RESEARCH ARTICLE
www.advmat.de

Self-Amputating and Interfusing Machines
Bilige Yang, Amir Mohammadi Nasab, Stephanie J. Woodman, Eugene Thomas,
Liana G. Tilton, Michael Levin, and Rebecca Kramer-Bottiglio*
aggregation(interfusion).[6]Selfamputation, or autotomy, is the ability
for animals to amputate appendages toescape danger.[5] For example, lizards
will voluntarily cast oﬀ a tail if seized bya predator[7] and crabs shed injured appendages that might otherwise hinder their
movement.[8] In a colony of insects (especially ants), biologists have observed andcharacterized the dynamic aggregation ofindividuals and how such aggregation aidsin functional behaviors through a collective arrangement of morphologies.[6,9] For
example, by rearranging the mechanicalconnection between individual ants, an ant
colony can link up to form a bridge to crossa gap on the forest ﬂoor or form into theshape of a ball to ﬂoat on water.[10,11] Bothautotomy and aggregation are functional
methods of shape change that require
the subtraction or addition of mass, andprior works suggest that especially subtractive inspirations are often overlooked forsynthetic systems.[12,13]
A key functional feature that enables bothself-amputation and dynamic aggregationin animals is the ability to form and breakconnections, both within a body (individual scale) and between
bodies (group scale) (Figure 1A). On the individual scale, biological autotomy tells us that the intentional separation of appendages is achieved by actively adjusting the tensile strength
along a predetermined fracture plane that connects extremities
and the rest of the body,[14–17] and animals lacking control of thisfracture plane cannot autotomize.[7,18] On the group scale, there
are clear ties between animals that connect/disconnect and modular robotics. Myriad options for reversible connections exist for
rigid-material modular robots, enabling adaptive body conﬁgurations and controls under changing circumstances.[19] In parallel,emerging soft robots also promise adaptability to changing tasks
and environments.[1] The intersection of soft and modular robots
oﬀers soft modular robots,[20] which are made of predominantly
soft materials and require equally soft reversible joints that do notinterfere with the novel functions aﬀorded by body compliance.
Many connectors for soft robots rely on mechanical forces
and include simple linkages like metal pins,[21] plastic sockets,[22]screws and bolts,[23] and push ﬁttings.[24] These connectors havehigh strength due to strong mechanical coupling but require precise alignment and manual handling to connect. More importantly, the rigid materials used in these structures reduce the overall ﬂexibility and compliance of the soft system.

Biological organisms exhibit phenomenal adaptation throughmorphology-shifting mechanisms including self-amputation, regeneration,and collective behavior. For example, reptiles, crustaceans, and insects
amputate their own appendages in response to threats. Temporary fusion
between individuals enables collective behaviors, such as in ants that
temporarily fuse to build bridges. The concept of morphological editing often
involves the addition and subtraction of mass and can be linked to modular
robotics, wherein synthetic body morphology may be revised by rearranging
parts. This work describes a reversible cohesive interface made ofthermoplastic elastomer that allows for strong attachment and easy
detachment of distributed soft robot modules without direct human handling.
The reversible joint boasts a modulus similar to materials commonly used insoft robotics, and can thus be distributed throughout soft robot bodies
without introducing mechanical incongruities. To demonstrate utility, thereversible joint is implemented in two embodiments: a soft quadruped robotthat self-amputates a limb when stuck, and a cluster of three soft-crawlingrobots that fuse to cross a land gap. This work points toward future robots
capable of radical shape-shifting via changes in mass through autotomy andinterfusion, as well as highlights the crucial role that interfacial stiﬀness
change plays in autotomizable biological and artiﬁcial systems.

1. Introduction
Many biological organisms can adapt their morphology andbehavioral control policy toward changing tasks and environments.[1] Among examples of such adaptive morphology,[2–4] twoparticularly striking methods are self-amputation[5] and dynamic
B. Yang, A. M. Nasab, S. J. Woodman, E. Thomas, L. G. Tilton,
R. Kramer-Bottiglio
School of Engineering & Applied Science
Yale University
9 Hillhouse Avenue, New Haven, CT 06511, USA
E-mail: rebecca.kramer@yale.edu
M. Levin
Department of Biology
Allen Discovery Center at Tufts University
200 Boston Ave. Suite 4604, Medford, MA 02155, USA
M. Levin
Wyss Institute for Biologically Inspired Engineering
Harvard University
3 Blackfan Cir, Boston, MA 02115, USA
The ORCID identiﬁcation number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/adma.202400241

DOI: 10.1002/adma.202400241

Adv. Mater. 2024, 2400241

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www.advmat.deFigure 1. Overview of the reversible joint concept. A) Self-amputation and interfusion in biological systems and their artiﬁcial counterparts. B) Schematicfor the structure and mechanism of the reversible joint. C) Joint tensile strength versus joint compliance for relevant connection mechanisms in theliterature. The mechanical[24] and magnetic connectors[25,27,28] are strong but stiﬀ, while the electrostatic connector[31] is compliant but weak. An ideal
reversible joint is both strong and compliant.

Magnetic forces have also been utilized to create connection mechanisms for soft robots.[25–28] By embedding permanent
magnets into the outside surfaces of soft bodies, soft modules cansnap together and establish a strong magnetic bond. Yet magnets
also introduce undesired rigidity and weight into the soft system.
Furthermore, the magnetic force is typically constant or challenging to modulate, and large attractive forces often require manual
manipulation with the risk of tearing the soft materials. Electromagnets can produce a reversible magnetic force but face similardisadvantages of rigidity and weight, and have thus far only been
applied to rigid modular robots.[29,30]
Several additional connective mechanisms exist that may offer advantageous compliance in the context of soft robotics. Electrostatic force can be generated between thin membranes with
conductive carbon traces under high voltage.[31] Even though thesystem can remain soft, electroadhesion is several orders of magnitude weaker than the aforementioned mechanical and magnetic connections, making electroadhesion suitable only for extremely light modules. Micro-patterning the surface of soft structures creates dry adhesives for soft modules based on van der
Waals interactions.[32] However, van der Waals connections are

Adv. Mater. 2024, 2400241

still relatively weak (compared to magnetic and mechanical connectors) as well as limitingly directional, and detachment can require manual manipulation. Wang et al. illustrated the feasibilityof using hot-melt adhesives, a type of thermoplastic polymer, asa reversible connection for pick-and-place applications.[33] Relatedly, Huang et al. showed a programmable adhesive hydrogel that
can attach and detach conductive objects via the application ofvoltage.[34] López-Díaz et al. also showed a hydrogel-based connection that uses hydration-triggered self-healing for reversible
adhesion capacity, although the connection time given thereinis 24 h.[35] These approaches lack cool-state compliance, applicability to non-conductive materials, and suﬃciently rapid connective reversibility, respectively. However, such prior works point toward an emerging category of stimuli-responsive reversible adhesives suitable for integration with soft robots for programmableself-amputation, interfusion, and reconﬁguration.
Herein, we introduce a novel material architecture using thermally responsive thermoplastic elastomer for the attachment anddetachment of soft robot components or bodies. We considered
the following requirements: 1) Tunable bonding strength (highfor attachment and low for detachment); 2) Many attach/detach

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www.advmat.decycles without substantial performance degradation; and 3) Compliance matching to common structural materials in the soft
robotics literature. Using a bi-continuous structure of thermoplastic elastomer as the basis for our approach, we characterize
the formulation and geometry to optimize the reversible joint for
the requirements above. Further, we prove the utility of the reversible joint in autotomizing and interfusing soft robot demonstrations.

2. Results
2.1. Bicontinuous Thermoplastic Foam (BTF) as a Soft Reversible
Joint
We achieved a temperature-modulated reversible joint based ona bicontinuous thermoplastic foam structure. The thermoplastic used in this work is styrene-isoprene block copolymer (SIS),
which is a rubbery solid at room temperature (≈25 °C) and melts
into a liquid at elevated temperature (≈140 °C). We sought to utilize the heat-tunable strength of SIS for an artiﬁcial joint that
could be broken oﬀ and rejoined based on temperature—a “reversible joint.” To achieve such a joint, we ﬁrst plasticized SIS
with paraﬃn oil to suppress its modulus into a range representative of silicones commonly used in soft robotics, e.g., DragonSkin 10,[36] as well as decrease its melting point (see details inExperimental Section). SIS does not inherently bond to silicones
and urethanes, so we devised a bicontinuous foam structure comprised of an SIS scaﬀold infused with a silicone matrix (Figure 1B
and Experimental Section), which we termed Bicontinuous Thermoplastic Foam (BTF). This foam structure allows SIS to melt
and combine with its counterpart on another soft body cohesively
while still adhering to the silicone matrix (and the rest of the soft
robotic body) via mechanical bonding (Figure 1B).
Reversible connections are accomplished as follows. Two silicone bodies each have a layer of BTF on their exposed surfaces.
The foams are heated above the melting point of SIS, such that
the SIS in the BTF melts into a viscous liquid. The silicone matrix in the BTF holds the molten SIS like a sponge, preventing itfrom ﬂowing out. Upon contact between two BTF surfaces (without requiring direct pressure), the molten SIS from both sides
combines into a continuous liquid mass due to the cohesive force
between SIS molecules. As the joint cools, the SIS solidiﬁes, connecting the silicone bodies. To disconnect, the joint is heated suchthat the SIS melts and weakens substantially. The two sides of thejoint are then easily separated. Even after separation, each side
retains its bicontinuous structure of SIS in a silicone scaﬀold,
which enables multiple repeated connections.
An ideal joint for soft modular robots, beyond facilitating reversible connections, would exhibit high strength, cyclically stable bonds, and match the compliance of materials commonlyused to make soft bodies. We conducted a comprehensive mechanical assay to evaluate the BTF joint’s ﬁt with this ideal criterion. We ﬁrst conducted tensile tests on a joint to ascertain
its maximum tensile strength (Figure S1, Supporting Information), a typically reported metric for other reversible connection
systems. Figure 1C plots the tensile strength of the BTF joint(29.5 kPa, SD = 0.395 kPa) along with several other joints used
for soft bodies as a function of their joint compliance. The connection strength of the BTF joint was 454× greater than that ofAdv. Mater. 2024, 2400241

the electrostatic joint,[31] and even on par with rigid connectors
such as magnetic joints[25] and mechanical joints.[24] The BTF
joint boasts both high strength and high compliance, as indicated by its position in the upper right corner of Figure 1C. One
reported mechanical joint also falls toward the desired target ofhigh strength and high compliance,[24] but the joint cannot besoftened for disconnection, limiting its utility for modular soft
robotic applications.
We also conducted T-peel tests as a representative loading condition for soft joints used on robot systems (subjecting them to a
combination of tensile and shear stresses). T-peel tests were carried out with BTF joint samples (ﬁve for each condition) cast atop
an inextensible fabric-reinforced silicone substrate (Figure 2A).
At room temperature (25 °C), the joint exhibited an average bondstrength of 6.84 N cm−2 (68.4 kPa, SD = 9.61 kPa). With a suppressed melting temperature of Tm = 115 °C due to plasticization (see Figure S2, Supporting Information Text, and Experimental Section), heating the joint to a disconnection temperatureof 120 °C decreased its bond strength by more than 30×, to 0.21 N
cm−2 (2.1 kPa, SD = 1.25 kPa) (Figure 2B). Tests at an intermediate disconnection temperature of 65 °C yielded an average bondstrength of 0.53 N cm−2 (5.3 kPa, SD = 1.26 kPa) (Figure 2B).
The proximity of the peak force at an intermediate temperature
to that above the SIS melting temperature suggests that we do notalways need to heat to above the Tm to facilitate disconnection. Indeed, probing the force response between 25 and 65 °C reveals arapid and monotonic decrease in peak forces in this temperature
range (Figures S3 and S4, Supporting Information).
2.2. Performance Variation with Thickness and Pore Size
To understand the inﬂuence of key design variables on the resulting bond strength of the reversible joint, we conducted further
T-peel tests on BTF specimens varying thickness per side, t, andaverage pore size of the SIS foam, d (Figure 2A,C,D). The maximum stress a joint can sustain at 25 and 65 °C roughly increased
with an increase in the thickness of the joint (Figure 2C). Thisﬁnding is consistent with literature documenting the eﬀect of the
thickness on the peel strength of adhesive joints.[37] Namely, as
the joint thickness increases, a larger volume of polymer is subjected to deformation per unit area of detachment, increasing thetotal work expended while peeling. Past the Tm of SIS at 120 °C,
the maximum stress stayed approximately constant as joint thickness increased, and we thus speculate that the separation force ofthe liquid–liquid interface does not vary with the change in bulk
thickness.[38] Collectively, the T-peel results reveal that the ratio
between the bond strengths at room temperature and above Tm
is positively correlated with t.
To further understand the strength of the connection during
peeling, we constructed a 2D ﬁnite element model implementing a J-integral method where a constant force is applied to thejoint to calculate the energy release rate (G) at the tip of an edge
crack (see Figure S5, Supporting Information). G is proportional
K2

to the square of the model-I fracture toughness (KI ): G= EI′ .[39]A lower G means that the joint will tear at a higher maximum
force. The simulation showed that, as joint thickness increased,
G decreased for the room-temperature BTF joint. For SIS foam
at 120 °C, the simulation predicted increasing G with increasing

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www.advmat.deFigure 2. Mechanical characterization of the bicontinuous thermoplastic foam (BTF) reversible joint. A) A schematic of the T-peel test setup. (Inset)
Microscope images of the reversible joint before and after 150 cycles of “heat-connect-cool-heat-disconnect.” The round circles are the silicone network,
while the SIS network ﬁlls the space between the circles. The bright blue bars on the top and bottom are the silicone substrate. t is the thickness of the
BTF on one side. d is the pore size of the SIS foam. Scale bar, 2 mm. B) Force response of reversible joint for standard samples of 1 cm2 contact areaand 1.5 mm thickness at 25, 65, and 120 °C. Error clouds are one SD over ﬁve samples. C) Maximum sustained stress for joint samples of diﬀerent
thicknesses at diﬀerent temperatures, while holding pore size range at 850–1000 μm. Error bars are one SD over ﬁve samples. D) Maximum sustained
stress for joint samples of diﬀerent pore sizes of the SIS foam at diﬀerent temperatures, holding thickness constant at 1.5 mm. Error bars are one SD. E)
Representative force response of joint standard samples of 1 cm2 contact area and 1.5 mm thickness after cyclic (dis)connections. Additional samples
and averages are provided in Figure S6 (Supporting Information). F) Maximum sustained force and stress of joints after cyclic (dis)connections. Error
bars are one SD over three samples. G) Force response of a joint under a cyclic displacement of 14 mm at room temperature.

thickness, which was consistent with the experimental results
showing a slight decline in the pull-oﬀ force (Figure 2C).
Pores in the SIS foam were created with sacriﬁcial sugar particles and later ﬁlled in by silicone (see Experimental Section
and Figure S2, Supporting Information). The sacriﬁcial sugar
particles were infused with packing density ϕ = 0.74 across all
samples.[40] Pore sizes are normally an important determinantof the mechanical properties of foams.[41–43] We were surprised,
then, to ﬁnd that varying pore size had minimal eﬀect on the
maximum stress sustained by the BTF joint (Figure 2D). Several
studies in the literature also report that varying pore size does not
aﬀect the strength of foams, granted the porosity (packing fraction of pores) is held constant.[44,45] Consequently, streamlined
manufacturing of the material structure is possible using larger
particles that are much easier to incorporate into the host SIS material.

2.3. Cyclic Strength and Repeatability
Having quantiﬁed the impact of key design parameters on thereversible joint’s adhesion strength, we proceeded to characterize the cyclic performance. Two tests were conducted. First,
to see how repeated connection and disconnection aﬀects joint
strength, we conducted experiments containing a series of steps:
1) heating up two sides of the joint to connect, 2) contacting, andcooling down to ﬁrm up the connection, and 3) heating up theAdv. Mater. 2024, 2400241

joint again to disconnect. We did the above steps over 0 (pristine), 50, 100, and 150 cycles and tested the ﬁnal cooled-down
joint strength by pulling it to failure (Figure 2E,F). Figure 2E
shows representative force responses, while averages across multiple samples (N = 3) are given in Figure S6 (Supporting Information). The ultimate strength of the joint decreased with increasing cycle number but still remained at 67% of the pristine maximum force after 150 cycles. After 150 cycles, a portion of themelted SIS exited the silicone matrix, causing gradual connection degradation (inset of Figure 2A). Most deterioration of thejoint happened within the ﬁrst 50 cycles and thereafter degradation slowed (Figure 2F). Extrapolating past cycle 150 (based on alinear approximation after cycle 50), we predict that the joint canhold 50% of its original strength at around cycle 250. For most
systems that are designed with a safety factor of 2, 50% of theinitial strength keeps the system operational. Further, since morphological adaptation enabled by connection and disconnectionis unlikely to be a frequent functional need, we believe that joints
operational through 250 cycles are suﬃcient for most soft modular robots.
For the second cyclic test, we sought to evaluate the behaviorof the joint when it experienced repeated loading in its cool, connected phase. We conducted cyclic tests for which we repeatedly
subjected the room temperature joint to a ﬁxed displacement andrecorded the resulting holding force. Figure 2G shows one representative sample, while additional samples and averages are
given in Figure S7 (Supporting Information). We started with a

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www.advmat.deFigure 3. A soft quadruped robot performing self-amputation. A) A schematic of the quadruped robot equipped with reversible joints and copper heaters.
The robot is 12 cm long and 7 cm wide. B) The robot walked in an unstructured environment. C) A rock unexpectedly fell onto the back limb of the robot,
trapping the robot. D) One reversible joint was weakened via Joule heating of the copper heater. E) The robot kept walking and left the stuck limb behind.

14 mm displacement at the joint, which corresponds to the joint’s
initial maximum peel load or 6 N (on average). After 50 and 200
cycles, the joint held a force of 2.5 N (≈40% of initial) and 1.4 N(≈25% of initial), respectively. Conducting the same test with asmaller displacement of 12 mm (see Figure S7, Supporting Information), which corresponds to 55% of the maximum peel load or
3.60 N, we ﬁnd that after 200 cycles, the joint held 1.17 N (32.5%of the force at the start). Therefore, we deduce that even smaller
displacements weaken the joint under large (>200) cycle numbers.
Having veriﬁed the mechanical strength and cyclic stability of
the reversible joint, we moved to assess its application in modular
soft robotic systems.

adverse condition—having part of the body pinned beneath aheavy object (Figure 3C)—the robot was able to self-amputate
a limb to escape via heating of the thermally responsive joint(Figure 3D,E). After the self-amputation, the robot walked away
as a three-legged robot. This demonstration shows the eﬀectiveness of on-demand joints for the adaptability and autonomy ofrobots facing unstructured and hazardous environments.
In unstructured environments, abundant contaminants may
interfere with joint functionality. To test joint robustness, we deliberately exposed the melted interface to soil and water, respectively, and measured bond strength afterward (Figure S8, Supporting Information). We found that bond strength decreased byonly 6% after soil contact and by 20% after water soaking. These
tests demonstrate the suitability of our on-demand joints for realworld environments.

2.4. Self-Amputation of a Soft Quadruped Robot
We showcase a soft robot, an adaptation of the well-known soft
quadruped robot introduced by Shepherd et al.,[46] equipped withreversible joints navigating an environment with pre-planned
hazards (Movie S1, Supporting Information). Between the limbs
and torso of the soft robot, reversible soft joints were added together with a ﬂexible copper heater (Figure 3A). The robot was
teleoperated via a direct line of sight. Under undisturbed conditions, the robot walked forward (Figure 3B). Subjected to an

Adv. Mater. 2024, 2400241

2.5. Interfusion for Collaborative Robots
Environments can engender hazardous conditions that are diﬃcult for single artiﬁcial units to navigate. The ability to connect
and disconnect on-demand beneﬁts a group of robots aiming tocomplete a task that is impossible for a single robot (Movie S2,
Supporting Information). We constructed a group of soft crawlers
with reversible joints at their ends (Figure 4A) and built a

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www.advmat.delocomotion course with a gap in the path. A single soft crawler
was eﬀective at locomoting on ﬂat ground. Yet, when faced withthe wide crevice, it failed to cross and dropped into the gap(Figure 4B,C). However, a group of robots with reversible joints
could interfuse and collaborate to cross the gap. Three crawlers
ﬁrst lined up, one behind another (in contact), by crawling forward, and then the soft joints were heated to connect with theirneighbors (Figure 4D–F). After the joints cooled and strengthened, the crawlers collectively walked over the gap by utilizing
the added length of the body (Figure 4G). After the crossing wascomplete, the joints were heated again to detach, and the modules walked away separately (Figure 4H,I). This demonstrationshows how reversible joints can enable soft modular robots withindependent and collective behaviors. The soft robotic modules
are capable of joining with other modules to make use of an editable anatomy, and also capable of continued task performance
independently if cut oﬀ from the main body, which is a prodigious feat of dynamic plasticity that can counteract the eﬀects of
unexpected experiences or damage.

3. Conclusion

Figure 4. A cluster of interfusing soft crawlers. A) A schematic for the soft
crawler (with an actuator body), equipped with reversible joints and copper
heaters on both ends. The crawler is 7 cm long and 4 cm wide. B,C) A single crawler could crawl on ﬂat ground but fell into a gap that is too wideto cross. D–I) A cluster of crawlers interfused with each other, and thencrossed the gap collaboratively. After the crossing, the reversible joints
were heated up again to release individual modules so they again operate independently.

Adv. Mater. 2024, 2400241

In this work, we observed the general principle that a reversible
joint is key to realizing certain shape-based adaptations in biological organisms. The ability to voluntarily sever appendages when
encountering danger is a survival skill for a wide range of animals, as is the ability to dynamically aggregate for collective function. Both approaches underscore the importance of reversible
joints in biological adaptation, either as an individual or as a collective.
We see potential for soft artiﬁcial systems with reversible joints
for shape change via mass addition and subtraction. This work is
the ﬁrst instantiation of a fully soft reversible joint, which enabled
a self-amputating robot and an interfusing system of robots. The
reversible joint presented herein boasts inactive compliance andstrength similar to common elastomers and active weakening
for easy separation. Moreover, such synthetic systems can shedlight on the algorithms used by multiscale natural biological systems in which structure, size, and composition change dynamically (e.g., the collective cell behavior of embryonic morphogenesis in multicellular creatures, metamorphosis, remodeling, andcancer).[47,48]
Mechanically, our reversible joint can bear 68.4 kPa (6.84 N
per 1 cm2 contact area) at room temperature, which is muchstronger than benchmark compliant joints (17.5× that of van der
Waals force connectors and 1052× that of electrostatic connectors), while maintaining the same compliance (modulus) as thehost robot’s body. At an elevated temperature (65 °C), the joint
strength decreases to less than one-tenth of the original for easy
detachment. We have also demonstrated repeated connection
and disconnection of the joint over 250 cycles while maintaining more than 50% of its original strength. Finally, the reversible
joint does not require precise alignment for attachment and detachment. A 1.5 mm thick and 1 cm2 area BTF requires ≈400 Jof energy to reach the (dis)connection temperature, promising
untethered operation.
The superior performance of this reversible joint material
architecture allows for a ﬁrst-of-its-kind demonstration of aself-amputating soft robot that can escape from entrapment

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www.advmat.deunder a rock and a group of interfusing soft crawlers thatcan cross a gap too wide for one alone. These demonstrations are uniquely enabled by bio-inspired fracture planes withtunable stiﬀness, and show that autonomy and interfusion
may be promising survival mechanisms for soft robots invariable environments.

4. Experimental Section
Reversible Joint Fabrication: To decrease the melting point ofpolystyrene-block-polyisoprene-block-polystyrene (SIS; 432393, Sigma
Aldrich) from 140 to 115 °C for easier processing, pristine SIS wasplasticized with paraﬃn oil (18512-1L, Sigma Aldrich). Brieﬂy, SIS pellets
and paraﬃn oil were added to a beaker in a 60:40 ratio by weight, andthe mixture was thinned with toluene (adding 2× the mixture’s weight intoluene). This ratio of SIS and paraﬃn was selected after the initial testingof diﬀerent ratios (see Appendix, Figure S9, Supporting Information). This
mixture was placed on a hot plate at 80 °C for 4 h with occasional manual
stirring to help the dissolution of SIS. Then the toluene was allowed toevaporate overnight, resulting in plasticized SIS. See Appendix, Figure S2(Supporting Information) for illustration.
Plasticized SIS was fabricated into an open-cell foam using round sugar
particles (SUGLETS, Colorcon) as a sacriﬁcial material. To incorporate thesugar particles into the SIS, the SIS was kept at an elevated temperature(to suppress the SIS viscosity), with pressure applied so the SIS ﬂowed
into the gaps between the sugar particles. A cylindrical steel herb press(BouPower, Amazon) was ﬁlled with closely packed sugar and the plasticized SIS. The cylinder was then put into an oven at 120 °C for 4 h, occasionally turning the handle of the press to sustain the pressure on the SIS.
The result was a disk of closely packed round sugar with SIS in between.
This disk was then put into room temperature water to dissolve away thesugar overnight, yielding an open-cell foam of plasticized SIS. To ﬁnalize
the bicontinuous thermoplastic foam (BTF), the SIS foam was manuallypressed into pre-cure silicone (DragonSkin 10, Smooth-On). After curing,the BTF was cut into appropriate sizes with an X-ACTO knife for use as areversible joint.
T-Peel and Cyclic Tests: For both peel and cyclic mechanical tests, a materials testing machine (Instron 3345) was used to record the force during
the peeling and the distance of separation for the joints. The temperature
conditions were created via a digital heat gun (N2030, Neu Master) blowing directly on the samples.
For the T-peel tests (Figure 2A–D), the speed of peeling was 60 mm
min–1 and the samples were peeled to a displacement of 60 mm. The Tjunction was made of two strips of muslin fabric (4 × 1 cm) coated insilicone (Smooth-Sil 950, Smooth-On), one being pulled upward and one
downward. The silicone layer emulated a silicone-based body interface,
and the muslin fabric limited the amount of stretch in the substrate to
make sure deformation and breakage occurred at the joint. Each strip had
a BTF layer (1 × 1 cm in contact area with various thicknesses) glued on via
silicone adhesive (Silpoxy, Smooth-On). The two BTF layers were touched
together while heated to 120 °C for 3 min and then cooled down to room
temperature before testing. The heating step ensured that the two pieces
were fused to each other.
For the cyclic tests, ﬁrst, the process of “heat-connect-cool-heatdisconnect” was repeated with the BTF joint for 0, 50, 100, and 150 cycles,with four respective sets of samples. Then, the resultant joint was pulled
to failure at 60 mm min–1 to record the maximum force (Figure 2E,F).
Second, for a fused joint at room temperature, samples were repeatedly
strained to a set displacement and then returned to the original state for
200 cycles (Figure 2G).
Heating and Cooling for Joint Demonstration: Copper heater: In boththe soft quadruped and soft crawler demonstrations, an embedded copper
coil was used to heat up the reversible joint to 120 °C for (dis)connection.
This copper coil was fabricated using a ProtoLaser U4 (LPKF, Germany)to cut out a serpentine pattern on a 1 × 4 cm piece of single-sided ﬂexible
PCB Board (thickness = 0.005 in, FlexPCB, Pulsar). The resulting copper

Adv. Mater. 2024, 2400241

coil had a resistance of 0.8 Ω. A constant current of 2 A (input power of
3.2 W) was supplied to each joint for ≈2 min before the joint was heatedto the break-oﬀ/reattach temperature (120 °C).
Cooling: To complete fusing, the joint should cool to room temperature(25 °C). The cooling time varied based on environmental conditions suchas temperature and air circulation. In the laboratory tests, it was found the
BTF took ≈3 min to cool. The rate of cooling depends on the BTF’s surface
area, with a larger surface area allowing for faster heat transfer.

Supporting Information
Supporting Information is available from the Wiley Online Library or fromthe author.

Acknowledgements
This work was supported by the National Science Foundation (NSF) under grant no. EFMA-1830870 and IIS-1955225, as well as by the Oﬃce ofNaval Research under award N00014-21-1-2417. Any opinions, ﬁndings,
and conclusions or recommendations expressed in this material are thoseof the authors and do not necessarily reﬂect the views of the Oﬃce ofNaval Research. S.J.W. was supported by the NASA NSTGRO Fellowship(80NSSC22K1188).

Conﬂict of Interest
The authors declare no conﬂict of interest.

Data Availability Statement
The data that support the ﬁndings of this study are available from the corresponding author upon reasonable request.

Keywords
autotomy, modular robotics, soft robotics
Received: January 5, 2024
Revised: April 25, 2024
Published online:[1] D. Shah, B. Yang, S. Kriegman, M. Levin, J. Bongard, R. KramerBottiglio, Adv. Mater. 2021, 33, 2002882.
[2] K. Gotthard, S. Nylin, Oikos 1995, 3.
[3] B. H. Robison, Mar. Freshwater Behav. Physiol. 1999, 32, 17.
[4] J. S. Reidenberg, Anat. Rec. 2007, 290, 507.
[5] T. L. Maginnis, Behav. Ecol. 2006, 17, 857.
[6] N. J. Mlot, C. A. Tovey, D. L. Hu, Proc. Natl. Acad. Sci. 2011, 108, 7669.
[7] A. R. Clause, E. A. Capaldi, J. Exp. Zool. Comp. Exp. Biol. 2006, 305,965.
[8] Z. Emberts, C. W. Miller, D. Kiehl, C. M. St. Mary, Behav. Ecol. 2017,28, 1047.
[9] F. J. Vernerey, T. Shen, S. L. Sridhar, R. J. Wagner, J. R. Soc., Interface
2018, 15, 20180642.
[10] S. Goss, S. Aron, J.-L. Deneubourg, J. M. Pasteels, Naturwissenschaften 1989, 76, 579.
[11] M. Tennenbaum, Z. Liu, D. Hu, A. Fernandez-Nieves, Nat. Mater.
2016, 15, 54.

2400241 (7 of 8)© 2024 Wiley-VCH GmbH

15214095, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adma.202400241 by Yale University, Wiley Online Library on [16/07/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License

www.advancedsciencenews.com

www.advmat.de[12] G. S. Adams, B. A. Converse, A. H. Hales, L. E. Klotz, Nature 2021,
592, 258.
[13] Q. T. Davis, S. Woodman, M. Landesberg, R. Kramer-Bottiglio, J.
Bongard, in 2023 IEEE Int. Conf. on Soft Robotics (RoboSoft), IEEE
2023, pp. 1–6.
[14] I. Wilkie, Nat. Mater. 1978, 186, 311.
[15] L. Bickell-Page, G. Mackie, Philosophical Transactions of the Royal Society of London. Series B: Biological Sciences 1991, 331, 155.
[16] I. Wilkie, Cell Tissue Res. 1979, 197, 515.
[17] K. W. Sanggaard, C. C. Danielsen, L. Wogensen, M. S. Vinding, L. M.
Rydtoft, M. B. Mortensen, H. Karring, N. C. Nielsen, T. Wang, I. B.
Thøgersen, J. J. Enghild, Plos one 2012, 7, e51803.
[18] W. E. Dobson, J. Exp. Mar. Biol. Ecol. 1985, 94, 223.
[19] K. Gilpin, D. Rus, IEEE Rob. Autom. Lett. 2010, 17, 38.
[20] C. Zhang, P. Zhu, Y. Lin, Z. Jiao, J. Zou, Adv. Intell. Syst. 2020, 2,
1900166.
[21] M. A. Robertson, J. Paik, Sci. Rob. 2017, 2, 9.
[22] S. Kurumaya, B. T. Phillips, K. P. Becker, M. H. Rosen, D. F. Gruber,
K. C. Galloway, K. Suzumori, R. J. Wood, Soft Rob. 2018, 5, 399.
[23] Z. Jiao, C. Ji, J. Zou, H. Yang, M. Pan, Adv. Mater. Technol. 2019, 4,1800429.
[24] J.-Y. Lee, W.-B. Kim, W.-Y. Choi, K.-J. Cho, IEEE Rob. Autom. Lett. 2016,
23, 30.
[25] A. Vergara, Y.-s. Lau, R.-F. Mendoza-Garcia, J. C. Zagal, PloS one 2017,12, e0169179.
[26] J. Zou, Y. Lin, C. Ji, H. Yang, Soft Rob. 2018, 5, 164.
[27] W. Wang, N.-G. Kim, H. Rodrigue, S.-H. Ahn, Mater. Horiz. 2017, 4,367.
[28] J.-Y. Lee, K.-J. Cho, in 2017 14th Int. Conf. on Ubiquitous Robots andAmbient Intelligence (URAI), IEEE, 2017, pp. 65–67.
[29] P. Moubarak, P. Ben-Tzvi, Rob. Auton. Syst. 2012, 60, 1648.
[30] B. T. Kirby, B. Aksak, J. D. Campbell, J. F. Hoburg, T. C.
Mowry, P. Pillai, S. C. Goldstein, in 2007 IEEE/RSJ Int. Conf.

Adv. Mater. 2024, 2400241

on Intelligent Robots and Systems, IEEE, 2007, pp. 2787–
2793.
[31] J. Germann, M. Dommer, R. Pericet-Camara, D. Floreano, Adv. Rob.
2012, 26, 785.
[32] Y. A. Tse, S. Liu, Y. Yang, M. Y. Wang, in 2020 3rd IEEE Int. Conf. on
Soft Robotics (RoboSoft), IEEE, 2020, pp. 150–155.
[33] L. Wang, F. Iida, IEEE/ASME Trans. Mechatron. 2012, 18, 1397.
[34] J. Huang, Y. Liu, Y. Yang, Z. Zhou, J. Mao, T. Wu, J. Liu, Q. Cai, C. Peng,
Y. Xu, B. Zeng, W. Luo, G. Chen, C. Yuan, L. Dai, Sci. Rob. 2021, 6, 53.
[35] A. López-Díaz, J. De La Morena, F. Ramos, E. Vázquez, A. S. Vázquez,in 2022 Int. Conf. on Robotics and Automation (ICRA), IEEE, 2022, pp.
7124–7130.
[36] S. Park, K. Mondal, R. M. Treadway III, V. Kumar, S. Ma, J. D. Holbery,
M. D. Dickey, ACS Appl. Mater. Interfaces 2018, 10, 11261.
[37] A. N. Gent, G. R. Hamed, Polym. Eng. Sci. 1977, 17, 462.
[38] A. Ghouﬁ, P. Malfreyt, D. J. Tildesley, Chem. Soc. Rev. 2016, 45, 1387.
[39] W. Soboyejo, Mechanical Properties of Engineered Materials, CRC
Press, Boca Raton 2002.
[40] T. C. Hales, arXiv preprint math/9811071, 1998.
[41] N. Tuncer, G. Arslan, E. Maire, L. Salvo, Mater. Sci. Eng., A 2011, 530,633.
[42] B. Jiang, Z. Wang, N. Zhao, Scr. Mater. 2007, 56, 169.
[43] E. Ciecierska, M. Jurczyk-Kowalska, P. Bazarnik, M. Gloc, M. Kulesza,
M. Kowalski, S. Krauze, M. Lewandowska, Compos. Struct. 2016, 140,67.
[44] X. Xia, X. Chen, Z. Zhang, X. Chen, W. Zhao, B. Liao, B. Hur, J. Magnesium Alloys 2013, 1, 330.
[45] B. van Bochove, D. W. Grijpma, Eur. Polym. J. 2021, 143, 110223.
[46] R. F. Shepherd, F. Ilievski, W. Choi, S. A. Morin, A. A. Stokes, A. D.
Mazzeo, X. Chen, M. Wang, G. M. Whitesides, Proc. Natl. Acad. Sci.
2011, 108, 20400.
[47] M. Levin, Biochem. Biophys. Res. Commun. 2021, 564, 114.
[48] M. Levin, Front. Psychol. 2019, 10, 2688.

2400241 (8 of 8)© 2024 Wiley-VCH GmbH

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