Creating a Future Prosthesis for Today’s Military Amputees

Imagine a prosthesis that saves up the energy created when walking, for use during another part of the process. Visualize a prosthesis that reduces the amount of energy necessary to propel oneself forward, making it easier for amputees to walk. Researchers at Arizona State University’s (ASU) Polytechnic campus and the Military Amputee Research Program at Walter Reed Army Medical Center are working to create just that – a powered prosthetic device based on lightweight energy-storing springs.

The new device, a spring ankle with regenerative kinetics (SPARKy), is the first-of-its-kind smart, active and energy-storing transtibial prosthesis. The project is a multiphase effort led by ASU’s Human Machine Integration Lab, Arise Prosthetics, which aids in fitting the device, and Robotics Group Inc., which designs embedded processors and motor amplifiers.

Direct crossover

Thomas Sugar, PhD, associate professor for the department of engineering at ASU’s Polytechnic Campus, is the principal investigator of this project, administered by the U.S. Army Medical Research & Materiel Command and part of Telemedicine and Advanced Technology Research Center. His research group in the Human Machine Integration Laboratory works to build wearable robots for stroke rehabilitation and powered prosthetic ankles.

“In stroke rehabilitation, we have been designing powered ankle foot orthoses as part of a National Institutes of Health grant,” Sugar told O&P Business News. “We saw a direct crossover to powered prosthetic ankles.”

In 2002, his team applied the spring technology to medical robots.

A patient walking on a treadmill with a powered prosthetic ankle
Image: Joseph Hitt

According to an ASU news release, SPARKy is expected to provide functionality with enhanced ankle motion and push-off power comparable to the gait of an able-bodied individual. Existing technology in prosthetic devices is largely passive and requires the amputee to use 20% to 30% more energy to propel themselves forward when walking compared to an able-bodied person.

SPARKy provides 100% of the required power for gait and 100% of the ankle motion. By providing the user with the kinetics and kinematics comparable to able-bodied gait, researchers expect the patient’s gait symmetry and speed to improve, explained Joseph K. Hitt, active duty Lieutenant Colonel in the U.S. Army and the team leader on the project.

The device delivers 250 W of power, with approximately 50 W coming from a small motor. The rest of the power is distributed by the uniquely tuned and controlled helical springs that are in series with the motor.

“This is probably the most efficient robotic powered prosthesis in the world,” Hitt said. “Since the motor requirement is so low, we hope to be able to use a portable amount of batteries to power SPARKy for a whole day of use.”

Testing methods

Sugar’s team consists of three students who collaborate on all aspects of the project: Hitt, Matthew Holgate, a doctoral student who models the motor and power requirements, and Ryan Bellman, an honors undergraduate student and an expert in computer aided design. They began trial walking tests with SPARKy in June.

After 3 weeks of testing, the right transtibial amputee has increased his self-selected treadmill pace to 2.2 mph from 1 mph with his own prosthesis. The patient, who is part of the project’s objective to support military amputees, also reached and sustained 3.7 mph on the treadmill.

“We have seen that the peak output power at the ankle can be 2 to 4 times greater than the peak input power,” Sugar said.

He said this outcome is made possible because both the patient and the motor store energy in the gait cycle during stance and release the energy quickly during push off.

The team noted that the patient expressed, “Even though [SPARKy] is heavier, it feels lighter than my foot.”

Following the trial, Sugar’s team planned to fit a volunteer with the prosthesis for a demonstration to the Army in December. According to Hitt, subsequent testing will include motion capture analysis, which the team anticipates will more clearly indicate symmetry improvements.

In the second year of the SPARKy project, the research team will develop a controller to determine the patient’s intent.

“Does the person want to start or stop? Does the person want to climb stairs? Does the person want to walk faster or slower?” Sugar asked. “Batteries are always the Achilles heel to any wearable robot project. We are investigating new batteries.”

A patient walking on a treadmill with a powered prosthetic ankle A patient walking on a treadmill with a powered prosthetic ankle A patient walking on a treadmill with a powered prosthetic ankle
A patient is walking at 3.7 mph on a treadmill with a powered prosthetic ankle. SPARKy is a lightweight, energy efficient, and powerful device using a tuned spring for a given body mass. At push off, the mechanical power out of the device is 2 to 4 times greater than mechanical power into the device. This boost in power is possible by storing energy in the stance phase. A spotter is holding a safety switch that can turn off the robot.
Images: Joseph Hitt

Future goals

The final year of the project will test the transition from walking to running, culminating with the functionality to support walking in a daily environment, which is expected in 2009.

“The rehabilitation community has come a long way in the advancement of prosthetic componentry [and] the robotics community has made great strides in biomechanical system development and control,” Hitt said. “What I see in the future [is the] robotics community and the rehab community merging. [First], they have to better understand what the other knows and does.”

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Stephanie Z. Pavlou is a staff writer for O&P Business News

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