Method for measuring stable and reproducible electrode-tissue impedance

Abstract

The present invention is a method for measuring stable and reproducible electrode-tissue impedance, comprising preconditioning an electrode-tissue interface. Further aspect of the invention is a stimulation system for a visual prosthesis generating a stimulation signal to precondition the electrode-tissue interface, comprising a computer; software, loaded in the computer, adapted to perform a stimulating method for a visual prosthesis having a plurality of electrodes; a video processing unit; and an implanted neuron-stimulator.

Claims

1. A method for measuring stable and reproducible electrode-tissue impedance, comprising: providing a stimulation system including a computer; software, loaded in the computer, adapted to perform a stimulating method for a visual prosthesis having a plurality of electrodes; a video processing unit; and an implanted neuron-stimulator including at least one electrode; preconditioning the electrode-tissue interface by delivering a first stimulation signal, including charge balanced biphasic square wave current pulses for a duration suitable to precondition the electrode interface to the electrode, the duration being between 1 second and 10 minutes; stopping preconditioning at the end of the duration; measuring an impedance on the electrode at similar electrode-tissue interface conditions to the preconditioning step, after said preconditioning step within a predetermined delay; transmitting the impedance wirelessly to a computer; and estimating perceptual thresholds based on the impedance.

2. The method for measuring stable and reproducible electrode-tissue impedance according to claim 1 wherein the electrode-tissue impedance is measured in vivo.

3. The method for measuring stable and reproducible electrode-tissue impedance according to claim 1 wherein preconditioning of the electrode-tissue interface is carried out one electrode at a time or in groups of electrodes.

4. The method for measuring stable and reproducible electrode-tissue impedance according to claim 1 wherein preconditioning of the electrode-tissue interface is carried out for all electrodes simultaneously.

5. The method for measuring stable and reproducible electrode-tissue impedance according to claim 4 further comprising varying a combination of variable pulse-width amplitude and frequency is used to control preconditioning of the electrode-tissue interface.

6. The method for measuring stable and reproducible electrode-tissue impedance according to claim 1, comprising preconditioning at 10-300 μA at 15-120 Hz with 0.1-2.0 ms pulse width.

7. The method for measuring stable and reproducible electrode-tissue impedance according to claim 1, comprising preconditioning at 30-150 μA at 30-120 Hz with 0.2-1.0 ms pulse width.

8. The method for measuring stable and reproducible electrode-tissue impedance according to claim 1, comprising: preconditioning at 80-120 μA at 40-60 Hz with 0.4-0.6 ms pulse width.

9. The method for measuring stable and reproducible electrode-tissue impedance according to claim 1, wherein the step of measure is measuring with charge balanced square wave current pulses.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a chart of average impedance data for 250 μm D electrodes.

(2) FIG. 2 shows a chart of average impedance data for 500 μm D electrodes.

(3) FIG. 3 shows a chart of correlation PCIT vs. back telemetry.

(4) FIG. 4 shows a chart of correlation PCIT vs. back telemetry.

(5) FIG. 5 shows a chart for 1 min. pre-conditioning PCIT vs. back telemetry.

(6) FIG. 6 shows a chart for 3 min. pre-conditioning PCIT vs. back telemetry.

(7) FIG. 7 shows a chart of average electrode impedance.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(8) The present invention is an improved method for measuring stable and reproducible electrode-tissue impedance. The method comprises the preconditioning of an electrode-tissue interface. In-vivo electrode-tissue impedance measurements in prostheses show great variations, depending on the stimulation history on the electrodes. The variations are significant especially for micro-electrodes in an electrode array.

(9) Any changes in impedance values can be observed when the stimulation is turned off for a period of a time, for example overnight. This is due to the variation in the electrode/tissue interface. The interface can be protein absorption and/or ion absorption on the electrode surface. The variations alter the electrode/electrolyte interface. This causes a variation of the electrode impedance.

(10) Although, a double layer capacitance is much more sensitive to such change, both resistance and capacitance are altered by the interface change. It has been surprisingly shown that preconditioning of the electrode-tissue interface can be achieved by simulating the electrodes with a small stimulation current or voltage for a certain period of time.

(11) The stimulation current amplitude is chosen to cause non-faradaic reactions in the electrode-tissue interface. Such reactions reduce the absorptions and diffusion layer thickness. Therefore a uniform electrode-tissue interface is reached quickly.

(12) The measurements are performed at similar electrode-tissue interface conditions, thus variations for each electrode on an array are reduced. For example in a 16 electrode array, a three minute preconditioning stimulation with a biphasic, charge balanced square wave pulse current (Tx=Ty=Tx=0.3 ms, current=10 μA, 50 Hz) reduces the standard deviation of electrode-tissue impedance from 4.8 to 2.3 for the individual electrodes and from 7.44 to 4.06 for the whole array. Further, the measurement results were much more reproducible with pre-conditioning than without.

(13) In the context of the present disclosure Back Telemetry stands for Back Telemetry Voltage Waveform, which is the information received back from an implant via a RF link. Voltage waveform Rs is the resistance of the electrode material and electrolyte calculated by taking the measured incidental voltage change at the start of current stimulation divided by the current used (Ohm's Law: Resistance=Voltage/Current). The waveform is obtained through back telemetry of the electronics, hence labeled Back Telemetry Rs. These measurements are shown and explained in the following tables 1-5 for patients A, B, and C.

(14) In the context of the present disclosure PCIT stands for Portable Cochlear Implant Tester. PCIT is a device (from the Advanced Bionics Corporation) that provides quick impedance measurements of the electrodes. These measurements are shown and explained in the following tables 6-12 for patients A, B, and C.

(15) TABLE-US-00001 TABLE 1 Patient A 250 μm D Back Telemetry Rs Electrode Measurement 1 Measurement 2 M1 1 8.2 8.7 L1 2 9.9 9.6 M2 3 9.4 8.9 L2 4 8.5 8.8 M3 5 10.4 10.0 L3 6 8.7 8.8 M4 7 8.5 7.6 L4 8 8.9 8.8 M5 9 10.4 10.1 L5 10 8.7 8.6 M6 11 9.9 9.6 L6 12 9.6 9.6 M7 13 10.1 10.4 L7 14 9.0 9.0 M8 15 8.9 8.6 L8 16 10.6 10.7 Average 9.4 9.2 St. dev. 0.78 0.81

(16) TABLE-US-00002 TABLE 2 Patient B 250 μm D Back Telemetry Rs Electrode Measurement 1 Measurement 2 Measurement 3 M1 1 11.3 11.1 12.0 L1 2 9.0 9.5 12.1 M3 5 11.2 11.5 11.4 L3 6 11.2 11.3 11.7 M6 11 13.9 13.3 13.5 L6 12 10.5 10.2 10.6 M8 15 11.2 12.3 11.8 L8 16 11.1 10.8 11.9 Average 11.2 11.3 11.9 St. dev. 1.34 1.17 0.80

(17) TABLE-US-00003 TABLE 3 Patient B 500 μm D Back Telemetry Rs Electrode Measurement 1 Measurement 2 Measurement 3 M2 3 9.4 9.2 9.5 L2 4 7.4 8.0 7.9 M4 7 8.8 8.5 8.9 L4 8 8.1 8.0 8.6 M5 9 9.5 9.6 9.9 L5 10 8.2 8.0 8.3 M7 13 8.8 9.3 9.0 L7 14 8.1 8.3 8.6 Average 8.5 8.6 8.8 St. dev. 0.73 0.67 0.65

(18) TABLE-US-00004 TABLE 4 Patient C 250 μm D Back Telemetry Rs Electrode Measurement 1 Measurement 2 M1 1 11.9 10.8 L1 2 10.4 10.0 M3 5 11.1 10.4 L3 6 10.4 9.9 M6 11 11.2 10.4 L6 12 10.1 11.8 M8 15 10.6 9.8 L8 16 12.9 12.9 Average 11.1 10.7 St. dev. 0.93 1.08

(19) TABLE-US-00005 TABLE 5 Patient C 500 μm D Back Telemetry Rs Electrode Measurement 1 Measurement 2 M2 3 8.3 8.1 L2 4 7.4 7.0 M4 7 7.7 7.6 L4 8 7.6 7.3 M5 9 8.2 8.1 L5 10 8.2 6.8 M7 13 8.8 8.5 L7 14 6.8 6.8 Average 7.9 7.5 St. dev. 0.64 0.65

(20) TABLE-US-00006 TABLE 6 Patient A 250 μm D PCIT Initial Electrode Measurement 1 Measurement 2 M1 1 21.8 28.6 L1 2 24 26.5 M2 3 23.3 29.3 L2 4 23.5 23.9 M3 5 30.3 31.9 L3 6 25.8 26.3 M4 7 17.2 12.1 L4 8 29.4 24.3 M5 9 38.2 31.5 L5 10 31.5 25 M6 11 33.1 35.1 L6 12 31 30.4 M7 13 34.4 36.2 L7 14 35.9 29.5 M8 15 27.8 22.3 L8 16 43.9 46.8 Average 29.4 28.7 St. dev. 6.83 7.44

(21) TABLE-US-00007 TABLE 7 Patient B 250 μm D PCIT Initial Electrode Measurement 1 Measurement 2 Measurement 3 M1 1 33.6 31.5 33.2 L1 2 20.0 32.5 33.1 M3 5 31.5 30.7 27.2 L3 6 33.2 30.9 27.6 M6 11 49.5 40.2 47.7 L6 12 26.7 25.5 20.9 M8 15 43.5 36.4 46.3 L8 16 40.5 38.7 52.1 Average 34.8 33.3 36.0 St. dev. 9.44 4.83 11.30

(22) TABLE-US-00008 TABLE 8 Patient B 500 μm D PCIT Initial Electrode Measurement 1 Measurement 2 Measurement 3 M2 3 18.6 17.3 17.9 L2 4 15.3 24.1 20.9 M4 7 21.1 20.1 19.6 L4 8 16.7 17.3 17.9 M5 9 19.8 19.8 18.5 L5 10 20.3 16.1 16.0 M7 13 21.4 21.1 24.0 L7 14 18.8 19.0 22.0 Average 19.0 19.4 19.6 St. dev. 2.13 2.55 2.59

(23) TABLE-US-00009 TABLE 9 Patient C 250 μm D PCIT Initial Electrode Measurement 1 Measurement 2 M1 1 26.9 30.5 L1 2 31.2 30.8 M3 5 27.1 28.3 L3 6 27.1 28.6 M6 11 35.1 38.4 L6 12 35.7 36.7 M8 15 32.0 35.1 L8 16 70.1 75.7 Average 35.7 38.0 St. dev. 14.36 15.68

(24) TABLE-US-00010 TABLE 10 Patient C 500 μm D PCIT Initial Electrode Measurement 1 Measurement 2 M2 3 15.9 15.0 L2 4 16.0 15.6 M4 7 17.3 17.1 L4 8 22.0 22.3 M5 9 18.1 19.9 L5 10 17.7 18.1 M7 13 24.7 22.3 L7 14 17.0 16.6 Average 18.6 18.4 St. dev. 3.12 2.86

(25) TABLE-US-00011 TABLE 11 Patient A After 3 min Conditioning 250 μm D Electrode Measurement 1 Measurement 2 M1 1 17.7 20.9 L1 2 30.2 29.3 M2 3 25.1 25.5 L2 4 15.9 16.5 M3 5 21.4 22.2 L3 6 19.9 19.6 M4 7 15.7 13.2 L4 8 18.7 17.9 M5 9 23.6 24.6 L5 10 18.4 21.2 M6 11 23.1 22.0 L6 12 18.4 19.3 M7 13 22.1 21.7 L7 14 21.5 17.6 M8 15 17.2 15.9 L8 16 23.7 24.1 Average 20.8 20.7 St. dev. 3.83 4.06

(26) TABLE-US-00012 TABLE 12 Patient B After 3 min Conditioning 250 μm D Electrode Measurement 1 Measurement 2 Measurement 3 M1 1 24.7 25.0 26.4 L1 2 15.9 23.4 22.5 M3 5 24.4 25.9 26.0 L3 6 24.0 29.2 25.2 M6 11 29.7 28.9 25.1 L6 12 20.7 19.4 20.1 M8 15 21.5 24.0 24.1 L8 16 22.0 20.2 25.0 Average 22.9 24.5 24.3 St. dev. 3.96 3.58 2.08

(27) TABLE-US-00013 TABLE 13 Patient B After 3 min Conditioning 500 μm D Electrode Measurement 1 Measurement 2 Measurement 3 M2 3 18.6 19.7 19.3 L2 4 13.8 17.7 16.8 M4 7 16.6 15.3 16.0 L4 8 15.2 16.5 15.5 M5 9 17.7 19.4 17.4 L5 10 14.3 15.5 13.5 M7 13 16.4 17.2 15.8 L7 14 14.0 14.4 14.4 Average 15.8 17.0 16.1 St. dev. 1.78 1.92 1.79

(28) TABLE-US-00014 TABLE 14 Patient C After 3 min Conditioning 250 μm D Electrode Measurement 1 Measurement 2 M1 1 25.5 19.0 L1 2 21.4 17.9 M3 5 16.4 17.5 L3 6 15.9 15.5 M6 11 20.1 18.6 L6 12 19.6 21.6 M8 15 20.0 17.8 L8 16 30.7 27.8 Average 21.2 19.5 St. dev. 4.86 3.78

(29) TABLE-US-00015 TABLE 15 Patient C After 3 min Conditioning 500 μm D Electrode Measurement 1 Measurement 2 M2 3 13.6 14.0 L2 4 11.0 12.9 M4 7 11.7 12.9 L4 8 12.4 13.5 M5 9 12.7 13.8 L5 10 12.2 12.8 M7 13 14.9 15.0 L7 14 12.6 12.5 Average 12.6 13.4 St. dev. 1.19 0.82

(30) TABLE-US-00016 TABLE 16 Patient A After BT Recording Measurement 1 Measurement 2 250 μm D Electrode No Conditioning 1 min Conditioning M1 1 43.8 20.8 L1 2 37.1 21.1 M2 3 23.8 17.7 L2 4 19.8 17.1 M3 5 30.9 22.9 L3 6 26.4 15.3 M4 7 13.1 11.6 L4 8 34.9 20.9 M5 9 43.4 24.4 L5 10 17.0 15.1 M6 11 20.1 18.7 L6 12 32.4 19.4 M7 13 41.4 24.2 L7 14 15.7 15.1 M8 15 13.6 12.3 L8 16 29.9 22.7 Average 27.7 18.7 St. dev. 10.58 4.04

(31) TABLE-US-00017 TABLE 17 Patient B After BT Recording 250 μm D Measurement 1 Measurement 2 Measurement 3 Electrode No Conditioning No Conditioning 1 min Conditioning M1 1 37.9 36.4 23.3 L1 2 19.9 35.9 20.9 M3 5 32.0 24.4 24.8 L3 6 33.6 31.0 24.2 M6 11 38.2 35.1 24.1 L6 12 27.3 24.5 19.4 M8 15 39.7 34.3 23.9 L8 16 40.4 24.3 16.3 Average 33.6 30.7 22.1 St. dev. 7.10 5.49 2.99

(32) TABLE-US-00018 TABLE 18 Patient B After BT Recording 500 μm D Measurement 1 Measurement 2 Measurement 3 Electrode No Conditioning No Conditioning 1 min Conditioning M2 3 22.5 21.3 18.1 L2 4 15.3 26.8 14.9 M4 7 23.1 19.0 15.4 L4 8 17.7 17.8 14.2 M5 9 19.2 21.6 15.7 L5 10 17.0 18.3 13.7 M7 13 21.7 21.4 15.2 L7 14 18.1 19.8 14.2 Average 19.3 20.8 15.2 St. dev. 2.82 2.85 1.36

(33) TABLE-US-00019 TABLE 19 Patient C After BT Recording Measurement 1 Measurement 2 250 μm D Electrode No Conditioning 1 min Conditioning M1 1 39.4 31.2 L1 2 41.9 24.0 M3 5 21.5 19.7 L3 6 21.3 15.4 M6 11 22.1 12.4 L6 12 32.8 24.6 M8 15 25.3 20.8 L8 16 27.1 27.0 Average 28.9 21.9 St. dev. 8.19 6.13

(34) TABLE-US-00020 TABLE 20 Patient C After BT Recording Measurement 1 Measurement 2 500 μm D Electrode No Conditioning 1 min Conditioning M2 3 15.7 16.4 L2 4 15.5 15.1 M4 7 18.1 17.3 L4 8 24.1 12.3 M5 9 20.3 16.7 L5 10 11.2 8.5 M7 13 19.9 16.2 L7 14 11.8 10.1 Average 17.1 14.1 St. dev. 4.40 3.34

(35) TABLE-US-00021 TABLE 21 Patient A Final (2 hrs after Back Telemetry) 250 μm D Electrode Measurement 1 Measurement 2 M1 1 17.9 24.7 L1 2 19.8 23.4 M2 3 17.2 20.1 L2 4 11.0 14.8 M3 5 25.5 21.0 L3 6 22.2 17.8 M4 7 10.8 11.0 L4 8 22.7 17.3 M5 9 32.3 23.0 L5 10 12.8 14.1 M6 11 18.6 19.0 L6 12 23.2 17.2 M7 13 33.0 25.3 L7 14 13.1 14.4 M8 15 12.2 11.2 L8 16 23.2 18.1 Average 19.7 18.3 St. dev. 6.94 4.47

(36) TABLE-US-00022 TABLE 22 Patient B Final (2 hrs after Back Telemetry) 250 μm D Electrode Measurement 1 Measurement 2 M1 1 40.7 42.2 L1 2 19.9 41.0 M3 5 36.4 39.3 L3 6 34.2 40.6 M6 11 40.2 39.8 L6 12 28.5 29.1 M8 15 40.9 35.9 L8 16 40.4 30.0 Average 35.2 37.2 St. dev. 7.52 5.09

(37) TABLE-US-00023 TABLE 23 Patient B Final (2 hrs after Back Telemetry) 500 μm D Electrode Measurement 1 Measurement 2 M2 3 26.2 24.7 L2 4 17.6 16.6 M4 7 22.9 22.5 L4 8 17.4 14.0 M5 9 22.2 23.9 L5 10 19.4 18.4 M7 13 21.3 21.8 L7 14 18.6 19.6 Average 20.7 20.2 St. dev. 3.03 3.72

(38) TABLE-US-00024 TABLE 24 Patient C Final (2 hrs after Back Telemetry) 250 μm D Electrode Measurement 1 Measurement 2 M1 1 43.1 30.2 L1 2 45.8 31.0 M3 5 28.6 23.9 L3 6 28.9 21.2 M6 11 29.7 20.9 L6 12 32.3 29.9 M8 15 28.2 32.1 L8 16 48.6 50.1 Average 35.7 29.9 St. dev. 8.65 9.31

(39) TABLE-US-00025 TABLE 25 Patient C Final (2 hrs after Back Telemetry) 500 μm D Electrode Measurement 1 Measurement 2 M2 3 16.1 15.7 L2 4 19.4 16.0 M4 7 19.4 10.2 L4 8 23.8 15.3 M5 9 21.7 18.2 L5 10 13.0 10.5 M7 13 20.7 17.7 L7 14 15.1 13.0 Average 18.7 14.6 St. dev. 3.63 3.05

(40) FIGS. 1 and 2 are based on the measurements stated in previous tables 1-25 and show the advantageous effect of preconditioning of electrodes. The results can be summarized in the following table 26:

(41) TABLE-US-00026 TABLE 26 After Back Telemetry After 30 After 2 Hours After 3 After Back minutes (Intermittent Initial - No minutes Telemetry 1 minute stimulation on Pre- Pre- After 30 Pre- single Diameter Measurement Conditioning Conditioning minutes Conditioning electrodes) 250 umD Impedance 32.70 21.70 29.70 20.40 26.70 (kohms) Standard 10.08 4.04 8.64 4.60 10.41 deviation 500 umD Impedance 19.00 15.00 19.10 14.60 18.50 (kohms) Standard 2.57 2.24 3.63 2.53 4.03 deviation

(42) The table shows clearly that the preconditioning yields lower values for impedance (e.g. 32.70 v. 21.70 and 19.00 v. 15.00) and lower values for standard deviation (e.g. 10.08 v. 4.04 and 2.57 v. 2.24).

(43) The pre-conditioning stimulation is performed on electrodes as for about 1 and about 3 minutes at 30-200 μA/30-120 Hz/0.1-0.1-0.1 ms-1.0-1.0-1.0 ms pulse width, preferred but not limited at about 100 μA/60 Hz/0.3-0.3-0.3 ms pulse width.

(44) FIG. 1 shows results for 250 μm electrodes PCIT and FIG. 2 shows results for 500 μm electrodes PCIT. The charts show average values obtained from three patients (A, B, and C). Patient A obtained an array with 16 electrodes with 250 μm and patients B and C obtained an array with 8 electrodes with 250 μm and 8 electrodes with 500 μm diameters. The previous tables 1-25 show the measurements which lead to the average values summarized in table 26 and are shown in the charts in FIGS. 1 and 2.

(45) FIGS. 3 to 6 show correlation of PCIT measurements and effects of pre-conditioning of electrodes. The charts in FIGS. 3-6 show an increase in improvement of the correlation. FIGS. 3 and 4 both show measurements without pre-conditioning. FIG. 3 shows correlation of PCIT vs. back telemetry voltage waveform impedance initial PCIT vs. voltage waveform Rs. FIG. 4 shows correlation of PCIT vs. back telemetry voltage waveform impedance after voltage waveform measurement PCIT vs. voltage waveform Rs. FIG. 5 shows correlation of PCIT vs. back telemetry voltage waveform impedance after voltage waveform measurement PCIT (1 minute pre-conditioning) vs. voltage waveform Rs. FIG. 6 shows correlation of PCIT vs. back telemetry voltage waveform impedance after voltage waveform measurement PCIT (3 minute pre-conditioning) vs. voltage waveform Rs. FIG. 6 shows the best correlation with 3 minutes of pre-conditioning.

(46) FIG. 7 shows an average electrode impedance (n=10) measured in a canine's eye. In-vivo electrode impedance changes measured in a canine's eye are shown therein. There were two groups of electrodes measured: one group with the electrodes in contact with retina tissue and the other with all the electrodes away from the tissue in saline. The impedance data from 10 electrodes for each group were averaged. The electrode/tissue impedance decreased dramatically upon electrical stimulation for the electrolyte in contact with the retina tissue while the electrode/electrolyte impedance had no significant changes for the electrodes being away from the tissue.

(47) Accordingly, what has been shown is a method for measuring stable and reproducible electrode-tissue impedance, comprising preconditioning of an electrode-tissue interface. While the invention has been described by means of specific embodiments and applications thereof, it is understood that numerous modifications and variations could be made thereto by those skilled in the art without departing from the spirit and scope of the invention. It is therefore to be understood that within the scope of the claims, the invention may be practiced otherwise than as specifically described herein.