Hydrogels are three-dimensional crosslinked networks of hydrophilic polymers. They can absorb large amounts of water or biological fluids, which causes them to swell while maintaining their 3D structure without dissolving. The ionic crosslinking of alginate with cations involves the formation of “egg-box” structures, where cations coordinate between adjacent alginate chains. The type and valence of the cations strongly influence the properties of alginate hydrogels. Divalent cations (Ca$^{2+}$, Ba$^{2+}$, Sr$^{2+}$) and trivalent cations (Fe$^{3+}$) effectively crosslink alginate chains through coordination with carboxyl and hydroxyl groups. The release of an active compound from hydrogels occurs via diffusion, which depends on the structure and material properties. The key factors influencing release include pore size, interactions between the active substance and the polymer network, and the degree of crosslinking. In my master’s thesis, I prepared 2% wt. alginate hydrogels using Ca$^{2+}$, Sr$^{2+}$, Ba$^{2+}$, Cu$^{2+}$ and Fe$^{3+}$ ions at a concentration of 0.25%. I investigated their combinations in ratios of 25/75, 50/50 and 75/25. The aim of the study was to examine the influence of these cations on the mechanical properties of the hydrogels and on the release rate of glucose. First, I determined the mechanical properties of the samples using a rheometer. Based on these measurements, I used the generalized Maxwell model to calculate the shear modulus, which was then used to determine the crosslinking density. Using the Peppas–Merrill equation, I subsequently estimated the average pore size within the hydrogel network and the corresponding diffusion coefficients of glucose in the aqueous medium. In addition to mechanical characterization, I also performed glucose release tests in water. I measured the concentration of released glucose during the first 60 minutes at 10-minute intervals, followed by 30-minute intervals during the next hour. UV–Vis spectroscopy was used to determine glucose concentration, and then I calculated the diffusion coefficients. The results were compared with the values calculated from rheological measurements. The findings of the study demonstrate that combinations of ionic crosslinkers do not follow a principle of linear averaging, but instead produce pronounced synergistic or antisynergistic effects that influence both mechanical behavior and diffusion processes. Ion mixtures can mechanically strengthen or loosen the polymer network, thereby increasing or decreasing glucose diffusion. The results confirm that by combining different ionic crosslinkers, it is possible to precisely control stiffness, crosslinking density, pore size, and consequently the release rate of active compounds.
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