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Animals exploit soft structures to move effectively in complex natural environments. These capabilities have inspired robotic engineers to incorporate soft technologies into their designs. The goal is to endow robots with new, bioinspired capabilities that permit adaptive, flexible interactions with unpredictable environments. Here, we review emerging soft-bodied robotic systems, and in particular recent developments inspired by soft-bodied animals. Incorporating soft technologies can potentially reduce the mechanical and algorithmic complexity involved in robot design. Incorporating soft technologies will also expedite the evolution of robots that can safely interact with humans and natural environments. Finally, soft robotics technology can be combined with tissue engineering to create hybrid systems for medical applications.
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Soft
robotics:
a
bioinspired
evolution
in
robotics
Sangbae
Kim
1
,
Cecilia
Laschi
2
,
and
Barry
Trimmer
3
1
Massachusetts
Institute
of
Technology,
Cambridge,
MA,
USA
2
The
BioRobotics
Institute,
Scuola
Superiore
Sant’Anna,
Pisa,
Italy
3
Tufts
University,
Medford,
MA,
USA
Animals
exploit
soft
structures
to
move
effectively
in
complex
natural
environments.
These
capabilities
have
inspired
robotic
engineers
to
incorporate
soft
technolo-
gies
into
their
designs.
The
goal
is
to
endow
robots
with
new,
bioinspired
capabilities
that
permit
adaptive,
flexible
interactions
with
unpredictable
environments.
Here,
we
review
emerging
soft-bodied
robotic
systems,
and
in
particular
recent
developments
inspired
by
soft-
bodied
animals.
Incorporating
soft
technologies
can
potentially
reduce
the
mechanical
and
algorithmic
com-
plexity
involved
in
robot
design.
Incorporating
soft
technologies
will
also
expedite
the
evolution
of
robots
that
can
safely
interact
with
humans
and
natural
envir-
onments.
Finally,
soft
robotics
technology
can
be
com-
bined
with
tissue
engineering
to
create
hybrid
systems
for
medical
applications.
Soft
biological
materials
inspire
a
new
wave
of
robotics
Human-made
manufacturing
robots
are
mostly
designed
to
be
stiff
so
that
they
can
perform
fast,
precise,
strong,
and
repetitive
position
control
tasks
in
assembly
lines.
Com-
mon
actuators
in
such
robotic
systems
are
composed
of
rigid
electromagnetic
components
(e.g.,
magnets,
copper,
and
steel
bearings)
or
internal
combustion
engines
made
of
steel
and
aluminum
alloys.
By
contrast,
in
the
animal
world
soft
materials
prevail.
The
vast
majority
of
animals
are
soft
bodied,
and
even
animals
with
stiff
exoskeletons
such
as
insects
have
long-lived
life
stages
wherein
they
are
almost
entirely
soft
(maggots,
grubs,
and
caterpillars).
Even
animals
with
stiff
endoskeletons
are
mainly
com-
posed
of
soft
tissues
and
liquids.
For
example,
the
human
skeleton
typically
contributes
only
11%
of
the
body
mass
of
an
adult
male,
whereas
skeletal
muscle
contributes
an
average
42%
of
body
mass.
In
addition,
parts
of
animal
bodies
that
play
supportive
roles
in
locomotion
(e.g.,
diges-
tion,
gas
and
heat
exchange,
and
motor
control)
are
highly
deformable
as
well.
Studying
how
animals
use
soft
materials
to
move
in
complex,
unpredictable
environments
can
provide
invalu-
able
insights
for
emerging
robotic
applications
in
medi-
cine,
search
and
rescue,
disaster
response,
and
human
assistance.
All
these
situations
require
robots
to
handle
unexpected
interactions
with
unstructured
environments
or
humans.
Soft
robotics
aims
to
equip
robots
for
the
unpredictable
needs
of
such
situations
by
endowing
them
with
capabilities
that
are
based
not
in
control
systems
but
in
the
material
properties
and
morphology
of
their
bodies
(Figure
1)
[1].
Soft
robotics
is
a
growing,
new
field
that
focuses
on
these
mechanical
qualities
and
on
the
integra-
tion
of
materials,
structures,
and
software.
In
the
same
way
that
animal
movements
are
based
on
the
tight
inte-
gration
of
neural
and
mechanical
controls,
soft
robotics
aims
to
achieve
better
and
simpler
mechanisms
by
exploit-
ing
the
‘mechanical
intelligence’
of
soft
materials.
In
this
article
we
introduce
robotic
systems
that
are
fundamentally
soft
and
highly
deformable
[2].
These
robots
are
differentiated
from
other
approaches
in
which
the
machines
are
built
using
hard
materials
and
compliance
is
achieved
using
variable-
stiffness
actuators
and
compli-
ant
control
[3].
We
discuss
the
key
biomechanical
features
of
three
soft
animals
that
are
used
as
inspiration
for
different
soft
robotic
systems
and
suggest
future
directions
where
soft
robotics
can
be
integrated
with
tissue
engineer-
ing
for
medical
applications.
Lessons
from
biology
Soft
materials
are
essential
to
the
mechanical
design
of
animals,
and
their
body
structures
have
coevolved
with
the
central
nervous
system
to
form
a
completely
integrated
neuromechanical
control
system.
These
soft
components
provide
numerous
advantages,
helping
animals
negotiate
and
adapt
to
changing,
complex
environments.
They
con-
form
to
surfaces,
distribute
stress
over
a
larger
volume,
and
increase
contact
time,
thereby
lowering
the
maximum
impact
force.
Soft
materials
also
lend
themselves
to
highly
flexible
and
deformable
structures,
providing
additional
functional
advantages
to
animals,
such
as
enabling
en-
trance
into
small
apertures
for
shelter
or
hunting.
Simple
examples
include
the
soft
paws
of
mammalian
runners
that
damp
the
force
of
impact
when
their
legs
strike
the
ground,
and
the
soft
finger
pads
and
skin
of
arboreal
animals
that
assist
climbing
by
conforming
to
surfaces
for
better
grip
or
adhesion.
Ultimately
it
is
probably
the
ecological
niche
that
deter-
mines
the
evolutionary
tendency
to
be
stiff
or
soft.
Animals
that
do
not
need
to
travel
quickly
or
exert
high-impact
forces
do
not
need
a
permanently
stiff
skeleton
and
can
instead
develop
highly
deformable
bodies
that
allow
them
to
exploit
behaviors
and
environments
unavailable
to
Review
0167-7799/$
see
front
matter
ß
2013
Elsevier
Ltd.
All
rights
reserved.
http://dx.doi.org/10.1016/
j.tibtech.2013.03.002
Corresponding
author:
Kim,
S.
(sangbae@mit.edu)
Keywords:
bio-inspired
robot;
soft
robotics.
Trends
in
Biotechnology,
May
2013,
Vol.
31,
No.
5
287
skeletal
animals.
The
octopus
can
mimic
its
surroundings,
caterpillars
can
conform
to
their
host
plants
to
be
cryptic,
and
all
of
them
can
squeeze
through
gaps
smaller
than
their
unconstrained
body.
These
are
important
lessons
for
building
soft
robots.
For
all
of
their
advantages,
soft
biological
structures
have
some
important
limitations.
Soft
animals
tend
to
be
small
because
it
is
difficult
for
them
to
support
their
own
body
weight
without
a
skeleton.
All
of
the
extremely
large
soft
invertebrates
are
found
either
in
water
(squid
and
jellyfish)
or
underground
(giant
earthworms),
where
their
body
is
supported
by
the
surrounding
medium.
Similar
limitations
would
apply
to
soft
robots
and
necessitate
care-
ful
selection
of
materials
to
match
size
as
well
as
function.
Additionally,
the
high
deformability
and
energy-absorbing
properties
of
soft
tissues
prevent
them
from
exerting
large
inertial
forces
and
limit
how
fast
soft
animals
can
move
from
place
to
place.
This
does
not
prevent
different
parts
of
the
body
from
moving
quickly
under
low
loads.
Octopuses
can
extend
their
limbs
quickly
by
exploiting
the
fixed
volume,
low-aspect
ratio
geometry
of
their
arms
[4],
and
carnivorous
caterpillars
can
strike
their
prey
within
a
few
hundred
milliseconds
[5].
However,
these
considerations
make
it
likely
that
terrestrial
soft
robots
bigger
than
a
mouse
or
rat
will
incorporate
stiff
components
for
better
performance,
taking
advantage
of
high
flexibility.
Soft-bodied
animals
and
soft-bodied
robots
One
problem
with
developing
robots
that
use
soft
materials
is
that
we
currently
have
no
general
theory
of
how
to
control
such
unconstrained
structures.
Robotics
engineers
have
begun
to
develop
this
knowledge
by
building
robot
models
based
on
the
neuromechanical
strategies
that
soft-
bodied
animals
use
to
locomote,
chiefly
annelids
(earth-
worms
and
leeches)[6],
molluscs
(primarily
the
octopus)[7],
and
insect
larvae
(caterpillars)
[8].
Worms
and
worm-like
robots
From
a
biomechanical
perspective,
worms
are
fixed-
volume
hydrostats.
They
mimic
the
mechanical
actions
of
a
lever
by
transforming
force
and
displacement
through
Pascal’s
principle.
Contraction
of
longitudinal
muscles
shortens
the
body
and
increases
its
diameter,
whereas
contraction
of
circumferential
muscles
decreases
the
diam-
eter
and
elongates
the
body
[9,10]
(Figure
2).
Worms
achieve
locomotion
by
creating
traveling
waves
of
contrac-
tion
and
expansion
using
their
cylindrical
segments,
a
process
that
is
analogous
to
intestinal
peristalsis.
The
directions
of
the
locomotion
and
the
traveling
wave
can
be
the
same
or
opposite,
depending
on
the
timing
of
contact
with
the
terrain
[11].
Many
worm-like
robots
have
been
developed
based
on
hydrostatic
structures,
with
a
range
of
hard
and
soft
(A)
(D)
(E) (F)
(B)
(C)
TRENDS in Biotechnology
Figure
1.
Recent
development
of
robots
that
incorporate
soft
materials.
(A)
A
soft
gripper
composed
of
a
flexible
sac
filled
with
granular
materials
that
can
grasp
a
wide
range
of
objects
by
vacuum
pressure
control
[56].
(B)
A
soft
manipulator
modeled
on
the
characteristic
muscle
structure
of
the
octopus
[7].
(C)
The
GoQBot,
capable
of
the
ballistic
rolling
motion
observed
in
caterpillars
[8].
(D)
A
multigait
soft
walker
powered
by
compressed
air
[39].
(E)
The
Meshworm,
which
attains
peristaltic
locomotion
by
contracting
its
body,
made
of
compliance
mesh
[6].
Review Trends
in
Biotechnology
May
2013,
Vol.
31,
No.
5
288
actuators.
One
example
uses
pressure
actuators
with
air
valves,
metal
springs,
and
thermoplastic
bearings
[12],
and
an
annelid
robot
uses
a
stack
of
dielectric
elastomers
mounted
on
a
printed
circuit
board
inside
a
silicone
skin
to
generate
worm-like
movement
[13].
Many
worm-like
robots
have
used
shape–memory
alloy
(SMA)
actuators,
pioneered
in
the
worm-like
crawler
[14]
and
later
in
a
jointed,
segmented
worm
robot
that
mimics
how
nema-
todes
swim
[15].
The
Meshworm
is
the
most
recent
device
to
use
the
SMA
technology
(Figure
1E)
[6].
The
Meshworm
is
based
on
a
constant-length
design
rather
than
the
constant-volume
design
that
worms
use.
Radial
SMA
con-
traction
in
one
segment
causes
radial
expansion
of
an
adjacent
segment,
and
propulsion
is
derived
from
peristal-
tic
waves
of
ground
contacts.
Linear
potentiometers
that
detect
the
length
of
each
segment
provide
feedback.
Using
iterative
learning,
the
duration
of
each
SMA
actuation
is
adjusted
to
maximize
either
the
speed
of
the
Meshworm
or
its
traveling
distance
and
energy
consumption.
Steering
is
achieved
by
replacing
two
of
the
passive
tendons
with
longitudinal
SMA
coils.
Activation
of
one
coil
shortens
one
side
of
the
robot
and
biases
its
movements
in
that
direction.
This
robot
demonstrates
a
key
feature
of
soft
technology:
it
can
be
hit
repeatedly
with
a
hammer
and
still
function
reliably.
Caterpillars
and
caterpillar-like
robots
Although
sometimes
confused
with
worms,
the
larval
stages
of
insects
have
a
completely
different
anatomy
and
locomotion
strategy.
Burrowing
species
such
as
fly
larvae
(maggots)
and
sedentary
Hymenoptera
larvae
(e.g.,
wasps)
generally
lack
limbs,
but
butterfly
and
moth
larvae
are
highly
active
climbing
animals
with
well-
developed
gripping
appendages
called
prolegs.
Although
their
bodies
appear
to
be
segmented,
there
are
no
internal
divisions
between
these
segments,
just
a
single
continuous
body
cavity
called
the
hemocele.
Caterpillar
musculature
is
surprisingly
complex,
with
as
many
as
2000
motor
units
distributed
throughout.
There
are
no
circumferential
mus-
cles,
only
longitudinal
muscles,
oblique
muscles,
and
many
small
muscles
attached
to
the
limbs
and
other
body
parts
(Figure
3A).
Caterpillars
can
adjust
pressure
to
increase
body
stiffness
so
that
they
can
cantilever
their
body
across
a
gap,
but
they
do
not
appear
to
use
pressure
as
a
major
control
variable
for
most
other
movements
[16–18].
Caterpillars
crawl
and
climb
by
exerting
compressive
forces
on
the
substrate
(the
so-called
‘environmental
skel-
eton
hypothesis’)
[19,20]
and
controlling
the
release
of
body
tension.
Waves
of
muscular
contraction
do
not
appear
to
be
tightly
coordinated
[21,22]
but
serve
primarily
to
redistrib-
ute
mechanical
energy
stored
in
elastic
tissues
[23].
The
coordination
of
movement
is
determined
by
controlling
the
timing
and
location
of
substrate
attachment
by
means
of
hooks
at
the
tip
of
the
prolegs
[24,25].
The
hooks
grip
in
a
purely
passive
way,
but
release
is
actively
accomplished
by
a
single
pair
of
retractor
muscles
controlled
by
three
motoneurons
[26,27].
This
is
remarkable
because
a
single
proleg
can
produce
sufficient
grip
to
prevent
any
forward
Longitudinal
muscle fibers
Circular muscle
fibers
Coelom
Longitudinal muscle
Circumferenal muscle
Oligochaeta
(A)
(B)
(C)
TRENDS in Biotechnology
Figure
2.
Earth
worm-inspired
robot.
(A)
Muscular
structure
of
Oligochaeta,
which
forms
antagonistic
pairs
without
skeleton
or
joint.
(B)
A
mesh
structure
that
contains
longitudinal
and
circumferential
artificial
muscles,
creating
an
antagonistic
pairing
similar
to
the
pairing
in
Oligochaeta.
(C)
Demonstration
of
various
actuation
modes.
Review Trends
in
Biotechnology
May
2013,
Vol.
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No.
5
289
locomotion.
Grip
release
must
therefore
be
completely
reliable
regardless
of
the
shape
or
texture
of
the
substrate.
It
is
unlikely
that
the
retractor
muscles
are
controlled
with
great
precision
or
adjusted
with
every
step
to
compensate
for
changes
in
attachment.
It
is
more
likely
that
very
soft
parts
of
the
proleg
are
deformed
to
redirect
automatically
muscle
forces
to
ensure
hook
release
from
the
substrate.
The
system
appears
to
be
an
excellent
example
of
morpho-
logical
computation
and
illustrates
how
important
the
embodiment
process
will
be
in
the
design
of
soft
robots
[1].
These
caterpillar-like
robots
demonstrate
an
important
attribute
of
highly
deformable
devices:
they
can
morph
to
exploit
other
body
shapes.
As
an
example,
the
GoQBot
(Figure
3B)
has
an
elongated
narrow
body
that
can
be
deformed
into
a
circle.
When
done
quickly,
this
change
releases
enough
stored
elastic
energy
to
produce
ballistic
rolling
locomotion
(Figure
3C)
[8].
The
GoQBot
changes
conformation
within
100
ms,
generating
approximately
1
G
acceleration
and
200
rpm,
enough
to
propel
the
10-cm-long
robot
at
a
linear
velocity
of
200
cm/s.
Octopus
and
octopus-like
robots
Some
of
the
most
elaborate
and
intricate
soft-bodied
move-
ments
are
accomplished
by
cephalopods
(e.g.,
octopus
and
squid).
Cephalopods
can
change
their
shape
to
mimic
the
environment
or
other
animals,
and
they
can
deform
their
bodies
to
fill
completely,
for
example,
a
cubic
box.
This
remarkable
physical
fluidity,
together
with
an
ability
to
manipulate
objects,
has
made
the
octopus
an
attractive
model
[28].
Each
octopus
arm
is
packed
with
muscles
organized
into
distinct
anatomical
groups
[29,30].
A
central
block
of
SMA coil actuators
Major muscle group in Manduca
Longitudinal
Local
Oblique
40ms
80ms
180ms
120ms
200ms
(A)
(B)
(C)
TRENDS in Biotechnology
Figure
3.
Caterpillar-inspired
robot.
(A)
The
caterpillar
as
a
model
organism
for
studying
the
control
of
soft-bodied
movements
(tobacco
hornworm
Manduca
sexta
shown
here).
Each
segment
contains
many
longitudinal
and
oblique
muscles.
(B)
A
soft
silicone-elastomer
robot
(GoQbot)
that
mimics
the
body
of
Manduca
with
paired
longitudinal
shape–memory
alloy
(SMA)
coil
actuators.
(C)
Rapid
ballistic
rolling
that
exploits
the
morphability
and
elastic
storage
of
a
soft
body,
achieved
by
coordinated
contraction
of
SMAs.
Review Trends
in
Biotechnology
May
2013,
Vol.
31,
No.
5
290
transverse
muscle
sends
fibers
peripherally
to
interdigi-
tate
with
bundles
of
longitudinal
muscle
fibers.
Both
are
surrounded
by
three
sets
of
oblique
muscle
layers
that
spiral
in
left
and
right
helices
along
the
length
of
the
arm.
The
arm
articulates
the
shape
by
shortening,
elongation,
bending,
or
torsion,
and
forces
can
be
distributed
by
local-
ized
or
global
stiffening
[30].
Muscle
tissues
maintain
a
constant
volume,
which
allows
the
octopus
to
exploit
the
hydrostatic
exchange
of
displacement
and
force.
By
stereo-
typical
movements,
it
has
been
shown
that
octopuses
can
simplify
control
by
reducing
the
degrees
of
freedom.
For
example,
in
a
behavior
called
‘arm
reaching’,
a
wave
of
stiffening
and
straightening
forms
a
propagating
passive
bend
[31–33].
Similarly,
localized
bending
of
the
arm
(pseudojoints)
can
be
seen
in
some
forms
of
fetching
move-
ment
[32,34].
However,
the
extraordinary
intricacy
of
most
octopus
movements
[35,36]
cannot
be
explained
by
such
stereotyped
movements
alone
but
presumably
involves
local
control
by
the
50
million
peripheral
neurons
within
each
octopus
arm
[33,37].
A
variety
of
octopus-inspired
robots
have
been
devel-
oped,
mostly
using
the
broad
concept
of
compartmental-
ized
deformation
to
produce
limbed
locomotion
[38,39].
Some
solutions
for
soft
manipulators,
such
as
the
OctArm
robot,
use
pneumatic
muscles
that
can
bend
in
all
direc-
tions
[2].
The
pneumatic
approach
is
used
in
walking
robots
composed
of
layers
of
silicone
elastomers
contain-
ing
embedded
channels
that
can
be
pressurized
by
fluid
or
air.
Through
careful
design
of
chamber
size,
wall
thick-
ness,
and
geometry,
selective
inflation
and
deflation
of
these
cavities
can
produce
a
variety
of
walking
gaits
[39].
With
the
development
of
pumps,
valves,
and
power
sup-
plies
that
are
compatible
with
highly
deformable
body
structures,
it
will
be
possible
to
construct
extremely
intri-
cate
embedded
pneumatic
networks
capable
of
high-
resolution,
complex
movements.
Another
approach
for
a
completely
soft
manipulator
is
based
more
directly
on
the
anatomy
and
mechanisms
of
octopus
arm
movements
[40,41],
specifically
on
the
imita-
tion
of
the
longitudinal
and
transverse
arrangement
of
soft
actuators,
as
in
muscular
hydrostats
[42,43].
A
plastic
fiber
braid
constitutes
the
highly
deformable
mechanical
struc-
ture
of
this
robot
arm
[7],
whereas
soft
actuators
comprised
of
SMA
springs
[44]
are
arranged
transversely
and
longi-
tudinally
to
produce
the
local
deformations
[45]
shown
in
Figure
4.
Global
bending
is
obtained
with
longitudinal
cables.
The
arm
works
in
water,
exploiting
the
interaction
with
the
environment,
as
observed
in
the
animal
model,
Octopus vulgaris
SMA arficial muscle
(A)
(B)
(C)
(D)
TRENDS in Biotechnology
Figure
4.
Octopus-inspired
robot.
(A)
Octopus
(Octopus
vulgaris)
grasping
a
human
finger
with
one
arm.
(B)
An
octopus-like
robot
arm
wrapping
around
a
human
wrist,
in
water.
(C)
Details
of
an
octopus-like
robot
arm.
The
external
braid
represent
the
mechanical
structure
of
the
arm,
allowing
for
local
and
global
deformations
while
keeping
the
arm
shape
(reproduced
with
permission
from
Massimo
Brega,
The
Lighthouse).
(D)
Details
of
the
SMA
springs
that
generate
local
diameter
reductions
(reproduced
with
permission
from
Massimo
Brega,
The
Lighthouse).
Review Trends
in
Biotechnology
May
2013,
Vol.
31,
No.
5
291
and
can
elongate,
shorten,
bend,
and
stiffen.
A
similar
approach,
but
using
silicone
and
cables,
has
led
to
the
first
soft
robot
with
both
manipulation
and
locomotion
capabil-
ities
[46].
In
this
case,
the
octopus
locomotion
strategy
in
water
has
been
synthesized
and
applied
in
the
design
of
the
robot.
It
consists
of
pushing
with
the
rear
arms,
which
is
achieved
by
exploiting
the
effect
of
water
on
gravity,
the
shortening/elongation
functions
of
the
arm,
the
adhesion
of
the
arms
to
the
substrate,
and
the
ability
to
stiffen
parts
of
the
arms.
The
result
is
a
six-limbed
robot
capable
of
both
locomotion
in
water
and
grasping
objects
by
wrapping
one
limb
around
them.
Soft
technologies
in
robotics
and
challenges
Actuation
One
of
the
biggest
challenges
in
soft
robotics
is
designing
flexible
actuation
systems
capable
of
high
forces,
to
repli-
cate
the
functionality
of
muscles
in
the
animal
body.
The
ability
of
soft
animals
to
change
body
shape
depends
on
a
large
number
of
muscles
being
distributed
over
the
body.
Currently
there
are
three
popular
actuation
techniques.
The
first
technique
is
to
use
dielectric
elastomeric
actua-
tors
(DEAs)
made
of
soft
materials
that
actuate
through
electrostatic
forces
an
important
development
in
the
quest
for
artificial
muscles
[47,48].
Despite
its
relatively
high
performance
metric
(high
strain/stress
and
mass-
specific
power),
this
technique
has
limitations.
(i)
Most
designs
that
use
DEAs
require
a
rigid
frame
that
pre-
strains
the
elastomer.
A
few
designs
work
without
rigid
frames,
but
they
yield
very
low
stress,
and
their
fabrication
process
is
complex
[49].
(ii)
The
reliability
of
the
compliant
electrodes
used
in
these
designs
needs
improvement.
(iii)
The
technique
requires
high
voltages,
which
is
undesirable
for
many
applications.
The
second
technique
is
to
use
SMAs,
which
are
popular
choices
for
soft
actuation
due
to
their
high
mass-specific
force.
Because
the
strain
is
relatively
low
(5%)
in
the
most
common
nickel–titanium
alloys,
engineers
often
create
coils
from
a
thin
wire
to
amplify
the
overall
strain
[8,42,50].
This
allows
SMAs
to
be
formed
into
highly
flexi-
ble
threadlike
springs
that
can
be
integrated
into
a
soft
structure.
However,
force
generation
in
SMAs
depends
on
temperature
change,
so
robust
temperature
control
in
various
thermal
conditions
is
a
challenge.
The
most
input
energy
is
consumed
by
heating
SMA
wire
itself,
therefore,
efficiency
is
very
poor
(1%).
Moreover,
overheating
or
overstraining
can
easily
cause
permanent
damage
to
the
actuator.
The
third
technique
is
to
use
compressed
air
and
pres-
surized
fluids.
This
technique
has
provided
powerful
actu-
ation
systems
for
soft
materials
since
the
1950s.
Contractile
devices
such
as
McKibben
actuators
(made
of
a
fiber
braid)
that
are
deformed
by
pressurized
air
can
produce
relatively
high
forces
and
displacements,
but
they
require
high
power
and
complex
compressed
air
supply
systems.
However,
a
soft
orthotic
device
that
uses
pneumatic
actuators
has
recently
been
developed
using
this
technique
[51].
Alternatively,
compressed
air
and
fluid
can
deform
soft
body
parts
directly
using
networks
of
channels
in
elastomers
to
inflate
chambers
and
create
motion
in
tethered
robots
[39].
Such
a
hydraulic
network
was
used
to
change
the
skin
color
of
a
soft
robot,
mimicking
animal
camouflage
strategies
[52].
Stiffness
modulation
A
critical
technology
for
soft
robotics
is
stiffness
modula-
tion.
Soft
systems
need
stiffness
in
order
to
apply
inten-
tional
forces
to
a
specific
task,
such
as
tissue
sampling.
Soft
robotics
technologies
have
looked
to
animal
models
for
ways
to
vary
body
stiffness
as
needed
for
a
given
task.
For
example,
muscles
transition
from
a
passive
(low
stiff-
ness)
to
an
active
(high
stiffness)
state
[53].
This
property
is
used
not
only
for
actuation
but
also
to
help
distribute
forces
or
to
dissipate
energy
to
maintain
stable
locomotion
[54].
An
interesting
example
of
variable
stiffness
is
a
soft
grip-
per
based
on
particle
jamming
[55,56].
Granular
material
is
loosely
enclosed
in
a
sac
to
create
a
soft
and
flexible
structure
that
can
conform
to
the
shape
of
objects
that
it
is
pressed
against.
After
the
sac
conforms
to
an
object,
pres-
sure
inside
the
sac
is
reduced
with
a
vacuum
pump,
causing
the
granular
filling
to
pack
firmly
to
create
a
stiff
structure
that
can
grasp
the
object
with
relatively
low
applied
force.
A
similar
idea
has
been
implemented
in
a
laminated
tubular
structure
to
create
a
variable-stiffness
tube
for
laparoscopic
applications
[57].
The
pneumatic
network
architecture
used
for
this
structure
modulates
stiffness
by
controlling
the
pressure
of
compressed
air
[39].
Soft
materials
Although
conventional
rigid
robots
articulate
discrete
joints
that
are
designed
to
have
negligible
impedance,
soft
robots
articulate
their
entire
body
structure
as
a
continu-
um.
To
minimize
the
force
required
to
cause
deformation,
the
body
should
be
made
of
low-modulus
materials
(such
as
elastomers).
Silicone
rubber
is
a
popular
choice
for
body
fabrication
due
to
its
availability
in
low
modulus
(as
low
as
05-00
durometer)
that
allows
high
strain
and
the
conve-
nience
of
a
room-temperature
vulcanizing
process.
It
is
also
a
good
biocompatible
material
for
medical
applications.
For
future
alternative
material
choice,
a
recently
developed
tough
and
highly
stretchable
hydrogel
[58]
can
serve
as
a
soft
body
material
that
may
integrate
tissue-engineered
materials
by
providing
scaffolding.
Dissolvable
robots
made
of
soft,
biodegradable
materials
could
be
used
to
deliver
drugs
to
specific
tissues
[59].
New
techniques
are
needed
to
model
and
control
the
environmental
interactions
of
soft-bodied
robots.
Known
robotics
techniques
for
kinematic
and
dynamic
modeling
cannot
be
directly
used
in
soft
robotics
because
the
struc-
ture
is
a
continuum
and
deformation
is
highly
nonlinear
owing
to
large
strain.
Several
constitutive
models
for
large
deformations
of
rubber-like
materials
have
been
developed
[60,61],
but
soft
robots
usually
have
heterogeneous
struc-
tures
with
complex
boundary
conditions,
so
accurate
dy-
namic
modeling
of
such
systems
is
still
challenging.
Most
current
approaches
for
modeling
direct-continuum
materi-
als
in
soft
robotics
are
limited
to
kinematic
analysis
[62,63].
Future
convergence
with
tissue
engineering
Soft
materials
open
up
new
prospects
for
bioengineered
and
biohybrid
devices
[64].
Researchers
have
created
a
Review Trends
in
Biotechnology
May
2013,
Vol.
31,
No.
5
292
flexible
biohybrid
microsystem
that
models
the
alveolus–
capillary
interface
of
the
human
lung
[65].
A
soft
material
allows
the
interface
to
be
rhythmically
stretched,
reprodu-
cing
the
cyclical
mechanical
effects
of
breathing.
By
grow-
ing
cardiac
muscle
cells,
researchers
have
developed
a
tissue-engineered
jellyfish
that
can
swim
[66].
Significant
advances
have
been
made
in
developing
biomaterials
suit-
able
for
minimally
invasive
surgery
(MIS)
soft
robots,
such
as
soft,
transient
electronics
[67]
and
a
tissue
growth
scaffold
made
from
biopolymers
such
as
silk.
A
locomotive
‘bio-robot’
is
fabricated
by
growing
muscle
cells
on
a
3D
printed
hydrogel
structure
[68].
A
soft
robot
could
be
designed
with
biomaterials
that
release
therapeutic
agents
locally
[69]
or
that
deposit
materials
that
the
body
can
use
as
a
scaffold
for
tissue
repair
[70].
Soft
robots
built
from
biological
materials
and
living
cells
would
inherit
the
advantages
of
these
materials:
they
have
extraordinary
potential
for
self
assembly
(from
molecular
structures
to
integrated
devices);
they
are
powered
by
energy-dense,
safe,
hydrocarbons
such
as
lipids
and
sugars;
and
they
are
biocompatible
and
biodegradable,
making
them
a
potentially
green
technology.
The
primary
robotic
components
needed
are:
(i)
actuators
(synthetic
or
living
muscles);
(ii)
a
mobile
body
structure
(built
from
biopoly-
mers
in
any
desired
configuration);
and
(iii)
a
supply
of
biofuel
(e.g.,
mobilizing
glucose
or
lipid
reserves
in
the
body
cells
of
the
robot).
Such
robots
could
be
built
(or
grown)
by
using
parallel
fabrication
methods,
therefore,
they
also
have
great
potential
for
tasks
that
require
disposable
devices
or
swarm-like
interactions.
New
challenges
lie
in
the
selection
of
appropriate
tissue
sources
and
in
interfacing
them
with
synthetic
materials
and
electronics.
Concluding
remarks
Recent
work
on
soft
technologies
embodied
in
robotic
systems
has
been
greatly
inspired
by
the
study
of
soft-
bodied
animals.
The
investigation
of
biological
examples
is
playing
a
vital
role
in
developing
new
robotic
mecha-
nisms,
actuation
techniques,
and
algorithms.
To
construct
robots
that
implement
the
biomechanical
intelligence
of
soft-bodied
animals,
we
need
new
active
soft
materials.
Developing
soft
muscle-like
actuation
technology
is
still
one
of
the
major
challenges
in
the
creation
of
fully
soft-
bodied
robots
that
can
move,
deform
their
body,
and
modulate
body
stiffness.
Soft
technologies
will
greatly
assist
the
development
of
robots
capable
of
substantial
interaction
with
an
environ-
ment
or
human
users
by
providing:
(i)
safer
and
more
robust
interactions
than
are
currently
available
with
conventional
robotics;
(ii)
adaptive
behaviors
that
use
mechanical
intelli-
gence
and
therefore
simplify
the
controllers
needed
for
physical
interaction;
and
(iii)
cheaper
and
simpler
robotic
components.
Soft
robotics
has
particular
utility
for
medical
applications.
Soft
materials
may
enable
robotic
devices
that
are
safe
for
use
in
medical
interventions,
including
diagno-
sis,
drug
therapy,
and
surgery.
For
example,
soft
robotics
may
expedite
the
development
of
MIS
techniques.
A
soft-
bodied
MIS
robot
might
cause
less
tissue
trauma
than
rigid
instruments
during
insertion
and
navigation
through
soft
tissues
and
complex
organ
geometries.
In
the
near
future,
we
will
be
able
to
engineer
biohybrid
soft
robotic
systems
for
medical
interventions
by
combining
biocompatible
soft
materials
and
tissue-engineered
cells.
The
applications
of
soft
robotics
will
drive
the
conver-
gence
of
technologies.
To
create
a
generation
of
soft
robots
in
real-world
situations
requires
seamless
integration
of
various
disparate
fields
such
as
mechanical,
electrical,
bioengineering,
material
science,
and
medicine.
We
envi-
sion
that
such
technological
convergence
eventually
allows
for
prosthetic
limbs
and
organs
that
consist
of
artificial
robotic
components
and
tissue
engineered
materials.
Appendix
A.
Supplementary
data
Supplementary
data
associated
with
this
article
can
be
found,
in
the
online
version,
at
doi:10.1016/j.tibtech.
2013.03.002.
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