The multifactorial modification of the structure and properties of alginate matrix was conducted using partially acetylated cellulose nanocrystals. Fourier-transform infrared spectroscopy and thermogravimetric analysis indicated the absence of chemical interactions between the polymer matrix and the filler. The surface texture was examined using optical microscopy and scanning electron microscopy, along with a reconstruction of its 3D model. With an increase in the content of nanoparticles in the composite, the following was revealed. Firstly, the roughness and density of the arrangement of surface elements increased, while their size decreased. Secondly, at pH values < 7, the puncture resistance increased, whereas the swelling coefficient of the films decreased. In Hanks solutions, the low solubility of the films was established, as well as a higher swelling coefficient at pH > 7. Thirdly, the contribution of donor centers to the free surface energy, cytocompatibility of composite films, and adhesion of fibroblasts to the surface increased. The hematological tests of the composites showed a procoagulant effect. Summarizing the data, we propose a model that explains the influence of nanocrystals and their concentration on the formation of the observed composites’ structure and their physicochemical and biological properties. The main driving forces of structurization are the factor of the excluded volume and interactions in a heterogeneous colloidal system.

In this study, the effect of citrate-modified magnetite nanoparticle (MNP) concentration on the microstructure and properties of composite alginate-containing hydrogels and xerogels was investigated. Data from electron and optical microscopy, FTIR, STA, and mechanical testing demonstrated that MNP concentration directly influences both functional properties and microstructure. Adjusting this parameter allows fine-tuning of the morphology, shifting it from uniform dispersion to particle aggregation, and eventually to sedimentation-driven stratification. At 1 wt%, MNPs are incorporated into the interchain spaces of alginate, leading to partial disruption of the polymer network structure and a reduction in mechanical strength. At 5 wt% MNPs, maximum mechanical strength is achieved due to the formation of oriented clusters uniformly distributed throughout the sample, forming a rigid reinforcing framework. At 10 wt%, the formation of numerous smaller but denser clusters introduces stress concentration sites, leading to a significant decrease in mechanical strength. At 20 wt%, sedimentation of large magnetite clusters during hydrogel formation leads to stratification into a lower MNP-rich reinforcing layer and an upper polymer-rich elastic layer, which remain connected through the continuous alginate matrix, providing a combination of high strength and rigidity with preserved elasticity. All samples showed procoagulant activity, while higher MNP content enhanced fibroblast adhesion and proliferation relative to unmodified alginate. Strong UV–Vis absorption in MNP-containing composites points to their potential to shield bioactive components from unwanted photo-induced degradation. Regulation of structure driven by magnetite concentration opens prospects for the development of alginate matrices with tailored magnetic sensitivity, physicochemical properties, and cytohemocompatibility.