Investigation of coagulation process of wet-spun sodium alginate polymannuronate fibers with varied functionality using organic coagulants and cross-linkers

Alginate fibers composed of mannuronate blocks were manufactured via a wet spinning process by varying organic coagulants and cross-linkers. As cross-linkers, both ionic (Ca²⁺, Ba²⁺, and Al³⁺) and covalent (citric acid) were used, and as coagulants, dimethyl sulfoxide (DMSO), dimethylformamide (DMF), and tetrahydrofuran (THF) were selected. Fiber properties depended on the ionic size, valence, and chemical structure of the cross-linkers. Among the used cross-linkers, Al³⁺ being the smallest ion, could diffuse inside the polymer solution easily and form metal-carboxylate coordination with three mannuronate chains. However, Ca²⁺ and Ba²⁺ both were very large compared to it, and polymer chains were positioned at about 48° for Ca²⁺ and in a parallel manner for Ba²⁺ after forming inter and intrachain metal-carboxylate bonds. Citric acid underwent an esterification reaction with the hydroxyl groups of mannuronate chains. Coagulants stabilized the cross-linking process and formed hydrogen bonds with the polymer chains. Depending on the cross-linkers and coagulants, the fiber diameter ranged from 217 μm to 830 μm and tensile modulus ranged from 88 MPa to 2 MPa. Ca²⁺, and then Ba²⁺ ions were more effective as cross-linkers since they produced fibers with superior mechanical properties followed by citric acid and Al³⁺. For all cross-linkers except citric acid, when DMSO was used as a coagulant, the tensile modulus was the highest. This indicates that DMSO better stabilized the cross-linking process during coagulation. In phosphate buffer saline (PBS), only Ba²⁺ cross-linked fibers could retain their original structure for 24 h, and fiber formed with coagulant DMSO swelled the least because of its compact structure. It also lost the least percentage of weight during 6 weeks. Thermal properties of the samples were also different as Ba²⁺ and Ca²⁺ cross-linked samples were more resistive to high temperature than other samples. 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay illustrated the non-cytotoxicity of all the manufactured fibers.