Sodium Alginate: A Versatile Polysaccharide with Diverse Physico-Chemical, Mechanical, and Biological Properties

General Description

Sodium alginate, a versatile polysaccharide, showcases diverse physico-chemical, mechanical, and biological properties. Its molecular weight and pH influence viscosity, gel properties, and solubility characteristics. The polymer's mechanical behavior is impacted by its molecular composition, concentration, and glucuronic acid content, displaying pseudoplasticity and thermal sensitivity. In terms of biological aspects, sodium alginate is FDA-approved for its safety, biocompatibility, and biodegradability, finding applications in drug delivery and metal binding scenarios. Despite its biocompatibility, careful consideration of factors affecting immunogenicity and biodegradation pathways is necessary for specific biomedical applications. Overall, sodium alginate's multifaceted properties make it a valuable component in various industries.

Figure 1. Sodium alginate

Physico-Chemical Properties

Molecular Weight and Viscosity

Sodium alginate, a widely used polysaccharide in various industries, exhibits distinct physico-chemical properties. Firstly, its molecular weight ranges from 32,000 to 400,000 with long M and G chains, resulting in a polydispersion index between 1.5 and 3. The viscosity of alginate solutions is affected by both the molecular mass and pH, with lower pH leading to increased viscosity due to protonation of carboxyl groups, enabling hydrogen bond formation. Higher molecular weight enhances gelling rate and physical gel properties, such as tensile strength, elasticity, and viscosity. However, excessive molecular weight can yield overly viscous solutions, unsuitable for certain applications such as cell immobilization matrix preparation, where high viscosity may compromise cell viability during mixing.

Solubility and Stability

Secondly, sodium alginate's solubility in cold water is gradual, yielding a viscous solution, while it remains insoluble in alcohol, chloroform, and ether. Its solubility depends on pH, molecular weight, ionic strength, ion nature, and concentration. Finally, sodium alginate demonstrates stability when conditioned as dry powder, remaining compatible with most anionic substances and exhibiting higher stability against external factors compared to its solution form. It forms a gel of alginic acid at low pH, redissolving at elevated pH, and withstands short periods of alkaline environments. Moreover, short-term high-temperature exposure for sterilization is feasible, but prolonged exposure reduces viscosity. ¹

Mechanical Properties

Sodium alginate, a naturally derived polysaccharide, exhibits unique mechanical properties influenced by its molecular composition and environmental conditions. Its viscosity is primarily contingent on the molecular weight and concentration of the polymer, as well as the glucuronic acid content, which enhances solubility and gel formation. Sodium alginate solutions are pseudoplastic, showing a significant change in viscosity when diluted or subjected to shear stress. Temperature also impacts the viscosity of sodium alginate. Differential scanning calorimetry (DSC) analyses reveal that alginate films exhibit lower rigidity at elevated temperatures, indicating thermal sensitivity. Below 100°C, noncovalent bonds maintain the polymer's integrity under oscillatory deformation, although constant stirring can disrupt this equilibrium. Commercial sodium alginate solutions typically show viscosities ranging from 20 to 400 centipoise (cP) at 20°C. Additionally, sodium alginate's mucoadhesive properties, due to free carboxyl and hydroxyl groups, promote bioadhesion through hydrogen bonding with mucin, enhancing drug delivery efficiency by increasing contact time and bioavailability at mucosal sites. ¹

Biological Properties

Biocompatibility

Sodium alginate possesses notable biological properties, rendering it suitable for various applications in the food, biomedical, and pharmaceutical industries. It has been approved by the FDA due to its lack of toxicity, immunogenicity, as well as its biocompatibility and biodegradability. In the realm of pharmaceuticals, sodium alginate's safety and non-toxic nature allow its utilization as an excipient in oral dosage forms, where it can bind to heavy metals in the intestine, offering protection against their adverse effects. However, when considering applications in implantology or intravenous administration, factors influencing biocompatibility and immunogenicity, such as chemical composition, purification process, and residual contaminants, must be carefully evaluated to mitigate potential fibrosis and immune responses.

Medical Applications

Furthermore, sodium alginate's biodegradability is a crucial consideration, as enzymatic degradation in mammals is limited due to the absence of alginase. To address this, studies suggest that structural modifications can facilitate oxidative and ionic reticular degradation pathways. Consequently, meticulous preparation and purification processes are essential to ensure the removal of potential contaminants, making sodium alginate safe for biomedical use. ²

Reference

1. Frent OD, Vicas LG, Duteanu N, et al. Sodium Alginate-Natural Microencapsulation Material of Polymeric Microparticles. Int J Mol Sci. 2022; 23(20): 12108.

2. Batista PS, de Morais AM, Pintado ME, de Morais RC. Extracellular Sugar-Based Biopolymers Matrices. Springer; Berlin/Heidelberg, Germany: 2019. Alginate: Pharmaceutical and Medical Applications; 649–691.

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