2024 7th International Conference on Soft Robotics (RoboSoft)San Diego, CA, USA. April 14-17, 2024

Performance Enhancement of a Morphing Limb
for an Amphibious Robotic Turtle

2024 IEEE 7th International Conference on Soft Robotics (RoboSoft) | 979-8-3503-8181-8/24/$31.00 ©2024 IEEE | DOI: 10.1109/ROBOSOFT60065.2024.10521924

Jiefeng Sun, Brandon Lin, Luis A. Ramirez, Esteban Figueroa,
Robert Baines, Bilige Yang, Erick Marroquin, and Rebecca Kramer-Bottiglio(a)Abstract— Terrestrial and aquatic animals exhibit appendages adapted to the propulsion physics of their primary
habitats. Terrestrial appendages typically assume stiff andload-bearing form factors, while aquatic appendages tend to
adopt flexible and streamlined profiles. Bio-inspired robots
with synthetic appendages often mirror this dichotomy of
specialization: they are designed with fixed legs or flippers
for locomotion on land or in water, respectively. Appendages
that adjust their shape and stiffness can serve to specializea robot’s propulsion physics on demand, enabling transitions
between multiple environments. Herein, we report a morphinglimb combining layer jamming and pouch-based pneumatic
actuation that rapidly and efficiently switches between a flexible
flipper for swimming and a rigid leg for walking/crawling.
The internal pouch actuator contributes to pressure that jams
the external layers of the limb, which we refer to as positive
pressure-reinforced jamming. We quantify the extent of shapemorphing conferred by the pouch actuator, the maximum
load-bearing capability of the limb in leg mode, and thehydrodynamic characteristics of the limb in flipper mode. We
find that the new limb boasts better performance than previous
designs with respect to morphing shape, speed, efficiency, andhydrodynamics. Crucially, we also find that positive pressurereinforced jamming increases the leg’s compressive strength by
30% relative to just jamming the layers via negative pressure.
With its own lightweight and compact electronics system, themorphing limb is a plug-and-play component for building an
untethered multi-environment robot.

Morphing limb(b)Flipper mode

5 cm

Leg mode

9 cm

Portable pneumatic system

Fig. 1. (a) An amphibious robotic turtle with a morphing limb for multienvironment locomotion. (b). The morphing limb can transition between aflipper shape and a leg shape. (c) A portable pneumatic system facilitates
tether-less operation of the limb’s morphing process.

I. I NTRODUCTION
Adapting robot limb morphology has proven to be a viablestrategy to navigate diverse terrains and transition between
different physical environments (i.e., water vs. land) [1], [2].
The optimization of morphing limbs is a complex challenge,
due to the different shapes and stiffnesses required for effective locomotion in different environments [3]. For example,
in water, limbs should ideally function as hydrodynamic
surfaces, sustaining distributed forces while accommodatingpressure gradients created by propulsive movements [4], [5].
On land, limbs should support the robot’s weight while
exhibiting spring-like behavior for agile movement [6].
Many strategies have been explored to manipulate thestiffness and geometry of robotic limbs [7], [8]. For example,
researchers developed robots with limbs capable of melting
and solidifying thermoplastic joints, allowing them to adapt

their movement kinematics on demand [9]; adjusting theirleg lengths using linear actuators to optimize their cost oftransport on different surfaces [10]; adapting their structuralconfiguration from wheel-like to flipper-like for amphibious
propulsion [11]; and changing their inflation state to improve
traction on land or increase propulsion in water [12].
Prior work demonstrated a robotic limb that transitions
from a flipper shape (for aquatic swimming) to a load-bearing
leg shape (for land walking or crawling) via a thermallyresponsive variable stiffness epoxy adhered to pneumatic
bending actuators [13]. Neutrally in the flipper shape, theepoxy could be warmed above its glass transition temperature
to soften it for morphing. The pneumatic bending actuators
could then morph the limb into its leg shape, and the epoxy
was cooled to hold the new shape passively. Warming thelimb would again soften the epoxy and return the limb to its
flipper shape. Realizing these morphing limbs in a quadruped
robot yielded sea turtle- and land tortoise-inspired multienvironment robotic transitions [1].
The version of the morphing limb presented in [13] and [1]included a thermally-responsive material, which coupled its
energetic cost of morphing to the environmental temperature. A more recent (though unpublished at the time of

The authors are with the Department of Mechanical Engineering andMaterials Science, Yale University, New Haven, CT, 06511, USA. (email:rebecca.kramer@yale.edu)
This project was sponsored by the Office of Naval Research, United States
under award N00014-21-1-2417. L.R. was supported by a National Science
Foundation (NSF) Graduate Research Fellowship (DGE-1752134). B.L. andE.F. were supported by REU supplements to NSF EFMA-1830870.

979-8-3503-8181-8/24/$31.00 ©2024 IEEE

(c)

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Version
V1: Epoxy [1]V2: Jamming
V3: This work

Weight
350 g
450 g
260 g

Morphing Energy
10360 J
146.5 J
95 J

Actuation Pressure
30 psi30 psi
2 psi

Flipper Thickness
16 mm
30 mm
10 mm

Max Load (Vertical)
600 N
450 N
220 N

Glide Ratio
1.4
1.1
2

TABLE IC OMPARISON OF PERFORMANCE METRICS BETWEEN ITERATIONS OF MORPHING LIMBS FOR THE AMPHIBIOUS ROBOTIC TURTLE .

II. M ORPHING LIMB DESIGN

this writing) version of the limb uses topologically altered“kirigami” laminar jamming [14] to morph between thehydrodynamic flipper and load-bearing leg shapes. With apressure-responsive layer jamming system for variable stiffness, the jamming limb decouples its morphing performancefrom the environmental temperature and further achieves
rapid stiffness adjustments.
However, the kirigami layer jamming limb design faced
two critical issues. First, generating sufficient geometricchanges through the pneumatic bending actuators proved
arduous. Morphing from flipper to leg shapes required pressures exceeding 30 psi, which made it difficult to untether
the robot. Second, the switch from epoxy to layer jamming
as a variable stiffness material increased the limb’s thickness,
improving load-bearing, but sacrificing hydrodynamics.
In this study, we introduce a refined morphing limb designthat captures the advantages of both the epoxy limb’s hydrodynamic profile and the jamming limb’s approach to rapid,
environment-agnostic stiffness switching (Fig. 1a,b). The
limb retains laminar jamming for variable stiffness but uses a
new fluidic actuation strategy. We replaced the prior external
fluidic actuators with a single pouch actuator, which couples
the inflation that drives the flipper-to-leg cross-section shapechange with the curvature of the limb’s tip (the “foot”), thus
reducing the total actuator count from three to one per limb.
Since the limb’s compressive modulus increases with bothdecreasing pressure in the layer jamming components andincreasing pressure in the pouch actuator, we call this design“positive pressure-reinforced jamming.”
The stiffness contribution from the pouch actuator allows for a substantial reduction in the number of layers
within the laminar jamming components. Hydrodynamic
characterizations suggest that the new limb’s flipper profile
is improved relative to prior versions, both when jammed(stiff) and unjammed (soft). Utilizing onboard 2.5 W pumps(Fig. 1c), the limb executes morphing sequences within 38 s,
consuming 95 J of energy. A comparative analysis of thelimb against prior designs is shown in Table I. We note that
tethered pumps drive the prior designs, denoted as V1 andV2 in Table I, due to high-pressure requirements and thatthese pumps consume relatively more energy.
Pouch actuator (b)(a)
Layer jamming
systemSleevePouch
Actuator

As shown in Fig. 2, for morphing into and maintaining
flipper and leg shapes, the limb combines kirigami layer
jamming [14] with an internal pouch actuator. The internal
pouch deforms the layers and induces a bend in the limb’s tip
through a cable mechanism, while layer jamming functions
as a variable stiffness component that softens for morphingand stiffens to hold a target shape.
A. Pouch actuator
The pouch actuator is a pneumatic artificial muscle constructed from a thin, inextensible, yet highly flexible filmthat is heat-formed into a pouch. Typically, pouch actuators generate linear contraction with little radial expansion.
Our approach leverages radial expansion and contraction
in tandem to alter the limb’s cross-sectional shape (andconsequently, its bending stiffness) as well as the curvatureof the tip, respectively. Moreover, the pouch’s expansion
reinforces the strength of the layer jamming components,
akin to the principles of positive pressure jamming [15].
(a)

2)

1)
Fold

C
Hood

6)

3)

Heat
seal

C/ 2

Tubing

5)

Wf

Wf
Hot glue

PET
sheet

4)
L

(b)

Wf

W

Deflated pouch

Wf

Inflated pouch

C=260 mmFig. 3. (a) The fabrication process of the pouch actuator. 1: Fold a sheetof air-tight film in half. 2: Seal the folded sheet at the designated width. 3:
Create internal folds along the sides of the sheet. 4: Create seals at the top
and bottom of the pouch. A thin PET sheet is inserted at the top to create
an opening for the pneumatic tubing. 5: Insert the pneumatic tubing andseal it within the pouch using hot glue. 6: Seal a thin strip of air-tight film
to the top of the pouch to create a hood for anchoring the pouch within thecircular housing. (b) The pouch actuator before and after inflation.

Cable anchors
Cable
Kirigami
LayersFig. 2.
The working principle of the shape morphing limb. (a) The
limb cross-section. (b) The cable mechanism bends the tip during actuator
contraction.

Initial trials employing a conventional pouch actuator withno folding [16] revealed inadequate contractile displacement375

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Second silicone layer with fabricFirst silicone layer

Cast silicone walls

Demold silicone walls

4 hr cure time

PET layers

Flip and repeat once
Completed cross-section

Attach tubing
and cable anchors

4 hrcure time

Bond top fabric layer

Completed cross-section

Completed fabric layer

Silicone-infused fabricInfused fabric
Silicone layer
Infused fabricFig. 4.

Bond first fabric layer

Silicone wall
8 PET layers

Remove excess
fabricFig. 5. The fabrication process of the layer jamming system part A andB, with the integration of the topologically augmented (kirigami) layers.

Fabrication of the silicone-infused double-layer fabric (SIDF).

at maximum expansion. To address this limitation, we created pouches with folds to generate the necessary contractile
displacement to achieve tip bending [17]. Fig. 3 illustrates the
fabrication steps for the folded pouches. For the pouch material, we opted for heat-sealable film (amine-free polyethylene
from McMaster-Carr, 4663T9), as heat sealing simplifies the
manufacturing process. Alternative materials such as heatsealable nylon fabrics and silicone-infused fabric could beconsidered to enhance the pouch’s strength and durability.
We denote the pouch’s initial circumference (unfolded
width) as C, the side-fold widths as Wf , the pouch length asL, and the final pouch width as W . The folding ratio Wf /W
is correlated to the contraction generated when the pouch’s
length, L, is held constant [17]. Based on our experimental
findings, a folding ratio exceeding 0.5 results in folding
overlaps and increases the risk of pouch leakage due tothe additional layers that must be heat-sealed. Therefore, we
opted for a folding ratio of 0.5, resulting in C = 4W = 8Wf .
This ratio achieved the desired tip bending, as demonstrated
in Section III-A.

The complete fabrication procedure for the kirigami layer
jamming system is shown in Fig. 5. First, a silicone wall
conforming to the limb’s shape is cast. Subsequently, thissilicone wall is affixed to the SIDF housing eight layers of
PET sheets. These PET sheets are laser-cut into the desired
limb shapes, denoted as A and B. The choice of limb shapeis detailed in [13]. To optimize the jammed-to-unjammed
flexural stiffness ratio, we selectively removed material fromthe sheets to augment the second moments of area of thejammed and unjammed states, following guidelines proposedin [14]. A silicone tube is threaded through a hole in theenvelope surrounding the jamming sheet and sealed withadhesive (Silpoxy, Smooth-on Inc.).
C. Morphing limb assembly
Fig. 6a shows an exploded view of the morphing limb.
We first sew parts A and B of the jamming system together
along their edge, as shown in Figure 6b. We then insert thepouch between the two jamming walls, using a sleeve to
reduce the friction. An assembly comprising two clamps acircular housing, and a carbon fiber rod serves to anchor
the pouch actuator and interface to the shoulder joints of
our amphibious robot body. A cable (fishing line) is fixed
between the pouch and the limb’s tip, to pull the tip inward
when the pouch contracts.

B. Layer jamming system
The layer jamming system comprises silicone-infuseddouble-layer fabric (SIDF) serving as an airtight envelope
in which cut polyethylene terephthalate (PET; 0.01 in thick)sheets are positioned. The SIDF is composed of two layersof silicone-impregnated fabric enveloping a central layer ofsilicone. The outer layer of fabric is resilient to external penetration, particularly when the limb encounters rough terrain,
while the inner layer serves to protect against abrasion fromthe PET sheets contained inside.
The fabrication of SIDF involves three steps, depicted
in Fig. 4. Initially, an airtight layer of silicone (EcoFlex50, Smooth-on Inc.) is cast onto a smooth surface. After
curing, additional silicone is evenly applied atop the airtightlayer, and a layer of fabric (poly-cotton and polyester) isimmediately placed on it and flattened via scraping motions
with a ruler to ensure a secure interfacial bond. This process
also facilitates complete silicone infusion into the fabric.
Subsequently, the assembly is flipped over, and another layerof fabric is bonded in the same manner.

D. Electrical system
We designed a compact and portable electrical system tofacilitate the untethered mobility of future amphibious robots
equipped with the limb. The electrical system consists of
two micro diaphragm pumps (Dynaflo, 2000 series), four
valves (V1-V4; Park, X-Valves, 30 psi, normally closed),
and two pressure sensors (S1, S2; 015PGAA5, 030PGAA5;
Honeywell). An Arduino Nano is used as an onboard microcontroller, sending signals to the motor driver (TB6612FNG)
and the transistor array (TBD62083APG-ND), which respectively drive the two pumps and the four valves and sample
the pressure data.
As illustrated in Fig. 7a, the system employs two pumps:
P1 is responsible for inflating the pouch actuator, while
P2 manages the vacuuming of the jamming system. Given
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(a)

(a)
Assembledflipper

ATM
V1

Clamp
Circularhousing

Part A

Container

P1

P2

V2

V3

rbo
n
rod fiber

Pouch

S1

S2
Positive pressure
Negative pressure
ATM line
Layer jamming
systemPouch
Sleeve

o
ch

bl
Ca

n
ea

rs

Part B

Cable(b)

Sewing

V4

Ca

(b)

(c)

Inﬂation
Jamming
Deﬂation
Unjamming

Cable anchors

Fig. 6. (a) The fully assembled limb, displayed with an exploded view. (b)The sewing method to attach Parts A and B of the layer jamming system.
(c) Cable anchors constrain the cable that pulls the tip of the limb inwards.

V1
1
0
0
0

V2
0
0
1
0

V3
0
1
0
0

V4
0
0
0
1

P1
1
0
0
0

P2
0
1
0
0

Fig. 7. (a) A schematic of the pneumatics system. (b) The truth table ofthe pneumatic control logic.

260, and 300 (all mm). As shown in Fig. 8d, the diametric
expansion increases with increasing pressure for all pouch
sizes tested, and plateaus after 1.5 psi. The pouch with thelargest expansion is C = 260 mm, which is slightly larger
than the maximum designed circumference of the layer jamming system (220 mm in Fig. 8b). We suspect that the layer
jamming system plastically deforms due to pouch actuationto accommodate the pouch’s 260 mm circumference, but not
so much to accommodate even larger pouches.
Fig. 8e shows that the tip bends by more than 70◦ after
the pouch inflates to 2 psi, which prevents it from uncurlingwhile the robot is walking. We also observed that the tip
bending angle increases when the layer jamming system isengaged (Pj = −13 psi), because the clearance between the
SIDF and PET layers reduces (Fig. 8c).
Fig. 8f depicts the expansion and bending angle whenPp = 2 psi and Pj = −13 psi. The maximum bending angle
occurs with C = 220 mm; the largest expansion is with C= 260 mm. We surmise that C = 260 mm did not have thelargest tip bending due to the physical constraints imposedby the layer jamming system. We chose C = 260 mm for
subsequent characterizations, since it achieves the largest
expansion and has a tip bending angle larger than 60◦ (theminimum required to prevent uncurling).

the limb’s intended application for an amphibious robot, it
necessitates enclosure within a waterproof housing to ensure
protection (labeled as “Container” in Fig. 7a). Additionally,
the exposed tubing incorporates an actively controlled valve(V1), serving as a safeguard against water ingress during
underwater operations. The pneumatic pressure sensors play
an integral role by continuously measuring the pressure
within both the pouch and the jamming system, enabling
closed-loop control of the pumps. For reference, Fig. 7bprovides the truth table for the pneumatic valves during themorphing sequence, where “1” indicates that the valves or
pumps are active, and “0” signifies that they are deactivated.
III. M ORPHING LIMB CHARACTERIZATION
A. Limb shape actuation
When the internal pouch actuator inflates, the limb’s crosssection transforms from flat to oval. To characterize therelationship between the pouch pressure (Pp ) and the limb’s
diameter expansion (d), we conducted experiments shown in
Fig. 8. We placed a marker atop the flipper to visually trackthe expansion while slowly inflating the pouch (Fig. 8a).
Using a recording and subsequent tracking software (Tracker;
physlets.org/tracker), we analyzed the changes in d as afunction of Pp . The coupled bending angle of the limb’s
tip was also tracked. Three color markers were distributedon the edge of the tip. Since the three dots are almost on astraight line, the bending angle was calculated as the angle
between a vector through the two tracking dots (close to thetip) and the horizontal line.
Because the layer jamming system that constrains thepouch has varying widths (Fig. 8b), different pouch circumferences were tested to optimize the expansion: 180, 220,

B. Compression testing in leg mode
During terrestrial gaits, the morphing limb (in leg mode)needs to withstand compression loads from various angles.
To determine the limb’s load-bearing capability, an Instron
3345 equipped with a 1 kN load cell was used to measureforce-displacement curves for the limb in vertical and horizontal orientations corresponding to waking and crawling.
We tested five limb conditions in the vertical orientation,
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Vacuum
Inflation

Expansion
tracking dot

Pneumatics
control

Pj = -13 psi

Pp

20
d

Pj

180 mm
220 mm
260 mm
300 mm0.5
1
1.5
Pressure, Pp (psi)

2

(f)

70
60
50
40

180 mm
220 mm
260 mm
300 mm

30
20
10
0

0

0.5
1
1.5
Pressure, Pp (psi)100

Expansion, d (mm)300C2=220 mm

α2 > α1Clearance

α2
(e)

40

0

α1

C1=180 mm

Bending angle
tracking dots

Morphing limb

50

10

Pj = 0

2

Pp= 2

80

Expansion, dBending Angle

100
80

60

60

40

40

20

20

0

180
220
260
300
Pouch Circumference, C (mm)Bending Angle, α2 (°)

Expansion, d (mm)

(d)

(c)

(b)

Pumps

Bending Angle, α1 (°)(a)

0Fig. 8. (a) The setup for the pouch actuation tests. (b) The inner circumference of the layer jamming system varies between 180 and 220 mm. (c)Bending angle increases following the jamming of the limb. Images of the limb before jamming display a large amount of slack between the cable andthe fabric; this slack is removed upon jamming. (d) Limb expansion vs. pouch pressure before jamming. (e) Limb tip bending angle vs. pouch pressure
before jamming. (f) The maximum values for limb expansion and tip bending angle after jamming.

listed in Fig. 9a: 1) unjammed flipper (pouch deflated, jamming off), 2) jammed flipper (pouch deflated, jamming on),3) jammed leg (pouch initially inflates, then jamming holds
shape with pouch deflated), 4) unjammed leg (pouch inflated,jamming off), and 5) positive pressure-reinforced jammedleg (pouch inflated, jamming on). The positive pressurereinforced jammed leg shows a 30% increase in maximum
load-bearing capability (peak force) when compared with the
jammed leg or unjammed leg. Assuming the robot has its
weight (18.5 kg) equally distributed between its four limbs,
the maximum load that a limb needs to withstand is 20 N.
The results suggest that all limb conditions would sufficiently
support the robot, except the flipper conditions.
Fig. 9b shows a loading condition representative of acrawling gait. The results suggest that the limbs should be rotated to the side (seamed edges contacting the ground) duringcrawling to maximize the stiffness and mitigate buckling at
the limb-shoulder interface, regardless of the limb condition.

values, where lift forces experienced overcome drag forces,
are highlighted green in the plots. Flow rates of 0.3 and0.4 m/s result in similar performance patterns to the epoxy
predecessor, with increased glide ratios occurring around10◦ -15◦ . These desirable glide ratios were attained by thejamming flipper at each tested velocity. In contrast, theprevious epoxy flipper was unable to reach glide ratios above
1 for speeds lower than 0.3 m/s. As the angle of attack
increases, both flipper designs experience a decline in glide
ratio characterized by mild fluctuations, ending in ratios
around 1 at 25◦ .
IV. C ONCLUSION
In this work, we proposed a refined morphing limb design
intended for the next generation of the amphibious robotic
turtle. The new morphing limb boasts temperature-agnostic
morphing performance, addressing prior issues related tothe use of thermally responsive materials, and drastically
speeding up shape and stiffness transitions. Moreover, the
new limb leverages a simplified morphing mechanism consisting of a single internal pouch actuator that applies positive
pressure to the external layer jamming system. This positive
pressure-reinforced jamming scheme reduces the number oflayers needed to sustain a stable leg configuration undercompressive loads. Thanks to the reduction in overall limb
thickness due to positive pressure-reinforced jamming, thelimb achieves favorable glide ratios at a wide range of flow
velocities and angles of attack. Lastly, as a complement to
the new morphing limb, we introduced a compact pneumatic
control system. The improved morphing limb and its control
module provide the foundation necessary to step toward thegoal of an untethered, amphibious quadruped robot capableof autonomous multi-environment transitions.

C. Hydrodynamic testing in flipper mode
We measured lift and drag forces on the limb (in flippermode) in a laminar flow tank (Fig. 10a) using the same
setup (1503 Gal recirculating system, 2.25 m × 0.45 m ×
0.45 m) and method as in [13]. The angle of attack of thelimb with respect to the incoming flow was adjusted. The
lift and drag forces were acquired by a 6-axis Gamma DAQ
F/T Transducer (ATI industrial), and a National Instruments
DAQ (USB-6212) was used with a Matlab data acquisition
toolbox to record force data.
Fig. 10b plots the glide ratio (FL /FD ) with respect tothe angle of attack (θ) at flow rates of 0.2-0.4 m/s. The
hydrodynamic performance of the flipper in its jammed (stiff)and unjammed (soft) states are juxtaposed with those of
the epoxy-based flippers from [13]. Desirable glide ratio
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Condition
Positive pressure-reinforced jammed leg
Unjammed leg
Jammed leg with released pouch
Jammed ﬂipper
Unjammed ﬂipper

Pp (psi)
2
2
2
0
0
0

Positive pressure-reinforced Leg
jammed leg
mode

200

(a)

Pj (psi)
-13
0
-13
-13
0

FD

FL

(b)

1.5

Flipper
mode

1

0.5
0

0.3 m/s

2.5
Jammed leg withreleased pouch

Unjammed flipper

0

0

40

20

(b)

80

①

80
60
Displacement (mm)Force (N)

① Positive pressure-reinforced
jammed leg - Side

40

20
③ Unjammed flipper - Side
④ Unjammed flipper - Top

0

0

10

20

30
40
50
Displacement (mm)FL / F D

10.5
010 20 30 40 50
Angle of attack, θ (°)

00

10 20 30 40 50
Angle of attack, θ (°)

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② Positive pressure-reinforced
jammed leg - Top④

0.4 m/s

2.5

Fig. 10. (a) Setup for hydrodynamic testing. (b-d) Lift and drag force ratios
for the morphing limb both jammed (stiff) and unjammed (soft), comparedwith the epoxy-based limb [13], respectively at 0.2, 0.3, and 0.4 m/s.

Side

②

1

0

120

50

1.5

0.5

Top

60

③

100

10 20 30 40
Angle of attack (°)

2

1.5

Jammed flipper

50

0

(d)

2
FL / F D

Force (N)

Flow

(c)

100

JammedUnjammed
Epoxy

2

θ

Unjammed leg

150

0.2 m/s

2.5

FL / F D

(a)

60

70

Fig. 9. (a) Vertical compression tests, representative of upright gaits, forfive different limb inflation and stiffness conditions. (b) Horizontal loading
tests, representative of crawling gaits, for two different limb conditions.

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