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Impedance detection of the electrical resistivity of the wound tissue around deep brain stimulation electrodes permits registration of the encapsulation process in a rat model

Kathrin Badstübner
Kathrin Badstübner
, 
Marco Stubbe
Marco Stubbe
, 
Thomas Kröger
Thomas Kröger
, 
Eilhard Mix
Eilhard Mix
 et 
Jan Gimsa
Jan Gimsa
   | 17 avr. 2017
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Article Category: Articles
Publié en ligne: 17 avr. 2017
Pages: 11 - 24
Reçu: 22 déc. 2016
DOI: https://doi.org/10.5617/jeb.4086
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© 2017 Kathrin Badstübner, Marco Stubbe, Thomas Kröger, Eilhard Mix, Jan Gimsa, published by Sciendo
This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 3.0 License.

Figure 1

Photographs of tips (left) and schematic drawings (right) of (A) a unipolar electrode (200 μm wire diameter) and (B) a bipolar electrode (125 μm wire diameter). The electrode shafts were insulated with 25 μm polyesterimide. All electrode tips were bared for 100 μm.
Photographs of tips (left) and schematic drawings (right) of (A) a unipolar electrode (200 μm wire diameter) and (B) a bipolar electrode (125 μm wire diameter). The electrode shafts were insulated with 25 μm polyesterimide. All electrode tips were bared for 100 μm.

Figure 2

Illustration of the impedance Z* of unipolar and bipolar electrodes measured in the calibration solution. For clarity, not all 401 measuring points per spectrum were plotted, and error bars are only provided as examples. Above 30 kHz, all error bars were smaller than the size of the measuring point symbols. (A) Complex plots illustrate the fitting procedure for RBulk using a straight line and a semicircle. For α, see equation (1). The plots indicate that CPE effects are pronounced in a wider frequency range in the spectrum of the bipolar electrode compared with the unipolar electrode. Measuring points for 2, 30, 240, and 1700 kHz are filled. (B) Plot of the real and imaginary parts of the impedance of the bipolar electrode over the frequency. In (A), the characteristic frequency fRC is located above the center of the semicircle. (C) Equivalent circuit of the electrode impedance and its surrounding medium. RBulk and CBulk describe the properties of the bulk medium in contact with the bared electrode contacts. Cadd summarizes additional capacitances in the measuring setup. The CPE describes the impedance of the electrode effects at the metal-medium interface. For the calibration solution, RBulk = RCal.
Illustration of the impedance Z* of unipolar and bipolar electrodes measured in the calibration solution. For clarity, not all 401 measuring points per spectrum were plotted, and error bars are only provided as examples. Above 30 kHz, all error bars were smaller than the size of the measuring point symbols. (A) Complex plots illustrate the fitting procedure for RBulk using a straight line and a semicircle. For α, see equation (1). The plots indicate that CPE effects are pronounced in a wider frequency range in the spectrum of the bipolar electrode compared with the unipolar electrode. Measuring points for 2, 30, 240, and 1700 kHz are filled. (B) Plot of the real and imaginary parts of the impedance of the bipolar electrode over the frequency. In (A), the characteristic frequency fRC is located above the center of the semicircle. (C) Equivalent circuit of the electrode impedance and its surrounding medium. RBulk and CBulk describe the properties of the bulk medium in contact with the bared electrode contacts. Cadd summarizes additional capacitances in the measuring setup. The CPE describes the impedance of the electrode effects at the metal-medium interface. For the calibration solution, RBulk = RCal.

Figure 3

Schemes illustrating the geometries used in the finite-element simulations of the unipolar and bipolar electrodes. (A) Locations of the electrodes (vertical line) and the counter electrode of the unipolar electrode (dashed). (B) Locations of the electrode tips in the central, spherical domain with the finer mesh. The diameters of the outer and inner spherical domains were 80 and 2 mm, respectively.
Schemes illustrating the geometries used in the finite-element simulations of the unipolar and bipolar electrodes. (A) Locations of the electrodes (vertical line) and the counter electrode of the unipolar electrode (dashed). (B) Locations of the electrode tips in the central, spherical domain with the finer mesh. The diameters of the outer and inner spherical domains were 80 and 2 mm, respectively.

Figure 4

Implanted unipolar DBS electrode with gold-wire counter electrode. (A) Scheme of a sectional plane through the scull with: (1) unipolar Pt/Ir electrode, (2) connecting cables of stimulating and counter electrode, (3) gold-wire counter electrode, and (4) anchor-screw that was tightened to the skull on the left hemisphere. All components were embedded in an adhesive-glue bridge of biocompatible dental acrylic (5) that fixed the mounting to the skull. (B) Explanted DBS mounting. (C) Details of the surgical treatment. The operational area is spread by surgical clips. The connecting cables of the stimulating and counter electrodes are covered with biocompatible dental acrylic.
Implanted unipolar DBS electrode with gold-wire counter electrode. (A) Scheme of a sectional plane through the scull with: (1) unipolar Pt/Ir electrode, (2) connecting cables of stimulating and counter electrode, (3) gold-wire counter electrode, and (4) anchor-screw that was tightened to the skull on the left hemisphere. All components were embedded in an adhesive-glue bridge of biocompatible dental acrylic (5) that fixed the mounting to the skull. (B) Explanted DBS mounting. (C) Details of the surgical treatment. The operational area is spread by surgical clips. The connecting cables of the stimulating and counter electrodes are covered with biocompatible dental acrylic.

Figure 5

Numerical results of the field strength distributions in the central plane of the unipolar and bipolar electrodes. The 200 μA constant-current setting at medium conductivities of 0.1307 S/m led to potentials of 1.56 V and 3.69 V against the counter electrode at ground potential (0 V) for the unipolar and bipolar electrodes, respectively. Please note that the distributions are not axially symmetric for the bipolar electrode.
Numerical results of the field strength distributions in the central plane of the unipolar and bipolar electrodes. The 200 μA constant-current setting at medium conductivities of 0.1307 S/m led to potentials of 1.56 V and 3.69 V against the counter electrode at ground potential (0 V) for the unipolar and bipolar electrodes, respectively. Please note that the distributions are not axially symmetric for the bipolar electrode.

Figure 6

In vivo changes in Real(Z*) for bipolar electrodes in the frequency range of 100 Hz to 100 kHz over 13 days. A characteristic decrease at the second day after implantation was identified in each of the four animals used in the experiments. For a clearer presentation, SEMs are only provided for the filled points as examples.
In vivo changes in Real(Z*) for bipolar electrodes in the frequency range of 100 Hz to 100 kHz over 13 days. A characteristic decrease at the second day after implantation was identified in each of the four animals used in the experiments. For a clearer presentation, SEMs are only provided for the filled points as examples.

Figure 7

Complex plots of the impedances of unipolar (A, A1) and bipolar (B, B1) electrodes in dependence on the day after implantation. Each spectrum was measured in a frequency range of 100 Hz to 10 MHz. For clarity, not all measuring points were plotted. (A1) and (B1) are zooms of (A) and (B).
Complex plots of the impedances of unipolar (A, A1) and bipolar (B, B1) electrodes in dependence on the day after implantation. Each spectrum was measured in a frequency range of 100 Hz to 10 MHz. For clarity, not all measuring points were plotted. (A1) and (B1) are zooms of (A) and (B).

Figure 8

In vivo time courses of the effective specific resistivities of the brain tissue that surrounded four bipolar and nine unipolar electrodes (mean values ± SEM). The vertical dotted line indicates the DBS initiation.
In vivo time courses of the effective specific resistivities of the brain tissue that surrounded four bipolar and nine unipolar electrodes (mean values ± SEM). The vertical dotted line indicates the DBS initiation.

Figure 9

Individual plots of the four bipolar electrodes summarized in Figure 8. Despite the qualitatively similar pictures, the plots suggest a strongly individual behavior. Missing experimental points were caused by data-logging problems with the impedance setup.
Individual plots of the four bipolar electrodes summarized in Figure 8. Despite the qualitatively similar pictures, the plots suggest a strongly individual behavior. Missing experimental points were caused by data-logging problems with the impedance setup.

Comparison of experimentally obtained parameters in calibration solution with numerical cell constants. The capacitances that correspond to Rcal were calculated from the experimental cell constants with equation (2).

ElectrodeMeasurementsCorresponding capacitanceNumerical results
Rcal ± SEM [Ω]σ [Sm-1]γ ± SEM [μm]Ccal ± SEM [pF]γ [μm]
bipolar17,544 ± 1,9330.1307460 ± 650.33 ± 0.05418
unipolar7,631 ± 780.13071049 ± 130.74 ± 0.01989

Experimental in vivo conditions

Bipolar electrodeUnipolar electrode
Animal-group size49
Counter electrodeIntracerebral shorter electrodeSubcutaneous gold wire (Figure 4)
Stimulating signal60 μs rectangular constant current (200 μA negative) pulses of 130 Hz repetition rate; capacitive compensation current between the pulses
eISSN:
1891-5469
Langue:
Anglais
Périodicité:
Volume Open
Sujets de la revue:
Engineering, Bioengineering and Biomedical Engineering, Biomedical Electronics, Life Sciences, Biophysics, Medicine, Biomedical Engineering, Physics, Spectroscopy and Metrology
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