**2. Ulvan properties**

Ulvales (Chlorophyta) are very common seaweeds distributed worldwide. The two main *genera Ulva* and *Enteromorpha* are sadly known for being involved in processes detrimental for the aquatic environment. Indeed this algal biomass proliferates very quickly in eutrophic coastal and lagoon waters in the form of "green tides" leading up to hypoxia and death of most of aquatic organisms (Morand & Brian, 1996). Environmental concerns arise also for the disposal of this huge biomass that is mostly left do degrade on the shore creating nuisance problems, so that its exploitation could represent a remedy to related environmental and economical concerns.

To date, this biomass has very low added value and its use is limited to food consumption (Bobin-Dudigeon et al., 1997) composting (Mazè et al., 1993) and methane production

Ulvan: A Versatile Platform of Biomaterials from Renewable Resources 79

The most striking feature that distinguishes the chemical composition of Ulvan from that of the other polysaccharides of marine origin is, therefore, the presence of uncommon sugar such as iduronic and sulphated rhamnose displaying a close similarity with mammalian glycosaminoglycans. To this view Ulvan and related polysaccharides could represent an abundant and cheap feedstock for the substitution of heparinoid substances commonly used in biomedical applications solving the problems related to their isolation and purification

The physical properties of polymeric materials are deeply affected by the association and conformation assumed by the constituting chains in the final product. The balance between ordered crystalline and disordered amorphous structures dictates the ultimate mechanical properties of the polymeric material. Indeed the possibility of forming crystalline regions inside a polymeric structure could even generate physical crosslinks between the chains inducing ultimately to the formation of stiff networks, as in the case of polyvinyl alcohol (Ricciardi et al, 2005). The achievement of suitable mechanical properties for a material to be used in biomedical applications, namely tissue engineering, represent a key requirement to fulfil since the final product must provide a physical support for the cell growth and

Past investigations on this issue revealed an essentially disordered conformation of Ulvan (Paradossi et al., 1999) mainly induced by the heterogeneous chemical composition of this polysaccharide. The local regularity given by the repeating aldobiuronic units, denominated as A3s and B3s (Figure 3), is believed to be sufficient for the formation of transient "junctionzones" responsible for the formation of the weak gel that ulvan is known to perform in nature (Paradossi et al., 2002). The stability of these ordered structures can be affected by the attractive and repulsive interactions that form between the functional groups of the polysaccharide, and in particular by the electrostatic forces. Ulvan is an anionic polyelectrolyte as it contains carboxylic and sulphate groups inside its structure, so that its net charge strongly depends on the pH and ionic strength of the working medium. The net charge on Ulvan is found to affect the conformation of its polymeric chains and ultimately controls the order to disorder transitions given by the locally regular sequences (Paradossi et al., 2002). The conformational change from an ordered structure present in the uncharged chain, i.e. the protonated form of ulvan, toward a disordered state, happens when a critical charge density is reached and is induced only in the chemically regular portions of the chains. The structures of the ordered sequences have been hypothesized on the basis of molecular modelling calculations and are compatible with the formation of helical conformations inside homogeneous portions of the chains containing the repeating units A3s

The presence of ordered structures limited only in the regular sequences of the Ulvan polymeric chains is not sufficient to provide enough "junction-zones" for the preparation of a material with mechanical properties suitable for biomedical applications. For this purpose Ulvan has to be modified through the introduction of chemical groups or molecules that

The possibility of chemically modifying Ulvan is strongly dependent on the physical availability of its functional groups so that its solubility and morphology in the working

(Alban et al., 2002).

differentiation.

**2.1.2 Ulvan conformation** 

and B3s (Paradossi et al., 2002).

increase the number of "junction-zones".

**2.1.3 Ulvan morphology and solubility** 

(Brand & Morand, 1997) but as it will be stressed in the following part of the chapter, the chemicals and polymers of this underexploited biomass along with their abundance, biological properties and "renewability" represent a potential source to be explored.
