Ulvan: A Versatile Platform of Biomaterials from Renewable Resources

Abstract

Biomass represents an abundant renewable resource for the production of bioenergy and biomaterials and its exploitation could lead to overcome the dependence from petroleum resources. Indeed fossil energy and chemical sources are not unlimited and there is a critical need to turn the current way of life back to a sustainable manner. The conversion of biomasses into high value chemicals, energy and materials is nowadays gaining more and more attention and represents the final goal of the “Industrial Biorefinering”. Indeed Biorefinery aims at the optimum exploitation of biomass resources for the production of materials that eventually might replace the conventional products from fossil/non renewable resources, thus decisively contributing to the development of a sustainable system. The great challenge in which Biorefinering is involved is the possibility of creating high value products from low value biomasses. In this view, the feasibility of using starting materials obtainable from organic waste sources (agricultural, municipal and industrial waste) or having harmful effects on the environment (algae) as feedstock can represent the strategy of election for the production of sustainable materials. To this aim algae could represent a potentially advantageous biomass to be explored since they are very abundant and cheap and very often involved in uncontrolled proliferation processes detrimental for marine and aquatic environments (Barghini et al., 2010, Chiellini et al., 2008, 2009, Fletcher, 1996). Today most of the naturally produced and harvested algal biomass is an unused resource and often is left to decompose on the shore creating waste problems (Morand et al., 2006). The current use of this huge underexploited biomass is mainly limited to food consumption and as bio-fertilizer, but its potentiality as renewable and sustainable feedstock for energy and material production is gaining more and more attention (Demirbas A. & Demirbas M.F., 2011). Indeed microalgae have been considered to be an excellent source for biodiesel production since are characterized by high growth rates and high population densities, ideal for intensive agriculture and may contain huge lipid amounts, needed for fuel production (Christi, 2007). Besides biodiesel, algae can be cultivated and can be used as a feedstock for the production of bioethanol (John et al., 2011). In particular macroalgae (seaweed) can produce huge amount of carbohydrates per year (Matsumoto et al., 2003) that suitably processed through specific fermentation processes would provide renewable and sustainable biofuel. Algae represent also an advantageous resource of chemicals and building block materials that can be tailored through proper biorefinering processes according to the different envisaged applications. The rising demand for natural instead of synthetic materials especially in biomedical applications where high biocompatibility and no adverse effects for the host organism are required (Mano et al., 2007), has led to an outburst of scientific papers involved in the study of biobased materials. Among these, polysaccharides could represent the best candidate since abundant, biocompatible and displaying a pronounced chemical versatility given by the great number of chemical functionalities present in their structures. The list of known natural carbohydrates is continuously growing, owing to new discoveries in animal and plant material (Tsai, 2007). They can be used in their native form or after proper chemical modifications made according to the final applications (d’Ayala et al., 2008). The use of polysaccharides of animal origin (e.g. heparin and hyaluronic acid) in biomedical applications is not straightforward since it can raise concerns about immunogenicity and risk of disease transmission (Stevens, 2008) Indeed these materials require very accurate purification treatments aimed to free them from biological contaminants, in contrary to polysaccharides of plant (e.g. cellulose and starch) or algal origin (e.g. alginate). Polysaccharides of algal origins are gaining particular attention due to their abundance, renewability (Matsumoto et al., 2003) and to their peculiar chemical composition not found in any other organisms. Over the last few years medical and pharmaceutical industries have shown an increasing interest in alginate (d’Ayala et al., 2008), an anionic polysaccharide widely distributed in the cell walls of brown algae. This biopolymer has been largely used for its gel forming properties. Due to its non-toxicity, unique tissue compatibility, and biodegradability, alginate has been studied extensively in tissue engineering, including the regeneration of skin (Hashimoto et al., 2004), cartilage (Bouhadir et al., 2001), bone (Alsberg et al., 2001), liver (Chung et al., 2002) and cardiac tissue (Dar et al., 2002). A very intriguing feature that distinguishes algal biomass from other resources is that it contains large amounts of sulphated polysaccharides, whose beneficial biological properties (Wijesekara et al., 2011) prompt scientists to increase their use in the biomedical fields. Indeed the presence and the distribution of sulphate groups in these polysaccharides are reported to play an important role in the antiviral (Damonte et al., 2004), anticoagulant (Melo et al., 2004), antioxidant (Rocha de Souza et al., 2007) and anticancer (Athukorala et al., 2009) activity of these materials. The chemical composition of the sulphated polysaccharides extracted from algae, including the degree and the distribution of the sulphate groups, varies according to the species, and the ecophysiological origin of the algal sources (Rioux et al., 2007). Anyhow, a structural differentiation depending on the different taxonomic classification of the algal origin, has been found. According to the mentioned classification the major sulphated polysaccharides found in marine algae include fucoidan from brown algae, carrageenan from red algae and ulvan obtained from green algae (Figure 1). Ulvan polysaccharides possess unique structural properties since the repeating unit shares chemical affinity with glycoaminoglycan such as hyaluronan and chondroitin sulphate due to its content of glucuronic acid and sulphate (Figure 2). This resemblance and the possibility of obtaining this material from cheap and renewable resources make it worthwhile a deeper investigation on the biological activity and the feasibility of using this polysaccharide for biomedical applications.