U.S. patent application number 16/265669 was filed with the patent office on 2019-08-01 for systems and methods to sense stimulation electrode tissue impedance.
The applicant listed for this patent is Stimwave Technologies Incorporated. Invention is credited to Patrick Larson, Richard LeBaron, Laura Tyler Perryman.
Application Number | 20190232057 16/265669 |
Document ID | / |
Family ID | 67391266 |
Filed Date | 2019-08-01 |
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United States Patent
Application |
20190232057 |
Kind Code |
A1 |
Perryman; Laura Tyler ; et
al. |
August 1, 2019 |
SYSTEMS AND METHODS TO SENSE STIMULATION ELECTRODE TISSUE
IMPEDANCE
Abstract
A method includes: transmitting a first set of radio-frequency
(RF) pulses to an implantable wireless stimulator device such that
electric currents are created from the first set of RF pulses and
flown through a calibrated internal load on the implantable
wireless stimulator device; in response to the electric currents
flown through a calibrated internal load, recording a first set of
RF reflection measurements; transmitting a second set of
radio-frequency (RF) pulses to the implantable wireless stimulator
device such that stimulation currents are created from the second
set of RF pulses and flown through an electrode of the implantable
wireless stimulator device to tissue surrounding the electrode; in
response to the stimulation currents flown through the electrode to
the surrounding tissue, recording a second set of RF reflection
measurements; and characterizing an electrode-tissue impedance by
comparing the second set of RF reflection measurements with the
first set of RF reflections measurements.
Inventors: |
Perryman; Laura Tyler;
(Pompano Beach, FL) ; Larson; Patrick; (Surfside,
FL) ; LeBaron; Richard; (Miami Beach, FL) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Stimwave Technologies Incorporated |
Pompano Beach |
FL |
US |
|
|
Family ID: |
67391266 |
Appl. No.: |
16/265669 |
Filed: |
February 1, 2019 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62624982 |
Feb 1, 2018 |
|
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
A61N 1/36171 20130101;
A61N 1/37223 20130101; A61N 1/3614 20170801; A61N 1/3787 20130101;
A61N 1/08 20130101; A61N 1/36125 20130101; A61N 1/36175 20130101;
A61N 1/36075 20130101 |
International
Class: |
A61N 1/36 20060101
A61N001/36; A61N 1/372 20060101 A61N001/372; A61N 1/378 20060101
A61N001/378; A61N 1/08 20060101 A61N001/08 |
Claims
1. A method to adjust stimulation by an implantable wireless
stimulator device to surrounding tissue, the method comprising:
transmitting, from an external pulse generator and via electric
radiative coupling, a first set of radio-frequency (RF) pulses to
the implantable wireless stimulator device such that electric
currents are created from the first set of RF pulses and flown
through a calibrated internal load on the implantable wireless
stimulator device; in response to the electric currents flown
through the calibrated internal load, recording, on the external
pulse generator, a first set of RF reflection measurements;
transmitting, from the external pulse generator and via electric
radiative coupling, a second set of radio-frequency (RF) pulses to
the implantable wireless stimulator device such that stimulation
currents are created from the second set of RF pulses and flown
through an electrode of the implantable wireless stimulator device
to tissue surrounding the electrode; in response to the stimulation
currents flown through the electrode to the surrounding tissue,
recording, on the external pulse generator, a second set of RF
reflection measurements; and characterizing an electrode-tissue
impedance by comparing the second set of RF reflection measurements
with the first set of RF reflections measurements.
2. The method of claim 1, further comprising: in response to
characterizing the electrode-tissue impedance as resistive,
adjusting one or more input pulses to be transmitted by the
external pulse generator to the implantable wireless stimulator
device such that stimulus currents created from the input pulses on
the implantable wireless stimulator device are adjusted to
compensate for a resistive electrode-tissue impedance.
3. The method of claim 2, wherein adjusting input pulses comprises:
maintaining a steady-state delivery of electrical power to the
implantable wireless stimulator device such that electrical energy
is extracted from the input pulses as fast as electrical energy is
consumed by the implantable wireless stimulator device to (i)
generate the stimulus currents with one or more pulse parameters
that have been varied to accommodate the resistive electrode-tissue
impedance, and (ii) deliver the stimulus currents from the
electrode on the implantable wireless stimulator device to the
surrounding tissue.
4. The method of claim 3, wherein the pulse parameters comprise: a
pulse width, a pulse amplitude, and a pulse frequency.
5. The method of claim 1, further comprising: in response to
characterizing the electrode-tissue impedance as capacitive,
adjusting one or more input pulses to be transmitted by the
external pulse generator to the implantable wireless stimulator
device such that stimulus currents created from the input pulses
and delivered by the electrode on the implantable wireless
stimulator device to the surrounding tissue are adjusted to
compensate for a capacitive electrode-tissue impedance.
6. The method of claim 5, wherein adjusting input pulses comprises:
maintaining a steady-state delivery of electrical power to the
implantable wireless stimulator device such that electrical energy
is extracted from the input pulses as fast as electrical energy is
consumed by the implantable wireless stimulator device to (i)
generate the stimulus currents with one or more pulse parameters
that have been varied to accommodate the capacitive
electrode-tissue impedance, and (ii) deliver the stimulus currents
from the electrode on the implantable wireless stimulator device to
the surrounding tissue.
7. The method of claim 6, wherein the pulse parameters comprise: a
pulse width, a pulse amplitude, and a pulse frequency.
8. The method of claim 1, further comprising: based on results of
characterizing the electrode-tissue impedance, automatically
choosing a stimulation session by determining input pulses to be
transmitted by the external pulse generator to the implantable
wireless stimulator device such that stimulus currents are created
on the implantable wireless stimulator device and delivered by the
electrode on the implantable wireless stimulator device to the
surrounding tissue.
9. The method of claim 8, wherein determining input pulses
comprises: updating the second set of radio-frequency (RF) pulses
to obtain updated second set of RF reflection measurements; and
comparing the updated second set of RF reflection measurements with
the first set of RF reflection measurements.
10. The method of claim 9, wherein updating and comparing are
performed iteratively until desired RF reflection measurements are
obtained.
11. The method of claim 1, further comprising: automatically
performing fault checking according to results of characterizing
the electrode-tissue impedance.
12. The method of claim 11, wherein automatically performing fault
checking comprises: detecting a damaged wire in a circuit leading
to the electrode on the implantable wireless stimulator device.
13. A system comprising: an implantable wireless stimulator device
comprising: a first non-inductive antenna; one or more electrodes;
and a circuit between the first non-inductive antenna and the one
or more electrodes, the circuit comprising: a calibrated internal
load that represents a pre-determined load condition on the one or
more electrodes; an external pulse generator comprising: a second
non-inductive antenna configured to: transmit, via electric
radiative coupling, a first set of radio-frequency (RF) pulses to
the first non-inductive antenna on the implantable wireless
stimulator device such that electric currents are created from the
first set of RF pulses and flown through the calibrated internal
load on the implantable wireless stimulator device; and transmit,
via electric radiative coupling, a second set of radio-frequency
(RF) pulses to the first non-inductive antenna on the implantable
wireless stimulator device such that stimulation currents are
created from the second set of RF pulses and flown through an
electrode of the implantable wireless stimulator device to tissue
surrounding the electrode; and a reflection sensor subs-system
coupled to the second non-inductive antenna and configured to: in
response to the electric currents flown through the calibrated
internal load, obtain a first set of RF reflection measurements;
and in response to the stimulation currents flown through the
electrode to the surrounding tissue, obtain a second set of RF
reflection measurements; and a signal processor in communication
with the reflection sensor subs-system and configured to:
characterize an electrode-tissue impedance by comparing the second
set of RF reflection measurements with the first set of RF
reflections measurements.
14. The system of claim 13, wherein the reflection sensor
subs-system comprises: a directional coupler coupled to the second
non-inductive antenna and configured to detect a radio frequency
(RF) signal reflected from the first non-inductive antenna; and a
radio frequency (RF) phase detector coupled to the directional
coupler and configured to detect phase differences between the RF
signal reflected from the first non-inductive antenna and an RF
signal transmitted from the second non-inductive antenna to the
first non-inductive antenna.
15. The system of claim 14, wherein the reflection sensor
subs-system further comprises: an analog-to-digital converter (ADC)
coupled to the directional coupler and configured to convert the RF
signal reflected from the first non-inductive antenna into digital
recordings.
16. The system of claim 15, wherein the signal processor is a
digital signal processor.
17. The system of claim 13, wherein the signal processor is further
configured to: in response to characterizing the electrode-tissue
impedance as resistive, adjust one or more input pulses to be
transmitted by the external pulse generator to the implantable
wireless stimulator device such that one or more stimulus pulses
created from the input pulses and delivered by the electrode on the
implantable wireless stimulator device to the surrounding tissue
are adjusted to compensate for a resistive electrode-tissue
impedance.
18. The system of claim 13, wherein the signal processor is further
configured to: in response to characterizing the electrode-tissue
impedance as capacitive, adjust one or more input pulses to be
transmitted by the external pulse generator to the implantable
wireless stimulator device such that stimulus currents created from
the input pulses and delivered by the electrode on the implantable
wireless stimulator device to the surrounding tissue are adjusted
to compensate for a capacitive electrode-tissue impedance.
19. The system of claim 13, wherein the signal processor is further
configured to: based on results of characterizing the
electrode-tissue impedance, automatically choose a stimulation
session by determining input pulses to be transmitted by the
external pulse generator to the implantable wireless stimulator
device such that stimulus currents are created on the implantable
wireless stimulator device and delivered by the electrode on the
implantable wireless stimulator device to the surrounding
tissue.
20. The system of claim 13, wherein the signal processor is further
configured to: automatically perform fault checking according to
results of characterizing the electrode-tissue impedance.
Description
RELATED APPLICATIONS
[0001] This application claims benefit of U.S. Provisional
Application No. 62/624,982, filed Feb. 1, 2019, which is
incorporated by reference in its entirety.
TECHNICAL FIELD
[0002] This application relates generally to systems and methods to
operation of an implantable stimulator device that has been
implanted inside a subject.
BACKGROUND
[0003] Modulation of excitable tissue in the body by electrical
stimulation has become an important type of therapy for patients
with chronic disabling conditions, including pain, movement
initiation and control, involuntary movements, vascular
insufficiency, heart arrhythmias and various other modalities. A
variety of therapeutic intra-body electrical stimulation techniques
can be utilized to provide therapeutic relief for these conditions.
For instance, devices may be used to deliver stimulatory signals to
excitable tissue, record vital signs, perform pacing or
defibrillation operations, record action potential activity from
targeted tissue, control drug release from time-release capsules or
drug pump units, or interface with the auditory system to assist
with hearing.
SUMMARY
[0004] In one aspect, some implementations provide a method to
adjust stimulation by an implantable wireless stimulator device to
surrounding tissue, the method including: transmitting, from an
external pulse generator and via electric radiative coupling, a
first set of radio-frequency (RF) pulses to the implantable
wireless stimulator device such that electric currents are created
from the first set of RF pulses and flown through a calibrated
internal load on the implantable wireless stimulator device; in
response to the electric currents flown through the calibrated
internal load, recording, on the external pulse generator, a first
set of RF reflection measurements; transmitting, from the external
pulse generator and via electric radiative coupling, a second set
of radio-frequency (RF) pulses to the implantable wireless
stimulator device such that stimulation currents are created from
the second set of RF pulses and flown through an electrode of the
implantable wireless stimulator device to tissue surrounding the
electrode; in response to the stimulation currents flown through
the electrode to the surrounding tissue, recording, on the external
pulse generator, a second set of RF reflection measurements; and
characterizing an electrode-tissue impedance by comparing the
second set of RF reflection measurements with the first set of RF
reflections measurements.
[0005] Implementations may include one or more of the following
features. In response to characterizing the electrode-tissue
impedance as resistive, the method may include adjusting one or
more input pulses to be transmitted by the external pulse generator
to the implantable wireless stimulator device such that stimulus
currents created from the input pulses on the implantable wireless
stimulator device are adjusted to compensate for a resistive
electrode-tissue impedance. Adjusting input pulses may include:
maintaining a steady-state delivery of electrical power to the
implantable wireless stimulator device such that electrical energy
is extracted from the input pulses as fast as electrical energy is
consumed by the implantable wireless stimulator device to (i)
generate the stimulus currents with one or more pulse parameters
that have been varied to accommodate the resistive electrode-tissue
impedance, and (ii) deliver the stimulus currents from the
electrode on the implantable wireless stimulator device to the
surrounding tissue. The pulse parameters may include: a pulse
width, a pulse amplitude, and a pulse frequency.
[0006] The method may include: in response to characterizing the
electrode-tissue impedance as capacitive, adjusting one or more
input pulses to be transmitted by the external pulse generator to
the implantable wireless stimulator device such that stimulus
currents created from the input pulses and delivered by the
electrode on the implantable wireless stimulator device to the
surrounding tissue are adjusted to compensate for a capacitive
electrode-tissue impedance. Adjusting input pulses may include:
maintaining a steady-state delivery of electrical power to the
implantable wireless stimulator device such that electrical energy
is extracted from the input pulses as fast as electrical energy is
consumed by the implantable wireless stimulator device to (i)
generate the stimulus currents with one or more pulse parameters
that have been varied to accommodate the capacitive
electrode-tissue impedance, and (ii) deliver the stimulus currents
from the electrode on the implantable wireless stimulator device to
the surrounding tissue. The pulse parameters may include: a pulse
width, a pulse amplitude, and a pulse frequency.
[0007] The method may further include: based on results of
characterizing the electrode-tissue impedance, automatically
choosing a stimulation session by determining input pulses to be
transmitted by the external pulse generator to the implantable
wireless stimulator device such that stimulus currents are created
on the implantable wireless stimulator device and delivered by the
electrode on the implantable wireless stimulator device to the
surrounding tissue. Determining input pulses may include: updating
the second set of radio-frequency (RF) pulses to obtain updated
second set of RF reflection measurements; and comparing the updated
second set of RF reflection measurements with the first set of RF
reflection measurements. Updating and comparing may be performed
iteratively until desired RF reflection measurements are
obtained.
[0008] The method may further include: automatically performing
fault checking according to results of characterizing the
electrode-tissue impedance. Automatically performing fault checking
may include: detecting a damaged wire in a circuit leading to the
electrode on the implantable wireless stimulator device.
[0009] In another aspect, some implementations provide a system
that includes: an implantable wireless stimulator device including:
a first non-inductive antenna; one or more electrodes; and a
circuit between the first non-inductive antenna and the one or more
electrodes, the circuit comprising: a calibrated internal load that
represents a pre-determined load condition on the one or more
electrodes; an external pulse generator including: a second
non-inductive antenna configured to: transmit, via electric
radiative coupling, a first set of radio-frequency (RF) pulses to
the first non-inductive antenna on the implantable wireless
stimulator device such that electric currents are created from the
first set of RF pulses and flown through the calibrated internal
load on the implantable wireless stimulator device; and transmit,
via electric radiative coupling, a second set of radio-frequency
(RF) pulses to the first non-inductive antenna on the implantable
wireless stimulator device such that stimulation currents are
created from the second set of RF pulses and flown through an
electrode of the implantable wireless stimulator device to tissue
surrounding the electrode; and a reflection sensor subs-system
coupled to the second non-inductive antenna and configured to: in
response to the electric currents flown through the calibrated
internal load, obtain a first set of RF reflection measurements;
and in response to the stimulation currents flown through the
electrode to the surrounding tissue, obtain a second set of RF
reflection measurements; and a signal processor in communication
with the reflection sensor subs-system and configured to:
characterize an electrode-tissue impedance by comparing the second
set of RF reflection measurements with the first set of RF
reflections measurements.
[0010] Implementations may include one or more of the following
features. The reflection sensor subs-system includes: a directional
coupler coupled to the second non-inductive antenna and configured
to detect a radio frequency (RF) signal reflected from the first
non-inductive antenna; and a radio frequency (RF) phase detector
coupled to the directional coupler and configured to detect phase
differences between the RF signal reflected from the first
non-inductive antenna and an RF signal transmitted from the second
non-inductive antenna to the first non-inductive antenna. The
reflection sensor subs-system may further include: an
analog-to-digital converter (ADC) coupled to the directional
coupler and configured to convert the RF signal reflected from the
first non-inductive antenna into digital recordings.
[0011] The signal processor may be a digital signal processor. The
signal processor may be further configured to: in response to
characterizing the electrode-tissue impedance as resistive, adjust
one or more input pulses to be transmitted by the external pulse
generator to the implantable wireless stimulator device such that
one or more stimulus pulses created from the input pulses and
delivered by the electrode on the implantable wireless stimulator
device to the surrounding tissue are adjusted to compensate for a
resistive electrode-tissue impedance. The signal processor may be
further configured to: in response to characterizing the
electrode-tissue impedance as capacitive, adjust one or more input
pulses to be transmitted by the external pulse generator to the
implantable wireless stimulator device such that stimulus currents
created from the input pulses and delivered by the electrode on the
implantable wireless stimulator device to the surrounding tissue
are adjusted to compensate for a capacitive electrode-tissue
impedance. The signal processor may be further configured to: based
on results of characterizing the electrode-tissue impedance,
automatically choose a stimulation session by determining input
pulses to be transmitted by the external pulse generator to the
implantable wireless stimulator device such that stimulus currents
are created on the implantable wireless stimulator device and
delivered by the electrode on the implantable wireless stimulator
device to the surrounding tissue. The signal processor may be
further configured to: automatically perform fault checking
according to results of characterizing the electrode-tissue
impedance.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIG. 1 depicts a high-level diagram of an example of a
wireless stimulation system.
[0013] FIG. 2 depicts a more detailed diagram of an example of the
wireless stimulation system.
[0014] FIG. 3A is an illustration of an example of an
implementation of the microwave field stimulator (MFS) transmitter
for wireless power transfer to an implanted dipole antenna.
[0015] FIG. 3B is another illustration of another example of the
implementation with a directional coupler and power detector.
[0016] FIGS. 4A-4D illustrate examples of normalized reflection
versus normalized power under various loading conditions.
[0017] FIG. 5 illustrates an example of a flow chart.
[0018] Like reference symbols in the various drawings indicate like
elements.
DETAILED DESCRIPTION
[0019] In various implementations, systems and methods are
disclosed for applying one or more electrical impulses to targeted
excitable tissue, such as nerves, for treating chronic pain,
inflammation, arthritis, sleep apnea, seizures, incontinence, pain
associated with cancer, incontinence, problems of movement
initiation and control, involuntary movements, vascular
insufficiency, heart arrhythmias, obesity, diabetes, craniofacial
pain, such as migraines or cluster headaches, and other disorders.
In certain embodiments, a device may be used to send electrical
energy to targeted nerve tissue by using remote radio frequency
(RF) energy without cables or inductive coupling to power an
implanted wireless stimulator device. The targeted nerves can
include, but are not limited to, the spinal cord and surrounding
areas, including the dorsal horn, dorsal root ganglion, the exiting
nerve roots, nerve ganglions, the dorsal column fibers and the
peripheral nerve bundles leaving the dorsal column and brain, such
as the vagus, occipital, trigeminal, hypoglossal, sacral, coccygeal
nerves and the like.
[0020] A wireless stimulation system can include an implantable
stimulator device with one or more electrodes and one or more
conductive antennas (for example, dipole or patch antennas), and
internal circuitry for detecting pulse instructions, and
rectification of RF electrical energy. The system may further
comprise an external controller and antenna for transmitting radio
frequency or microwave energy from an external source to the
implantable stimulator device with neither cables nor inductive
coupling to provide power.
[0021] In various implementations, the wireless implantable
stimulator device is powered wirelessly (and therefore does not
require a wired connection) and contains the circuitry necessary to
receive the pulse instructions from a source external to the body.
For example, various embodiments employ internal dipole (or other)
antenna configuration(s) to receive RF power through electrical
radiative coupling, and the received RF power is used to power the
implantable stimulator device. This allows such devices to produce
electrical currents capable of stimulating nerve bundles without a
physical connection to an implantable pulse generator (IPG) or use
of an inductive coil.
[0022] In some implementations, a passive relay module may be
configured as an implantable device to couple electromagnetic
energy radiated from an external transmitting antenna to a wireless
implantable stimulator device. In one example, the implantable
device includes two monopole coupler arms connected to each other
by a cable. One monopole coupler arm may be implanted in a parallel
configuration with the external transmitting antenna such that
linearly polarized electromagnetic waves radiated from the external
transmitting antenna are received by this monopole coupler arm.
Through the cable, the received electromagnetic waves may propagate
to the other monopole coupler arm. In a reciprocal manner, this
monopole coupler arm may radiate the received electromagnetic
energy to the receiving antenna of the stimulator device. To
effectively radiate the received electromagnetic energy to the
receiving antenna of the stimulator device, parallel alignment of
this other monopole coupler arm and the receiving antenna again may
be used. In some cases, lengths of the monopole arms and length of
the cable can be tailored to improve transmission efficiency, for
example, at a particular operating frequency.
[0023] Some implementations utilize non-battery wireless power
transfer implants, a new class of devices that can be constructed
in very small form factors, enabling a minimal surgical incision
and potentially unlimited product life, free of limitations and
complications associated with battery powered devices. However,
wireless power transfer faces various challenges. An implanted
antenna is ideally very small in size to pass through a needle or
cannula in order to enable a minimally invasive surgery. Generally
a small antenna receives less RF power than a larger antenna,
meaning the efficiency of power transfer to a very small antenna
can be poor. Compounding the problem is the limited RF power that
can be delivered by the external transmitting source because the
Specific Absorption Rate (SAR) of RF inside the human body must be
kept within safety limits. As such, optimum power transfer
efficiency (or minimum path loss) must be maintained during
wireless power transfer for implantable medical devices. To affect
optimum power transfer, the external transmitting antenna must be
aligned on the body in a favorable position relative to the
implant. Estimating the location of the implant was historically
only feasible using a medical imaging system, such as x-ray or
ultrasound. Some implementations disclosed herein enable locating
the in-situ receiver antenna, without the use of complex and
expensive medical imaging techniques.
[0024] Further descriptions of exemplary wireless systems for
providing neural stimulation to a patient can be found in
commonly-assigned, published PCT applications PCT/US2012/23029
filed Jan. 28, 2011 and published Aug. 2, 2012, PCT/US2012/32200
filed Apr. 11, 2011 and published Oct. 11, 2012, PCT/US2012/48903,
filed Jan. 28, 2011 and published Feb. 7, 2013, PCT/US2012/50633,
filed Aug. 12, 2011 and published Feb. 21, 2013 and
PCT/US2012/55746, filed Sep. 15, 2011 and published Mar. 21, 2013,
the complete disclosures of which are incorporated by
reference.
[0025] FIG. 1 depicts a high-level diagram of an example of a
wireless stimulation system. The wireless stimulation system may
include four major components, namely, a programmer module 102, a
RF pulse generator module 106, a transmitting (TX) antenna 110 (for
example, a patch antenna, slot antenna, or a dipole antenna), and
an implanted wireless stimulator device 114. The programmer module
102 may be a computer device, such as a smart phone, running a
software application that supports a wireless connection 104, such
as Bluetooth.RTM.. The application can enable the user to view the
system status and diagnostics, change various parameters,
increase/decrease the desired stimulus amplitude of the electrical
pulses, and adjust feedback sensitivity of the RF pulse generator
module 106, among other functions.
[0026] The RF pulse generator module 106 may include communication
electronics that support the wireless connection 104 and the
battery to power the generator electronics. In some
implementations, the RF pulse generator module 106 includes the TX
antenna embedded into its packaging form factor, while in other
implementations, the TX antenna is connected to the RF pulse
generator module 106 through a wired connection 108 or a wireless
connection (not shown). The TX antenna 110 may be coupled directly
to tissue to create an electric field that powers the implanted
wireless stimulator device 114. The TX antenna 110 communicates
with the implanted wireless stimulator device 114 through an RF
interface. For instance, the TX antenna 110 radiates an RF
transmission signal that is modulated and encoded by the RF pulse
generator module 110. The implanted wireless stimulator device of
module 114 contains one or more antennas, such as dipole
antenna(s), to receive and transmit through RF interface 112. In
particular, the coupling mechanism between antenna 110 and the one
or more antennas on the implanted wireless stimulation device of
module 114 utilizes electrical radiative coupling and not inductive
coupling. In other words, the coupling is through an electric field
rather than a magnetic field.
[0027] Through this electrical radiative coupling, the TX antenna
110 can provide an input signal to the implanted wireless
stimulator device 114. This input signal contains energy and may
contain information encoding stimulus waveforms to be applied at
the electrodes of the implanted wireless stimulator device 114. In
some implementations, the power level of this input signal directly
determines an applied amplitude (for example, power, current, or
voltage) of the one or more electrical pulses created using the
electrical energy contained in the input signal. Within the
implanted wireless stimulator device 114 are components for
demodulating the RF transmission signal, and electrodes to deliver
the stimulation to surrounding neural tissue.
[0028] The RF pulse generator module 106 can be implanted
subcutaneously, or it can be worn external to the body. When
external to the body, the RF generator module 106 can be
incorporated into a belt or harness design to allow for electric
radiative coupling through the skin and underlying tissue to
transfer power and/or control parameters to the implanted wireless
stimulator device 114. In either event, receiver circuit(s)
internal to the wireless stimulator device 114 can capture the
energy radiated by the TX antenna 110 and convert this energy to an
electrical waveform. The receiver circuit(s) may further modify the
waveform to create an electrical pulse suitable for the stimulation
of neural tissue.
[0029] In some implementations, the RF pulse generator module 106
can remotely control the stimulus parameters (that is, the
parameters of the electrical pulses applied to the neural tissue)
and monitor feedback from the wireless stimulator device 114 based
on RF signals received from the implanted wireless stimulator
device 114. A feedback detection algorithm implemented by the RF
pulse generator module 106 can monitor data sent wirelessly from
the implanted wireless stimulator device 114, including information
about the energy that the implanted wireless stimulator device 114
is receiving from the RF pulse generator and information about the
stimulus waveform being delivered to the electrodes. In order to
provide an effective therapy for a given medical condition, the
system can be tuned to provide the optimal amount of excitation or
inhibition to the nerve fibers by electrical stimulation. A closed
loop feedback control method can be used in which the output
signals from the implanted wireless stimulator device 114 are
monitored and used to determine the appropriate level of neural
stimulation for maintaining effective therapy, or, in some cases,
open loop control can be used.
[0030] FIG. 2 depicts a detailed diagram of an example of the
wireless stimulation system. As depicted, the programming module
102 may comprise user input system 202 and communication subsystem
208. The user input system 221 may allow various parameter settings
to be adjusted (in some cases, in an open loop fashion) by the user
in the form of instruction sets. The communication subsystem 208
may transmit these instruction sets (and other information) via the
wireless connection 104, such as Bluetooth or Wi-Fi, to the RF
pulse generator module 106, as well as receive data from module
106.
[0031] For instance, the programmer module 102, which can be
utilized for multiple users, such as a patient's control unit or
clinician's programmer unit, can be used to send stimulation
parameters to the RF pulse generator module 106. The stimulation
parameters that can be controlled may include pulse amplitude,
pulse frequency, and pulse width in the ranges shown in Table 1. In
this context the term pulse refers to the phase of the waveform
that directly produces stimulation of the tissue; the parameters of
the charge-balancing phase (described below) can similarly be
controlled. The patient and/or the clinician can also optionally
control overall duration and pattern of treatment.
TABLE-US-00001 Stimulation Parameter Table 1 Pulse Amplitude: 0 to
25 mA Pulse Frequency: 0 to 20000 Hz Pulse Width: 0 to 2 ms
[0032] The RF pulse generator module 106 may be initially
programmed to meet the specific parameter settings for each
individual patient during the initial implantation procedure.
Because medical conditions or the tissue properties can change over
time, the ability to re-adjust the parameter settings may be
beneficial to ensure ongoing efficacy of the neural modulation
therapy.
[0033] The programmer module 102 may be functionally a smart device
and associated application. The smart device hardware may include a
CPU 206 and be used as a vehicle to handle touchscreen input on a
graphical user interface (GUI) 204, for processing and storing
data.
[0034] The RF pulse generator module 106 may be connected via wired
connection 108 to an external TX antenna 110. Alternatively, both
the antenna and the RF pulse generator are located subcutaneously
(not shown).
[0035] The signals sent by RF pulse generator module 106 to the
implanted wireless stimulator device 114 may include both power and
parameter-setting attributes in regards to stimulus waveform,
amplitude, pulse width, and frequency. The RF pulse generator
module 106 can also function as a wireless receiving unit that
receives feedback signals from the implanted wireless stimulator
device 114. To that end, the RF pulse generator module 106 may
contain microelectronics or other circuitry to handle the
generation of the signals transmitted to the device 114 as well as
handle feedback signals, such as those from the stimulator device
114. For example, the RF pulse generator module 106 may comprise
controller subsystem 214, high-frequency oscillator 218, RF
amplifier 216, a RF switch, and a feedback subsystem 212.
[0036] The controller subsystem 214 may include a CPU 230 to handle
data processing, a memory subsystem 228 such as a local memory,
communication subsystem 234 to communicate with programmer module
102 (including receiving stimulation parameters from programmer
module), pulse generator circuitry 236, and digital/analog (D/A)
converters 232.
[0037] The controller subsystem 214 may be used by the patient
and/or the clinician to control the stimulation parameter settings
(for example, by controlling the parameters of the signal sent from
RF pulse generator module 106 to the stimulator device 114). These
parameter settings can affect, for example, the power, current
level, or shape of the one or more electrical pulses. The
programming of the stimulation parameters can be performed using
the programming module 102, as described above, to set the
repetition rate, pulse width, amplitude, and waveform that will be
transmitted by RF energy to the receiving (RX) antenna 238,
typically a dipole antenna (although other types may be used), in
the implanted wireless stimulation device 214. The clinician may
have the option of locking and/or hiding certain settings within
the programmer interface, thus limiting the patient's ability to
view or adjust certain parameters because adjustment of certain
parameters may require detailed medical knowledge of
neurophysiology, neuro-anatomy, protocols for neural modulation,
and safety limits of electrical stimulation.
[0038] The controller subsystem 214 may store received parameters
in the local memory subsystem 228, until the parameters are
modified by new data received from the programming module 102. The
CPU 206 may use the parameters stored in the local memory to
control the RF pulse generator circuitry 236 to generate a pulse
timing waveform that is modulated by a high frequency oscillator
218 in the range from 300 MHz to 8 GHz (preferably between about
700 MHz and 5.8 GHz and more preferably between about 800 MHz and
1.3 GHz). The resulting RF signal may then be amplified by RF
amplifier 226 and then sent through an RF switch 223 to the TX
antenna 110 to reach through depths of tissue to the RX antenna
238.
[0039] In some implementations, the RF signal sent by TX antenna
110 may simply be a power transmission signal used by the wireless
stimulation device module 114 to generate electric pulses. In other
implementations, a digital signal may also be transmitted to the
wireless stimulator device 114 to send instructions about the
configuration of the wireless stimulator device 114. The digital
signal is used to modulate the carrier signal that is coupled onto
the implanted antenna(s) 238 and does not interfere with the input
received on the same stimulator device to power the device. In one
embodiment the digital signal and powering signal are combined into
one signal, where the digital signal is used to modulate the RF
powering signal, and thus the wireless stimulation device is
powered directly by the received digital signal; separate
subsystems in the wireless stimulation device harness the power
contained in the signal and interpret the data content of the
signal.
[0040] The RF switch 223 may be a multipurpose device such as a
dual directional coupler, which passes the RF pulses to the TX
antenna 110 with minimal insertion loss while simultaneously
providing two low-level outputs to the feedback subsystem 212; one
output delivers a forward power signal to the feedback subsystem
212, where the forward power signal is an attenuated version of the
RF pulse sent to the TX antenna 110, and the other output delivers
a reverse power signal to a different port of the feedback
subsystem 212, where reverse power is an attenuated version of the
reflected RF energy from the TX Antenna 110. The reflected RF
energy and/or RF signals from the wireless stimulator device 114
are processed in the feedback subsystem 212.
[0041] The feedback subsystem 212 of the RF pulse generator module
106 may include reception circuitry to receive and extract
telemetry or other feedback signals from the wireless stimulator
device 114 and/or reflected RF energy from the signal sent by TX
antenna 110. The feedback subsystem may include an amplifier 226, a
filter 224, a demodulator 222, and an A/D converter 220.
[0042] The feedback subsystem 212 receives the forward power signal
and converts this high-frequency AC signal to a DC level that can
be sampled and sent to the controller subsystem 214. In this way
the characteristics of the generated RF pulse can be compared to a
reference signal within the controller subsystem 214. If a
disparity (error) exists in any parameter, the controller subsystem
214 can adjust the output to the RF pulse generator 106. The nature
of the adjustment can be, for example, proportional to the computed
error. The controller subsystem 214 can incorporate additional
inputs and limits on its adjustment scheme such as the signal
amplitude of the reverse power and any predetermined maximum or
minimum values for various pulse parameters.
[0043] The reverse power signal can for example be used to detect
fault conditions in the RF-power delivery system. In an ideal
condition, when the TX antenna 110 has perfectly matched impedance
to the tissue that it contacts, the electromagnetic waves generated
from the RF pulse generator 106 pass unimpeded from the TX antenna
110 into the body tissue. However, in real-world applications a
large degree of variability may exist in the body types of users,
types of clothing worn, and positioning of the antenna 110 relative
to the body surface. Since the impedance of the antenna 110 depends
on the relative permittivity of the underlying tissue and any
intervening materials, and also depends on the overall separation
distance of the antenna from the skin, in any given application
there can be an impedance mismatch at the interface of the TX
antenna 110 with the body surface. When such a mismatch occurs, the
electromagnetic waves sent from the RF pulse generator 106 are
partially reflected at this interface, and this reflected energy
propagates backward through the antenna feed.
[0044] The dual directional coupler RF switch 223 may prevent the
reflected RF energy propagating back into the amplifier 226, and
may attenuate this reflected RF signal and send the attenuated
signal as the reverse power signal to the feedback subsystem 212.
The feedback subsystem 212 can convert this high-frequency AC
signal to a DC level that can be sampled and sent to the controller
subsystem 214. The controller subsystem 214 can then calculate the
ratio of the amplitude of the reverse power signal to the amplitude
of the forward power signal. The ratio of the amplitude of reverse
power signal to the amplitude level of forward power may indicate
severity of the impedance mismatch.
[0045] In order to sense impedance mismatch conditions, the
controller subsystem 214 can measure the reflected-power ratio in
real time, and according to preset thresholds for this measurement,
the controller subsystem 214 can modify the level of RF power
generated by the RF pulse generator 106. For example, for a
moderate degree of reflected power the course of action can be for
the controller subsystem 214 to increase the amplitude of RF power
sent to the TX antenna 110, as would be needed to compensate for
slightly non-optimum but acceptable TX antenna coupling to the
body. For higher ratios of reflected power, the course of action
can be to prevent operation of the RF pulse generator 106 and set a
fault code to indicate that the TX antenna 110 has little or no
coupling with the body. This type of reflected-power fault
condition can also be generated by a poor or broken connection to
the TX antenna. In either case, it may be desirable to stop RF
transmission when the reflected-power ratio is above a defined
threshold, because internally reflected power can result in
unwanted heating of internal components, and this fault condition
means the system cannot deliver sufficient power to the implanted
wireless stimulation device and thus cannot deliver therapy to the
user.
[0046] The controller 242 of the wireless stimulator device 114 may
transmit informational signals, such as a telemetry signal, through
the antenna 238 to communicate with the RF pulse generator module
106. For example, the telemetry signal from the wireless stimulator
device 114 may be coupled to its dipole antenna(s) 238. The
antenna(s) 238 may be connected to electrodes 254 in contact with
tissue to provide a return path for the transmitted signal. An A/D
(not shown) converter can be used to transfer stored data to a
serialized pattern that can be transmitted on the pulse-modulated
signal from the internal antenna(s) 238 of the wireless stimulator
device 114.
[0047] A telemetry signal from the implanted wireless stimulator
device 114 may include stimulus parameters such as the power or the
amplitude of the current that is delivered to the tissue from the
electrodes. The feedback signal can be transmitted to the RF pulse
generator module 116 to indicate the strength of the stimulus at
the nerve bundle by means of coupling the signal to the implanted
RX antenna 238, which radiates the telemetry signal to the external
(or remotely implanted) RF pulse generator module 106. The feedback
signal can include either or both an analog and digital telemetry
pulse modulated carrier signal. Data such as stimulation pulse
parameters and measured characteristics of stimulator performance
can be stored in an internal memory device within the implanted
stimulator device 114, and can be sent via the telemetry signal.
The frequency of the carrier signal may be in the range of at 300
MHz to 8 GHz (preferably between about 700 MHz and 5.8 GHz and more
preferably between about 800 MHz and 1.3 GHz).
[0048] In the feedback subsystem 212, the telemetry signal can be
down-modulated using demodulator 222 and digitized through an
analog to digital (A/D) converter 220. The digital telemetry signal
may then be routed to a CPU 230 for interpretation. The CPU 230 of
the controller subsystem 214 can compare the reported stimulus
parameters to those held in local memory 228 to verify the wireless
stimulator device 114 delivered the specified stimuli to tissue.
For example, if the wireless stimulation device reports a lower
current than was specified, the power level from the RF pulse
generator module 106 can be increased so that the implanted
wireless stimulator device 114 will have more available power for
stimulation. The implanted wireless stimulator device 114 could
alternatively generate telemetry data in real time, for example, at
a rate of 8 Kbits per second. All feedback data received from the
implanted stimulator device 114 can be logged against time and
sampled to be stored for retrieval to a remote monitoring system
accessible by the health care professional.
[0049] The RF signals received by the internal antenna(s) 238 may
be conditioned into waveforms that are controlled within the
implantable wireless stimulator device 114 by the control subsystem
242 and routed to the appropriate electrodes 254 that are placed in
proximity to the tissue to be stimulated. For instance, the RF
signal transmitted from the RF pulse generator module 106 may be
received by RX antenna 238 and processed by circuitry, such as
waveform conditioning circuitry 240, within the implanted wireless
stimulator device 114 to be converted into electrical pulses
applied to the electrodes 254 through electrode interface 252. In
some implementations, the implanted wireless stimulator device 114
contains between two to sixteen electrodes 254.
[0050] The waveform conditioning circuitry 240 may include a
rectifier 244. The rectified signal may be fed to the controller
242 for receiving encoded instructions from the RF pulse generator
module 106. The rectifier signal may also be fed to a charge
balance component 246 that is configured to create one or more
electrical pulses such that the one or more electrical pulses
result in a charge balanced electrical stimulation waveform at the
one or more electrodes. The charge-balanced pulses are passed
through the current limiter 248 to the electrode interface 252,
which applies the pulses to the electrodes 254 as appropriate.
[0051] The current limiter 248 insures the current level of the
pulses applied to the electrodes 254 is not above a threshold
current level. In some implementations, an amplitude (for example,
current level, voltage level, or power level) of the received RF
pulse directly determines the amplitude of the stimulus. In this
case, it may be particularly beneficial to include current limiter
248 to prevent excessive current or charge being delivered through
the electrodes, although current limiter 248 may be used in other
implementations. Generally, for a given electrode having several
square millimeters surface area, it is the charge per phase that
should be limited for safety (where the charge delivered by a
stimulus phase is the integral of the current). But, in some cases,
the limit can instead be placed only on the current amplitude. The
current limiter 248 can automatically limit or "clip" the stimulus
phase to maintain the phase within the safety limit.
[0052] The controller 250 of the stimulator 205 may communicate
with the electrode interface 252 to control various aspects of the
electrode setup and pulses applied to the electrodes 254. The
electrode interface 252 may act as a multiplex and control the
polarity and switching of each of the electrodes 254. For instance,
in some implementations, the wireless stimulator 106 has multiple
electrodes 254 in contact with tissue, and for a given stimulus the
RF pulse generator module 106 can assign one or more electrodes to
1) act as a stimulating electrode, 2) act as a return electrode, or
3) be inactive. The assignment can be effectuated by virtue of RF
pulse generator module 106 sending instructions to the implantable
stimulator 205.
[0053] Also, in some implementations, for a given stimulus pulse,
the controller 250 may control the electrode interface 252 to
divide the current among the designated stimulating electrodes.
This control over electrode assignment and current control can be
advantageous because in practice the electrodes 254 may be
spatially distributed along various neural structures, and through
strategic selection of the stimulating electrode location and the
proportion of current specified for each location, the aggregate
current distribution in tissue can be modified to selectively
activate specific neural targets. This strategy of current steering
can improve the therapeutic effect for the patient.
[0054] In another implementation, the time course of stimuli may be
manipulated. A given stimulus waveform may be initiated and
terminated at selected times, and this time course may be
synchronized across all stimulating and return electrodes; further,
the frequency of repetition of this stimulus cycle may be
synchronous for all the electrodes. However, controller 250, on its
own or in response to instructions from pulse generator 106, can
control electrode interface 252 to designate one or more subsets of
electrodes to deliver stimulus waveforms with non-synchronous start
and stop times, and the frequency of repetition of each stimulus
cycle can be arbitrarily and independently specified.
[0055] In some implementations, the controller 250 can arbitrarily
shape the stimulus waveform amplitude, and it may do so in response
to instructions from pulse generator 106. The stimulus phase may be
delivered by a constant-current source or a constant-voltage
source, and this type of control may generate characteristic
waveforms that are static, e.g. a constant-current source generates
a characteristic rectangular pulse in which the current waveform
has a very steep rise, a constant amplitude for the duration of the
stimulus, and then a very steep return to baseline. Alternatively,
or additionally, the controller 250 can increase or decrease the
level of current at any time during the stimulus phase and/or
during the charge-balancing phase. Thus, in some implementations,
the controller 250 can deliver arbitrarily shaped stimulus
waveforms such as a triangular pulse, sinusoidal pulse, or Gaussian
pulse for example. Similarly, the charge-balancing phase can be
arbitrarily amplitude-shaped, and similarly a leading anodic pulse
(prior to the stimulus phase) may also be amplitude-shaped.
[0056] As described above, the wireless stimulator device 114 may
include a charge-balancing component 246. Biphasic stimulating
current pulses ensure that no net charge appears at the electrode
after each stimulation cycle and the electrochemical processes are
balanced to prevent net DC currents. The wireless stimulator device
114 may be configured to ensure that the resulting stimulus
waveform has a net zero charge. Charge balanced stimuli are thought
to have minimal damaging effects on tissue by reducing or
eliminating electrochemical reaction products created at the
electrode-tissue interface.
[0057] In some implementations, the charge balance component 246
uses a DC-blocking capacitor(s) placed electrically in series with
the stimulating electrodes and body tissue, between the point of
stimulus generation within the stimulator circuitry and the point
of stimulus delivery to tissue. In a multi-electrode stimulator,
one charge-balance capacitor(s) may be used for each electrode or a
centralized capacitor(s) may be used within the stimulator
circuitry prior to the point of electrode selection. The stimulus
waveform created prior to the charge-balance capacitor, called the
drive waveform, may be controlled such that its amplitude is varied
during the duration of the drive pulse. The shape of the stimulus
waveform may be modified in this fashion to create a
physiologically advantageous stimulus.
[0058] In some implementations, the wireless stimulator device 114
may create a drive-waveform envelope that follows the envelope of
the RF pulse received by the receiving dipole antenna(s) 238. In
this case, the RF pulse generator module 106 can directly control
the envelope of the drive waveform within the wireless stimulator
device 114, and thus no energy storage may be required inside the
stimulator itself. In this implementation, the stimulator circuitry
may modify the envelope of the drive waveform or may pass it
directly to the charge-balance capacitor and/or electrode-selection
stage.
[0059] In some implementations, the implanted wireless stimulator
device 114 may deliver a single-phase drive waveform to the charge
balance capacitor or it may deliver multiphase drive waveforms. In
the case of a single-phase drive waveform, for example, a
negative-going rectangular pulse, this pulse comprises the
physiological stimulus phase, and the charge-balance capacitor is
polarized (charged) during this phase. After the drive pulse is
completed, the charge balancing function is performed solely by the
passive discharge of the charge-balance capacitor, where is
dissipates its charge through the tissue in an opposite polarity
relative to the preceding stimulus. In one implementation, a
resistor within the stimulator facilitates the discharge of the
charge-balance capacitor.
[0060] In the case of multiphase drive waveforms the wireless
stimulator may perform internal switching to pass negative-going or
positive-going pulses (phases) to the charge-balance capacitor.
These pulses may be delivered in any sequence and with varying
amplitudes and waveform shapes to achieve a desired physiological
effect.
[0061] In some implementations, the amplitude and timing of
stimulus and charge-balancing phases is controlled by the amplitude
and timing of RF pulses from the RF pulse generator module 106, and
in others this control may be administered internally by circuitry
onboard the wireless stimulator device 114, such as controller 250.
In the case of onboard control, the amplitude and timing may be
specified or modified by data commands delivered from the pulse
generator module 106.
[0062] Referring to FIGS. 3A to 3B, some implementation use the
microwave field stimulator (MFS) transmitter for wireless power
transfer, as illustrated in system level diagram 300. The MFS may
include a digital signal processor 301, gain control 302,
phase-locked loop 303, gating amplifier 304, pulse-amplitude input
matching network 305, boost regulator 306, radio-frequency (RF)
amplifier 307, pulse-amplitude harmonic filter 308, antenna 309,
tissue boundary 310, passive neural stimulator 311, directional
coupler 312, analog-digital converter (ADC) 313, and receiving
dipole antenna 314. As illustrated, implanted electrodes may be
used to pass pulsatile electrical currents of controllable
frequency, pulse width and amplitudes. A variety of therapeutic
intra-body electrical stimulation techniques may be utilized to
treat conditions that are known to respond to neural
modulation.
[0063] Digital signal processor 301 may generate pulse parameters
such as pulse width, amplitude, and repetition rate. Digital signal
processor 301 may feed pulse parameters to gain control 302, which
can include a digital to analog converter (DAC). Gain control 302
may generate RF envelope 302A to gating amplifier 304. Digital
signal processor 301 may feed phase-locked loop 303 with stimulus
timing control 301A, which is a voltage signal that drives crystal
XTAL 303A to generate RF carrier burst 303B. RF carrier burst 303B
arrives at gating amplifier to modulate RF envelope 302A such that
RF pulse 304A is generated to feed pulse-amplitude input matching
network 305.
[0064] Output from pulse-amplitude input matching network 305 is
provided to RF amplifier 307 under a bias voltage from boost
regulator 306. Subsequently, a harmonic filter 308 mitigates
harmonic distortions and feeds the filtered output as a high power
RF pulse to antenna 309. The high power RF pulse is transmitted
from antenna 309 through skin layer 310 to reach receiver dipole
antenna 314 of the implanted neural stimulator device 311 so that
therapies are applied at tissue electrodes.
[0065] In some implementations, the Rx antenna 238 and Tx antenna
110 may exhibit mutual coupling. In some implementations the mutual
coupling of the Rx antenna 238 and Tx antenna 110 may be observed
for the purpose of assessing the state of the Implanted Neural
Stimulator 114.
[0066] In some implementations an estimated geometric factor may be
included in the measurement normalization that may account for the
change in mutual coupling for various thicknesses of tissue that
separates the Rx antenna 238 from the Tx antenna 110.
[0067] Some implementations incorporate RF complex impedance
measurement via F Sensor subsystem, which may include an RF phase
detector 312A, shown in FIG. 3B. The reflected RF signal is
received at antenna 110 and routed via directional coupler 312 to
analog-digital converter (ADC) 313, and from this signal the RF
impedance, or reflection coefficient, may be calculated.
[0068] In some implementations, the wireless stimulation system 100
may utilize the RF reflection measurements to obtain the impedance
at the electrode-tissue interface of FIG. 3A. At the implantation
site, the extracellular environment around implanted electrodes can
change due to insertion-related damage and the presence of the
electrodes (foreign material) in the tissue, both of which
instigate formation of scar tissue, a compact sheath of cells and
extracellular matrix surrounding the implant. Some studies have
found that this encapsulating tissue can alter electrical impedance
relative to normal (or unscarred) tissue. Since a change of the
electrode-tissue impedance may alter the effectiveness of the
Implanted Neural Stimulator 114, it would be advantageous for the
wireless stimulation system 100 to have the capability of assessing
the impedance of the electrode-tissue interface.
[0069] In some implementations, based on the deduced impedance of
the electrode-tissue interface, the strategy for stimulation can be
modified to compensate for the impedance. For example, if the
electrode-tissue impedance is found to be highly resistive, the
wireless stimulation system 100 may compensate by providing higher
voltage to the current driver within the Implantable Neural
Stimulator 114.
[0070] As discussed in detail through FIGS. 1-2, the RF waveform
112 is received by the stimulator's Rx antenna 238 and subsequently
rectified via an RF-to-DC bridge 244 connected to the feed point of
the Rx antenna. The received energy is stored in a capacitor in the
Implantable Neural Stimulator 114. The energy stored in the
capacitor is a function of the charge held by the capacitor and the
voltage across the capacitor.
[0071] In some implementations, the signal received at the F Sensor
of FIG. 3B is processed to deduce the state of charge of the
capacitor in the Implanted Neural Stimulator 114. In more detail,
the time-varying currents and voltages at the rectifier and
capacitor act to create a variable RF impedance at the feed point
of Rx antenna 238. When the capacitor has low charge, the RF
current flows freely across the bridge 244, which is a near RF
short circuit at the feed point. When the capacitor approaches full
charge, the RF current is impeded, such that there is a near RF
open circuit at the feed point. It follows that the complex
impedance at the feed point of Rx antenna 238 is an indicator of
the state of charge of the capacitor in the Implanted Neural
Stimulator 114. Because the dynamic impedance at the Rx antenna is
coupled to the Tx antenna 110, the impedance at the Rx antenna can
be observed by the F Sensor of FIG. 3B, and from this measurement,
the RF pulse generator module 106 can deduce the charge of the
capacitor in the Implanted Neural Stimulator 114.
[0072] In some implementations, the transmitted RF signal 112 can
be judiciously selected to maintain the voltage on the capacitor in
the Implanted Neural Stimulator 114 at a desired, constant level.
For example, the RF pulse rate and width can be strategically
selected to maintain a steady-state delivery of power to the
stimulator such that energy is delivered at the same rate that it
is consumed by the stimulator circuitry.
[0073] In some implementations, the state of charge of the
capacitor in the Implanted Neural Stimulator 114 is an indicator of
the stimulator's present operational state and environment. The
voltage at the capacitor will decay proportionally to the rate at
which energy is depleted by the load connected to the capacitor.
The load may encompass the load at the stimulator's electrodes (the
tissue) and the load of the circuitry associated with transferring
charge from the capacitor to the electrodes.
[0074] In some implementations, the rate at which charge is
depleted from the capacitor in the Implanted Neural Stimulator 114
depends on the stimulus parameters, the electrode-tissue load, and
the internal circuitry of the stimulator. By virtue of such
dependence, the rate of charge depletion from the capacitor can be
used to determine the impedance of the electrode-tissue interface.
The rate of charge depletion may reveal an RF impedance
characteristic of the Rx antenna 238 from which the
electrode-tissue impedance can be extracted. For example, if the
electrode-tissue impedance is mostly resistive and is sufficiently
low (for example, z=300-500.OMEGA.), the intended stimulus current
will be driven to the targeted tissue, and the charge on the
capacitor will deplete at an expected rate. In contrast, if the
electrode-tissue impedance has a high-value resistance or is
dominated by series-capacitance, the intended stimulus current may
not be delivered. An example of high-value resistance is
demonstrated in FIG. 4D below. If the programmed stimulus current
is not delivered to the tissue, the charge on the capacitor will
deplete at a lower rate than expected. In both cases, the rate of
charge depletion may revealed the RF impedance characteristic of
the Rx antenna, and from this rate a deduction about the impedance
of the electrode-tissue interface can be made.
[0075] In some implementations, a circuit internal to the Implanted
Neural Stimulator 114 may allow connection of the current-driver
circuit to a calibrated internal load.
[0076] In some implementations, a calibrated internal load in the
Implanted Neural Stimulator 114 may be programmed to specific
impedance values. In these implementations, the calibrated internal
load can be placed anywhere on the Implanted Neural Stimulator
114.
[0077] In some implementations, the Implanted Neural Stimulator 114
may drive current into the calibrated internal load while either
the current or the load is swept through a range of values, and the
corresponding family of unique complex RF reflection coefficients
may be captured for reference. Subsequently, when the stimulator is
configured to drive current through the electrode-tissue load,
which is unknown, the RF reflection coefficient curve may be
captured and compared to the family of reference curves. By
matching the curve of the unknown electrode-tissue load to the
curve of a known load, the electrode-tissue impedance may be
deduced.
[0078] In some implementations, a circuit internal to the Implanted
Neural Stimulator 114 may facilitate a system self-check to
ascertain the suitability of the wireless stimulation system 100 to
provide stimulation therapy. For example, for a system self-check,
the stimulator may drive various currents into an internal load,
and for each current level the average RF power is swept while the
RF reflection is observed. These measurements may be used for a
reference to compare to electrode-tissue load measurements during
the self-check.
[0079] For purpose of the analysis it may be advantageous to view
the change of the measurand (RF reflection, or reverse RF voltage)
relative to its initial value, and the change in the independent
variable (average RF power) relative to its initial value. For
example, for measurand V, the change would be (V-V.sub.min).
Further, it may be advantageous to normalize the data. The data
shown in FIG. 4A through 4B was normalized to the reference
measurements as follows: The maximum and minimum RF reflections
(reverse RF voltages, V.sub.max and V.sub.min) of these reference
measurements were extracted, and the difference was used as the
normalization factor. For example, the subsequent measurand changes
were normalized as follows:
v=(V-V.sub.min)/(V.sub.max-V.sub.min),
where V is the measured reverse RF voltage, and v (lower case)
implies normalized. Similarly the independent variable changes were
normalized as follows:
p=(P-P.sub.min)/(P.sub.max-P.sub.min),
where P is the average transmitted RF power and p (lower case)
implies normalized.
[0080] FIG. 4A shows the reference measurements, normalized change
in RF reflection versus normalized change in RF average power for
three different currents driven into the internal load. The curves
show: 1) minimum current through the internal load (circles), 2)
medium current through the internal load (triangles), 3) maximum
current through the internal load (squares). In some cases, minimum
can be around 0.1 mA; medium can be around 6 mA; and maximum can be
around 12 mA. In the case of low current, the charge on the
capacitor builds steadily higher as the average RF power is
increased. This is indicated on the plot (circles) by the rising
trajectory of the RF reflection. In the case of medium current, the
charge on the capacitor remains relatively flat as the average RF
power is increased. This is indicated on the plot (triangles) by
the relatively flat trajectory of the RF reflection. For the case
of high current, the charge on the capacitor remains relatively
flat (but at different level) as the average RF power is increased.
This is indicated on the plot (squares) by the relatively flat
trajectory (at a higher level) of the RF reflection.
[0081] In some implementations, the wireless stimulation system 100
self-check may include various fault checks. For example, when the
Implanted Neural Stimulator 114 is energized but not programmed to
drive stimulus current, the RF reflection should be similar to that
shown for the minimum-current case of FIG. 4A. This is shown in
FIG. 4B, where the RF reflection for the un-configured stimulator
(dashed line) is overlaid onto the plots of FIG. 4A.
[0082] Further, based on the reference measurements shown in FIG.
4A, it is feasible for the system to test for a fault in the
stimulus-current driver (such as a broken electrode). For example,
the Implanted Neural Stimulator 114 could be programmed to drive
stimulus current through selected pairs of electrodes, and the RF
reflection for each pair is compared against the reference
measurements in FIG. 4A. When the stimulator is programmed to drive
a given stimulus current through tissue, the RF reflection should
be similar to that of when the same current is driven through the
internal load. However, should there be a damaged wire at the
electrode, for example, the current through the circuit would be
blocked, meaning the RF reflection would resemble the
minimum-current curve of FIG. 4A. This case is shown in FIG. 4C,
plotted with the reference measurements of FIG. 4A.
[0083] Further, during normal operation, the tissue impedance may
be unknown, however, it will likely be within an expected range,
and a fault-check could verify this condition. To illustrate, FIG.
4D shows the RF reflection (dashed line) when a 500.OMEGA. resistor
is connected in series with the electrodes, and a medium current is
delivered. The result shows the RF reflection falls within the
expected range. However, for example, if the result showed the RF
reflection overlaid the low-current curve, a fault would be
evident.
[0084] By capturing the reflected RF signal and applying the
analysis methods described herein, it is possible to measure the
impedance at the electrode-tissue interface. Furthermore, by
measuring said impedance, the system may adjust stimulus parameters
to compensate, thereby maintaining the efficacy of stimulation.
[0085] FIG. 5 shows an example of a flow chart 500 for implementing
stimulation adjustment on an implantable wireless stimulator device
based on sensing of tissue-electrode impedance. As illustrated by
step 502, a first set of radio-frequency (RF) pulses are
transmitted from an external pulse generator (such as RF pulse
generator module 106) to an implantable wireless stimulator device
(such as implanted neural stimulator 114) via non-inductive
electric radiative coupling. Consistent with discussions from FIGS.
1-3, electric currents are created from the first set of RF pulses
and flown through a calibrated internal load on the implantable
wireless stimulator device. The calibrated internal load represents
a load condition that is pre-determined and imposed on, for
example, an electrode on implanted neural stimulator 114.
[0086] In response to the electric currents flown through the
calibrated internal load, flow chart 500 proceeds to recording, on
the external pulse generator, a first set of RF reflection
measurements (504). This recording measures, for example, RF
signals reflected from the implantable neural stimulator 114 and
received by RF pulse generator module 106.
[0087] Next, a second set of radio-frequency (RF) pulses are
transmitted, from the external pulse generator and via electric
radiative coupling, to the implantable wireless stimulator device
such that stimulation currents are created from the second set of
RF pulses and flown through an electrode of the implantable
wireless stimulator device to tissue surrounding the electrode
(506). Here, the stimulation currents flow through the stimulator
circuitry, the electrode, and the electrode-tissue interface.
[0088] In response to the stimulation currents flown through the
electrode to the surrounding tissue, a second set of RF reflection
measurements is recorded on the external pulse generator (508).
This second set of reflection measurements are based on RF signals
reflected from the implantable neural stimulator 114 and received
by RF pulse generator module 106.
[0089] By comparing the second set of RF reflection measurements
with the first set of RF reflections measurements, an
electrode-tissue impedance is characterized (510). When the
electrode-tissue impedance is characterized as resistive, one or
more input pulses to be transmitted by the external pulse generator
to the implantable wireless stimulator device can be adjusted such
that stimulus currents created from these input pulses on the
implantable wireless stimulator device are likewise adjusted to
compensate for a resistive electrode-tissue impedance. When the
electrode-tissue impedance is characterized as capacitive, one or
more input pulses to be transmitted by the external pulse generator
to the implantable wireless stimulator device can be adjusted such
that stimulus currents created from these input pulses on the
implantable wireless stimulator device are likewise adjusted to
compensate for a capacitive electrode-tissue impedance. Here, the
adjustment of input pulses involves maintaining a steady-state
delivery of electrical power to the implantable wireless stimulator
device such that electrical energy is extracted from the input
pulses as fast as electrical energy is consumed to generate the
stimulus currents with one or more pulse parameters that have been
varied to accommodate the resistive electrode-tissue impedance.
Such stimulus currents are delivered from the electrode to the
surrounding tissue. Examples of pulse parameters include: a pulse
width, a pulse amplitude, and a pulse frequency.
[0090] Based on results of characterizing the electrode-tissue
impedance, a stimulation session can be automatically chosen. The
selection process may include: determining input pulses to be
transmitted by the external pulse generator to the implantable
wireless stimulator device such that stimulus currents are created
on the implantable wireless stimulator device and delivered by the
electrode on the implantable wireless stimulator device to the
surrounding tissue in a manner that, for example, maintains therapy
consistency despite variations in electrode-tissue impedance. In
one instance, the second set of radio-frequency (RF) pulses may be
updated to obtain updated second set of RF reflection measurements;
and then the updated second set of RF reflection measurements may
be compared with the first set of RF reflection measurements. In
this instance, the updating and comparing steps can be performed
iteratively until desired RF reflection measurements are
obtained.
[0091] The characterizing step may also lead to automatic fault
checking according to results from such characterization. In one
instance, automatic fault checking includes automatic detecting a
damaged wire in a circuit leading to the electrode on the
implantable wireless stimulator device, as illustrated in, for
example, FIG. 4C.
[0092] A number of implementations have been described.
Nevertheless, it will be understood that various modifications may
be made. Accordingly, other implementations are within the scope of
the following claims.
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