BION2 technology will provide outgoing telemetry of data from various modalities of sensing in order to achieve FES. The sensing modalities that appear immediately feasible include bioelectrical signal recording from the existing electrodes, range-finding between implants (based on implants quantifying the strength of the outgoing RF transmission of another implant), and acceleration (based on incorporating a microelectromachined silicon [MEMS] sensor in the implant package).
The BION2 incorporates a novel power and data transmission scheme called "suspended carrier" that reduces the power requirement about 5-fold from the BION1 amplitude-modulated 2-MHz carrier. The power oscillator can be switched completely off and on again within two carrier cycles and with minimal power loss by electronically opening the tank circuit at the instant when all of the energy in the circuit is in the form of charge stored on its tuning capacitor (Troyk, Heetderks and Loeb, U.S. Patent #5,649,970, July 22, 1997). All of the outgoing telemetry and some of the low-level sensing will occur during periods of carrier suspension.
Figure 3.7 illustrates a typical scheme for using multiple BION2 implants to achieve the relatively simple clinical FES function of assisting a patient with weak voluntary grasp. This application has the advantage that all of the muscles that must be monitored and stimulated are located in the forearm, where BIONs can be easily powered and controlled by a transmission coil embedded in the sleeve of a garment. A somewhat simpler scheme has been tested clinically, using transcutaneous stimulation of digit flexors triggered by mechanical monitoring of voluntary wrist flexion.26
Functional grasp of objects includes several distinct postural strategies, each requiring the coordinated recruitment of many muscles.27 The example chosen is a relatively crude palmar prehension task in which an object is captured by curling
FIGURE 3.6 A. Insertion of BION1 implants into supraspinatus (not illustrated) and middle deltoid muscle using an insertion tool that permits application of search stimuli to identify the correct site. B. Extrusion of BION1 implant into desired site in middle deltoid. C. Daily self-administered TES session requires donning of transmission coil configured similar to a heating pad and push-button operation of Personal Trainer™ into which clinician has downloaded one or more customized exercise programs.
© 2001 by CRC Press LLC
the fingers around the object and forcing it into the palm. Many patients with strokes and cervical spinal cord injury can initiate the first phase of such a grasp, which is to stabilize the wrist in extension so that the subsequent contraction of the long finger flexors is not dissipated by their flexion action at the wrist. In Figure 3.7, BIONS #0 and #1 in the two wrist extensors detect the onset of this voluntary activity by monitoring their electromyographical (EMG) signals. The external controller then coordinates the electrical stimulation of the other wrist and digit muscles, monitoring the resulting hand motion by measuring the distances between the various implants as the muscles change length. Changes in the shape of a contracting muscle may cause changes in the recruitment properties of an intramuscular stimulator and in the force-generating capabilities of stimulated motor units. In the example, BIONS #5 and #6 in the critical long digit flexor muscle each monitor the M-wave produced by the stimulation from the other implant as well as the distance between the two implants. The external controller can then compensate for such effects in order to maintain the desired level of grip force.
Many other FES applications can be imagined using different combinations of stimulation and sensing (see Chapters 2 and 5 for additional examples). All are likely to require some form of sensory feedback to replace the reflexive control of the spinal circuitry for normal voluntary behaviors and some form of command signal detection based on mechanical or electrical sensing of a part of the neuromuscular apparatus that remains under voluntary control. As the peripheral interfaces available through BION technology become more sophisticated, the limiting factor in performance will shift first to the control algorithms for using the sensory feedback and then to the integration of the sensory and motor components with the cognitive and volitional functions of the brain itself. These challenges are covered in other chapters in this volume.
FIGURE 3.7 BION2 implants will be capable of various sensing modalities as well as generating stimulation pulses. As illustrated in the icon key, a given implant can be commanded to sense bioelectrical signals such as EMG for a programmable sensing interval and programmable gain. While the power RF carrier is turned off (state T), the implant telemeters out a digitized value corresponding to the area of the EMG waveform. For position sensing, a given implant can emit a standardized RF burst (ping) while other implants record and digitize the local field strength of the ping for later outgoing telemetry. The RF carrier can exist in one of two on-states: D for continuous modulation to encode command data addressed to individual implants and C for continuous unmodulated carrier, which maximizes field strength to precharge the power storage capacitors in the implants to function during subsequent carrier-off periods. Data sent to each implant include synchronization codes to permit various implants to begin sensing and sending data back upon detection of particular carrier states. The state diagram at the bottom shows the sequence of functions in each implant for one typical frame of control, with frames repeated at 25 frames/sec. Each implant can be sent a command in 160 to 270 |s, depending on the number of new parameters required for its next function. Back-telemetry of one 8-bit data byte (plus formatting bits) from one implant requires 256 |s.
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