IEEE Robotics & Automation Magazine - September 2013 - 21

augment the quadriceps muscles to help
those with impaired strength or control
of one leg. Early analysis showed that the
biggest technological problem would
likely be the actuator, simply because the
forces to help lift a patient are quite substantial, requiring over 80 nm of torque
at the knee joint from an actuator that
should be as light as possible, ideally
weighting less than 1 kg. The high
torque is needed to assist in such everyday activities as sit-stand transfers and
stair ascent, but the actuator must also
allow normal walking while permitting
the leg to swing freely and quickly. These
conflicting requirements for high torque
and high speed could not be met using
the available technology, and we worked
on nine different approaches, including
various electrostatic motors and several
types of continuously variable transmissions, before finding a breakthrough
solution. The final product design uses
a pair of inexpensive brushless dc
motors in a patented system architecture
to effectively build a high-speed,
high-torque, battery-powered, electronically controlled, continuously variable
linear actuator.
However, developing the actuator was
only part of the challenge. Once we had a
working actuator design, other difficult
engineering challenges still needed to be
met, such as coupling the device securely
to the leg, making a single device to work
on the left or right legs of small and large
patients, and developing noninvasive
sensors and control algorithms to determine and respond to the patient's
intended movements (Figure 2).
All during the engineering development phase, we were still not sure how
patients would respond. The first clear
indication of benefits came during our
first tests with brain injury patients in
2008. The patients were at first hesitant to
try the device but were soon excited to
move around under their own control
while being assisted. Then, as now, the
first reactions to using the device are
often quite emotional. Sometimes
patients who have been mostly confined
to a wheelchair for years are able to move
around again for the first time since their
stroke or injury. Tears are shared among
patients, caregivers, and therapists alike

Figure 2. The AlterG Bionic Leg.

when they realize they now have new
hopes for recovery.
After the first successful tests with
patients, the work of commercializing
the system still lay ahead. We developed
the quality systems needed to meet the

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Food and Drug Administration (FDA)
requirements for manufacturing a medical device, ramped up a production
facility, and developed our marketing
and sales strategies to address what we
knew to be a huge potential market. In
the United States alone, there are nearly
800,000 stroke patients every year, and
more than 400,000 of those patients
require postacute care for rehabilitation
of mobility deficits.
As the product matured and we
headed for a commercial launch, we
completed the first clinical studies that
quantitatively demonstrated significant
improvements in poststroke and partial
spinal cord injury patients following traditional therapy augmented by use of the
bionic leg. The improvements shown
were with patients in a chronic state
(more than one year after the injury),
(continued on p. 102)

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september 2013

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IEEE ROBOTICS & AUTOMATION MAGAZINE

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