Implantable Sensors Improve Control for Prosthetic Limbs

The main components of the prosthesis control system. (Source: Richard Weir, U. Colorado) When a muscles contracts it generates electricity. This electrical activity remains in the muscles of an amputated limb. It has long been a goal to use these natural muscle signals to enhance the control of hand and arm prosthetics. Previously, researchers did this by attaching two external sensors to the surface of the skin of the residual limb. The sensors were connected to a prosthesis controller by wires.

Detecting muscle signals on the surface of the skin limited the number of control sites available and only gave users the ability to open and close the prosthesis. Now, researchers are developing novel sensors that can be implanted directly into the muscles of the residual limb to create more control sites. These sensors wirelessly transmit intramuscular electrical signals from within the body to external electronics that command a prosthesis to move in a more natural way.

Richard Weir, Ph.D. and his team at the University of Colorado are refining this technology, with the goal of enabling individual finger control of a prosthetic hand. The key technology is known as an implantable myoelectric sensor (IMES) system, which enables an increased number of control sites to be used, providing enhanced, intuitive, simultaneous control of the many joints in the prosthetic hand. Because they are placed directly into the muscle, IMES sensors are more specific than the sensors currently placed on the skin.

Dr. Weir holds one of the tiny sensors to be implanted in each forearm muscle to send signals to the prosthetic hand. (Source: Richard Weir, U. Colorado)

The potential exists to implant each of the human forearm’s 18 muscles with a sensor that wirelessly transforms all those impulses into signals that can be used to control hand and individual finger movements. Current prosthetics have only two sensors, which send two signals: one to open and one to close the hand.

Participants were able to activate multiple muscles to simultaneously generate multiple signals. This was a significant result because it demonstrated that forearm muscles retained the independent activity necessary for multiple prosthetic finger control. Additional research on the interactions between finger, thumb and wrist muscles was critical for designing and controlling an artificial hand with similar capabilities to that of the human hand.An important aspect of the work was determining the control potential and limitations of individual muscles. Using intramuscular electromyographic (EMG) electrodes, the group examined the ability of study participants to send signals from specific forearm muscles to specific fingers of the prosthetic hand.

The original IMES (IMES1) was developed in the first five-year portion of an NIBIB Bioengineering Research Partnership (BRP) that was awarded to the Weir laboratory, then at the Rehabilitation Institute of Chicago. Weir's group has since moved to the University of Colorado, Denver/Anschutz Medical Campus. Additional partnership collaborators include the Illinois Institute of Technology, Chicago; Sigenics, Inc,. in Chicago; and the Alfred Mann Foundation in Valencia, CA.

The Mann Foundation licensed the technology developed under this first BRP, and with additional funding and resources of their own, further developed the IMES1 for use in a first clinical trial that was conducted at Walter Reed Medical Center. Staff Sergeant James Sides, who lost his forearm in Afghanistan, was implanted with eight myoelectric sensors that enable him to control the first prosthetic hand tested in a human subject using the IMES system. The novel prosthesis moves similarly to a human hand, allowing Sides to lift small items such as a drinking glass; he can also perform other daily tasks such as use an ATM—which involves taking out his wallet, inserting and removing money and credit cards, and returning the wallet to his pocket—all with the prosthetic right forearm and hand.

The collaborators are further refining the technology, under the second five year portion of the NIBIB partnership, to allow the IMES to transfer more data at a higher data transfer rate, with fewer mistakes and using far less power. All of this is necessary to make the device more robust and clinically viable. The ultimate goal of the collaborators is to make the device commercially available to benefit people with arm amputations.

Source: NIBIB/NIH [1]