Characterization of Sulfated Alginate Hybrid Gels for Tissue Engineering
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Tissue engineering is a field aiming to replace damaged tissue while reducing the great need of organ donors the world is facing today. Alginates are linear co-polymers consisting of the two monosaccharides β-D-mannuronic acid (M), and its 5-epimer α-L-guluronic acid (G). They can form hydrogels, and are candidates for use in tissue engineering scaffolds. Alginate is readily available at a low cost, and its hydrogels meet requirements of scaffolds such as mechanical strength and good biocompatibility, but lack the ability to interact with cells and proteins. The aims of this study was to investigate properties of alginate/alginate-sulfate mixtures for tissue engineering applications. This included the gel strength, osmotic stability in physiological solutions, distribution in gels, and interactions with the growth factor FGF. Mixing of formamide and chlorosulfonic acid with alginate LF200S yielded, as shown by ICP-MS, sulfated alginate with degrees of sulfation (DS) depending on the concentration of chlorosulfonic acid, volume of reaction mixture and the solubility of the alginate. Use of SEC-MALLS revealed some depolymerization of the alginate during the sulfation process, and showed that sulfation of alginate increases its susceptibility to acid hydrolysis at 95\celsius\ and pH 5.6. Use of alginate/alginate-sulfate mixtures was shown by swelling studies to have a higher osmotic stability in a physiological solution than pure alginate-sulfate beads when comparing total sulfate content. Increased content of sulfate in the beads generally led to decreased stability. Some hybrid beads containing low proportions of alginate-sulfate showed a higher stability than beads made from pure alginate. Increased proportions of alginate-sulfate in mixtures led to decreasing gel strengths, as shown by a longitudinal compression test. The Young's modulus of the gels ranged from 310 kPa (135 kPa when corrected for syneresis) to 26 kPa (16 kPa), placing them in the same range as several tissues. Interactions between FGF and alginate-sulfate in solution increased in strength for an increasing DS or a higher proportion of alginate-sulfate in mixtures. Gel disks made from mixtures all released FGF gradually into the surrounding medium. Gels containing alginate-sulfate still retained some FGF after 12 days, in contrast to the pure alginate gels. The mechanical strength, ability to interact with FGF, osmotic stability and tunable distribution make alginate/alginate-sulfate mixtures possible candidates for tissue engineering applications.