Interfibre Bridging in Bacterial Nanocellulose via Co-Culture-Derived Polyhydroxybutyrate and Solvent-Free Blending Approaches

Bacterial nanocellulose (BNC) is a renewable polymer valued for its strength and purity, but its brittleness and hydrophilicity limit wider application. Incorporating biodegradable polyester polyhydroxybutyrate (PHB) offers a pathway to functional, scalable composites. We establish two complementary routes for producing bacterial nanocellulose-polyhydroxybutyrate composites. In-situ co-cultures of Komagataeibacter rhaeticus (KR) and Cupriavidus necator (CN) were optimised through inoculation timing, medium screening, and pH buffering. 2-(N-morpholino)ethanesulfonic acid (MES) at 50 mM stabilised culture conditions, improved cellulose output, and enabled PHB co-localisation of 4% total wet weight. These natural incorporation levels provided benchmarks for a solvent-free blending strategy, in which powdered PHB was introduced into plasticised sterilised BNC using Gellan gum, Glycerol, PEG400, and CaCl₂ at loadings of 0.1%, 0.3%, 0.7%, and 2.0% (approximately 10%, 30%, 70%, and 200% relative to dry BNC mass) and heat pressed. Blended films reproduced co-culture PHB levels and tolerated up to 0.7% (wet weight) before shrinkage and brittleness were observed. Heat pressing promoted PHB diffusion between cellulose fibrils, enhancing interfibre bonding; in blended films at 0.3 % PHB (heat-pressed), this yielded a 6.3-fold increase in ultimate tensile strength and a 9.5-fold increase in Young's modulus. Co-culturing defined the biological starting point, while blending enabled scalable processing and systematic characterisation, offering complementary routes to manufacture BNC-PHB composites.