U.S. patent application number 14/857121 was filed with the patent office on 2016-01-07 for implantable neurostimulators having reduced pocket stimulation.
The applicant listed for this patent is Boston Scientific Neuromodulation Corporation. Invention is credited to Kerry Bradley, Dongchul Lee, Michael A. Moffitt.
Application Number | 20160001083 14/857121 |
Document ID | / |
Family ID | 41603875 |
Filed Date | 2016-01-07 |
United States Patent
Application |
20160001083 |
Kind Code |
A1 |
Moffitt; Michael A. ; et
al. |
January 7, 2016 |
IMPLANTABLE NEUROSTIMULATORS HAVING REDUCED POCKET STIMULATION
Abstract
Neurostimulators and methods of using neurostimulators are
provided. The neurostimulator is implanted within a tissue pocket
of a patient, and electrical energy is conveyed from the
neurostimulator to stimulate a target tissue site remote from the
tissue pocket. The neurostimulator has a case with which one or
more electrodes are associated. The electrical energy is returned
to the electrode(s) in a manner that prevents, or at least reduces,
pocket stimulation that may otherwise occur due to the return of
electrical energy to the case of the neurostimulator.
Inventors: |
Moffitt; Michael A.;
(Valencia, CA) ; Lee; Dongchul; (Agua Dulce,
CA) ; Bradley; Kerry; (Glendale, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Boston Scientific Neuromodulation Corporation |
Valencia |
CA |
US |
|
|
Family ID: |
41603875 |
Appl. No.: |
14/857121 |
Filed: |
September 17, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12629814 |
Dec 2, 2009 |
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14857121 |
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61119662 |
Dec 3, 2008 |
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Current U.S.
Class: |
607/62 |
Current CPC
Class: |
A61N 1/36175 20130101;
A61N 1/3756 20130101; A61N 1/36185 20130101; A61N 1/37205 20130101;
A61N 1/372 20130101 |
International
Class: |
A61N 1/36 20060101
A61N001/36 |
Claims
1. (canceled)
2. A method, comprising: delivering monopolar neurostimulation to
targeted neural tissue using a neurostimulator that includes an
electrically conductive outer case implanted within a tissue pocket
and a lead extending away from the outer case toward the targeted
neural tissue, the outer case being configured to provide a
hermetically sealed compartment for internal electronics that are
operably connected to the outer case and the lead to transmit
electrical stimulation pulses through tissue between the lead and
the outer case, wherein the monopolar neurostimulation includes a
plurality of stimulation pulses having an amplitude and a pulse
width; detecting tissue pocket stimulation, wherein the tissue
pocket stimulation occurs with high current density at the outer
case which is attributable at least in part to geometry of the
outer case; responding to detection of tissue pocket stimulation
including reducing the pulse width of stimulation pulses in
response to the detection of tissue pocket stimulation and
delivering the monopolar neurostimulation using the electrical
pulses with the reduced pulse width.
3. The method of claim 2, wherein the decreased pulse width is in
the range of 10-100 .mu.s.
4. The method of claim 3, wherein the decreased pulse width equal
to or less than 50 .mu.s.
5. The method of claim 2, further comprising responding to reducing
the pulse width by automatically increasing the amplitude of the
electrical pulses.
6. The method of claim 2, wherein the outer case for the
neurostimulator used to deliver the monopolar neurostimulation has
an area equal to or less than 1000 mm2, thereby contributing to
high current density.
7. The method of claim 2, wherein the outer case for the
neurostimulator used to deliver the monopolar neurostimulation has
an area equal to or less than 200 mm2, thereby contributing to high
current density.
8. The method of claim 2, wherein the outer case for the
neurostimulator used to deliver the monopolar neurostimulation has
length less than 35 mm, thereby contributing to high current
density.
9. The method of claim 2, wherein the outer case for the
neurostimulator used to deliver the monopolar neurostimulation has
diameter less than 5 mm, thereby contributing to high current
density.
10. The method of claim 2, wherein delivering monopolar
neurostimulation to targeted neural tissue includes delivering
cathodic energy to the targeted neural tissue.
11. The method of claim 2, wherein delivering monopolar
neurostimulation to targeted neural tissue includes delivering
cathodic energy to the targeted neural tissue.
12. A method for operating a neurostimulator that includes an
electrically conductive outer case implanted within a tissue pocket
and a lead extending away from the outer case toward targeted
neural tissue, the method comprising: delivering monopolar
neurostimulation to the targeted neural tissue including
transmitting electrical stimulation pulses have an amplitude and a
pulse width through tissue between the lead and the outer case;
detecting tissue pocket stimulation, wherein the tissue pocket
stimulation occurs with high current density at the outer case
which is attributable at least in part to geometry of the outer
case; responding to detection of tissue pocket stimulation
including reducing the pulse width of stimulation pulses in
response to the detection of tissue pocket stimulation and
delivering the monopolar neurostimulation using the electrical
pulses with the reduced pulse width.
13. The method of claim 12, wherein the decreased pulse width is in
the range of 10-100 .mu.s.
14. The method of claim 13, wherein the decreased pulse width equal
to or less than 50 .mu.s.
15. The method of claim 12, further comprising responding to
reducing the pulse width by automatically increasing the amplitude
of the electrical pulses.
16. The method of claim 12, wherein the outer case for the
neurostimulator used to deliver the monopolar neurostimulation has
an area equal to or less than 1000 mm2, thereby contributing to
high current density.
17. A method for operating a neurostimulator that includes an
electrically conductive outer case implanted within a tissue pocket
and a lead extending away from the outer case toward targeted
neural tissue, the method comprising: delivering monopolar
neurostimulation to the targeted neural tissue including
transmitting electrical stimulation pulses have an amplitude and a
pulse width through tissue between the lead and the outer case,
wherein the outer case has an area equal to or less than 200 mm2;
detecting tissue pocket stimulation, wherein the tissue pocket
stimulation occurs with high current density at the outer case
which is attributable at least in part to geometry of the outer
case including the area of the outer case; responding to detection
of tissue pocket stimulation including reducing the pulse width of
stimulation pulses to a pulse width within the range of 10-100
.mu.s in response to the detection of tissue pocket stimulation and
delivering the monopolar neurostimulation using the electrical
pulses with the reduced pulse width.
18. The method of claim 17, wherein the outer case for the
neurostimulator used to deliver the monopolar neurostimulation has
an area equal to or less than 200 mm2 and the decreased pulse width
is equal to or less than 50 .mu.s.
19. The method of claim 17, further comprising responding to
reducing the pulse width by automatically increasing the amplitude
of the electrical pulses.
20. The method of claim 17, wherein the geometry of the outer case
for the neurostimulator includes a length less than 35 mm and a
diameter less than 5 mm.
21. The method of claim 17, wherein delivering monopolar
neurostimulation to targeted neural tissue includes delivering
cathodic energy to the targeted neural tissue, and wherein the
tissue pocket includes larger nerves than nerves within the
targeted neural tissue.
Description
RELATED APPLICATION
[0001] The present application claims the benefit under 35 U.S.C.
.sctn.119 to U.S. provisional patent application Ser. No.
61/119,662, filed Dec. 3, 2008, The foregoing application is hereby
incorporated by reference into the present application in its
entirety.
FIELD OF THE INVENTION
[0002] The present inventions relate to tissue stimulation systems,
and more particularly, to systems and methods for preventing or
reducing inadvertent stimulation of tissue adjacent an implantable
neurostimulator.
BACKGROUND OF THE INVENTION
[0003] Implantable neurostimulation systems have proven therapeutic
in a wide variety of diseases and disorders. Pacemakers and
Implantable Cardiac Defibrillators(ICDs) have proven highly
effective in the treatment of a number of cardiac conditions (e.g.,
arrhythmias). Spinal Cord Stimulation (SCS) systems have long been
accepted as a therapeutic modality for the treatment of chronic
pain syndromes and the application of tissue stimulation has begun
to expand to additional applications, such as angina pectoris and
incontinence. Deep Brain Stimulation (DBS) has also been applied
therapeutically for well over a decade for the treatment of
refractory Parkinson's Disease, and DBS has also recently been
applied in additional areas, such as essential tremor and epilepsy.
Further, in recent investigations, Peripheral Nerve Stimulation
(PNS) systems have demonstrated efficacy in the treatment of
chronic pain syndromes and incontinence, and a number of additional
applications are currently under investigation. Furthermore,
Functional Electrical Stimulation (FES) systems such as the
Freehand system by NeuroControl (Cleveland, Ohio) have been applied
to restore some functionality to paralyzed extremities in spinal
cord injury patients.
[0004] Each of these implantable neurostimulation systems typically
includes one or more electrode carrying stimulation leads, which
are implanted at the desired stimulation site, and a
neurostimulator (i.e., an implantable pulse generator (IPG))
implanted remotely from the stimulation site, but coupled either
directly to the stimulation lead(s) or indirectly to the
stimulation lead(s) via a lead extension. For example, in the
context of SCS, the electrode lead(s) are typically implanted along
the dura of the spinal cord, with the electrode lead(s) exiting the
spinal column, where they can generally be coupled to one or more
electrode lead extensions. The electrode lead extension(s), in
turn, are typically tunneled around the torso of the patient to a
subcutaneous pocket (typically in the chest or abdomen) where the
neurostimulator is implanted.
[0005] The neurostimulation system may further comprise a handheld
patient programmer to remotely instruct the neurostimulator to
generate electrical stimulation pulses in accordance with selected
stimulation parameters. The handheld programmer, which may take the
form of a remote control (RC) may, itself, be programmed by a
clinician, for example, by using a clinics programmer (CP), which
typically includes a general purpose computer, such as a laptop,
with a programming software package installed thereon.
[0006] Thus electrical pulses can be delivered from the
neurostimulator to the stimulation electrode(s) to stimulate or
activate a volume of tissue. In particular, electrical energy
conveyed between at least one cathodic electrode and at least one
anodic electrodes creates an electrical field, which when strong
enough, depolarizes (or "stimulates") the neurons beyond a
threshold level, thereby inducing the firing of action potentials
(APs) that propagate along the neural fibers.
[0007] Electrical pulses may be delivered from the neurostimulator
to the stimulator electrode(s) in accordance with a set of
stimulation parameters and provide the desired efficacious therapy
to the patient. A typical stimulation parameter set may include the
electrodes that are sourcing (anodes) or returning (cathodes) the
stir stimulation current at any given time, as well as the
amplitude, duration, and rate of the stimulation pulses.
[0008] Stimulation energy may be delivered to the electrodes during
and after the lead placement process in order to verify that the
electrodes are stimulating the target neural elements and to
formulate the most effective stimulation regimen. The regimen will
dictate which of the electrodes are sourcing current pulses
(anodes) and which of the electrodes are sinking current pulses
(cathodes) at any given time, as well as the magnitude, duration,
and rate of the electrical current pulses. The stimulation regimen
will typically be one that provides stimulation energy to all of
the target tissue that must be stimulated in order to provide the
therapeutic benefit, yet minimizes the volume of non-target tissue
that is stimulated. In the case of SCS, such a therapeutic benefit
is "paresthesia," i.e., a tingling sensation that is effected by
the electrical stimuli applied through the electrodes.
[0009] Electrical energy may be transmitted to the tissue in a
monopolar or multipolar (e.g., bipolar, tripolar, etc.) fashion.
Monopolar delivery occurs when a selected one or more of the lead
electrodes is activated along with the case of the neurostimulator,
so that electrical energy is transmitted between the selected
electrode and the case. Bipolar delivery occurs when two of the
lead electrodes are activated as anode and cathode, so that
electrical energy is transmitted between the selected electrodes.
Tripolar delivery occurs when three of the lead electrodes are
activated, two as anodes and the remaining one as a cathode, or two
as cathodes and the remaining one as an anode.
[0010] In the context of SCS, the neurostimulator case is used as a
cathodic return electrode, and the lead electrodes are used as
anodic stimulating electrodes. The neurostimulator case is selected
as the cathodic return electrode, because it is relatively far away
from the stimulation site, and because it has a large surface area,
resulting in relatively small current densities.
[0011] This pocket stimulation problem exacerbated when
microstimulators are used. A "microstimulator" is an implantable
neurostimulator it which the body or case of the device is compact
(typically on the order of a few millimeters is diameter by several
millimeters to a few centimeters in length). For example, the
Bion.RTM. microstimulator (manufactured and distributed by Boston
Scientific Neuromodulation Corporation) is a tiny fraction of the
size of the Precision.RTM. IPG. Typically, the cases of the
microstimulators carry electrodes for producing the desired
electrical stimulation current. Microstimulators of this type
(i.e., microstimulators with leadless electrodes) are implanted
proximate to the target tissue to allow the stimulation current to
stimulate the target tissue to provide therapy for a wide variety
of conditions and disorders. In these cases, it is, of course,
desired for the pocket in which the microstimulator is implanted to
be stimulated.
[0012] However, it y sometimes be desirable to connect one or more
short, flexible stimulation leads to a microstimulator, as
described in U.S. patent application Ser. No. 09/624,120, filed
Jul. 24, 2000, which is expressly incorporated herein by reference.
The use of such leads may permit electrical stimulation to be
directed more locally to target tissue a short distance from the
microstimulator, while allowing the microstimulator to be located
in a more surgically convenient site. In this case, stimulation of
the imply implantation pocket is undesirable.
[0013] Because the case of a microstimulator is relatively small,
the current density the surface of the case may be quite high when
the microstimulator is operated in a monopolar mode. For example,
the surface area on the case of a Precision.RTM. IPG is 3882
mm.sup.2, whereas the surface area of the anodic surface of the
Bion.RTM. microstimulator is approximately 50 mm.sup.2. If this
anodic surface were used with a leaded Bion.RTM. microstimulator,
undesired and perhaps annoying or painful stimulation in the
implantation pocket might be expected.
[0014] Attempts have been made to prevent or, at least reduce,
inadvertent pocket stimulation when operating a neurostimulator in
a monopolar mode. For example, it is known to coat a portion of the
neurostimulator case (e.g., the edges where current density is the
greatest) with an insulative material in order to reduce pocket
stimulation (see Toshimi Yajima, et al. "Effects of Muscle
Potential Depression and Muscle Stimulation Caused by Different
Insulation Coating Configurations on Cardiac Pacemakers: The Use of
Insulative Coatings to Try to Reduce Pocket Stimulation," J Artif
Organs (2005) 8:47-50; Davies T, "Do Permanent Pacemakers Need an
Insulative Coating? Results of Prospective Randomized Double-Blind
Study," Pacing Clin. Electrophysiol. 1997 Oct. 20 (10 Pt
1):2394-7). However, coating a portion of the neurostimulator case
necessarily increases the current density of the uncoated portions,
thereby potentially increasing the chance that pocket stimulation
will occur adjacent these higher current density sections.
Furthermore, new edges are created between the coated and uncoated
portions of the neurostimulator case, thereby creating higher
current densities at these new edges.
[0015] There, thus, remains a need to provide an improved
neurostimulator and technique that prevents, or at, least,
minimizes inadvertent pocket stimulation,
SUMMARY OF THE INVENTION
[0016] In accordance with the present inventions, a method of
providing therapy using a neurostimulator implanted within a tissue
pocket of a patient is provided. The method comprises initially
conveying pulsed electrical energy from the neurostimulator,
thereby stimulating a target tissue site remote from the tissue
pocket, and returning the initially conveyed pulsed electrical
energy to the neurostimulator, thereby causing stimulation of the
tissue pocket.
[0017] The method further comprises detecting the tissue pocket
stimulation, and decreasing the pulse width (e.g., to a value in
the range 10-100 .mu.s, and perhaps even equal to or less than 50
.mu.s) in response to the detection of the tissue pocket
stimulation. The method further comprises subsequently conveying
pulsed electrical energy having the decreased pulse width from the
neurostimulator, thereby stimulating the target tissue site, and
returning the subsequently conveyed pulsed electrical energy to the
neurostimulator, wherein any stimulation of the tissue pocket is
prevented, or at least decreased.
[0018] In one method, the pulsed electrical energy is conveyed from
the neurostimulator to at least one stimulation lead connected to
the neurostimulator. In another method, the pulsed electrical
energy is conveyed between a case of the neurostimulator and the
target tissue site. In still another method, the pulse amplitude of
the subsequently conveyed pulsed electrical energy is increased.
For example, the pulse width can be manually decreased, while the
pulse amplitude is automatically increased in response to the
manual decrease of the pulse width.
[0019] Other and further aspects and features of the invention will
be evident from reading the following detailed description of the
preferred embodiments, which are intended to illustrate, not limit,
the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] The drawings illustrate the design and utility of preferred
embodiments of the present invention, in which similar elements are
referred to by common reference numerals. In order to better
appreciate how the above-recited and other advantages and objects
of the present inventions are obtained, a more particular
description of the present inventions briefly described above will
be rendered by reference to specific embodiments thereof, which are
illustrated in the accompanying drawings. Understanding that these
drawings depict only typical embodiments of the invention and are
not therefore to be considered limiting of its scope, the invention
will be described and explained with additional specificity and
detail through the use of the accompanying drawings in which:
[0021] FIG. 1 is plan view of one embodiment of a tissue
stimulation system arranged in accordance with the present
inventions;
[0022] FIG. 2 is a perspective view of a microstimulator used in
the tissue stimulation system of FIG. 1;
[0023] FIG. 3 is a block diagram of the electronic circuitry
contained within the microstimulator of FIG. 2;
[0024] FIG. 4 is a plot of strength-duration curves for an
exemplary small nerve fiber and an exemplary large nerve fiber;
[0025] FIG. 5 is a perspective view of one embodiment of the
microstimulator of FIG. 2 that can be operated to reduce pocket
stimulation;
[0026] FIG. 6 is a plot of a stimulation pulse and temporally
spaced return pulses that can be generated by the microstimulator
of FIG. 5 to reduce the pocket stimulation;
[0027] FIG. 7 is a perspective view of another embodiment of the
microstimulator of FIG. 2 that can be operated to reduce pocket
stimulation;
[0028] FIG. 8 is a perspective view of still another embodiment of
the microstimulator of FIG. 2 that can be operated to reduce pocket
stimulation;
[0029] FIGS. 9A-9C are perspective views of other embodiments of
the microstimulator of FIG. 2 that can be operated to reduce pocket
stimulation; and
[0030] FIGS. 10A-10B are perspective views of still other
embodiments of the microstimulator of FIG. 2 that can be operated
to reduce pocket stimulation.
DETAILED DESCRIPTION OF THE EMBODIMENTS
[0031] Turning first to FIG. 1, an exemplary tissue stimulation
system 10 generally includes a neurostimulator, and in particular,
microstimulator 12, an implantable stimulation lead 14, an external
control device in the form of a remote controller RC 16, and a
clinician's programmer (CP) 18. The stimulation system 10 may also
include an external trial stimulator (not shown) for testing the
effects of the stimulation prior to implantation of the
microstimulator 12, and an external charger 20 for recharging the
microstimulator 12.
[0032] It should be noted that, although the neurostimulator is
described as being a is microstimulator herein, the present
invention may be used with used with any type of implantable
electrical circuitry used to stimulate tissue where it is desirable
to prevent pocket stimulation. It should also be noted that,
although the microstimulator 12 is described herein as being
battery-powered, the microstimulator 12 alternatively be
radio-frequency (RF)-controlled. Ultimately, the architecture of
the microstimulator 12 will depend on the context in which it is
intended to be used.
[0033] For example, for some patients, the use of a stimulator for
only a few hours per day or week will improve the symptomatology of
the ailment or ailments suffered by the patient. In such patients,
RF-controlled devices provide an adequate amount of stimulation if
used intermittently, e.g., for only a few hours per day, to greatly
decrease the incidence of the symptoms. For many other patients,
however, a continuous or intermittent stimulation throughout the
day is needed. These patients may best utilize a stimulator that
has a self-contained power source sufficient to deliver stimulation
for several hours and that can be recharged repeatedly, if
necessary. Thus, the use of a stimulator with a rechargeable
battery, thus, provides these patient the portability needed to
free the patient from reliance on RF power delivery.
[0034] For purposes of the specification, it is sufficient to note
that RF-controlled stimulators receive power and control signals
from an extracorporeal device via inductive coupling of a modulated
RF field. Battery-powered stimulators incorporate a power source
within the device itself, but rely on RF control, inductive
linking, or the like to program stimulus sequences and to recharge
the power source, when needed. Whether RF-controlled or
battery-powered the implanted stimulator may be commanded to
generate pulsed electrical stimulation energy (i.e., a temporal
series of electrical pulses) in accordance with a set of
stimulation parameters, including pulse amplitude, pulse width,
pulse rate, etc. Further details discussing battery-powered
stimulators are disclosed in PCT Publication WO 98/7926, WO
98/43700, and WO 98/43701, and U.S. Patent Publication No.
2008/0097599, which are expressly incorporated herein by
reference.
[0035] Thus, once the microstimulator 12 and stimulation lead 14
are implanted, the RC 16 may be used to telemetrically control the
IPG 14 via a bi-directional RF communications link 22. Such control
allows the microstimulator 12 to be turned on or off and to be
programmed with different stimulation parameter sets. The RC 16 may
also be operated to modify the programmed stimulation parameters to
actively control the characteristics of the electrical stimulation
energy output by the microstimulator 12.
[0036] The CP 18 provides clinician detailed stimulation parameters
for programming the microstimulator 12 in the operating room and in
follow-up sessions. The CP 18 may perform this function by
indirectly communicating with the microstimulator 12, through the
RC 16, via an IR communications link 24. Alternatively, the CP 18
may directly communicate with the microstimulator 12 via an RF
communications link (not shown). The external charger 20 is a
portable device used to transcutaneously charge the microstimulator
12 via an inductive link 26. Once the microstimulator 12 has been
programmed, and its power source has been charged by the external
charger 20 or otherwise replenished, the microstimulator 12 may
function as programmed without the RC 16 or CP 18 being
present.
[0037] The stimulation lead 14 carries a plurality of electrodes 28
arranged in an array. In the illustrated embodiment, the
stimulation lead 14 is a flexible percutaneous lead, and to this
end, the electrodes 28 are arranged in-line along the stimulation
lead 14. In the illustrated embodiment, the stimulation lead 14
includes four collinear electrodes 28, but may include as little as
two electrodes 28 and as many as sixteen or more electrodes 28. In
alternative embodiments, the electrodes 8 may be arranged in a
two-dimensional pattern on a single paddle lead. The electrodes 28
may be composed of a noble or refractory metal or compound, such as
platinum, iridium, tantalum, titanium, titanium nitride, niobium,
or alloys thereof, in order to avoid corrosion or electrolysis,
which could damage the surrounding tissues and/or stimulation lead
14.
[0038] The use of the stimulation lead 4 permits electrical
stimulation to be directed more locally to targeted tissue sites a
short distance from the tissue pocket in which the microstimulator
14 will be implanted. In one embodiment, the leads are no longer
than 120 mm. The stimulation lead 14 is preferably less than 5 mm
in diameter, and more preferably less than1.5 mm in diameter. The
stimulation lead 14 further comprises a proximal connector (not
shown) and wires (not shown) electrically connecting the electrodes
28 to the proximal connector. Further details regarding the use of
stimulation leads with microstimulators are disclosed in U.S.
patent application Ser. No. 09/624,130, filed Jul. 24, 2000 which
is expressly incorporated herein by reference.
[0039] As shown in FIG. 1, a single microstimulator 12 and
stimulation lead 14 are implanted under the skin 29 of the patient,
with the microstimulator 12 disposed into a subcutaneous pocket 30,
and the electrode array 28 of the stimulation lead 14 disposed at a
target tissue site 32 remote from the subcutaneous pocket 30. Nerve
bundles at the target tissue site 32 may carry somatic sensor axons
supplying receptors in skin and muscle and somatic motor axons
supplying skeletal muscle, as well as autonomic axons supplying
visceral and glandular structures and smooth muscle. In alternative
techniques, multiple microstimulators 12 with respective
stimulation leads 14 or a single microstimulator 12 with multiple
stimulation leads 14 may be implanted to achieve greater
stimulation of the target tissue site 32.
[0040] The microstimulator 12 may be implanted into the patient
with a surgical insertion tool specifically designed for this
purpose, as described in U.S. Pat. No 6,582,441, which is expressly
incorporated herein by reference, or may be placed, for example,
via a small incision and through a small cannula. Alternatively,
the microstimulator 12 may be implanted via conventional surgical
methods, or may be inserted using other endoscopic or laparoscopic
techniques. A more complicated surgical procedure may be required
for the purposes of fixing the microstimulator 12 in place. The
stimulation lead 14 may be implanted into the patient using
suitable means, such as an endoscope or laparoscope and mated to
the microstimulator 12.
[0041] Referring now to FIG. 2, the microstimulator 12 includes a
case 34 and electronic circuitry 38 (shown in FIG. 3) contained
within the case 34. A preferred microstimulator 12 is sufficiently
small to permit its placement near structures with very little
discomfort. As such, the case 34 may have an area equal to or less
than 1000 mm, and preferably, less than 200 mm.sup.2, and may have
a diameter less than 5 mm and a length less than 35 mm. The shape
of the microstimulator 12 may be determined by the structure in
which it will be implanted, the surrounding area, and the method of
implantation. In the illustrated embodiment, the case 34 takes the
form of a narrow, elongated body with an oblong cross-section, but
other shapes, such as rounded cylinders, spheres, disks, and
helical structures, are possible. The outer case 34 is composed of
an electrically conductive, biocompatible material, such as
titanium, and forms a hermetically sealed compartment wherein the
internal electronics are protected from the body tissue and fluids.
In the illustrated embodiment, the case 34 or a portion thereof may
serve as an electrode.
[0042] The microstimulator 12 further includes connector 36 to
which the proximal end of the stimulation lead 14 can be mated,
thereby electrically coupling the electronic circuitry to the
electrode array 28. In particular, as shown in FIG. 3, the
electronic circuitry 38 is also electrically coupled to the
connector 36 via one or more output terminals 40 (in this case,
eight terminals for the eight electrodes) and electrically coupled
to the case 34 via one or more electrical terminals 42 (in this
case, a single terminal), so that the electrical energy can be
conveyed to the electrodes 28, and thus, the tissue target site 32,
and then returned at the case 34.
[0043] The electronic circuitry 38 further includes analog
stimulation circuitry 44 in the form of pulse generation circuitry
that delivers the electrical stimulation energy in the form of a
pulsed electrical waveform to the electrode array 28 in accordance
with a set of stimulation parameters. Such stimulation parameters
may comprise electrode combinations, which define the electrodes
(including the case 34) that are activated as anodes (positive),
cathodes (negative), and turned off (zero), and electrical pulse
parameters, which define the pulse amplitude (measured in milliamps
or volts depending on whether the microstimulator 12 supplies
constant current or constant voltage to the electrode array 28),
pulse duration (or pulse width) (measured in microseconds), and
pulse rate (measured in pulses per second).
[0044] The stimulation circuitry 44 may, e.g., include a single
current or voltage source (not shown) for conveying stimulation
energy to selected ones of the electrodes 28 as a group and
returning the stimulation energy to selecting ones of the
electrodes as a group, or multiple current or voltage sources (no
shown) for independently conveying stimulation energy to selected
ones of the electrodes and independently returning the stimulation
energy to selected ones of the electrodes.
[0045] In any event, electrical stimulation will occur between two
(or more) activated electrodes, one of which may be the case 34.
Simulation energy may be transmitted to the tissue in a monopolar
or multipolar (e.g., bipolar, tripolar, etc.) fashion. Monopolar
stimulation occurs when selected ones of the lead electrodes 28 are
activated along with the case 34 of the microstimulator 12, so that
stimulation energy is transmitted between the selected lead
electrodes 28 and the case 34. Bipolar stimulation occurs when two
of the lead electrodes 28 are activated as anode and cathode, so
that stimulation energy is transmitted between the selected
electrodes 28. Tripolar stimulation occurs when three of the lead
electrodes 28 are activated, two as anodes and the remaining one as
a cathode, or two as cathodes and the remaining one as an anode.
Thus, when the microstimulator 12 is operated in a monopolar mode,
the stimulation circuitry 44 will convey electrical stimulation
energy to the lead electrodes 28 and return the electrical
stimulation to the case 34. When the microstimulator 12 is operated
in a multipolar mode, the stimulation circuitry 44 will convey
electrical stimulation energy to lead electrodes 28 and return the
electrical stimulation energy at different lead electrodes 28.
[0046] In the illustrated embodiment, when the microstimulator 12
is operated in the monopolar mode, the electrical energy that is
conveyed from the microstimulator 12 to the target tissue site is
cathodic (i.e., the activated lead electrodes 28 are cathodes), and
the electrical energy returned to the case 34 of the
microstimulator 12 is anodic (i.e., the case 34 serves as an
anode). However, the assignment of anodic and cathodic current to
the lead electrodes 28 and case 34 will ultimately depend on the
application in which the system 10 is intended to use. That is, if
the nerve fibers at the target tissue site 32 are to be stimulated
with cathodic current, the lead electrodes 28 will be cathodes and
the case 34 will be an anode. On the other hand, if the nerve
fibers at the target tissue site 32 are to be stimulated with
anodic current, the lead electrodes 28 will be anodes and the case
34 will be a cathode. In any event, the lead electrodes 28 and the
case 34 wilt be oppositely polarized.
[0047] The stimulation energy may be delivered between electrodes
as monophasic electrical energy or multiphasic electrical energy.
Monophasic electrical energy includes a series of pulses that are
either all positive (anodic) or all negative(cathodic). Multiphasic
electrical energy includes a series of pulses that alternate
between positive negative. For example, multiphasic electrical
energy may include a series of biphasic pulses, with each biphasic
pulse including a cathodic (negative) stimulation pulse and an
anodic (positive) recharge pulse that is generated after the
stimulation pulse to prevent direct current charge transfer through
the tissue, thereby avoiding electrode degradation and cell trauma.
That is, charge is conveyed through the electrode-tissue interface
via current at an electrode during a stimulation period (the length
of the stimulation pulse), and then pulled back off the
electrode-tissue interface via an oppositely polarized current at
the same electrode during a recharge period (the length of the
recharge pulse).
[0048] In the illustrated embodiment, the microstimulator 12 is
battery-powered, and thus, the electronic circuitry 38 includes an
energy storage device 46; for example, a replenishable or
rechargeable battery, such as a lithium ion battery, an
electrolytic capacitor, a super- or ultra-capacitor, or the like).
If the energy storage device 46 is replenishable or rechargeable,
an external charger (not shown) can be used to charge the power
source via an inductive link. The energy storage device 46 is
configured to output a voltage used to supply the various
components of the electronic circuitry 38 with power. The energy
storage device 46 also provides power for any stimulation current
generated by the stimulation circuitry 44.
[0049] The microstimulator 12 further comprises telemetry circuitry
48 (which includes an antenna in the form of an inductive coil and
transceiver) for receiving programming data (including stimulation
parameters), transmitting status data to and from the remote
controller 16, and receiving power from an external device, which
may be the remote controller 16. The microstimulator 12 further
comprises control circuitry 50 (which may be embodied in an
integrated circuitry (IC) chip) for operating the stimulation
circuitry 44 in accordance with a set or sets of stimulation
parameters (e.g., selection of activated electrodes, pulse
amplitude, pulse width, pulse rate, etc.), and a memory 52 for
storing the stimulation parameters. The memory 52 may be any type
of memory unit, such as, but not limited to, random access memory
(RAM), status RAM (SRAM), EEPROM a hard drive, or the like. Thus,
the use of the control circuitry 50 and memory 52 allow the
stimulation parameters to be adjusted to setting that are safe and
efficacious with minimal discomfort for each individual. Specific
stimulation parameters may provide therapeutic advantages for
different patients or for various types and classes of ailments.
For instance some patients may respond favorably to intermittent
stimulation, while others may require continuous stimulation for
treatment and relief.
[0050] In addition, different stimulation parameters may have
different effects on different tissue. Therefore, stimulation
parameters may be chosen to target specific neural or other cell
populations and/or to exclude others, or to increase activity in
specific neural or other cell populations, and/or to decrease
activity in others. For example, a relatively low pulse rate (i.e.,
less than 100 pulses per second (pps)) may have an excitatory
effect on surrounding neural tissue, leading to increased neural
activity ("excitatory stimulation"), whereas a relatively high
pulse rate (i.e., greater than 100 pps) may have an inhibitory
effect, leading to decreased neural activity ("inhibitory
stimulation"). As another example, a relatively low pulse amplitude
(typically less than 15 mA), but dependent on the distance between
electrodes and nerve fibers) are likely to recruit relatively large
diameter fibers (e.g., A-.alpha. and/or A-.beta. fibers), while not
recruiting relatively small diameter fibers (e.g., A-.delta. and/or
C fibers). In the illustrated embodiment, the pulse rate may be in
the range of 2-20 pps. The pulse duration may be in the range of
50-350 microseconds, and the amplitude may be in the range of 1-5V
at about 1-50 mA,
[0051] Significant to the present inventions, the microstimulator
12 is designed in a manner that prevents, or at least reduces, the
pocket stimulation phenomenon when operated in a monopolar
mode.
[0052] For example, in the case where the nerve stimulated in the
tissue pocket 30 are smaller than the nerve fibers stimulated at
the target tissue site 32 for therapy, stimulation energy having
relatively short pulse widths might be used to selectively
stimulate the larger nerve fibers at the target tissue site 32
without stimulating the smaller nerve fibers at the tissue pocket
30. The difference between the amplitude at which smaller nerve
fibers are stimulated and the amplitude at which large nerve fibers
are stimulated allows the amplitude of the electrical energy to be
adjusted within a usage range where the larger nerve fibers at the
target tissue site 32 are stimulated, while the smaller nerve
fibers at the tissue pocket 30 are not stimulated. Reducing the
pulse width of the electrical stimulation effectively increases
this usage range.
[0053] The benefits of this technique can be better appreciated
with reference to FIG. 4, which illustrates the strength-duration
curves for smaller nerve fibers (solid line) and larger nerve
fibers (dashed line). Notably, a strength-duration curve represents
the pulse amplitude and pulse width needed to stimulate a nerve
fiber of a specified diameter, and the usage range with respect to
the strength-duration curve is the variance from the
strength-duration curve that maintains the stimulation energy
between the point at which it is perceived by the patient (i.e.,
the point at which stimulation at the tissue target site 32 (shown
in FIG. 1) is perceived) and the point at which it is uncomfortable
for the patient (i.e., the point at which stimulation at the tissue
pocket 30 is perceived). It should be noted that at shorter pulse
widths, the usage range between large and small nerve fibers is
greater (assuming a pulse width of 50 .mu.s, the usage range is 60%
of the amplitude that stimulates the large nerve fibers) than the
usage range between larger and small nerve fibers (assuming a pulse
width of 250 .mu.s, the usage range is 41% of the amplitude that
stimulates the large nerve fibers). It follows from this that using
stimulation energy with small pulse widths is more selective, and
therefore, can be more easily used to avoid stimulation of the
relatively small nerve fibers in the tissue pocket 30 (shown in
FIG. 1), while stimulation energy with larger pulse widths is not
as selective, and therefore, more difficult to avoid stimulation of
the relatively small nerve fibers in the tissue pocket 30.
[0054] In one exemplary manner that uses stimulation energy with
smaller pulse widths to avoid pocket stimulation, pulsed electrical
energy is initially conveyed from the microstimulator 12 to the
lead electrodes 28, thereby stimulating the target tissue site 32
remote from the tissue pocket 30. The initially conveyed pulsed
electrical energy is then returned to the case 34 of the
neurostimulator, potentially causing stimulation of the tissue
pocket 30. If stimulation of the tissue pocket 30 is detected
(e.g., by the patient experiencing discomfort in that region and
relaying this information to the clinician), the pulse width of the
stimulation energy is decreased (e.g., in the range 10-100 .mu.s
and preferably equal or less than 50 .mu.s).
[0055] The pulsed electrical energy is subsequently conveyed from
the microstimulator 12 to the lead electrodes 28, thereby
stimulating the target tissue site 32 again. The step of
subsequently conveying the pulsed electrical energy may be
performed by simply continuing the conveyance of the initially
electrical energy or by ceasing conveyance of the pulsed electrical
energy and then again conveying the pulsed electrical energy, in
any event, the subsequently conveyed pulse electrical energy is
then returned to the case 34 of the microstimulator 12 again. For
the purposes of this specification, the term "initial" with
reference to electrical energy conveyance does not necessarily mean
the first time the electrical energy is conveyed by the
microstimulator 12, but rather merely presupposes that there will
be a subsequent conveyance of electrical energy.
[0056] Because the subsequently conveyed electrical energy is more
selective than the initially conveyed electrical energy due to the
decreased pulse width, stimulation of the tissue pocket 30 will be
decreased, if not eliminated altogether. The pulse width of the
stimulation energy can be further decreased to prevent stimulation
of the tissue pocket 30 if not already eliminated. It should be
noted that, as illustrated in the strength-duration curves in FIG.
4, the point at which a nerve fiber at a specific size is
stimulated increases as the pulse width of the stimulation energy
decreases, thereby potentially losing stimulation of the target
tissue site 32 if adjustments in the amplitude are not made.
[0057] As a result, it may be desirable to increase the amplitude
of the stimulation energy as the pulse width of the stimulation
energy is decreased. Adjustment of the pulse width and amplitude of
the stimulation energy can be manually performed via operation of
the remote controller 16 or automatically performed, such as
described in U.S. patent application Ser. No. 11/553,447, entitled
"Method of Maintaining Intensity Output While Adjusting Pulse Width
or Amplitude" and U.S. patent application Ser. No. 12/606,050
entitled "System and Method for Automatically Adjusting Pulse
Parameters to Selectively Activate Nerve Fibers " which are
expressly incorporated herein by reference.
[0058] As another example of preventing pocket stimulation, the
microstimulator 12 may be provided with spatially segmented
electrodes (in this case, three electrodes E1-E3) that are
associated with the case 34, as shown in FIG. 5. Although the case
electrodes E1-E3 are illustrated as completely covering the outer
surface of the case 34, the electrodes E1-E3 can be disposed on or
form only a portion of the outer surface of the case 34. The
microstimulator 12 is configured to independently return the pulsed
electrical energy to the respective electrodes, thereby reducing or
preventing inadvertent stimulation of the tissue pocket 30 (shown
in FIG. 1).
[0059] The electrodes E1-E3 may form the structure of the case 34
itself, or may he formed of electrically conductive and
biocompatible material mounted to the outside of the case 34. The
shape of the electrodes 28 be ring-shaped, as shown, or radial or
disk-like. Ultimately, the shape of the electrodes 28 will depend
upon the shape of the case 34. Although three ring-shaped
electrodes E1-E3 are shown to form the case 34 illustrated in FIG.
5, a different number of electrodes with different shapes can be
used. For example, as shown in FIG. 7, the same-shaped case 34 is
used, but two electrodes E1-E2 can form the two halves of the case
34 that are coupled together in a clam-shell arrangement.
[0060] In one technique for independently returning electrical
energy to the case electrodes 28, a temporally segmented current
waveform can be used, in particular, the electrical pulses returned
to the respective electrodes 28 are temporally interleaved, as
shown in FIG. 6. For example, for each stimulation pulse delivered
to the lead electrodes 28, a pulse or a train of pulses can be
returned to the first case electrode E1, then a pulse or a train of
pulses can be returned to the second case electrode E2, and then a
pulse or a train of pulses can be returned to the third case
electrode E3. In this case, each return pulse necessarily has a
shorter duration than the stimulation pulse. The pulses can be
sequentially returned to the electrodes E1-E3, as shown in FIG. 6,
or alternatively, can be randomly returned to the electrodes E1-E3.
This process can be repeated for the same stimulation pulse, such
that multiple pulses can be returned, as illustrated in FIG. 6.
[0061] It can be appreciated that the foregoing technique minimizes
temporal summation of voltage at or across the neural membranes by
virtue of the temporal spacings of the return pulses, and minimizes
spatial summation of the voltage at or across the neural membranes
by virtue of spacing of the electrodes 28. In alternative
embodiments any of the lead electrodes 28, in combination with the
case electrodes 28, can be used to return the electrical
energy.
[0062] In another technique for independently returning electrical
energy to the case electrodes, the case electrodes are selectively
activated; for example, via operation of the remote controller 16,
thereby changing the electrical field within the tissue pocket 30
(shown in FIG. 1). Thus, the combination of case electrodes that
reduces or eliminates the pocket stimulation can be selected. For
example, electrical energy can be conveyed to the lead electrodes
28 and returned to the case 34 for different combinations of
activated case electrodes. Based on patient feedback, the
combination of case electrodes with the best result can then be
selected as the return electrodes.
[0063] If the stimulation circuitry 44 of the microstimulator 12
(shown FIG. 3) only comprises a single current or voltage source,
the relative currents returned to the activated case electrodes
cannot be controlled. However, if multiple current or voltage
sources are provided, the fractionalized currents returned to the
activated case electrodes can be controlled. For example, as shown
in FIG. 7, one case electrode E1 can return 65% of the current, and
another case electrode E2 can return 35% of the current.
[0064] In alternative embodiments, selected ones of the lead
electrodes 28, in combination with the activated case electrodes
E1-E2, can be used to return the electrical energy. In this case,
only a portion of the electrical energy is returned to the case
electrodes E1-E2. For example, the return current can be split 55%,
35%, and 10% between a selected case electrode E1, another selected
case electrode E2, and a selected lead electrode 28,
respectively.
[0065] As another example of preventing pocket stimulation
segmented electrodes can be associated with the case 34 of the
microstimulator 12 in the same manner described above with respect
to FIGS. 5 and 7. In this case, however, four electrodes are
provided, as shown in FIG. 8, with electrode E1 forming the main
body of the case 34, electrode E2 forming the bottom edge of the
case 34 that r meets the connector 36, electrode E3 forming the top
edge of the case 34, and electrode E4 forming the flat top surface
of the case 34.
[0066] Because geometrical features, such as corners and edges
(e.g., the electrodes E2 and E3), are know to exhibit higher
current densities as opposed to other geometrical features, such as
smooth or fiat surfaces (e.g., the electrodes E1 and E4), these
higher current density regions may be more likely to cause
stimulation than the other regions of the case 34. Thus, the
current densities on the case electrodes E1-E4 are made as uniform
as possible by decreasing the variances of the current densities to
a relatively low value, (e.g., 25% or less, and preferably 10% or
less . As a result, the maximum current density on the case
electrodes E1-E4 is minimized, thereby minimizing or completely
eliminating pocket stimulation.
[0067] The current densities on the case electrodes E1-E4 can be
made more uniform by varying the relative impedances of the case
electrodes E1-E4, with the electrodes that normally have higher
current densities (e.g., electrodes having edges and corners, such
as electrodes E2 and E3) having a relatively higher impedance, and
the electrodes that normally have lower current densities (e.g.,
the electrodes having smooth or flat surfaces such as electrodes E1
and E4) having a relatively lower impedance. For example, FIG. 8
illustrates a circuit representation of the case electrodes E1-E4
and associated impedances R1-R4. As there shown, the input terminal
42 of the stimulation circuitry 44 (shown in FIG. 3) is coupled in
parallel to the tour case electrodes E1-E4. It should be noted,
however, that the input terminal 42 can alternatively be coupled in
series to the four case electrodes E1-E4.
[0068] The relative impedances of the case electrodes E1-E4 can be
changed, e.g., by making the electrodes from materials with
different resistivities. For example, the edge electrodes E2, E3
may be composed of a material that has a higher resistivity than
that of the flatter body and top electrodes E1, E4, thereby
decreasing the current density exhibited by the edge electrodes E2,
E3 relative to the current density that they would have exhibited
had they been made of the same material as the body and top
electrodes E1, E4.
[0069] As another example, passive components (not shown), such as
resistors, can be located between the input terminal 42 of the
stimulation circuitry 44 and the particular case electrodes, such
as the edge electrodes E2, E3, with the anticipated higher current
densities. In this manner, the passive components decrease the
current density on the edge electrodes E2, E3 relative to the
current density that would have been on the electrodes E2, E3
absent the passive components. Alternatively, passive components
can be located between the input terminal 42 of the stimulation
circuitry 44 and all four case electrodes E1-E4 with the combined
resistance value of the passive component or components associated
with each of the electrodes increasing with the anticipated current
density,
[0070] Although the case electrodes E1-E4 have been described as
being coupled to a single input terminal 42, in alternative
embodiments the case electrodes E1-E4 can be respectively coupled
to a plurality of input terminals. In this case, if the stimulation
circuitry 44 comprises multiple current or voltage sources, the
electrical currents at the respective input terminals can be
independently adjusted by increasing or decreasing the current or
voltage of the sources. For example, for the edge electrodes E2, E3
(i.e., the case electrodes where a high current density is
anticipated), the electrical current at the respective input
terminals coupled to these electrodes can be set to be relatively
low, whereas for the flat or smooth electrodes E1, E4 (i.e., the
case electrodes where a low current density is anticipated), the
electrical current at the respective input terminals coupled to
these electrodes can be set to be relatively high.
[0071] As another example of preventing pocket stimulation, the
current density on the case electrode(s) can be reduced by using an
external lead with additional electrodes that are not intended for
stimulation therapy. In this case, the electrical energy is
conveyed from the microstimulator 12 to the lead electrodes 28,
thereby stimulating the target tissue site 32 remote from the
tissue pocket 30 (shown in FIG. 1), while the conveyed electrical
energy is returned to both the case electrode(s) and the external
lead.
[0072] For example, in one embodiment illustrated in FIG. 9A, the
stimulation lead 14, itself, serves as the external lead, with
additional electrodes 50 located on the proximal end of the
stimulation lead for returning the electrical energy along with the
case electrode(s). In another embodiment illustrated in FIG. 9B, an
external lead 52 that is separate from the stimulation lead 14 is
provided. In still another embodiment, an external lead 54 in the
form of an electrical conductor is used to return the electrical
energy along with the case electrode(s). The electrical conductor
54 could, e.g., be composed of a flexible or rigid hypotube or wire
composed of a biocompatical, electrically conductive, material,
such as stainless steel or platinum. If the electrical conductor 54
is made from a hypotube, it can be etched for flexibility.
[0073] In either of the embodiments in FIGS. 9A-9C, the additional
electrodes 50 can be directly electrically coupled to the case 34
via the connector 36, thereby reducing the current density on the
case electrode(s) For the purposes of this specification, an
element is directly electrically coupled to another element if no
active components are located between the elements. In the
embodiments of FIGS. 9A and 9B, the electrodes 50 can be ganged
together using a common lead conductor (not shown). Alternatively,
any of the additional electrodes 50 can be electrically coupled to
input terminals of the stimulation circuitry 42 that are not
already used to return electrical energy from the case electrodes.
In this case, selected ones of the additional electrodes 50 may be
activated in the case where the stimulation circuitry 42 has a
switch and/or fractionalized currents can be assigned to the
activated electrodes 50 in the case where the stimulation circuitry
42 includes multiple current or voltage sources.
[0074] As another example of prevent pocket stimulation, the
current density on the case electrode(s) can be reduced by using an
external lead with additional electrodes that are not intended for
stimulation therapy. In this case, the electrical energy is
conveyed from the microstimulator 12 to the lead electrodes 28,
thereby stimulating the target tissue site 32 remote from the
tissue pocket 30 (shown in FIG. 1), while the conveyed electrical
energy is returned to both the case electrode(s) and the external
lead.
[0075] As another example of preventing pocket stimulation, the
current density case electrode(s) can be reduced by using an
expandable electrode that, like the previous embodiment, is not
intended for stimulation therapy. In this case, the electrical
energy is conveyed from the microstimulator 12 to the lead
electrodes 28, thereby stimulating the target tissue site 32 remote
from the tissue pocket 30 (shown in FIG. 1), while the conveyed
electrical energy is returned to both the case electrode(s) and the
expandable electrode.
[0076] For example, in one embodiment illustrated in FIG. 10A, an
expandable electrode 56 is directly electrically coupled to the top
of the case 34 via a conductor 58, and is therefore used to return
electrical energy along with the case electrode(s). To allow for
its expansion, the electrode 56 includes a plurality of stacked
blades 58 that are tied together at one end. When the electrode 56
is in a non-expanded geometry, the stacked blades 58 are folded
together (left side of FIG. 10A), so that it can be easily
introduced into the patient's body during implantation. To place
the electrode 56 in an expanded geometry, the stacked blades 58 are
radially folded out, much like a fan (right side of FIG. 10A),
thereby increasing the surface area of the electrode 56 after
implantation. In the illustrated embodiment, the shape of the
blades 58 is rectangular, although other shapes may be used,
including those with rounded edges or triangular shapes.
[0077] In another embodiment illustrated in FIG. 10B, an expandable
electrode 60 is directly electrically coupled too the top of the
case 34 via a conductor 62, and is therefore used to return
electrical energy along with the case electrode(s). To allow for
its expansion, the electrode 60 includes a plurality of stacked
blades 62 much like the expandable electrode 56 described above,
except that the stacked blades 82 are tied to each other at both
ends. When the electrode 60 is in a non-expanded geometry, the
stacked blades 62 are folded together (left side of FIG. 10B), so
that it can be easily introduced into the patient's body during
implantation. To place the electrode 60 is in an expanded geometry,
the stacked blades 62 are laterally folded out, much like blinders
(right side of FIG. 10B), thereby increasing the surface area of
the electrode 60 after implantation. In the illustrated embodiment,
the shape of the blades 62 is rectangular, although other shapes
may be used, including those with rounded edges or triangular
shapes.
[0078] Although particular embodiments of the present inventions
have been shown and described, it will be understood that it not
intended to limit the present inventions to the preferred
embodiments, and it will be obvious to those skilled in the art
that various changes and modifications may be made without
departing from the spirit and scope of the present inventions.
Thus, the present inventions are intended to cover alternatives,
modifications, and equivalents, which may be included within the
spirit and scope of the present inventions as defined by the
claims.
* * * * *