Biocoatings: challenges to expanding the functionality of waterborne latex coatings by incorporating concentrated living microorganisms

Biocoatings concentrate living, nongrowing microbes in nanoporous adhesive polymer films. Any microbial activity or trait of interest can be intensified and stabilized in biocoatings. These films will dramatically expand the functionality of waterborne coatings. Many microbes contain enzyme systems which are unstable when purified. Therefore, thin polymer coatings of active microbes are a revolutionary approach to stabilize living cells as industrial or environmental biocatalysts. We have demonstrated that some microbes survive polymer film formation embedded in nontoxic adhesive waterborne binders by controlling formulation and drying. Biocoatings can be a single layer of randomly oriented microbes or highly structured multilayer films combining monolayers of different types of microbes on solid, porous, or flexible substrates. They can be formed by drawdown or ink-jet deposition, convective sedimentation assembly, dielectrophoresis, or coated onto or embedded within papers. Controlled drying generates nanoporous microstructure; the pores are filled with a carbohydrate glass which stabilizes the entrapped dehydrated microbes. When the coating is rehydrated, the carbohydrates diffuse out generating nanopores. The activity of biocoatings can be 100s of g L⁻¹ (coating volume) h⁻¹ stabilized for 100–1000s of hours, and therefore, they represent a new approach to process intensification (PI) using thin liquid film bioreactors. A current challenge is that many microbes being engineered as environmental, solar, or carbon recycling biocatalysts do not naturally survive film formation. The mechanisms of dehydration damage that occur during biocoating formulation, ambient drying, and during dry storage have begun to be studied. Critical to preserving microbe viability are minimizing osmotic stress, toxic monomers, biocides, and utilizing polymer chemistries that generate strong wet adhesion with arrested coalescence (nanoporosity). Therefore, controlling desiccation, drying rate/uniformity, and residual moisture are important. Optimization of biocoating activity can be affected at multiple stages—cellular engineering prior to coating (preadaptation), formulation, deposition (film thickness), film formation/drying (generates microstructure), dry storage (minimize metabolic activity), and rehydration. Gene induction (activation) leading to enzyme synthesis following rehydration has been demonstrated. However, little is known about gene regulation in nongrowing microbes. Challenges to optimizing biocoating activity include generating stable film porosity, strong wet adhesion, control of residual water content/form/distribution, and nondestructive measurement of entrapped microbe viability and activity. Indirect methods to measure viability include vital staining, enzyme activity, reporter genes, response to light, confocal fluorescent microscopy, and ATP content. Microbes containing stress-inducible reporter genes can be used to monitor cell stress during formulation, film formation, and drying. Future cellular engineering to optimize biocoatings includes desiccation tolerance, light reactivity (photoefficiency), response to oxidative stress, and cell surface-to-polymer or substrate adhesion. Preservation of microbial activity in waterborne coatings could lead to high intensity biocatalysts for environmental cleaning, gaseous carbon recycling, to produce H₂ or electricity from microbial fuel cells, delivery of probiotics, or for biosolar energy harvesting.