Chemical, physical and biological properties of alginates and their biomedical implications
Graphical abstract
Introduction
Alginates occur both as structural components in marine brown algae (Phaeophyceae) as well as capsular polysaccharides in some bacteria (Draget, Moe, Skjåk-Bræk, & Smidsrød, 2006). Commercial alginates are at present still exclusively extracted from algal sources although production by microbial fermentation is technically feasible. Industrial applications of alginates, based on their gelling, viscosifying and stabilizing properties, account for the quantitatively most important uses of alginates. In comparison, emerging and knowledge demanding speciality applications within biotechnology and medicine are based on biological effects of the alginate molecule, of alginate molecular building blocks, or alginate's unique, gentle and almost temperature independent sol/gel transition in the presence of multivalent cations (e.g. Ca2+), which makes alginate highly suitable as an immobilization matrix for biocatalysts such as living cells. Advanced research within these areas is promoting a further detailed understanding of structure–function relationships of the alginate molecule.
Section snippets
Physicochemical properties
Alginates constitute a family of linear binary copolymers, consisting of (1 → 4) linked β-d-mannuronic acid (M) and α-l-guluronic acid (G) residues (see Fig. 1a and b). Chemical composition and sequence may vary widely between algae species and even between different parts of the algae and the time of year when it is harvested. Alginate with a very high content of guluronic acid, which is of importance for the mechanical properties of the alginate gel, can be prepared from special algal tissues
Alginate gels and their properties
A direct mixing of alginate and multivalent cations rarely produces homogeneous gels due to the very rapid and irreversible binding of such ions. A controlled introduction of cross-linking ions is possible by the two fundamental methods for preparing an alginate gel: the diffusion method and the internal setting method (Smidsrød & Draget, 1996). The diffusion setting technique, which is the only method to be described in this paper, is characterized by allowing a cross-linking ion (e.g. Ca2+)
Traditional biomedical alginate applications
Alginates have for many years been used as devices in various human health applications, such as excipients in drug delivery (DDS), wound dressings, as dental impression materials and in some formulations preventing gastric reflux. One advantage of using alginates in oral tablet formulation is their property of preserving a solid-like attribute (gel) also at gastric (acid) conditions due to the formation of an alginic acid gel. This property allows for the protection of delicate compounds
Potential biomedical and pharmaceutical alginate applications
Over the last couple of decades, new knowledge about the impact of chemical composition and sequential arrangement of alginate on biological systems points towards more advanced biomedical devices as well as pharmaceutical properties of alginate in its own right.
Conclusions
Although alginates have been exploited for several decades as devices in a multitude of pharmaceutical and biomedical applications, there has this far not been any pharmaceutical claims on the alginate molecule itself. We may now be in the midst of an inflection point where this may change in the near future.
Acknowledgements
The authors would like to thank Prof. Em. Olav Smidsrød for valuable discussions, and the Norwegian Research Council and Helse Midt-Norge for financial support.
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