Ionotropic Crosslinking of Alginate as a Pathway to Versatile Gels for Diverse Applications

abstract

English

Hydrogels are soft materials composed of a three-dimensional polymer network capable of retaining large amounts of water. They are characterized by biocompatibility, low toxicity, and tunable properties, which makes them particularly suitable for use in medicine—such as wound dressings, drug carriers, or supportive structures for cell growth. One of the most commonly used gelling polymers is sodium alginate, a natural polysaccharide derived from brown seaweed. Its ability to easily react with divalent or trivalent cations, especially Ca²⁺, allows for rapid gel formation without the need for toxic crosslinking agents. However, the external gelation method, which relies on the diffusion of ions into the solution, often results in gels with an inhomogeneous structure—featuring a densely crosslinked surface and a less crosslinked core. Here, we present an optimized method of internal ionic gelation of alginate that enables homogeneous crosslinking throughout the entire volume. It employs a combination of calcium carbonate (CaCO₃) and D-glucono-δ-lactone (GDL), which gradually hydrolyses, lowers the pH, and allows for the controlled release of Ca²⁺ ions. This results in uniform network formation without diffusion gradients. An additional advantage of this method is the ability to control gelation time and the final pH by adjusting the CaCO₃:GDL ratio. In the experimental part, gels were prepared using various molar ratios of CaCO₃:GDL, and their gelation behaviour, pH, and mechanical properties were analysed. Increasing the amount of GDL accelerated gelation and lowered the pH, which often led to bubble formation. At low GDL-to-carbonate ratios, gelation was slow or incomplete due to insufficient acidification. Higher calcium carbonate concentrations improved the mechanical strength of the gels, but also increased syneresis, the process of water release from the polymer network, which can lead to volume loss and structural changes over time. Based on these results, the optimal CaCO₃:GDL molar ratio was determined to be in the range of 1:1.25 to 1:1.75, which ensures the formation of clear, homogeneous gels with nearly neutral pH (6.7–7.7), minimal bubble formation, and high mechanical stability. We have implemented an optimized internal gelation method for the development of multicomponent hydrogels tailored for modern applications, particularly in nerve and muscle tissue regeneration. The developed electroactive, soft, and biocompatible hydrogel—based on alginate, BSA, and the conductive polymer PEDOT:PSS—can provide mechanical support, conform to the application site, and enable electrical stimulation of cells, thereby promoting their growth and differentiation. Thanks to controlled gelation, the hydrogel can be applied directly to damaged tissue, where it adapts to the shape and subsequently crosslinks. This property is advantageous for the development of smart wound dressings and implantable materials for advanced applications in tissue engineering and regenerative medicine. In a second application, ionotropic alginate crosslinking was utilized for the development of agricultural bioinoculants. This approach leverages the self-entrapment of the plant growth-promoting bacterium Azotobacter vinelandii in hydrogels formed by crosslinking alginate produced directly by the bacterium. Notably, the natural presence of CaCO3 in the cultivation medium is advantageously employed to facilitate this process.

hydrogel, sodium alginate, internal ionic gelation, regenerative medicine, bioinoculants

KOUŘILOVÁ, L.; ŠČOTKOVÁ, R.; SMILEK, J.; FOHLEROVÁ, Z.; SEDLÁČEK, P.