Biocoatings, which confine living bacteria within a colloidal polymer layer, have great potential to be used to perform useful functions such as hydrogen production, carbon capture or nitrification. There are three main challenges to overcome to increase the viability and chemical yield of the confined bacteria: low porosity which reduces the permeability of small molecule metabolites, high osmotic stress and also cell dehydration which bacteria experience during the drying process of established coating methods. This thesis presents my research which studies techniques to increase the porosity and reduce osmotic stress and cell dehydration in the creation of biocoatings.

In the coagulant gelation method of coating, a substrate decorated with salts is dipped into a charge-stabilised colloidal dispersion. When the salts dissolve into solution, they destabilise the colloid to create a gel layer, which is then coalesced to make a continuous coating. In this thesis, I first explore the methods of coagulant gelation and the addition of clay nanotubes to obtain a porous microstructure. I find that although both techniques individually result in an increase in surface porosity (by factors of ×10^2 ), the combination of the two techniques acts to reduce the overall surface porosity.

I then showcase a promising new method of biocoating formation to overcome the problems posed by the desiccation of bacteria and the poor mechanical stability of biocoatings. I use glassy polymer colloids with a low solids content and achieve partial particle coalescence by immersing the gel layer in water at temperatures above the polymer’s glass transition temperature. Coalescence is driven by the reduction of the polymer/water interfacial area in a process called wet sintering. This process creates a porous glassy coating while completely avoiding a drying step. I observed, via scanning electron microscopy, an unusual coarsening process in which sub-micrometer voids grow within the coating. I discovered that the resultant microstructure can be tuned via the temperature of wet sintering and the time of immersion. I found that the void size grows proportional to the power of 0.3 of reduced time. Wet sintering by immersion was found to increase the water permeability, in comparison to a dry sintered non-gelled film, by a factor of ~ ×10, achieving the aims of increasing porosity whilst reducing osmotic stress.

This thesis presents research on the viability of bacteria trapped within coatings formed using this new method, using adenosine triphosphate (ATP) assays to study the viability. As a model system, E. coli bacteria were found to be more viable in biocoatings formed using the method of wet sintering by immersion than by dry and moist sintering. Encapsulated bacteria in biocoatings formed by wet sintering by immersion in water were found to have a lower viability than immersion in phosphate-buffered saline (PBS) solution, due to osmotic pressure from water during wet sintering decreasing the number of viable cells. Luria Broth (LB) was found to be the optimal liquid choice for immersion. The coagulant concentration could be tuned to ensure high viability, with a 0.2 M concentration of calcium nitrate tetra-hydrate resulting in the greatest viability. Keeping the coatings immersed after film formation was required to obtain the highest viability in comparison with drying the films in air and rehydrating.

In the final parts of this research, I explored the coating of 3D structures using coagulant gelation, including how this technique may be used to coat the inside of channels without plugging the channel. I also showcased how X-ray Computed Tomography (CT) may be used to image gel coatings within 3D structures.

This research showcased ways to overcome the current challenges of low porosity, high osmotic stress and cell dehydration in the creation of biocoatings. The findings of this research can be applied to a number of systems, for example biocoatings containing nitrifying or photosynthesising bacteria, which may be used to combat climate change. With genetic modification, the E. coli used as the model system in this research can produce hydrogen biofuels and potentially be used within biocoatings in a novel reactor design.