IEEE Robotics & Automation Magazine - December 2014 - 25

Structured Textiles for Load Distribution
In addition to the architecture of the exosuit transferring
forces over the body effectively, the suit itself must be comfortable and have high axial stiffness. We accomplish this with
structured textiles made from specially designed patterns and
materials. As a concrete example, we consider the design of
the waist attachment in the multiarticular exosuit in Figure 2,
which is also shown in Figure 3(a) and (b).
Figure 3(a) shows the front view of the waist attachment,
with lines showing the forces within the garment. The exosuit
is designed to distribute forces from a node on the crease of
the hip (shown with a circle) up to both sides of the waist. On
the opposite side of the body, the forces are delivered to the
top of the iliac crest of the pelvis; on the same side, forces follow two paths both above and below the iliac crest for
improved load distribution. For the suit to be comfortable, the
forces must be distributed as evenly as possible over the body
to avoid points of high pressure that may cause discomfort or
restrict blood flow [33], [34].
To achieve this pattern of load distribution, we use the
suit layout shown in Figure 3(b). The waist attachment is
composed of three different textiles, layered and oriented in
different directions. The majority of the fabric is a plain
weave nylon, chosen due to its high dimensional stability (it
holds its shape) and its higher stiffness in extension as compared to other fabrics. This fabric, like all woven fabrics, has
threads in two perpendicular directions, the warp and the
weft. The fabric is strongest and stiffest in these directions
(the principal fabric axes) since along them the threads are
pulled lengthwise. In a direction 45c from either of these
axes, the fabric is less stiff since the weave structure of the

Iliac
Crest

(a)

Seam Lines

Spandex
Inset

Webbing
Reinforcement
(On Reverse
Side)

Principal
Fabric
Axis

Principal
Axes

(b)
200
Force (N)

Figure 2(c) shows how our multiarticular exosuit applies
moments at the hip and ankle simultaneously with the underlying muscles during 30-60% of the gait cycle, which extends
from one heel strike to the next for a given leg. During this
stage of the gait, the calf muscles and tendons push the body
up and forward, and the hip muscles and ligaments swing the
leg forward. Initially, the calf and hip absorb power by stretching as the body's center of mass falls downward and forward
over the planted foot. After around 50% in the gait cycle, this
absorbed power is returned to the body as the tendons and
ligaments elastically recoil. The muscles in the calf and hip
actively contract to supplement this returned power with
additional energy. Our exosuit absorbs and transmits power
in this manner as well: with the actuators held at a fixed
length initially, the exosuit material itself stretches and the tissue under the suit compresses as the body falls forward. This
induces a tension in the suit and absorbs power from the
body. Thus, the multiarticular exosuit architecture has the
unique property in that the exosuit only becomes tense when
the body is in the correct position for forces to be applied.
After the period of power absorption, the suit retracts elastically, returning the energy to the body. This is supplemented
by the actuators contracting, starting at 40% in the gait cycle
to propel the body upward and forward.

150
100

Webbing
Nylon 0°
Nylon 45°

50
0

4
5
Strain (%)

(c)
Figure 3. (a) The front view of the waist belt with arrows indicating
force paths throughout the garment; (b) the view of the reverse
side of the left leg of the waist belt in (a), showing key features
for load distribution; and (c) the results from testing three textiles
showing different strains under load and different hysteresis
(photos courtesy of Harvard Biodesign Laboratory).

fabric must support forces instead of just the thread. The relative strains of the fabric in different directions are shown in
Figure 3(c), which is the result of evaluating the mechanical
properties of swaths of fabric 5-cm wide in an Instron
mechanical testing machine.
To best utilize the fabric to convey forces in the desired
pattern, we use three panels oriented so that the principal fabric axes are parallel to the desired force paths. The strain of
the fabric under load matters greatly since large displacements will reduce the stiffness of the exosuit and require
increased power when actuated [35].
The nylon base layer is further stiffened by seat belt webbing (Seat belt Planet, Inc.) in the main load distribution
paths across the body and around the side of the leg. As
shown in Figure 3(c), this webbing has a much lower strain
than the nylon fabric (0.3 versus 3.2% under a 200-N load)
December 2014

IEEE ROBOTICS & AUTOMATION MAGAZINE

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