Cable Bacteria—Electrifying Electronic Biological Materials with Record Intrinsic Electrical Properties—From Transistors to Billie Eilish

Cable bacteria, a group of filamentous electroactive bacteria, have developed a unique energy metabolism and parallel fibre structures demonstrating electron transport for conduction lengths up to 1 cm and with fibre conductivities exceeding 10 S/cm. Conduction measurements carried out in high vacuum excluded the possibility of ionic conduction, but the fundamental charge transport mechanisms remain unknown. The observed electron transport in cable bacteria over distances in the order of centimeters is remarkable in the biological world.<br/><br/>Cable bacteria as ‘living electrical wires’ are of fundamental interest to better understand the underlying biological processes, but are also potentially interesting as alternative organic electronic materials for the emerging field of bioelectronics as <i>e.g.</i> biocompatible electrical connections and circuits, conductive composite materials, (nano-) sensors, transistors,... In order to investigate the intrinsic electrical properties and underlying transport mechanisms, our approach is to study them with a range of macroscopic and nanoscale solid state electrical characterisation techniques, in combination with structural and analytical techniques ranging from SEM to ToF-SIMS.<br/><br/>The electrical circuitry in a single cable bacterium is visualized with nanoscopic resolution using conductive atomic force microscopy (C-AFM). Combined with perturbation experiments, it is demonstrated that electrical currents are conveyed through a parallel network of conductive fibers embedded in the cell envelope, which are electrically interconnected between adjacent cells. This structural organization provides a fail-safe electrical network for long-distance electron transport in these filamentous microorganisms.<br/><br/>Impedance spectroscopy provides an equivalent electrical circuit model, which demonstrates that dry cable bacteria filaments function as resistive biological wires. Temperature-dependent electrical characterization reveals that the conductivity can be described with an Arrhenius-type relation over a broad temperature range (-195°C to +50°C), demonstrating that charge transport is thermally activated with a low activation energy of 40-50 meV. Furthermore, cable bacterium filaments can be utilized as the channel in a field-effect transistor.<br/><br/>The measured intrinsic electrical properties result very similar to for instance organic semiconductors, situating them in the context of electrical functional materials between semiconductors and conductors. These electrifying ‘bad guy’ microorganisms – with from various perspectives behave as they are not supposed to – are attracting growing interest and in the long-term could open novel avenues in emerging domains such as bioelectronics, biodegradable electronics and electronic biological materials (e-biologics). As a proof-of-principle for electrical transmission capabilities through cable bacteria filaments, the music track “Bad Guy” of Billie Eilish is transported through them. Cable Bacteria are rock & roll.