2.1. Alginate Structure, Chemistry and Purity
Alginates are polysaccharides which consist of linear (unbranched) 1,4 linked residues of β-d-mannuronic acid (M) and its C5-epimer α-l-guluronic acid (G) (Figure 1). The alginate molecular structure contains blocks of consecutive G or M monomers (-GGG- or -MMM-) or blocks of alternating monomers (-MGMG-). The G content of most algal alginates varies between 30% and 70%. The blocks vary considerably in length and distribution depending on from what species and part of the seaweed the alginate is extracted. The chemical composition and distribution of blocks in the alginate molecule play a major role in their capability of forming ionic gels.
Figure 1  The structure of alginate shown as the segment of ..MMGG.. residues [18]. Epimerisation of the M residues changes the conformation of the sugar from 4C1 to 1C4 [19,20].   At neutral pH alginate has a polyanionic character due to the pKa values d-mannuronic and l‑gulronic acid of 3.38 and 3.65, respectively [21]. Hence, acidification below pKa leads to insoluble alginic acid, whereas alginate molecules in solution have an extended random coil conformation due to intramolecular electrostatic repulsion between neighboring negative charges. This results in highly viscous solutions of alginate even at low concentrations where the viscosity is influenced by the ionic strength, temperature and molecular weight [21].
Commodity alginates, while having similar physicochemical properties, may contain contaminants inducing adverse cell reactions or undesired and uncontrolled cell to matrix interactions. Cells do not have receptors that recognize alginates and regular commercially available alginates can be considered as inert if they are of ultrapure quality. Impurities that should be considered and controlled in alginates for biomedical applications are presented in ASTM F 2067 and relate to the level of endotoxins, protein contaminants, elemental impurities and microbial bioburden [22]. The presence of residual endotoxins will, for example, interact with the liposolysaccharide (LPS) receptor CD14 [23]. CD14 is involved in different cell signaling pathways related to management of sepsis and can induce secretion of cytokines and upregulation of adhesion molecules. To ensure consistent cellular behavior in the presence of alginate biomaterials, the use of well-characterized and highly purified alginates is essential.