Biomedical Engineering Reference
In-Depth Information
The most striking feature that distinguishes the chemical composition of Ulvan from that of
the other polysaccharides of marine origin is, therefore, the presence of uncommon sugar
such as iduronic and sulphated rhamnose displaying a close similarity with mammalian
glycosaminoglycans. To this view Ulvan and related polysaccharides could represent an
abundant and cheap feedstock for the substitution of heparinoid substances commonly used
in biomedical applications solving the problems related to their isolation and purification
(Alban et al., 2002).
2.1.2 Ulvan conformation
The physical properties of polymeric materials are deeply affected by the association and
conformation assumed by the constituting chains in the final product. The balance between
ordered crystalline and disordered amorphous structures dictates the ultimate mechanical
properties of the polymeric material. Indeed the possibility of forming crystalline regions
inside a polymeric structure could even generate physical crosslinks between the chains
inducing ultimately to the formation of stiff networks, as in the case of polyvinyl alcohol
(Ricciardi et al, 2005). The achievement of suitable mechanical properties for a material to be
used in biomedical applications, namely tissue engineering, represent a key requirement to
fulfil since the final product must provide a physical support for the cell growth and
differentiation.
Past investigations on this issue revealed an essentially disordered conformation of Ulvan
(Paradossi et al., 1999) mainly induced by the heterogeneous chemical composition of this
polysaccharide. The local regularity given by the repeating aldobiuronic units, denominated
as A
3s
and B
3s
(Figure 3), is believed to be sufficient for the formation of transient “junction-
zones” responsible for the formation of the weak gel that ulvan is known to perform in
nature (Paradossi et al., 2002). The stability of these ordered structures can be affected by the
attractive and repulsive interactions that form between the functional groups of the
polysaccharide, and in particular by the electrostatic forces. Ulvan is an anionic
polyelectrolyte as it contains carboxylic and sulphate groups inside its structure, so that its
net charge strongly depends on the pH and ionic strength of the working medium. The net
charge on Ulvan is found to affect the conformation of its polymeric chains and ultimately
controls the order to disorder transitions given by the locally regular sequences (Paradossi et
al., 2002). The conformational change from an ordered structure present in the uncharged
chain, i.e. the protonated form of ulvan, toward a disordered state, happens when a critical
charge density is reached and is induced only in the chemically regular portions of the
chains. The structures of the ordered sequences have been hypothesized on the basis of
molecular modelling calculations and are compatible with the formation of helical
conformations inside homogeneous portions of the chains containing the repeating units A
3s
and B
3s
(Paradossi et al., 2002).
The presence of ordered structures limited only in the regular sequences of the Ulvan
polymeric chains is not sufficient to provide enough “junction-zones” for the preparation of
a material with mechanical properties suitable for biomedical applications. For this purpose
Ulvan has to be modified through the introduction of chemical groups or molecules that
increase the number of “junction-zones”.
2.1.3 Ulvan morphology and solubility
The possibility of chemically modifying Ulvan is strongly dependent on the physical
availability of its functional groups so that its solubility and morphology in the working
