Strong Arm Tactics

We have the technology,” TV doctors and engineers intoned more than 30 years ago as they turned Steve Austin into the Six Million Dollar Man. But that was science fiction back in the seventies. The truth is that, even today, bionic reality falls far short of that sci-fi fantasy.

We may have the technology within the decade, however, if everything pans out the way the Defense Advanced Research Projects Agency intends for its new Revolutionizing Prosthetics 2009 program. No, the new artificial arm DARPA is funding won’t be stronger than the natural human arm, but it will look, feel and function very like the real thing, sweat included.

Vanderbilt engineers are responsible for both the strength and the sweat.

Professor of Mechanical Engineering Michael Goldfarb is creating the “works” of the new arm—the power, movement and control systems. When finished, their device will be connected directly to the wearer’s central nervous system, either through the peripheral nerves on the shoulder or inside the brain, itself. The arm will thus be thought-controlled and virtually indistinguishable from a natural arm.

It is rocket science

Instead of using heavy batteries or requiring connection to a wall outlet, the new arm will be steam-powered by rocket fuel, releasing moisture comparable to the perspiration a normal arm would release. This power system is much lighter than the state-of-the-art artificial arms available currently, and because it is a pneumatic system it will also provide smoother, more fluid control of the apparatus.

Goldfarb and his team were selected to design the crucial elements of the hand and arm because of their prior work developing a strap-on lower-extremity exoskeleton that they designed to help soldiers carry heavy loads over long distances and difficult terrain. They are coupling the innovations from this prior research with a complex new system of servo valves and sensors to achieve precision control of the arm and to deliver feedback to the wearer much like the neural feedback experienced from a natural arm.

Developing a thought-controlled, lifelike arm is a highly ambitious goal, and DARPA has pulled together top experts in prosthetics and robotics throughout the nation to achieve it. But from the perspective of those who have lost limbs in Afghanistan and Iraq, it’s a long overdue invention. Despite great efforts in research and development to create artificial arms that are functional and practical, even the best of these devices are too heavy and too cumbersome, or they must be plugged into an electrical outlet.
Today many amputees simply make do with their remaining natural arm, or resort to using World War II technology-based artificial arms with hooks.

“Even the state-of-the-art artificial arm doesn’t allow a user to drink a glass of water,” Goldfarb says. “We need a paradigm shift.”

Goldfarb and his team are indeed shifting the paradigm by harnessing fluid power to give the arm needed strength without excess weight. Their new power and control systems eliminate the need for batteries and motors, give smoother and more precise control, and enable 21 degrees of freedom for the arm as a whole, including 17 degrees of freedom for the hand. That’s a big improvement over the 3 degrees of freedom the state-of-the-art myoelectric (controlled by electrical signals from the
body) arm allows.

“We will leverage the actuation technology we have already developed with the exoskeleton project to design the artificial arm,” Goldfarb says. “Our goal is to develop a biomimetic prosthesis that has the strength and power approaching an intact human arm.”

Sweat included

Not to mention a decent appearance. The arm will look as lifelike as possible, with breathable, sweat-able “skin.”

Working under the skin will be 9 pneumatic actuators, 13 sensors, support structure, and a computer to help monitor and control the motions. To achieve the desired fidelity between thought and action, Goldfarb and his team have developed a system of sensors that sense the position and direction of movement of the arm that work with pneumatically driven valves that produce precise movements in response to control signals from the wearer’s central nervous system.

Energy-dense rocket propellant is released into a thermometer-size reaction chamber filled with iridium pellets, which serve as a catalyst separating the fuel into steam and oxygen. The flow is controlled by the servovalves, which control the flow of fuel in relation to the signal from the nervous system. The exhaust from the system of steam and oxygen will be vented through the porous “skin.”

Goldfarb and his team have already achieved some of their goals for dexterity and control of the arm, using a nitrogen-powered cool gas system for power. The next phase of development will involve strengthening the design of the AA battery-size servo valves so that they can function accurately and efficiently under the pressures and heat produced by catalyzed rocket fuel. The valves will need to work at 500 degrees Fahrenheit.

“The heat will be mostly contained within the system, and you can put your hand over the exhaust,” Goldfarb says.

DARPA expects the technology to be ready for clinical trials in only four years. Johns Hopkins University Applied Physics Laboratory is the lead institution of the $30.4 million project, $2.7 of which will go to Vanderbilt.