Abstracts

Rationale: Currently, only two devices have been approved by the Food and Drug Administration for the treatment of epilepsy via neural stimulation, vagal nerve stimulation (VNS) or Responsive Neuro-Stimulation (RNS). Although both systems can interface with larger neural networks, the electrodes' area of activation only extends 4mm from the surface of the stimulating electrode. Extending the electrode's area of influence by at least one millimeter would increase this volume by over 95%. One method for increasing the extent of activation is to enhance the conductivity and reduce the impedance of the brain adjacent to the electrode. Our work demonstrates the influence of carbon nanotubes (CNTs) over the electrical impedance and potential neuromodulation safety implications. Methods: A 0.6% agarose gel phantom which replicates both the conductivity and porosity of the brain tissue was cast around a pair of 8-contact Spencer Probe depth electrodes (Ad-Tech) spaced 10mm center to center. A 4V peak-to-peak alternating current (AC) sinusoidal waveform was applied to the agarose gel phantom. Voltage and current responses were measured with a 100MHz bandwidth 1GS/s Rigol digital oscilloscope. The observed differences in amplitude and phase between the voltage and current waveforms where recorded and used to compute the impedance relative to frequency. A frequency response analysis (FRA) was then performed by testing impedance at 200 different frequencies through a logarithmic sweep from 50Hz to 1MHz. The results were recorded and analyzed via a custom LabVIEW program. FRAs where performed for 3 independent pairs of contacts to obtain a control dataset, and the results were averaged. For calculating the influence of CNTs, nine 2L injections of 95% pure metallic FITC-functionalized CNTs at a concentration 0.25 mg/mL were delivered in a grid-like pattern creating a conductive CNT cloud (C3) between 3 independent pairs of testing contacts. One FRA was performed for each independent pair of contacts. A student's paired t-test was performed (a=0.05) to determine if a difference in mean existed between the baseline (without CNTs) relative to the test condition (with CNTs) over the frequency range of 50Hz to 1KHz. Results: A student's paired t-test showed a statistically significant difference in impedance magnitude (p < 0.00001) and phase (p < 0.001) when CNTs were injected. Conclusions: Metallic-type CNTs could serve as a way to enhance the extent of activation by an electrode via direct neurostimulation by reducing the brain's local impedance. Results demonstrated a statistical significant decrease in impedance magnitude when injecting nanotubes into a brain phantom. Reducing impedance directly translates to potentially neuromodulating the maximum extent of an epileptogenic circuit. Additionally, reduced local impedance could enable the delivery of larger amounts of current while maintaining safety parameters due to the increased effective surface area of the C3. Funding: 1. Mary Keane Fund 2. RUSH University Medical Center