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Proceedings ArticleDOI

A distributed transducer system for functional electrical stimulation

02 Sep 2001-Vol. 1, pp 397-400

AbstractImplanted transducers for functional electrical stimulation (FES) powered by inductive links are subject to conflicting requirements arising from low link efficiency, a low power budget and the need for protection of the weak signals against strong RF electromagnetic fields. We propose a solution to these problems by partitioning the RF transceiver and sensor/actuator functions onto separate integrated circuits. By amplifying measured neural signals directly at the measurements site and converting them into the digital domain before passing them to the transceiver the signal integrity is less likely to be affected by the inductive link. Neural stimulators are affected to a lesser degree, but still benefit from the partitioning. As a test case, we have designed a transceiver and a sensor chip which implement this partitioning policy. The transceiver is designed to operate in the 6.78 MHz ISM band and consumes approximately 360 /spl mu/W. Both chips were implemented in a standard 0.5 /spl mu/m CMOS technology, and use a 3 V supply voltage.

Topics: Transceiver (53%), Signal integrity (53%), CMOS (51%)

Summary (2 min read)

1. Introduction

  • Sensor chip coil close to the skin surface, and route the antenna signal to a single integrated transceiver/transducer.
  • This is however not practical since biocompatible interconnect solutions like Cooper cable' have a high impedance at DC and RF.
  • The authors adopted a solution with simple digital communication protocols between the central transceiver and control chip, and the outlying transducer chips.
  • The sensor chip includes an amplifier for cuff electrode signals, which is itself the subject of another article [2] , an AD converter and bus interface logic.
  • The transceiver includes a direct conversion receiver (actually a homodyne), a load modulation circuit for transmission of data out of the system, control logic, supply regulators and references.

2. System description

  • The following is a description of the transceiver chip and the aspects relating to communication between it and the other parts of the system.
  • Since the main focus of this article is on the way in which partitioning of functions can solve some of the problems in an implanted system of this type, the sensor chip will only be described inasmuch as it relates to the top-level design of the system.
  • The principle of partitioning applies to any transducer type that is relevant in an implanted system, so the internals of the sensor chip will not be emphasized.
  • Figure 2 shows atoplevel diagram of the system.

2.2. The power supply

  • The transceiver chip provides the supply voltage for the other chips in the system.
  • Two strands out of four in the Cooper cable have been assigned to Vss and VD,.
  • Because of the helically wound construction of the cable, and the Pt-Ir composition, the impedance is relatively high at all frequencies.
  • To provide a stable supply voltage for the outlying chips, it is therefore necessary to add decoupling capacitors to the supply lines at the transducer ends.
  • A stimulator chip can require relatively large current pulses from the supply while delivering a stimulus.

2.3. Transceiver

  • The target bit rate for the system was 50kbiv's, and the receiver is designed to handle up to 10Okbit/s. Data is transmitted out of the system by load modulation.
  • The reflected impedance seen by the external transmitter is varied by changing the load seen by the secondary LC circuit.
  • A switch is connected between the terminals of the antenna, and by closing the switch, a maximum change in the reflected impedance is obtained.
  • This simple scheme has the disadvantage of stopping power transfer to the system during load modulation, and other load modulation circuits have been designed to avoid this [9] .
  • The duty cycle of the switch closure is however so low in their case that the reduction in average power transfer is small.

2.4. Interchip communication

  • The line drivers are class AB circuits which have a quiescent current consumption of 5pA each, and can slew the line voltages with a 50 pA current.
  • This type of driver was chosen instead of faster types because this is more than sufficient for the purpose, and by limiting the slewing currents the supply transients are reduced.
  • The high and low voltages on the signal lines are 1.OV and 0.3V respectively instead of the full supply range, again to reduce supply transients and power consumption.

3. Measurement results

  • The data link to the transceiver chip was tested by using a Class D transmitter driving the inductive link, with a data rate of 50kbitfs and 20% ASK modulation and Manchester encoding.
  • The data transfer functions according to the specifications, and higher data rates can easily be supported with minor modifications.

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A distributed transducer system for functional electrical stimulation
Gudnason, Gunnar; Nielsen, Jannik Hammel; Bruun, Erik; Haugland, Morten
Published in:
Proceedings on 8th IEEE International Conference on Electronics, Circuits and Systems
Link to article, DOI:
10.1109/ICECS.2001.957763
Publication date:
2001
Document Version
Publisher's PDF, also known as Version of record
Link back to DTU Orbit
Citation (APA):
Gudnason, G., Nielsen, J. H., Bruun, E., & Haugland, M. (2001). A distributed transducer system for functional
electrical stimulation. In Proceedings on 8th IEEE International Conference on Electronics, Circuits and Systems
(Vol. 1) https://doi.org/10.1109/ICECS.2001.957763

A
distributed
transducer
system
for
functional
electrical stimulation
Gunnar Gudnason, Jannik H. Nielsen and Erik Bruun
Orsted-DTU, Technical University
of
Denmark,
DK-2800
Lyngby
gg@oersted.dtu.dk
Morten Haugland
Center
for
Sensory-Motor Interaction,
Aalborg
University,
DK-9220
Aalborg
Abstract
Implanted transducers for functional electrical stim-
ulation (FES) powered by inductive links are subject to
conflicting requirements arising from
low
link eflciency, a
low power budget and the need for protection of the weak
signals against strong RF electromagneticJields.
We propose a solution to these problems by partition-
ing the RF transceiver and sensor/actuator functions onto
separate integrated circuits.
By
amplifying measured neu-
ral signals directly at the measurements site and convert-
ing them into the digital domain before passing them to the
transceiver; the signal integrity is less likely to be affected
by the inductive link. Neural stimulators are afected to a
lesser degree, but still benejit from the partitioning.
As
a test case, we have designed a transceiver and a
sensor chip which implement this partitioning policy. The
transceiver is designed to operate in the
6.78MHz
ISM
band and consumes approximately
360pW
Both chips
were implemented in a standard
0.5
,um
CMOS
technol-
ogy, and use a
3
V
supply voltage.
1.
Introduction
The subject of this article is a solution for several
problems which plague designers of inductively powered
biomedical implants for functional electrical stimulation.
Inductive links are commonly employed to power and
control such implants since they provide a wireless con-
nection, which is desirable since percutaneous wires pro-
vide an infection path into the body. Inductive links also
eliminate the need
for
an implanted battery, which might
eventually need to be replaced. The price which must be
paid for these advantages is the relatively high power dis-
sipation in the external (non-implanted) apparatus due to
the
low
efficiency of the link, and a degradation of the sig-
nals under examination, because of the high electromag-
netic fields. While the
RF
signals normally lie far outside
the biological signal band, they can easily desensitize an
amplifier designed for a
10-2OpV
signal range.
The signal amplitude quoted above is typical for sig-
nals obtained using cuff electrodes
[
11. Stimulators con-
nected to cuff electrodes generate much larger signals,
so
they are not affected to the same extent as sensors by the
external
RF
field.
We propose therefore a physical partitioning of the sig-
nal processing functions, in order to limit the transmission
w
transmitter
\
Figure
1.
A
partitioning example for a
FES
system con-
taining a transceiver chip and sensor and actuator chips.
The last two are not necessarily connected to the same
nerve trunk.
distances for sensitive signals, and to allow optimal place-
ment of critical functions of the system. The conflicting
design criteria which can be accommodated to a large ex-
tent by partitioning are:
The
RF
part of the system should be close to the skin
surface for better power transmission, and also for
better link bandwidth. The proximity may also be
an
advantage for surgical access.
The transducers should be placed close to the active
sites, which can be relatively deep inside the body.
Long routing of transducer signals should be
avoided, as the strong electromagnetic field will
induce an
RF
interferer overlaid on the desired sig-
nal. Physical separation of the transducer from the
RF
link also reduces the problems, since the field
falls off rapidly with distance (as
1
/r3
in the far-field
limit).
The system must contain a considerable amount
of
digital logic,
for
control, timing, and buffering of
data. The logic will inevitably couple switching
noise onto the supplies and into the substrate. Using
low-noise logic solutions like current-steering logic
[3]
can eliminate the problem, but the static supply
current of CSL and other ultra-low noise logic fami-
lies makes them unsuitable as the system complexity
passes a certain point. Placing the sensors on sepa-
rate chips isolates them from the logic supply noise.
One option which was examined, and which alleviates
some of the problems above, is to place only the receiver
0-7803-7057-010
1
/$
10.00
0200
1
IEEE.
397
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Sensor chip
Figure
2.
A
simplified block diagram of the system, showing the main parts of the transceiver and sensor chips.
Differential signals are shown with a single line for clarity
coil close to the skin surface, and route the antenna signal
to a single integrated transceiver/transducer. This is how-
ever not practical since biocompatible interconnect solu-
tions like Cooper cable' have a high impedance at DC and
RF.
We adopted a solution with simple digital communica-
tion protocols between the central transceiver and control
chip, and the outlying transducer chips. The protocol was
designed
so
that it could be accommodated along with the
power supply wires in a simple 4-wire flexible cable. In
this specific test case, we implemented a simple system
with one transceiver chip and one sensor chip. The sensor
chip includes an amplifier for cuff electrode signals, which
is itself the subject of another article [2], an AD converter
and bus interface logic. The transceiver includes a direct
conversion receiver (actually a homodyne), a load modu-
lation circuit for transmission of data out of the system,
control logic, supply regulators and references.
2.
System description
The following is a description of the transceiver chip
and the aspects relating to communication between it and
the other parts of the system. Since the main focus of this
article is on the way in which partitioning of functions
can solve some of the problems in an implanted system of
this type, the sensor chip will only be described inasmuch
as it relates to the top-level design of the system. The
principle of partitioning applies to any transducer type that
is relevant in an implanted system,
so
the internals of the
sensor chip will not be emphasized. Figure 2 shows atop-
level diagram of the system.
2.1.
Thelink
The use
of
inductive links for power and data transmis-
sion is described in great detail elsewhere, see for example
By separating the transceiver chip and the antenna, it is
possible to reduce the distance between the external trans-
[5,61.
mitter antenna (a tuned LC circuit) and the internal one to
15-20mm. The coupling coefficient for normal coil ge-
ometries at this distance is on the order of
0.01-0.05.
This
permits far better power transfer than if the transceiver
were placed together with the transducer, and makes load
modulation a viable method to extract information from
the system. Systems with smaller coupling coefficients
must resort to active transmission of signals to the outside
[4], which increases the power consumption.
The chosen modulation method is PAM with Manch-
ester encoding and a modulation index of approximately
0.2. This is compatible with high-efficiency class D or
E
transmitter configurations
[7,
81.
The carrier frequency is
6.78MHz
which coincides with one of the
ISM
(indus-
trial, scientific and medical) bands. The target bit rate is
5Okbitls.
2.2.
The power supply
The power is extracted from the
RF
carrier by means
of
a hll-wave bridge rectifier, which is implemented by
us-
ing diode-connected P-channel
MOS
transistors in a com-
mon N-well, as shown in figure
3.
The standard CMOS
process does not offer high-quality floating diodes,
so
an-
other solution must be chosen. The available elements
are three types of p-n junction and diode-connected
MOS
transistors, but these all suffer from high substrate currents
and/or parasitic elements which divert some of the current
from its intended path. The
PMOS
rectifier bridge is ac-
companied by parasitic vertical PNP transistors which can
divert some of the input current to
V,,
instead of to
VDD.
'Produced
by
Finetech
Medical
Ltd.
398
Figure
3.
The power conversion circuit.
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100
mA
lOmA
1
mA
100pA
10pA
lOOnA
Figure
4.
The measured
shunt
regulator current
We have solved this problem by dimensioning the
PMOS
transistors
so
that they are biased in weak inversion over
the entire operating range. Since the nominal threshold
voltage is about
0.6V,
there is a range of drain-gate volt-
ages for M1 in figure
3
(corresponding to the emitter-base
potential in the parasitic
PNP)
where the
MOS
current
dominates the current through the bipolar transistor by a
large factor. The current-handling capability
of
the recti-
fiers can be increased for a given maximum parasitidmain
current ratio, by increasing the width (and area)
of
the
transistors. The main penalty is an increased input capac-
itance. Our experimental data show that the parasitic cur-
rent ratio is smaller than
0.005
for a system supply current
of
200pA
and an input capacitance of 2pF.
All
the tran-
sistors in figure
3
have
W/L
=
800/0.5
in micrometers.
The transducer chip contains an active shunt regulator
which provides protection against excessive input power.
The transmitted power
is
a very strong function of antenna
spacing,
so
an unprotected circuit can easily be burned
out.
The
shunt
circuit
is
basically
a
pass
transistor be-
tween
V,,
and
Vss,
and a feedback loop which compares
V,,
to a bandgap reference voltage.
As
the supply volt-
age approaches the trigger point (which was set equal to
the maximum supply voltage for the technology), the reg-
ulator begins shunting current from the supply, and the
current changes by
4
decades over a short supply voltage
interval (see figure
4).
The performance of the power supply conditioning sys-
tem is shown in figure
5.
The conversion
of
the
RF
power
to a DC supply voltage and the overvoltage protection is
carried out mostly on-chip, with the only external compo-
nent being the energy storage capacitor
C,.
The transmit-
ter used in these measurements was running at a relatively
high power level, witnessed by the fact that the full supply
voltage is reached at a separation of 120mm. Despite the
strong dependence of the magnetic field amplitude on the
separation
d,
the on-chip shunt succeeds in maintaining
the supply voltage at the nominal level at all separations.
The transceiver chip provides the supply voltage for the
other chips in the system. Two strands out of four in the
Cooper cable have been assigned to
Vss
and
VD,.
Because
of the helically wound construction of the cable, and the
Pt-Ir composition, the impedance
is
relatively high at all
frequencies. To provide a stable supply voltage for the
Figure
5.
The measured on-chip supply voltage
as
a
function of the separation between the transmitter and
transceiver antennas.
outlying chips, it is therefore necessary
to
add decoupling
capacitors to the supply lines at the transducer ends.
A
stimulator chip can require relatively large current pulses
from the supply while delivering a stimulus.
2.3.
Transceiver
The receiver part of the transceiver chip is a direct-
conversion receiver. Since the input signal has a larger
amplitude than the supply range, an attenuator with a gain
of approximately
0.1
is inserted in the signal path. The
input signal is tracked by a
PLL
which also provides the
system clock (the block diagram does not show a clock
divider circuit which divides the clock frequency down to
1.7MHz).
The output of the
PLL
and the input signal are put
through a mixer, and the mixer output is filtered in a 4th-
order differential G,,< filter. The filter output is directed
to
a mixed analog-digital level detector, which detects
the sign of the transmitted data bits. Instead of using a
partly analog level detector, the normal processing method
would be to sample the filter output for further digital
sig-
nal processing. This does however require more sophisti-
cated digital circuits than we were willing to implement,
and possibly requires more supply current.
The target bit rate for the system was 50kbiv's, and the
receiver is designed to handle up to 10Okbit/s.
Data
is
transmitted out of the system by load mod-
ulation. The reflected impedance seen by the external
transmitter is varied by changing the load seen by the
secondary
LC
circuit.
A
switch is connected between
the terminals
of
the antenna, and by closing the switch,
a maximum change in the reflected impedance is ob-
tained. This simple scheme has the disadvantage of stop-
ping power transfer to the system during load modulation,
and other load modulation circuits have been designed to
avoid this
[9].
The duty cycle of the switch closure is how-
ever
so
low in our case that the reduction in average power
transfer is small.
2.4.
Interchip communication
The cable type that we used as a reference for
our
inter-
connects is, as mentioned before, a 4-strand biocompati-
399
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ble cable type by the name of Cooper cable. In addition
to being biocompatible, the cable is wound in a helical
pattern and embedded in silicone,
so
it can be stretched.
The stretching reduces the probability of damage to the
surrounding tissue.
Because
of
the biological constraints that the cable
must fulfill, it does not have very good electrical prop-
erties, and the use of the cable must be adjusted accord-
ingly. We measured the series resistance at
DC
of a repre-
sentative length
of
the cable, and found it to be 2
10
Rlm.
The capacitance between any two wires varies from
20-
80pF/m because of the lack of symmetry, and the capaci-
tance
of
each wire to the surroundings is about SOpFlm.
No
clock signal is sent across the connection between
the chips, Instead, we use an asynchronous handshake
mechanism to control data transfer, and an internal oscil-
lator in the sensor chip to provide a time base.
The line drivers are class
AB
circuits which have a qui-
escent current consumption of
5pA
each, and can slew
the line voltages with a
50
pA
current. This type of driver
was chosen instead of faster types because this is more
than sufficient for the purpose, and by limiting the slew-
ing currents the supply transients are reduced. The high
and low voltages on the signal lines are
1.OV
and
0.3V
respectively instead of the full supply range, again to re-
duce supply transients and power consumption.
3.
Measurement results
Most of the basic functions of the transceiver and sen-
sor
chips have been measured, and they behave according
to specifications.
The results for the power conversion circuit show that
the use of PMOS transistors in weak inversion as rectifiers
is an ideal solution for pure CMOS technologies. Cur-
rent throught the substrate are eliminated by placing ev-
erything in an N-well, and the effect
of
parasitic bipolars
are all but eliminated.
The active shunt regulator and other regulators and ref-
erences on the chip are up to the design criteria. Specifi-
cally, the shunt regulator consumes negligible supply cur-
rent within the normal voltage range, with a very sharp
rise in current as the trigger point is exceeded. Previ-
ous systems use passive zener-based regulators, whose
gradual
I-V
characteristic provides insufficient overvolt-
age protection for low-voltage
CMOS
technologies.
The data link
to
the transceiver chip was tested by using
a Class
D
transmitter driving the inductive link, with a data
rate of 50kbitfs and
20%
ASK
modulation and Manch-
ester encoding. The data transfer functions according to
the specifications, and higher data rates can easily be sup-
ported with minor modifications.
4.
Conclusion
We have demonstrated a partitioning scheme for im-
planted sensor and actuator devices which places the sig-
nal processing functions where they are needed. By us-
ing this scheme, it is possible to provide better isolation
of weak biological signals from strong external distur-
bances, while simultaneously reducing the overall power
Figure
6.
Chip die microphotograph.
consumption by placing the transceiver closer to the exter-
nal interface. The communication between separate parts
of the system has been adapted to existing biocompatible
interconnect methods.
We have implemented a simple system consisting of a
transceiver and a single sensor chip, but the concept can
easily be extended to more general combinations of sen-
sors and actuators, in order to created a complete neural
stimulation system with a closed feedback loop.
[I]
G.
E.
Loeb and
R.
A. Peck, “Cuff electrodes for chronic
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of
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T.
Lehmann,
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implantable CMOS
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H.-T.
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J.
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M.
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de N. Donaldson and
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P.
R.
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and
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Citations
More filters

Journal ArticleDOI
TL;DR: The present study confirms that the proposed biological signal sensing device is suitable for various implanted applications following an appropriate biocompatible packaging procedure.
Abstract: This study presents an implantable microcontroller-based bi-directional transmission system with an inductive link designed for biological signal sensing. The system comprises an external module and an implantable module. The external module incorporates a high-efficiency class-E transceiver with amplitude modulation scheme and a data recovery reader. The transceiver sends both power and commands to the implanted module, while the reader recovers the recorded biological signal data and transmits the data to a personal computer (PC) for further data processing. To reduce the effects of interference induced by the 2 MHz carrier signal, the implanted module uses two separate coils to perform the necessary two-way data transmission. The outward backward telemetry circuitry of the implanted module was based on the load-shift keying (LSK) technique. The transmitted sensed signal had a 10-bit resolution and a read-out rate of 115 kbps. The implanted module, measuring 4.5 ? 3 ? 1.2 cm3, was successfully verified in animal experiment in which the electroneurogram (ENG) signal was recorded from the sciatic nerve of New Zealand rabbits in response to nociceptive stimulation of foot. The reliable operating distance of the system was within about 3.5 cm with an efficiency of around 25%. Our present study confirms that the proposed biological signal sensing device is suitable for various implanted applications following an appropriate biocompatible packaging procedure.

70 citations


Cites background or methods from "A distributed transducer system for..."

  • ...However, the magnetic coupling signal of the inductive coupling may be several orders of magnitude larger than these recorded biological signals (Gudnason et al 2001)....

    [...]

  • ...The impedance reflection technique, utilizing a LSK modulation method, is generally adopted in implantable biomicrosystem applications (Smith et al 1998, Gudnason et al 2001, Lanmüller et al 1999)....

    [...]


PatentDOI
TL;DR: Disclosed is a tactile sensory system consisting of set of sensors that work by measuring impedance among plurality of electrodes that is related to canonical physical representations used to describe stimuli to be sensed.
Abstract: Disclosed is a tactile sensory system consisting of set of sensors that work by measuring impedance among plurality of electrodes. The electrodes are deployed on a substantially rigid structure that is protected form the direct contact with external objects by overlying deformable structures. These mechanical structures have similarities to the biological relationships among the distal phalanx, overlying finger pulp and covering skin and nail. Signal information is extracted form these sensors that is related to canonical physical representations used to describe stimuli to be sensed.

53 citations


Journal ArticleDOI
TL;DR: A communication and control architecture with several novel features that address the functional requirements of the implanted interfaces of BION2 to achieve efficient fine control of muscle force while minimizing fatigue is described.
Abstract: BION2 is a system based on injectable neuromuscular implants whose main goal is to restore the functional movement of paralyzed limbs. To achieve this objective, the functional requirements of the implanted interfaces include not only stimulation but also integrated sensors in order to detect patient intention, to provide servocontrol of muscle activation and to sense posture to inform more global motor planning and coordination. The technical constraints for managing the system include the efficient use of forward and reverse telemetry channels with limited capacity, minimization of adverse consequences from errors in data transmission or intermittent loss of power to the implants, and ability to adjust stimulation rates and phases to achieve efficient fine control of muscle force while minimizing fatigue. This paper describes a communication and control architecture with several novel features that address these requirements

44 citations


Cites background from "A distributed transducer system for..."

  • ...Some systems provide stimulation and sensing in different devices [8]....

    [...]


Patent
02 Jun 2011
Abstract: Disclosed herein is a functional electrical stimulation (FES) device and system. In one embodiment, sequential bipolar pulse stimulation may be provided to an area of a living body via one or more electrode leads applied to the area via a FES device comprising a current pulse generating circuit comprising output nodes for operative coupling to the one or more electrode leads, and configured for operative coupling to a voltage supply. The current pulse generating circuit generally comprises positive and negative stimulation paths drawing from the voltage supply to respectively apply positive and negative currents through the area via the one or more electrode leads. In one example, the stimulation paths comprise respective capacitive elements, a capacitance ratio of which dictating, at least in part, an amplitude ratio of the positive and negative currents, wherein periodic alternative activation of the stimulation paths provides the sequential bipolar pulse stimulation. In another example, each path comprises a respective charging element and a respective activation switch, wherein each respective charging element is charged by the voltage supply and discharged upon activation of the respective activation switch to generate positive and negative current pulses respectively, such that a pulse rise time of the positive and negative current pulses is predominantly dictated by a switching speed of each respective switch. Systems and uses for these devices, and FES in general, are also described.

37 citations


Patent
03 Jul 2007
Abstract: A novel system and method for restoring functional movement of a paralyzed limb(s) or a prosthetic device. Stimulating one or more muscles of a patient using an implanted neuromuscular implants and sensing the response of the stimulated muscle by the implants, wherein the sensing the response is not limited to data related to patient's movement intention, the posture, muscle extension, M-Wave and EMG. A communication and control protocol to operate the system safely and efficiently, use of forward and reverse telemetry channels having a limited bandwidth capacity, and minimizing the adverse consequences caused by errors in data transmission and intermittent loss of power to the implants. Adjusting stimulation rates and phases of the stimulator in order to achieve an efficient control of muscle force while minimizing fatigue and therefore providing for smooth movements and dynamic increase of the strength in patient's muscle contraction.

29 citations


References
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Journal ArticleDOI
TL;DR: The most effective design was one in which a thin sheath of silicone rubber was wrapped around and intra-operatively sealed to a longitudinally slit, tripolar cuff made by dip-coating silicone over stranded stainless steel leads that were prepositioned on a mandrel using polyvinyl alcohol as a temporary adhesive.
Abstract: A comparative study of 5 different designs of nerve cuff electrodes was undertaken to determine their relative merits for stimulating and recording whole-nerve activity over extended periods of chronic implantation on large and small peripheral nerves in 8 cats. Four of the designs represent novel fabrication strategies, including 2 based on flexible, thin-film substrates and 2 based on dip-coating silicone elastomer on a cylindrical mandrel. Various advantages and shortcomings of these materials and designs are discussed in the context of the biophysical factors that influence these electrophysiological interfaces, particularly the problem of recording microvolt-level neurograms in the presence of millivolt-level electromyograms from adjacent muscles in freely behaving subjects. The most effective design was one in which a thin sheath of silicone rubber was wrapped around and intra-operatively sealed to a longitudinally slit, tripolar cuff made by dip-coating silicone over stranded stainless steel leads that were prepositioned on a mandrel using polyvinyl alcohol as a temporary adhesive. When properly installed, these electrodes had stable impedances, recruitment thresholds and relatively interference-free recording properties for the duration of this study (up to 9 weeks).

238 citations


Journal ArticleDOI
TL;DR: A new method of desensitizing the gain of an inductive link to the mutual coupling of its inductors when the coupling varies due to geometrical misalignments is described.
Abstract: This paper describes a new method of desensitizing the gain of an inductive link to the mutual coupling of its inductors. When the coupling varies due to geometrical misalignments [1] this design method guarantees good efficiency and a large bandwidth. The mathematics for four link combinations are presented, and examples of the link efficiency and bandwidth for one of the combinations are shown and discussed.

231 citations


Journal ArticleDOI
Abstract: Using the reflected impedance property of an inductive couple (transformer), a modulation method, load-shift keying using circuit configuration modulator (LSK-CCM), was developed to perform data transmission from an implantable telemeter. With a very simple circuit, this method utilizes a radio-frequency electromagnetic field induced with a single pair of coils to transmit power into the implant and data out of it. >

222 citations


Journal ArticleDOI
TL;DR: A theory of coupled resonant coils has been developed which makes possible the design of radio frequency transcutaneous links of simultaneously high overall efficiency and good displacement tolerance while keeping circuitry simple while keep circuitry simple (particularly in the implanted receiver).
Abstract: A theory of coupled resonant coils has been developed which makes possible the design of radio frequency transcutaneous links of simultaneously high overall efficiency and good displacement tolerance while keeping circuitry simple (particularly in the implanted receiver). Series-tuned transmitter coils were used, obtaining high efficiency. In the first example a stimulator which has excellent displacement tolerance because it works at critical coupling is designed. The second example shows how the theory was used when the voltage in the implant was regulated. The design process involvesad hoc compromises between disparate quantities (e.g. efficiency against voltage transfer ratio), rendering a standard design procedure unsuitable. Fortunately, the derived theoretical formulae are simple enough for every design to be considered from fundamentals, based on coil parameters. Extensive coil loss data are presented here for the frequency band 0·2 to 20 MHz.

215 citations


Journal ArticleDOI
TL;DR: The development of a high-Q approximation, which simplifies the design procedure is presented, and the closed-loop class E circuit shows great promise, especially for circuits with unusually low coefficients of coupling.
Abstract: The use of a multifrequency transmitter coil driver based on the class E topology is described. The development of a high-Q approximation, which simplifies the design procedure is presented. A closed-loop controller to compensate for transmitter and receiver variations, and a method of data modulation using synchronous frequency shifting are described. The closed-loop class E circuit shows great promise, especially for circuits with unusually low coefficients of coupling. Currents of several amperes, at radio frequencies, can easily and efficiently be obtained. >

209 citations