**2.3 Biocompatibility**

Alginate has excellent biocompatibility, that has been widely assessed. However, some gaps of biocompatibility between alginate and alginate complex still exist. For purified alginate gels, the relative amounts and distribution of M and G have an effect on biocompatibility [16]. For example, M. Otterlei et al. reported that alginates with low G were approximately 10 times more potent inducing cytokine production compared with high G alginates [7]. S.K. Tam et al. found that gel beads prepared with alginate contenting intermediate guluronate (IntG, 44% G) exhibited better biocompatible than high guluronate content (HiG, 71% G). There are no inflammatory reactions around alginate implants [17]. For the alginate complex, the impurities from alginate-based materials, such as heavy metals, proteins, and polyphenolic compounds, have the potential to cause an immunogenic response. A multi-step extraction procedure reduces the concentration of impurities and will not cause foreign body reactions. Meanwhile, the biocompatibility properties of alginate are attributed to hydrophilicity, chain migration, and water-absorbing [18]. Swelling properties contribute to enhance biocompatibility, that limiting the adsorption of proteins and cells of immune response [19].

#### **2.4 Degradation**

Degradability of biomaterials, as a critical property, contributes to providing a biomimetic microenvironment for use in cell delivery, survival, and expansion [20]. Hydrolytic is the main reason for degradation, which will be initiated spontaneously after contacting water-based fluids [21]. The degradation rate is related to the molecular weight of alginate. Generally, with the increase of molecular weight, the amounts of reactive sites for hydrolysis degradation decrease, that reduce the degradation rate [22]. Degradation, accompanied by adjusting the structure and molecular weight distribution, has an effect on the mechanical properties. In the physiological environment, alginate will undergo rapid degradation [23]. For alginate hydrogel, the degradation process will be accomplished by releasing the divalent ions into the surrounding environment. However, some residues of alginate still exist and will not be removed [24]. Many physical and chemical methods have been used to control alginate degradation, including ultrasonic, ultraviolent, gamma irradiation, and partial oxidation.
