Biological Nanofibres from Electroactive Microorganisms Towards e-Biologics

In nature, biological ‘nanofibers/nanowires’ in electroactive microorganisms such as <i>Geobacter sulfurreducens</i>, <i>Shewenalla oneidensis</i> and the more recently discovered <i>Cable Bacteria</i> demonstrate remarkable electrical transport properties. These electroactive bacteria are receiving growing attention from diverse research fields, motivated by a fundamental interest in the underlying long-range transport mechanisms and in the potential future role in emerging domains such as bio-electronics, biodegradable electronics, and electronic biological materials (e-biologics). In the long-term these materials could open novel avenues for the growing problem of electronic waste in the upcoming era of ubiquitous electronics (cfr. the <i>Internet of Things</i>). Moreover, the emergence of biological electronic materials could play a role next to synthetic organic electronic materials in the diverse “<i>More than Moore”</i> technology platforms for future electronic applications, where new materials and heterogeneous integration technologies are indispensable for future breakthroughs and towards next generation sustainable electronics. The fundamental study of the electro-optical properties of biological nanofibers from Cable Bacteria and other electroactive microorganisms, and their behaviour in bio-hybrid electronic devices can therefore be situated in this broader technological context.<br/><br/>Cable Bacteria are filamentous microorganisms consisting of more than 10⁴ cells, forming unbranched filaments of up to several centimeters long, characterized by a distinct morphology with parallel ridges along the length of the filament. They have developed a unique energy metabolism which requires charge transport over centimeter distances. Electrical measurements at X-LAB on ‘dry’ cable bacteria filaments out of their wet natural aquatic habitat provided the first direct evidence of intrinsic electrical conductivity through the use of electrical probe measurements, with nanofiber conductivity values over 10 S/cm and an exceptionally long electron transport distance in the order of 1 cm. While nanometer-scale electron transport is known to occur in prokaryotes, chloroplasts and mitochondria, and micrometer-scale electrical currents are measured in the nanowire appendages of bacteria and archaea, the centimeter-scale electron transport by cable bacteria extends the known length scale of biological transport by several orders of magnitude.<br/><br/>To disclose the underlying electrical transport mechanisms occurring in Cable Bacteria several techniques are being introduced. Impedance spectroscopy provides an equivalent electrical circuit model, which indicate that dry Cable Bacteria filaments function as resistive biological wires. Recently, proof-of-principle biohybrid demonstrators have been prepared to investigate the electrical signal transmission possibilities in a broad frequency range. 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) indicative for hopping transport. Furthermore, when cable bacterium filaments are utilized as the channel in a field effect transistor, they show n type transport with electron mobility values of ~ 0.1 cm²/Vs at room temperature and display a similar Arrhenius temperature dependence as the earlier mentioned conductivity.<br/>Overall, the obtained results so far demonstrate that the intrinsic electrical properties of the conductive fibres in Cable Bacteria are comparable to synthetic organic semiconductor materials, and so they offer promising perspectives for both fundamental studies of biological long-range electron transport as well as alternative organic electronic materials for the emerging field of bioelectronics, biohybrid electronics and for visionary technologies such as biodegradable electronics and other <i>“More than Moore”</i> technology platforms.