General Hyaluronic Acid Information
Hyaluronic acid, sometimes referred to a hyaluronan or HA, is the most abundant glycosaminoglycan in the body with being an important major component of several tissues throughout the body. While it is abundant in extracellular matrices, hyaluronan also contributes to tissue hydrodynamics, movement and proliferation of cells, and participates in a number of cell surface receptor interactions.
Hyaluronic acid is a polymer of disaccharides, themselves composed of D-glucuronic acid and N-acetyl-D-glucosamine, linked via alternating β-(1→4) and β-(1→3) glycosidic bonds. Hyaluronic acid can be 25,000 disaccharide repeats in length. Polymers of hyaluronic acid can range in size from 5,000 to 20,000,000 Da in vivo.
Rheological Properties of Hyaluronic Acid
As a polymer in solution, shows non-Newtonian and viscoelastic behavior. Non-Newtonian liquids exhibit viscosity dependence upon the applied shear conditions. The most common type of non-Newtonian behavior being shear-thinning, where viscosity decreases with increasing shear rate.
This characteristic is beneficial as it allows for the product to be manipulated and handled with greater ease, such as when pumped and filled in manufacturing processes or dispensed via a needle as occurs in extrusion 3D bioprinting. Once at rest, however the product regains its viscosity, helping maintain its position.
Hyaluronic acid is highly soluble and often exhibits very poor mechanical properties with rapid degradation behavior in vivo. Hyaluronic acid has been chemically and crosslinker-modified to improve its properties, including mechanical properties, viscosity, solubility, degradation, and biologic properties.
The chemical structure of hyaluronic acid can be covalently modified where the three most commonly used sites includes the carboxylic groups, hydroxyl group, and –NHCOCH3 groups.
Methacrylated Hyaluronic Acid (PhotoHA®)
PhotoHA® is a methacrylated hyaluronic acid that can be photocrosslinked in the presence of a photoinitiator and light. Tunability is accomplished by altering the HA concentration, photocrosslinking time and intensity, or by adjusting the photoinitiator concentration.
Crosslinked PhotoHA® hydrogels are strong and optically clear (as seen in the image above).
Rheological testing using Irgacure and UV light (365 nm) demonstrate a high rate of crosslinkability, tunability, and strength for PhotoHA® hydrogels (right).