Poster B7 - High-Performance Magnetically Actuated Mxene-Based Microelectrodes for Neural Interfacing

Friday, October 25, 2024

10:00 AM - 11:00 AM EST

Location: Exhibit Hall E, F & G

Presenting Author(s)

Brayden Davis

Undergraduate Research Assistant
The Joint Department of Biomedical Engineering at The University Of North Carolina and North Carolina State University
Chapel hill, North Carolina, United States

Primary Investigator(s)

Wubin bai, PhD (he/him/his)

Assistant Professor
Applied Physical Sciences
Chapel Hill, North Carolina, United States

Introduction: Neural interfaces offering bidirectional communication with targeted tissue are crucial in developing treatments and studying neurological diseases.1–4 Intraneural electrodes penetrate neural tissue for neuron-level interfacing. In contrast, epineural electrodes, applied to the surface of neural tissue, are generally preferred for therapeutic applications due to their noninvasive nature.1,3 However, creating a reliable and efficient nerve-electrode connection remains challenging for these interfaces.2

For optimal signal acquisition and stimulation, epineural electrodes must demonstrate superior electrochemical attributes, including low interfacial impedance spectrums, substantial charge storage capacities, high charge injection limits, and excellent biocompatibility.1,5 Flexible electrodes enhance signal fidelity by conforming to target tissues and decreasing contact impedance.5 However, epineural electrode attachment methods, such as wrapping or mechanical wedging, cause long-term tissue deformations, potentially leading to harmful immune responses.6–10

Recent research has explored using soft magnetic robotics for adaptable neural interface placement, focusing on their utility in navigation. However, magnetic actuation's electrochemical benefits remain unexplored - specifically, its potential to enhance conformational contact to decrease contact impedance.11–13 Integrating materials that demonstrate superior electrochemical attributes into these soft magnetic interfaces is crucial for advancing these modalities. Mxene (Mx), particularly Ti3C2Tx, offers superior charge injection limits and electrochemical impedance spectrums over traditional materials like gold and carbon-based electrodes.14,15 However, integrating Mx onto an elastomeric substrate often requires elaborate fabrication methods.16 This work aims to develop a simple materials integration platform to deposit Mx onto a magnetic elastomer, characterize the resulting materials' electrochemical performance, and elucidate the electrochemical utility of magnetic actuation in epineural interfacing.

Materials and

Methods: To fabricate the electrodes, a solution of Mx and poly(3,4-ethylene dioxythiophene):polystyrene sulfonate (PEDOT:PSS) was drop-cast on a glass slide, dried, and annealed to create a composite film (MxP). A mixture of Fe3O4 microparticles and polydimethylsiloxane was then drop-cast atop the MxP film. The electrode trace was then etched using laser ablation, and the electrodes were insulated using a shadow mask method. The resulting electrodes are displayed in Figure 1.
For electrochemical characterizations, a three-electrode setup in phosphate-buffered saline (PBS), with an Ag/AgCl reference electrode, a platinum counter electrode, and an MxP working electrode, was utilized . Cyclic voltammetry was used to investigate MxP's charge storage capacity and electrochemical water window. Voltage transient measurements using a biphasic current square wave were used to assess the neural interface's charge injection limit. Electrochemical impedance spectroscopy measurements, ranging from 1 Hz to 1 MHz, were conducted over multiple days on an MxP electrode immersed in PBS at 37°C to evaluate its interfacial impedance spectrum stability.
An electromechanical bending fatigue test was used to evaluate MxP's mechanical robustness compared to Mx. The change in initial resistance across each film was tracked over approximately 8000 cycles. To validate the robust attachment of the interface due to actuation, an in vitro dynamic contact impedance test was conducted. This test utilized a gelatin-based spinal cord phantom tissue immersed in PBS to mimic the spinal cord environment. A stimulation was applied and the voltage across the phantom was analyzed and compared between an actuated and non-actuated electrode.

Results, Conclusions, and Discussions: The results indicate that despite MxP containing approximately 40% wt PEDOT:PSS, there was no statistically significant difference in interfacial impedance spectrums between pure Mx electrodes and MxP electrodes across the frequency range of 1 Hz to 1 MHz, giving insight into MxP’s self-assembly characteristic (Figure 2a, 2d). Further elucidating MxP's self-assembly phenomenon, the energy dispersive spectroscopy results demonstrate a higher concentration of titanium on the side of the film used for the electrochemical characterizations (Figure 3). Furthermore, the charge injection limits of Mx and MxP were statistically similar, indicating comparable stimulation performance between the two materials, consistent with findings by Driscoll et al.17 Initial results revealed that MxP demonstrated enhanced stability over pure Mx films, with 1 kHz impedance measurements showing greater consistency over multiple days when immersed in PBS at 37°C. This stability suggests that the composite self-assembly method significantly enhances Mx's stability, offering insight into a potential solution for stabilizing Mx for chronic stimulation applications.17

Additionally, while MxP exhibited similar electrochemical performance to pure Mx, it seemingly demonstrated superior electromechanical performance. In preliminary trials, MxP films lasted 1.2 times longer in bending cycles than pure Mx films, which is comparable to what was found by Liu et al. in a similar composite film (Figure 3g).18 These results, in combination with the electrochemical performance of MxP, suggest that MxP is an ideal candidate for integration into soft robotic interfaces for adaptable neural interfacing applications. Furthermore, an increased voltage amplitude applied to the phantom spinal cord model with magnetic actuation was also observed, indicating enhanced contact impedance on the phantom (Figure 4e). These preliminary results suggest that magnetic actuation can improve the electrochemical performance of neural interfaces. However, further characterization is necessary to understand the specific enhancements magnetic actuation provides fully.
In summary, MxP matches pure Mx in electrochemical performance but offers improved electromechanical properties and stability, making it more suitable for advanced applications in adaptable neural interfacing. The potential benefits of magnetic actuation in these contexts warrant additional investigation to elucidate its full impact on electrochemical performance.

Acknowledgements (Optional): References
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