Electrical Impedance Properties of Deep Brain Stimulation Electrodes during Long-Term In-Vivo Stimulation in the Parkinson Model of the Rat - Biomedical Engineering Systems and Technologies - page 267

2.3 Electric Impedance Spectroscopy (EIS)

Electric impedance spectroscopy (EIS) is a common measuring technique for deter-
mining the electrical properties of tissues [4]. It is used in a wide range of applica-
tions, such as breast cancer detection [10], the monitoring of the lung volume [1] and
in material sciences. EIS is nondestructive and therefore suitable for the characteriza-
tion of the DBS electrodes during the encapsulation process.
Equipment. The EIS measurements were conducted with an impedance spectrometer
Sciospec ISX3 (Sciospec Scientific Instruments, Pausitz, Germany) and a test fixture
HP16047D (Hewlett-Packard, Japan) connected to a personal computer with Scios-
pec-measuring software. The two connectors of the test fixture were connected to the
DBS and the counter-electrodes.
Measurement. To characterize the electrode properties during encapsulation, the
impedance was recorded in the frequency range from 100 Hz to 10 MHz over a period
of two weeks after implantation. Frequency range, amplitude, number of points and
the averaging of the impedance spectrometer were programmed by the measuring-
software (Sciospec). 401 frequency points were logged which were distributed equi-
distantly over a logarithmic frequency scale. The measuring voltage (peak to peak)
was 12.5 mV PP . The measuring-software logged the measuring data of the impedance
spectrometer (real and imaginary parts of the impedance vs. frequency) by saving
them as a data file.
Before each measurement, the impedance spectrometer was calibrated by open,
short and load measurements. Each measurement was repeated three times to improve
the statistical significance. The measurements were repeated every day for one week
and every second day during the second week.
The stimulation pulse usually applied in DBS has a frequency of 130 Hz and a
pulse width of 60 µs. Because of the steep slopes of the needle-shaped pulse, the sig-
nal is rich in high harmonic frequencies [5]. For this reason, we measured the imped-
ance within the wide frequency range from 100 Hz to 10 MHz, which is beyond the
range of up to 10 kHz reported by Lempka et al. [12].
Impedance Theory. The electrical impedance describes the magnitude ratio between
the applied AC voltage and the resulting current flowing with a certain phase shift.
Mathematically speaking, the impedance Z* is a complex number with the unit [Ω],
which is composed of a r eal (Z´) and an orthogonal imaginary part (Z´´) marked by
the complex unit j = √ 1 :
Z* = Re(Z*) + j ·Im(Z*) = Z´+ j Z´´ (1)

For interpretation of the measuring data, an equivalent circuit model is required to be
fitted to the measuring data. The aim was to model electrochemical processes and
adherent cell growths by combinations of resistors, capacitors and constant phase
elements [12].
Data Analysis. The logged data were transferred to Matlab (The MathWorks™, Ver-
sion 7.9.0.529) to calculate means and standard deviations. For their graphic represen-
tation, they were finally copied to Sigma Plot 11.0 (Systat Software, 11.0, Build
11.2.0.5).