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Multifunct. Mater. 1 (2018) 012001

https://doi.org/10.1088/2399-7532/aad378

TOPICAL REVIEWRECEIVED

Functional ﬁbers for robotic fabrics

22 March 2018
REVISED

12 June 2018
ACCEPTED FOR PUBLICATION

13 July 2018
PUBLISHED

28 August 2018

Trevor L Buckner

and Rebecca Kramer-Bottiglio1

School of Engineering & Applied Science, New Haven, CT, 06510, United States of America
1Author to whom any correspondence should be addressed.
E-mail: trevor.buckner@yale.edu and rebecca.kramer@yale.edu
Keywords: soft robotics, wearable, fabric, ﬁber, artiﬁcial muscle, variable stiffness, textile

Abstract
Fabrics have conventionally been passive materials with static properties, leveraging the mechanical,
optical, and thermal properties of networks of ﬁbers. Recently, however, the emergence of functionalﬁbers with dynamic properties has begun to disrupt this conventional deﬁnition. Engineers havebegun to explore new materials and manufacturing processes for actuating, sensing, and variablestiffness ﬁbers, and the integration of these functional ﬁbers into dynamic, robotic fabrics. This review
discusses recent developments in functional ﬁbers and speculates on their utility in future robotic
fabrics.

1. Introduction
A fabric is a thin, ﬂexible assembly of interlacing ﬁbers with adequate mechanical strength to be considered acohesive structure. Fabrics can be manufactured from a variety of materials, both natural and synthetic, toprovide desired properties in terms of durability, weight, comfort, protection, appearance, breathability, andmore. In contrast to other sheet materials such as paper or plastic, a fabric can typically recover its shape after
bending or crumpling, and has sufﬁcient strength and tear resistance for use in many commonplace andtechnical applications [1, 2]. As an inactive yet highly customizable material, fabric ﬁnds wide-ranging use inapplications such as clothing, bedding, and furniture, to more specialized cases such as backpacks, protective
ﬂame-resistant garments, resilient canvas in sails, and weather-resistant tarps and tents. Such qualities and usecases have established fabrics as an effective and adaptable manufacturing material.
Recently, researchers have begun to realize new responsive materials and manufacturing methods that have
led to the development of functional ﬁbers with active stimulus response [3–6]. With these resources in reach,
we see an opportunity for the emergence of a new class of planar soft robots based on fabrics. These robotic fabrics
will extend the beneﬁts of conventional fabrics to include intelligent garments and robust adaptive structuresthat leverage the diverse properties of technical ﬁbers. A robotic fabric, then, is a functionalized network of
ﬂexible, responsive ﬁbers, thereby enabling sensing, actuation, and stiffness control embedded in a single
conformable substrate. By treating fabric as the foundation of a robot, it can be transformed from passive
equipment to active machinery that will positively impact the manufacturability, transportability, andadaptability of complex robotic systems. Robotic fabrics will enable a new method of on-demand machinereconﬁguration through adaptable structure (ﬁgure 1), as well as enhanced compatibility with wearables.
Furthermore, by designing components to be ﬁber-based, they can be integrated into a fabric structure using
well-established fabric manufacturing techniques, such as weaving, knitting, and sewing.

2. Grand challenges and scope of review
The development of robotic fabrics presents many technical challenges, two of which we will highlight in thisreview. The ﬁrst challenge is the need to generate adequate forces in thin-body or ﬁber form. Although someexamples do exist [7], fabrics that exhibit active motion are rare. Traditional robot actuators are not soconstrained, and hence can make use of heavy electric motors or bulky pressurized ﬂuid reservoirs to generate© 2018 IOP Publishing Ltd

Multifunct. Mater. 1 (2018) 012001

T L Buckner and R Kramer-Bottiglio

Figure 1. Simulation of a hypothetical robotic fabric. The robot transforms from a neutral planar state to a legged ‘walking’ form.

large, predictable forces. Such components can generally be scaled in size to ﬁt the required load and are readily
available. In contrast, a robotic fabric, which by its nature is a thin entity, would be restricted to components
which are small and ﬂexible, such as ﬁbers and yarns.
A second challenge is establishing rigidity control to achieve move-and-hold operations or load-bearing
structures in a fabric. While the inherent softness of a robotic fabric may allow for extreme conformational
adaptability, light weight, and resistance to trauma, these same properties become disadvantageous whenattempting to apply forces to other bodies or to directly manipulate the environment in which the robot
operates. In order to overcome this challenge, robotic fabrics require a method of selectively adjusting theirstiffness to create rigid load paths on demand. Variable-stiffness control of this type has become an important
topic in the realm of soft robotics, and a variety of solutions have resulted with varying levels of feasibility. These
include using pressurized balloons [8], granular jamming [9], and changing shape to make use of geometricstiffening [10]. However, not all methods are scalable for integration into a fabric.
To demonstrate the progress that has been made toward conquering these two challenges, we will review
both geometries and materials as applied to robotic fabric components. First, we will present ﬁber geometries
that are commonly used with ﬁber-based actuators to enhance the performance of the bulk actuating material.
This information will be critical in forming a reasonable comparison between the performances of various ﬁber
actuators. Second, we will expand our discussion from individual ﬁbers to assembled fabric units. We will
discuss fabric manufacturing methods and the effect of the resulting fabric structure on performance. Third, we
will present the state-of-the-art in responsive, motion-generating materials that have demonstrated feasibility of
being manufactured into thin actuating ﬁbers for integration into fabrics. Fourth, we will review some candidate
methods of achieving variable structural stiffness in the form of a ﬁber. As a benchmark, it is convenient tocompare actuators and load-bearing structures to human skeletal muscle, due to its well-documented properties
and wide relatability. The performance of human muscle can vary somewhat between specimens, but we will
provide a general range of properties.
In presenting this information, we build upon the vast amount of work that has been done in ‘smart textiles’or ‘e-textiles’ as they are sometimes referred to [11–13]. E-textiles represent a set of technologies which endeavor
to functionalize common fabrics, usually by introducing ﬂexible [14–16] or miniaturized [17, 18] versions ofexisting electrical components. Several comprehensive reviews have been published which summarize various
stationary fabric-based components, including electrically conductive fabric materials [13, 19], energy collectors[2, 20, 21], and several types of sensors [11, 19, 22]. We recognize that many of these precursor technologies will
form the basis of robotic fabrics, however we differentiate a robotic fabric from these works by the stipulationthat it must also provide active motion control, such as actuation and stiffness tuning. As such, this review will
focus speciﬁcally on those technologies that contribute to motion generation in a robotic fabric and have been
demonstrated as viable responsive ﬁbers. Given the wide variety of potential fabric substrates and integration
methods, an evaluation of the performance of each ﬁber within an assembled fabric unit is beyond the scope of
this review. Rather, we intend this work to serve as a reference for future development in this area, both as ashowcase of the state-of-the-art in relevant topics as well as to provide collected data to aid researchers indesigning robotic fabrics that meet speciﬁc design requirements.

2

Multifunct. Mater. 1 (2018) 012001

T L Buckner and R Kramer-Bottiglio

Table 1. Direct comparison of ﬁber geometry effects on actuator performance,
assuming comparable ﬁber diameters (qualitative evaluation scale: ++, +, o,−, − − from the highest to the lowest values). Upward and downward arrows
indicate properties that are more favorable when high (#) or when low ($).

Geometry

Strokelength#Pulling
force #

Stiffness $Actuation
speed #

Monoﬁlament
Twist
Coil
WritheHierarchical

−−
o
++
−
+

++
−
−−
o
+

++
−
−−
+
+

−−
++
o
o
+

3. Fiber morphology
As evident by observing a length of rope or the fabric of modern clothing, a given set of ﬁbers can be joined in avariety of conﬁgurations, both functional and stylistic, to achieve an aggregate unit. At the most fundamental
level, this includes collecting smaller ﬁbers into bundles, twisting into cords, and coiling into loops. Different ﬁberarrangements offer different beneﬁts, from stretchability, strength, and thickness, to decoration and texture.
In the context of ﬁber actuators, single monoﬁlament ﬁbers are often not the most effective way to achieve
motion. Similar to how the difﬁcult task of lifting a heavy weight by a rope can be made more manageable byintroducing a pulley, actuator ﬁlaments can be arranged in favorable conﬁgurations that enhance performance,
perhaps at the expense of some other property. A simple demonstration of this can be performed with a piece ofplain cotton string. By twisting one end and holding the other ﬁxed, the string will become thicker and shorter inlength as it begins to coil upon itself; in exchange, the string now has an internal contracting force which will pullback to its shortened state when stretched apart. Here we will cover some of these geometric conﬁgurations that
prepare inert ﬁbers to generate motion and can be used in combination with active materials to amplify theireffects. A qualitative summary is presented in table 1.
3.1. Twisted bundles
Pulling axially on a single linear ﬁber (ﬁgure 2(a)) will cause it to quickly reach a maximum extension, after
which point any further extension requires breaking apart atomic bonds. As a result of this, individual ﬁbers
formed from bulk material will have a relatively high stiffness in tension, which corresponds to short stroke
length if used as an actuator. One method to combat this is to twist a bundle of many smaller ﬁbrils together intoa single yarn (ﬁgure 2(b)). Now, when pulling on this larger aggregate ﬁber, the individual ﬁbrils have freedom torelease their internal torsion and straighten in the direction of tension to some degree before the tensile limit isreached. This effectively results in an aggregate ﬁber that is lower in stiffness (has more conformational freedom)than the individual ﬁbrils, thus allowing a larger actuating stroke.
Another beneﬁt of utilizing many small ﬁbrils in composite rather than a solid homogenous ﬁber is theincrease in the surface area to volume ratio. Many of the materials to be mentioned in this review rely heavily on
the rate at which an outside stimulus, such as heat, can fully penetrate the structure. In this case, a higher surface
area will generally induce actuation more rapidly and aid in activating the full cross-section of the ﬁber more
completely resulting in a higher actuation force.
3.2. Coils
Coils are one of the simplest conﬁgurations that can improve the performance of an actuating ﬁber. Rather thantwisting multiple small ﬁbrils together, a coil is generated from a single ﬁber by wrapping it around a central axis
in the shape of a spring (ﬁgure 2(c)). When a coiled spring is stretched, it produces radial twist at the crosssection of the ﬁber, rather than direct prismatic stretching. By distributing this twist along the length of the ﬁber,
a spring takes advantage of the sum of many small atomic displacements to provide a large overall macroscalestrain. Thus, a coil can be elongated to many times that of bulk material, at the cost of a moderate reduction in
stiffness. Tighter coils will produce a higher pulling stress due to their increased stiffness, while wide coils willproduce a larger strain [23].
It should be noted that as an actuator, a coil works best against a pre-strain. An unloaded coil will experience
very little contraction when activated and would gain no beneﬁt over bulk material. With pre-strain, however, a3Multifunct. Mater. 1 (2018) 012001

T L Buckner and R Kramer-Bottiglio

Figure 2. (a)–(e) Geometric conﬁgurations used to tune the capabilities of an actuating ﬁber. (a) Typical unmodiﬁed ﬁber. (b) Twisted
bundle of small ﬁbrils. (c) Coil. (e) Writhe. (e) Hierarchical combination of a twisted bundle and writhe. (f)–(i) General methods of
combining multiple ﬁbers into a fabric structure. (f) Fibers woven in parallel will enhance the output actuator stress. (g) Fibers knitted
together allow a greater range of motion, resulting in enhanced stroke length. (h) Braided ﬁbers will enhance actuator stress as well as
fabric strength and stiffness. (i) Fibers sewn directly onto another fabric substrate allows for greater control over load paths. (j)–(q)Illustration depicting fabric component parts and their proper names. (j) Staple ﬁbers. (k) Filament ﬁbers. (l) Illustration of a staple
spun yarn. (m) Illustration of a ﬁlament spun yarn. (n) Monoﬁlament yarn is a single ﬁber acting as a whole yarn. (o) Illustration of a2-ply yarn. (p) Realistic simulations of different knit structures, with a single yarn in orange to depict the path of the yarn. (q) Realistic
weave simulations. (j)–(q) Reproduced with permission [2]. Copyright 2014, The Royal Society of Chemistry.

coil can work to pull the load back into to the initial unstretched length, resulting in a larger effective stroke
length.
As an example, coils are most often seen used with shape memory alloys (SMA) [24–28]. An SMA spring can
far exceed the bulk-material 4% strain limit, reaching strains of 50% or more, at the expense of pulling force,
reduced to about 70% of the original [25]. There are a few rare examples of other materials being made into coils
to increase actuating strain. Strips of liquid crystal ﬁlm cut in a particular orientation can be made into a coiled
actuator that ampliﬁes the natural curling motion of the ﬁlm [29]. Similar work has been done with ionicpolymer-metal composite (IPMC) actuators [30]. Conductive polymer actuators have been made with the ﬁbers
oriented helically in a tube [31].

3.3. Writhes
By ﬁxing one end and turning the other, straight ﬁbers can be overtwisted to the point that coils spontaneously
form along the length (ﬁgure 2(d)). These internally twisted coils are known as ‘writhes’ [32]. While they may
appear similar to a wrapped coil spring, the additional internal angular strain imparts an improved pulling stress.
This can also serve as an alternative geometry to simple coiled structures when the chosen material is too ﬂexible
to maintain the spring shape created by wrapping alone. However, this type of inserted-twist actuator can also
result in reduced stroke length for materials that expand when activated. This expansion causes some ‘untwist’ inthe coil, which might counteract some of the linear contraction of the writhe, resulting in an overall lower stroke
length of the thread [33, 34]. On the other hand, some actuators rely entirely on the strain-augmenting feature of
the writhe conﬁguration to achieve any amount of usable stroke and do so with remarkable success. This
incredibly useful conﬁguration allows creation of actuators from materials as common as nylon ﬁshingline [23, 32].
4

Multifunct. Mater. 1 (2018) 012001

T L Buckner and R Kramer-Bottiglio

Table 2. Direct comparison of whole actuating fabric morphologies, assuming comparable ﬁber diameters(qualitative evaluation scale: ++, +, −, − − from the highest to the lowest values). Upward and downward
arrows indicate properties that are more favorable when high (#) or when low ($).
Geometry

Stroke length #Pulling force #Stiffness $Strength #Customizability #

Weave
Knit
Braid
Sew

−−
++
−
−−

++
−−
+
—

+
−−
++
−

+
−
++
−−

−−
−
+
++

Table 3. Standard yarn sizes and typical uses.
Size

Name

Diameter (mm)

Uses

0
1
2
3
4
5
6
7

Lace
Super ﬁne
Fine
Light
MediumBulky
Super bulky
Jumbo

0.64–0.85
0.85–1.81
1.41–2.12
1.69–2.31
2.12–2.82
3.18–4.23
4.23–5.08
N/A

Lace, threadLace, socks
Baby clothes
Normal clothing
Blankets, sweaters
Scarves, rugs
Thick sweaters
Artisan designs

3.4. Hierarchical twist insertion
By progressively introducing additional levels of twist, coils, and writhes, groups of ﬁbers can achieve a
hierarchical geometrical structure (ﬁgure 2(e)). This results in a compounding effect that can be designed toincrease both the effective strain and stress to even higher levels [35, 36]. Lima et al [37] explored a series of thesehierarchical yarn conﬁgurations to maximize either contraction or torsion. As mentioned earlier, to achieve
optimal contraction, the ﬁber must be constrained such that a minimal amount of overall unwinding occurs
upon ﬁber volume expansion. This arrangement is formed by using a consistent winding direction for coils
along the entire length of the ﬁber, as well as clipping both ends in some manner to prevent them from rotating .

4. Assembled fabric morphology
By leveraging existing fabric manufacturing methods, robotic fabrics can be made inexpensively and rapidly,
easing the transition from foundational technology to mass-production and commercialization. Here, we
present a brief overview of how this process might work (ﬁgures 2(j)–(q)). Responsive ﬁbers are created throughthe appropriate process speciﬁc to that material, be it melt extrusion, wet drawing, coating of an inert ﬁber, or
another method. Fibers may be relatively short, known as staple ﬁbers, or may form long, continuous ﬁbers,
known as ﬁlament ﬁbers [1]. These ﬁbers can then be spun into a thicker thread or yarn; shorter ﬁbers will be
more difﬁcult to spin together and result in a more ‘hairy’ yarn. The resulting yarn may be further spun to takeadvantage of twist insertion, coiling, writhes, or any combination of these depending on the desired properties,
as discussed in section 3. A complete yarn can then be intertwined with other yarns in a particular pattern via
weaving, knitting, braiding, or sewing. The manner in which individual actuating ﬁbers are joined together intoa fabric will have some impact on the performance of the actuators in aggregate. As with the geometry of a single
ﬁber, different overall fabric structures will have distinct advantages and disadvantages. A qualitative summary ispresented in table 2.
The feasibility of a particular pattern will also depend on the diameter of the ﬁber being used (table 3).
Typical yarns are categorized into seven primary size ranges, the thickest of which, labeled ‘super bulky’, isapproximately 4.23–5.08 mm in diameter. Larger ﬁbers (‘jumbo’) are generally considered novelty yarns that arereserved for artisan textiles and may not be compatible with manufacturing machinery. Minimizing the ﬁberdiameter will generally lead to greater ﬂexibility in manufacturing. For instance, twisted nylon thread ﬁbers [32]or carbon nanotube ﬁbers [38] have been created at the ‘lace’ or ‘super ﬁne’ thread size (<1 mm diameter) andcould be threaded through a needle for sewing. Somewhat thicker ﬁbers at the ‘ﬁne’ or ‘light’ size could easily be
woven or knitted, such as low-melting-point-material-core tubes [39]. A ‘bulky’ or larger ﬁber, such as a thickstrand of acrylonitrile butadiene styrene (ABS) [27] or a jamming tube [40] may not feed properly throughavailable machinery and would instead need to be afﬁxed to a fabric base with a sewing machine.
5Multifunct. Mater. 1 (2018) 012001

T L Buckner and R Kramer-Bottiglio

4.1. Weaving
A weave is a pattern of crossing ﬁbers, the warp and the weft, that alternately pass over and under one another,interlaced row by row as the fabric is created (ﬁgure 2(f)). This conﬁguration is easy to produce and has been
automated for centuries. The end effect is multiple aligned ﬁbers cooperating in sum to amplify any forces
linearly with the width of the section of cloth [41]. Additionally, thermally-responsive woven actuators havebeen used to increase actuation cycle speed by dissipating heat over a larger surface area than a single large ﬁberof similar cross-section [23]. The strain of actuation however is limited to that of a single ﬁber, since all ﬁbers will
experience a similar strain as they are pulled in parallel. One other drawback of this conﬁguration is that theactuation is limited to one or two directions unless multiple layers of cloth are stacked upon each other.
More unique patterns can be generated by allowing the weft ﬁbers to skip over a number of warp ﬁbers
before interlacing. Weaving demonstrates this in three main construction techniques; plain weaving, twill
weaving and satin weaving (ﬁgure 2(q)). All other techniques are variations of these main techniques. A plain
weave is formed by having each warp ﬁbre pass alternately under and over each weft ﬁber, and will tend to be ofhigher strength. A twill weave instead has warp ﬁbers weave over and under two or more weft ﬁbers at a time,
resulting in a diagonal pattern and a fabric that is slightly smoother to the touch and more pliable. In a satin
weave, the warp ﬁbers pass over four or more weft ﬁbers before weaving under one weft ﬁber. This gives the
fabric a very tight weave and an even, smooth sheen on one side and high pliability [42].
4.2. Knitting
A knit is a teardrop-shaped loop which extends on either side into legs that attach to other adjacent knit loops[43]. The fabric is built one row at a time by feeding new loops through loops in the previous rows, eventuallyforming a planar chain-like structure (ﬁgure 2(g)). This process can be semi- or fully automated by using aknitting machine, of which there are several varieties. Modern knitting machines typically consist of rows ofhooks or needles which can be manipulated in sequence to produce knits of varying design. Larger needles canbe chosen to handle thicker yarns as needed. Knitted fabrics are loose and highly extensible, thereby drasticallyincreasing the overall stroke length of an actuating fabric. Additionally, a knitted fabric can be made of single
continuous strand. In one application using SMA wire, a simple knit pattern produced a strain of 80% when
activated [44]. By alternating the direction from which each loop is pulled through the previous one, front-toback or back-to-front, the actuation response can be further tailored from a curling motion, to a contractile
motion, to a buckling motion, or any combination of these.
4.3. Braiding
A braid is an interlacing of multiple ﬁbers in which all strands lie approximately parallel with each other(ﬁgure 2(h)). As opposed to weaving, in which ﬁbers cross in perpendicular groups to form a rectangular shape, abraid is generally long and narrow. Braids can be as simple as a ﬂat three-strand ribbon, or become quite
complex, involving many intertwining strands to form elegant three-dimensional patterns. A braided rope will
generally be thicker but stronger than a simply twisted rope of the same material due to the mechanicalinterference of interlaced strands. Braiding machines are commonly used to produce ﬁshing lines, rope, andpower cable shielding. As such, they can be made to handle a variety of materials including both natural andsynthetic yarns, and even metal wire. Modern braiding machines can be designed to produce varying braidstyles, and some can be programmed to generate more complex and non-traditional shapes.
The nearly aligned nature of a braided structure beneﬁts from the combined pulling force of multiple ﬁbers
in parallel. Without a perpendicular crossweave inhibiting the motion, braids are more ﬂexible than weaves andcan produce considerable force in the ﬁber direction. In thermally activated materials, braided actuators havebeen shown to have improved heat transfer, and therefore actuate more quickly while consuming less power,
than weaves with the same number of parallel strands. By changing the number of ﬁbers and the order ofinterlace, a braid can be tailored to a particular force and strain requirement as needed [45].
4.4. Sewing
Rather than serving as the base of the fabric structure itself, an active ﬁber can instead be sewn onto anotherfabric substrate. Sewing allows a robotic fabric to be manufactured with very speciﬁc actuation paths that would
otherwise be impossible using weaving or knitting. A thick ﬁber can simply be placed onto the fabric substrate as
desired, and a thinner inert needle thread can be used to sew it in place (ﬁgure 2(i)). This process, known ascouching, has been used to afﬁx thick actuating ﬁbers by hand onto a commercial cloth [28], and has also been
demonstrated with variable-stiffness ﬁbers by ﬁrst softening and bending them into a desired shape before
sewing [27, 46]. However, sewing by hand is a time-consuming and inaccurate process, and is not scalable,
particularly if a robotic fabric requires many ﬁbers working together to generate sufﬁcient force.
6

Material

7

a

Coil
Writhe
Twist
Twist+Twist
Twist+Writhe
Twist
WritheWritheTwist+Writhe
Twist+Knit

Actuation stress[kPa]#

Actuation
Strain#Activation time[s]$

Relaxation time[s]$

Work density[J/kg]#

Energyefﬁciencya#

Cycle lifea#

Sources

100–500
280
280
34 000
92

30%
35%
35%
2%
0.4%

0.2–0.3
>60
1
25
250

0.2–0.3
>60
2
—
160

9.434
—
—
*
95 240
—

40%
<10%
—
<18%
—

>109
—
—
500 000—[3, 50][51, 52][51, 52][3][41]

200 000
46 800*46 800
10 000
19 0001600
14 500—
19 000
—
—
2160
—
1904
16 000
2000
720

4%
50%
75%
2%
—
14%
10%
4%
34%
0.3%
16%
8.9%
14%
45%
25%
600%
7%

2
2
2
0.4
0.5
0.2
0.05
—
2
—
—
—
—
—
1s
10
<1

10
10
10
0.4
0.2
—
2.5
—
15
—
—
—
—
—
1s
N/A
<1

<16%
—
—
< 10%
—
<5.4%
—
<1%
<1%
<1%
<1%
—
—
—
3.2%
—
<90%

300 to 107
—
—
<5000
—
<3000
—
1.2×106
1.2×106—————>10 000—
106[3, 25, 53][25, 53][25, 53]
[54]
[55]
[56]
[37]
[23]
[23]
[23]
[23]
[57]
[58]
[58]
[59]
[60]
[61]

Values for Energy Efﬁciency and Cycle Life are gathered from the work by Mirvakili and Hunter [6].

*

155
1226
*
1839
1100
*
4.85
2200
836
—
2480
—
2630
—
—
1523
650
—
*
2800

T L Buckner and R Kramer-Bottiglio

Human skeletal muscle
Liquid crystal elastomers
Liquid crystal elastomers+CNT
Conductive polymer (polypyrrole)
Conductive polymer (polypyrrolecoated yarn)
Shape-memory alloy (NiTi)
Shape memory alloy (NiTi)
Shape memory alloy (NiTi)
Electromechanical CNT
Electromechanical CNT
Electromechanical CNT
Parafﬁn-inﬁltrated CNT
Nylon
Nylon
Polyethylene
Polyethylene
PET
SpandexSpandexSpandex+CNT
Shape-memory polymer (cold-drawn)
Dielectric elastomer

Fiber geometry

Multifunct. Mater. 1 (2018) 012001

Table 4. Values for actuator ﬁber performance as found in the literature. Some values were not reported for every material, and some have been derived by the authors from other reported values. CNT=Carbon Nanotube,
Twist=Twisted-bundle conﬁguration, Coil=Untwisted coil conﬁguration, Writhe=Writhe conﬁguration, Knit=Knit of several ﬁbers, X+X=Hierarchical combination of two conﬁgurations, *=Not explicitly reported in theliterature but calculated or extrapolated by the authors from ﬁgures or other reported information. Upward and downward arrows indicate properties that are more favorable when high (#) or when low ($).

Multifunct. Mater. 1 (2018) 012001

T L Buckner and R Kramer-Bottiglio

It is also possible to afﬁx a ﬁber by sewing machine. The thick ﬁber can be run through the bobbin [47],which will minimize the bending required and allow for ‘bulky’ ﬁbers (4 mm) or irregularly shaped ﬁbers to be
used [48]. This method can easily generate simple straight lines. Alternatively, the ﬁber can be fed into themachine and used as the needle thread. This does, however, place some limitations on the ﬁber properties. To
feed properly through a normal sewing machine, the ﬁber must have a consistent diameter, resist elongation, be
ﬂexible enough to form small loops, and be torque-free to avoid tangles or kinks [49]. This naturally means thatmany ﬁbers, particularly those which are easily stretched or are overtwisted may not be suitable.

5. Actuating ﬁbers
A wide variety of materials exist that will respond to a stimulus by changing in shape or size, which can beharnessed to generate motion in a fabric. These materials often have tradeoffs in performance, excelling incertain aspects while another material might perform better in a different area. As such, some key metrics areimportant when comparing the effectiveness of actuating ﬁbers and will be brieﬂy summarized here. It should be
noted that some other important characteristics, including stress relaxation, creep, and density, are notconsistently characterized or reported throughout the literature to derive a meaningful comparison, and so will
not be included here for compactness of presentation. A summary of these materials is collected in table 4.
Actuation Strain. The majority of actuating ﬁbers operate under a principle of contraction to generate an
actuation stroke. In this review, all stroke lengths will be reported in terms of tensile strain as a percentage of theoriginal ﬁber length.
Actuation Stress. As a ﬁber contracts, the force of actuation generally is not constant and changes over theduration of the stroke. However, the maximum stress can be assumed to be the smaller of the ultimate tensile
strength or the product of the maximum strain and the stiffness. In this review, all actuating forces have beennormalized to an actuation stress based on the outer diameter of the actuator to accommodate any unusual ﬁber
surface geometries due to coiling or twisting of ﬁlaments. This reported value will be the blocking stress if
available, or the highest reported stress otherwise.
Actuation Energy Density. As an addendum to the actuation stress and strain, this value reports the total
amount of work performed per actuator unit mass or volume by a typical actuation stroke. This information
serves to reveal some less-effective materials that may, for example, pull exceptionally heavy loads but only
negligible distances.
Actuation and Relaxation Time. An important factor in actuation is the amount of time not only to actuate,
but also to return to the unactuated state. For instance, an actuator may demonstrate a reasonably rapid
contraction time, but reliance on thermal sensitivity in the material could result in a long cooling time, often on
the order of tens of minutes, rendering the initial burst of speed much less useful. In this review, the relaxation
time will be reported for room temperature in air, if available.
Energy Efﬁciency. In a robotic fabric system, any energy expenditure will require some power source. An onboard battery will have a limited supply, making low-energy systems very attractive. This value will be reported
as a ratio of output work to input energy.
Cycle Life. Some materials require intense energy to activate the actuation mechanism, and therefore degrade
quickly. As such, we have listed the largest reported number of cycles for each material.
A robotic fabric relies upon very small-scale actuators. In the case of actuating ﬁbers, this size restriction
greatly limits the potential force generation, as output force scales positively with actuator cross-section.
However, power output, or speed of actuation, increases at smaller dimensions (faster mass/heat transfer, or
higher surface area/volume ratios). Below, we list a series of materials which are able to provide motion by
exploiting various physical phenomena.
5.1. Human muscle (benchmark)Human muscle experiences a contracting strain of approximately 25%–30%, with a maximum actuating stressof 350 kPa, although sustained force is about 30% of the peak value [50]. This results in an energy density of
about 9.43 J kg−1, or 10 kJ m−3. Human muscle is also extremely fast-acting, allowing both contraction andrelaxation times of approximately 0.2–0.3 ms. It is also very robust, allowing on the order of 109 cycles before
failure due to the live tissue continuously regenerating itself [50, 52, 62, 63]. In general, most artiﬁcial actuatingﬁbers are able to greatly surpass human muscle in terms of pulling stress; the limiting factors are stroke lengthand speed of actuation.
5.2. Aligned amorphous polymers
Some polymer products, such as ﬁshing line, are known for their high strength-to-weight ratio. This is partially
due to the polymer chains being highly aligned in the direction of stress. Some polymers of this type have regions
8

Multifunct. Mater. 1 (2018) 012001

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Figure 3. (a)–(f) Aligned amorphous polymer threads coiled into actuators and placed into fabrics. (a) A spool of coiled muscle(235 μm outer diameter) made from 125 μm-diameter nylon 6/6 monoﬁlament sewing thread by a continuous process, which has
been prestretched to provide space between coils for actuation, and the same ﬁber with an insulated copper wire wrap forelectrothermal actuation, without prestretch, pictured on the Left and Right, respectively. (b), (c) Closeup photographs of the ﬁbers in
A. (d) Woven fabric made from coiled, 225 μm-diameter nylon sewing thread muscle. (e) Stitches made by sewing the coiled ﬁber intoa polymer sheet using a conventional sewing machine. (f) Machine-knitted fabric made from a coiled 225 μm-diameter nylon sewing
thread muscle. (g) Liquid crystal elastomer ﬁber. (h) Fiber contraction due to collapse of mesogen alignment. Fiber warmed with aheating coil to lift a load. (i)–(k) Shape memory recovery of SMP composite loosely woven fabric with SMP yarns at 50 °C withrecovery times (i) 0 s, (j) 30 s and (k) 60 s. (l), (m) SMA in a knit pattern. (l) The ﬂat elongated prototype in the cold martensite stateand (m) the contracted prototype in the hot austenite state. (n)–(p) Cellulose-based threads coated with CP polypyrrole for actuation.
(n) A bobbin with industrially manufactured CP-coated yarn. (o) A knitted actuator unit. (p) A knitwear structure comprisingconductive polymer-coated yarns (black yarn) knitted together with normal (white) yarn. (q), (r) Dielectric elastomer ﬁber created by
dip-coating a string. (q) A typical specimen. (r) Cross-section of an actuator with one layer. (s-w) Carbon nanotube ﬁbers joined
together by a twist and writhe hierarchical conﬁguration. (s) 5 and (t) 20-ply twisted actuators. (u)–(w) Enlarged images of (t). (x), (y)Electromechanical bending motion generated by sewing carbon nanotube ﬁbers into Kapton ribbon upon applying a pulsed current.
(a)–(f) Reproduced with permission [32]. Copyright 2016, National Academy of Sciences. (g), (h) Reproduced with permission [51].
Copyright 2003, American Chemical Society. (i)–(k) Reproduced with permission [65]. Copyright 2007, Sage Publications. (l), (m)Reproduced with permission [44]. Copyright 2013, IOP Publishing. (n)–(p) Reproduced with permission [41]. Copyright 2017 The
Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. Distributed under a Creative
Commons Attribution NonCommercial License 4.0 (CC BY-NC). (q-r) Reproduced with permission [61]. Copyright 2010, Springer
Nature. (s-y) Reproduced with permission [46]. Copyright 2015, Wiley.

of amorphous structure, allowing individual chains some freedom of movement. When exposed to heat, these‘loose’ chains are able to slide past each other into a more entropically favorable conformation, generally
resulting in an overall reduction in length in the direction of polymer alignment, with a slight expansion in theperpendicular direction correlated to Poisson’s ratio [64]. Several materials exhibit this behavior to differing
degrees. In general, high-modulus materials will provide high pulling forces yet experience very little
contraction. Drawn polyethelyne, for example, experiences a very small thermal contraction of only 0.3%, yet
has been successfully fabricated into an actuating ﬁber capable of extremely high pulling stresses, up to 190 MPa

9Multifunct. Mater. 1 (2018) 012001

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when made into a writhe [23]. Twisted polyethylene terephthalate (PET) yarns on the other hand could reach acontraction of 8.9%, but with a pulling stress of only ∼2.16 MPa [57].
Nylon is becoming popular as another such material that, in addition to being very inexpensive and readily
available as commercial ﬁbers in the form of ﬁshing line or nylon sewing thread, experiences contraction on parwith SMA, as high as 4% [23]. By taking advantage of writhe geometry, Haines et al developed intertwined ﬁber
actuators with an impressive 34% contraction and 19 MPa pulling stress (based on the coil diameter) with anactuation cycle time of 17 s in air [23]. Increasing the diameter of the coils led to a stroke length of up to 49% at
the expense of pulling force (ﬁgures 3(a)–(f)).
One new promising ﬁber actuator uses Spandex as its core material. Spandex ﬁbers experience a contractionof about 14% at approximately 130 °C, compared to the 4% contraction of nylon 6,6 at 240 °C [58]. With thefurther beneﬁt of a twisted-bundle/writhe conﬁguration, the Spandex actuator was able to reach a maximum of
45% contraction, with 0.758 MPa pulling stress (extrapolated from the data). However, cycle times are upwardsof 160 s in air due to long cooling times [58]. An alternative approach involves spinning carbon nanotube thread
and Spandex together, and then knitting the resulting twisted ﬁber into a thin mesh tube only 1.7 mm indiameter, rather than using a monolithic coil. The resulting long knitted tube can be actuated by Joule heating attemperatures as low as 70 °C, and requires only 7.5 s to complete an actuation cycle. This mesh has a blocking
force of about 16 MPa, and can contract up to 25% [59].
One difﬁculty in working with amorphous polymer actuators is the need for precise temperature control.
Thermal contraction for nylon 6,6 requires rather high temperatures around 240 °C [23], yet melting occurs at
only 265 °C, putting the ﬁber in danger of rapid degradation. Some more stable methods of temperature control
via Joule heating have used a conductive wrapping of carbon nanotube sheets [66], and coating with conductive
silver paint [34]. Electroless metal plating of aluminum [67], copper [68], silver [69], or gold [70] could also be
options to enable thermal activation by Joule heating.
5.3. Liquid crystal elastomers
Liquid Crystal (LC) materials are so-called because, in addition to a solid and liquid state, they have a uniqueintermediary phase with behavior somewhere in between. This secondary phase is enabled by the interactions oflarge, rod- or disk-like monomers, known as mesogens [71], attached to the polymer chains. When cooled to thesolid state, these rods are aligned in a regular crystal. With the gradual addition of heat, the material enters thesecondary nematic phase, wherein the mesogens become somewhat dislodged from their rigid crystal structure,
but are still generally oriented in the same direction. Upon further heating, the mesogens are more and more free
to completely reorient themselves, and the material begins to approach something more akin to an isotropic
liquid phase. A strand of Liquid Crystal polymer at this stage will experience a contractile force along its length as
the unaligned mesogens are able to collapse upon themselves into a tighter conformation. This contractile
response can be excited by direct application of heat, or indirectly due to exposure to light [52, 72], electricalﬁelds, pH changes, or other stimuli [73].
Liquid Crystal polymers are classiﬁed into three major network topologies. Liquid Crystal Polymers (LCP)
contain the mesogens within the polymer chain backbone itself, and have found use as high-performance, highmodulus polymers, such as Kevlar or Vectra, and undergo very little contraction when stimulated. Liquid
Crystal Networks (LCN) are highly crosslinked polymers that retain some of the high-performing properties of
LCPs, but also experience a moderate amount of thermal expansion when stimulated. Due to the anisotropy of
the aligned mesogens in the network, this expansion can be directed along a particular axis or curve, although thedegree of crosslinking limits this motion. Liquid Crystal Elastomers (LCE) are a subset of LCNs, where thedegree of crosslinking is minimal, allowing the polymer backbone to become more ﬂexible and the bulk material
to behave like an elastomer. LCEs exhibit the greatest degree of contraction due to the large changes in mesogenalignment order that are possible. Upon actuation, The loose polymer chain structure will primarily collapse
along the single axis of initial mesogen alignment, leading to much larger strains than a typical polymer would
experience [73].
Of the Liquid Crystal topologies, only LCEs show real promise as ﬁber actuators, since they exhibit thegreatest amount of strain. LCE material can also easily be formed into a ﬁber. After combining the necessarypolymer components and waiting for the mixture to begin crosslinking, a pair of tweezers are dipped into the
mixture and pulled away as quickly as possible, generating a long strand. These strands can then be set aside toﬁnish the curing process [51], after which they can be integrated into a fabric.
A highly deformable LCE ﬁber has been developed that, when warmed inside of a heating coil, experiences
up to 35% strain and 280 kPa of blocking stress [51] (ﬁgures 3(g), (h)). The limiting factor for actuation time is
efﬁcient dispersion of heat into the ﬁber from the external source. Doping with conductive particles such asgraphite [52] or carbon nanotubes [51] has been shown to decrease this response time from 60 s in some cases toless than 1 or 2 s.
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5.4. Shape memory polymers
A shape memory polymer (SMP) contains a high degree of chemical crosslinking. When heated, the polymer
reaches a viscoelastic state where it can be easily deformed and if held in place, the polymer will maintain the new
conﬁguration upon cooling. The crosslinks then act like stretched springs, ready to pull the polymer back into its
original shape once it is heated again and free to move. The recovery stress from this return to the original shape
can be employed as an actuation mechanism.
As with other polymers, SMPs can easily be manufactured into long fabric-ready ﬁbers via wet-spinning [74]or extrusion [65], and have already been utilized as a woven fabric component, often as self-unwrinklingmechanism in clothing [75] (ﬁgures 3(i)–(k)). SMP ﬁbers have also been woven into fabrics for use as ‘smart
curtains’ that open or close depending on incident heat from the Sun [65]. In some early experiments,
polyurethane block copolymers were spun into ﬁbers and found to exhibit a strong shape memory effects. A
ﬁber stretched to 65% elongation strain was able to recover all but 10% strain with a maximum recovery stress of
11 MPa, although recovery time was not provided [74].
Recently, very high-strain SMPs have been developed. Fibers of this material can be stretched to 600% of theinitial length and, with enough time, fully recover this length. Furthermore, this recovery can be accomplishedusing relatively low contact heat from the human body. A fully stretched ﬁber can pull with a stress of more than
2 MPa, although this stress tapers off approximately linearly toward zero as the ﬁber returns to its original
length [60].
Overall, SMPs seem promising for the amount of strain and recovery stress they deliver. One downside,
however, is that the shape memory effect is limited to one stroke until an external force is applied to deform the
SMP again; an SMP will not naturally spring back into an ‘unactuated’ state. This necessitates the use of someother antagonistic actuator which must ﬁght the contraction force to ‘reload’ the SMP ﬁber for the next stroke orcold-draw the ﬁber while it is cooled [60].

5.5. Shape memory alloys
Shape memory alloys (SMA) are metals which experience a change of atomic lattice structure when heated abovesome transition temperature. In the low-temperature martensite state, the metal is arranged in a face-centered
tetragonal lattice. When heated to the austenite state, the lattice rearranges into the slightly more dense bodycentered cubic lattice [3]. One of the most popular SMAs, nickel-titanium, also known as Nitinol, experiences a
4% reduction in length during this transition, which can be used as an actuating mechanism [25](ﬁgures 3(l), (m)).
Beyond this base change of length, however, SMA actuators can be made much more effective by coiling
SMA wire into a spring. This makes use of the property that an SMA wire in the hot austenitic state has a spring
constant that is 2-3 times higher than that of the same spring in the martensitic state [25]. To avoid the coil
returning to its original straight wire conﬁguration, the SMA can be annealed at a high temperature, setting a
new ‘default’ shape for the martensite phase.
In a typical actuation cycle, the NiTi coil begins in the cool martensitic state and is stretched with an initialload. The coil is then heated until the austenite transition activates, at which point the spring constant increases,
and the actuator retracts until the load and higher internal spring force are balanced. Due to their high electricalresistance, SMA actuators can be heated to the austenite transition state by simple application of electricalcurrent which causes Joule heating. This affords a very easily controlled system since no external heat source isrequired.
As a straight wire, SMA can provide considerable blocking stress, up to 200 MPa, although with thelimitation of only 4% strain. However, when formed into a coil, SMA wire can provide considerable strain in
exchange for some pulling force; careful control of the annealing temperature can result in SMA springs with
50% contraction while still pulling with an impressive 46.8 MPa stress [3]. What’s more, a coil can be furthertransformed by pulling it ‘inside-out’ into a writhe-like formation. The added internal torsional strain in thewire results in not only an increased stroke length and actuation stress, but also helps the actuator pull its load thefull stroke length [25].
SMA unfortunately suffers from at least two problems. The ﬁrst, as typical of any thermally responsive
actuator, is the long cooling times. Submersion in water has proven effective, allowing millisecond actuation
cycles, however this is not always practical. Other methods have involved lagging the wire with thermallyconductive paste with some success, reducing cooling times from 10 s to less than 2 s [53]. The second issue,
perhaps more severe, is the rather quick degradation of the actuators. After only hundreds or even tens ofactuation cycles, the austenitic ‘activated’ position can begin to deteriorate, resulting in shorter and shorter
strains, until the SMA is re-annealed [3].
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5.6. Conductive polymers
Conductive polymers (CP) are an unusual group of materials that have a molecular chain structure andmechanical properties typical of polymers, yet possess surprisingly high electrical conductivity, on par withmetals (copper). A conductive polymer is designed such that the backbone consists of alternating sigma and pi
bonds, which leaves open orbital positions for electrons to ﬂow from atom to atom along the length of the chain.
This conductivity in a polymer is interesting in itself, but it can also be leveraged to produce motion. Given a
supply of ions, such as an electrolyte bath [63], a voltage differential can be applied to a strand of CP, which
creates either a lack or abundance of electrons at the orbital sites. Ions ﬂow in or out of the polymer matrix tobalance the charge, creating a net volume change, and thus motion. There may also be some amount of motion
due to absorption of solvent, as well as rearrangement of the polymer chains to accommodate the ﬂux of ions [3].
One major drawback of this system is the need for an ion reservoir. While most demonstrations have takenplace in the presence of a liquid electrolyte solution, there have been some more recent attempts to incorporate alayer of electrolyte into a multilayer system, such that the device can perform in open air. These multilayer CP
actuators tend to provide only bending motion as one outer layer swells with ions and the opposite side loses
them, but some limited progress has been made toward the development of linear, open-air CP actuators[76, 77]. This reliance on ion exchange also means that the actuation times are highly dependent on the surfacearea-to-volume ratio of the actuator; a thick ﬁber can take several minutes to fully actuate, but a ﬁlm or bundle ofthinner ﬁbers may require only a few seconds [78]. CP actuators have been successfully formed into woven andknitted fabrics by taking an existing cloth made from cellulose-based yarns and coating it with an active CP layer.
This method allowed the designers to forgo the process of weaving or knitting the CP directly as a ﬁber [41](ﬁgures 3(n)–(p)).
CP ﬁber actuators have consistently demonstrated high stress outputs, typically on the order of 5 MPa, andeven as high as 35 MPa [3]. The voltages required for actuation are also typically very low, only requiring 1–2 V,
although higher voltages will speed actuation times [3, 79]. As a CP actuator approaches equilibrium with its
load, it has also been reported the current ﬂow also tapers off toward zero. Thus a catch state is available, withessentially no energy being expended in order to maintain a constant force [3]. Despite these advantages, CP
actuators suffer from very small strains, typically on the order of 1%–2% [78]. Some research has reported
strains has high as 30% [80], however this capability quickly degrades, losing half its actuation length after only
10 cycles.

5.7. Dielectric elastomers
When a thin, dielectric elastomer ﬁlm is coated on both sides with a stretchable electrode material such as carbon
grease, it can be made to act as a parallel plate capacitor. By applying a high voltage across the two electrodes, thetwo sides are drawn together, compressing the ﬁlm in one direction and expanding in the other two, providing
an actuation mechanism [81]. Actuation cycles can be extremely rapid for such ﬁlms since the motion is only
dependent on the presence or absence of an electric ﬁeld. Dielectric elastomers can be rolled up or manufactured
directly into a tube to achieve a ﬁber-like structure [82, 83].
As an extending rather than contracting actuator, dielectric elastomers are often used antagonistically withother mechanisms [84]. While dielectric elastomers in general have demonstrated extremely high extending
stroke length (215%), and even respectable actuation pressure (2 MPa) [81], they require extremely highvoltages, on the order of kilovolts, which can be difﬁcult to control as well as dangerous. Dielectric actuators also
run the risk of self-destruction; if the spacing between electrodes becomes small enough, the dielectric ﬁller canevaporate at that point, causing the opposing electrodes to be shorted together [5].
A few attempts have been made at incorporating dielectric elastomers into a thin ﬁber-like form factor. One
method simply uses a hollow, 0.9 mm-diameter, elastomer tube ﬁlled with conductive silver grease, and an outercoating of carbon-black-embedded elastomer, forming inner and outer electrodes. Silicone tubes reachedextensions of 7% and blocking stress of 0.24 MPa, while polyurethane tubes reached strains of only 0.6%, butblocking stresses of 0.82 MPa [85]. Another approach involves repeatedly dip-coating the ﬁber in alternating
conducting and non-conducting blends of poly-styrene-ethylene-butylene-styrene (SEBS) and carbon black,
resulting in a multilayer, coaxial ﬁber 0.4 mm in diameter. This approach had similar results as the previously
mentioned case, about 7% elongation and reported actuating stress of 0.72 MPa. It was noted that by using manythin layers rather than one large layer, the activation voltage could be lowered, and it is surmised that with layersof 30–40 um, actuators requiring fewer than 1000 volts could be made [61] (ﬁgures 3(q), (r)). One group has
made use of twist insertion to create a dielectric elastomer actuator that actually generates a contracting stroke,
although with electric ﬁelds upwards of 10.3 million volts per meter (MV/m) of dielectric thickness, and, with anobserved maximum contraction of only 4.3% [86].
12Multifunct. Mater. 1 (2018) 012001

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5.8. Carbon nanotube ﬁbers
Carbon nanotubes have been the focus of a ﬂurry of innovation in recent years. They exhibit extraordinary
tensile strength, as well as high electrical conductivity [55]. Carbon nanotubes can be deposited into othermaterials to create a composite with enhanced properties, such as increased stiffness, resistance to fracture, andconductivity.
Interestingly, recent developments have provided a method of spinning forests of grown carbon nanotubes
into pure carbon nanotube aggregate ﬁbers, similar to how cotton strands are spun into twisted threads. This
breakthrough allows the handling of these microscopic particles on a macroscale as a visible yarn. These carbon
nanotube yarns are extremely strong in tension and retain electrical conductivity on par with metals, yet can be
woven and threaded like any other soft, ﬂexible fabric material [54].
Multiple strategies have been developed to exploit these new carbon nanotube ﬁbers as actuators, the ﬁrst ofwhich utilizes a guest material inﬁltrated into the gaps between spun threads. One promising example of thisuses parafﬁn wax, which is chosen for its widely customizable state transition temperature, ability to easily wet
the nanotube ﬁbers, and large volume changes at it heated. By Joule heating the ﬁber with application ofelectrical current, the inﬁltrated ﬁber quickly warms and expands, inducing linear contraction and untwisting.
Wax-inﬁltrated actuators of this type demonstrate strains between 5.1% and 10%, with 16.4 MPa stress and
5.5 MPa stress, respectively. Experiments with microscale threads show very fast heating and cooling cycles, forinstance a 20 μm-diameter thread can heat in 15 ms and cool in 25 ms, although any increase in diameter to raisepulling force will increase those times. A 150 μm-diameter thread increased the cooling time to 2.5 s, whileheating time remained at only 50 ms [37].
Another strategy uses bare carbon nanotube thread without any guest material. In this case, the actuation isdriven by electromagnetic force between individual carbon nanotube strands rather than thermal expansion of
an interstitial material. By applying a small amount of current to a twisted carbon nanotube thread composed ofmany small strands, Ampere’s Law states that current ﬂow in parallel wires will cause magnetic attraction
between adjacent ﬁbers, resulting in a net torsion and contraction of the ﬁber. The helical twist angle can be
optimized to further amplify the contraction and torsional effects (between 20° and 50°). In general, increasing
twist leads to higher contractions, although with bare CNT strands too sharp of an angle forces strands closer toeach other, causing current to short between strands rather than ﬂow in parallel and reducing the effect ofAmpere’s Law. In a simple single-ply ﬁber (meaning only one level of ﬁbers are twisted together), a linear
contraction of about 2% is found, with a pulling stress of about 10 MPa [56]. Winding multiple ﬁbers together ina multi-ply yarn, as well as taking advantage of overtwisting into writhes, can greatly enhance the actuation
stroke, reaching up to 14% strain at a reduction to 1.6 MPa of pulling stress [38] (ﬁgures 3(s)–(y)). Since thismethod of actuation does not rely on the transfer of heat as many others do, it has extremely fast actuation times,on the order of 200 ms for both contraction and elongation.

6. Structural stiffness control
Generating motion is only one half of the solution toward effective actuation in ﬂexible robots. A soft robot may
be capable of force generation, but the effectiveness of this motion is weakened without a supporting structure to
guide and stabilize it. The possibility of decoupling actuation and structure control, then, becomes very
intriguing in the context of robotic fabrics. Looking again to the example of human muscle ﬁbers, I W Hunterand S Lafontaine state that stiffness control is one of the most important features of skeletal muscle; since it caneffectively only generate tensile forces, it is necessary to have an opposing force to extend the muscle after
contraction, usually achieved by having opposing muscles around a joint [50]. The stiffness of the joint can bevaried by the nervous system independently of the torque generated by the limb via co-contraction of the
opposing muscles. It is the ability of the nervous system to independently set limb force and stiffness that gives
rise to much of a limb’s mechanical versatility [50]. It should be noted that in a human, this stiffness change
occurs on the level of the limb, but not on the single ﬁber level. A mechanical joint is already in place in the bone
structure, about which the antagonistic muscles can contract. In a robotic fabric, this is not the case. Decoupledstiffness and actuation becomes even more critical because a soft robotic fabric may not have deﬁned skeleton or
any supporting structure. The capacity to selectively generate joints and bones of varying stiffness not only
provides a supportive structure for load handling, but also serves to shape the paths of actuation. Reducing thedegrees of freedom in this way can also help to simplify the complicated problem of control of soft robots, whicharises due to the nonlinearities of ﬂexible materials.
Below, we present a summary of some key metrics that are important to variable-stiffness materials. A
summary of these materials is collected in table 5.
13

Multifunct. Mater. 1 (2018) 012001

Table 5. Values for variable-stiffness ﬁber performance as found in the literature. Some values were not reported for every material, and some have been derived by the author from other reported values. ABS=acrylonitrile butadienestyrene, PLA=polylactic acid, LMPA=Low Melting Point Alloy. *=Not explicitly reported in the literature but calculated or extrapolated by the author from ﬁgures or other reported information. Upward and downward arrows
indicate properties that are more favorable when high (#) or when low ($).
Method14

Human skeletal muscle (single ﬁbers)
Phase-change polymers (ABS)
Phase-change polymers (PLA)
LMPA (ﬁeld’s metal)
Thermally sensitive Elastomer (propylene-ethylene)
Coil collision
Layer jamming (ﬂat polypropylene sheets)
Layer jamming (rolled mylar tube)
Weave friction
Tendon jamming

Hard modulus [MPa] #Soft modulus [MPa] $Stiffness ratio #Softening yime $

50
820
5100
888
37
*
1000
*
100 000
*
930
—
*
7855

10
160
3800
1.2
1.5
*
41
*
1000
—
—
*
150

5
5.1
1.3
740
24.7
24
100
—
—
50

∼1 s
15 s
15 s
*
10 s
6s
*
2s
<1 s
*
<1 s
*
<1 s
*
<1 s

Stiffening time $Max strength [MPa] #Sources∼1 s
Minutes*
Minutes*
Minutes*
Minutes
*
15 s
<1 s
*
<1 s
*
<1 s
*
<1 s

—
44.8
57.8
*
6.5
—
—
32
—
—
—

[3, 87]
[27]
[27]
[39]
[87]
[23]
[88]
[40]
[89]
[90]

*

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Figure 4. (a)–(d) Variable-stiffness low melting point alloy tube woven into a cotton fabric. (a) Schematic representation of the threecomponents: LMPM core, silicone encapsulation, and conductive heating wire. (b) Manufactured LMPM-core wire with outer
diameter of 1 mm. (c) Variable-stiffness ﬁber loaded with a 200 g load. Left: in the solid state, the ﬁber sustains the load; right: whencurrent is applied and the metal melts, the ﬁber deforms under the load. (d) An example custom rigid shape held by the the LMPM
ﬁber. (e), (f) PLA thermoplastic ﬁbers sewn into a section of fabric for stiffness control. (g), (h) A conductive elastomer composite that,when activated with electrical current, rapidly and reversibly softens to 1%–10% of its initial elastic rigidity, shown with power on (g)and off (h). (i), (j) Closeup view of tubular shape of device with a large, central hollow section for tool passage shown on the left side (i),and the layer jamming scales shown on the right side (j). (k) Many wires bound together in an enclosed in an airtight chamber. Becauseof its structure, this element can bend in two directions and hold its position when vacuum is applied. (l)–(n) Modular tension-beaddesign prototyped via 3D printing. (l), (m) Body beads are composed of spherical bearing surfaces to enable friction locking and a hole
to allow control wires to pass through. (n) A large 6:1 mockup of the assembled tension-bead structure. (a)–(d) Reproduced withpermission [39]. Copyright 2016, Wiley. (e), (f) Reproduced with permission [46]. Copyright 2014, IEEE. (g), (h) Reproduced withpermission [87]. Copyright 2015, IOP Publishing. (i), (j) Reproduced with permission [94]. Copyright 2013, IEEE. (k) Reproduced
and adapted with permission [88]. Copyright 2002, IEEE. (l)–(n) Reproduced with permission [95]. Copyright 2010, IEEE.

Elastic Modulus in Hard and Soft States. The effectiveness of variable-stiffness materials is largely determinedby the stiffness ratio between the softened and stiffened states. A larger change means that the material is able tobear a respectable load when rigid without sacriﬁcing the ease of reshaping the device when soft.
Hard-to-Soft Cycle Time. A variable-stiffness material’s transition time between hard and soft states is often aconcern when working in tandem with actuators. Transition times will be reported for materials and systems
operating at room temperature in air, if available.
Maximum Strength. As a structural component, a variable-stiffness material must be able to support somedegree of load before failure. A very rigid element is not effective if it is also prone to fracture.
6.1. Human muscle (benchmark)As individual skeletal muscle ﬁbers contract, they have been recorded to reach a stiffness change of 5–10 times,
from approximately 10 MPa to 50 MPa when ﬂexed [50]. However, this value is much smaller than the typicalobservation of a live subject. Whole muscle may experience some additional stiffening due to pressure fromadjacent ﬁbers during contraction as they expand radially. However, the largest stiffness gains are apparent on
the level of an entire limb. When placed antagonistically about a common joint, the stiffness of opposing muscles
is added together as they contract simultaneously. With this in mind, human skeletal muscle is able to change thestiffness about a joint by a factor of approximately 50, which is used to enhance control and tune force
output [91].
6.2. Low melting point materials
Melting is one obvious way to achieve very large stiffness changes in a structure. Several low melting point
metallic alloys and polymers have been discovered that, as solids, are extremely rigid at room temperature, but
become liquid with only a small application of heat. As such, low melting point materials (LMPM) have alreadybeen used in a variety of ways in an attempt to create variable-stiffness components [92].
One drawback is the need to somehow contain the liquid material. When melted, the risk of leaking can be
detrimental to the functionality of the device. Another problem is the cooling time; if a material has been melted
to allow motion, holding the new position while the material slowly returns to the rigid state requires acontinuous expenditure of energy. Without some method of active cooling, such as air or water ﬂow toconvectively absorb heat, these materials can take several minutes before becoming solid again [93].
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For use as a ﬁber, molten metal can be inserted into a small-diameter silicone tube by suction. Coils of
resistive heating wire are wrapped along the length of the tube, providing a method of electrically heating themetal. This metal-core ﬁber can be made long and thin, and when heated becomes perfectly ﬂexible so as to be
woven into fabric as in one demonstration (ﬁgures 4(a)–(d). By pre-stretching the elastomer tube as the metal isinserted, it accumulates a restorative spring force, whereby it can pull any disjointed segments of metal that may
have broken apart back together into one solid continuum, self-healing the next time the metal is melted. These
devices experienced a 700× change in stiffness from 888 MPa to about 1.2 MPa when melted [39]. Similar coresheath ﬁbers have been developed using a polymer blend as the phase-change material [96].
LMPMs can also be incorporated into an existing fabric by various means, such as coating via wetting [97],thin ﬁlm lamination [98], or bonding small microencapsulated particles to the fabric surface [99]. By applying
the material to a pre-made fabric, much of the complexity involved in manufacturing directly with variablestiffness ﬁbers can be avoided, such as maintaining the ‘soft’ state so as to feed properly through a machine.
However, these above applications were designed for uses other than structural control, and so did not report on
the stiffness-changing behavior.
6.3. Glass-transition polymers
There are many commercial polymers available that exhibit stiffness change at low temperatures (∼30–100C).
Unlike metals which undergo a sharp solid-liquid phase transition, these polymers undergo a glass transition,softening gradually over a wider temperature range from a glassy to a rubbery state due to greater mobility of theconstituent polymer chains. In practice, thermoplastic polymers can be used as variable-stiffness materials byheating the polymer slightly beyond the glass transition temperature, stopping the heating process before thematerial reaches a polymer melt state and ﬂows like a viscous liquid. Alternatively, thermoset polymers can also
be exploited for their glass-transition behavior, but will not melt into a liquid-like state due to the crosslinks
joining the polymer chains together, keeping the material a solid.
Strands of polymer have already been sewn onto existing cloth for use as a variable-stiffness structure(ﬁgures 4(e), (f)). These strands consist of a resistive Nitinol wire coated with polylactic acid (PLA) polymer,
which can be softened by Joule heating through the wire. This variable-stiffness ﬁber experienced a change instiffness in about 15 s [46]. Later work made use of the Nitinol SMA wire as an integrated actuation element. A
combined ﬁber of PLA or ABS and a Nitinol coil experienced changes in stiffness of about 5.1 GPa to 3.8 GPa,and 820 MPa to 160 MPa respectively. In open air, these ﬁbers required approximately 5 minutes to fully cool toa rigid state [27].
As with low melting point materials, this method suffers from long cooling times. Additionally, since thematerial does not fully melt, the change in stiffness is comparatively low; either there is still some signiﬁcant levelof ﬂexure occurring even in the rigid state (ABS), or the soft state is perhaps not compliant enough to be used incertain applications (PLA).
6.4. Thermally sensitive elastomers
Expanding on the idea of softening polymers, one variable-stiffness method makes use of carbon-black particle
additives to induce conductivity into a propylene-ethylene elastomer. The elastomer can then be cut into narrow
strips to be used as a variable-stiffness ﬁber. By applying 150 volts to the elastomer, it becomes warm by Joule
heating and softens from 37 MPa to 1.5 MPa in about 6 s (ﬁgures 4(g), (h)). Again, no cooling time is given, but
this is assumed to take several minutes. In the stiff state, the material is still rather ﬂexible - on par with leather; inthe soft state it is comparable to human skin. Rather than being used as a load-bearing structure, staggered
segments of the material can be activated and deactivated to guide the motion of any actuators by promoting
bending in one area more than in others [87].
6.5. JammingA stack of overlapping thin sheets of material that are normally able to slide over one another freely can become
rigidly ﬁxed in place upon application of a pressure differential. This pressure forces the multiple layers together
into a high-friction scenario, causing the layers to behave together as a single, thicker sheet. The stiffness of thisthick stack can easily be calculated as proportional to the cube of the thickness. This effect of inducing friction tolock a system into position is known as jamming.
Jamming systems of varying approach can be designed to ﬁt different geometries as needed. The most basic
jamming structure involves a simple stack of thin ﬂexible ﬁlms held in an air-tight enclosure to hold vacuum
pressure. This structure can be modiﬁed into a long, thin tube by simply wrapping the layers in a cylindricalfashion and cutting away slits to prevent buckling of the material when bent. Such an effort has been
demonstrated in the context of medical endoscopes [40, 94]. Although the demonstrated product is 22 mm in
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diameter, it could be made much smaller by reducing the size of the hollow core, which makes up 15 mm of thatmeasurement (ﬁgures 4(i), (j)).
An alternative conﬁguration is to use an air-tight enclosed collection of wires rather than ﬂat sheets. The
wires in parallel act as a long ﬁber, allowing bending motion in any direction rather than being constrained to
one dimension (ﬁgure 4(k)). Although no performance data was provided on this particular arrangement, it was
expected to perform similarly to the basic stacked layer conﬁguration, which can change from 1GPa to 100GPa
when vacuum is applied to stack of 15 polypropylene sheets [88]. This method is extremely fast-acting since it
can be controlled pneumatically. However, the need for an air supply adds external bulk to the overall product.
Another approach to jamming involves using internal tension in the ﬁber itself as the source of friction
rather than vacuum pressure. For example, when a coiled actuator has contracted to its minimum length,
perhaps due to excessive overtwist in the ﬁber or a very small load, the adjacent coils may begin to come incontact. In the case of a thermally expanding material, further heating at this point will cause the coils to press
more ﬁrmly against each other. For example, upon coil contact at ∼130 °C, nylon 6,6 muscle expands at a ratecomparable to the ﬁber’s radial thermal expansion. The coiled structure stiffens with increasing temperature,
producing a 24-fold increase in nominal tensile modulus [23], allowing the coiled actuator to serve double-duty
as both actuator and stiffening mechanism.
A similar effect can be achieved by using ﬁber tension to provide the friction pressure. The general design
involves threading an actuating ﬁber through a series of small shaped beads and afﬁxing it to the ends. The
actuating core, when contracted, will pull the beads together and lock them in position by friction [90, 95, 100].
This method is capable of changes in stiffness of up to 50× depending on the friction of the materials and beadgeometry, and has been validated using beads as narrow as 1 mm in diameter (ﬁgures 4(j)–(m)).
The weave of a fabric itself can create enough friction between its ﬁbers that it can become locked in place
after being ﬂexed. The locking is continuous, and so requires that the actuator be able to manipulate the fabricwith enough force to overcome this friction when motion is desired [89]. In this case, the jamming pressure
between ﬁbers being provided by the dense structure of the weave. However, the loading capability of thisstructure has not been reported.

7. Integrated robotic fabrics and applications
In this section we review some of the assembled fabric systems that have been developed in the context ofwearables (ﬁgure 5(a)), object manipulation (ﬁgure 5(b)), and self-reconﬁguration (ﬁgure 5(c)).
7.1. Wearables
Robotic fabric technology is highly suited to wearable devices due to its conformable nature, breathability, andease of integration into traditional fabrics, and consequently has various applications in responsive clothing forboth aesthetic and assistive purposes [104]. Smart clothing that uses SMA threads has been shown to adjust
garment ﬁt [7] (ﬁgure 5(a1)) or roll up sleeves in response to rising temperature [105]. Clothing has also beentreated with SMP to recover from wrinkling by applying heat (ﬁgure 5(a4–5)) or avoid shrinkage when washed[75] (Figure 5(a3)). Functional ﬁbers have even enhanced fashionable garments with decorative features that
self-animate [7, 103] (ﬁgure 5(a2)). Variable-stiffness ﬁbers made from PLA or LMPM have been implemented
as adjustable splints or casts that easily form to the contours of the body and then lock into place, with thepossibility of softening and reshaping as necessary [39, 46] (ﬁgure 5(a6-10)). SMA wires programmed toconstrict inward can be knitted into dynamic compression stockings [101, 106] (ﬁgure 5(a11)) or sewn into aspacesuit to combat orthostatic hypotension [107], and in extreme cases afﬁxed to cuffs to act as an emergency
tourniquet [108]. Additionally, active orthotics prototypes have been made by afﬁxing SMA coils to a fabricsleeve and worn about the lower leg, allowing the garment to selectively support different areas of the leg at
different points of the gait. However, the result was ultimately too slow for real-world use due to long coolingtimes [109].
7.2. Object manipulation
A robotic fabric can impart motion onto other objects by interacting with them. However, due to the thin andlightweight nature of fabric, generating sufﬁcient force while maintaining stability and balance to act on a
separate body is difﬁcult unless the robotic fabric is afﬁxed to some external rigid support, such as lab stand or atesting rig. Therefore, we designate three categories of object manipulation examples that use robotic fabrics:(1) cases where the robotic fabric is afﬁxed to a rigid support, (2) cases where the robotic fabric is only attached tothe manipulation target, and (3) cases demonstrating grasping techniques. There are several examples of fabrics
ﬁxed to a support, then contracting to lift a hanging weight. One example used thermally activated Spandex thathas been knitted into a long tube [59] to lift a load (ﬁgure 5(b1)), and another used SMA wires sewn into muslin
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Figure 5. (a) Examples of robotic fabric wearables. (a1) Rising hem line of a dress through the use of hand-stitched Nitinol wires. (a2)Animated ﬂowers that open and close over a 15 s interval. (a3) Wool garment treated with an SMP coating maintains size uponlaundering. (a4) Wrinkle-free effect of fabric treated with SMP in comparison with that of the fabric not treated with SMP. (a5) After
spraying hot steam, the treated fabric recovers its ﬂat shape. (a6-9) Variable-stiffness low melting point alloy tube woven into a cotton
fabric. (a6) Prototype of a variable-stiffness ﬁnger splint. (a7) Woven fabric made using the cotton as the weft and the variable-stiffnesstube as the warp. (a8-9) Variable-stiffness cast holding two distinct positions. (a10) Variable-stiffness ﬁnger cast using thermoplasticﬁbers. (a11). SMA compression garment. (b) Examples of robotic fabric object manipulation. (b1) Spandex/CNT knitted tube that
contracts by 25% while lifting a 25 MPa load as a result of electrothermal actuation. (b2) Characterization of linear force generated bya sewn-SMA actuator unit using a materials testing machine. (b3) A knitted CP actuator unit. (b4) Knitted actuator unit drives a leverarm in a LEGO setup. (b5-6) Knitted actuator unit lifting a 2.0 g load. (b7) Hand-sewn SMA into muslin fabric and wrapped around afoam block, showing initial state, maximum bending position, and the ﬁnal relaxed state. (b8) Application of the same robotic fabrictowards a compression mode, showing initial state, maximum compression, and the ﬁnal relaxed state. (b9) A knitted SMA ring
actuates from the initially compressed state to lift the applied load in the expanded state. (b10) A knitted SMA belt which actuates fromthe initially expanded cylinder to a contracted cylinder. (b11-14) A mechanical gripper. (b11) A fabric actuator curls to grip a smallumbrella. (b12) Lifting up the umbrella. (b13) Dropping the umbrella into a container. (b14) Release of umbrella from the gripper byunrolling the fabric. (c) Examples of robotic fabric self-reconﬁguration. (c1-6) Various SMA knit patterns designed to produce
different shape tranformations. (c1) An initial ﬂat SMA knit rectangle. (c2) Curling motion. (c3) Arching motion. (c4) Accordion
motion. (c5) Folding motion. (c6) Combination of arching and folding. (c7) Shape memory recovery creating a curled shape on one
half of a fabric over a period of 20 s. (c8-9) Two knitted SMA fabric patterns that generate a ball (c8) and half-moon (c9) 3D surface
texture when activated. (c10) Fabric woven with SMA wires crumples up over a period of 60 s when activated. (c11) A smart partition
that opens when activated. (c12) Fabric woven with SMA wires that ﬂattens out over a period of 30 s when activated. (c13-14) Robotic
fabric with active variable-stiffness ﬁbers. (c13) Initial stiff state with a hanging 200 g mass. (c14) Actuated and then stiffened fabricholding its new shape. (a1-2) Reproduced with permission [7]. Copyright 2005, IEEE. (a3-5) Reproduced and adapted with permission[75]. Copyright 2012, IOP Publishing. (a6-9) Reproduced with permission [39]. Copyright 2016, Wiley. (a10) Reproduced withpermission [46]. Copyright 2014, IEEE. (a11) Reproduced with permission [101]. Copyright 2017, Association for Computing
Machinery, Inc. (b1) Reproduced and adapted with permission [59]. Copyright 2016, American Chemical Society. (b2) Reproducedwith permission [28]. Copyright 2015, IEEE. (b3-6) Reproduced with permission [41]. Copyright 2017 The Authors, some rights
reserved; exclusive licensee American Association for the Advancement of Science. Distributed under a Creative Commons
Attribution NonCommercial License 4.0 (CC BY-NC). (b7-8) Reproduced and adapted with permission [28]. Copyright 2015, IEEE.
(b9-10) Reproduced and adapted with permission [44]. Copyright 2013, IOP Publishing. (b11-14) Reproduced with permission [36].
Copyright 2017, Wiley. (c1-6) Reproduced and adapted with permission [44]. Copyright 2013, IOP Publishing. (c7, 10-12)
Reproduced and adapted with permission [102]. Copyright 2007, Sage Publications. (c8-9) Reproduced with permission [103].
Copyright 2003, Emerald Publishing Limited. (c13-14) Reproduced with permission [27]. Copyright 2016, IEEE.

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fabric to pull against a materials testing machine [28] (ﬁgure 5(b2)). A more intricate setup used a strip of knitted
CP fabric to drive a lever arm in a LEGO setup [41] (ﬁgure 5(b3-6)). In contrast, actuating fabrics that do not
require a rigid test rig to function are hard to ﬁnd. One such fabric made from SMA sewn into muslin could be
wrapped around a soft structure such as a foam block and used to deform and bend the block into a desired
shape [28] (ﬁgure 5(b7-8)) and even lock it into position with the addition of stiffness-changing PLA ﬁbers [27].
A knitted ring of SMA was able expand into a tall, free-standing tube, lifting a stack of washers placed atop it [44](ﬁgure 5(b9)). In cases where the manipulation target is not already attached to the fabric, some kind of grasping
technique might be needed. By changing the ﬁber arrangement of the knitted SMA ring mentioned above, themotion was made so it could contract inward upon an object like a belt [44] (ﬁgure 5(b10)). Alternatively, arudimentary gripper has been made by curling an active fabric around small objects [36], although the weight ofthese objects (cocktail umbrellas) suggests that the gripping strength is extremely small (ﬁgure 5(b11-14)).
7.3. Self-reconﬁguration
Self-reconﬁgurable robots are of interest for adaptability in unstructured environments, multi-functionality toincrease task performance, and compactability/deployability during transport and use. Robotic fabrics offer aunique solution to these needs, as 3D shapes can be formed out of 2D fabrics, and potential modes of actuation
can extend beyond the hinge-like bending utilized by self-folding origami-inspired robots, to include
continuum-body bending, twisting, buckling, expanding, and contracting. While a fully integrated robotic
fabric of this complexity has not yet been developed, some signiﬁcant shape-changing demonstrations do exist.
With carefully planned knitting arrangements, SMA fabric can be designed to deform in speciﬁc patterns,
including bending, rolling, arching, contracting, expanding, or any combination of these [44] (ﬁgure 5(c1-6)).
Woven SMA fabric pieces have been made to curl into tubes (ﬁgure 5(c7)), contract in length (ﬁgure 5(c10)), andﬂatten out after being folded [102] (ﬁgure 5(c12)). Similarly, a ﬂat piece of fabric with embedded SMA has been
made which can alter its surface from a 2D plane to a bumpy 3D texture [103] (ﬁgure 5(c8-9)). In a practicalapplication, an SMA woven panel has been made which can act as a smart partition, opening up when activated[102] (ﬁgure 5(c11)). By introducing stiffness-changing PLA ﬁbers, a robotic fabric with SMA wires sewn in has
been made that can not only crumple into a compact form against an opposing load, but also lock in its newshape after moving [27] (ﬁgure 5(c13-14)).

8. Future outlook
The concept of robotic fabrics falls in line with recent trends exploring alternatives to traditional, rigid robot
structures. However, the unique approach of using fabrics as the robotic framework introduces a new, exciting
dimension of possibility to complementary concepts such as soft robotics, reconﬁgurable robotics, robotic
materials, and origami-inspired robotics. A great deal of preliminary technologies in wearable electronics ande-textiles have paved the way for fabrics that generate force and motion, and we believe a conﬂuence of factors
now make it possible to further explore the rich robotic fabric design space: current interest in wearables andhuman-robot interfaces is high, novel processes are allowing functional materials to be fabricated in ﬁber form,
and well-established textile manufacturing techniques are poised to streamline ﬁber-based technology
breakthroughs into commercial products. While the current state-of-the-art is limited to examples where one or
two types of functional ﬁbers are integrated within an existing fabric substrate or comprise the fabric itself,
robotic fabrics may include any combination of active and responsive ﬁbers in any number of arrangements,
yielding an immense design space for the concept, which is representative of both the promise and challenge ofthis emerging technology.
In the future, we envision rolls of commercially available robotic fabrics, which could be purchased by theyard and easily programmed to meet the demands of variable tasks and environments. We envision selfreconﬁgurable ‘origami’-inspired machinery that can later be crumpled and stowed for compact storage. We
envision consumer clothing that will actively adjust itself and assist the wearer beyond being a simple covering.
Such visions call on other challenges such as the development of complex design tools, compact or ﬁber-form
power supplies, distributed computation, and thin-body control. As with most emerging concepts, thearchitecture of a robotic fabric at its full potential is unknown. The deﬁnition of robotic fabrics may very well
evolve as new technologies are discovered and new challenges are encountered. Yet, with the ‘toolbox’ oftechnologies reviewed herein, we hope to facilitate the transition of commonplace fabrics from passive
constructs to a promising new class of active, task-assistive machinery.

Acknowledgments
This work was supported by the Air Force Ofﬁce of Scientiﬁc Research under award number FA9550-16-1-0267.
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ORCID iDs
Trevor L Bucknerhttps://orcid.org/0000-0002-6602-8511

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