Characterizing DBS Electrode Impedance

The impedance properties of clinical DBS electrodes (Model 3387, Medtronic Inc., Minnesota, USA) were measured in vivo following acute implantation in the cat brain and in vitro in a near-physiological saline electrolyte buffered with carbonate and phosphate at concentrations found in interstitial fluid (137 mM NaCl, 29 mM NaHCO₃, 1.7 mM Na₂HPO₄, and 0.7 mM NaH₂PO₄) infused with a mixture of gas (5% CO₂, 6% O₂, and 89% N₂) to maintain a pH of 7.4 (Cogan et al 2004 (link)). The clinical DBS electrode (Model 3387) includes a linear array of 4 cylindrical electrode contacts (1.5 mm length, 1.27 mm diameter, 5.98 mm² surface area) separated by insulating rings (1.5 mm length). Impedance was measured with sinusoidal currents at 41 frequencies evenly distributed on a log scale between 1 Hz and 10 kHz, and at five root-mean-square amplitudes of 0.01 mA, 0.02 mA, 0.05 mA, 0.1 mA and 0.2 mA. As well, the voltage transients generated by applying symmetrical biphasic square currents (200 μs per phase and 1 ms per phase) were measured at 9 amplitudes from 0.01 mA to 5 mA. The measurements were replicated three times at each amplitude level for each of the four electrode contacts.