Biomedical Engineering Reference
In-Depth Information
nature of the polymer and of the „junction-zones“ represent the key parameters that mainly
affect the above-mentioned properties and can be adjusted according to the addressed
applications.
Hydrogels based on polysaccharides are usually characterized by poor mechanical
properties due to their impressive water uptake and swelling that lead to the formation of
wide opened pore structures that ultimately weakens the scaffold architecture (LaNasa et
al., 2010). The presence of charged groups like sulphate and carboxylate in the structure of
Ulvan would lead to an even more accentuated absorption of water molecules, hampering
the preparation of mechanically stable hydrogels. The strategies for increasing the
mechanical properties are several and comprise the preparation of interpenetrating
networks with other polymers like polycomplexes formed between oppositely charged
polyelectolytes (Hamman, 2010), the preparation of composite hydrogels mixed with
inorganic additives (Pavlyuchenko & Ivanchev, 2009) or the use of hydrophobic
comonomers as in the preparation of hydrogels by radical crosslinking (Li et al., 2003).
All these strategies strongly affect the chemical nature of the original biopolymers and the
properties of the final hydrogels can be very different from those of the native materials
comprising their biocompatibility and bioactivity. A strategy to improve the mechanical
properties of polysaccharides and in particular Ulvan is based on a proper choice of the
nature and amount of “junction-zones” that crosslink the polymer. Indeed both the increase
of the crosslinking or the preference for chemical instead of physical crosslinking would
positively affect the mechanical properties of the final hydrogels, leading to more compact
structures and dimensionally shaped architecture. Both type of crosslinking have been
conducted on Ulvan and are worth of mentioning to get a deeper insight on the possible
applications and future developments of using this biopolymer in the biomedical fields.
3.1 Ulvan-based physical hydrogels
Ionotropic gelation is a kind of physical crosslinking based on the ability of polyelectrolytes
to give hydrogels in the presence of counter-ions. Alginate is a naturally occurring
polysaccharide obtained from marine brown algae that spontaneously form reticulated
structures in the presence of divalent or polyvalent cations (Patil et al., 2010). The
mechanism involves the cooperative interactions of the carboxylate groups of alginate with
the polyvalent cations present in solution to form “junction-zones” between the chains that
crosslink the matrix in an insoluble polymeric network.
Due to its polyanionic nature Ulvan is expected to show a similar behavior but its gel
formation has the unique characteristic of involving borate esters (Haug, 1976). The optimal
conditions for the preparations of the hydrogels requires the presence of boric acid and
calcium ions at slightly basic conditions (pH 7.5) giving hydrogels with storage modulus of
about 250 Pa (Lahaye & Robic, 2007). Higher ion concentrations, different pH, and even
phosphate buffering ions are detrimental to the gel.
The mechanism of gel formation is not yet completely unveiled but is proposed to proceed
through the formation of borate esters with Ulvan 1,2-diols followed by crosslinking via
Ca
2+
ions (Haug, 1976). Since the gel is thermoreversible the “junction-zones” that crosslink
the polymer are thought to involve weak linkages, likely based on labile borate ester groups
and ionic interactions easily disrupted by thermal treatments (Lahaye & Robic, 2007).
Calcium ions would bridge complexes and/or stabilize the borate esters (Fig. 4a) but also
sulphate and carboxylic acid groups were later on proposed to coordinate to Ca
2+
(Figure
3b,c) (Lahaye & Axelos, 1993) and contribute to the gel formation.
