On the microstructural anisotropy and mechanical properties of bacterial nanocellulose obtained by static culture

The microstructural organization of bacterial nanocellulose is still not completely understood. Although its morphological and mechanical anisotropy is commonly recognized, it remains unclear whether the forces stabilizing the cellulose fiber network are comparable in different planes. In this study, we characterized the microstructure of statically produced BNC according to its horizontal and vertical planes. The tensile mechanical response in both planes was also investigated and complemented by finite element modeling. The intrinsic 3D anisotropy of BNC microstructure, obtained by static culture, was demonstrated by scanning electron microscopy, even without a stratified layered microstructure. A mechanism based on swelling-induced stretching of BNC in the vertical direction is suggested to explain the observed fiber reorganization during membrane growth: high morphological anisotropy of the fibers in the upper surface partially dissipates towards depth. Moreover, the distinct tensile mechanical response observed for both planes (Young's modulus: 1.9 MPa horizontal and 0.9 MPa vertical) supports the 3D anisotropy of static cultured BNC. However, the similar maximum tensile strength (0.39-0.41 MPa) suggests that fiber networking and cohesion are similar in orthogonal orientations. The similar cross-linking density in both planes (5.8-6 × 10⁸ counts/mm³) determined by stimulated emission depletion microscopy suggests the same conclusion.