## 3 Results and Discussion
In this regard, an important step in the creation of any collagen material is the development of cross-linking conditions that provide mechanical strength and stability for the duration that the material is used. In this case, cross-linking agents were used for collagen fibers of various natures [23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34]. This work considers collagen with copolymers of acrylates; therefore, a non-traditional approach was used. Cross-linking agents for acrylate fragments, such as TEDMA and AA, were added into the initial reaction mixture to form an additional ionic \"crosslink\" in the composite after neutralization of the final product. The formation of cross-links was expected (Figure 2a,b). The synthesis of copolymer CCC-1 using photocatalysts was carried out similarly to that described earlier [10], by adding TEDMA and AA. The organic and aqueous phases were separated after synthesis. The aqueous phase consisted of a stable homogeneous solution of a white polymer product (Figure 3a). Purification of the grafted copolymer from AA and TEDMA residues was carried out by toluene extraction, and samples were dried in a vacuum. Insignificant amounts of organic polymer (less than 1%) were isolated from toluene extract after synthesis by precipitation with ethanol. The study of the nitrogen content in the polymer product, isolated and neutralized from the aqueous phase, indicated the formation of collagen along with acrylate graft copolymer formation (Figure 4). This value, as well as that of the collagen content, were close to that of a copolymer without cross-linking additives. The results of the elemental analysis did not change after washing the copolymer with chloroform in the Soxlet extractor. Figure 1: Schemes of different interactions between the hydroxyl radical and the substrate during photocatalytic polymerization. Figure 3: Photographs of (**a**) CCC-1 solution and (**b**) CCC-3 gel. Figure 2: Scheme of (**a**) covalent cross-linking involving TEDMA; and (**b**) ionic cross-linking in a composite with acrylic acid fragments after neutralization of the final product. The most interesting information can be gathered from the SEM results of CCC-1 (Figure 5c,d) in comparison to the CC (Figure 5a) and PMMA-CC graft copolymers (Figure 5b). It is likely that, due to ionic cross-linking (Figure 2b) during the polymer solution neutralization and to the use of TEDMA, a matrix with a fine cellular structure was formed. Examples of such cross-linking have been described previously in the literature [24; 26]. At high magnification, collagen fibers densified by the grafted synthetic fragment can be observed (Figure 5d). The CCC-1 copolymer in water is a homogeneous phase, as shown in Figure 3a, and no coagulation occurs when an acidic solution is neutralized by alkali. At the same time, it was not possible to estimate the molecular weight parameters of the resulting CCC-1 copolymer, because it was retained on a filter with a pore size of 0.45 \(\upmu\)m during the preparation of the polymer solution for GPC analysis. The filtered aqueous phase contained CCC-1 (\(\sim\)98% of the polymer product in solution), which was not integrated into the matrix, and low-molecular-weight collagen with MM \(\sim\)10 kDa and \(\sim\)20 kDa. In the next series of experiments, the CC content was increased in order to suppress the formation of a synthetic polymer. In the case of the copolymer CCC-2, after synthesis, the organic phase was absent. This is extremely important because the process did not require an organic phase separation step. The aqueous phase, as in the previous experiment, consisted of a stable homogeneous solution of a white polymer product. The increase in CC in the reaction mixture led to a slight increase in the nitrogen content in the polymer in comparison to the first synthesis, as well as a corresponding decrease in the content of the synthetic fragment in the polymer. The organic polymer was not isolated from toluene extract after synthesis by precipitation with ethanol.