What do paintball pistols, earth movers, and a first-of-its-kind orthosis prototype have in common? They're all driven by fluid power, the technology that allows engineers to vastly magnify human or mechanical strength by harnessing the physics of compressed fluids (hydraulics) or gases (pneumatics). Though these technologies are integrated into a variety of powered-orthosis prototypes currently under development across the country, all but one of these devices are leashed to external electricity sources and computing power. In contrast, the Portable Power-Assist AFO is the world's first cordless (i.e., untethered) power-assisted AFO. Because it carries its own "muscles and brain," it is able to benefit patients in a far greater variety of situations. The AFO is intriguing in another way as well: it represents an unusual collaboration between heavy industrial interests and O&P through a rare grant from the National Science Foundation (NSF).
The project is being developed by a team of researchers led by Géza Kogler, PhD, CO, a research scientist in the O&P program at the Georgia Institute of Technology (Georgia Tech), Atlanta, and Elizabeth Hsiao-Wecksler, PhD, an associate professor of mechanical engineering at the University of Illinois at Urbana-Champaign. According to Kogler, the team includes approximately 25 researchers from a wide variety of disciplines, including clinical care, thermodynamics, and even acoustics.
Industrial Allies
This multidisciplinary approach seems appropriate considering the unusual source of the grant series that has funded the project since 2006. It is provided by the Center for Compact and Efficient Fluid Power (CCEFP), an NSF Engineering Research Center tasked with developing hydraulic and pneumatic technology for use in new arenas. The CCEFP states on its website, "Because of its superior power-to-weight ratio, fluid power is ideal for applications like orthoses where space and weight are at a premium." According to the Center, the AFO project serves the Center's goal of making fluid power "more acceptable and its use more widespread in human interface devices by making it quieter, safer, and cleaner than current technology allows."
This Portable Power-Assist AFO prototype is powered by the pneumatic force of paintball-style CO2 canisters, but future versions will likely use tiny engines to concentrate air or power hydraulic systems.(Image courtesy of the Georgia Institute of Technology.)
Rehabilitative Power
Though fluid power is used worldwide for moving almost unthinkably heavy objects, such as the locks of canals, Kogler emphasizes that the Portable Power-Assist AFO has limited power, and for an excellent reason. He says that the AFO is "power-assisting," meaning that it doesn't completely replace the function of lost muscle power, as would a fully powered device designed for people with paraplegia. Instead, it is designed to help patients with partial paralysis by supplementing their remaining abilities. The addition of partial power offers extremely important benefits to patients.
"Right now," Kogler says, "almost all that practitioners can do with a passive AFO is restrict movement to improve gait function. But oftentimes when you block movement, it perturbs another part of the walking cycle, so ideally we would like to provide or augment that lost function. Without providing power, we can't fully restore gait."
He notes that while functional electrical stimulation (FES) devices already on the market actively restore some function, they have particular limitations that his team's AFO could improve upon.
"First of all, some pathologies may never be able to benefit from FES," Kogler explains. "Second, FES devices are very successful at providing dorsiflexion assistance, but the power requirements for plantarflexion are three times that for dorsiflexion. FES control systems may not be able to generate in the individual's muscles as much power as the patient needs, and no matter how well the FES system works, patients may need to be rehabilitated some before their muscles can even provide full power." He adds that he fully expects to see FES integrated in some future power-assist AFOs, perhaps as a control input, but the one his team is developing does not utilize it.
Ultra-fine, flexible pneumatic lines allow the AFO’s air-canister power to reach where driveshafts cannot.
Engineering Challenges
To achieve the needed capacity for unprecedented lightweight power, control, and portability, the team has been exploring several engineering methods. The current iteration of the AFO uses compressed gas cylinders such as those used in paintball pistols. In a bid to provide more continuous power, the second-generation AFO will employ an engine approximately the size of a pack of cigarettes that constantly compresses air into an accumulator. Developing the miniaturized engine engenders sticky engineering problems, Kogler concedes, such as heat and noise, and a team of thermal engineers and acoustical engineers are working on these problems. Concurrently, the team is also exploring the feasibility of a hydraulic version, to take advantage of the greater power assistance that hydraulics can provide.
The control system introduces an additional level of complexity. Currently, the AFO is guided by input from a series of embedded sensors. "It uses these to determine the position of the foot in space and to know where and when the device should provide power assistance in either dorsiflexion or plantarflexion," Kogler says. "And we're now adding accelerometers to redouble the inputs, to better detect when the power needs to be initiated or turned off."
Portable Rehabilitation
This approach offers many potential therapeutic benefits. In the grant application for the AFO's current round of funding, the team notes that some 6.2 million Americans who live with conditions such as stroke, polio-related gait problems, multiple sclerosis (MS), cerebral palsy (CP), or acute trauma could benefit from the use of the Portable Power-Assist AFO.
For such patients, the AFO would serve as a rehabilitative, rather than daily-wear device, Kogler believes. "I envision it packaged in a suitcase and being provided to the patient for use on a treadmill fairly soon after a loss of foot and ankle function. The device could be programmed so that it holds the [patient's] foot and ankle in the correct gait positions, rather than having a physical therapist do it for them on the treadmill. The patient would be trained and then sent home with the AFO." He adds that after the patient performed power-assisted treadmill walking at home or in a local gym, session data from the AFO could be uploaded to a computer and sent to the orthotist or physical therapist for progress monitoring.
"They could evaluate how much muscle power was used versus the power provided by the device, and the combination of those two could be adjusted remotely for the next two weeks of training. That would be a significant reduction in cost compared to on-site therapy," Kogler says. Additionally, an orthotist or physical therapist looking to strengthen a particular muscle group might be able to tune a version of the device to provide resistance training as well.
Positive Early Results
Though the AFO is still in prototype form, data published in the Journal of Rehabilitation Research & Development (JRRD) demonstrates the AFO's feasibility on three healthy wearers and one with cauda equina syndrome. (Shorter, K. A., Kogler, G. F., Loth. E., Durfee, W. K., and Hsiao-Wecksler, E.T. A portable powered ankle foot orthosis for rehabilitation. JRRD 48(4)2011.) Kogler says that the team has also found positive results with a number of additional patients with gait pathologies on whom data is still unpublished. Kogler has also worn the device himself.
"When the power is turned on, you don't even notice it," he says. "It's kind of like you have springs on your ankle. It has some weight to it, but any sensation of additional mass is negated by the supplemented power assistance. The experience is quite remarkable."
Morgan Stanfield is a freelance communications specialist and journalist based in Boulder, Colorado. She can be reached at
