IEEE Robotics & Automation Magazine - December 2014 - 23

increases the metabolic cost of accelerating and decelerating
them (8%/kg for mass at the feet versus 1-2%/kg for mass at
the waist) [20]. Due to these effects, wearing such devices
often disrupts the natural biomechanics of walking, leading
to discomfort or increased metabolic expenditure.
For scenarios in which an assistive device would be worn
for extended periods of time, such as endurance augmentation, load carriage, or potential medical applications, avoiding
increased metabolic expenditure is especially important. A
few devices have been able to reduce the metabolic cost of
certain activities, including tethered walking [7], [21], untethered walking with load [22], or stationary activities such as
squatting [23] and hopping [24].
Wearable Robots
Our long-term goal is to create a portable wearable robot that
assists the wearer during walking and can reduce his/her metabolic expenditure compared to regular walking. To work
toward these goals, we have proposed a new paradigm in assistive device design, which we call soft clothing-like exosuits [25],
[26]. These are devices that use textiles to interface with the
body and apply joint torques via tensile forces over the outside
of the body in parallel with the muscles, using the bone structure to support compressive loads. Previous research at Harvard focused on the exciting approach of designing soft wearable robots that could use actuators and sensors that were
sufficiently compliant so as to not restrict movement [27]-
[29]. In addition, work at Chuo University proposed a pneumatically powered orthosis that used low forces to assist hip
flexion and encourage longer steps during walking [30].
Compared with these prior approaches, we are focusing
on systems intended to assist with forward propulsion during walking. A significant challenge with this approach is
ensuring that the exosuits we describe have sufficient bandwidth and force-generating capability to apply biologically
relevant torques to the joints of the wearer during walking.
In comparison with rigid exoskeleton devices, exosuits
have a number of advantages: they can be very light and have
extremely low inertias, which reduces the metabolic cost of
wearing them; they intrinsically transmit moments through
the biological joints since they can only apply tensile forces;
and they are low profile and can be worn underneath regular
clothing so that the wearer can either blend in with normal
society or can take advantage of protective outerwear. Since
they are composed of textiles, they are easy to put on and take
off and can adapt easily to anatomical variations. A key feature of exosuits is that, if the actuated segments are extended,
the suit length can increase so that the entire suit is slack, at
which point wearing an exosuit feels like wearing a pair of
pants and does not restrict the wearer whatsoever. An effective exosuit for gait augmentation meets three requirements:
1) it leaves the user in full control over his/her own gait, 2) it
introduces minor to no kinematic changes to natural gait, and
3) it assists the lower body during walking. Figure 1 shows
two examples of exosuits designed by our lab, including an
early pneumatically powered exosuit and a more recent elec-

(a)

(b)

Figure 1. Two soft exosuits developed by our lab: (a) an early
pneumatically powered design that controls each of the joints in
the leg in both directions in the sagittal plane and (b) the latest
multiarticular design aiding ankle plantarflexion and hip flexion
actuated by geared motors driving Bowden cables (photos
courtesy of Harvard Biodesign Laboratory).

tromechanically driven exosuit. Exosuits do have a few drawbacks, however, including being able to transmit lower maximum forces than rigid-frame devices, not supporting
compressive loads, and presenting challenging requirements
for sensing and actuation. Table 1 provides a summary of the
differences between rigid exoskeletons and exosuits.
Table 1. A comparison of the key features of rigid
exoskeletons versus exosuits.
Feature

Rigid Exoskeletons

Exosuits

Construction of Metal, plastic, etc.
leg components

Textiles

Mode of
operation

Torques, tension, and
compression forces

Tensile forces
only

Joint alignment,
system
adjustability

Alignment and
Alignment and
adjustability are difficult adjustability are
or require complex
easily achieved
mechanisms

Bulkiness,
inertia

Can be bulky and high Very low profile
inertia, requiring energy and low inertia
to move

Bandwidth

Very high due to rigid
frame

Low to medium
due to compliant
suit and human
interface

Maximum
torques

1-10x the nominal
biological torques

0.1-1x the
nominal biological
torques

Effect on gait

Usually alter normal
walking kinematics

Little to no effect
on kinematics

December 2014

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Table of Contents for the Digital Edition of IEEE Robotics & Automation Magazine - December 2014
