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Compliance supply-limited driving of iridium oxide (AIROF) electrodes for maintenance in a safe operating region

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Although neural stimulating electrodes that use an activated iridium oxide film coating have existed for more than 20 years, the design of electronic circuits to preserve the electrode electrochemical integrity has received only minimal attention. In contemplating the implantation, in humans, of perhaps hundreds of electrodes for use in visual prostheses, it is essential that methods be defined for driving of the electrodes so that deterioration of the AIROF does not occur. We have developed a simple driving technique that limits the cathodic and anodic voltage excursions of any stimulating electrode within the "water window."
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10th Annual Conference of the International FES Society
July 2005 – Montreal, Canada
Compliance supply-limited driving of iridium oxide (AIROF) electrodes
for maintenance in a safe operating region.
Philip R. Troyk1, Stuart Cogan2, Glenn A. DeMichele3
1 Illinois Institute of Technology, IIT Center, Chicago, IL
2 EIC Laboratories, Norwood MA.
3 Sigenics, Inc., Lincolnshire, IL
troyk@iit.edu
Abstract
Although neural stimulating electrodes that
use an activated iridium oxide film coating
have existed for more than 20 years, the
design of electronic circuits to preserve the
electrode electrochemical integrity has
received only minimal attention. In
contemplating the implantation, in humans, of
perhaps hundreds of electrodes for use in
visual prostheses, it is essential that methods
be defined for driving of the electrodes so that
deterioration of the AIROF does not occur.
We have developed a simple driving technique
that limits the cathodic and anodic voltage
excursions of any stimulating electrode within
the “water window.”
1. INTRODUCTION
AIROF electrodes have been proposed for use
in neural prostheses within the central nervous
system and in the periphery, and research
directed at their fabrication and measurement
has been reported over the past two decades
[1,2,3]. AIROF provides a significantly higher
charge capacity, per phase, than other candidate
electrode materials [4], particularly those
belonging to the noble metal class, e.g.
platinum. Since the mechanism of charge
injection for metal electrodes is primarily
faradaic reactions, it has been long-recognized
that anodic and cathodic voltage excursions that
drive the electrode potential (relative to
Ag|AgCl) beyond –0.6V and +0.8V result in
damage to the AIROF film due to the onset of
oxidation or reduction of water. Damage to an
ARIOF electrode can be virtually immediate
and irreversible, frequently taking the form of
delamination of the ARIOF film, as shown in
Figure 1. Attempts to recover the electrode, in-
vitro, or in-vivo, by reactivation are generally
unsuccessful. In addition, operation outside of
this “water window” may cause significant pH
changes within the surrounding tissue.
Although the symmetric biphasic constant
current waveform has historically been
regarded as the standard for driving not only
AIROF, but also other metal electrodes, it has
recently been shown that arbitrary use of this
waveshape may damage the electrodes as the
voltage excursions move outside of the water
window, and that an asymmetric waveshape can
increase the safe charge capacity of the AIROF
electrode [5]. We have devised an automatic
method of generating an asymmetric constant
current stimulating waveform that maintains
operation within the water window while
maximizing the allowable stimulus charge
injection.
2. METHODS
Typically a biphasic constant current driver
consists of an anodic current source, and an
cathodic current sink, connected to an anodic
and cathodic compliance supply, respectively.
Most often, these compliance supplies are in
excess of the water window and therefore create
the potential for damage to the electrode as the
constant current drivers attempt to maintain
10 µ
Figure 1. SEM photograph of an AIROF electrode showing delamination
due to cathodic voltage excursions below –0.6V vs Ag|AgCl. This damage
is irreversible and severely compromises the charge injection capacity of
the electrode.
10th Annual Conference of the International FES Society
July 2005 – Montreal, Canada
constant current drive, using only the
compliance supplies as a limit.
In an attempt to avoid damage to the electrode,
a maximum charge injection limit is typically
defined for each electrode and is often based
upon the assumed surface area, a percentage
utilization of the total charge (as measured by
low-frequency CV curves), or is based upon a
published figure for the safe charge injection
for AIROF. This same strategy is sometimes
applied to platinum electrodes as well. The
significant point is that the charge injection
limit is based upon assumptions for the
electrode area, and this defines an apriori
charge injection limit. This method ignores
geometry specific effects, as well as shifts in
the access resistance once the electrode is
implanted. Consequently, prior studies often
report an abrupt onset of electrode damage as
the stimulation charge/phase is increased, and
we suspect that such damage is caused by
voltage excursions that are outside of the water
window.
Our approach adopts a rule of cutting back the
cathodic current (for cathodic-first stimulation)
as necessary to prevent the cathodic voltage
excursion from exceeding –0.6V vs. Ag|AgCl.
Similarly, during the anodic recharge phase, the
anodic recharge current is cutback so as to
prevent the electrode voltage from exceeding
+0.8V vs. Ag|AgCl. This method is illustrated
in Figure 3.
Figure 4 shows how this rule is practically
implemented for a biphasic electrode driver. In
actual circuitry, the method is based upon the
fact that all electronic current source, and sink,
circuits must operate with a finite voltage drop
across them. Once the voltage across the circuit
is reduced to zero, so does the current. Anodic
and cathodic compliance supplies are generated
by adding the desired anodic bias, and the
desired cathodic limit to a buffered
measurement of the reference electrode. Prior
to the stimulation pulse, the anodic current
source charges the electrode to the anodic bias
limit. As the electrode voltage approaches the
anodic limit, the anodic current source
automatically shuts off due to the reduction in
the operating voltage across the current source.
Similarly, during the cathodic stimulation pulse,
the cathodic current sink can only continue to
sink current while the electrode voltage is more
positive than the desired cathodic limit. Once
the limit is reached, the cathodic driver
automatically reduces current.
If considering the stimulation electrode to be a
simple series RC circuit, then use of this
method would result in the current being
reduced to zero as soon as the capacitor voltage
reached the compliance limits. This could,
conceivably, uncomfortably limit the charge
capacity of the electrode. However, a typical
electrode is better characterized by a distributed
RC network. Therefore, only a modest cutback
in the currents are needed to hold the voltage
excursion within the desired window.
It seems almost ridiculously simple that to
implement this method, one merely needs to
adjust the compliance supplies of any biphasic
constant current driving circuit. Derivation of
the compliance supplies, relative to the
reference electrode, is a slight complication.
However, by adding a safety margin, one might
even use a fixed power supply for each of the
compliance supplies.
We fabricated a hardware system that
implements the strategy of Figure 4.
Electrode
Electrode
Anodic recharge
Cathodic current
Cathodic
Anodic
Figure 3. Depiction of electrode current and voltage waveforms for
constant current cutback method. Both the cathodic and the anodic drivers
automatically reduce the electrode current to prevent voltage excursions
outside of the water window.
Anodic bias limit
Cathodic limit
Anodic current
source
Cathodic current
source
AIROF electrode
Reference electrode
A
+
Desired anodic bias
_
Desired cathodic limit
Anodic bias limit
Cathodic limit
Anodic current
source
Cathodic current
source
AIROF electrode
Reference electrode
A
+
Desired anodic bias
_
Desired cathodic limit
Figure 4. Circuit implementation of the current cutback method.
Once the value of the electrode voltage reaches either of the
compliance supplies the respective current source automatically
cuts backs the current preventing the electrode from exceeding
the limit.
10th Annual Conference of the International FES Society
July 2005 – Montreal, Canada
Microelectrodes that were fabricated at both the
Huntington Medical Research Institutes, and
the Laboratory of Neural Control at the NIH
were used to determine the effectiveness of the
protective method for a variety of electrodes.
Electrodes areas ranged between 1000 and 2000
µm2. Pulsing was done in phosphate-buffered
saline (PBS) having a concentration of
0.0125M NaCl, 0.0014M NaH2PO4·H2O, and
0.005M Na2HPO4·7H2O at a pH of ~7.3. This
reduced PBS concentration (~1/16 of normal)
was used to simulate physiologic extra-cellular
fluid. The PBS was open to air and thus
contained ~0.004M dissolved oxygen. A
Ag|AgCl reference electrode was used to
monitor the working electrode potential and
derive the anodic and cathodic compliance
supply limits.
3. RESULTS
A typical waveform set of electrode voltage and
current is shown in Figure 5 for a 1000 µm2
AIROF microelectrode. Under these
conditions, with the cathodic cutback, the
maximum safe charge injection was 1.86
mC/cm2. In comparison, had the current pulse
been constant, a charge injection of 2.25
mC/cm2 would have resulted. For only this
modest 17% increase, the cathodic excursion
would have crossed the –0.6V limit, and
electrode the likelihood of electrode damage
would have been significantly greater. And,
this comparison does not take into account that
using a symmetric waveform would not have
permitted the anodic bias to be as great as
+0.7V, as seen in Figure 5.
4. DISCUSSION AND CONCLUSIONS
One complicating factor arises for electrodes
that have excessively high access resistances, or
lead designs with large resistances (due to small
conductors). For both of these cases, the IR
drop during stimulation can be substantial and
one is tempted to add the IR drop to the
compliance supply limit.
We believe this approach to be misguided, at
least in the case of excessive electrode access
resistances. If a substantial portion of the
allowable voltage excursion is defined by the
access resistance drop, then dynamic
measurement of that access resistance becomes
crucial to maintaining safe operation of the
electrode. Sufficiently accurate computation of
the access resistance, on a pulse-by-pulse basis
would be required. For the case of lead
resistance, the IR drop might be more
predictable, but would still be a function of the
actual stimulus current. In our designs, we
ignore the IR drops, and adopt a conservative
rule of never allowing the electrode voltage to
more outside of the water window. One also
needs a stable reference electrode. In several
tests, we have examined the feasibility of using
the counter electrode as a stable reference
electrode. For a large area platinum wire, we
have found this method to be acceptable for
both in-vitro and in-vivo studies, although the
open circuit potential of the platinum wire
needs to be at least initially measured, and a
suitable safety factor added to the derivation of
the compliance supplies.
References
[1] X. Liu X, D. B. McCreery, R. R. Carter, L. A. Bullara, T. G.
H. Yuen, W. F. Agnew, “Stability of the interface between neural
tissue and chronically implanted intracortical microelectrodes,”
IEEE Trans. Rehab. Eng., vol. 7, pp. 315-326, 1999.
[2] D. J. Anderson, K. Najafi, S. J. Tanghe, D. A. Evans, K. L.
Levy, J. F. Hethke, X. Xue, J. J. Zappia, K. D. Wise, “Batch-
fabricated thin-film electrodes for stimulation of the central
auditory system,” IEEE Trans. Biomed. Eng., vol. 36, pp. 693-
704, 1989.
[3] J. D. Weiland, D. J. Anderson, “Chronic neural stimulation
with thin-film, iridium oxide electrodes at high current densities,”
IEEE Trans. Biomed Eng., vol. 35, pp. 911-918, 2000.
[4] X. Beebe, T. L. Rose, “Charge injection limits of activated
iridium oxide electrodes with 0.2 ms pulses in bicarbonate
buffered saline,” IEEE Trans. Biomed. Eng., vol. 35, pp. 494-495,
1988.
[5] S. F. Cogan, P. R. Troyk, J. Ehrlich, T. D. Plante, D. B.
McCreery, L. Bullara, “Charge-injection waveforms for iridium
oxide (AIROF) microelectrodes,” Proceedings of EMBS
Conference, Cancun, Mexico, September 17-21, pp 1960 – 1963,
2003.
Acknowledgements
Funding by the Brain Research Foundation, private
donations, and NIH grant R01 EB002184
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
0 100 200 300 400 500 600 700 800 900 1000
time (usec)
Electrode Voltage
-80
-60
-40
-20
0
20
40
60
80
Electrode Current (uA)
Figure 5. Voltage (red, upper) and current (yellow, lower)
waveforms for an AIROF electrode driven with the current cutback
method.
... Between the short pulses, the power supply applies a conducting bias, so that electrodes are charged to the bias level without illumination. Under both schemes it is advantageous to bias the electrodes to one end of the 'water window' (−0.6 V or +0.8 V with respect to Ag/AgCl [23]) to maximize charge injection. Since there are ohmic losses across the electrolyte, voltages outside the water window would be necessary to fully charge the electrodes to their safe limits. ...
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