A simple electrical method shows how protein nanowires carry both electrons and ions, offering a clearer way to design soft, tunable materials for body-compatible electronics.
(Nanowerk Spotlight) Electronic devices and living systems process information in fundamentally different ways. Microchips move electrons through metal circuits. Cells move ions, charged atoms like sodium or potassium, across protein-lined membranes. One system relies on fast electronic flow. The other on slow, tightly regulated chemical signals. For technologies designed to interact with biology, such as sensors embedded in tissue or devices that track internal states, this mismatch creates a foundational problem. Traditional electronics are fast but rigid, dry, and unable to communicate in the language used by living systems.
To function at this interface, materials must do more than carry electrons. They must also support the motion of ions. Ideally, they should be soft, biologically compatible, and tunable at the molecular level. Proteins offer all of this. They are biodegradable, can be produced using genetic instructions, and can organize themselves into precise structures. But most proteins are poor conductors. And even when they show some ability to carry charge, it remains unclear whether that charge moves as electrons or as ions.
This lack of clarity has made it difficult to engineer better protein-based materials. Standard measurement methods often detect total conductivity without distinguishing between different types of charge carriers. As a result, researchers cannot say with confidence how a protein film conducts or how changes to its structure or environment affect that behavior. For those trying to build electronic systems that work inside or alongside the body, that uncertainty has real consequences.
A study published in Advanced Materials (“Mixed Ionic and Electronic Charge Transport in Conductive Protein Fibers Revealed with DC Electrical Measurements”) at McGill University introduces a method that addresses this problem directly. Using time-resolved direct current electrical measurements and a microstructured electrode design, the researchers measured how charge flows through different protein nanowires. They found clear and measurable differences between ionic and electronic contributions. Their work provides a tool for exploring and improving protein-based materials that operate at the interface between biology and electronics.
The study focuses on three protein systems. The first is the M13 bacteriophage, a virus composed entirely of protein that assembles into long filaments. It has been used widely as a molecular scaffold but has not been well characterized for electrical transport. The second is a modified form of curli fibers, which are bacterial proteins that naturally form amyloid-like structures. These were engineered to contain aromatic amino acids that can support electron movement. The third is a type of protein nanowire produced by expressing Geobacter pilus genes in E. coli. These fibers are known to support long-range electron transport and serve as a benchmark for comparison.
How the design of interdigitated electrodes improves measurements of M13 phage films. A) Diagram and photo of the microfabricated electrodes. B) Electrical tests on dried phage films show a non-linear response at 50% humidity. C) When voltage is applied, current starts high and then gradually levels off to a steady value. D) The steady current increases in a straight-line relationship with voltage. E) A simplified circuit model explains the behavior, including resistance from the contacts, resistance from the film itself, and a capacitor effect created by the electrode design. (Image: Reprinted from DOI:10.1002/adma.202507906, CC BY) (click on image to enlarge)
To study conductivity, the researchers created microelectrodes with fine interlocking fingers. The small scale of the electrode spacing increases the current signal from low-conductivity films. Each protein material was cast as a thin film across the electrode array. A constant voltage was applied and the resulting current was measured over time. This time-resolved measurement captures both an initial decay in current and a stable plateau. The early decay corresponds to ionic motion, where charged species slowly migrate or redistribute in response to the electric field. The steady-state current reflects electronic conduction, where electrons move through molecular structures.
By analyzing both phases of the current, the researchers were able to extract separate contributions from ionic and electronic transport. They used a simplified electrical circuit model to describe the system. This model includes contact resistance between the protein and the electrode, resistance from the protein film itself, and a capacitance term that reflects the ability of the material to store charge. From the time-dependent current response, they calculated these parameters for each protein sample.
The measurements showed that e-PN nanowires, derived from Geobacter proteins, conduct mainly through electronic transport. Their behavior remained stable across different humidity levels. In contrast, both curli fibers and M13 bacteriophage films showed mixed conduction, with significant ionic contributions that increased with humidity. This suggests that water plays a key role in enabling ion transport in these materials, while the intrinsic electronic conduction remains low.
To confirm the role of water and ionic species, the researchers systematically changed the humidity around the samples. They found that higher humidity increased both the transient and steady-state currents in curli and M13 samples. This was attributed to greater ionic mobility and better hydrogen bonding networks, which can assist both ionic and electronic movement. The e-PN samples, however, were largely unaffected by these changes, reinforcing their identity as primarily electronic conductors.
The study also examined the impact of residual processing additives on conductivity. During purification, M13 bacteriophage preparations often retain sodium chloride and polyethylene glycol. To control for this, the team dialyzed the samples to remove these additives, then added them back in controlled amounts. Adding salt increased the transient current without affecting the steady-state value, indicating enhanced ionic conduction. Adding polyethylene glycol increased both types of current, possibly by promoting water retention and facilitating hydrogen bonding that supports electron transport. These experiments helped clarify how external factors influence the two charge transport modes.
From these data, the team calculated conductivity values in the range of ten to the minus eleven siemens per centimeter for M13 and curli films under dry conditions. For e-PN, the value was closer to ten to the minus eight. These differences were consistent with earlier studies but now could be interpreted more precisely, separating ionic and electronic behavior. Capacitance values also differed sharply between materials. The high capacitance of e-PN films suggests they can store charge more effectively, which could be relevant for applications that rely on signal retention or buffering.
The method developed here is notable for its simplicity and precision. It uses direct current measurements with standard equipment, yet provides detailed information about the nature of charge transport. It avoids the complexity and limitations of alternating current impedance spectroscopy or transistor-based gating systems. It is also compatible with small sample sizes and low-conductivity materials, making it suitable for screening engineered protein variants or testing environmental effects.
The findings also clarify structural factors that influence conductivity. Although the M13 phage and e-PN nanowires have similar dimensions and both contain aromatic residues, their conductivities differ by orders of magnitude. This may be due to differences in how aromatic side chains are spaced or oriented, or how the protein fibers pack together. The pVIII protein in M13, for example, has a gap between two key aromatic residues that may disrupt long-range electron hopping. In contrast, the pilA monomer used in e-PN forms a tighter and more continuous path for charge movement.
By combining structural insight with sensitive measurements, this study sets a foundation for rational design of protein-based electronic materials. The M13 phage, although not yet as conductive as e-PN, has many features that make it a promising platform. It is genetically modifiable, easy to produce at scale, and self-assembles into uniform filaments. With targeted mutations and structural refinements, it may be possible to enhance its electronic performance while preserving its biocompatibility and processability.
This method can also be extended beyond protein fibers. Any soft or hybrid material where electronic and ionic charges coexist could benefit from this type of measurement. That includes hydrogels, mixed conductors, and materials used in neural interfaces or implantable devices. The ability to distinguish charge types and quantify them precisely is critical for building systems that must operate in complex biological environments.
For now, the technique offers researchers a practical way to test and compare protein-based conductors with minimal instrumentation. It also opens a path toward systematic engineering of charge transport properties, informed by measurable parameters rather than indirect indicators. As protein materials move toward real applications in bioelectronics, this type of analysis will be essential.
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