Taking into account the trends in the field of green chemistry and the desire to use natural materials in biomedical applications, (bio)polyelectrolyte complexes ((bio)PECs) based on a mixture of chitosan and gelatin seem to be relevant systems. Using the approach of self-assembly from the dispersion of the coacervate phase of a (bio)PEC at different ratios of ionized functional groups of chitosan and gelatin (z), hydrogels with increased resistance to mechanical deformations and resorption in liquid media were obtained in this work in comparison to a hydrogel from gelatin. It was found that at z ≥ 1 a four-fold increase in the elastic modulus of the hydrogel occurred in comparison to a hydrogel based on gelatin. It was shown that hydrogels at z ≈ 1 had an increased sorption capacity and water sorption rate, as well as increased resistance to the in vitro model environment of phosphate-buffered saline (PBS) solution containing lysozyme at 37 °C. It was also shown that in PBS and simulated gastric fluid (SGF) solutions, the effect of the polyelectrolyte swelling of the hydrogels was significantly suppressed; however, at z ≥ 1, the (bio)PEC hydrogels had increased stability compared to the samples at z < 1 and based on gelatin.

In this work, using small-angle X-ray scattering from synchrotron radiation, the macromolecular structure of chitosan and gelatin polyelectrolytes and their mixtures at various pH values and ratios was studied to determine the size and shape of primary supramolecular (bio)PEC. Analysis of the scattering profiles of the initial solutions of chitosan and gelatin with the building of the pair distance function showed the formation of single-modal distributions with a maximum molecular size of 46 and 32.2 nm, respectively. Ab initio reconstruction of the macromolecule’s shape showed the formation of objects shaped like an oblate spheroid. In mixtures of chitosan and gelatin at a pH below the isoelectric point, it was found that the scattering structures correspond to the initial biopolymers. However, it is observed that values of the aspect ratio at a ratio above 1:10 gradually increase, which indicates a slight elongation of the average particle and indirectly indicates the formation of dissipative structures of (bio)PEC. In mixtures at a pH above the isoelectric point, it was shown that at ratios above 1:5, the formation of primary supramolecular complexes is observed, which is accompanied by an increase in zero-scattering intensity by about three times, maximum molecular size by two to two-and-a-half times relative to the initial polymers, and the formation of elongated structures corresponding to the cylinder (swollen spiral). It may be a consequence of the increased efficiency of the polyelectrolyte associative interaction between chitosan and gelatin.