Biomedical Engineering Reference
In-Depth Information
Fig. 4. Proposed mechanisms for the formation of Ulvan-based ionotropic hydrogels
through a) Ca
2+
stabilization of borate esters (Haug, 1976) or the participation of b)
carboxylate and c) sulphate groups (Lahaye & Axelos, 1993).
The gel behaviour of Ulvan is different from that of most polysaccharides which usually
involve tight junction zones of ordered molecular structures like helices or flat buckled
ribbons (Stephen, 1995). Ulvan gel results from the aggregation of bead-like structures
interconnected by more hydrophilic polymeric fractions. This behaviour would be
undoubtedly favoured by the necklace-type ultrastructure assumed by the polymer in
solution, whose formation has been shown to be promoted by ionic interactions (Robic et al,
2009). To this view, the positive role of boric acid can also be related to its reactivity towards
the neutrally charged hydroxyl moieties of Ulvan and subsequent substitution with charged
borate ester groups, thus contributing to create additional charges on the beads surface of
the polysaccharide and favouring their association.
The mechanical properties of these ionotropic hydrogels are usually poor due to their
intrinsic weakness and tend to get worse when used in contact with body fluids due to the
ion exchange phenomena that occurs between Ca
2+
that stabilizes the network and the
monovalent cations like K
+
and Na
+
present in the physiological liquids (LeRoux et al.,
1999). These hydrogels found limited applications in tissue engineering due to their
mechanical instability and uncontrolled dissolution in physiological conditions (Atala &
Lanza, 2002).
3.2 Ulvan-based chemical hydrogels
In order to overcome the problems related to the mechanical instability of the physically
gelled Ulvan and extend the range of their potential applications, the strategy of chemical
crosslinking of Ulvan was undertaken (Morelli & Chiellini, 2010).
A smart and relatively innovative technique of obtaining chemically crosslinked hydrogels
is represented by their photopolymerization in the presence of photoinitiators using visible
or ultraviolet (UV) light.
Photopolymerization is used to convert a liquid monomer or macromer to a hydrogel by
free radical polymerization in a fast and controllable manner under ambient or
physiological conditions. The mechanism is triggered by visible or UV light that interact
with light-sensitive compounds called photoinitiators to create free radicals that can initiate
polymerization of species containing suitably reactive groups (typically double bonds).
