How Researchers Are Building a Lighter Adaptive Ankle Without the Bulky Motor
Reading Time: 4 minutes
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Summary:
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NAU doctoral student Sam Hopkins is developing a wearable ankle-foot orthosis to help children with cerebral palsy move independently.
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The ankle orthosis uses smart, adjustable springs instead of a motor, which makes it lighter than other options.
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The device is built from carbon fiber tubing and 3D-printed carbon-reinforced nylon, is designed to work without individual fitting, and is estimated to cost $2,000 per leg.
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An NIH F31 fellowship funds the work. Currently, there are no test results or availability dates.
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A research team at Northern Arizona University (NAU) is developing a wearable ankle device designed to provide some of a robotic exoskeleton’s adaptability without the weight of a motor. Doctoral student Sam Hopkins is creating it in the Biomechatronics Lab of mechanical engineering professor Zach Lerner, with support from a National Institutes of Health (NIH) fellowship.
The device is an ankle-foot orthosis—a brace worn on the leg, not a prosthesis. It is designed to help children with cerebral palsy move more independently by supporting their own ankle rather than replacing a limb. The design challenge, though, will be familiar to anyone following prosthetic technology: how to build something that adapts to different movements without becoming heavy and bulky.
Where current ankle orthotics fall short
According to NAU’s report, existing ankle orthotics tend to be stiff, resembling a ski boot more than a moving joint. This rigidity works well for walking, but it can make activities that require a wider range of motion more difficult, such as sitting down or standing up, climbing stairs, and entering and exiting a car.
Hopkins describes current options as two ends of a spectrum. Passive, spring-based devices excel at a single function but can’t adapt to the wearer’s changing activity. Meanwhile, fully powered, motorized exoskeletons can perform many tasks, but their weight and bulk remain drawbacks. He says his device is designed to bridge the two.
How the device is built
Instead of a motor, the device uses smart, adjustable springs. Hopkins says that keeps it mostly passive while still giving it the adaptability he is after, and that it makes the device lighter than other options available.
The frame is made of carbon-fiber tubing with 3D-printed carbon-reinforced nylon components, a material that NAU’s report says can match aluminum’s strength. The design is flexible enough to fit different users without individual adjustments. Hopkins estimates that, with these materials and no customization, the cost would be approximately $2,000 per leg. That figure is the researcher’s own estimate, not a confirmed price, and NAU does not give a timeline for testing or availability.
The inspiration behind it
Hopkins is in his fourth year in Lerner’s lab and received an NIH F31 fellowship, which supports doctoral students in health-related research training. The funding supports his work on an orthosis for children with cerebral palsy.
His interest in the field began in childhood. He watched his grandfather struggle with everyday activities and needed help moving as he battled cancer. Around the same time, he went to the movies and saw Iron Man use technology to augment an injured body. These experiences led Hopkins to wonder whether technology could help injured people manage everyday tasks effortlessly in real life.
The Biomechatronics Lab creates wearable robotic exoskeletons that help restore neuromuscular function and enhance mobility. Hopkins has also contributed to developing a device intended to relieve pain associated with knee osteoarthritis.
Why this development is worth watching
While Hopkins’ device is not a prosthesis, the challenges it tackles may interest anyone following prosthetic technology: how much adaptability can a device offer before its weight, bulk, and cost make it inaccessible?
The project offers one possible answer: adjustable springs instead of a motor, strong yet lightweight materials, and a flexible design that requires no custom fitting. Whether it delivers on that promise will depend on testing, and we don’t yet have details.
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