As part of a multi-institutional team consisting of Northwestern University, IIT, the Rehabilitation Institute of Chicago, the Alfred Mann Foundation (AMF), the University of Colorado, and Sigenics, Inc, a prosthesis control system using Implantable Myoelectric Sensors (IMES) has been developed that consists of multiple single-channel implanted EMG sensors which provide control signals for control of artificial limbs. EMG signals generated by the residual muscles at each implant site are amplified and digitized by the IMES; an extracoporeal Telemetry Controller (TC) within the limb prosthesis controls a time division multiplexing (TDM) sequence to provide power and orchestrate RF transmissions from each implant over a common inductive link. Each IMES is wirelessly powered and telemeters an EMG signal. The TC decodes the received EMG signals from all of the IMES devices, and passes the multi-channel EMG data to a Prosthesis Controller. Locating the IMES in separately innervated muscles, each IMES can be treated as an independent control site with minimal cross-talk or interference. IMES signals appear stable and robust because fibrous tissue holds the devices in place and they are not affected by muscle motion.
Compared to using surface EMG electrodes, IMES can provide multiple degrees of freedom of control. Where once an amputee would have only two degrees provided by surface EMG, IMES can provide 6-8 degrees of freedom simultaneously. This allows an amputee to perform the functions of grasp and wrist rotation at the same time, thus providing significantly enhanced limb control.
How They Work
Each IMES device is powered by a common transmitter coil within the TC. Each IMES has its own address, like a small cell phone network. The TC both powers and receives the digitized EMG signals sensed by the implanted IMES devices. The TC coordinates the EMG transmissions from all implanted IMES contained within the residual limb.
A digital time-domain-multiplexed coordination between the IMES allows for multiple implants within the same residual limb. Up to 32 IMES implants may be controlled from a single TC, with each IMES being assigned a unique time-slot within a master time-domain frame. Each IMES implant can also be independently configured to adjust the gain, bandwidth, and sampling for its EMG signal. Transmission of EMGs can be one of two forms: full-bandwidth (raw) EMG on a 6.78 MHz carrier (Band 2), or integrated EMG on a 60 kHz carrier (Band 1).
Physically, the IMES are constructed similarly to other AMF devices. The electronic chip, substrate, and inductive link coils are housed in a sealed tubular ceramic structure. Implantable lifetime is estimated to be in excess of 80 years.
Even today, limb prosthetics typically use technologies that were developed at the end of the second World War. These prosthetics (the “split hook” arm being one of the most common) are reliable and reasonably effective, but give their users only a very limited subset of the abilities and movements allowed by a natural limb. To solve this, it has long been a goal to use the body’s natural signals to control an active prosthetic limb by tapping into the electrical activity of muscles or nerves left in the residual limb.
One way to accomplish this is to use surface electrodes to take electromyogram data from muscles through the skin. The use of surface EMG (SEMG) signals for control of prosthetic limbs, however, has been historically plagued by unreliability of the SEMG electrodes due to movement artifacts, wire breakage, inconvenience of applying and removing the electrodes, maintenance of skin condition, and repeatability of placement. In addition, it is difficult to obtain more than two or three degrees of freedom from the extra-corporeal surface sites.
While modern electric prosthetic hands often use SEMG control, they are generally limited to a single degree of freedom (opening and closing). Current prosthetic arms requiring multi-degree of-freedom control most often use sequential control – the user manually switches the SMEG control to multiple control functions. While modern robotic limb techniques offer the promise of prosthetic limbs that behave and appear like biological limbs, one major factor limiting the development of more sophisticated hand/arm prostheses is the difficulty in finding stable biological control sources to control the many degrees of freedom required to replace a natural hand and/or arm.
Starting in 2002, the IMES team partners collaborated within the structure of an NIH Bioengineering Research Partnership Grant to develop the IMES1 system. Recent clinical deployment of the IMES1 system in 2014, as supported by AMF, has demonstrated the potential of the IMES system as a clinical prosthesis limb control technology. Starting in 2011, the IMES team is pursuing research on the IMES2 system under a second phase five-year NIH-funded Bioengineering Research Partnership.