A simple relation between pendant groups of polymers in hydrogels is introduced to determine the crosslink density of (complex) hydrogel systems (mixtures of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) modified nanocellulose, alginate, scleroglucan and Laponite in addition of crosslinking agents). Furthermore, the rheological properties and their great potential connection to design complex hydrogel systems with desired properties have been thoroughly investigated. Hydrogel structures governing internal friction and flow resistance were described by the predominant effect of ionic, hydrogen, and electrostatic interactions. The relationship between rheological properties and polymer-polymer interactions in the hydrogel network is explained and expressed in a new mathematical model for determining the crosslink density of (crosslinked) hydrogels based on single or mixture of polymer systems. In the end, the combined used of rheology and low field nuclear magnetic resonance spectroscopy (LF-NMR) for the characterization of hydrogel networks is developed.

Controlled release technology has a great potential in pharmaceutical and medical applications to ensure high efficacy of treatment, reduces the aggressive action of the medicines per patient, decreases the cost of treatment and reduces the side effects of the drug as well. In this research, hydrogels from biopolymers were designed for potential use in the drug release systems. The main objective was the manipulation of alginate and 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) - oxidized cellulose nanofibers hydrogels crosslinking density by changing the biopolymer and crosslinker concentrations. Rheological measurements of prepared hydrogels were performed to determine the viscosity as a biomedical applicability factor and for determining shear modulus as a basis for theoretical mesh size calculations. The homogeneity of the hydrogel was confirmed by NMR verifying the validity of the mesh size calculations at the same time. In the last stage, the improved mathematical model was developed taking into account the concentration of crosslinker and the concentration of biopolymer in hydrogel as well. The designed model is the first step for the preparation of hydrogels with specific properties.