural way, leading to longer residence time in the body with improved elasticity.