News
Prosthetic Promise
With demand growing, mesofluidic technology is one path to lighter, more effective artificial limbs
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The Robotics & Energetic Systems group’s Lonnie Love tests a prosthetic finger prototype powered by a mesofluidic system.
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But that was long ago in a galaxy far, far away. In reality, the loss of a limb is a devastating injury that entails a lifetime of rehabilitation and adaptation. At best, prosthetics give back only a fraction of the lost functionality of a real leg or arm.
Events of the past several years have renewed interest in prosthetics; in fact, prosthetics technologies typically see the most advances during times of conflict. A 2008 visit by an official of a prosthetics firm revealed a number of ORNL technologies that could be applied to improving life for people who have lost limbs.
Lonnie Love is part of a team that is developing a system for artificial limbs based on mesofluidics, a technology similar to the hydraulic systems that power large equipment. Except in this case the technology is scaled down to power equipment for a person.
A mesofluidic prosthetic system (meso means it’s on a scale between micro and macro) would enable control and motion with pressurized fluid, much as high-lifts and front-end loaders are powered, except on a much smaller scale.
“Most prosthetics today are powered by electric motors, with very low power-to-weight ratio. We’re looking at miniaturizing hydraulics,” Lonnie says. “Hydraulics have a power-to-weight ratio that is 10 to 20 times higher, and these systems can also be cheaper.”
Years ago the conventional wisdom held that electric servo motors would supplant hydraulic or, on a smaller scale, fluidic systems. But for some applications, particularly where there are weight limits, fluidics make more sense.
Lonnie and his teammates Randy Lind and John Jansen of the Robotics and Energetic Systems group have devised a mechanical finger composed of just 25 moving parts (as opposed to, in some cases, thousands). It can generate 20 pounds of force with its self-contained fluidic “muscles,” is simple to operate, reliable, rugged and lightweight.
“The key is the control valves,” Lonnie says. “Hydraulic force is proportional to pressure--2,000 pounds per square inch is a lot of force. The flow rate of the fluid controls velocity, and the control valve controls how much fluid goes to the artificial joint’s actuator, or muscle.”
The Measurement Science & Systems Engineering Division group has designed tiny valves that provide a crucial combination of high pressure at low flow to mimic the operation of a true muscle. The prototype system works with simple mineral oil, channeled through a two-thousandths of an inch diameter valve port.
“There are lots of low-flow, low-pressure systems and high-flow, high-pressure systems, but combining high pressure and low flow is the key to making artificial hands and feet that work similar to the real limbs,”
Lonnie says. “These valves are the keys to making controllable hands and feet.”
Recently Doug McCormack, chief executive officer of the prosthetic firm OrthoCare Innovations, visited ORNL accompanied by the company’s chief technical officer, David Boone. They received tours and briefings on ORNL technologies and capabilities that could be applied to advanced prosthetics, such as superhydrophobic materials, carbon foam, e-beam curing and low-cost manufacturable titanium.
“By the end of the day they felt like they were in a grocery story,” Lonnie says.
McCormack, whose company’s headquarters is in Washington, D.C., is trying to overcome the cyclical nature of prosthetics research funding, which spikes during wartime and ebbs when injuries subside in peace. His relationships with lawmakers are bolstered by the fact that he, himself, wears a prosthetic leg.
The company is attempting to establish a research center in Oklahoma City, where OrthoCare Innovations has a manufacturing facility. Lonnie, Art Clemons and others recently participated in a technology showcase there, also attended by Sen. James Inhofe, a congressional supporter of prosthetics research.
Lonnie believes prosthetics technologies, which haven’t seen a great deal of advancement since the 1970s, are poised to benefit from materials and systems technology advances of the past few decades.
“They [OrthoCare] sent me prosthetic parts and asked what we could do. Half the weight on a prosthetic foot is only the metal fasteners. Cliff Eberle is working on a high-strength composite made with e-beam curing. You could make everything out of composites with tremendous weight savings,” Lonnie says.
Other ORNL materials that apply to better prosthetics include carbon foam, whose heat transferring properties could make wearing artificial limbs more comfortable, as would the superhydrophobic material, which could alleviate the discomfort caused by perspiration at prosthetic-limb interfaces. ORNL’s new low-cost method for casting titanium could have a major lightweight benefit. OrthoCare also asked if parts could be coated in nickel, which makes them resistant to infection.
ORNL-developed materials may soon be joined with Defense Advanced Research Projects Agency-funded control systems. One is nerve reinnervation, being developed by Dr. Todd Kuiken of the Rehabilitation Institute of Chicago, who attached the arm nerves of a Dayton, Tenn., double amputee, Jessee Sullivan, to healthy chest muscles. Sullivan has learned to move his prosthetic arms by simply “thinking” about what he wants them to do.
Other approaches to prosthetic control are implanting “bions,” or electronic capsules, in the nerves of lost limbs, which would work with a cuff at the interface with the prosthetic and brain-machine interfaces that would transmit brain activity to the limbs.
“Quadriplegics could really benefit from this,” Lonnie says, describing fluidic-based systems that would help power enfeebled limbs. “It’s far out, but there is lots of potential.”
That potential is being recognized by the National Science Foundation and DARPA, who are active in advancing the state of the art in prosthetics. Advances in hydraulic and fluidic technologies stalled after a spurt of advances in electric motors occurred.
“The best books on hydraulics were written in the ’50s, ’60s and ’70s,” says Lonnie, who recalls that his Georgia Tech mentor was dubious about his choice of hydraulics as a specialty. Funding at the time was nearly nonexistent. Now, however, that same professor is the first Distinguished Chair in Fluid Power at Georgia Tech.”
Combat injuries from current conflicts, which have been dominated by blunt-force trauma, have bumped up the demand for new and better artificial limbs.
One new technology that is not so “far out” is a mesofluidic alignment device that would help patients properly align their prosthetic legs with the residual limb, which is typically a difficult procedure requiring multiple refits.
To someone like OrthoCare’s McCormack, who has experienced the challenges presented by the loss of a limb, ORNL is a font of promise directed toward improving shattered lives and moving advanced prosthetics out of space-operas and into reality.
“He was absolutely giddy when he left,” Lonnie says of McCormack’s visit. “His excitement is something I’ll never forget.”
