INCORPORATING FES
CONTROL SOURCES INTO IMPLANTABLE STIMULATORS
P. Strojnik*, S. Pourmehdi**, H.Peckham*,
*Case
Western Reserve University Cleveland , USA
**NeuroControl Corporation,
SUMMARY
With growing demands for more sophisticated neural prostheses, control sources are becoming increasingly important. As the cosmetic appearance plays an important role in the acceptance of neural prosthesis, a need is emerging to move the control sources out of sight and implant them under the skin. Implantable electronic transducers and natural bio-electric signals are considered to be most suitable for the purpose. Recently, an implantable joint angle transducer has been implanted in a C6/C5 tetraplegic volunteer to control hand movement. In a parallel effort, an EMG-processing stimulator-telemeter is being developed to include residual electromyographic activity as a control source for patients with C5 level lesions.
STATE OF THE ART
In
the development of multichannel implantable stimulators, backtelemetry has been
known for at least a decade /1/. Initially, it was intended for monitoring
stimulation electrodes and for general implant housekeeping /2/. With the
emerging need for implantable control signals and sources, implanted telemetry
became a necessity.
For
practical reasons, a single external antenna and a single implantable package
became the preferred configuration for forward and back telemetry. In this
configuration, implanted sensors are connected to the implanted stimulator
package, which contains sensor powering and signal processing circuits as well
as back-telemetry electronics. Several modulation schemes have been used for
backtelemetry, including reflected load modulation and separate transmitter
circuits /3,4/.
MATERIALS AND METHODS
There
are a number of commercial sensors available that can help control an
implantable prosthesis. Among them are angle transducers, accelerometers,
linear displacement transducers, force transducers, and, to name just a few. Virtually none of them exists in an
implantable form and few can be used in an implant because of their size, power
consumption or supply voltage. Depending
on their purpose they have to be modified to reside in the implant package or
at a remote location, connected to the main package by power supply and signal
lead wires.
EMG
signals have been used as a control source in many prosthetic applications.
Recently, myoelectric signals recorded from sternocleidomastoid muscles have
been proposed for ipsilateral hand control /5/. Following
this lead in the implantable environment, a stimulator-telemeter design was
proposed with two EMG channels and 12 stimulation channels to facilitate hand
control for patients that have no voluntary control over wrist motion. It was
suggested that EMG activity of one or two target muscles, monitored by
implanted electrodes, be amplified, rectified and bin integrated. The end
values of the integrated EMG signals would then be digitized and sent to the
external controller.
RESULTS
Two versions of implantable stimulators-telemeters have been designed to work with implantable control sources. Each one consists of a conventional multichannel stimulator circuit and a specialized control signal processing circuit. Backtelemetry is provided by a reflected impedance modulation.
Implantable
Joint Angle Transducer (IJAT).
IJAT is a magnetic transducer connected to the stimulator package by a set of lead-wires. It consists of a miniature magnet and a magnetic sensor. The magnet is implanted in the lunate carpal bone in the wrist and an array of magnetic sensors is implanted in the head of the radius /6/. The change of the wrist position changes the magnetic field, which is recorded by the sensor. This information is telemetered to the external controller and processed for hand grasp and release. To conserve energy, the sensor is pulse-powered, as needed, on the request from the external controller. Two tetraplegic patients have been equipped with the IJAT, thus replacing the external wrist position transducer.
Myoelectric
Signal Processor (MES)
MES
electronic circuit or the MES Processor is the main characteristic of the
second version of stimulators-telemeters. Connected to implanted EMG
electrodes, it measures EMG signal within a defined time window and sends the
final value of a bin-integrated EMG to the external controller.
Fig 1.
shows the MES version of the implantable stimulator
telemeter. It consists of the stimulator-telemeter in the upper section of the
drawing and the MES processor in the dashed block below.
The
MES multiplexer enables the use of a single amplifier chain for two EMG sensing
channels and can also disconnect both electrodes from the MES processor. It
saves valuable space in the implant case and reduces power consumption.
The
preamplifier is a differential, DC coupled two-stage amplifier with a fixed
gain of 50. It is AC coupled to a programmable gain amplifier with gains of 2,
5, 10 and 20. The amplified EMG is fed into a simplified full wave rectifier
followed by a 3.3 ms integrator. The value of the integrated signal is grabbed
at the end of the integrating window and can be transmitted to the external
controller.
The
MES processor is controlled by the external controller via a custom designed
Application Specific Integrated Circuit (ASIC), which is part of the implant’s
electronic circuit. The ASIC continuously maintains communication with the
external control unit and carries out its commands.
The
MES circuit can be instructed to select between two signals for back-telemetry:
the bin-integrated EMG signal or the “raw” EMG signal, sampled at 1kHz. The latter option is designed for muscle EMG
characterization, external EMG processing and system diagnostics. Because of
the limited space in the implant package, there are no separate filters
When
using conventional surface technology, simultaneous EMG recording and
electrical stimulation is possible only by totally separating EMG and
stimulation circuits and by blanking the stimulation pulse from the EMG signal. In the implant package, the stimulation and
the EMG circuits share the same electrical ground, which is connected to the
metal titanium case. This way, the titanium case is the return electrode for
the stimulation current and also the reference electrode for the EMG amplifier.
Both during stimulation pulse and also
during the recharge phase of stimulation, current flows through the titanium
case, thus compromising the reference of the EMG amplifier.
Several
measures were taken to minimize the influence of the stimulation artifact on
the EMG reading and to guarantee a clean EMG observation time window. First,
the pulses of all stimulation channels, normally equidistantly spaced in time,
are grouped together so that their artifact is not spread throughout the
stimulation sequence. Second, during the EMG measuring window, the electrode
recharge currents are disconnected. Third, the EMG integrator is opened only
during the EMG window. Fourth, outside the EMG window, the variable gain is set
to minimum. Fifth, during stimulation, the front-end multiplexer can entirely
disconnect the EMG processor from the EMG electrodes.
Two
sets of MES circuit tests have been performed. The first test was designed to
demonstrate the MES circuit ability to reject stimulation artifacts. A small
Plexiglas basin was filled with saline solution to mimic tissue environment.
Epimysial stimulating electrodes, epimysial EMG electrodes and a titanium
indifferent-reference electrode were positioned in the solution in various
geometric configurations to imitate possible real life situations. In addition,
a 1mV PP, 200 Hz sinusoidal signal, simulating EMG activity was injected into
the saline solution via two stainless steel needles. The integrator output was
monitored at different amplifier gains and with and without the simulated EMG
activity. Fig.2. shows the results for a configuration with the sensing
electrodes positioned between the stimulating electrode and the return
electrode, 2cm away from the stimulating electrode.
The second set of tests was performed using surface EMG and stimulating electrodes on the forearm of a healthy subject. Again, usable EMG recording was obtained for different electrode configurations.
Technology
Hybrid thick film technology on a ceramic substrate is used for production of electronic circuits, with three (IJAT version) and four conductive layers (MES version) respectively. The hybrid circuit is mounted into a titanium case with eight bipolar feedthroughs for lead attachment and two monopolar feedthroughs for the antenna coil. A laser-welded titanium lid hermetically closes the enclosure. The case and the antenna are encapsulated in epoxy resin and conformally coated with silicone.
DISCUSSION
Implantable
control sources, in connection with backtelemetry and digital signal
processing, represent the next step in integration of implantable neural
prosthesis. A former tetraplegic user of an external wrist transducer was able
to switch to an implantable Joint Angle transducer and use it in a matter of
hours. EMG has been shown to contain enough information to control simple
functional tasks in hand grasp and release. Availability of implantable EMG
processor in a stimulator case will allow subjects with high cervical lesions
to control hand movements based on minimal muscle control. Much work has to be
done to implant other transducers, presently used externally on the patient,
and integrate them into the implantable systems
/1/ Strojnik, P., Whitmoyer, D., Schulman, J., "An Implantable
Stimulator for All Seasons", Proc. 10th Int. Symp.
on External Control of Human Extremities,
/2/ Strojnik,
P., Meadows, P., Schulman, J.H., Whitmoyer, D., "Modification
of A Cochlear Stimulation System for FES Applications", Basic and Applied Myology, BAM 4(2): 129-140, 1994;