Biomedical Engineering Reference
In-Depth Information
medium could affect deeply its reactivity. Ulvan has been shown to dissolve only in water
due to its charged and highly hydrophilic nature. Nevertheless, the obtained solutions are
not transparent, indicating the formation of microaggregates of polymeric material not fully
dispersed in the solvent. Indeed TEM analysis of Ulvan revealed the presence of aggregates
of spherical shaped forms partially linked by strands-like filaments (Robic et al., 2009). This
necklace-like ultrastructure is usually formed by polyelectrolyte material in poor solvent
conditions (Dobrynin, 2008) so that even water can not be considered a good solvent for
Ulvan. The large presence of methyl groups provided by the rhamnose repeating unit has
been considered responsible for the unusual hydrophobic behavior of this highly charged
polysaccharide (Robic et al., 2009).
The unusual low intrinsic viscosity of Ulvan in solution can also be ascribed to the presence
of condensed spherical shaped aggregates not typical for polyelectrolytes whose
conformation usually expands in the form of charged filaments and leads to an increase in
the viscosity (Dobrynin et al., 1995). The formation of microaggregates in solution does not
allow also a reliable mass analysis of Ulvan, whose different type of aggregation affects
deeply the peak distributions usually found on the GPC chromatograms (Robic et al., 2009).
Being a polyelectrolyte, both the ionic strength and the pH of the dissolving medium would
affect the solubility and the morphology of Ulvan. Indeed the association of the bead-like
aggregates in a necklace-type ultrastructure is promoted by the ionic interactions of
carboxylated groups as demonstrated by its rupture at pH below the pKa of glucuronic acid
(3.28) (Robic et al. 2009). In basic conditions (pH 13) the bead-like structures resulted to
collapse into a dense homogeneous network likely prompted by the ionic interactions of
carboxylate and sulphate groups. The type and amount of counter-ion in solution could also
contribute to chain expansion or condensation as demonstrated by the aggregative
propensity of Ulvan at low NaCl concentration observed by light scattering and rheological
measurements (Lahaye & Robic, 2007).
The tendency of Ulvan to form aggregates in aqueous solution and its insolubility in almost
every organic solvents limit the number of functional groups available for chemical
modifications thus hampering its potential versatility. But its great number of reactive
groups still present on the “free” surface exposed outside the aggregate and the possibility
to optimize the solvent variables (pH and ionic strength) that affect the dispersion of the
polymer in solution make Ulvan a suitable reactive platform, tailorable according to the
envisaged application.
2.2 Biological activity
The possibility of using bio-based materials in almost every technological field and
particularly in biomedical applications is challenging and can be considered the strategy of
election for limiting environmental concerns and create a virtuous circle of sustainability.
Biomaterials possess the essential prerequisite of renewability and biocompatibility and as
such are worth of deep investigations as main candidates for the substitution of synthetic
petroleum-based materials, well known for being not renewable and often not
biocompatible.
Biodegradability represents also an important property possessed by biomaterials and it is
especially required in materials used for biomedical applications with specific reference to
tissue engineering and regenerative medicine. Not only the material has to be safe but also
the products of degradation should be non-toxic and easily cleared from the body.
Biomaterials that other than being renewable, biocompatible and biodegradable are able to
