**1. Introduction**

#### **1.1 Alginate: Structure and chemical properties**

Alginate belongs to a class of linear polysaccharides consisting of a repeated alternating 1–4 glycosidic bond linked to hexuronic acids: α-L-guluronic acid (G-block) and β-D-mannuronic acid (M-block) subunits are randomly distributed along the chain. They can be arranged into homopolymeric sequences of MM or GG blocks and heterogenous or alternating MG blocks, as shown in **Figure 1** [1]. The physiological and rheological properties of alginate are strongly influenced by the uronic acid

#### **Figure 1.**

*Structural properties of alginate. (1) polysaccharide subunits of alginate. (2) Homopolymeric and heterogeneous sequences.*

compositions (M/G ratio) and by the distribution of the different blocks along the chains [2]. The molecular proportions of G and M subunits in any given alginate vary depending on the source from which it is extracted as well as on the season and the ecological conditions of algal growth [3]. More specifically, the proportions of MM, MG, and GG blocks modulate the physical properties of alginate, where alginates with higher G contents tend to have higher gelling properties. In contrast, those with higher M are associated with higher viscosity [4]. Additionally, alginates with high M/G ratios generate elastic gels, while brittle gels result from alginates with low M/G ratios [1, 5]. Thus, the chemical composition of alginate is fundamental to its intended applications. Alginates have been extracted and purified from different sources including cell walls of brown algae (also known as brown seaweeds), bacterial capsules of *Azotobacter sp*. and *Pseudomonas sp*. They provide flexibility and strong structure to algae to buffer the impact of strong water waves, and support bacteria in biofilm formation, adherence, and colonisation [6] (**Figure 2**).

#### **1.2 Physical properties of alginates**

#### *1.2.1 Solubility*

Being a polymer of acidic sugar residues, alginate exists in solution as a negatively charged viscous solution and as metal salts (such as sodium, magnesium potassium salts of alginate) in solid states. Among these, sodium alginates are water soluble, resulting in solutions of varying viscosity, depending on the M/G ratio and the alginate molecular weight (MW). Although high MW alginate exhibits greater gel strength, especially when it has more significant G-block content, they are challenging to handle because of the high viscosity of its resulting solution [7]. The water-soluble salt of alginate has an excellent capacity to retain a large amount of water and some amounts of organic matter [8], which is characteristic of hydrogels. Due to its polydispersity, MW of alginate is usually taken as average over the wide distribution of MW ranging from 32,000 to 400,000 g/mol, depending on the source from which the *Alginate-Based Applications in Biotechnology with a Special Mention to Biosensors DOI: http://dx.doi.org/10.5772/intechopen.110737*

#### **Figure 2.** *Illustration of alginate polymer.*

alginates were extracted [9]. The MW distribution of alginate has been shown to modulate the viscosity of the pre-gel solution and the rigidity of the gel, thus it influences the choice of alginate for specific applications.
