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IEEE ROBOTICS AND AUTOMATION LETTERS, VOL. 1, NO. 2, JULY 2016

Active Variable Stiffness Fibers for Multifunctional
Robotic Fabrics
Michelle C. Yuen, R. Adam Bilodeau, and Rebecca K. Kramer

Abstract—In this letter, we introduce active variable stiffness
fibers that are made from shape memory alloy and thermally
responsive polymers. This combines the actuation of shape memory alloy with the variable stiffness of a thermoplastic using
electric current as the stimulus. By combining both actuation andvariable stiffness functions, the multifunctional fibers can moveto a new position then hold that position without requiring additional power. We explore the possibility of tuning the fibers to meetvarying structural and performance demands by selecting different thermoplastics with different glass transition temperatures.
Finally, we integrate the active variable stiffness fibers into a fabric to demonstrate multifunctional robotic fabrics that can control
the motion of soft, compliant bodies from their surface.
Index Terms—Soft material robotics, novel actuators for natural machine motion, flexible robots.

I. I NTRODUCTIONT

HE emerging field of soft robotics aims to bring highly
deformable electromechanical systems for applications
in wearables, search-and-rescue, medical devices, exploratory
robots, and more. This field actively pushes forward nonconventional actuators, sensors, and structures while removing
the frames, motors, and linkages associated with traditional
robots. To achieve versatility in soft robotic design, we envision robotic fabrics bringing actuation, sensing, and stiffness
control to the exterior of integrated systems. Robotic fabrics
are 2D fabrics that can wrap around any highly deformable 3D
object (e.g. an inflatable balloon or foam) to create a soft robot.
These fabrics are currently expanding the possibilities of wearable and conformable robotics by adding function to textiles
and manipulating soft-bodied objects [1]–[3]. With no fixed
shape of their own, robotic fabrics can exist on the surface of
deformable objects and in spaces that do not have a pre-definedgeometry or volume.
In this letter, we introduce active variable stiffness fibers(AVS fibers) made from a combination of shape memory alloy
and thermally responsive polymers (e.g. thermoplastics). This
combination couples the actuation of shape memory alloy with

Manuscript received August 31, 2015; accepted January 3, 2016. Date of
publication January 19, 2016; date of current version February 29, 2016.
This paper was recommended for publication by Associate Editor X. Liu andEditor Y. Sun upon evaluation of the reviewers’ comments. This work was
supported by an Early Career Faculty Grant NNX14A052G from National
Aeronautics and Space Administration’s Space Technology Research Grants
Program. The work of M. C. Yuen was supported by the National Science
Foundation Graduate Research Fellowship under Grant DGE-1333468. M. C.
Yuen and R. A. Bilodeau are co-first authors.
The authors are with the School of Mechanical Engineering, Purdue
University, West Lafayette, IN 47907 USA (e-mail: yuenm@purdue.edu;
rbilodea@purdue.edu; rebeccakramer@purdue.edu).
Digital Object Identifier 10.1109/LRA.2016.2519609

Fig. 1. Active variable stiffness fabric composed of active variable stiffness
fibers sewn onto inextensible muslin fabric. (a) Unactuated, stiff AVS fabricwith a hanging 200 g mass. (b) Actuated then stiffened AVS fabric holding a
200 g mass. (c) Unactuated, stiff AVS fabric acting as a bridge supporting a
200 g mass on a small plate. (d) Unactuated, softened AVS fabric sinking under
the weight of a 200 g mass. Scale bars are 2 cm.

the passive softening/holding capabilities (variable stiffness) of
a thermally responsive material using only a single input: electric current. We have previously demonstrated variable stiffness
using a thermoplastic in a fabric-based system [4] and theintegration of shape memory alloy (SMA) into fabric to form
compliant and actuating planar structures [1]. This letter combines these two bodies of work into an integrated, active,
variable stiffness material that demonstrates performance as amultifunctional active variable stiffness fabric (AVS fabric). By
combining both actuation and variable stiffness functions, the
AVS fabric can change the position of a soft structure and holdthe new position without requiring additional power. Figure 1
shows the AVS fiber sewn into muslin fabric (an inextensible
cotton textile) to form an AVS fabric.

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YUEN et al.: ACTIVE VARIABLE STIFFNESS FIBERS

This letter contributes the following innovations: (1) integrated actuation and variable stiffness functions in a single
fiber, enabled by a novel room-temperature manufacturing technique that avoids activation of the thermally responsive materials during processing, (2) coupled and un-coupled actuationand variable stiffness, achieved by tuning the thermoplastic’s
glass-transition temperature either higher or lower than theactivation temperature of the SMA actuator, and (3) multifunctional fabrics with integrated active variable stiffness fibers.
The integrated fabric demonstrates both actuation and variable
stiffness on-demand. The fabric acts as a connecting medium
where multiple fibers can be integrated and activated in parallel, thereby increasing overall lifting and holding capacity.
Figure 1(a)–(b) demonstrates the AVS fabric lifting a 200 g
mass and holding it suspended even after power has been shut
off to the system. The serpentine pattern of the stiffness element
enables the fabric to support the 200 g mass suspended between
two blocks when not activated (Figure 1(c)). After being softened, the fabric is then allowed to deform under the weight(Figure 1(d)).
II. P REVIOUS W ORK
Variable stiffness actuators for soft machines have taken
two approaches: system-level and material-level. System-levelvariable stiffness actuators contain elastic and/or damping elements or employ control algorithms to tune the stiffness and
impedance of the device [5]–[11]. Variable stiffness actuators
may also derive their stiffness control at the material level.
For example, the McKibben actuator is a widely-used soft
pneumatic actuator, consisting of a braided mesh that shortens in length and increases in stiffness as it is inflated with air(the stiffness and actuation are coupled) [12], [13]. Similarly,changes in both volume and rigidity have been demonstrated
by swelling of polymer gels [14]. Wang, et al. developed acomposite system using SMA wire actuators and a low-melting
point fusible alloy as a variable stiffness component, thenencapsulated both in a silicone elastomer [15].
Multifuctional fibers have been developed by researchers
to duplicate the tensile strength and actuation capabilities ofnatural muscle fibers. Approaches include the use of carbon
nanotubes to create a conductive, strong fiber [16]–[18], with adisadvantage of small displacements (less than 5%), micron sizing, and an inability to maintain a structural shape. Other recent
work created fibers using polymer structures [19], but thesefibers also only provide actuation and tensile strength without
structural rigidity.
In the broader community of robotic fabrics, shape memory alloy wires are a common actuator choice due to easeof integration by stitching [1], felting [2], weaving [20],[21], and other fixture methods [3]. Previously demonstrated
active fabrics have been used in robotic applications [1],kinetic and technical garments [2], [22], and self-deploying
structures [20], [23]. Our previous work demonstrated variable stiffness fibers integrated into fabrics to distribute stiffness control over a surface [4]. These fibers leveraged theglassy-rubbery transition in thermoplastics for variable stiffness
function.

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Fig. 2. Active variable stiffness fibers. (a) Diagram of the composition of an
AVS fiber. (b) PLA-based AVS fiber (the PLA core and the SEBS coating areboth transparent). (c) ABS-based AVS fiber. Scale bars are 1 cm.

III. M ATERIALSThe active variable stiffness (AVS) fibers were composed ofan actuating shape-memory alloy (SMA) wire and a variable
stiffness (VS) fiber, bound together with a polymer rubber as
seen in Figure 2. The SMA wire actuator, composed of a nickeltitanium (NiTi) alloy (Dynalloy, 0.508 mm dia.), changes shape
to return to a programmed helical coil when heated above its
activation temperature (70 ◦ C - 75 ◦ C). The VS fibers werecomposed of a heating element coated with a film of thermoplastic that changes stiffness in response to temperature. Our
previous work utilized unprogrammed NiTi wire (Dynalloy,
0.254 mm dia.) encased in a thin coating of polylactic acid
(PLA) (Open Source Printing, LLC) [4]. Here, we built on
this work and also introduced acrylonitrile butadiene styrene(ABS) (MakerBot Industries, LLC) as a variable stiffness coating. Both types of VS fibers reduced in stiffness when thethermoplastic was heated above its glass-transition temperature(Tg ): 55 ◦ C - 65 ◦ C for the PLA [4], and 105 ◦ C for the ABS(manufacturer specified). Though ABS has a lower stiffness
than PLA, we included it in this study because it has a higher Tg
than the SMA transition temperature. This allows us to couple
and decouple the variable stiffness effect from the SMA actuation, using the PLA and ABS as the stiffening material, respectively. The SMA actuator and VS fibers were bound together byencapsulating them with polystyrene-block-poly(ethylene-ranbutylene)-block-polystyrene (SEBS) (Sigma-Aldrich, Mw =89, 000), a polymer rubber that withstands the operating temperatures of the fibers and remains rubbery even below room
temperature.
We note here that although the NiTi heating wire used in theVS fibers can also exhibit the shape-memory effect if properly
programmed, in our work it was used purely for its efficacy asa Joule heater. In the following discussions, the heating wire in
the VS fibers will be referred to as NiTi wire; the SMA actuator
will be referred to as SMA wire.

IV. M ANUFACTURING M ETHODS
A. Variable Stiffness Fibers
In this work, we manufactured two varieties of AVS fibers:
ABS-based and PLA-based. Drawing upon our previous work,

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Fig. 3. Manufacturing ABS-based active variable stiffness fibers. (a) Coating
ABS solution onto a heating NiTi wire for variable stiffness fiber.
(b) Programming active SMA wire into a helical coil in a 390 ◦ C oven.
(c) Twisting the programmed SMA wire onto the variable stiffness fiber.
(d) Encapsulating the twisted wires with SEBS. (e) Completed AVS fiber.

PLA-based variable stiffness fibers are manufactured by drawing 0.254 mm diameter NiTi wire through melted PLA thermoplastic as described by Chenal, et al. [4]. Our resulting fiber
diameter was 1.50±0.28 mm (95% confidence).
In order to process the ABS, an amorphous polymer withno true melting point, we developed a solvent-based approachto manufacture the VS fibers at room temperature (25 ◦ C).
ABS filament (1.75 mm dia.) was cut into approximately 1 cmlong pellets and then placed in a solvent (2-butanone, SigmaAldrich) at a 50% polymer (by weight) concentration. The
mixture homogenized over a 12hr period, and was then transferred into a syringe tipped with a T-junction (4 mm innerdia.). The solution was allowed to degas for 4hrs. To coat thewire, the NiTi wire was fed through the T-junction, as shownin Figure 3(a), at a constant rate of approximately 30 cm/min.
Simultaneously, pressure was applied to the syringe to ensure
the T-junction was full of the ABS mixture. At this rate ofpull, the resulting thickness of the variable stiffness fiber was
2.08 ± 0.80mm (95% confidence).

IEEE ROBOTICS AND AUTOMATION LETTERS, VOL. 1, NO. 2, JULY 2016

Fig. 4. Setup for characterization of AVS fibers. (a) Stiffness characterization
using a 3-point bending setup. (b) Force and load characterization setup. Scalebars are 2 cm.

programming coil to prevent internal torsion, as demonstrated
in Figure 3(c). This twist had a pitch of approximately 0.25
(ABS) and 0.4 (PLA) rotations per cm, influenced by the diameter of the VS fiber. The low-pitch twist bound the SMA
wire and the VS fiber with a slight mechanical connection in
anticipation of the next processing step. The twisted pair wasthen passed through a T-junction filled with a polymer solution (Figure 3(d)) composed of SEBS polymer and toluene(30% polymer, by weight), prepared in a manner similar to
the ABS 2-butanone solution, to bond the two components
together. After the toluene evaporated from the SEBS coating,
the AVS fiber was complete (Figure 3(e)). The final diameterof the ABS-based AVS fiber was 2.78 ± 0.20mm (95% confidence); the final diameter of the PLA-based AVS fiber was
2.04 ± 0.18mm (95% confidence). Because of the VS fibers’
small diameter, the twisted SMA can be approximated as linear
along the length of the VS fiber, allowing the SMA to deform
without interference when activated.
V. C HARACTERIZATION
A. Stiffness Characterization

B. SMA Actuator
The SMA actuator was programmed through the following
series of steps. The wire was coiling tightly onto a 9.525 mm
diameter shaft (Figure 3(b)) as tension was applied to the wire.
The wire was then secured onto the shaft using two collars. The
shaft was then placed in an oven at 390 ◦ C for 10min and thenquenched and dried [24]. This cycle was repeated 10 times toensure shape fixity.
C. Integration into AVS Fiber
After the coating on the VS fiber solidified either by cooling or solvent evaporation, the SMA wire and VS fiber weretwisted around each other in the same direction as the SMA’s

We characterized the AVS fibers with the thermoplastic in
the inactive (stiff, glassy) state and active (soft, rubbery) state.
We did this with a 3-point bending test using an Instron 3345
as seen in Figure 4(a). The distance between supports was 5 cm
and the nose radius was 5 mm. In our tests, strain was applied
at a rate of 0.01(mm/mm)/min. The glassy state tests were performed at room temperature (25 ◦ C). To achieve the rubbery
state, the VS fibers, contained within the AVS fibers, were activated for 5s at I = 0.75A before the start of the test to transition
the thermoplastic into the rubbery state. Each fiber specimen
was tested three times in both glassy and rubbery states.
The bending modulus, calculated as the ratio of the bending
stress to the bending strain, was determined for three specimens of each type of AVS fiber. The 3-point bending test

YUEN et al.: ACTIVE VARIABLE STIFFNESS FIBERS

Fig. 5. Stress-strain plot for two AVS fibers in a 3-point bending setup. A PLAbased and an ABS-based AVS fiber were tested repeatedly in stiff (cold) andsoftened (hot) states. The shaded area around each average line represents a
95% confidence interval.
TABLE I
B ENDING M ODULUS OF ABS- AND PLA-BASED AVS F IBERS W ITH THE
S TIFFENING T HERMOPLASTIC H EATED (>T G ) AND C OOLED (25 ◦ C).
± VALUE IS 95% C ONFIDENCE ON M EAN

provided a set of stress-strain data points that follow a linear trend in the elastic range of the material (Figure 5). We
fit a straight line to each data set to obtain the bending modulus from the slope of fitted line, given in Table I. This tableshows the bending modulus fluctuated from one specimen toanother. We attributed the variation between the values fromeach specimen to two sources: manufacturing inconsistency
and geometric anisotropy. Manufacturing inconsistency refers
to variation in the thermoplastic and SEBS coating thicknesses,
which resulted in variance in the cross-sectional geometry of
both the VS fibers and the AVS fibers. The geometric anisotropyof the AVS fibers arises from the change in position of the
SMA wire relative to the loading direction as the fiber is rotatedduring the 3-point bending test. The maximum stiffness configuration occurs when the loading direction is aligned withthe axis passing through the center of both the VS fiber and
SMA, and the minimum stiffness configuration occurs when
the loading direction is orthogonal to this axis. To ensure thatthe measured values are within reason, we performed a stiffness analysis of the cross-section by integrating the second
moment of area and elastic modulus to determine the overall bending modulus of the AVS fibers in their stiffened state.
By considering the upper and lower extremes for both coating

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Fig. 6. Stiffness characterization plot for a single ABS-based AVS fiber softened with various currents. At high currents, the SMA actuator begins toactivate during the stiffness test resulting in an increased modulus.

thicknesses and geometric anisotropy, we calculated possible
modulus ranges as 0.453-4.80GPa and 2.11-7.70GPa for the
ABS-based and PLA-based fibers, respectively. These ranges
include the values that we measured (Table I), confirming thatthe spread of the experimental values is within reason.
The bending modulus was also determined as a function ofcurrent for a single representative ABS-based AVS fiber specimen (Figure 6). We varied the current from 0A to 1A, focusing
on the currents near the transition from the stiff to soft state. A
sigmoidal curve fit to the experimental data follows the trend
expected of a thermoplastic’s modulus going through its Tg[25]. However, the trend breaks at the higher currents (represented by the 1A data point) when the heated VS fiber is hot
enough to trigger activation of the adjacent SMA actuator, transitioning the SMA from its flexible martensite phase into a stiff
austenite phase, thereby increasing the general bending modulus of the AVS fiber. This curve provided us with a range ofcurrents to use that would soften the ABS-based AVS fiber. The
PLA-based AVS fibers were not characterized in this work, asa similar analysis appears in previous work done by Chenal,
et al. [4].
The average modulus values presented in Table I shows that
the ABS-based fibers are softer than the PLA fibers both when
stiffened and softened. To determine how the stiffnesses of thefibers affect actuation, we subsequently performed force tests
on the fibers.

B. Force and Load Characterization
To characterize the force produced by the AVS fibers, 10 cmlong AVS fiber segments were clamped in an Instron 3345 fitted with a 50N load cell (Figure 4(b)). Throughout the test, thespecimens were held in tension at a fixed length to measure
the force produced by the isometrically actuating SMA when
the VS fiber was both stiff and softened.
In contrast to the stiffness tests, only the ABS-based AVS
fibers were tested for force in both their stiffened and soft
state. The PLA-based fibers were tested only in their soft state,

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IEEE ROBOTICS AND AUTOMATION LETTERS, VOL. 1, NO. 2, JULY 2016

Fig. 7. Plot of force produced by an ABS-based AVS fiber. Hot: VS fiber wassoftened before SMA actuation. Cold: VS fiber was not softened before SMA
actuation. Cold-to-Hot: VS fiber was softened 30s after the SMA was actuated.
The shaded area around each average line represents a 95% confidence interval.

Fig. 8. Plot of force produced by an ABS-based AVS fiber as a function of
applied current. The VS fiber was softened prior to the start of each run. The
currents applied ranged from 0.5A to 1.75A in increments of 0.25A. The shaded
area around each average line represents a 95% confidence interval.

TABLE IIF ORCE P RODUCED BY THE SMA ACTUATOR P ORTION OF ABS- AND
PLA-BASED AVS F IBERS IN THE S OFTENED AND S TIFFENED S TATE .
± VALUE IS 95% C ONFIDENCE ON M EAN“Hot”, and “Cold-to-Hot”. The plot clearly shows that the fiber
follows the zero-force trend when the fiber is stiff, but once thefiber is softened, the actuating SMA is able to produce force.
This demonstrates complete decoupling of the actuation andstiffening components in the ABS-based AVS fibers.
We further characterized a single ABS-based AVS fiber for
the force produced as a function of current applied (Figure 8).
For all of these experiments, the VS fiber was softened priorto actuating the SMA, identical to the “Hot” tests shown in
Figure 7. We measured the amount of force produced by thefiber over 60s of applied current across a range from 0.5A
to 1.75A. As expected, increasing the input current results
in increased output force, showing that the incorporation of
the SMA actuator in an AVS fiber does not change the SMA
actuation behavior [24].
In addition to the force generation experiments, we also characterized the load capacity of the fibers and SMA wire in theirstiffened, coiled shape. The fibers and wire were first actuated(without load) then cooled into a fully contracted coil. This
stiffened coil was then placed in the same Instron setup shownin Figure 4(b). We applied a constant linear displacement of
10 mm/min to the coil and measured the resulting load. The
resulting load-extension curve indicated that the coils behaved
as linear springs with spring constants of 3.85 ± 2.91 N/m
for the SMA wire, 22.2 ± 6.28N/m for the PLA-based AVS
fibers, and 51.8 ± 14.8N/m for the ABS-based AVS fibers (95%
confidence).

because the PLA stiffness cannot be decoupled from the SMA
actuation. Since the Tg of PLA is below the activation temperature of SMA, when the SMA is powered the PLA will soften
regardless of whether or not the VS fiber is powered. This effectdoes not occur in the ABS-based fibers because the Tg of ABS
is above the activation temperature of SMA—only overheating
the SMA will cause the ABS to transition into the rubbery state.
We averaged the tests from multiple segments of the ABSbased AVS fiber (Table II). When the ABS was softened, the
SMA was able to produce 2.25 ± 0.17N of actuation force. In
contrast, when the ABS was stiff, the testing apparatus was
unable to detect any pulling forces from the SMA wire. The
negative force values result from the hot SMA reducing thewhole fiber’s modulus slightly. This test demonstrates that theABS-based AVS fiber will maintain its position when the VS
fiber is inactive, even when the SMA is powered. The results
also show that the ABS-based AVS fibers produce higher forcescompared to the PLA-based AVS fibers. This is likely due to aboth the PLA having a higher bending modulus than ABS in its
soft state (Table I) and the soft ABS adding heat to the SMA
wire.
In Figure 7, we plotted the data from a single ABS-based
AVS fiber showing the force being generated from the active
SMA wire over time for 60s. The fiber was subjected to 3
operating conditions: 1) actuation of the SMA without softening the ABS, 2) actuation of the SMA after softening theABS, and 3) actuation of the SMA for 30s prior to softening
the ABS. These conditions are shown in Figure 7 as “Cold”,

VI. A PPLICATIONS
A. Single-Fiber Active Lifting and Passive Holding
To demonstrate the “move and hold" potential of AVS fiber,
we lifted a 47 g mass with the AVS fiber and then removed
power to the system, allowing the fiber to passively hold theweight. As shown in Figure 9, we compared a programmed
SMA actuation wire with the PLA-based and ABS-based AVS
fibers. The bare SMA actuator wire is identical to the actuation

YUEN et al.: ACTIVE VARIABLE STIFFNESS FIBERS

713

wire in the AVS fibers. To set up the test, we clamped a 15 cm
segment of the fiber to a support, then hung a mass from the free
end of the fiber, applying a downward force. The bottom leadsof the fiber were attached to the weight making it a common
ground for the fiber. A small copper wire of negligible mass
(0.2 g) was attached to the weight and traveled with the weight
to maintain electrical contact.
For each test, after hanging the fiber, the NiTi wire washeated briefly to soften the variable stiffness thermoplastic(Figure 9(I)). Current was then applied to the SMA actuator
to lift the weight. Both the VS fiber and SMA wire were powered until the SMA actuator achieved maximal displacement(Figure 9(II)). At this point, all power to the fibers was shutoff, and the entire AVS fiber was allowed to cool. The AVS
fiber relaxation was recorded 15 seconds after the power was
shut off (Figure 9(III)) and 5 minutes later (Figure 9(IV)) when
the fibers had completely cooled. The SMA wire was tested
using this same procedure, without the initial step of heating up
a variable stiffness fiber prior to actuation.
Figure 9 presents the results of this test. The bare SMA wire
lifted the bolt higher than the AVS fibers by 1 cm, but oncethe power was disconnected the weight dropped 11 cm (85%)(Figure 9(a)). The PLA-coated AVS fiber lifted the bolt 12 cm,
but lost 7 cm (58%) of this initial displacement during cooling(Figure 9(b)). In comparison, the ABS-based fiber lifted the bolt
up 12 cm and then, upon cooling, only lost 4 cm (33%) of its
actuation height (Figure 9(c)).
According to the previously calculated spring constants, thedisplacement (the distance between actuation height and final
height) for each fiber type was predicted as 11 cm, 2.08 cm
and 0.89 cm for the SMA wire, PLA-based, and ABS-based
fibers, respectively. While the actual displacement of the SMA
wire was consistent with the predicted displacement, the actual
displacement of the AVS fibers was greater than predicted.
We attribute this discrepancy to the cooling and re-stiffening
period of the thermoplastic. As the AVS fibers cool down
(Figure 9(III)), the ABS is able to drop below its Tg and stiffen
even while the SMA is still at its activation temperature. In
contrast, the PLA remains soft while the SMA cools below
its activation temperature. With no lifting force, the weight
can stretch out the PLA-based fiber more than the ABS-based
fiber. This explains why both fibers dropped more than theirspring constants predicted, as well as why the PLA-based fiber
dropped an additional 5 cm when the ABS-based fiber dropped
only an additional 3 cm.
It is important to note that the bare SMA wire actuated significantly faster than the other two AVS fibers. Though we did
not record the data precisely, the overall activation time for the
SMA wire was 20-30s shorter than the tests for the AVS fibers(about 60-70s).
Fig. 9. Comparison of an actuation cycle of (a) an SMA wire, (b) a PLA-based
AVS fiber, and (c) an ABS-based AVS fiber. The top of the fiber is fixed anda 47 g mass is attached to the hanging end of the fiber. The mass acts as anelectrode, with a small copper wire attached to it. From top to bottom: (I) the
fiber is not actuated, (II) the fiber is softened and fully actuated, (III) all poweris turned off to the fiber, and it is allowed to cool for 15s, (IV) the fiber iscompletely cooled. For scale, each square is 1 cm2 .

B. AVS Fabric for Conformable Robotic Skins
The second test demonstrates the potential of these AVS
fibers to be used in robotic fabric. We sewed a long ABS-based
AVS fiber to a strip of cotton fabric, creating a fabric capable

714IEEE ROBOTICS AND AUTOMATION LETTERS, VOL. 1, NO. 2, JULY 2016

Fig. 10. AVS fabric attached to a cantilevered foam block to create a soft robotic lifting arm structure. One end of the foam block is fixed; the other end is freeand supports a 50 g mass. (a) AVS fabric is stiff, (b) AVS fabric is softened and fully actuated, (c) AVS fabric is stiff and cooling, and (d) Robotic arm structure
sustaining mass for extended period of time with no additional power input. Scale bars are 2 cm.

of manipulating objects from their surface. The AVS fiber
adds both structure and function the fabric, giving the fabric
the ability to mechanically manipulate objects (Figure 10).
Figure 1(a)–(b) shows the AVS fabric used in this demonstration in both a flattened and actuated state. The holding
capability of the ABS-based fibers from the first demonstration encouraged us to choose these fibers over the PLA-based
fibers in our final demonstration. We stitched the AVS fiber at
2 cm intervals in a serpentine pattern consisting of six lines.
This stitch length adequately secured the AVS fibers to thefabric without constricting the AVS fiber deformation during
actuation.
We attached the AVS fabric to the top of a foam block asshown in Figure 10(a) to simulate a soft robotic arm. The foam
block was fixed in a cantilever position with a 50 g mass suspended from the free end. In Figure 10(a), the fabric is stiffwith the mass causing the foam block to sag. After softening theVS fiber with 0.75A of current and actuating the programmed
SMA for 30s at 1.5A, the fully actuated state of the AVS fabricis attained (Figure 10(b)), lifting the mass 10 cm. Immediately
after full actuation, power was disconnected to the fiber and the
AVS fabric was left to cool. While the AVS fabric was coolingto a glassy, stiffened state, the spring-force of the curled foam
block and the weight of the 50 g mass pulled the foam arm partially down from the fully actuated position (Figure 10(c)). The
soft robotic arm structure then sustained the weight 5.75 cm
higher than its original position with no additional energy input,
for 2hrs. Figure 10(d) shows the structure holding the weight,
with the weight only dropping an additional 0.75 cm from the 2minute mark in Figure 10(c). This small drop in position can be
attributed both to creep in the polymer and the AVS fabric nothaving fully cooled at the 2 minute mark.
VII. C ONCLUSION AND F UTURE W ORK
In this work, we have demonstrated active variable stiffness
fibers for use in conformable robotic applications. The active
variable stiffness fibers are comprised of two functional elements intertwined: a variable stiffness fiber and an actuating
shape memory alloy wire. Both fibers are activated by thermalenergy generated by Joule heating. The active variable stiffness

fibers can actuate and sustain displacements even after powerhas been disconnected from the system.
We introduced active variable stiffness fibers using two different thermoplastics: PLA and ABS. Because the Tg of PLA
is below the SMA activation temperature, the variable stiffness effect is coupled to SMA actuation. In contrast, the Tgof ABS is higher than the SMA activation temperature, andthus the variable stiffness and actuation are decoupled. The
fibers were characterized for their change in stiffness and forforce production. We studied the behavior of individual AVS
fibers in performing a “move-and-hold" operation, demonstrating an improved holding capacity in the ABS-based due tothe decoupled stiffening and actuation. The ABS-based AVS
fibers were used to manufacture an active variable stiffness fabric by stitching the fibers onto cotton fabric. By attaching theplanar AVS fabric to a soft foam block, motion and stiffness
control was imparted upon the foam. Actuation of the fabriccaused the foam block to bend and lift a mass, then proceed tohold the deformation without continuous actuation via stiffening of the fabric. This demonstrated that the 1D capabilities ofmultifunctional fibers can be extended to 2D surfaces and 3D
structures.
Future work will focus on optimization of the manufacturing process and further characterization of the fibers. An
improved manufacturing framework would increase control
over the cross-sectional geometry of the fibers, allowing for
greater customizability and uniformity of the fibers’ function. Full characterization of the stiffness and force production
would enable a better understanding of the behaviors and functional coupling of the fibers. Additionally, future work may
explore the use of other materials, such as alternatives to thestiffening agent and encapsulation polymer, as it is possible
to tune the fibers precisely to structural and/or performancerequirements. Additionally, a parametric study to determine
the effect of different diameters of wire, thickness of polymer
coatings, and twisting pitch may contribute to future design
optimization. Future work will also explore the effect of different or more complex sewing patterns on the performance of
AVS fabrics, as well as the addition of sensing elements for proprioceptive feedback, enabling closed-loop control of a robotic
fabric.

YUEN et al.: ACTIVE VARIABLE STIFFNESS FIBERSACKNOWLEDGMENT
The authors would like to thank Edward White for assistancewith manufacturing and Prof. Kendra Erk (Purdue University)
for assistance with the materials selection. Any opinion,
findings, and conclusions or recommendations expressed inthis material are those of the authors and do not necessarily reflect the views of the National Aeronautics and Space
Administration or the National Science Foundation.
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