**2. Alginate**

British Chemist E.C.C. Stanford in 1881 first described Alginate and available as the most common and abundant polysaccharide in the brown algae having 40% of the dry matter. It is composed of sodium, calcium, magnesium, strontium and barium ions in a gel form located in the intercellular matrix. It is mostly utilised for industrial purpose because of its capacity of retaining water, gelling, viscosifying and stabilising properties [16].

Alginate has been broadly investigated and employed for many biomedical implementations because of its biocompatibility, low toxicity, low cost, and mild gelation by adding of divalent cations such as Ca2+ [17, 18]. It is different from the other hydrocolloids like agar and carrageenan as it is collected from brown seaweed (Phaeophycean) exactly from outer layer cell wall because inner layer mostly made up of cellulose [19]. These alginate molecules allow both flexibility and mechanical strength to the algae [20, 21]. Apart from brown algae, it can also collected from different bacteria species such as *Azotobacter* and *Pseudomonas* [22]. Brown algal species like *Laminaria hyperborean*, *Macrocystis pyrifera*, *Laminaria digitata* and *Ascophyllumnodosum* are extensively used for commercial alginate preparation, while species like *Sargassum spp.*, *Laminaria japonica*, *Ecklonia maxima* and *Lessonia nigrescens*are utilised only if other brown seaweeds not accessible as these gave low and weak production of alginates [23, 24]. Although alginates can be produced both from algae and bacteria, mostly algae are used for commercial production [25].

### **3. Structure and composition**

Alginates are unbranched polysaccharides that are made up of 1, 4 linked β-D-mannuronic acid (M) and its C-5 epimer, α-L-guluronic acid (G). It is composed of M sequences (M-blocks) and G sequences (G-blocks) distributed with MG sequences (MG-blocks) [25]. The D-mannuronate was thought to be the main component of alginate before Fischer and Dörfel discovered the α-l-guluronate residue [26]. Later, fractional precipitation with manganese and calcium salts revealed that alginates are also block copolymers with a different ratio of guluronate

#### *Curcumin-Alginate Mixed Nanocomposite: An Evolving Therapy for Wound Healing DOI: http://dx.doi.org/10.5772/intechopen.98830*

to mannuronate depending on the natural source [27]. Alginate is now understood to be a group of linear copolymers made up of 1,4-linked β-D-mannuronate (M) and α-l-guluronate (G) residues. Consecutive G residues (GGGGGG), consecutive M residues (MMMMMM) and alternating M and G residues (GMGMGM) make up the blocks (**Figure 1**). The M and G contents of alginates extracted from various sources vary, as does the length of each block, and over 200 different alginates are currently manufactured [28]. The G-block content of L-hyperborean stems is 60%, compared to 14 to 31% for other commercially available alginates [29]. Only the G-blocks of alginate are thought to engage in the formation of hydrogels by intermolecular cross-linking with divalent cations (e.g., Ca2+). The composition (i.e., M/G ratio), sequence, G-block length, and molecular weight of alginate and its resulting hydrogels are thus critical factors influencing their physical properties [30]. The length of the G-block and the molecular weight of alginate gels are usually increased to improve mechanical properties. Different alginate sources manufacture polymers with a variety of chemical structures, for example, bacterial alginate derived from Azotobacter has a high concentration of G-blocks and its gels are relatively stiff [31]. The stability of the gels, the rate of drug release from gels, and the phenotype and function of cells encapsulated in alginate gels are all influenced by their physical properties [18].

Commercially available sodium alginates have molecular weights ranging from 32,000 to 400,000 g/mol. For sodium alginate in 0.1 M NaCl solution at 25°C, the parameters of the Mark–Houwink relationship; [ƞ] = KMv a) are K = 2 103 and a = 0.97, where [ƞ] is intrinsic viscosity (mL/g) and Mv is viscosityaverage molecular weight (g/mol) [32]. As the carboxylate groups in the alginate backbone become protonated and form hydrogen bonds, the viscosity of alginate solutions increases as the pH decreases, peaking around pH 3 to 3.5. The physical properties of the resulting gels can be improved by increasing the molecular weight of alginate. An alginate solution made from a high molecular weight polymer, on the other hand, becomes extremely viscous, making it difficult to work with [33]. The pre-gel solution viscosity and post-gelling stiffness can be regulated independently by manipulating the molecular weight and its distribution. By combining high and low molecular weight alginate polymers, the elastic modulus of gels can be greatly increased while the viscosity of the solution is minimally increased [18, 34].

**Figure 1.**

*Diagrammatic representation of alginates: (a) molecular chain, (b) M and G block distribution [25].*
