In situ visualization of the tensile deformation mechanism of bacterial cellulose network

Bacterial cellulose (BC) pellicles are strong hydrogels composed of nanofibril networks. These hydrogels are considered attractive materials for synthetic biology, in which biological systems or modules are designed with user-defined functions. To develop BC-based materials with tailored mechanical properties, elucidation of the tensile deformation mechanism is essential. Therefore, in this study, BC hydrogels were fluorescently labeled, and the fiber network under tensile deformation was observed in situ using a device for simultaneous confocal laser scanning microscopy and uniaxial tensile deformation. As a result, strain-dependent deformation modes were identified and the generation of stress paths (stress-loaded fiber segments) during deformation was visualized. Furthermore, characteristic relaxation spectra of the nanofiber network were obtained from stress-relaxation measurements, revealing the existence of a first-order relaxation mode at approximately 1 s and higher-order relaxation modes over a long time period of 10²-10⁵ s. On this basis, we proposed a tensile deformation model of the BC hydrogel characterized by rearrangements of fiber segments accompanied by cleavage of cross-links. This model is expected to facilitate synthetic biology using BC hydrogels.