Hydrogels based on natural biopolymers such as alginate and cellulose represent an important area of research due to their non-toxicity, biocompatibility, biodegradability, renewability, tunable properties, and wide range of applications. Depending on the desired application of the hydrogel, its mechanical properties, electrical conductivity, water content, smart properties (the hydrogel's response to external stimuli), and structural and surface properties can be modified. In this master's thesis, I focused on studying the mechanical and flow properties and electrical conductivity of hydrogels. The mechanical properties can be adjusted through cross-linking by changing the type and altering the concentration of the cross-linking agent, by adding nanomaterials, or by combining different polymers or biopolymers. Calcium ions are commonly used crosslinkers, especially in alginate hydrogels, as they enable immediate and reversible crosslinking. Electrical conductivity depends on the presence of conductive nanomaterials, electrolytes and the mobility of ions within the hydrogel network. Graphene nanoplatelets (GNPs) are excellent additives for improving the rheological properties and conductivity of the material, enabling the development of hydrogels for biomedical and technological applications. As part of my master's thesis, I studied the effect of different concentrations of graphene nanoplatelets and calcium ions on the mechanical properties and electrical conductivity of hydrogels. For this purpose, alginate and cellulose hydrogels with varying concentrations of graphene nanoplatelets and calcium ions were prepared. The preparation was carried out under controlled conditions, ensuring homogeneous dispersion of the nanoplatelets in the hydrogel network. The mechanical properties were determined using a rheometer, and the shear viscosity, as well as elastic and viscous shear modulus were analyzed using frequency and current tests. The obtained results were processed using Maxwell and Cross rheological models. Electrical conductivity was determined by measuring the electric current and voltage with an ohmmeter using the two-probe method. The results confirm that the final properties of hydrogels are not determined by individual components but are the result from a precise balance between calcium ion concentration, GNP content, and network microstructure. Optimal mechanical and conductive properties are achieved at optimal, rather than maximum, concentrations of both components. Calcium ions establish the basic hydrogel network, while GNP further strengthens and structures it at sufficient concentrations.
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