'Robotic Tendon’ Puts Spring Back in Steps

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By Miki Fairley

New device could provide amputees with a more powerful stride.

When complete, the aptly nicknamed SPARKy device-short for Spring Ankle with Regenerative Kinetics-is expected to provide functionality with enhanced ankle motion and push-off power comparable to that of an able-bodied person.

Doctoral candidates Matthew Holgate, left, and Joseph Hitt show off SPARKy, the name given to the Spring Ankle with Regenerative Kinetics. Sparky also just happens to be the name of ASU's Sun Devils teams' mascot.
Doctoral candidates Matthew Holgate, left, and Joseph Hitt show off SPARKy, the name given to the Spring Ankle with Regenerative Kinetics. Sparky also just happens to be the name of ASU's Sun Devils teams' mascot.

A research team at Arizona State University's (ASU) Polytechnic campus, Mesa, has teamed up with the Military Amputee Research Program at Walter Reed Army Medical Center, Washington DC, to create the next generation of powered prosthetic devices based on lightweight, energy-storing springs.

Headed by Thomas Sugar, PhD, assistant professor of engineering, the ASU team is the first to apply regenerative kinetics to design a lightweight prosthetic device, according to the university. Other devices are using large motors combined with harmonic drives and a monopropellant, or extremely high-pressure oil.

The team has described SPARKy as a "robotic tendon" because of its bionic properties. "What we hope to create is a robotic tendon that stretches springs when the ankle rolls over the foot, thus allowing the springs to thrust or propel the artificial foot forward for the next step," Sugar says. "Because energy is stored, a lightweight motor can be used to adjust the position of a uniquely tuned spring that provides most of the power required for gait. Thus, less energy is required from the individual."

Existing technology in prosthetic devices is largely passive and requires amputees to use 20 to 30 percent more energy to propel themselves forward when walking compared to an able-bodied person, Sugar points out.

Sugar defines a gait cycle as "the natural motion of walking, starting with the heel strike of one foot and ending with the heel strike of the same foot. The cycle can be split into two phases: stance and swing. We are concerned with storing energy and releasing energy [regenerative kinetics] in the stance phase."

Signs of Success

SPARKy is a powered prosthetic device based on lightweight, energy-storing springs.
SPARKy is a powered prosthetic device based on lightweight, energy-storing springs.

Sugar's team, including doctoral students Joseph Hitt and Matthew Holgate, and Barrett Honors College student Ryan Bellman, already has proof that SPARKy is working. In recent experiments with able-bodied subjects outfitted with a robotic ankle orthosis, or a powered-assist device, the researchers found that the spring-and-motor combination was able to amplify the motor power by threefold. This significant finding allows SPARKy to be downsized from a 6-7kg system to a 1kg (2.2 lb.) system, which is significant weight savings for those who use a prosthesis.

"We expect this device to revolutionize prosthetics, and it will be especially helpful for military personnel wounded in active duty," Hitt says.

Prosthetist Plays Role

The project is a multi-phased effort led by ASU's Human Machine Integration Laboratory; Arise Prosthetics, Phoenix, Arizona; and Robotics Group Inc., Scottsdale, Arizona. Arise Prosthetics is helping with the fitting of the device, and Robotics Group is designing embedded processors and motor amplifiers.

Mark Werner, CPO, FAAOP, of Arise Prosthetics, is excited about the project. "I met Thomas Sugar while serving as a consultant on a separate project," Werner told The O&P EDGE . "We quickly developed interest in each other's work. I received a bachelor of science degree in mechanical engineering before becoming a CPO, and I have always wanted to use that background to help develop new technology for amputees.... [Sugar] was an engineering professor who was doing research to apply his robotic tendon' technology to restore human function."

SPARKy Timeline

According to Thomas Sugar, PhD, the SPARKy project will be completed in three, year-long phases with the following milestones:

  • Year One: Walking on a treadmill.
  • Year Two: Over-ground walking indoors.
  • Year Three: Adjustable stiffness to allow for walking and running.

Werner's primary involvement with the SPARKy project is to lend his expertise as a CPO to guide the team regarding prosthetic designs and issues related to interfacing with the human body, he explains. "In addition, I inform the researchers about the current technology used by prosthetists and provide feedback about the limitations of this current technology. I will also assist in the fitting of the SPARKy prosthesis once it is ready for human trials... The most exciting part is yet to come-when an amputee gets to walk using SPARKy. Based on those findings, my hope is to help Dr. Sugar's team find other applications for this technology."

The first phase of SPARKy featuring the robotic tendon is expected to be ready for demonstration in December. "We are working on one clinical trial currently," Sugar says. "It started on June 1, 2007, and will end in December 2007. We have shown very good walking on the device," he adds.

Miki Fairley is freelance contributing editor and writer for The O&P EDGE based in southwest Colorado. She can be contacted via e-mail at miki.fairley@gmail.com