## 3 Results and Discussion
### Characterization of BNC/\(\kappa\)-Car Composites
#### 3.2.4 Water Holding Capacity, Water Retention Rate, and Swelling Properties  
Water holding capacity, water retention rate and swelling properties are very important parameters with regard to medical applications of biomaterials. They decide of water diffusion in membrane and thus the ability of transporting the nutrients and active compounds throughout the biomaterial. The high water retention capacity enables to maintain the moist environment that is crucial for the proper cell growth and favorable interactions with the environment in tissue engineering applications. The results of WHC measurements for the pure BNC and composites with \(\kappa\)-Car are shown in Figure 5. The highest water content was observed for BNC and BNC/\(\kappa\)-Car\({}_{0.2}\) (it reached 137 and 133 g water/g of their dry mass, respectively). However, we observed a decline in WHC for higher concentrations of \(\kappa\)-Car (0.5 and 0.8%). WHC was reduced to 109 gwater/g\({}_{\text{dry mass}}\) in case of BNC/\(\kappa\)-Car\({}_{0.8}\). WHC level is a consequence of structural arrangements of fibrils in BNC network and it is affected by the size of pores, crystallinity index, density, and width of fibers. The addition of gelling agents such as carrageenan or agar to the culture medium resulted in the changed cellulose network consisting of ribbon-like wide fibers [48]. The contact surface was decreased and less water molecules could be hold within the BNC structure [49]. On the other hand, the presumable reason of the reduced WHC in the case of BNC/\(\kappa\)-Car\({}_{2.0}\) and BNC/\(\kappa\)-Car\({}_{4.0}\) (117 and 123 gwater/g\({}_{\text{dry mass}}\)- respectively) was the presence of \(\kappa\)-Car aggregates, especially in surface layers, which filled the empty spaces between fibers. \begin{table}
\begin{tabular}{c c} \hline \hline
**Sample** & **CI (\%)** \\ \hline BNC & 87.6 \\ BNC/\(\kappa\)-Car\({}_{0.2}\) & 85.7 \\ BNC/\(\kappa\)-Car\({}_{0.5}\) & 85.3 \\ BNC/\(\kappa\)-Car\({}_{0.8}\) & 85.0 \\ BNC/\(\kappa\)-Car\({}_{2.0}\) & 79.8 \\ BNC/\(\kappa\)-Car\({}_{4.0}\) & 77.4 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Crystallinity index (CI) of BNC/\(\kappa\)-Car composites. Figure 4: XRD diffraction patterns of BNC/\(\kappa\)-Car composites. The swelling ability of hydrogels is essential in tissue engineering and drug delivery applications. During the process of swelling, the structure of the porous materials changes due to the growing distance between the fibers. The absorbed water molecules are directed to free spaces, fill them, and form hydrogen bonds with chains of polymers. Both BNC and \(\kappa\)-Car are excellent water absorbents, and therefore they have found numerous medical applications. However, their swelling profiles are different. Pure \(\kappa\)-Car, which can absorb 11.28 g water per 1 g of its dry mass, swells rapidly and the gel reaches the equilibrium state in 60 min (Figure 11). BNC swells more slowly and binds 87.75 \(\mathrm{S_{Water}}/\mathrm{S_{dry\;mass}}\), what is 7.8 times more compared to \(\kappa\)-Car. Water completely fills pores in BNC membranes within 3 h (Figure 6). In the case of BNC/\(\kappa\)-Car\({}_{0.2}\), BNC/\(\kappa\)-Car\({}_{0.5}\), and BNC/\(\kappa\)-Car\({}_{0.8}\), the amount of absorbed water decreased when the concentration of \(\kappa\)-Car in the culture medium increased (79.13, 64.26, and 53.27 \(\mathrm{S_{Water}}/\mathrm{S_{dry\;mass}}\), respectively). These results were consistent with the data obtained for BNC and pure \(\kappa\)-Car. The swelling ratio of BNC/\(\kappa\)-Car composites reflected both the water binding capacity of each of the two biopolymers and their percentages in the composites. The swelling profiles for BNC/\(\kappa\)-Car\({}_{2.0}\) and BNC/\(\kappa\)-Car\({}_{4.0}\) were different compared to the other composites. The plateau was achieved after 36 and 48 h, respectively.