NANOscientific

In the rapidly evolving field of bioelectronics, one of the grand challenges lies in bridging the material divide between biology and electronics. Our current technological landscape is dominated by rigid, inorganic semiconductors, while biological systems operate through soft, ionic mechanisms. Prof. Tobias Cramer of the University of Bologna addresses this critical disjunction through a fascinating study of organic mixed ionic electronic conductors (OMIECs) and a pioneering AFM-based technique that enables the quantitative imaging of electroswelling—a key electromechanical response in these materials.

This innovative work, presented at the 2024 NanoScientific Forum Europe (NSFE), provides deep insights into the electromechanical dynamics of OMIECs, particularly focusing on materials such as PEDOT:PSS and polypyrrole doped with dodecylbenzenesulfonate (PPy:DBS). These insights carry significant implications for designing bioelectronic interfaces, actuators, and organic electrochemical transistors (OECTs).

The Biological-Inorganic Divide
The human brain processes information through ions and neurotransmitters in a soft, aqueous environment. In contrast, modern electronics rely on electrons moving through solid-state semiconductors. Attempts to interface these disparate systems—such as using metal electrodes to connect neurons with circuits—often result in functional limitations and long-term biocompatibility issues. The search for materials that can mimic biological ionic transport while maintaining electronic conductivity is at the heart of OMIEC research.

OMIECs, such as PEDOT:PSS, offer a unique duality. They consist of a π-conjugated polymer network that allows electronic charge transport, embedded in a polyanionic matrix that supports ionic conduction. This dual functionality enables these polymers to interact intimately with biological systems while still interfacing with traditional electronic devices.

The Phenomenon of Electroswelling
When OMIECs undergo electrochemical doping (oxidation or reduction), ions and water are inserted or removed from the polymer matrix, leading to a volumetric change—an effect known as electroswelling. This phenomenon has been exploited in applications ranging from soft actuators to neural probes. However, electroswelling can also be problematic, leading to mechanical strain and delamination in thin films used in transistors and sensors.

Understanding the dynamics of electroswelling—its speed, magnitude, and underlying mechanisms—is essential for engineering OMIEC-based devices with greater reliability and performance. Traditional measurement techniques, however, fail to resolve these processes at the nanoscale with sufficient spatial and temporal resolution.

A New AFM Paradigm: mEC-AFM
To tackle these challenges, Prof. Cramer’s team developed a novel modulated ElectroChemical Atomic Force Microscopy (mEC-AFM) approach. By integrating the principles of electrochemical AFM with those of electrochemical strain microscopy, the method introduces an AC voltage modulation to microelectrodes coated with OMIEC films. This enables simultaneous acquisition of:
• Ionic current responses
• Height modulation (swelling amplitude)
• Phase shifts of mechanical deformation

These nanoscale measurements are achieved using PinPoint mode AFM, ideal for soft polymer films where conventional contact mode could damage the sample.

One of the key advantages of mEC-AFM is its ability to perform multidimensional spectroscopy—tracking the electromechanical response over a wide frequency range and film thicknesses.

The results can be visualized spatially (as images) and spectroscopically (as frequency-dependent plots), offering a uniquely holistic view of OMIEC behavior.

Key Findings in PEDOT:PSS

Using PEDOT:PSS as a model system, the group demonstrated that the swelling response scales linearly with the injected ionic charge—regardless of film thickness, frequency, or position. The electroswelling coefficient they extracted was approximately 0.08 nm per nanoCoulomb of charge, correlating directly with the hydration volume of sodium ions.

This linearity and consistency across experimental conditions pointed to a simple yet powerful model: the swelling is governed by the volume of hydrated cations entering the polymer matrix. Additional experiments with polyanions confirmed that swelling is indeed due to cation ingress and not anion egress.

Interestingly, the hydration radius measured inside the film was about 50% smaller than in free water, attributed to coordination of the ions with the polyanionic PSS matrix. This reduced hydration reflects the constrained environment within the polymer and provides a window into the microscopic interactions driving electromechanical behavior.

Complex Dynamics in Polypyrrole (PPy:DBS)

While PEDOT:PSS presents relatively stable and predictable electroswelling, other OMIECs like PPy:DBS exhibit more dramatic and complex responses. Thicker PPy:DBS films (up to 3 μm) showed electroswelling coefficients nearly 10 times higher than PEDOT:PSS, with a strong dependence on both thickness and electrochemical potential.

To unravel these effects, Prof. Cramer's team employed advanced impedance modeling using the Transfer Line Method (TLM). This technique models the film as a 1D transmission line of resistors and capacitors, allowing the reconstruction of subsurface swelling and charge profiles from spectroscopic data.

The analysis revealed that in reduced films, most swelling occurs deep within the bulk, whereas in oxidized states, swelling is confined to near the surface. These findings suggest that swelling is not merely a surface phenomenon but a result of dynamic microstructural rearrangements within the film—shifts that are essential for optimizing actuation performance.

Mapping Signal Propagation

In another extension of their work, the team applied mEC-AFM to study signal propagation along OMIEC channels.

In these experiments a modulated AC signal is injected from one side into the OMIEC channel to generate a combined excitation of potential and swelling.

The resulting electrochemical strain waves propagate along the channel and their local amplitude and phase shift is measured with the AFM experiment.

From these findings, the team could directly determine the phase velocity, that is the speed at which combined ionic/electronic signals propagate along thin OMIEC channels.

Interestingly, it was found that the speed is slower than signal propagation along biological axons, underscoring the limitations of synthetic mixed conductors.

This velocity was found to depend primarily on electronic mobility and ionic capacitance of the material. Notably, highly ionic hydrogels, while biocompatible, tended to slow down signal transmission due to excessive ion loading.

Implications and Outlook

The implications of Prof. Cramer’s research are both foundational and practical. His mEC-AFM technique opens a new window into the electromechanical behavior of soft, multifunctional materials, offering unprecedented resolution and interpretability.

The work also advances our understanding of how ions and electrons coexist and interact in confined, hydrated environments—insights critical to improving bioelectronic devices, sensors, and soft actuators.

Moreover, the research redefines how we view “material performance” in biointerfaces—not just in terms of conductivity or swelling, but in the spatiotemporal orchestration of ionic and electronic processes.

As artificial tissues, neural prosthetics, and wearable electronics become increasingly viable, tools like mEC-AFM will be indispensable in guiding their design and ensuring their compatibility with the body’s delicate electrochemical symphony.

Prof. Cramer and his team at the University of Bologna have thus illuminated a crucial frontier—where physics meets physiology, and where the future of bioelectronics takes shape not only through molecules but through movement.