Influence of the Processes Parameters on the Properties of The ...

Chapter 1.
Polylactide Based Bio-Materials
Figure 1.6: Electron micrograph and chemical HA structure.
[Varki et al., 1999]
3.1.2 Physicochemical and Biological Properties <strong>of</strong> HA
The inherent physicochemical structure and properties <strong>of</strong> HA−high molecular weight, β-glycosidic
linkages, internal hydrogen bonds, and interactions with <strong>the</strong> solvent - enable <strong>the</strong> molecule to behave in a
highly non-newtonian, gel-like manner, even in dilute solutions <strong>of</strong> HA [Chong et al., 2004]. The result is a
unique water-binding and high retention capacity – for example, one gram <strong>of</strong> HA is capable <strong>of</strong> holding up to
6 litres <strong>of</strong> water.
As a natural conjunction to its physicochemical properties, HA also has various roles at <strong>the</strong>
biological level [Garg and Hales, 2004a]. As an essential structural component in <strong>the</strong> extracellular matrix <strong>of</strong>
vertebrate tissues, HA regulates water balance and fills space, interacting with a variety <strong>of</strong> extracellular
molecules [Garg and Hales, 2004b]. HA is also known to activate intracellular signalling pathways and to
induce proliferative and migratory responses [Garg and Hales, 2004b].
Hyaluronic acid is fairly stable, partially because <strong>of</strong> <strong>the</strong> way that its disaccharide components are
positioned. The bulkier parts <strong>of</strong> <strong>the</strong> molecule are spaced far apart. By lowering this crowding, which is also
known as "steric hindrance," <strong>the</strong> molecule is able to be flexible, but also resistant to break down.
Hyaluronan is found in many tissues <strong>of</strong> <strong>the</strong> body, such as skin, cartilage, and <strong>the</strong> vitreous humour.
Therefore, it is well suited to biomedical applications targeting <strong>the</strong>se tissues. The first hyaluronan
biomedical product, Healon, was developed in <strong>the</strong> 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).
Hyaluronan is also used to treat osteoarthritis <strong>of</strong> <strong>the</strong> knee [Puhl and Scharf, 1997]. Such
treatments, called viscosupplementation, are administered as a course <strong>of</strong> injections into <strong>the</strong> knee joint, and
are believed to supplement <strong>the</strong> viscosity <strong>of</strong> <strong>the</strong> joint fluid, <strong>the</strong>reby lubricating <strong>the</strong> joint, cushioning <strong>the</strong> joint,
and producing an analgesic effect. It has also been suggested that hyaluronan has positive biochemical
effects on cartilage cells. However, some placebo-controlled studies have cast doubt on <strong>the</strong> efficacy <strong>of</strong>
hyaluronan injections, and hyaluronan is recommended primarily as a last alternative before surgery
[Karlsson et al., 2002; Holmes et al., 1988].
HA is an important component <strong>of</strong> skin, where it is involved in tissue repair. The skin needs an
optimum proportion <strong>of</strong> water to retain its s<strong>of</strong>tness and suppleness. Hydro regulative ingredients incorporated
into cosmetic emulsions provide <strong>the</strong> skin with moisture. Hyaluronic acid is a gel-like, water-holding
molecule that is <strong>the</strong> space filler and cushioning agent for skin. The remarkable ability <strong>of</strong> hyaluronic acid to
hold moisture ensures s<strong>of</strong>t, smooth, hydrated and elastic skin with <strong>the</strong> 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 <strong>the</strong> molecular size <strong>of</strong> <strong>the</strong> HA used in topical creams, it is unlikely and unproven that any
penetration <strong>of</strong> <strong>the</strong> dermis occurs. HA acts as a free radical scavenger, absorbing and degrading <strong>the</strong>m. When
skin is excessively exposed to UV B (wavelength 290 to 320 nm) rays and becomes inflamed (sunburn), <strong>the</strong>
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