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August 29, 2009

Revolutionizing Prosthetics 2009: APL-Led Team Advances Prosthetic Arm Technology

prosthetic armUnder Defense Advanced Research Projects Agency (DARPA) sponsorship, APL is leading a revolution in prosthetic arm capabilities, an effort that is producing prostheses with functional capabilities similar to those of native limbs. A multinational team led by APL is in the final stage of development, having sufficiently advanced technology development, systems design, and integration in early prototype work.

DARPA, which commissions “high-risk, high-reward” research to address urgent challenges for the Department of Defense, embarked on a path to revolutionize prosthetics in 2005 when it funded the creation of a mechanical arm that would look, feel, perform, and be controlled like a natural limb. DARPA came to APL to create and lead that complex program, challenging the Laboratory to advance basic research related to neural control mechanisms and the physiological function of the human limb in order to create a state-of-the-art mechanical arm.

Applying the systems engineering approach that is the Laboratory’s hallmark, APL assembled the Revolutionizing Prosthetics (RP) 2009 team, pulling in experts from across The Johns Hopkins University (including the Medical Institutions, Whiting School of Engineering, and Bloomberg School of Public Health) and more than 30 organizations in the United States, Canada, and Europe.

Successful Prototypes

prosthetic armIn the first phase of the project, completed in 2007, the team created two increasingly capable prototype limb systems. APL engineers led the development of Proto 1, a fully integrated prosthetic arm that could be controlled naturally, provide sensory feedback, and allow 8 degrees of freedom (DOF)—a level of control far beyond the current state of the art for prosthetic limbs.

APL engineers created a virtual integration environment for patient training, which allowed limb movements and control signals to be configured and recorded during clinical investigations. It also allowed prototype limb movement to be displayed as a virtual model on computer screens shared by partners collaborating on intricate applications. In such a virtual environment, patients test-drive limbs using their own inherent signals for control and provide feedback even before a prosthetic arm is manufactured and fitted, thus optimizing limb performance for each patient and speeding development.

The Proto 2 limb was developed under APL’s direction during 2008. Working closely with RP 2009 partners, Laboratory engineers provided expertise in systems engineering, neural signal analysis and fusion, controls development, neural system design, electronics and software development, limb system design, electronics, and software development.

The Proto 2 limb has 25 DOF, individual finger movements, and dexterity that approaches that of the human limb. APL designed and developed several of its components. Proto 2 was conceived as a benchtop system to show that the team could meet the stringent performance expectations for the final limb and that components would fit within the shape and size of a human arm.

Although Proto 2 was not originally intended to be used with a patient, DARPA asked that it be demonstrated at the DARPA Tech 2007 annual symposium. A Duke University student/team member, who also is an Iraq War amputee, quickly developed a way to demonstrate how the virtual integration environment could be used to train patients in grasp patterns and individual finger movements. That demonstration showed that Proto 2 had unprecedented mechanical agility, natural control, and sensory feedback.

Building on Early Accomplishments

armIn 2008, DARPA proceeded with the second phase of the RP 2009 project, in which the team added more advanced control and sensory feedback mechanisms and researched other sensory feedback and neural interface mechanisms. These features will give prosthesis users a sense of touch, temperature, pressure, and vibration, as well as a sense of where the limb is situated in space. A tactor, an electromechanical device placed on the surface of the chest, currently provides the feedback, but other technologies are being explored as well.

For a patient to operate the prosthesis using his or her own neural system, the team must develop a number of small wireless devices that can be surgically implanted (or injected) to allow access to intramuscular signals. The devices were tested and demonstrated within the virtual integration environment and are now moving through the Food and Drug Administration regulatory approval process.

Also in development is a small peripheral nerve interface device that can be used to record signals from the brain’s sensory and motor centers. The team is exploring how to incorporate these early devices into an integrated motor control and sensory feedback “tool kit” that provides suitable clinical options for a range of injury types and at various levels of corresponding invasiveness. Another challenge is to match the weight of the native limb, which is approximately 7–8 pounds for an average adult. This means that Proto 2 designs must be reduced by nearly 2 pounds.

There is no shortage of significant challenges ahead for the transdisciplinary team of experts. The RP 2009 program has already achieved significant advances in prosthetic limb research; more improvements await as the team works toward producing an aesthetic and multifunctional mechanical arm that enhances an amputee’s quality of life.