Development of Leaf-Inspired Functional Structures from Cellulose Nanofibers - Matrix Scaffolds for Solid-State Photosynthetic Cell Factories

This thesis pioneered an interdisciplinary investigation of the use of cellulose nanofibers as immobilization matrices for photosynthetic cells. This was achieved by exploiting the inherent properties of 2,2,6,6-tetramethylpiperidine-1-oxyl radical oxidized cellulose nanofibers (TCNF) to create hydrogel matrix structures with high mechanical stability, porosity, and biological compatibility in aqueous conditions. Moreover, this thesis developed analytical tools to reliably assess and optimize the crucial physical parameters of the hydrogel matrices and link them to the performance of the immobilized cells. Ultimately, the goal was to create tailored matrix structures that enhance the efficiency of photosynthetic cell factory platforms producing volatile chemicals. By screening different matrix structures based on TCNF and alginate as a reference, it was found that TCNF cross-linked with Ca2+ -ions and polyvinyl alcohol (PVA) yielded hydrogel matrices with high wet strength and good compatibility with photosynthetic cells. By establishing a surface-sensitive investigation setup using quartz crystal microbalance with dissipation monitoring (QCM-D), this thesis revealed that the photosynthetic cells are passively entrapped within the TCNF network, but direct attachment can be induced via a cationic surface treatment. Furthermore, this thesis developed a rheological measurement technique to study the behavior of the hydrogel matrices under shear stresses. It was found that alginate-based matrices have higher elastic and viscous moduli (G' and G'', respectively) at rest than TCNF-based matrices, but the latter have higher resistance to yielding. These properties can be explained through the innate structural differences between the materials and their interactions with Ca2+ -ions. Following a similar trend, TCNF-based matrices were discovered to have higher porosity and a more heterogeneous structure than Ca-alginate matrices. They also form hierarchical mesoporous networks with small additions of PVA or alginate. Finally, these results were combined with biological characterization techniques to assess how the matrix properties affect cell factory performance. TCNF and alginate matrices were observed to provide long-term cell viability to immobilized cells. The O2/CO2 exchange rates of the immobilized cells indicated that both the cells and matrices are in a dynamic state over time, affected by both matrix porosity and wet strength. Finally, TCNF matrices were shown to be more stable and enable higher hydrogen and ethylene production in submerged conditions. To gain further control in optimizing these properties, a method to prepare TCNF hydrogels with accurate porosity-density control was developed using osmotic dehydration. Overall, these findings highlight the prowess of TCNF-based hydrogels as versatile im-mobilization scaffolds and showcase how the development of efficient cell factory platforms can be assisted via interdisciplinary efforts.