What does sodium hyaluronate do?

Sodium hyaluronate (also known as hyaluronic acid or hyaluronan) is a natural substance. It is present in the body wherever moisture is stored or lubrication between layers of tissue is required to eliminate friction.

Examples are the vitreous body of the eye itself, the tear film, the joint cartilage, the synovial fluid in the joints, all the mucuous membranes of the body, but also the basic substance of the skin which consists of up to 55% sodium hyaluronate. 

Sodium hyaluronate takes on a variety of functions: 

  • Storage: it absorbs and stores moisture in enormous quantities
  • Lubrication
  • Transport medium for nutrients
  • Filter for inflammatory molecules.

It may help to visualise sodium hyaluronate as a dish of spaghetti (this is what the molecules look look like under an electronic microscope): a dense mesh that remains elastic and flexible in spite of its density and interwoven structure.

How does Sodium Hyaluronate work?

Sodium Hyaluronate is the ideal molecule in eye lubrication

As a result of their unique physical and chemical properties, sodium hyaluronate (SH) solutions are similar to natural tears:

Viscoelasticity
The most important property of SH solutions is their viscoelasticity, which combines the viscous properties of a liquid with the elastic properties of a solid, depending on the shear stress applied. When used as an ocular lubricant, the unique viscoelastic properties of SH increase the stability of the pre-corneal tear film and maximize tear film residence time.2, 3

 

When the eye is at rest (between blinks), the SH molecules are not subject to a shear stress, and form a randomly tangled meshwork that creates a viscous, stable, long-lastingand protective coating on the surface of the eye.


Between blinks SH forms a long-lasting protective coating
During blinking, shear stress causes the SH molecules in the solution to align in parallel with one another. As a result, the solution becomes less viscous, more elastic and spreads easily over the corneal surface. In addition, this reduction in viscosity prevents the SH from being cleared from the eye, providing better comfort than purely viscous fluids.


While blinking, SH molecules align and spread easily over the cornea

Angle of contact

Due to its low angle of contact12, VISMED® is highly effective in coating the cornea. The low angle of contact of VISMED® is achieved through the correct balance of molecular weight and concentration. 



Angle of Contact (α) 

Adherence to the mucin layer (Muco-adhesivity)

SH molecules adhere well to the mucin layer of the precorneal tear film, providing an effective and long-lasting coating of the cornea. In a comparative study, SH demonstrated markedly more muco-adhesion than polyacrylic acid hydrogels.4



Sodium Hyaluronate solutions are mucoadhesive 

Similar behaviour as the natural mucin layer (Mucomimesis)

The rheological properties of the pre-corneal tear film are largely determined by the mucus it contains.5 These properties are mimicked closely by SH in VISMED®. SH possesses viscoelastic properties similar to those of the mucin layer of the tear film - this is believed to contribute to the efficacy and tolerability of SH-based lubricant eye drops in the eyes.

Water retention

As a result of its coiled structure and large hydrophilic domains, SH attracts and retains a large amount of water,6 and therefore possesses the ability to retard water evaporation.7 Following instillation, SH solutions effectively moisturize the eye surface and prolong the beneficial wetting effect over time.



Sodium Hyaluronate has a polymeric structure that binds  large amounts of water  

Healing of superficial keratitis

The biomechanical properties of SH prevent further damage to the cornea by creating a protective coating on the corneal surface. In addition, SH has been shown to have a beneficial effect in the healing of superficial keratitis by promoting rapid cell migration.8

Free radical scavenging

SH molecules have been shown to have antioxidant properties9 and therefore effectively protect the ocular tissues against deleterious oxidative lesions caused by free radicals. Topical ophthalmic SH application has been shown to be safe and well tolerated in both short and long-term use.10, 11

References:

  1. Johnson ME et al. Graefe's Arch Clin Exp Ophthalmol 2006; 244: 109-12.
  2. 3 Snibson GR et al. Eye 1990; 4: 594-602.
  3. Saettone MF et al. Int J Pharm 1989; 51: 203-12.
  4. Tiffany JM. Adv Exp Med Biol 1994; 350: 267-70.
  5. Balazs EA, Band P. Cosmetics Toiletries 1984; 99: 65-72.
  6. Nakamura M et al. Cornea 1993; 12: 433-6.
  7. Gomes JAP et al. Br J Ophthalmol 2004; 88: 821-25.
  8. Debbasch C et al. Invest Ophthalmol Vis Sci 2002; 43: 3409-15.
  9. Nokazaki Y et al. Jpn Pharmacol Ther 1993; 21: 87-102.
  10. Sawa M et al. J Jpn Ophthalmol 1993; 97: 448-54.
  11. TRB Chemedica International SA: Data on file.