P. R. Troyk

Implantable Myoelectric Sensors (IMESs) for Intramuscular Electromyogram Recording

We have developed a multichannel electrogmyography sensor system capable of receiving and processing signals from up to 32 implanted myoelectric sensors (IMES). The appeal of implanted sensors for myoelectric control is that electromyography (EMG) signals can be measured at their source providing relatively cross-talk-free signals that can be treated as independent control sites. An external telemetry controller receives telemetry sent over a transcutaneous magnetic link by the implanted electrodes. The same link provides power and commands to the implanted electrodes. Wireless telemetry of EMG signals from sensors implanted in the residual musculature eliminates the problems associated with percutaneous wires, such as infection, breakage, and marsupialization. Each implantable sensor consists of a custom-designed application-specified integrated circuit that is packaged into a biocompatible RF BION capsule from the Alfred E. Mann Foundation. Implants are designed for permanent long-term implantation with no servicing requirements. We have a fully operational system. The system has been tested in animals. Implants have been chronically implanted in the legs of three cats and are still completely operational four months after implantation.

Inductively-coupled power and data link for neural prostheses using a class-E oscillator and FSK modulation

Despite the success of implantable batteries as commonly used in pacemakers, implantable neural prosthetic devices typically have power requirements that exceed the capability of reasonably-sized implantable batteries. Therefore, transcutaneous magnetic coupling remains the method of choice for powering implanted neural prostheses. Using the same inductive link for transfer of power and bidirectional telemetry is an attractive solution to powering and communicating with implanted devices thus avoiding percutaneous plugs, wires, or conduits. The Class-E power oscillator has been identified as a highly-efficient transmitter circuit for use as a means of transferring power to an implant. Although the high-Q nature of this topology makes rapid modulation difficult, it is feasible to use synchronous frequency-shift-keyed (FSK) modulation of the Class-E circuit thereby combining an efficient power transmitter with a highspeed data link. Using this method, the transmitter can be modulated on a cycle-by-cycle basis with little to no additional power loss. Within an implanted device, demodulation of the FSK transmitted carrier can be accomplished using a novel demodulation circuit.

Implantable myoelectric sensors (IMES) for upper-extremity prosthesis control- preliminary work

We are developing a multichannel/multifunction prosthetic hand/arm controller system capable of receiving and processing signals from up to sixteen Implanted MyoElectric Sensors (IMES). A BION/spl reg/ II package will house the implantable electrode electronics and associated circuitry. An external prosthesis controller will decipher user intent from telemetry sent over a transcutaneous magnetic link by the implanted electrodes. The same link will provide power for the implanted electrodes. Development of such a system will greatly increase the number of control sources available to amputees for control of their prostheses. This will encourage the design and fitting of more functional prostheses than are currently available.

Development of an implantable myoelectric sensor for advanced prosthesis control.

Modern hand and wrist prostheses afford a high level of mechanical sophistication, but the ability to control them in an intuitive and repeatable manner lags. Commercially available systems using surface electromyographic (EMG) or myoelectric control can supply at best two degrees of freedom (DOF), most often sequentially controlled. This limitation is partially due to the nature of surface-recorded EMG, for which the signal contains components from multiple muscle sources. We report here on the development of an implantable myoelectric sensor using EMG sensors that can be chronically implanted into an amputees residual muscles. Because sensing occurs at the source of muscle contraction, a single principal component of EMG is detected by each sensor, corresponding to intent to move a particular effector. This system can potentially provide independent signal sources for control of individual effectors within a limb prosthesis. The use of implanted devices supports inter-day signal repeatability. We report on efforts in preparation for human clinical trials, including animal testing, and a first-in-human proof of principle demonstration where the subject was able to intuitively and simultaneously control two DOF in a hand and wrist prosthesis.

Technical Details of the Implantable Myoelectric Sensor (IMES) System for Multifunction Prosthesis Control

The limitation of current prostheses is not the devices themselves but rather the lack of sufficient independent control sources. A system capable of reading intra muscular EMG signals would greatly increase the number control sources available for prosthesis control. We are developing a multichannel/multifunction prosthetic hand/arm controller system capable of receiving and processing signals from up to sixteen implanted bipolar differential electromyographic (EMG) electrodes. An external prosthesis controller will decipher user intent from telemetry sent over a transcutaneous magnetic link by the implanted electrodes. The same link will provide power for the implanted electrodes. This paper describes some of the technical aspects of the implant and telemetry design

Charge-injection waveforms for iridium oxide (AIROF) microelectrodes

The charge-injection limits of activated iridium oxide (AIROF) microelectrodes subjected to charge-balanced biphasic current pulsing are investigated as a function of anodic bias and asymmetry in the cathodic and anodic pulse widths. The use of asymmetric waveforms, in which the charge balancing anodic phase is delivered at a lower current density and longer pulse width, permits the use of anodic biasing to maximize charge-injection capacity. The need for more sophisticated driving waveforms and how these could be implemented in modern ASIC design to achieve optimal charge-injection is discussed.

Multichannel cortical stimulation for restoration of vision

Development of an intracortical visual prosthesis for restoration of vision, has been, and continues to be an elusive goal of neural prosthesis researchers. Our multi-institutional team has tested the feasibility of implanting and evaluating large numbers of stimulation/recording electrodes in an animal model. Using a combination of 8-electrode arrays and individual electrodes, 152 activated iridium microelectrodes were implanted in area V1 of a macaque. Visual stimuli were used to define a retinotopic map. Spatial coordinates for each electrode were used to train the animal to use electrical stimulation in performing a visual psychophysical task.

IMES: An Implantable Myoelectric Sensor

We present updated progress on the design, construction and testing of an upper-extremity prosthesis control system based on implantable myoelectric sensors. The miniature injectable implant consists of a single silicon chip packaged with transmit and receive coils. Preparation for human implantation of the IMES system is underway. As part of this process, critical design improvements in the IMES implant were required. Here we report improved functionality of the IMES implant, hardened protection against electrical malfunction and tissue damage.

Development of the boston retinal prosthesis

A small, hermetic, wirelessly-controlled retinal prosthesis was developed for pre-clinical studies in Yucatan mini-pigs. The device was implanted on the outside of the eye in the orbit, and it received both power and data wirelessly from external sources. The prosthesis drove a sub-retinal thin-film array of sputtered iridium oxide stimulating electrodes. The implanted device included a hermetic titanium case containing the 16-channel stimulator chip and discrete circuit components. Feedthroughs in the hermetic case connected the chip to secondary power- and data-receiving coils, which coupled to corresponding external power and data coils driven by a power amplifier. Power was delivered by a 500 KHz carrier, and data were delivered by frequency shift keying. Stimulation pulse strength, duration and frequency were programmed wirelessly from an external computer system. Through an ‘outbound’ telemetry channel, electrode impedances were monitored by an on-board analog to digital converter that sampled the output voltage waveforms. The final assembly was tested in vitro in physiological saline and in vivo in two mini-pigs for up to three months by measuring stimulus artifacts generated by the implants current drivers.

Decoding individuated finger flexions with Implantable MyoElectric Sensors

We trained a rhesus monkey to perform randomly cued, individuated finger flexions of the thumb, index, and middle finger. Nine Implantable MyoElectric Sensors (IMES) were then surgically implanted into the finger muscles of the monkeys forearm, without any observable adverse chronic effects. Using an inductive link, we wirelessly recorded EMG from the IMES as the monkey performed a finger flexion task. A principal components analysis (PCA) based algorithm was used to decode which finger switch was pressed based on the recorded EMG. This algorithm correctly decoded which finger was moved 89% of the time. These results demonstrate that IMES offer a safe and highly promising approach for providing intuitive, dexterous control of artificial limbs and hands after amputation.