Abstracts

Rationale: Conductivities of the neural tissue medium surrounding neuromodulating depth lead contacts are among the critical variables influencing the volume of neural activation during direct cortical stimulation therapy. Increasing the extent of activation in white matter by 1mm can significantly increase the volume of neural activation. Reducing the tissue impedance (Z) adjacent to the electrode contact can theoretically increase the extent of neural activation. Our study demonstrated the effect of fluorescein-thiosemicarbazide-functionalized metallic carbon nanotubes (fCNT) on Z, cellular uptake and biocompatibility.Electrical impedance spectroscopy (EIS) was carried out to evaluate (1) fCNT-induced conductivity (s) changes in a 0.6% agarose gel and (2) electric field magnitude (E) changes in response to s alterations, in vitro. (3) Biocompatibility was assessed using a fCNT cytotoxicity assay. (4) In situ immunofluorescence (IF) was performed after injection of fCNT in the hippocampal formation (HF) of F344 rats to evaluate cellular uptake. Finally, (5) s and propagation velocities of action potentials were simulated using Hodgkin and Huxley (HH) cable modeling. Methods: Impedance Spectroscopy: Changes in s were assessed via EIS (100mHz-1MHz) with different fCNT concentrations diffused in 0.6% agarose gel around a cylindrical depth lead.E-Field Magnitude Changes in Response to Alterations in Conductivity of the Medium: A tissue-box containing a depth lead with 4 conductive contacts was modeled to assess how alterations in s of the medium affected E. The finite element method (FEM) was used to solve for the electric potential. Cytotoxicity Testing: Cortical human astrocytes (HA) were cultured and treated with raw and fCNTs at 90%, 95%, and 99% purities. Cell viability was measured after 72hrs via an alamar blue assay.Cellular Uptake Test: 99% pure fCNTs were injected into the left dentate gyrus of F344 rats in order to evaluate cellular uptake via IF. Astrocytes and neurons were labeled in fresh frozen sections via GFAP and anti-synaptophysin. fCNT clusters were localized with confocal microscopy.Cable Modeling: A computational HH impulse-propagation cable model was used to analyze the effect of fCNT-induced s changes in the axonal intracellular medium. Changes in action potential production and propagation velocity were analyzed. Results: Z measurements showed statistically significant differences between the fCNT samples and controls (p< 0.05).  The cytotoxicity assay of 99% pure raw CNTs (25 ng/mL) and fCNTs demonstrated no statistically significant difference in HA viability. Hippocampal IF staining in F344 rats showed fCNT clustering 4 weeks after direct injection in the hippocampus, suggesting cellular co-localization and uptake near the injection point. E was sensitive to changes in s, as s decreased within a region, E increased. The HH model predicted intracellular s changes enhanced axon excitability and propagation velocity. Conclusions: Injecting putatively biocompatible fCNTs directly into a neural tissue medium can alter s, effectively altering the excitability of neural tissue and augmenting the volume of cortical activation. Such a strategy can also extend and shape a depth lead’s surface area available for direct brain stimulation therapy. Funding: Mary Keane Fund, Foglia Family Foundation, CONACYT