Pullulan Coating Preserves High Conductivity in Cable Bacteria Wires

The greening of electronics remains a grand societal challenge, with no radical improvement within sight. Sustainable solutions for electronics, such as biobased and transient materials, are hence receiving growing attention. Presently, there are no biobased alternatives to conventional conductors such as metals and organic polymers, as their conductivity is too low. The discovery of cable bacteria, which are filamentous microorganisms capable of conducting electricity over centimeter-scale distances, has the potential to change this. In cable bacteria, conductivity occurs through thin wires embedded in the cell envelope, displaying conductivities comparable to those of the best highly doped organic polymers. However, exposure to ambient air leads to a gradual loss of their conductivity. To enhance stability, a bioderived protective coating could be useful, thus retaining a fully biobased system. To this end, we investigated pullulan, a polysaccharide polymer primarily used in food packaging that is known for its excellent oxygen-barrier properties. Cable bacterium filaments protected with a film derived from a 10 wt % pullulan solution exhibited a 10-fold increase in conduction stability under ambient conditions compared to uncoated controls. Reducing ambient moisture also preserved the long-term conductivity of the cable bacteria, even in the absence of a protective coating, indicating that humidity plays a critical role in conductance deterioration. Our findings provide an important step toward further technological implementation of the highly conductive wires of cable bacteria and offer practical guidelines for developing biobased coatings for O₂-sensitive materials in electronics, thus contributing to the advancement of next-generation green technologies.