ÌÇÐÄVlog

Biocoatings are typically colloidal polymer films confining metabolically active, nongrowing bacteria. Depending on the species of confined bacteria, biocoatings find applications in wastewater treatment, biofuel production, carbon fixation, environmental remediation, biosensing, and more. However, the successful use of biocoatings faces numerous challenges, including a low permeability to reactants and metabolized products, osmotic stress on bacteria during drying of the coatings, and cell dehydration leading to bacteria death. Here, we address these challenges through two interlinked processing methods. (1) Coagulant gelation of the colloidal polymer dispersion creates a porous microstructure with high permeability. (2) Wet sintering by immersion in a liquid medium reduces osmotic stress and avoids desiccation of the bacteria. In a model system of Escherichia coli in an acrylic copolymer latex biocoating, these two methods yielded a cell viability that is approximately 500 times greater than the conventional method of biocoating formation using dry sintering in air at an elevated temperature. We have discovered that when lysogeny broth is used as the medium for wet sintering, the cell viability is significantly higher than that for other liquids. Increasing the salt concentration for coagulant gelation leads to thicker coatings (and hence more cells per area of the coating). However, cell viability decreases when the salt concentration is increased, so a compromise is needed. Metabolic activity of E. coli in a wet-sintered biocoating was demonstrated through the production of ethanol as a biofuel. These results hold promise for the future exploitation of biocoatings using a broad range of bacterial, especially desiccation-intolerant species.

Environmental regulations and consumer demand are driving the need for packaging materials to reduce or eliminate the use of petroleum-based plastics by replacing them with natural and degradable alternatives. Water flux through a barrier layer is governed by the differential in water activity between a high side (e.g., an aqueous solution) and a low side, such as humid air. We propose that a water-absorbent layer sandwiched between two barrier layers will act as a sponge and locally raise the water activity. It will thereby lower the activity differential across the first barrier layer and, hence, reduce the water flux through the multilayer for a set time period. We present a theoretical model that predicts the water flux through a multilayered structure of two or more barrier layers sandwiching hydrophilic layers that hold water according to an absorption isotherm. We use this model to evaluate the effects of layer thicknesses and distributions, the barrier permeability, the water-holding capacity, and initial water activity of the absorbent layers. We found that the water mass loss rate decreases when the thickness and water-holding capacity of the absorbent layer increase. The greatest reduction in the mass loss through a multilayer was achieved when the absorbent layer had an initial water activity of 0 (fully dried). The relative thicknesses of the barrier layers in the multilayer also have an impact on the water loss rate; thicker barrier layers on the low-activity side of the multilayer are the most effective in reducing the water flux. We have verified the model in an ideal system. We also investigated a multilayer structure of a waterborne emulsion polymer barrier coating and water-absorbing chitosan, which is a deacetylated form of chitin. Here, the chitosan layer offered little benefit in decreasing the water permeability because the layers were not thick enough and the permeability of the waterborne coating on its own is not sufficiently low. With our design concept for multilayer barriers containing absorbent layers and the underpinning theoretical model, we envisage materials systems with enhanced barrier properties while also using less petrochemically derived plastic.

•Thin, maltodextrin films were decorated with ink-jet printed insoluble cocoa butter patterns.•The heterogeneity Area Coverage (AC) affects the spreading dynamics of water droplets.•Area Coverage of 8% doubles the spreading time, while AC 26 % trebles it. AC < 1 % shows no significant effect.•Heterogeneities can affect wetting more than the molecular mass of the soluble substrate.•These results suggest surface design criteria to quantitatively control wetting. Recent studies have highlighted the complex mechanisms governing the spreading of a solvent onto a homogeneous soluble film, such as water on soluble polysaccharides. The presence of surface fat slows down the wetting of spray dried food powders, but this phenomenon is not yet understood quantitatively. In this study, surface heterogeneities were created by the ink-jet printing of cocoa butter onto water-soluble maltodextrin thin films to produce hydrophobic deposits with a range of area coverages. The spreading dynamics of water was studied controlling relative humidity. Area coverages above 1.2 % were found to decrease the contact line speed and increase the contact angle. The contact line was deformed by the deposits of cocoa butter, causing in some cases periodic decelerations followed by accelerations. Area coverages above 26 % led to a three-fold increase in the spreading time. These novel insights could help to design soluble heterogeneous surfaces meeting a desired wetting performance.

Bacteria are used in a range of sectors, such as wastewater treatment, bioremediation, or as soil additives. For these applications, live bacteria are encapsulated to protect them from mechanical damage and desiccation. Unlike other types of cargo, bacteria are not always required to be released because when encapsulated, they can interface with their environment and fulfill their roles via molecular transport through the capsule walls. The aims of encapsulation are then shifted away from delaying release to making capsules that are mechanically robust while permitting sufficient diffusion to support the metabolic activity of the bacteria. Here, we produced covalent hydrogel capsules from a water-in-oil (W/O) emulsion of aqueous poly(ethylene glycol) diacrylate (PEGDA) in hexadecane containing a UV-radical initiator. Upon initiation, PEGDA polymerization begins at the W/O interface to produce hydrogel capsules. We discovered three classes of capsule microstructures with differing levels of macroporosity that could be tailored by changing the polymerization conditions. Systematic investigations showed how the UV energy input and the PEGDA macromonomer concentration can be used to selectively create honeycomb, sponge like, or dense spherical capsules. To explain the sponge-like structure, we propose a capsule formation mechanism based on diffusion-limited aggregation of PEGDA microbeads. The structures resemble random-walk simulations of sticky beads and, furthermore, satisfy the theoretical volume fractions required for percolation. We successfully encapsulated live Mycobacterium smegmatis within the sponge structures, demonstrating biocompatibility. Importantly, the internal hydrogel microstructure allows the growth of bacteria. This mechanistic understanding is paramount for designing robust covalent capsules while optimizing porosity within hydrogel structures.