2204

IEEE ROBOTICS AND AUTOMATION LETTERS, VOL. 4, NO. 2, APRIL 2019

Morphing Robots Using Robotic Skins That
Sculpt Clay
Dylan S. Shah , Michelle C. Yuen , Liana G. Tilton, Ellen J. Yang, and Rebecca Kramer-Bottiglio

Abstract—Shape versatility is a mechanism that many animals
leverage to effectively interact-with and locomote-within the natural world. Toward the goal of shape-changing artificial systems,
we present morphing robots comprised of robotic skins and sculptable materials. Herein, we describe robotic skins—planar, skin-like
substrates with embedded actuation—that are wrapped around
sculptable materials in order to actively shape those materials into
different forms. Our approach is inspired by the art of sculpture,
where surface strains and pressures applied by hand allow clay to
be sculpted into nearly any desired shape. Replacing hands withrobotic skins, we achieve morphing capabilities in a robotic system.
We focus on an example robot in which two robotic skins are layered on a base sculptable material to induce both locomotion andmorphing behaviors, and show that morphing enables the robot
to overcome obstacles during locomotion. This letter is the first
instantiation of morphing robots based on sculpture-inspired surface manipulation of sculptable materials, where shape-changing
capabilities are expected to improve robot adaptability to meet thedemands of changing environments, overcome obstacles, or perform variable tasks.
Index Terms—Soft material
biologically-inspired robots.

robotics,

flexiblerobots,I. INTRODUCTIONOME biological organisms are able to adjust their bodystructure and behavior to accommodate a variety of environmental demands and external perturbations. For example,

S

Manuscript received September 10, 2018; accepted January 30, 2019. Dateof publication February 27, 2019; date of current version March 11, 2019. Thisletter was recommended for publication by Associate Editor L. Wen and Editor K.-J. Cho upon evaluation of the reviewers’ comments. This work was
supported by the National Science Foundation (NSF) under the EFRI program(EFMA-1830870). The work of D. S. Shah was supported by a NASA SpaceTechnology Research Fellowship (80NSSC17K0164). The work of M. C. Yuen
was supported by a NSF Graduate Research Fellowship (DGE-1333468). The
work of E. J. Yang was supported by a National Aeronautics and Space Administration (NASA) Connecticut Space Consortium Grant (NNX15AI12H).
(Corresponding author: Rebecca Kramer-Bottiglio.)
D. S. Shah, L. G. Tilton, E. J. Yang, and R. Kramer-Bottiglio are with the Department of Mechanical Engineering and Materials Science, School of Engineering and Applied Science, Yale University, New Haven, CT 06520 USA (e-mail:,dylan.shah@yale.edu; ltilton19@students.hopkins.edu; ellen.yang@yale.edu;
rebecca.kramer@yale.edu).
M. C. Yuen is with the Department of Mechanical Engineering and Materials Science, School of Engineering and Applied Science, Yale University, New Haven, CT 06520 USA, and also with the School of Mechanical Engineering, Purdue University, West Lafayette, IN 47907 USA (e-mail:,michelle.yuen@yale.edu).
This letter has supplementary downloadable material available at
http://ieeexplore.ieee.org, provided by the authors. The video shows a morphing robot that uses shape-change to overcome an obstacle during locomotion.
For more information, contact rebecca.kramer@yale.edu.
Digital Object Identifier 10.1109/LRA.2019.2902019

Fig. 1. A morphing robot is created by wrapping robotic skins around a sculptable material. This schematic shows the construction of a robot that changes its
shape to overcome an obstacle during locomotion. The example robot comprises
two robotic skins: a cable-driven morphing skin for shape-changing behavior,
and a pneumatic locomotion skin for rolling locomotion behavior.

amphibians can regenerate amputated limbs or remodel after
structural alteration [1], and metamorphosing creatures suchas the tadpole/frog or caterpillar/butterfly can reconfigure theirmorphology to fit completely different environmental life-styles
and operational needs.
Unlike their biological counterparts, current artificial systems(robots) are typically purpose-built for a specific function or finite collection of functions, and have therefore seen limited
transition into the natural, unstructured world. The burgeoning
use of soft materials in robot constructs (see reviews [2]–[4])has shown increased resiliency to damage [5], and clever designs have resulted in examples of soft robots that can use theirstructure to attain multiple locomotion gaits [6], [7]. However,in many cases, robot adaptability requires a change in morphology. Modular and reconfigurable designs have been suggested[7]–[10], although these systems reconfigure via removal andre-addition of parts relative to each other, and are often limited
to a discrete range of potential configurations defined by theconnections available to the modules. Herein, we introduce arobot that is self-morphing, employing robotic skins that transform the morphology of a constant-volume sculptable materialusing surface actuation.

2377-3766 © 2019 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.
See http://www.ieee.org/publications standards/publications/rights/index.html for more information.

SHAH et al.: MORPHING ROBOTS USING ROBOTIC SKINS THAT SCULPT CLAY

We previously introduced the concept of robotic skins thatmanipulate passive soft bodies [11], which is complementary
to the mainstream approach of passive skins with active bodies. Robotic skins are two-dimensional, skin-like, lightweightsubstrates with embedded actuators, which can be applied-to,
removed-from, and transferred-between the surface of any passive soft body to impart motion onto that body. In our prior work,
we attached robotic skins to a variety of elastically deformablebodies – such as inert foams, passive tensegrity structures, andhuman limbs – and generated a wide range of functions – including manipulation, locomotion, and active wearables. In each ofthese cases, the deformable body produced a restoring force,
returning to its original shape after the skin relaxed. Here, we
extend the robotic skin concept to moldable, sculptable bodies,treating the robotic skins as surface-based sculptors of the underlying body material to achieve morphing capabilities. Oncemorphed, the underlying material passively maintains its shape
via stress-relaxation, allowing the skin to de-activate and return
to a low energy-consumption state.
In this letter, we introduce the concept of morphing robots
based on wrapping robotic skins around sculptable material(Fig. 1). To prove the utility and feasibility of this concept,
we demonstrate a robot that changes its morphology to overcome an obstacle during locomotion. The example robot comprises two robotic skins layered onto each other and fixed to asculptable material body. The morphing skin is cable-driven andcontrols morphing behavior, sculpting the core material into various radially-symmetric shapes such as spheres, cylinders, anddumbbells. The locomotion skin contains thin, pneumatic bladders to produce rolling locomotion when inflated sequentially.
We explore several candidate sculptable materials by experimentally characterizing their behavior under compression, andcompare these results to in situ morphing operations to derive
insights on the interplay between the core material behavior anddesign of the morphing layers and actuation schemes.
II. MATERIALS AND MANUFACTURING
In order to achieve both locomotion and morphing behaviors,
we fabricated two robotic skins – one for each behavioral task
(which we refer to as the morphing skin and the locomotionskin). These functional layers can be combined on top of one
another, or even integrated by sewing or gluing.
A. Morphing Skin
When a human sculptor shapes clay, force is often exerted
downward onto the clay against a tabletop. However, this technique cannot be employed by a lightweight robotic skin since
its inertia is not large enough to stabilize itself. Alternatively,
circumferential squeezing is attainable since each force has anequal and opposite counterpart, all directed toward the center.
Potential methods of applying a circumferential squeezing
force include Pneunets [12] and PneuFlex bending actuators[13] and linearly contracting McKibben actuators [14].
However, we found that these bending actuators require acircumferential restraining layer to push the actuator into
the body. McKibben actuators, with their length wrapped

2205

Fig. 2. Fabrication of the cable-driven morphing skin (left) and pneumatic
locomotion skin (right).

around the circumference, have a tendency to flatten the clay
rather than squeeze circumferentially. The most promisingsolution to-date, presented here, uses cables wrapped around asculptable body to create radially-symmetric forces.
The morphing skin consists of multiple cables stitched inparallel on a Spandex fabric substrate (Fig. 2, left). First, theshorter edges of the fabric, corresponding to the termini of thecables, were reinforced with silicone elastomer (DragonSkin
10, Smooth-On). Paracord (1.18 mm microcord, Atwood RopeMfg.) was then stitched into the fabric (15 × 20 cm) at a stitch
spacing of 0.75 cm, and cable-to-cable spacing of 0.75 cm, fora total of 20 cables. 1 cm fabric snaps were attached along theshorter edges of the fabric, in the reinforcement tabs, to allow
the skin to be securely wrapped around the clay.
Practical morphing skin design requirements include cable tensile strength, sewability, and appropriate cable spacing.
While it may be advantageous to increase cable density to improve the resolution of morphing, as the number of cables increases, issues arising from cable management, cable-on-cable
friction, and complexity of associated hardware render the system infeasible. In contrast, sparse cable spacing can result in local pinching and an inability to translate the actuation to global
deformation of the clay body. Thus, the design of the morphingskin is driven by compatibility with the material properties of
the chosen clay.
During morphing operations, cables of the morphing skinwere pulled using a spooling motor (Fig. 3A, B). As shown in
Fig. 3B, the cables were fed through sheaths composed of stiff
tubing in bundles of five, effectively creating Bowden cables.
Five holes were drilled into the tubing, allowing each cable tobe fed in with minimal tangling; the holes at the surface of thesheath rested directly against the surface of the morphing skinduring contraction. The free ends of the cables were attached2206IEEE ROBOTICS AND AUTOMATION LETTERS, VOL. 4, NO. 2, APRIL 2019

Fig. 3. Photos of the skins and off-board, back-end hardware used to actuateeach skin. (A) Photo of the morphing skin. (B) A servomotor spools multiple
cables routed through a Bowden cable sheath. (C) Photo of the locomotion skin.
(D) A set of 8 pneumatic regulators controls the inflation and deflation of the
bladders.

to a spool wound by a servo motor (MX-106R, Dynamixel).
During contraction, the free end of the sheath rested directlyagainst the motors’ base plates, causing the sheath to undertakethe compressive load required to morph the clay.
B. Locomotion Skin
A wide range of robot locomotion strategies exist, including– but not limited to – quadrupedal walking [15], rolling (driven
by either controlled shifting of center of mass, or localized
extrusion of material) [16], [17], and caterpillar-inspired gaits[7]. Here we exploit rolling locomotion due to its compatibility
with a wide range of geometries, enabling the robot to utilize
similar gaits across many morphologies.
To achieve locomotion, a series of external-facing, parallel,
planar, pneumatic actuators were fabricated in a silicone elastomer film (Fig. 2, right). Silicone elastomer (Dragon Skin
10, Smooth-On) was rod-coated onto PET film to create a
20 × 15 × 0.2 cm layer. After curing, the bladder patterns
were lasercut into the PET film, leaving the silicone layer beneath uncut. The portions of the PET film not corresponding to
the bladders were peeled away to create a mask for the bladders.
Another 0.2 cm thick layer of silicone was then rod-coated overthe silicone and PET film masks to form the bladders. Upon
curing, excess silicone was trimmed from the edges of the skin,
exposing the ends of each of the PET bladders. These piecesof PET were removed by opening up the bladders with reverse
tweezers, spraying ethanol into the pocket to release the PET
from the silicone, and then pulling out the PET film with tweezers. After allowing the ethanol to evaporate from the bladder,silicone tubing (1/32” ID, McMaster-Carr) was inserted into
each bladder and adhered to the skin using silicone elastomer.
Small squares of muslin were dipped in silicone and placed on
the edges of the skins as attachment points for safety pins that
secured the edges of the skin while wrapping it around the clay
body.
Various bladder layouts can be fabricated. We considered
three layout designs: one, two, and many rows of bladders (arow runs horizontally across the skin as shown in Fig. 2). For the

Fig. 4. The morphing skin sculpts clay into different shapes. (A) A sphere ofclay is morphed into a cylinder using a morphing skin with cables wrapping
around the circumference of the clay. The sphere is morphed into a cylinder by
sequentially pulling the cables, starting from the innermost and moving outward.
(B) Longitudinally arranged cables are used to morph a dumbbell shape (i.e., acylinder with an on-demand joint) to an inchworm shape, generating a bending
motion about the generated hinge.

single-row design, the shape of the bladder, as well as defect-freefabrication, is more critical as any asymmetry will result in uneven inflation and thus unintended turning. With the double-row
design, asymmetry is less of an issue because each actuator only
operates at the ends of the skin. Beyond two rows of bladders,routing the inflation tubes without significant tangling becomesincreasingly complex and prohibitive. In this letter, we focus
on a design with two columns of parallel teardrop bladders asshown in Fig. 2. The inflation and deflation of each bladder
was controlled using digital pneumatic regulators as presented
in [18] (Fig. 3C, D).
III. ROBOTIC SKIN OPERATION
A. Morphing
Morphing behavior was achieved by wrapping the morphing skin around a sculptable body, such as clay. By pulling ona cable, radial contraction of the clay was attained (Fig. 4A).
Each cable can change the radius of the circular cross-section it
surrounds, so the resulting shapes span all radially-symmetricshapes, such as cylinders, dumbbells, and cones. By positioning cables longitudinally, bending motions can also be attained
for shape morphing needs or for inchworm-inspired locomotion (Fig. 4B). While the current work does not demonstrate
reversible morphing, preliminary work has suggested that thiscan be achieved through the addition of inextensible endcaps tothe morphing skin, and layering of morphing skins with orthogonal cable directions.
B. Locomotion
When wrapped around the clay body, the locomotion skins
operate by sequentially inflating their bladders to continuously

SHAH et al.: MORPHING ROBOTS USING ROBOTIC SKINS THAT SCULPT CLAY

Fig. 5. The locomotion skin operates by inflating the trailing-edge bladder,pushing the robot forward. At an inflation pressure of ≈3 psi, a half rotation
takes ≈30 s. (A) Only the locomotion skin is wrapped around clay, showing
the bladder expansion. (B) The morphing skin is wrapped over the locomotion
skin, showing that rolling can still occur when multiple skins are layered.

tip over, creating a rolling motion (Fig. 5A). Neglecting friction,
angular acceleration is proportional to torque (equivalently, tangential force from the bladders) and the mass moment of inertia
as α = T /J. Therefore, at low accelerations, rolling a chunkof clay can be done with relatively low forces. Here, we foundthat even small pressures of ≈3 psi were sufficient to inflate
the bladders quickly and forcefully enough to be practical inreal-time. Higher acceleration could be obtained by integratingfibers perpendicular to the bladders’ axes to allow operation atincreased pressure, as previously demonstrated by the transition
from PneuNets [12] to PneuFlex actuators [13].
C. Combined Morphing and Locomotion
Multiple robotic skins can be layered and fixed onto a sculptable body to achieve their corresponding behaviors in a single
system. Here, we layer the morphing skin and the locomotion
skin to realize a morphing, locomoting robot. With current hardware, placing the morphing skin underneath the locomotion skinleaves the locomotion skin slack during low-diameter configurations. Thus, to maximize the range of attainable shapes, themorphing skin is used as the outermost layer (Fig. 5B). This
arrangement has other benefits: it prevents interaction between
the cables and clay, it prevents the clay from drying out, thelocomotion skin promotes uniform morphing between cables,
and the morphing skin prevents the locomotion skin’s bladders
from over-inflating. We further note here that during the dozens
of tests performed in this study, the skins only failed due to operator error (for example, commanding the robot to over-inflatea silicone bladder).
IV. DESIGNING A MORPHING ROBOT SYSTEM
When designing a morphing robot system, it is critical tomatch the forces produced by the morphing skin with the2207

forces required to deform the sculptable body material, among
other considerations. Therefore, we sought to identify candidate
sculptable materials that: (1) have radial deformation force requirements that can be reasonably produced by a robotic skin;(2) exhibit stress-relaxation, to allow the robot to return to a lowpower state after morphing; (3) maintain cohesion over large
strains, to allow the robot to undergo significant shape-change
without fracturing.
We selected materials by first qualitatively determining thefeasibility of several candidate clay-like materials spanning awide range of material properties: Model Magic modeling compound (Crayola, Pty. Ltd.), Play-Doh modeling compound (Hasbro, Inc.), Protolina No. 1 soft modeling clay (Van Aken International), CoolSand moldable sand (Lisse Inc.), and Sands Alive
sand (MCA Sand Co. Ltd.). From this initial assessment, wechose Model Magic for its low stiffness and cohesiveness. We
then performed in situ tests wherein we deformed a section ofa cylindrical billet of Model Magic in a morphing skin using
a materials testing machine (Instron 3345) at various compression rates, verifying that commercially available servomotors
are suitable for morphing the clay. Seeking a simpler method
to characterize potential core materials, we then performed astandard unconfined compression test and an in situ test on eachcandidate material, to determine if and how a simple, standardized unconfined compression test could be used to determine
design specifications for the morphing system.
A. Morphing Skin Force Characterization
After initially selecting Model Magic as the “clay,” we measured the amount of force required to deform the clay by pullingthe cables of the morphing skin wrapped around the clay. For
each test, a billet of Model Magic was hand-shaped into a cylinder with initial diameter 6 cm (chosen so the morphing skin wasslightly pre-strained when attached) and initial length 15 cm.
The morphing skin was wrapped around the cylinder and enclosed in a polycarbonate tube which was secured to the base of
the materials testing system. To morph a region of the clay, a bundle of four cables, corresponding to a width w of 2.25 cm, wasattached to the load cell and pulled to a 100 mm displacement
at five different rates (80, 160, 320, 640, and 1000 mm/min),for five samples each, using the same skin for all tests. This
100 mm change in circumference corresponded to a final diameter of ≈4.4 cm within the squeezed region, and a final diameterof ≈6.2 cm in the region where the cables were not pulled.
Note that the diameter predicted by the 100 mm contraction is
2.8 cm. The difference between this prediction and the actual
squeezed diameter is primarily due to the clay bulging outward
between the cables. Though this inefficiency can be potentiallyminimized by increasing cable density, manufacturability andcable management would be adversely affected.
At a first estimation, we made some simplifying assumptions: (1) each test can be treated as quasi-static; (2) the clay
deformation is radially symmetric; (3) the surface pressure in
the squeezed region is homogeneous. Because morphing skins
are dynamically modifying their host body, the clay will, in
reality, experience non-uniform and time-variant stress. While2208

IEEE ROBOTICS AND AUTOMATION LETTERS, VOL. 4, NO. 2, APRIL 2019

Fig. 7. The stress-strain curves for several candidate materials under an unconfined compression test. Each curve shows the mean and ±1 standard deviation
across five tests.

nonzero, but small relative to the overall mechanical response.
Therefore, during design of the motor, gearing, and spoolingsystem, there is flexibility in choosing the morphing speed. The
force experienced at the full extent of compression was ≈70 N
at 80 mm/min, and ≈97 N at 1000 mm/min. With a spool ofradius 2 cm attached directly to the motor shaft, 1.4 Nm of
torque was required to morph the clay at a rate of 80 mm/min.
In comparison, the Dynamixel MX series servomotors utilized
here have advertised stall torques ranging from 2.4 to 8.4 Nm.
Fig. 6. Plot of the stress vs. strain behavior during in-situ radial compression
tests. Each curve shows the mean and ±1 standard deviation across five tests.
(A) Stress-strain curves for all strain rates, showing significant overlap acrossall speeds. (B) Stress-strain curves for the lowest and highest strain rates. The
schematic shows the experimental setup conceptually.

understanding the magnitude and nature of this stress distribution could potentially lead to increased morphing efficiency, thebehavior is material-specific and beyond the scope of this letter. Our assumptions allow us to approximate the normal stressacross the outer cylindrical shell as a function of strain in theradial direction from the force vs. extension data measured in
the in situ morphing skin tests (Fig. 6). In each test, as the cables
are pulled to displacement y, the skin presses inward radially sostrain  and stress σ are expressed by the following:=c1 − c2yr1 − r2
=
=
r1
c1
c1

(1)

σ=

F
F
=
2πr2 w(2πr1 − y)w(2)

where r1 and r2 refer to the initial and instantaneous radii of thesqueezed region, c1 and c2 refer to the initial and instantaneous
circumferences, F is the measured load, and w refers to thewidth of the cylindrical region being contracted.
The behavior at the various strain rates was similar enoughthat the confidence intervals overlap significantly (Fig. 6A). For
clarity, two representative rates – the slowest and fastest speeds
tested – are plotted separately (Fig. 6B). The rate dependence is

B. Clay Characterization
After verifying that morphing the clay is possible with theservomotors and is rate-independent over a range of tested
speeds (80–1000 mm/min), we then posed the question: “For agiven sculptable material, would a simple compression test giveenough information to design a compatible morphing skin?” To
address this question, we characterized the behavior of all fivecandidate sculptable materials with an unconfined cylindricalcompression test [19], [20]. Each test specimen was shaped ina cylindrical mold with a diameter of 6 cm and height of 6 cm
and set in between compression plates mounted to the crosshead
and base of the materials testing system (Fig. 7). Fracturing canbe seen in the increased standard devaiation of sandy materials,
particularly for Sands Alive at ≈15% strain. For water-basedmaterials (Play-Doh, Model Magic), new samples were usedduring testing to minimize the effect of drying. The material
was compressed at a rate of 12.73 mm/min to match the strain
experienced in a morphing operation (with cables pulled at at
80 mm/min; c = 2πr). Since motor torque decreases with angular velocity, a slower morphing rate, such as the one used here,
allows for less demanding motor requirements.
To attain the stress vs. strain relationship, we assume that theclay samples maintain constant volume and an approximately
cylindrical shape. With an initial geometry of r1 , h1 , and instantaneous geometry of r2 , h2 , we can determine the strain by2and the average surface stress in the morphing re = h 1h−h
1gion σav g by the measured load F divided by the instantaneous

SHAH et al.: MORPHING ROBOTS USING ROBOTIC SKINS THAT SCULPT CLAY

2209

Fig. 9. Stress vs. strain behavior of candidate materials during in situ radial
compression tests. The material specimens were prepared as in Fig. 8. The cablesof the morphing skin were pulled at 80 mm/min as in Fig. 6B. Representative
curves for each material are plotted.
Fig. 8. The clay was first wrapped in the locomotion skin (A-B), followed by
the morphing skin (C–D).

contact area, as displacement y is applied:σav g =

FF
= πr2 h
1 1
πr22

(3)

h 1 −yIdeally, the stress-strain curves obtained under this standard
test (Fig. 7) should match the stress-strain behavior experienced
when using a morphing skin. If so, simple compression tests
would enable calculation of the motor torques required to morph
the material. However, we found that the stress experienced
by the materials during in situ tests is significantly higher than in
the unconstrained compression test. For Model Magic, the stressat 50% strain during in situ tests was ≥15 kPa, in comparison
to just ≈4 kPa during the unconstrained tests. We hypothesizethat the discrepancy is mainly due to presence of material neighboring the morphing region, which itself experiences circumferential tension imposed by the morphing skin wrapped aroundthese adjacent sections. Despite these limitations, we found thissimple, unconfined compression test to be an efficient tool toquickly compare and evaluate feasibility of candidate materials.
For example, the simple compression tests revealed that Protolina (an oil-based technical clay) experiences stresses at least
an order of magnitude larger than the other materials, and wetherefore removed Protolina from further consideration.
To gain insight into the mechanics of the final multi-skin system, in situ morphing tests were performed on the remaining
candidate materials by wrapping the material with the locomotion skin followed by the morphing skin, as seen in Fig. 8. In
this configuration, the sand-based materials (Sands Alive, CoolSand) had a tendency to exhibit granular jamming at moderatestrains (≈15% radial strain), resulting in a sharp increase instress with added strain. In contrast, the water-based materials(Play-Doh, Model Magic) did not exhibit jamming and requiredconsistently less stress to achieve the same degree of morphing, making them more viable materials for the morphing robot
system (Fig. 9). Ultimately, Model Magic was easiest to workwith for logistical reasons (did not stick to fabric, and exhibiteda slight spring-back effect), and was therefore selected.
We also characterized the materials when wrapped in only
the morphing skin (without the underlying locomotion skin).
During this characterization, the sand-based materials did notexhibit jamming behavior. We hypothesize that the direct contact between cables in the morphing skin and the underlying
body material initiating cracking. With the additional locomotion skin base-layer, the material is under uniform external pressure, inducing the jamming effect. Cumulatively, these in situ
tests highlight the intricate interplay between robotic skins andthe materials they are intended to deform or morph.
C. Scalability of Morphing Robots
To design morphing robots at different length scales, changes
in motor requirements must also be considered. Consider twomorphing robots, robot A and robot B, with the same geometric proportions but at different length scales (initial radii
r1A = Kr1B , instantaneous radii r2A = Kr2B , and morphing
region wA = KwB , where K is an arbitrary scaling constant).
Both morphing robots are driven by the same DC motor at a fixedvoltage and the same sized cable spool rspool (i.e., same torqueoutput Tm otor and applied force Fm otor = Tm otor /rspool ).
While morphing at the same cable spooling rate ẏ, the average surface stress in the morphed region is:σA =Fm otorFm otor1== 2 σB
πr2A wA
πKr2B KwB
K

(4)

Thus, robot B is able to impart 1/K 2 times as much surface
stress onto the clay as robot A. If the morphing speed changes
such that the strain rate is held constant, the torque-speed availability of the motors must be considered. Assuming an ideal
DC motor which has an affine torque-speed curve connecting
stall torque Ts and no-load angular velocity ΩN L , as the cable pull rate increases (y˙A = K y˙B ), the available torques, andcorresponding surface forces, decrease as:TA = 1 − KΩ /Ω
B

NL

1 − ΩB /ΩN L



TB

(5)

Therefore, with fixed motor and spool parameters, higher
surface stress (and range of morphing strains) can be attainedby scaling down the robot dimensions. However, scaling down
introduces challenges in routing cables from the skin to themotors.

2210

IEEE ROBOTICS AND AUTOMATION LETTERS, VOL. 4, NO. 2, APRIL 2019transmitters. With the ability to morph clay, it would be possible
to traverse flat terrain quickly with a large-diameter cylinder.
Upon encountering more constrained pathways, the clay could
be morphed into a small profile. Here, we showed the robot in amorphed dumbbell shape that enables it to straddle an obstacle
and continue rolling. Throughout these contortions, the sensors
and transmitter payload would remain held in the clay body.
More broadly, applications include resource-limited conditions
where supply chains for materiel are sparse. For example, themorphing sequence shown in Fig. 4 could be used to transformfrom a rolling sphere to a pseudo-jointed robotic arm. With sucha morphing system, it would be possible to robotically morph
matter into different forms to perform different functions.
VI. DISCUSSION

Fig. 10. Layering the morphing skin over the locomotion skin allows the robot
to morph to complete tasks such as rolling over an obstacle.

V. APPLICATIONS
By wrapping both the locomotion and morphing skins arounda cylinder of clay (Fig. 8), both locomotion and shape-changing
behaviors were achieved. The utility of this approach wasdemonstrated via an example where the locomoting, morphingrobot encountered an obstacle in its path (Fig. 10). First, the initially cylindrical robot rolled on a flat surface (Fig. 10A). Upon
encountering an obstacle and failing to roll over it (Fig. 10B),
the cylinder was morphed into a dumbbell shape, which allowed
the robot to straddle the obstacle (Fig. 10C). Forward locomotion then proceeded, unencumbered by the obstacle (Fig. 10D,
E).
Applications where morphing and locomotion might serve ascomplementary functions are abundant. For the example skins
presented in this work, a search-and-rescue operation could use
the clay as a medium to hold a payload such as sensors orThis letter presents a class of sculpture-inspired robots withthe ability to shape-shift on-the-fly, which will enable adaptability in environments with uncertain or variable conditions. The
salient criteria of this class of morphable robots are: (1) The
robotic skin driving the morphing ability must be well-matchedto the material properties of the sculptable body (i.e., the stressgenerated by the morphing skin exceeds the stiffness of the clay);(2) Any robotic skin operating on a shape-changing body musthave distributed actuation and be highly flexible/stretchable tomaintain functionality across a wide range of body shapes; (3)The robotic skins must be able to function even while layeredon top of each other for the robot to access each of the corresponding behaviors (e.g., shape-change and locomotion).
The primary aim of this work is to present the idea of using
passive clay as the body of a robot, and applying robotic function at the surface to sculpt the body into arbitrary shapes. Asa first step towards this vision, we demonstrated morphing via
motor-driven spooling of cables wrapped around sculptable material (i.e., “clay”). We characterized various types of clay-likematerials to understand their mechanical response to appliedcompressive strain and found that unconfined compression tests
are only useful for qualitative comparisons between materialsof similar granular structure. Rather, in situ force-displacement
tests yield more accurate robot characterizations and can be used
to select appropriate body materials and robotic skin actuators
for this class of robots.
The robotic skins presented in this letter utilize inherently
stretchable substrates. By attaching the skins with pre-strain,
the skins maintain contact with the clay body, distributing stressacross the surface of the clay over a range of body shapes.
Additionally, layering the two skins we showed in this letter does
not restrict the independent skin functions, and in some ways
enhances them. In the obstacle avoidance example shown in
Fig. 10, though the locomotion skin is wrapped around the clay
underneath the morphing skin, the stretchability of the morphingskin’s Spandex can accommodate inflation of the locomotion
skin’s bladders. Similarly, the locomotion skin’s elastomer isresilient to the compression applied by the morphing skin and isable to stretch to accommodate the larger diameter at the endsof the dumbbell.
Furthermore, while we showed only two skins in this work,
more skins with other functions could be layered and integrated

SHAH et al.: MORPHING ROBOTS USING ROBOTIC SKINS THAT SCULPT CLAY

2211

into a single, multifunctional robot. However, as the number ofskins increases, logistical issues such as cable management andskin interactions must be considered.
While we demonstrated morphing between various radiallysymmetric shapes, the concept of surface-driven shape-change
can extend to asymmetric deformation (e.g., contraction or bending along the length of the clay as shown in Fig. 4B). Returning
to our inspiration, we note that sculptors use pinching and smearing motions to create fine features and complex shapes. For arobotic system, incorporating a mechanism to press small bending actuators against the sculptable material would be one possible way to achieve this motion. Other future work includes evaluation of further candidate materials to measure compatibility
with both large-scale shape-changes and finer sculpting motions.

[2] D. Rus and M. T. Tolley, “Design, fabrication and control of soft robots,”
Nature, vol. 521, no. 7553, pp. 467–475, May 2015.
[3] C. Laschi, B. Mazzolai, and M. Cianchetti, “Soft robotics: Technologies
and systems pushing the boundaries of robot abilities,” Sci. Robot., vol. 1,
no. 1, 2016, doi: 10.1126/scirobotics.aah3690..
[4] S. Kim, C. Laschi, and B. Trimmer, “Soft robotics: A bioinspired evolution
in robotics,” Trends Biotechnol., vol. 31, no. 5, pp. 287–294, May 2013.
[5] M. T. Tolley et al., “A resilient, untethered soft robot,” Soft Robot., vol. 1,
no. 3, pp. 213–223, Aug. 2014.
[6] R. F. Shepherd et al., “Multigait soft robot,” Proc. Nat. Acad. Sci.,
vol. 108, no. 51, pp. 20 400–20 403, Dec. 2011.
[7] J. Zou, Y. Lin, C. Ji, and H. Yang, “A reconfigurable omnidirectional soft
robot based on caterpillar locomotion,” Soft Robot., vol. 5, pp. 164–174,
Jan. 2018.
[8] S. W. Kwok et al., “Magnetic assembly of soft robots with hard components,” Adv. Functional Mater., vol. 24, no. 15, pp. 2180–2187, 2014.
[9] S. A. Morin et al., “Using click-e-bricks to make 3D elastomeric structures,” Adv. Mater., vol. 26, no. 34, pp. 5991–5999, Sep. 2014.
[10] M. A. Robertson and J. Paik, “New soft robots really suck: Vacuumpowered systems empower diverse capabilities,” Sci. Robot., vol. 2, no. 9,
2017, doi: 10.1126/scirobotics.aan6357.
[11] J. W. Booth et al., “OmniSkins: Robotic skins that turn inanimate objects
into multifunctional robots,” Sci. Robot., vol. 3, no. 22, Sep. 2018, doi:
10.1126/scirobotics.aat1853.
[12] F. Ilievski, A. D. Mazzeo, R. F. Shepherd, X. Chen, and G. M. Whitesides, “Soft robotics for chemists,” Angewandte Chemie, vol. 123, no. 8,
pp. 1930–1935, Feb. 2011.
[13] V. Wall, R. Deimel, and O. Brock, “Selective stiffening of soft actuators
based on jamming,” in Proc. IEEE Int. Conf. Robot. Autom., May 2015,
pp. 252–257.
[14] F. Daerden and D. Lefeber, “Pneumatic artificial muscles: Actuators for
robotics and automation,” Eur. J. Mech. Environmental Eng., vol. 47, no. 1,
pp. 11–21, 2002.
[15] A. Sprwitz, A. Tuleu, M. Vespignani, M. Ajallooeian, E. Badri, and A. J.
Ijspeert, “Towards dynamic trot gait locomotion: Design, control, and experiments with Cheetah-cub, a compliant quadruped robot,” Int. J. Robot.
Res., vol. 32, no. 8, pp. 932–950, 2013.
[16] S. Kriegman, N. Cheney, and J. Bongard, “How morphological development can guide evolution,” Sci. Rep., vol. 8, no. 1, Sep. 2018, Art.
no. 13934.
[17] E. Steltz, A. Mozeika, N. Rodenberg, E. Brown, and H. M. Jaeger, “JSEL:
Jamming skin enabled locomotion,” in Proc. IEEE/RSJ Int. Conf. Intell.
Robots Syst., Oct. 2009, pp. 5672–5677.
[18] J. W. Booth, J. C. Case, E. L. White, D. S. Shah, and R. Kramer-Bottiglio,“An addressable pneumatic regulator for distributed control of soft robots,”
in Proc. IEEE Int. Conf. Soft Robot., Apr. 2018, pp. 25–30.
[19] S.-S. Park, “Effect of fiber reinforcement and distribution on unconfinedcompressive strength of fiber-reinforced cemented sand,” Geotextiles Geomembranes, vol. 27, no. 2, pp. 162–166, Apr. 2009.
[20] S. R. Kaniraj and V. G. Havanagi, “Compressive strength of cement stabilized fly ash-soil mixtures,” Cement Concrete Res., vol. 29, no. 5, pp. 673–
677, May 1999.

VII. CONCLUSIONS
Typically, roboticists need to know the functions that a robot
is meant to accomplish a priori, and this set of functions defines
the shape of the robot. In order to accomplish a broader rangeof tasks or to be able operate in environments with unknown orhighly variable conditions, one approach is to allow the robot toadapt its shape.
In this letter, we have presented robotic skins that wrap around
sculptable material as a way to create shape-changing robots.
Pairing morphing and locomotion behaviors together illustrates
the potential of morphing robots that derive versatility fromtheir ability to adapt their shape. The robotic skins were fabricated as stretchable planar substrates with embedded actuation.
The morphing skin used cables to squeeze the core material
into different shapes, and the locomotion skin utilized sequential inflation of bladders to roll the system. We showed how
mechanical characterization of the sculptable material can beused to inform the actuation requirements of the morphing skin.
Finally, we demonstrated an application where locomotion was
posited as the performance task, and morphing was required toovercome an obstacle.
REFERENCES[1] D. Lobo, E. B. Feldman, M. Shah, T. J. Malone, and M. Levin, “Limbform:
A functional ontology-based database of limb regeneration experiments,”
Bioinformatics, vol. 30, no. 24, pp. 3598–3600, 2014.