Electrochemical Evaluation of a Multi-Site Clinical Depth Recording Electrode for Monitoring Cerebral Tissue Oxygen

Abstract

The intracranial measurement of local cerebral tissue oxygen levels-PbtO₂-has become a useful tool for the critical care unit to investigate severe trauma and ischemia injury in patients. Our preliminary work in animal models supports the hypothesis that multi-site depth electrode recording of PbtO₂ may give surgeons and critical care providers needed information about brain viability and the capacity for better recovery. Here, we present a surface morphology characterization and an electrochemical evaluation of the analytical properties toward oxygen detection of an FDA-approved, commercially available, clinical grade depth recording electrode comprising 12 Pt recording contacts. We found that the surface of the recording sites is composed of a thin film of smooth Pt and that the electrochemical behavior evaluated by cyclic voltammetry in acidic and neutral electrolyte is typical of polycrystalline Pt surface. The smoothness of the Pt surface was further corroborated by determination of the electrochemical active surface, confirming a roughness factor of 0.9. At an optimal working potential of -0.6 V vs. Ag/AgCl, the sensor displayed suitable values of sensitivity and limit of detection for in vivo PbtO₂ measurements. Based on the reported catalytical properties of Pt toward the electroreduction reaction of O₂, we propose that these probes could be repurposed for multisite monitoring of PbtO₂ in vivo in the human brain.

Keywords: brain tissue oxygen; in vivo monitoring; multi-site clinical depth electrode.

Figure 1

(A) General view of a recording site on the Auragen^TM Probe; (B) and (C) High-resolution micrographs of the recording surface; (D) EDS elemental analysis of the surface (ROI indicated in inset with orange rectangle); (E) Different regions of the recording surface were targeted for EDS elemental analysis, indicated with red and yellow “+” signs, respectively; (F) and (G) show the elemental composition spectrum of the lighter (conductive) regions (predominantly Pt) and of the darker (non-conductive) region (predominantly Al and O), respectively.

Figure 2

Pseudo-color map of the relative distribution of different elements on the EDS analyzed surface. Blue—platinum; green—aluminum; red—carbon, yellow—oxygen. Pt and Al distribution are complementary. Note that Al and O overlap, suggesting Al-O as a substrate for the Pt overcoat. Uniform distribution of C is in line with contamination.

Figure 3

Reversible redox reaction of Ru(III)(NH₃)₆ in 0.5 M KCl at increasing scan rates (from 20 mV s⁻¹ in black to 200 mV s⁻¹ in green) and respective Ip vs. ν^1/2 plot for determination of the electrochemical surface area of the depth electrode recording site.

Figure 4

Electrochemical behavior in acidic and neutral electrolyte media. (A) Successive cyclic voltammograms (25^th scan) at increasing scan rates (50−1000 mV s⁻¹) obtained in N₂ saturated 0.1 M H₂SO₄, detailing the typical Pt oxide formation and reduction, proton adsorption (2 peaks) and desorption (3 peaks), and double layer zones. (B) Comparative CV plots (0.2 V s⁻¹) recorded in N₂-saturated 0.05 M, pH 7.4 PBS (black line) and N₂-saturated 0.1 M, H₂SO₄ (red line) highlighting the positive shift in hydrogen evolution potential and increasing currents for Pt-oxide formation and reduction at lower pH on the Pt surface of the Integra Probe recording site.

Figure 5

Electrochemical impedance spectroscopy measurements obtained in K₄[Fe(II) (CN)₆]/K₃[Fe(III)(CN)₆] (5 mM) in KCl 0.5 M. (A) Impedance−frequency plot (Bode plot). Filled circles represent |Z| values, and open circles are those obtained for the phase shift. The red circle highlights the |Z| value at 1 kHz. (B) Complex plane electrochemical impedance spectrum (Nyquist plot) of experimental data (open circles). Red line shows fitting to the electrical equivalent circuit shown in the inset. R1, solution resistance; R2, electron or charge transfer resistance; W, Warburg impedance element; Q, constant phase element.

Figure 5

Electrochemical impedance spectroscopy measurements obtained in K₄[Fe(II) (CN)₆]/K₃[Fe(III)(CN)₆] (5 mM) in KCl 0.5 M. (A) Impedance−frequency plot (Bode plot). Filled circles represent |Z| values, and open circles are those obtained for the phase shift. The red circle highlights the |Z| value at 1 kHz. (B) Complex plane electrochemical impedance spectrum (Nyquist plot) of experimental data (open circles). Red line shows fitting to the electrical equivalent circuit shown in the inset. R1, solution resistance; R2, electron or charge transfer resistance; W, Warburg impedance element; Q, constant phase element.

Figure 6

Electrochemical behavior of O₂ reduction at the depth probe surface. (A) Average sensitivity as a function of the applied potential, obtained from calibration of 4 sites in PBS. (B) Average baseline current as a function of the applied potential, obtained in N₂-purged PBS. (C) Representative calibration obtained at −0.6 V vs. Ag/AgCl and the calibration curve (inset) of a single recording site of the Integra probe. n = 4.

Figure 6

Electrochemical behavior of O₂ reduction at the depth probe surface. (A) Average sensitivity as a function of the applied potential, obtained from calibration of 4 sites in PBS. (B) Average baseline current as a function of the applied potential, obtained in N₂-purged PBS. (C) Representative calibration obtained at −0.6 V vs. Ag/AgCl and the calibration curve (inset) of a single recording site of the Integra probe. n = 4.