Tailoring the Microstructures of Biocoatings using Coagulant Dipping and Wet Sintering by Immersion

Biocoatings, which confine living bacteria within a colloidal polymer layer, have applications including carbon capture and biofuel production. There is a need to increase the porosity of biocoatings to raise the permeability of small molecule metabolites, thereby increasing the viability and chemical yield of the confined bacteria. 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. To overcome the problems posed by the desiccation of bacteria and the poor mechanical stability of biocoatings, we have proposed a new method of film formation. We use glassy polymer colloids 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. We observed, via scanning electron microscopy, an unusual coarsening process in which sub-micrometer voids grow within the coating. We discovered that the resultant microstructure can be tuned via the temperature of wet sintering and the time of immersion. As a model system, E. coli bacteria were found to be more viable in coagulant-dipped biocoatings than in desiccated ones.