Medical Device Daily Washington Editor

When we think of steam engines, we are fairly likely to think of the Iron Horse, the nickname given to the 19th Century steam locomotive that pulled the trains of yesterday.

That technology is fading from use to pull trains, but another form of it is now powering a very small locomotive being designed to provide life-like movement for the artificial limbs of tomorrow, via development of a new high-tech prosthetic arm.

And while the military funding for research is probably better known for supporting research that develops materials, the Defense Advanced Research Projects Agency (DARPA) at the Department of Defense (DoD) is supplying the grant for this artificial limb research as well.

A team of six researchers at the Vanderbilt University (Nashville, Tennessee) School of Engineering is harnessing the power of vaporized hydrogen peroxide (pH) to drive tiny motors that replicate the movement of our muscles, tendons and bones more accurately than currently possible with commercially available prosthetics.

Lead researcher Michael Goldfarb, PhD, a professor of engineering at Vanderbilt, told Medical Device Daily that the Vanderbilt arm “does not have superhuman strength or capability, but it is closer in terms of function and power to a human arm than any previous” self-powered prosthetic.

Because his design can allow the wearer to curl a weight of up to 25 lbs, three or four times more than the current state-of-the-art prosthetic arms, “that means it has about 10 times as much power,” despite the fact that his pH-powered arm “hasn’t yet been optimized for strength or power.”

The power source for this unit — only about the size of a pencil — uses iridium to spark the heat-driven expansion of pH to provide the energy needed to move the mechanical components of the artificial arm.

“Iridium is expensive,” Goldfarb told Medical Device Daily, “so you start with alumina, [which is aluminum in a crystalline form] and you coat it with iridium.” He said that his lab fabricates the combination in “granules a little smaller than kitty litter.”

When the pH hits the iridium-coated alumina, the steam pushes spring-loaded valves that pull on monofilament belts that are, in turn, connected to the rods that replicate the function of tendons for most of the muscles in the arm. In the case of the wrist and elbow, however, the valves are strapped directly to the prosthetic “bones” which replicates the direct attachment of muscles to the elbow and wrist in the human arm.

Goldfarb said that the Vanderbilt arm is a much more natural replacement for a lost limb than any of the competition can offer. While the Vanderbilt is not yet able to allow the wearer to swat a fly with the same gusto as he or she could with a natural arm, “[w]e can achieve 75[%] to 80% of what an arm can do. In some things, we can move arguably faster.”

“The larger joints [in the Vanderbilt] are a little bit slower” than the smaller joints, Goldfarb said, adding, however, that the hardware offered by others is even slower yet, and offers fewer degrees of motion freedom. He said that the competition does not yet offer powered wrist joints and that because “they’re so much slower, people get annoyed with them.”

The first prototype ran on condensed gas, which operates at considerably cooler temperatures than the current version generates. When the researchers switched to pH as an energy source, they found that the conversion expanded the volume of the gas by a factor of 1,000. This generated heat as high as 450° Fahrenheit and they found that the monofilaments used to lash the valves to the actuators had trouble maintaining material integrity under such heated conditions.

This prompted a change to monofilaments made of polyether-etherketone (PEEK), which Goldfarb said is stronger than spider silk, despite all the hype about the tensile properties of the latter substance.

With a little more work, the team got the artificial limb’s wrists and fingers to bend and the forearm to rotate, a much more natural set of motions than currently available, Goldfarb said.

At present, the Vanderbilt arm is designed to accommodate only one pH canister, which will last for about 18 hours under ordinary use. When a full arm model comes out, the amount of room for storage of pH canisters will increase.

“Once you go into the upper arm, you almost double the space,” Goldfarb said. “What we would do is just have a larger cartridge” of perhaps 300 ml capacity, a 50% boost in capacity compared to the current canister of 200 ml. The move to a full-arm design might allow for multiple pH canisters, and in terms of engineering, should not create new dilemmas.

“It arguably gets easier, not harder” to replicate the mechanical behavior of the upper arm, Goldfarb said.

Once such an artificial arm is installed, the patient will need a neurological connection to the prosthetic in order to make use of it. Goldfarb said that neurobiologists are working on interfaces between prosthetics and the human nervous system, but this part of the task is still in preliminary stages.

The DARPA funding is part of a project known as Revolutionizing Prosthetics 2009, which Johns Hopkins University (Baltimore) is administering for DoD, and two other teams are moving forward with their designs as well. Consequently, Goldfarb cannot be certain that funding for the Vanderbilt arm will remain steady.

The agency wants a commercially viable model by 2009, hence the name of the program, and Goldfarb said that the novelty of using pH as a power source may extend the regulatory timeline for the Vanderbilt arm to beyond that date.

On the other hand, the device market is flush with cash, and Goldfarb said he is confident about the Vanderbilt arm’s prospects.

“We have made so much progress and gotten such positive feedback from the research community that I’m certain we’ll be able to keep going.”