SCIENCE ROBOTICS | RESEARCH ARTICLESENSORS

Sustainable manufacturing of sensors onto soft systems
using self-coagulating conductive Pickering emulsions
Sang Yup Kim1, Youngwoo Choo2, R. Adam Bilodeau1,3, Michelle C. Yuen1*, Gilad Kaufman2,
Dylan S. Shah1, Chinedum O. Osuji2†, Rebecca Kramer-Bottiglio1‡

Copyright © 2020
The Authors, somerights reserved;
exclusive licensee
American Association
for the Advancementof Science. No claim
to original U.S.
Government WorksINTRODUCTION

Compliant sensors are becoming important components in the fieldsof biomedicine (1–3), sports (4, 5), and robotics (6–8) as a way tocommunicate between the user and device by eliciting reliable state
information, such as body movements or deformations. Compliant andgeometrically complex soft robotic systems require integrated sensors
that can stretch and conform along surface contours while adding
negligible stiffness to the system (9–12). Over the last decade, conductive
composite materials have gained traction as wearable, stretchable
sensors because of their scalability in manufacturing processes (13–20),
an attribute that is difficult to achieve using other strategies such as
patterned conductive traces (21–24) or fluidic channels filled with
conductive liquid (25–28). Conductive composite-based sensors consistof polymer matrices and conductive fillers, and these sensors can be
designed to measure deformation via changes in electrical resistanceor capacitance. These devices are typically fabricated by a liquidmolding process that comprises a distribution of solid-phase fillers
into liquid-phase resins and subsequent casting of the filler/resin
mixture. Hence, obtaining sufficient resin flow during this process
is critical for quality control, leading to widespread use of solvents(e.g., toluene, cyclohexane, and hexane) to dissolve the polymer resin
and thin down the material (29–31).

1
Mechanical Engineering and Material Science, School of Engineering and Applied
Science, Yale University, 9 Hillhouse Ave., New Haven, CT 06511, USA. 2Chemical
and Environmental Engineering, School of Engineering and Applied Science, Yale
University, 9 Hillhouse Ave., New Haven, CT 06511, USA. 3School of MechanicalEngineering, Purdue University, 585 Purdue Mall, West Lafayette, IN 47907, USA.
*Present address: Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson Air Force Base, OH 45433, USA.
†Present address: Department of Chemical and Biomolecular Engineering, Universityof Pennsylvania, 220 S 33rd St., Philadelphia, PA 19104, USA.
‡Corresponding author. Email: rebecca.kramer@yale.edu

Kim et al., Sci. Robot. 5, eaay3604 (2020)

26 February 2020

However, critical issues arise because of the typically high concentration of such solvents in the resin/filler mixture [ca. >70 volume %(vol %)], such as prolonged postprocesses to evaporate the solvents (32)and a dangerous manufacturing environment (33). Moreover, solvents capable of dissolving polymer resins can also penetrate into cross-linked
polymer networks and cause undesirable swelling and degradationof the host structure (34), be it a robot or a human. For example, printing a cyclohexane-containing resin/filler mixture onto an inflated
latex balloon results in the immediate bursting of the balloon. The
use of nonconventional solvents, such as ethanol (EtOH) and water,in lieu of conventional solvents, has thus far been infeasible because
they poorly dissolve most polymer resins, leading to the creation ofan emulsion with two distinct phases when mixed instead of thedesired homogeneous solution.
Here, we addressed these challenges using an EtOH-based
Pickering emulsion stabilized by nanoparticles (35–37). Using
EtOH, instead of conventional solvents, enabled a multitude ofwearable and robotic systems, including self-sensing McKibben
actuators and hybrid robotic textiles, to be realized through a sustainable manufacturing process. Our Pickering emulsion allowed
us to print directly onto the bare polymer surfaces of all-soft actuators for monitoring surface strains. Pickering emulsions were also
printed onto conventional polymer textiles to fabricate wearable
sensors that measure strain values with high sensitivity and negligible
hysteresis.
RESULTS

Characteristics of self-coagulating conductive
Pickering emulsions
Our study was inspired by hemostasis, a natural bodily processachieved by blood plasma that delivers proteins and platelets to an
open wound (38). The wound healing process is rapid and efficient
1 of 12

Downloaded from http://robotics.sciencemag.org/ at Yale University on February 26, 2020

Compliant sensors based on composite materials are necessary components for geometrically complex systems such as wearable devices or soft robots. Composite materials consisting of polymer matrices and conductive fillers have facilitated the manufacture of compliant sensors due to their potential to be scaled in printing
processes. Printing composite materials generally entails the use of solvents, such as toluene or cyclohexane,
to dissolve the polymer resin and thin down the material to a printable viscosity. However, such solvents causeswelling and decomposition of most polymer substrates, limiting the utility of the composite materials. Moreover,many such conventional solvents are toxic or otherwise present health hazards. Here, sustainable manufacturing of sensors is reported, which uses an ethanol-based Pickering emulsion that spontaneously coagulates andforms a conductive composite. The Pickering emulsion consists of emulsified polymer precursors stabilized byconductive nanoparticles in an ethanol carrier. Upon evaporation of the ethanol, the precursors are released,
which then coalesce amid nanoparticle networks and spontaneously polymerize in contact with the atmospheric moisture. We printed the self-coagulating conductive Pickering emulsion onto a variety of soft polymeric systems, including all-soft actuators and conventional textiles, to sensitize these systems. The resulting
compliant sensors exhibit high strain sensitivity with negligible hysteresis, making them suitable for wearable
and robotic applications.

SCIENCE ROBOTICS | RESEARCH ARTICLE
because of the direct transportation of microscale substances andtheir spontaneous coagulation (39). We adopted the fundamentalsof hemostasis to composite materials and their manufacturing tofabricate composite-based sensors in a sustainable fashion. We useda Pickering emulsion that consists of carbon black (CB) as stabilizing
nanoparticles, polydimethylsiloxane (PDMS) precursors as the dispersed
phase, and EtOH as the continuous phase and directly print it ontopolymer substrates (e.g., sheet or textile) to infuse conductive composite
materials (Fig. 1). We call this emulsion a self-coagulating conductive
Pickering (SCP) emulsion. Upon printing of the SCP emulsion andevaporation of EtOH, the colloids relaxed onto the substrate, and theKim et al., Sci. Robot. 5, eaay3604 (2020)

26 February 2020

2 of 12

Downloaded from http://robotics.sciencemag.org/ at Yale University on February 26, 2020

low viscosity of the PDMS precursors promoted the release and fusionof precursors amid the CB conductive network. One-part PDMS
precursors were then rapidly polymerized in contact with atmosphericmoisture without additional processing steps involving heat or pressure.
As a result, a polymer substrate spontaneously acquired an infused
conductive composite while avoiding issues with swelling or degradationof the substrate.
We began by analyzing the morphology and rheological behaviorof SCP emulsions (Fig. 2). An SCP emulsion was prepared in under
10 min by sonicating as-received CB in an EtOH medium for 6 min,adding PDMS precursors to the CB/EtOH suspension, and subsequently sonicating for 2 min. Optical
micrographs of the emulsion show CB/
PDMS-precursor droplets in the size
range of a few micrometers while present
in the EtOH medium (Fig. 2A). Scanningelectronic microscope (SEM) imaging
performed on the CB/PDMS droplets
cured within the medium confirmed that
the CB/PDMS-precursor droplets arespherical in shape and covered with CB
nanoparticles (Fig. 2B). Freeze-fractured
specimens further revealed the core-shellstructure, where the CB shell encapsulates
the PDMS-precursor core.
Dynamic light scattering measurementsof SCP emulsions showed the coexistenceof residual CB nanoparticles and CB/
PDMS-precursor droplets in the emulsion
and that the droplet size tends to decrease
with increasing CB content (Fig. 2C).
The average diameter (<2.0 m) of CB/
PDMS-precursor droplets was expected
to facilitate a uniform CB distribution
in the PDMS matrix by suppressing theformation of resin-rich regions greaterthan the droplets’ sizes. We have notobserved milli/microscale agglomerations of CB nanoparticles in the resulting
composite, even at >20 weight % (wt %)of CB concentrations. Ultraviolet-visible(UV-vis) transmittance measurements
performed on the diluted SCP emulsion
revealed the stability of the emulsion
against sedimentation or creaming,using absorbance retention as an indication of the settling of substances(Fig. 2D). We achieved a stable emulsion
at CB concentration >10 wt % withrespect to PDMS precursor alone, retaining >95% of the absorbance, whichis in contrast to the unstable emulsion
after 2 weeks (see inset of Fig. 2D). The
higher stability obtained at higher conFig. 1. Illustration of SCP emulsion and the printing process. SCP emulsion consists of PDMS precursors stabilized
centrations of electrically conductiveby CB nanoparticles in an EtOH medium. Printing SCP emulsion onto polymer substrates initiates a cascade reaction
CB also facilitated the creation of electrithat spontaneously forms the conductive composite. Using EtOH as the medium enables a sustainable printing process
cal pathways in the final composite
without swelling or degradation of the polymer substrate, a typical drawback when using conventional suspension
material, which is essential for sensing
inks with harsh, often toxic solvents. The nondestructive characteristic of SCP emulsions unlocks opportunities toapplications.
integrate complaint sensors into soft systems.

SCIENCE ROBOTICS | RESEARCH ARTICLE

SCP emulsion printed onto barepolymer substrate
We created conductive composites on thebare surface of the cured Ecoflex 00-30
substrates by directly printing SCP emulsion and characterized the electro­
mechanical properties of the resulting
composites (Fig. 3). The sheet resistanceof the produced composite was dependent
on the CB concentration in the PDMS
matrix and the composite morphology(Fig. 3A). Higher CB content, with respect
to PDMS precursors, generally increased
the local electrical conductivity and also
stimulated the formation of wrinkles
and microscale cracks on the composite(see inset images of Fig. 3A). An SEM
image of a conductive composite at 15 wt %of CB concentration shows the wrinkled
surface morphology, and a transmission
electron microscopy (TEM) image further
Fig. 2. Material characteristics and rheological behavior of SCP emulsions. (A) Optical micrograph of CB/
reveals the presence of embedded CB
PDMS-precursor droplets within EtOH medium. (B) SEM images of a CB/PDMS droplet cured within EtOH medium
nanoparticles (Fig. 3B). Such wrinkles
and its freeze-fractured morphology. The cross-sectional image is false-colored to highlight the PDMS core (blue) and thefurther developed into microscale cracks
CB shell (red). (C) Dynamic light scattering results for the size distribution of the emulsion at various CB concentrations.
at >15 wt % CB content, presumably
The average diameter for the droplets is around 2 m. (D) Absorbance retention of the emulsions with varying CB
attributed to the clustering of colloids asconcentrations, obtained from UV-vis measurements at 450 nm. The inset image shows SCP emulsions at 5 and 15 wt %after 2 weeks. (E) The rheological profile of a PDMS precursor alone, a CB/PDMS suspension, and SCP emulsions.
EtOH evaporates (41), and the resulting
EtOH thins out a CB/PDMS suspension to a printable viscosity, and the colloids in SCP emulsions exhibit a shear-thinningcomposite morphology enabled low-­
behavior. (F) Release and coalescence of PDMS precursors when printed onto a glass slide and EtOH evaporates.
hysteresis, high-sensitivity strain sensing.
Coalesced PDMS precursors spontaneously polymerize in contact with atmospheric moisture.
However, at >25 wt % CB content, thecomposite was easily delaminated from
SCP emulsions exhibited shear-thinning behavior, which is beneficial the substrate, thus degrading its value as a sensor.
for the nozzle-­assisted printing process, and the EtOH content deterThe microstructure of conductive composites, consisting of wrinkles
mined the overall viscosity (Fig. 2E and fig. S1). For example, the SCP and microcracks, allowed the composite to stretch without introducing
emulsion containing 95 vol % of EtOH exhibited a 1000-fold lower damage by leveraging the opening of existing cracks (Fig. 3C).
viscosity compared to the CB/PDMS mixture alone. Such low viscosity For example, in the case of printing a control ink containing
allowed CB/PDMS-precursor droplets to create a uniform composite cyclohexane, a composite film devoid of wrinkles/cracks catalayer and permeate through various types of textile substrates. Upon strophically failed at the strain of 180% because the composite material
printing, the EtOH medium rapidly evaporated, CB/PDMS-precursor itself was insufficiently stretchable. However, when SCP emulsion at
droplets relaxed onto the substrate, and PDMS precursors were re- 20 wt % was printed, the composite film could accommodate applied
Kim et al., Sci. Robot. 5, eaay3604 (2020)

26 February 2020

3 of 12

Downloaded from http://robotics.sciencemag.org/ at Yale University on February 26, 2020leased from the CB shell, fusing together
to form a monolithic composite (Fig. 2F).
The entire process was rapidly accomplished upon evaporation of the EtOH.
We surmise that the coalescence of CB/
PDMS-precursor droplets can be further
controlled by tuning the substrate material and texture (40). The resulting infused
droplets rapidly cured at ambient conditions in contact with the atmosphericmoisture because the precursor containshydroxy-­terminated PDMS and a cross-­linker [tris(cyclohexyl­amino)methylsilane],
which triggers a two-step curing reaction
when exposed to the moisture. As a result,relaxed CB/PDMS-precursor droplets
spontaneously became a conductive composite material.

SCIENCE ROBOTICS | RESEARCH ARTICLE

strain by opening up its existing cracks, as evidenced from the increased porosity shown in Fig. 3C. Furthermore, the crack opening
mechanism gave rise to the sharp increase in the electrical resistancewith increasing strain because the separation of crack planes induces
a sudden disconnection (see fig. S2 for details). We call this underlying
principle a crack opening sensing mechanism, which is a modification of the crack-driven model in (23). When the crack opening ratio
Kim et al., Sci. Robot. 5, eaay3604 (2020)

26 February 2020

is defined as (), our sensing mechanism predicts the normalized
electrical conductance across a crack (​​ S¯ ​​) as ​​  S¯ ​ = 1 − ​(see Materials and Methods for details).
This simple underlying principle provides a route to tailor therange of working strains by leveraging the width of existing cracks,
as shown in Fig. 3D. Compared to printing SCP emulsions onto a
pristine substrate, stretching the substrate before the printing decreased
4 of 12

Downloaded from http://robotics.sciencemag.org/ at Yale University on February 26, 2020

Fig. 3. Characteristics of conductive composites directly printed onto bare polymer substrates. (A) Sheet resistance of the composites and their surface morphology.
Wrinkles/cracks formed as EtOH evaporates become apparent with an increasing CB concentration. (B) SEM image of the composite surface at a 15 wt % CB concentration
and TEM inset image revealing uniformly distributed CB nanoparticles. (C) Porosity of composites as a function of applied strains. Composite microstructure consisting of
microscale cracks give rise to the stretchability (SCP emulsion), different from the crack-free composites that exhibit a catastrophic failure (control ink). (D) Measured crack
opening ratio in composites and the normalized electrical conductance predicted from the measured data and the inset crack opening model. Error bars indicate 1 SD.
Printing SCP emulsions onto a prestretched substrate (at  = 100%) decreases the initial crack width (at  = 0%), thereby increasing the working strain range fromca. 20 to 100%. (E) Hysteresis in mechanoelectrical responses of the composites produced on Ecoflex 00-30 substrates. Crack opening mechanism of the SCP emulsion
sensor grants negligible hysteresis and high sensitivity to strain within the working strain range. (F) Long-term fatigue behavior of the composite sensor produced from
SCP emulsions.

SCIENCE ROBOTICS | RESEARCH ARTICLE

SCP emulsions printed into textile substrates
To broaden the use of our sensors, we printed SCP emulsions into
textiles to embed sensing capabilities into wearables (Fig. 4). Upon
direct printing of SCP emulsions onto a textile, CB/PDMS-precursor
droplets permeated through the textile and formed a conductive
composite conformally around the textile fiber rovings (Fig. 4A).
Optical micrographs of a polyester/spandex textile onto which SCP
emulsion was printed reveal CB/PDMS composites infused into thetextile (Fig. 4B). The infused composites maintained the fiber architectureof the textile, rather than creating a thick composite layer on top,
thereby preserving porosity that grants breathability and comfort
for wearable applications (fig. S3). In addition, printing SCP emulsions
into textiles propelled evaporation of the solvent as the EtOH medium
separated from CB/PDMS-precursor droplets and rapidly absorbed
into the textile (fig. S4). This separation of active materials (CB/PDMS
precursor) from a vehicle (EtOH) grants an effective printing processthat locally infuses a sufficient amount of conductive material, evenafter just a single printing pass.
SEM imaging further confirms that individual fiber rovings ofthe textile were coated with the infused composites (Fig. 4C). The
infused composites exhibit microcracks on the top surface where
CB/PDMS-precursor droplets have penetrated through the textile
because of the ample fiber-interstitial spaces (ca. >10 m) that are
greater than the precursor droplet size (42). Such generic microcracks
further assisted in preserving inherent textile properties and rendereda strain-sensing ability that exploits aforementioned crack opening
behavior (23). The permeating CB/PDMS-precursor droplets allowed
the final composites to infuse deep into the textile and obtain aninterpenetrating structure with textile fibers for a secure bond (fig. S5).
TEM imaging reveals that CB nanoparticles were well distributed
along a PDMS matrix of 200-nm thickness to grant consistent electricalproperties, even when the composite layer was thinner than the precursor droplet diameter (Fig. 4D).
Consequently, printing SCP emulsions created quality conductive
composite-infused textiles that can be used as resistive strain sensors.
Kim et al., Sci. Robot. 5, eaay3604 (2020)

26 February 2020

Notably, polyester/spandex textile sensors exhibited a similar tensile
modulus to the neat textiles (Fig. 4E). The increment of the tensile
modulus induced by the SCP emulsion was <30%, which is significantly lower than the >200% for textiles onto which the control suspension was printed (fig. S6). Such minimal mechanical footprint of theinfused composite is advantageous for wearable applications, which
necessitate conformability and stretchability. The sheet resistance ofthe textile sensors was higher than that of the infused composites
alone because the textile fibers are not electrically conductive. Such
resistance of the textile reached the minimum value around 15 wt %of CB concentration, presumably because of the generic crack formation, a behavior also observed from the composites printed onto
bare polymer substrates (see Fig. 3A).
Stretching composite-infused polyester/spandex textiles increased
their electrical resistance with negligible hysteresis after the initial~10 cycles of Mullins softening (43), and CB concentrations in the
PDMS matrix modulated such electrical responses (Fig. 4F). For
example, a composite-infused textile at 10 wt % CB concentration
elicited an 810% increase in the resistance when stretched, which issignificantly higher than 260% for a textile at 15 wt % CB concentration.
Likewise, similar to the composite-printed bare polymer substrates,
these textile sensors exhibited high sensitivity to strains (i.e., gauge
factor >30) and negligible hysteresis. Uniaxial fatigue testing performed
at 17% strain reveals the consistent electrical response of a textile
sensor over 10,000 cycles, demonstrating the mechanical robustnessof the infused composite (Fig. 4G). Such fatigue resistance wasachieved through precise control of CB concentrations and printing
directions (fig. S7).
We also printed the SCP emulsion onto various textile substrates(i.e., kinesiology tape and cotton) and investigated their electricalcharacteristics (Fig. 4H). Both the strain sensitivity and the working
strain (linear response) of the textile sensors are governed by thesubstrate textile’s fiber architecture. For example, the working
strains are 4.6, 12, and 28% for textile sensors using kinesiology
tape, spandex, and cotton, respectively. Increasing the fiber packing
density in a textile results in higher working strain and an inversely
lower strain sensitivity. Such tunability of sensor characteristics
using substrate textiles further supports that the sensing mechanism
is associated with crack asperities rather than the composite’s
geometrical shape change. As a result, the strain sensitivity reacheda gauge factor of up to 150 when SCP emulsion was printed onto
kinesiology tape with coarsely packed fibers, enabling its use as anultrasensitive strain sensor.
Sensor integration onto all-soft systems through directly
printed sensors
When printed onto bare polymer surfaces, SCP emulsions enabled
direct strain monitoring of various types of all-soft systems widespreadin soft robotics (Fig. 5). SCP emulsions were printed onto the all-soft
systems to directly create strain sensors and maintain the original
pressure-responsive behaviors. SCP emulsion consisting of EtOH
broadened the selection of compatible substrates for printing by
mitigating the damage caused from the contained solvent. Ourpresented method is thus capable of printing composite sensors
onto a pressurized balloon, which has been impossible to achieve
because of the catastrophic failure of balloons when exposed toconventional solvents (movie S1).
When SCP emulsions were printed onto a pressurized latex balloon, the printed sensor could be used to monitor the inflation of the
5 of 12

Downloaded from http://robotics.sciencemag.org/ at Yale University on February 26, 2020

the initial crack separation (from 0 = 0.6 to 0.0), thereby increasing the range of working strains (from 20 to 80%). The electromechanical response of the composite sensors manifests the crack
opening sensing mechanism (Fig. 3E). A composite sensor produced
from a control ink exhibits high hysteresis and low stain sensitivity
because its electrical resistance is solely determined by the shapechange of the composite. On the other hand, composite sensors
printed from SCP emulsions reveal negligible hysteresis and highsensitivity to strains, driven by the crack opening sensing mechanism.
Furthermore, the appearance of hysteresis at  > 20% for the composite
produced on a pristine substrate is in agreement with the predictionin Fig. 3D (working strain of 20%), and the further increase in theresistance is expected to be from the geometrical shape change, notthe crack opening.
Cyclic tests performed on the composite fabricated onto a prestretchedsubstrate reveal consistent sensor responses when the composite is
stretched up to 100% (Fig. 3F). We expect that such reliable response
is due to the wrinkled/cracked composite structure that redistributes
applied strains and minimizes the strain concentration. In sum,printing SCP emulsions onto bare polymer substrates demonstrates
a facile, rapid, and safe process to implement quality sensors (i.e.,
negligible hysteresis and high strain sensitivity) directly onto soft
systems.

SCIENCE ROBOTICS | RESEARCH ARTICLE

Kim et al., Sci. Robot. 5, eaay3604 (2020)

26 February 2020

6 of 12

Downloaded from http://robotics.sciencemag.org/ at Yale University on February 26, 2020

Fig. 4. Morphology of CB/PDMS composites infused into textiles. (A) Illustration of the direct printing of SCP
emulsions into textiles. (B) Optical micrograph of the surface of a composite-infused textile. (C) SEM image of theinfused composite that contains generic microcracks on the surface. (D) TEM images of a thin CB/PDMS composite
layer bonded to the textile fiber. EDS result confirms the wet-out of the textile with the PDMS matrix. (E) Sheet
resistance of composite-infused textiles and the tensile modulus increment due to the composite. Error bars indicate
1 SD. Composites from SCP emulsions add negligible stiffness to the textile, unlike that from a control ink. (F) Electricalresistance of composite-­infused polyester/spandex textiles during uniaxial cyclic loading for the initial 100 cycles (N).
Highlighted curves indicate N = 50. (G) Long-term fatigue behavior of the composite-infused polyester/spandex textile.
The inset shows a detailed electrical resistance as a response to a sinusoidal uniaxial displacement. (H) Strain-responsive
electrical resistances of composite-infused textiles with various substrate textiles.

balloon (Fig. 5A and movie S2). The
printed sensor negligibly affected the inflation trajectory of the balloon, and its
electrical resistance increased as the balloon inflated. Such a resistance-inflation
relation enabled the estimation of thestate of inflation of the latex balloon
with ca. 9.8% error (see fig. S8). In addition, because the sensors do not
perturb the original inflation trajectoryof the balloon, SCP emulsions can be
printed at different angles with respect
to the balloon’s width (i.e., 0°, 45°, and90°) and used to elucidate the directionality of the balloon’s inflation/deflation(Fig. 5B).
We also printed SCP emulsions onto
typical soft robotic actuators, composedof Ecoflex 00-30 silicone elastomer, tomonitor the movements of the actuators.
For example, by implementing the composite sensors on an elastomeric gripper(44), we monitored the movement of its
fingers (Fig. 5C). Similarly, when SCP
emulsions were printed onto a fiber-­reinforced actuator, the reconfigured
motion of the actuator was tracked as
the location of a strain-limiting layer was
adjusted (Fig. 5D and movie S3). The
attached strain-limiting layer was easily
detached and reattached using a self-­adhesive layer (45), thereby applying
different bending strains to the sensor.
These demonstrations serve to collectively
showcase the viability of SCP emulsion
in producing sensors for all-soft systems.
To further demonstrate the capabilityof our composite sensors in prototypical
soft systems, we devised self-sensing
McKibben actuators, the simplest ofwhich are latex balloon actuators surrounded by a mesh sleeve that brings
about either extension or contraction
when pressurized (Fig. 6). To date, despite
the ubiquity of McKibben actuators in
the field of soft robotics, precisely monitoring their actuation has been difficult
to achieve. This challenge is attributed
to the requisite compliant nature of
McKibben actuators, as their intended
trajectories can be easily compromised
when sensors are separately attached.
Our self-sensing McKibben actuators
address this issue by seamlessly integrating
sensing capabilities into the actuators
using a balloon with an SCP-printed
trace (Fig. 6A). With the sensor embedded
inside of a contraction sleeve, the sensor
response tightly corresponds to theSCIENCE ROBOTICS | RESEARCH ARTICLE

Wearable textile sensors
To demonstrate the viability of SCP
emulsions, we used textile sensors tomonitor strains of arbitrarily shaped,
complex systems (Fig. 7). When SCP
emulsions were printed into kinesiology
tape, the produced textile sensor easilyadhered to the convex surface of a fabric-­
based pneumatic gripper finger using aself-adhesive layer (Fig. 7A). The self-­adhesive layer provided close contact
between the textile sensor and the gripper
finger, which yielded accurate strain measurements and prevented the sensor fromdelaminating from convex contours(fig. S9). We also show that textile sensors
could easily be printed into active clothes
that reveal specific motion artifacts present
in normal activity (Fig. 7B and movie S5).
For example, a kneepad fabricated usinga polyester/spandex textile enabled differentiation of gait while walking, climbing,and descending. Ready-made garments(i.e., kinesiology tapes and cotton gloves)
were also transformed into smart garmentswith strain-sensing ability when SCP
Fig. 5. All-soft systems with directly integrated strain sensors. (A) A latex balloon with an integrated composite
emulsion was directly printed on (fig. S10).
sensor. Volumetric expansion of the balloon is monitored at <10% of error from the sensor response. (B) A sphericallatex balloon with SCP emulsion traces printed at varied printing directions (i.e., 0°, 45°, and 90°) to examine any
We further showcased the potential
directionality in the volumetric change. Error bars indicate 1 SD. (C) A silicone rubber four-legged gripper, onto whichof SCP emulsions by devising a robotic
SCP emulsion traces are printed. The resistance (R) of the trace increases as the gripper inflates. (D) A fiber-reinforced
textile, referred to as an OmniSkin, thatsilicone rubber actuator with a printed SCP emulsion trace to monitor the strain along the length. A strain-limiting
can be wrapped around inert objects
layer is adhered onto the actuator at different locations (i.e., 90° and 180° with respect to the sensor) to create various
to create robotic systems on demandbending strains.
(Fig. 7C). Our OmniSkin is a hybridof sensors integrated onto bare polymer
actuation. Therefore, no additional sensors are necessary to measure substrates (self-sensing McKibben actuator) and textiles (textile sensor)
the strain of the actuators.
and eliminates the need for attaching separate sensors to monitor
The sensor response of the self-sensing McKibben actuators begins strains. In this way, the design and manufacturing processes were sigonly after the bladder has been inflated to connect with the external nificantly simplified compared to the original version of the OmniSkin
mesh, as any sensor information before this only describes the infla- (46), enabling seamless monitoring of multidirectional strains. Fortion of the inner balloon and not the contraction of the actuator. example, when applied to a passive foam to elicit inchworm locoAny additional inflation of the bladder decreases the resistance in a motion, our OmniSkin could monitor strains along the length (x)linear, predictable manner, which allows us to develop a direct correlation and circumference (y) of the foam, respectively (movie S6). Likewise,
Kim et al., Sci. Robot. 5, eaay3604 (2020)

26 February 2020

7 of 12

Downloaded from http://robotics.sciencemag.org/ at Yale University on February 26, 2020

between sensor response and the lengthof the actuator (Fig. 6, B and C). Suchelectromechanical responses of the sensor,
different from a freely expanding balloon,
are because the external strain-limiting
mesh constrains the expansion of the
bladder. The actuation-responsive change
in the resistance is apparent and repeatable, allowing for self-sensing closed-loop
control of the linear actuators (Fig. 6D
and movie S4). We anticipate our self-­
sensing McKibben actuators to open a
new avenue in numerous domains ofrobotics because they not only minimize
the footprint of robotic components but
also are easy to manufacture.

SCIENCE ROBOTICS | RESEARCH ARTICLE

Kim et al., Sci. Robot. 5, eaay3604 (2020)

26 February 2020

8 of 12

Downloaded from http://robotics.sciencemag.org/ at Yale University on February 26, 2020

sions onto numerous types of polymeric
substrates, including sheets and textiles,
to directly integrate sensing functionalities into soft-bodied systems. The
printed sensors leveraged the opening ofgeneric cracks that granted high sensitivity
to strains and negligible hysteresis.
Printing strain sensors directly onto
inflatable actuators demonstrated theviability of using SCP emulsions forthe integration of sensors in all-soft
systems. The EtOH medium of the
SCP emulsion enabled printing of the
conductive composite directly onto
pressurized all-soft systems, a processthat would otherwise lead to substratedamage due to the presence of conventional solvents in typical printableconductive composites. Such direct
integration of sensors into soft systems
unlocks opportunities to realize original soft actuators for which motion
can be monitored in a seamless fashion.
Our self-sensing McKibben actuator,
which exploits a composite-infusedballoon as its bladder, demonstrates thepractical impact of SCP emulsions andtheir applications in robotics. By using
the sensor data, the motion of the self-­
sensing McKibben actuator was effectively controlled in a closed loop witha state estimation error <3.0%.
We further demonstrated the capabilities of SCP emulsions by printing the
conductive composite into textile substrates
and realized wearable textile sensors. These
textile sensors exhibited pristine fiberarchitecture as the emulsion conformallycoated the fiber rovings, enabling stretchability and breathability. The gauge factor
Fig. 6. Self-sensing McKibben actuator. (A) Schematic of the self-sensing McKibben actuator, enabled by printingof these textile sensors varied from 7 to 150,
SCP emulsion onto an inflated balloon. Pin and Pout indicate input pressure and output pressure, respectively, and R
and their working strain (linear region)
denotes the electrical resistance of the sensor. (B) Photographs of the actuator that elicits the contraction as the inranged from 4 to 28%, depending on the
ternal balloon inflates. With the input pressure (Pin), the initial length (l) of the actuator is decrease by Δl. (C) Sensor
fiber architecture of substrate textiles. Most
responses collected from the actuator at various contraction states and the calculated error on the state estimationof all, SCP emulsions enabled the safe and(predicted length calculated from state estimation compared to ground truth length). The error is quantified as rms,
a root mean square of the error. (D) Closed-looped control of the self-sensing McKibben actuator with the controller
rapid manufacturability of state-of-the-art
matching the sensor output to a target value.
composite sensors (fig. S11).
We hypothesize that the electrical
we expect SCP emulsions to unlock the sensorization of soft systems conductivity of the produced composites would be simple to moduand facilitate the state feedback for a wide range of applications, in- late by substituting nanoparticles with alternate conductive fillers
cluding health monitoring, exploration, and robotic manipulation.
(e.g., carbon nanotubes and silver nanowires) in SCP emulsions(47, 48). Increasing electrical conductivity of the produced composite
will expand the potential application of SCP emulsions, such as
DISCUSSION
stretchable electronics or textile electronics. In addition, material
We have developed SCP emulsions that spontaneously form con- selections and processing parameters can be further investigatedductive composites upon printing. Our SCP emulsion used EtOH to enhance the quality of stand-alone composites and textile senas a carrier solvent, rather than the conventional solvents, rendering sors. Chemistry and viscosity of the precursor, solvent liberationa safe manufacturing process for conductive composites that are rate, and texture of the substrate are of interest. In the end, we anticicompatible with nearly any host substrate. We printed SCP emul- pate the presented materials and fabrication strategies herein toSCIENCE ROBOTICS | RESEARCH ARTICLE

Preparation and printing of
SCP emulsions
A mixture of CB (0 to 1 g) and EtOH
(100 ml) was prepared and sonicatedusing a tip sonicator Q700 from QSonica.
Sonication pulse (1-s on/1-s off) wasapplied at 70% for 3 min to break up CB
aggregates. PDMS resin (4 g) was thenadded to the CB/EtOH mixture, and an
identical sonication pulse was applied
for 1 min to emulsify the resin. Uponthe completion of the process, the emulsion
was moved onto 50-ml falcon tubes forcharacterization and/or sensor fabrication.
Direct printing was performed using
a custom-made direct printing setup.
The direct printing setup consists of acommercial three-dimensional printing
machine (Simple Metal, PrintrBot) of
which its filament feeder is substituted
with a syringe pump. Substrate was placedon the stage of the setup, allowing theplanar movement while the syringe pumpenabled the feeding at 3.5 ml/min towrite traces onto the substrate. Screen
Fig. 7. Application of composite-infused textile sensors. (A) Composite-infused kinesiology tape attached to a pneuprinting was performed by covering amatic fabric gripper to monitor the grip opening ratio (Φ). This self-adhesive textile sensor can conform to the surface
substrate with an impermeable polyethylene
contours and measure the curvature upon gripping motion. (B) Active clothes that are easy to don/doff and elucidate aterephthalate film where a specific shapespecific motion artifact present in normal human activities. Ready-made garments are also transformed into strain-sensing
was laser-cut. For the case of prestretched
wearables (see fig. S10 for details). (C) OmniSkin, a hybrid of sensorized soft systems where composite sensors are printed
bare polymer substrates, a sheet of Ecoflex
onto bare polymer substrates and textiles, respectively. This robotic textile can monitor bidirectional strains applied to a00-30 was stretched 100% and fixed using
passive foam during its inchworm motion via integrated self-sensing McKibben actuator (x) and textile sensor (y).
adhesive tape, after which the screen-­
printing procedure was performed. A
open a new avenue for forthcoming technologies beyond wearables polycarbonate blade was moved across the film once the Pickeringand robotics.
emulsion was poured to fill the open space of the film. The resulting
substrates were dried at room conditions, and infused composites were
cured subsequently. Prepared composite-­coated substrates were tested after
MATERIALS AND METHODS
at least 1 hour from fabrication to ensure the complete cross-linking
Materialsof the PDMS matrix.
Room temperature vulcanizing amine cure silicone (SEMICOSIL
Among the stretchable polymer substrates that we used (Ecoflex
964 UV/CLEAR) was obtained from Wacker (Adrian, MI), and CB 00-30, Dragon Skin 10, and latex), no significant disparity was
Kim et al., Sci. Robot. 5, eaay3604 (2020)

26 February 2020

9 of 12

Downloaded from http://robotics.sciencemag.org/ at Yale University on February 26, 2020

(BLACK PEARLS 2000) was purchasedfrom Cabot (Boston, MA). Silver conductive epoxy (part no. 8331-14G) was
obtained from MG Chemicals (Surrey,
British Columbia), and a two-part silicone
elastomer Ecoflex 00-30 was purchasedfrom Smooth-On (Macungie, PA). Conductive Velcro (part no. 1324) andconductive thread (part no. 641) werepurchased from Adafruit (New York
City, NY). Polyester Lycra/Spandexfour-way stretch fabric (LY 902) was
obtained from Paylessfabric (Iva, SC)
and used as is. Kinesiology tape (TexGold FP) and cotton gloves (Hycome
white gloves) were purchased from
Amazon (Seattle, WA).

SCIENCE ROBOTICS | RESEARCH ARTICLE
detected in the resulting composite morphology. In detail, althoughan SCP emulsion wet latex (marginal polarity) better than silicone
elastomers (negligible polarity), the relaxation of the colloids persisted
in a similar fashion across the substrates. We thus speculate that thecompatible wettability window for the SCP emulsion is large because
it is the medium of the SCP emulsion that determines the wettability(EtOH in this study), which works as a vehicle for the active materials(CB/PDMS-precursor droplets).

Characterization of the composites and sensors
The sheet resistance of composites, either printed onto barepolymer substrates or textiles, was measured by four-point measurements using a digital multimeter (model 34401A) from Agilent.
The probe for the measurement was prepared with a 1-mm tip spacing,and liquid metal (eutectic gallium-indium alloy) was used to provide a low contact resistance. At least five measurements were performed for each case. SEM was used to reveal the composite surface
morphology and the cross-sectional surface, and energy-dispersive
x-ray spectroscopy (EDS) was performed at 10.0 kV within the
SEM. TEM was performed using a Tecnai Osiris from FEI operated
at 200 kV, and specimens were sectioned using a Leica EM UC7
cryo-microtome. Optical imaging was performed to analyze strain-­
responsive crack opening behavior of the composites using a Smartzoom
5 from Zeiss. The porosity of textile sensors were calculated using a
conventional digital single-lens reflex camera with a backlightapplied to the textile. The trace of EtOH after printing onto thetextile was visualized by the slight addition of rhodamine dye to the
EtOH before the sonication.
To obtain sensor characteristics of composite-printed specimens,
we performed uniaxial cyclic tests using a dynamic testing machine
E300 from Instron with the frequency at 7 Hz and displacementof 10 mm for specimens with a 60-mm gauge length. A specimen
was cut to the dimension of l = 75 mm and w = 5 mm, and a copper
tape was attached to both ends of the composite and encapsulatedusing duct tape to avoid electrical contact with the grips when loaded
into the frame. Once a specimen was loaded, the copper tapes wereconnected to a custom-made Wheatstone bridge board consistingof three resistors (i.e., 100 kilohm and 8.2 megohm for bare polymer
Kim et al., Sci. Robot. 5, eaay3604 (2020)

26 February 2020

Crack opening sensing mechanism
The crack opening ratio was defined as () = l()/lmax, where l() is
the length of a crack at strain  and lmax is the length of a fully
opened crack. Assuming that crack asperities are uniformly populated
along the crack length (x direction) and each crack asperity provides
constant electrical conductance when a pair of crack asperities arein contact, the local conductance S across a crack was predicted as​lm​  ax​​−l​  ax​)​ ​		​
S = ​∫0​  ​​ ​Ga​  ​ ​dx (​l0​  ​​  ≤ l ≤ ​lm(1)where G a is the conductance from a pair of connected crack
asperities,  is the density of crack asperities along the crack length,
and l0 is the initial length of crack (see fig. S2). This crack opening–
driven mechanism was a modification of the crack asperity–driven
mechanism in (23) and can be normalized by S max (at l =0) to
simplify as	​​S¯​ = 1 −  (​​  0​ ​ ≤  ≤ 1)​	

(2)

where 0 is determined from the initial crack length l0.
Demonstration of soft systems with directly printed sensors
SCP emulsions were printed onto all-soft systems in their inflated
state to complement stretchability of the produced composites (49).
Commercial latex balloons were pressurized, and SCP emulsion
was printed onto the surface, thereby creating conductive composites.
For twisting latex balloons, a conductive copper tape enclosed bothends of the composite to provide electrical connection via conductive
threads. Thus, electrical resistance of the composite was in situ
monitored as the balloon inflated/deflated. For spherical latex balloons,
their pressure and pertinent electrical resistance were intermittently
measured as the balloon deflated. The pressure of the balloon wasmeasured through a pressure sensor (part no. 5 INCH-G-4V, Amphenol)
communicating with an Arduino, and the resistance of the sensor
was manually measured using a multimeter.
Soft systems consisting of a silicone elastomer matrix (Ecoflex
00-30, Smooth-On) were prepared in a similar fashion with the latex
balloons. A four-legged gripper was fabricated by casting the resin
in a mold for a pneumatic chamber and adhering a strain-limiting
muslin fabric (product no. 8808K11, McMaster-Carr) on the bottomof the chamber. The fiber-reinforced actuator and a strain-limiting
layer were prepared as described in (45). SCP emulsions were printed
onto these actuators after inflating them, and then, the pressurized air
was removed to provide electrical connection after 30 min using a
conductive carbon grease (part no. 8481-1, MG Chemicals). The electricalresistance of composites was in situ measured using a multimeter.
Self-sensing McKibben actuators were prepared by assembling acomposite-printed twisting latex balloon and a strain-limiting mesh
(PTN0.25BL, Techflex). Copper tapes enclosed both ends of thecomposite to provide robust electrical connection for alligator clips.
The composite sensor was connected to a voltage divider to enable
feedback control of the actuator. Ground truth sensor data wereobtained by taking a series of photographs while maintaining a
constant pressure in the unloaded actuator and measuring the
10 of 12

Downloaded from http://robotics.sciencemag.org/ at Yale University on February 26, 2020

Characterization of SCP emulsions
Dynamic light scattering was performed using a NanoBrook Omni
from Brookhaven Instruments. The emulsion was diluted by 100:1 in
EtOH and tested using the emission light of 350 nm and backscattermode. At least five measurements were performed for each case, anderror bars denote 1 SD. UV-vis measurement was performed usinga Cary 100 from Varian. The emulsion was diluted in EtOH by 200:1
in volume and measured in a crystal cuvette for 120 min. Absorbanceat a wavelength of 450 nm was used to quantify the settlement using
absorbance retention At/At = 0, where At denotes the measurement
after t min. Rheometer tests were performed using an ARES LS1from TA Instruments with parallel plates with a diameter of 15 mm
at the oscillatory strain of 1.0%. A solvent trap was used to preventthe evaporation of EtOH during the measurement and test. SEM
was performed using SU-70 from Hitachi. To image emulsified
PDMS droplets, we added a light amount of deionized water to theemulsion and stirred for 30 min to promote curing of PDMS droplets within the emulsion. Freeze-fractured specimens were preparedby casting this cured droplet to epoxy and fracturing it after freezing
using liquid nitrogen.

substrate and textile substrate, respectively) and a voltage follower
to maintain the consistent input voltage throughout the test. The
measured voltage from this board was directly sent to the software
Bluehill (Instron) that simultaneously records stress-strain data.

SCIENCE ROBOTICS | RESEARCH ARTICLE
resistance across the sensor. The correlation between pressure andactuator length was nearly perfect (<1%) and was used for subsequentcharacterizations of the sensor response (under the same conditions)to build up a statistically viable sensor-to-length correlation. For
closed-loop control of the actuators, an independent processor
board (Arduino UNO) used the voltage divider to measure theresistance of the sensor, compared it to the target resistance value,
and then sent appropriate command signals (in the form of a target
pressure via a proportional gain multiplier) to an in-house–designed
pressure regulator/sensor board.

SUPPLEMENTARY MATERIALS

robotics.sciencemag.org/cgi/content/full/5/39/eaay3604/DC1
Fig. S1. Rheological profile of the SCP emulsion at different EtOH concentrations.
Fig. S2. Crack opening mechanism for strain sensing.
Fig. S3. Optical images of composite-infused polyester/spandex textile.
Fig. S4. Separation of EtOH from CB/PDMS-precursor droplets on a textile.
Fig. S5. Cross-sectional SEM image of SCP emulsion–printed textile.
Fig. S6. Tensile moduli of composite-infused textiles.
Fig. S7. Fatigue resistance of composite-infused polyester/spandex textiles.
Fig. S8. Shape estimation of an inflatable latex balloon.
Fig. S9. Fabric gripper with a stand-alone sensor and a self-adhesive sensor.
Fig. S10. Strain sensing of composite-infused textiles fabricated from ready-made garments.
Fig. S11. Comparison with existing manufacturing processes for composite sensors.
Movie S1. Direct comparison of SCP emulsion (EtOH) and conventional suspension(cyclohexane) on a latex balloon.
Movie S2. Sensorized latex balloon using an SCP emulsion.
Movie S3. Fiber-reinforced actuator with an SCP emulsion–printed trace and a self-adhesive
strain-limiting layer.
Movie S4. Closed-loop control of a self-sensing McKibben actuator.
Movie S5. SCP emulsion–printed kinesiology tape to measure elbow flexion.
Movie S6. Hybrid robotic textile (OmniSkin) giving rise to an inchworm motion.
References (50–70)

REFERENCES AND NOTES

1. R.-H. Kim, D.-H. Kim, J. Xiao, B. H. Kim, S.-I. Park, B. Panilaitis, R. Ghaffari, J. Yao, M. Li, Z. Liu,
V. Malyarchuk, D. G. Kim, A. P. le, R. G. Nuzzo, D. L. Kaplan, F. G. Omenetto, Y. Huang,
Z. Kang, J. A. Rogers, Waterproof AlInGaP optoelectronics on stretchable substrates
with applications in biomedicine and robotics. Nat. Mater. 9, 929–937 (2010).
2. S.-K. Kang, R. K. J. Murphy, S.-W. Hwang, S. M. Lee, D. V. Harburg, N. A. Krueger, J. Shin,
P. Gamble, H. Cheng, S. Yu, Z. Liu, J. G. McCall, M. Stephen, H. Ying, J. Kim, G. Park,
R. C. Webb, C. H. Lee, S. Chung, D. S. Wie, A. D. Gujar, B. Vemulapalli, A. H. Kim, K.-M. Lee,
J. Cheng, Y. Huang, S. H. Lee, P. V. Braun, W. Z. Ray, J. A. Rogers, Bioresorbable siliconelectronic sensors for the brain. Nature 530, 71–76 (2016).
3. J. Xu, S. Wang, G. J. N. Wang, C. Zhu, S. Luo, L. Jin, X. Gu, S. Chen, V. R. Feig, J. W. F. To,
S. Rondeau-Gagné, J. Park, B. C. Schroeder, C. Lu, J. Y. Oh, Y. Wang, Y.-H. Kim, H. Yan,
R. Sinclair, D. Zhou, G. Xue, B. Murmann, C. Linder, W. Cai, J. B.-H. Tok, J. W. Chung, Z. Bao,Kim et al., Sci. Robot. 5, eaay3604 (2020)

26 February 2020

5.
6.

7.
8.

9.
10.
11.
12.
13.

14.

15.

16.

17.

18.

19.

20.
21.

22.

23.

24.

25.
26.

27.
28.

29.

11 of 12

Downloaded from http://robotics.sciencemag.org/ at Yale University on February 26, 2020

Demonstration of composite-infused textiles sensors
Composite-infused kinesiology tape was fabricated by direct-printing
an SCP emulsion onto the purchased kinesiology tapes. Conductive
threads were then stitched to each end of the sensory trace using a
conductive epoxy adhesive and used for the electrical connection.
The same was applied to composite-infused kneepads and cotton
gloves. The electrical resistance of the composite trace was measured
using a custom-made Wheatstone bridge board attached to an Arduino.
Robotic skins (OmniSkins) were prepared by first printing an
SCP emulsion onto muslin fabric. Then, we attached a self-sensing
McKibben actuator and three conventional McKibben actuators
onto the fabric. This robotic skin was wrapped around a passive
polyethylene foam, and pneumatic pressure (~100 kPa) was applied
to the actuators. Electrical resistance of the self-sensing actuatorand textile sensor was monitored using a Wheatstone bridge and an
Arduino microcontroller.

4.

Highly stretchable polymer semiconductor films through the nanoconfinement effect.
Science 355, 59–64 (2017).
J. A. Rogers, T. Someya, Y. Huang, Materials and mechanics for stretchable electronics.
Science 327, 1603–1607 (2010).
M. Amjadi, K.-U. Kyung, I. Park, M. Sitti, Stretchable, skin-mountable, and wearable strain
sensors and their potential applications: A review. Adv. Funct. Mater. 26, 1678–1698 (2016).
B. T. Quinlivan, S. Lee, P. Malcolm, D. M. Rossi, M. Grimmer, C. Siviy, N. Karavas,
D. Wagner, A. Asbeck, I. Galiana, C. J. Walsh, Assistance magnitude versus metabolic cost
reductions for a tethered multiarticular soft exosuit. Sci. Robot. 2, eaah4416(2017).
Y. Wei, Q. Xu, An overview of micro-force sensing techniques. Sens. Actuators A Phys. 234,
359–374 (2015).
M. Xie, K. Hisano, M. Zhu, T. Toyoshi, M. Pan, S. Okada, O. Tsutsumi, S. Kawamura,
C. Bowen, Flexible multifunctional sensors for wearable and robotic applications.
Adv. Mater. Dent. Tech. 4, 1800626 (2019).
D. Rus, M. T. Tolley, Design, fabrication and control of soft robots. Nature 521, 467–475(2015).
C. Laschi, B. Mazzolai, M. Cianchetti, Soft robotics: Technologies and systems pushing
the boundaries of robot abilities. Sci. Robot. 1, eaah3690 (2016).
M. A. Robertson, H. Sadeghi, J. M. Florez, J. Paik, Soft pneumatic actuator fascicles for highforce and reliability. Soft Robot. 4, 23–32 (2017).
T. J. Wallin, J. Pikul, R. F. Shepherd, 3D printing of soft robotic systems. Nat. Rev. Mater. 3,
84–100 (2018).
S. Lee, A. Reuveny, J. Reeder, S. Lee, H. Jin, Q. Liu, T. Yokota, T. Sekitani, T. Isoyama, Y. Abe,Z. Suo, T. Someya, A transparent bending-insensitive pressure sensor. Nat. Nanotechnol.
11, 472–478 (2016).
N. Matsuhisa, D. Inoue, P. Zalar, H. Jin, Y. Matsuba, A. Itoh, T. Yokota, D. Hashizume,
T. Someya, Printable elastic conductors by in situ formation of silver nanoparticles
from silver flakes. Nat. Mater. 16, 834–840 (2017).
D. J. Lipomi, M. Vosgueritchian, B. C. K. Tee, S. L. Hellstrom, J. A. Lee, C. H. Fox, Z. Bao,
Skin-like pressure and strain sensors based on transparent elastic films of carbon
nanotubes. Nat. Nanotechnol. 6, 788–792 (2011).
W. Hu, X. Niu, R. Zhao, Q. Pei, Elastomeric transparent capacitive sensors basedon an interpenetrating composite of silver nanowires and polyurethane. Appl. Phys. Lett.
102, 083303 (2013).
A. Atalay, V. Sanchez, O. Atalay, D. M. Vogt, F. Haufe, R. J. Wood, C. J. Walsh, Batch
fabrication of customizable silicone-textile composite capacitive strain sensors
for human motion tracking. Adv. Mater. Technol. 2, 1700136 (2017).
K. Li, H. Wei, W. Liu, H. Meng, P. Zhang, C. Yan, 3D printed stretchable capacitive sensors
for highly sensitive tactile and electrochemical sensing. Nanotechnology 29, 185501(2018).
K. R. Phillips, G. T. England, S. Sunny, E. Shirman, T. Shirman, N. Vogel, J. Aizenberg,
A colloidoscope of colloid-based porous materials and their uses. Chem. Soc. Rev. 45,
281–322 (2016).
T. G. Thuruthel, B. Shih, C. Laschi, M. T. Tolley, Soft robot perception using embedded soft
sensors and recurrent neural networks. Sci. Robot. 4, eaav1488 (2019).
C. Pang, G.-Y. Lee, T.-i. Kim, S. M. Kim, H. N. Kim, S.-H. Ahn, K. Y. Suh, A flexible and highly
sensitive strain-gauge sensor using reversible interlocking of nanofibres. Nat. Mater. 11,
795–801 (2012).
J. Kim, M. Lee, H. J. Shim, R. Ghaffari, H. R. Cho, D. Son, Y. H. Jung, M. Soh, C. Choi, S. Jung,
K. Chu, D. Jeon, S.-T. Lee, J. H. Kim, S. H. Choi, T. Hyeon, D. H. Kim, Stretchable siliconnanoribbon electronics for skin prosthesis. Nat. Commun. 5, 5747 (2014).
D. Kang, P. V. Pikhitsa, Y. W. Choi, C. Lee, S. S. Shin, L. Piao, B. Park, K.-Y. Suh, T.-i. Kim,
M. Choi, Ultrasensitive mechanical crack-based sensor inspired by the spider sensory
system. Nature 516, 222–226 (2014).
B. Xu, A. Akhtar, Y. Liu, H. Chen, W.-H. Yeo, S. I. Park, B. Boyce, H. Kim, J. Yu, H. Y. Lai,
S. Jung, Y. Zhou, J. Kim, S. Cho, Y. Huang, T. Bretl, J. A. Rogers, An epidermal stimulationand sensing platform for sensorimotor prosthetic control, management of lower backexertion, and electrical muscle activation. Adv. Mater. 28, 4462–4471 (2016).
C. Majidi, R. Kramer, R. J. Wood, A non-differential elastomer curvature sensor
for softer-than-skin electronics. Smart Mater. Struct. 20, 105017 (2011).
J. T. Muth, D. M. Vogt, R. L. Truby, Y. Mengüç, D. B. Kolesky, R. J. Wood, J. A. Lewis,
Embedded 3d printing of strain sensors within highly stretchable elastomers. Adv. Mater.
26, 6307–6312 (2014).
J.-Y. Sun, C. Keplinger, G. M. Whitesides, Z. Suo, Ionic skin. Adv. Mater. 26, 7608–7614(2014).
C. Larson, B. Peele, S. Li, S. Robinson, M. Totaro, L. Beccai, B. Mazzolai, R. Shepherd, Highly
stretchable electroluminescent skin for optical signaling and tactile sensing. Science 351,
1071–1074 (2016).
F. Hussain, M. Hojjati, M. Okamoto, R. E. Gorga, Review article: Polymer-matrix
nanocomposites, processing, manufacturing, and application: An overview. J. Compos.
Mater. 40, 1511–1575 (2006).

SCIENCE ROBOTICS | RESEARCH ARTICLE

Kim et al., Sci. Robot. 5, eaay3604 (2020)

26 February 2020

57. N. Lu, C. Lu, S. Yang, J. Rogers, Highly sensitive skin-mountable strain gauges based
entirely on elastomers. Adv. Funct. Mater. 22, 4044–4050 (2012).
58. Y. R. Jeong, H. Park, S. W. Jin, S. Y. Hong, S. S. Lee, J. S. Ha, Highly stretchable and sensitive
strain sensors using fragmentized graphene foam. Adv. Funct. Mater. 25, 4228–4236 (2015).
59. T. Yamada, Y. Hayamizu, Y. Yamamoto, Y. Yomogida, A. Izadi-Najafabadi, D. N. Futaba,
K. Hata, A stretchable carbon nanotube strain sensor for human-motion detection.
Nat. Nanotechnol. 6, 296–301 (2011).
60. L. Cai, L. Song, P. Luan, Q. Zhang, N. Zhang, Q. Gao, D. Zhao, X. Zhang, M. Tu, F. Yang,
W. Zhou, Q. Fan, J. Luo, W. Zhou, P. M. Ajayan, S. Xie, Super-stretchable, transparent
carbon nanotube-based capacitive strain sensors for human motion detection. Sci. Rep.
3, 3048 (2013).
61. M. Amjadi, Y. J. Yoon, I. Park, Ultra-stretchable and skin-mountable strain sensors using
carbon nanotubes-Ecoflex nanocomposites. Nanotechnology 26, 375501 (2015).
62. B.-U. Hwang, J.-H. Lee, T. Q. Trung, E. Roh, D.-I. Kim, S.-W. Kim, N.-E. Lee, Transparent
stretchable self-powered patchable sensor platform with ultrasensitive recognitionof human activities. ACS Nano 9, 8801–8810 (2015).
63. C. S. Boland, U. Khan, C. Backes, A. O’Neill, J. McCauley, S. Duane, R. Shanker, Y. Liu,
I. Jurewicz, A. B. Dalton, J. N. Coleman, Sensitive, high-strain, high-rate bodily motion
sensors based on graphene–rubber composites. ACS Nano 8, 8819–8830 (2014).
64. X. Xiao, L. Yuan, J. Zhong, T. Ding, Y. Liu, Z. Cai, Y. Rong, H. Han, J. Zhou, Z. L. Wang,
High-strain sensors based on ZnO nanowire/polystyrene hybridized flexible films.
Adv. Mater. 23, 5440–5444 (2011).
65. U.-H. Shin, D.-W. Jeong, S.-M. Park, S.-H. Kim, H.-W. Lee, J.-M. Kim, Highly stretchable
conductors and piezocapacitive strain gauges based on simple contact-transfer
patterning of carbon nanotube forests. Carbon 80, 396–404 (2014).
66. J.-H. Kong, N. S. Jang, S. H. Kim, J. M. Kim, Simple and rapid micropatterning of conductive
carbon composites and its application to elastic strain sensors. Carbon 77, 199–207 (2014).
67. S. Gong, D. T. H. Lai, Y. Wang, L. W. Yap, K. J. Si, Q. Shi, N. N. Jason, T. Sridhar, H. Uddin,
W. Cheng, Tattoolike polyaniline microparticle-doped gold nanowire patches as highly
durable wearable sensors. ACS Appl. Mater. Interfaces 7, 19700–19708 (2015).
68. S. Gong, D. T. H. Lai, B. Su, K. J. Si, Z. Ma, L. W. Yap, P. Guo, W. Cheng, Highly stretchy black
gold E-skin nanopatches as highly sensitive wearable biomedical sensors.
Adv. Electron. Mater. 1, 1400063 (2015).
69. Y. Zheng, Y. Li, K. Dai, Y. Wang, G. Zheng, C. Liu, C. Shen, A highly stretchable and stablestrain sensor based on hybrid carbon nanofillers/polydimethylsiloxane conductive
composites for large human motions monitoring. Compos. Sci. Technol. 156, 276–286(2018).
70. Y. Wang, J. Balowski, C. Phillips, R. Phillips, C. E. Sims, N. L. Allbritton, Benchtop
micromolding of polystyrene by soft lithography. Lab Chip 11, 3089–3097 (2011).
Acknowledgments: We thank J. Jacobs for assisting with demonstrations of the sensor and
E. Brown and A. M. Dollar for access to testing equipment. The pneumatic fabric gripper wasprovided by Otherlab for demonstration. Funding: This work was supported by a Small
Business Technology Transfer (80NSSC17C0030) from NASA and a Faculty Early Career
Development Award (CMMI-1454284) from NSF. C.O.O. and Y.C. acknowledge NSF support(DMR-1410568). R.A.B. acknowledges NSF support (EFRI-1830870). D.S.S. acknowledges NASAsupport (Space Technology Research Fellowship, 80NSSC17K0164). Author contributions:
S.Y.K. and R.K.-B. conceived the idea. S.Y.K., Y.C., and G.K. performed the material characterization.
S.Y.K., R.A.B., M.C.Y., and D.S.S. performed manufacturing and demonstration. C.O.O. and
R.K.-B. reviewed and commented on the manuscript. All authors participated in writing themanuscript. Competing interests: The authors declare that they have no competinginterests. Data and materials availability: All data are available in the main text or the
Supplementary Materials.
Submitted 12 June 2019
Resubmitted 9 January 2020
Accepted 6 February 2020
Published 26 February 2020
10.1126/scirobotics.aay3604
Citation: S. Y. Kim, Y. Choo, R. A. Bilodeau, M. C. Yuen, G. Kaufman, D. S. Shah, C. O. Osuji,
R. Kramer-Bottiglio, Sustainable manufacturing of sensors onto soft systems using self-coagulating
conductive Pickering emulsions. Sci. Robot. 5, eaay3604 (2020).

12 of 12

Downloaded from http://robotics.sciencemag.org/ at Yale University on February 26, 2020

30. E. L. White, M. C. Yuen, J. C. Case, R. K. Kramer, Low-cost, facile, and scalable manufacturingof capacitive sensors for soft systems. Adv. Mater. Technol. 2, 1700072 (2017).
31. S. Y. Kim, T.-W. Lim, N. R. Sottos, S. R. White, Manufacture of carbon-fiber prepreg
with thermoplastic/epoxy resin blends and microencapsulated solvent healing agents.
Compos. Part A Appl. Sci. Manuf. 121, 365–375 (2019).
32. M. S. Easmin, M. Z. I. Sarker, S. Ferdosh, S. H. Shamsudin, K. B. Yunus, M. S. Uddin,
M. M. R. Sarker, M. J. H. Akanda, M. S. Hossain, H. P. S. A. Khalil, Bioactive compounds
and advanced processing technology: Phaleria macrocarpa (sheff.) Boerl, a review.
J. Chem. Technol. Biotechnol. 90, 981–991 (2015).
33. E. Browning, Toxicity of Industrial Organic Solvents (Chemical Pub. Co., 1953);
http://agris.fao.org/agris-search/search.do?recordID=US201300637470.
34. J. N. Lee, C. Park, G. M. Whitesides, Solvent compatibility of poly(dimethylsiloxane)-based
microfluidic devices. Anal. Chem. 75, 6544–6554 (2003).
35. S. U. Pickering, CXCVI.—Emulsions. J. Chem. Soc. Trans. 91, 2001–2021 (1907).
36. A. Gelot, W. Friesen, H. A. Hamza, Emulsification of oil and water in the presence of finely
divided solids and surface-active agents. Colloids Surf. 12, 271–303 (1984).
37. R. M. Choueiri, E. Galati, H. Thérien-Aubin, A. Klinkova, E. M. Larin, A. Querejeta-Fernández,
L. Han, H. L. Xin, O. Gang, E. B. Zhulina, M. Rubinstein, E. Kumacheva, Surface patterningof nanoparticles with polymer patches. Nature 538, 79–83 (2016).
38. B. Furie, B. C. Furie, The molecular basis of blood coagulation. Cell 53, 505–518 (1988).
39. A. J. Singer, R. A. F. Clark, Cutaneous wound healing. N. Engl. J. Med. 341, 738–746 (1999).
40. S. Iijima, P. M. Ajayan, Substrate and size effects on the coalescence of small particles.
J. Appl. Phys. 70, 5138–5140 (1991).
41. B. Michen, C. Geers, D. Vanhecke, C. Endes, B. Rothen-Rutishauser, S. Balog, A. Petri-Fink,
Avoiding drying-artifacts in transmission electron microscopy: Characterizing the sizeand colloidal state of nanoparticles. Sci. Rep. 5, 9793 (2015).
42. S. Y. Kim, A. R. Jones, N. R. Sottos, S. R. White, Manufacturing of unidirectional glass/epoxy
prepreg with microencapsulated liquid healing agents. Compos. Sci. Technol. 153,
190–197 (2017).
43. J. Diani, B. Fayolle, P. Gilormini, A review on the Mullins effect. Eur. Polym. J. 45, 601–612(2009).
44. F. Ilievski, A. D. Mazzeo, R. F. Shepherd, X. Chen, G. M. Whitesides, Soft robotics
for chemists. Angew. Chem. 123, 1930–1935 (2011).
45. S. Y. Kim, R. Baines, J. Booth, N. Vasios, K. Bertoldi, R. Kramer-Bottiglio, Reconfigurable softbody trajectories using unidirectionally stretchable composite laminae. Nat. Commun.
10, 3464 (2019).
46. J. W. Booth, D. Shah, J. C. Case, E. L. White, M. C. Yuen, O. Cyr-Choiniere, R. Kramer-Bottiglio,
OmniSkins: Robotic skins that turn inanimate objects into multifunctional robots. Sci. Robot.
3, eaat1853 (2018).
47. A. Kamyshny, M. Ben-Moshe, S. Aviezer, S. Magdassi, Ink-jet printing of metallic
nanoparticles and microemulsions. Macromol. Rapid Commun. 26, 281–288 (2005).
48. Y. Kim, J. Zhu, B. Yeom, M. di Prima, X. Su, J.-G. Kim, S. J. Yoo, C. Uher, N. A. Kotov,
Stretchable nanoparticle conductors with self-organized conductive pathways. Nature
500, 59–63 (2013).
49. D.-Y. Khang, J. A. Rogers, H. H. Lee, Mechanical buckling: Mechanics, metrology,
and stretchable electronics. Adv. Funct. Mater. 19, 1526–1536 (2009).
50. S. Yao, Y. Zhu, Wearable multifunctional sensors using printed stretchable conductors
made of silver nanowires. Nanoscale 6, 2345–2352 (2014).
51. A. Frutiger, J. T. Muth, D. M. Vogt, Y. Mengüç, A. Campo, A. D. Valentine, C. J. Walsh,
J. A. Lewis, Capacitive soft strain sensors via multicore-shell fiber printing. Adv. Mater. 27,
2440–2446 (2015).
52. E. Roh, B.-U. Hwang, D. Kim, B.-Y. Kim, N.-E. Lee, Stretchable, transparent, ultrasensitive,
and patchable strain sensor for human-machine interfaces comprising a nanohybridof carbon nanotubes and conductive elastomers. ACS Nano 9, 6252–6261 (2015).
53. J. Shintake, E. Piskarev, S. H. Jeong, D. Floreano, Ultrastretchable strain sensors using
carbon black-filled elastomer composites and comparison of capacitive versus resistive
sensors. Adv. Mater. Technol. 3, 1700284 (2018).
54. Y. Zhang, C. J. Sheehan, J. Zhai, G. Zou, H. Luo, J. Xiong, Y. T. Zhu, Q. X. Jia, Polymerembedded carbon nanotube ribbons for stretchable conductors. Adv. Mater. 22,
3027–3031 (2010).
55. D. J. Cohen, D. Mitra, K. Peterson, M. M. Maharbiz, A highly elastic, capacitive strain gauge
based on percolating nanotube networks. Nano Lett. 12, 1821–1825 (2012).
56. M. Amjadi, A. Pichitpajongkit, S. Lee, S. Ryu, I. Park, Highly stretchable and sensitive strain
sensor based on silver nanowire–elastomer nanocomposite. ACS Nano 8, 5154–5163 (2014).

Sustainable manufacturing of sensors onto soft systems using self-coagulating
conductive Pickering emulsions
Sang Yup Kim, Youngwoo Choo, R. Adam Bilodeau, Michelle C. Yuen, Gilad Kaufman, Dylan S. Shah, Chinedum O.
Osuji and Rebecca Kramer-Bottiglio

Sci. Robotics 5, eaay3604.
DOI: 10.1126/scirobotics.aay3604

http://robotics.sciencemag.org/content/5/39/eaay3604

SUPPLEMENTARY
MATERIALS

http://robotics.sciencemag.org/content/suppl/2020/02/24/5.39.eaay3604.DC1

REFERENCES

This article cites 69 articles, 3 of which you can access for free
http://robotics.sciencemag.org/content/5/39/eaay3604#BIBL

PERMISSIONS

http://www.sciencemag.org/help/reprints-and-permissions

Use of this article is subject to the Terms of Service
Science Robotics (ISSN 2470-9476) is published by the American Association for the Advancement of Science, 1200
New York Avenue NW, Washington, DC 20005. The title Science Robotics is a registered trademark of AAAS.
Copyright © 2020 The Authors, some rights reserved; exclusive licensee American Association for the Advancementof Science. No claim to original U.S. Government Works

Downloaded from http://robotics.sciencemag.org/ at Yale University on February 26, 2020

ARTICLE TOOLS