**2. Structure and characterization**

## **2.1 Structure**

Alginate with molecular weight between 32,000 to 400,000 g/mol is mainly comprised of a sequence of liner polymers of β-(1–4)-D-mannuronic (M-blocks), α-L-guluronic acid (G-blocks), and inserted MG sequences (MG-blocks), with varying proportions and linear arrangements [4]. That organized in homogenous patterns with repeated G residues, repeated M residues, and heterogenous patterns with alternating G and M residues [5]. Alginate derived from different sources displays different M/G ratios and contents in M and G [6], leading to the change of molecule weight and physicochemical properties. These parameters are related to the characteristics and applications of alginate. Generally, alginate with high M units shows good biocompatibility and more immunogenic [7]. Alginate with high M units has soft and elastic properties, G-rich alginate exhibits hard and brittle characteristics [8, 9]. The rigidity of the chains increases in a sequence, MG < MM < GG, due to the electrostatic repulsion between charged groups. G-rich alginate gels have better mechanical stability (**Figure 1**) [5].

The electrostatic interactions between the carboxylate groups of G units and divalent cations, such as Ca2+, Fe2+, Mg2+, form an "egg-box" structure, which crosslinks to obtain the hydrogels. There are several hydroxy and carboxyl groups in the molecular structure of alginate. As the active site, more groups and side-chain molecules were introduced into the main chain to decorate the structure, which endow more features and expand its applications [10].

#### **2.2 Solubility and viscosity**

Sodium alginate exhibits slowly water-soluble and forms viscous and stable solution. At low solvent pH, more heterogeneous MG-blocks contribute to solute

**Figure 1.** *Chemical structure of alginate [5].*

than M-rich and G-rich alginate [11]. With the decrease of pH, the viscosity of alginate solution increase. When the range of pH is 3–3.5, the viscosity has the maximum. In the Mark-Houwink relationship ([*η*] = *KM*<sup>v</sup> a ), [*η*] is intrinsic viscosity (mL/g) and *M*v is the viscosity-average molecular weight (g/mol)[12]. For sodium alginate in 0.1 M NaCl solution at 25°C, K and *a*, the parameter of the Mark-Houwink relationship ([*η*] = *KW*<sup>v</sup> a ), are 2 × 10−3, *a* = 0.97 respectively.

The primary structure is associated with different amounts and sequential distribution of M and G, which affects the molecular weight and properties. For example, the viscous behavior is remarkably relevant to molecular weight during the preparation. Alginate with high molecular weight polymer could form an obviously viscous solution [13] and result in a higher elastic modulus in the gels [14]. Meanwhile, the alginate with a long chain shows a higher solution viscosity. For example, G-rich alginate displays more excellent water solubility than M-rich alginate [15].
