Prototype limb

An international team led by the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, MD, has developed a prototype of what they claim to be the first fully integrated prosthetic arm that can be controlled naturally and provide sensory feedback.

The arm also allows for eight degrees of freedom - a level of control far beyond the current state of the art for prosthetic limbs. Proto 1, developed for DARPA’s Revolutionizing Prosthetics Program, is a complete limb system that also includes a virtual environment used for patient training, clinical configuration, and to record limb movements and control signals during clinical investigations.

The DARPA prosthetics program is an ambitious effort to provide the most advanced medical and rehabilitative technologies for military personnel injured in the line of duty.

Over the last year, the APL-led Revolutionizing Prosthetics 2009 (RP 2009) team has worked to develop a prosthetic arm that will restore significant function and sensory perception of the natural limb. Proto 1 and its virtual environment system were delivered to DARPA ahead of schedule, and Proto 1 was fitted for clinical evaluations conducted by team partners at the Rehabilitation Institute of Chicago (RIC) in January and February.

The advanced degree of natural control and integrated sensory feedback demonstrated with Proto 1 are enabled by Targeted Muscle Reinnervation (TMR), a technique pioneered by Dr. Todd Kuiken at RIC that involves the transfer of residual nerves from an amputated limb to unused muscle regions in appropriate proximity to the injury.

In this case, the nerves were transferred to the pectoral area of the patient’s chest. This procedure provides for a more intuitive use of a prosthetic arm and allows for the natural sensation of grip strength and touch.

During clinical evaluation of the limb at RIC, Jesse Sullivan, a patient of Dr. Kuiken, demonstrated substantial improvements in functional testing, such as the ability to reposition his thumb for different grips, remove a credit card from a pocket, stack cups while controlling his grip force using sensory feedback verses vision, and to walk using the free swing mode of the limb for a more natural gait.

The limb system also includes a natural-looking artificial covering that was created using photographs of the patient’s native limb taken before the accident.

APL, which was responsible for much of the design and fabrication of Proto 1, and other team members are already hard at work on a second prototype, expected to be unveiled in late summer. It will have more than 25 degrees of freedom and the strength and speed of movement approaching the capabilities of the human limb, combined with more than 80 individual sensory elements for feedback of touch, temperature, and limb position.

Another exciting development is the functional demonstration of Injectable MyoElectric Sensor (IMES) devices - very small injectable or surgically implantable devices used to measure muscle activity at the source verses surface electrodes on the skin that were used during testing of the first prototype.

The IMES devices, coupled with TMR procedures, promise to increase the fidelity of control and can be used in certain injury scenarios to provide a significant improvement on their own merit.