Biomedical Engineering Reference
In-Depth Information
particular attention are gaining biopolymers of plant and algal origins due to their
abundance and minor concerns for purification (Stevens, 2008).
Ulvan could represent an advantageous versatile platform of “unique” sulphated
polysaccharides that along with their abundance and renewability would potentially
display the properties that match the criteria for biomedical applications. Despite the
promising properties related to this material, the use of Ulvan in the biomedical fields is not
yet reported and its potentiality still remain unveiled.
A base requirement for a material suitable for tissue engineering, regenerative medicine
and/or drug delivery is its insolubility in the physiological fluids not possible with most
classes of polysaccharides, whose high hydrophilicity make them very akin to water
molecules. Indeed materials used for the regeneration of organs or tissues must avoid
dissolution in contact with body fluids thus functioning as chemically and mechanically
stable scaffolds during the growth and differentiation of the implanted cells. Also polymeric
materials used for drug delivery must preserve their integrity or degrade slowly in order to
maintain a controlled release of the loaded drug. The use of polysaccharides in these types
of applications is possible only after proper chemical modifications aimed at making them
insoluble in aqueous solution. A possible strategy consists in decreasing the hydrophilicity
of these materials by introducing hydrophobic groups in their structures, but this often
leads to the obtainment of new class of materials, mostly semi-synthetic than naturals and
often very different from the original biomaterials. The strategy of election mostly followed
by biomaterial scientists in the last 50 years (Hoffmann, 2002) consisted simply in the
induction of “junction-zones” between the polymeric chains, inhibiting their dissolution
through the formation of permanent or temporary crosslinked networks displaying
hydrogel features. These structures maintain almost completely the chemical properties of
the original biopolymer comprising their affinity to water giving rise to swollen and not
dissolved polymeric scaffolds of natural origins. The maintained hydrophilic character of
hydrogels is particularly important in tissue engineering where the overall permeation of
nutrients and cellular products into the pores of the gel, determinant for the growth and
differentiation of the cells, is determined by the amount of water in the structure (Hoffmann,
2002).
Hydrogels can be chemically or covalently crosslinked and are defined as permanent when
the “junction-zones” between the constituting chemical chains are formed by covalent links.
If the new bonds are not susceptible to hydrolysis or enzyme recognition the formed
hydrogel can be stable indefinitely and not prone to degradation.
The formation of physical or temporary hydrogels is triggered when the “junction-zones”
between the polymeric chains are stabilized by weak forces such as electrostatic or
hydrophobic interactions. These interactions are reversible, and can be disrupted by changes
in physical conditions such as ionic strength, pH, temperature or application of stress.
Physical hydrogels are usually not homogeneous, since clusters of molecular entanglements
or hydrophobically- or ionically-associated domains can create in-homogeneities
(Hoffmann, 2002). This can lead to the formation of hydrogels with weak mechanical
properties not suitable for most conventional applications.
Apart from the biological activity displayed by these biomaterials, other important
attributes of hydrogels are their mechanical properties and degradation rates that must be
tuned according to the final application. The degree of crosslinking along with the chemical
