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Research Article

Printed and Laser-Activated Liquid Metal-Elastomer Conductors
Enabled by Ethanol/PDMS/Liquid Metal Double Emulsions
Shanliangzi Liu, Sang Yup Kim, Kristen E. Henry, Dylan S. Shah, and Rebecca Kramer-Bottiglio*
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sı Supporting Information*

ABSTRACT: Soft electronic systems require stretchable, printable conductors for applications in soft robotics, wearable
technologies, and human−machine interfaces. Gallium-basedroom-temperature liquid metals (LMs) have emerged as promising
candidates, and recent liquid metal-embedded elastomers(LMEEs) have demonstrated favorable properties such as stableconductivity during strain, cyclic durability, and patternability.
Here, we present an ethanol/polydimethylsiloxane/liquid metal(EtOH/PDMS/LM) double emulsion ink that enables a fast,scalable method to print LM conductors with high conductivity(7.7 × 105 S m−1), small resistance change when strained, andconsistent cyclic performance (over 10,000 cycles). EtOH, thecarrier solvent, is leveraged for its low viscosity to print the ink
onto silicone substrates. PDMS resides at the EtOH/LM interface and cures upon deposition and EtOH evaporation, consequently
bonding the LM particles to each other and to the silicone substrate. The printed PDMS−LM composite can be subsequently
activated by direct laser writing, forming high-resolution electrically conductive pathways. We demonstrate the utility of the double
emulsion ink by creating intricate electrical interconnects for stretchable electronic circuits. This work combines the speed,
consistency, and precision of laser-assisted manufacturing with the printability, high conductivity, strain insensitivity, and mechanicalrobustness of the PDMS−LM composite, unlocking high-yield, high-throughput, and high-density stretchable electronics.
KEYWORDS: stretchable electronics, liquid metal, double emulsion, laser sintering, liquid metal-embedded elastomer■

INTRODUCTION
Stretchable electronic systems including sensors,1,2 electroluminescent devices,3 energy storage devices,4,5 and integrated
circuits6 are essential for next-generation soft robots,7,8 wearable
technologies,9,10 and human−machine interfaces.11,12 Stretchable materials that can maintain high conductivity as well as
stable electrical and mechanical performance when strained areessential to the advancement of these soft electronic systems.
Strategies to make stretchable, conductive materials include
geometrical patterning of thin, rigid metal ﬁlms into
deterministic structures13 as well as embedding rigid conducting/semiconducting ﬁller materials into stretchable polymer
matrices.14 However, these materials usually suﬀer from low
maximum strains, limited by complex structures or rigidmaterials.6,14−17 Furthermore, stretching can lead to dramatic
losses in electrical conductivity, limiting applications in wearable
computation and soft robotics which require power and datatransmission with minimal losses when strained.
Over the last decade, gallium-based room-temperature liquid
metal (LM) alloys (EGaIn: 75.5% Ga/24.5% In; Galinstan:
68.5% Ga/21.5% In/10.0% Sn) have gained traction as afavorable material for stretchable electronics due to their highelectrical conductivity (3.4 × 106 S · m−1)18 and intrinsic© 2021 American Chemical Society

stretchablity. LM alloys spontaneously form a thin passivatingoxide skin (primarily Ga2O3) that allows them to be patterned
into nonspherical shapes and encourages adhesion tosurfaces.19−21 Emerging approaches for patterning LMs include
injection into microchannels,22 lithography,23,24 direct writing,25 and laser ablation.26
For many applications, signiﬁcant changes in interconnect
resistance could aﬀect signal transmission and complicate circuitdesign. Seeking improved electromechanical performance understrain,27−30 recent studies have introduced liquid metalembedded elastomer (LMEE) composites manufactured byshear mixing of the LM into a silicone matrix.31−35 The
elastomer walls surrounding the LM prevent conductivity as
cast, requiring the materials to be mechanically activated torupture the walls and form continuous electrically conductive
pathways. While the meandering conductive pathways attained
Received: December 30, 2020
Accepted: May 31, 2021
Published: June 14, 2021

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Figure 1. Illustration of the fabrication process of EPL double emulsion and PDMS−LM composite. (a) A two-step ultrasonication process creates
EPL double emulsion. The double emulsion consists of LM microparticles coated by PDMS precursors in an EtOH medium. (b,c) Printing the EPL
double emulsion onto a PDMS substrate creates a thin nonconductive composite material following EtOH evaporation and PDMS-precursor
polymerization. The PDMS precursors reside primarily at the interfaces between the LM particles (inset), acting as an adhesive. (d) Laser activationcoalesces the pristine, nonconductive structure into a continuously conductive, complex network.

by mechanical sintering in LMEEs yield a low resistance change
during stretching (compared to bulk conductor assumptions),36−38 the high viscosity of the composites prevents
high-resolution printing of LMEEs. Heavy particles may also
settle during the prolonged curing process, leading to nonuniform particle distribution throughout the thickness. Additionally, neighboring traces may be inadvertently connectedduring use due to natural mechanical sintering from handling or
rapid stretching. Prior eﬀorts to enhance processability andstability of LMEE composites include a reduction in LM loading(i.e., increasing silicone concentration31−35), often leading to
low conductivity, slow cure times, and poor shape retention.
To enable printability of LMEE composites, we look to ourprior results wherein bulk LM was made printable via emulsion
processing and then sintered, either mechanically or via laser, toenable conductivity of the printed traces. Prior work
demonstrated that creating an EtOH/LM (EL) single emulsion
by sonicating bulk LM in ethanol, spray printing the single
emulsion, and sintering the resulting LM particles on a substrate
can produce thin, uniform LM ﬁlms with high conductivity.39,40
Further, laser sintering of LM particles has been shown to be fastand scalable, relative to mechanical sintering.39−41 Yet, previous
laser-sintered LM particle ﬁlms were both highly sensitive tostrain and also only ﬂexible (not stretchable).41 A laser-activatedconductive trace experiences low adhesion to the substrate due
to unsintered particles beneath the top sintered layer, which act
as a lubricant between the sintered trace and the underlying
substrate, hence forming cracks and disrupting electricalpathways during stretching.41

In this work, we merge the processing advantages of LM
emulsions with the performance advantages of LMEEs using an
ethanol/polydimethylsiloxane/liquid metal (EtOH/PDMS/
LM; EPL) double emulsion. Unlike previous LM−silicone
mixtures, this double emulsion ink contains a high loading of LM(∼98.4 wt %), but it retains low viscosity and high stability (i.e.,
no particle settling). EtOH, the carrier solvent, is leveraged tofacilitate printing the ink onto silicone substrates to form thin,uniform LM particle ﬁlms. PDMS resides at the EtOH/LM
interfaces and rapidly cures upon printing and EtOH
evaporation, enabling the LM particles to form strong bonds
to each other and to silicone substrates. The printed PDMS−
LM composite ﬁlms can be easily activated to achieve highelectrical conductivity (7.7 × 105 S m−1) via direct laser writing,which is attributed to the small trace thicknesses and low
polymer concentrations. The laser-induced conductivity,
combined with the sustained adhesion of the composite to thesubstrate, enables excellent electrical performance and cyclic
reliability as well as provides a reliable way of rapidly creating
miniaturized conductive patterns. We demonstrate an intricate,high-resolution pattern of this stretchable conductor connected
as a part of an electrical circuit with an LED. Our double
emulsion ink opens up possibilities for achieving stretchable,
high-resolution, and device-on-demand conductive pathways,
which are implementable in both soft robotics and wearable
health monitoring devices.
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Figure 2. Material characteristics of EL single emulsion and EPL double emulsion. (a,b) SEM images of the as-printed EL single emulsion (a) and the
EPL double emulsion (b). Insets are the close views showing the interfaces between the neighboring particles. The boundaries between the particles
are false colored to highlight the PDMS precursors at the interfaces. (c) Histogram of particle size distributions for both the EL single emulsion (2.09 ±1.19 μm) and the EPL double emulsion (1.91 ± 1.21 μm). (d) EDS mappings of the top surface of the as-printed EPL double emulsion, indicating
higher silicon concentrations at the interfaces between the LM particles.

■

RESULTS AND DISCUSSION
We prepared the EPL double emulsion by ﬁrst sonicating
eutectic gallium−indium (eGaIn) in EtOH for 5 min, creating
an EL single emulsion. Then, we removed the supernatant(small eGaIn particles dispersed in EtOH), added PDMS
precursors (2−12 vol %) to the EL single emulsion, andsubsequently sonicated the whole system for 3 min. Following
sonication, we allowed the mixture to sit, separating out a top
cream layer, with segregated PDMS precursors and lighter,noncoated LM particles from the double emulsion. We removed
the cream, leaving behind the PDMS precursors, encapsulating
only the larger, heavier LM particles suspended in ethanol,
resulting in a EPL double emulsion (Figure 1a, Supporting
Information Figure S1a). When we added an excessive amountof PDMS (>12 vol %), the PDMS precursors formed unwanted
agglomerates in the EL single emulsion (Supporting Information Figure S1b). We observed the emulsion ink with 10 vol %PDMS precursors, and no agglomeration or sedimentation
appeared to occur over 5 h (Supporting Information Figure S2).
We also studied the rheological properties of the EPL double
emulsion with 10 vol % PDMS (Supporting Information FigureS3) and found that the ink shows an apparent viscosity of 2146
Pa s at a shear rate of 0.01 s−1, which is 6 orders of magnitude
higher than that of pure water, but it exhibits a pronounced shear
thinning behavior.
The EPL double emulsion consists of three components: (1)
EtOH as the continuous phase, (2) PDMS precursors toenhance stability of the LM particles in the suspension andadhesion of the LM particles to each other and the substrate post
deposition, and (3) LM enabling high electrical conductivity. In
the emulsion, thin layers of liquid PDMS cover the surfaces of
the LM microparticles, acting as a surfactant to stabilize thesuspension in EtOH (Figure 1a). When printing onto a silicone
substrate using a rod coating method (Figure 1b), EtOH
evaporates rapidly, releasing the PDMS−LM droplets onto thesubstrate. The LM microparticles relax, and the encapsulating
PDMS precursors ﬂow and coalesce, subject to both capillary
forces and gravity. The one-part PDMS precursors rapidly
polymerize in contact with the atmospheric moisture andeventually cure, adhering the LM microparticles to the substrate
and to each other. Upon printing, the entire curing process only
takes a few minutes due to the rapid evaporation of EtOH andthe small concentration of PDMS. High energy lasers can thenablate away some of the PDMS, activating the as-printed thinPDMS−LM composite (Figure 1c) into a continuously
conductive, complex network (Figure 1d) that is structurally
and functionally similar to previously reported LMEEs.
To study material characteristics, we compared themorphology, particle size distribution, and elemental distribution of as-printed EPL double emulsion with EL single emulsion
using scanning electron microscopy (SEM) imaging and energydispersive X-ray spectroscopy (EDS). SEM images of the asprinted EL emulsion reveal isolated LM particles with clear,
distinct boundaries (Figure 2a) and more strongly roundedshapes, while the EPL double emulsion (10 vol % PDMS)(Figure 2b) shows particles that are contiguous with each otherand have blurry, noncircular boundaries between the particles(Figure 2b, boundaries false colored). This phenomenon is
likely a result of the nonconductive PDMS precursors at theinterfaces between the neighboring particles, accumulating
electrons on the surface that deteriorates SEM image quality.
The PDMS precursors, subjected to capillary forces whencoalesced and cured, compress the particles into nonsphericalshapes. Histograms (Figure 2c) of particle sizes show thatpolydispersed distributions of particle sizes for both the EL
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Figure 3. Laser activation of the PDMS−LM composite. (a,b) SEM images of the transition between nonactivated and activated regions of the PDMSLM composite (a) and a detailed view of the activated, conductive region (b). (c) EDS mappings of the laser-activated PDMS−LM composite showing
a uniform elemental distribution of all three key elements: Ga, In, and Si. (d) SEM images showing cross-sectional views of the PDMS−LM composite
before and after laser activation. False coloring on the activated image highlights the distinct conductive, coalesced region on top.

Figure 4. Electromechanical characteristics of PDMS−LM composites. (a) Resistance and surface morphology of the PDMS−LM composites at
diﬀerent PDMS concentrations (2−12 vol %). The composites with lower PDMS concentrations exhibit more cracks and hence larger resistancevalues and variances. Error bars represent 1 standard deviation. Insets are the optical images showing cracks of the PDMS−LM composite. Scale bar:
200 μm. (b) Relative change in resistance as a function of uniaxial tensile strain of the PDMS−LM composites on PDMS substrates (pink, R/R0 ≈ 1.5± 0.01 at 100% strain, ﬁve samples), theoretical prediction based on bulk conductor assumptions (black, 3.4 × 106 S m−1), and other LM-based
conductors in the literature: PDMS-galinstan composite (purple),31 AgNP−Ga−In (green),29 and LM−silicone (orange).47 Reproduced withpermission.31 Copyright 2015 John Wiley and Sons. Reproduced with permission.29 Copyright 2018 John Wiley and Sons. Reproduced withpermission.47 Copyright 2015 John Wiley and Sons. (c,d), Relative resistance change of the PDMS−LM composite subjected to 10,000 cycles of 100%
uniaxial tensile strain. The resistance settling during the initial 10 cycles is highlighted in (c).

single emulsion (2.09 ± 1.19 μm) and the EPL double emulsion(1.91 ± 1.21 μm) are not signiﬁcantly diﬀerent, so diﬀerences inparticle geometry stem from a secondary inﬂuence. EDS
mappings (Figure 2d) of the as-printed EPL double emulsion

shows that gallium and indium are well-distributed throughout
all particles, as expected, while silicon is highly concentrated at
the interfaces between the LM particles. As PDMS is the only
source of silicon in the EPL double emulsion, the EDS mappings
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Figure 5. Demonstrations of the utility of the double emulsion ink to create stretchable electronics. Photographs of an intricate laser-activated maze
pattern on a PDMS−LM composite and acting as stretchable electrical interconnects. No evident reduction of the LED brightness was observed under
80% strain. The optical microscope image shows that the single trace width created by laser activation is as small as 37 μm.

during EtOH evaporation (Supporting Information FigureS6).45,46 The cracks randomly distribute throughout thecomposite, disrupting electrical pathways. This also caused
larger variances in the measured resistance of the composites
with lower PDMS content (Figure 4a). Once we increased the
PDMS content to 10 vol %, the ﬁnal thin-ﬁlm composite became
consistently uniform and crack-free on the PDMS substrate(Supporting Information Figure S7). We attribute the uniform
ﬁlm proﬁle to the liquid PDMS precursors around the LM
particles in the emulsion. As the EtOH evaporates, meniscusshaped liquid PDMS bridges form between the particles,
yielding capillary force that is remarkably large to overcome
the gravitational forces and bring the adjacent particles together(analytical estimation in Supporting Information Figure S8).
The PDMS precursors eventually cure, bonding the particles toeach other and to the surfaces of the PDMS substrates. Acting as
an adhesive, the PDMS precursor eﬀectively helps alleviate thecapillary pressure induced by the meniscus of the air−EtOH
interface between the particles, hence preventing crack
formation.46 In addition, due to the existence of the nonconductive PDMS after laser activation, the electrical conductivity of the PDMS−LM composites is lower than that ofbulk eGaIn.
To study the electromechanical performance of the PDMS−
LM composite, we measured the relative resistance change(R/R0) across a laser-activated trace as it underwent uniaxial
tensile strain. The composite exhibits stable electrical performance, experiencing small resistance change on PDMS at 100%
strain (R/R0 ≈ 1.5, Figure 4b). This value is smaller than thetheoretical prediction for classical incompressible, constantconductivity bulk conductors (R/R0 = (1 + ε)2, where ε is theapplied strain; R/R0 = 4 at 100% strain). Such strain-insensitive
behavior has been previously reported36 and attributed to thetortuosity of the meandering conductive pathways within LMEE
composites (Figure 3b). The applied global strain largely
contributes to rearrangement of the conductive pathways, still
supported by intact, cross-linked PDMS, rather than changing
the length or cross-sectional area of the coalesced network.
At low PDMS content, the PDMS−LM composite exhibits
inconsistent and unstable electrical responses during cyclic

clearly demonstrate that the interparticle bridges formed by theliquid PDMS precursors (originally encasing adjacent LM
particles) coalesce as EtOH evaporated.
The as-printed EPL double emulsionwhich, after EtOH
evaporation and PDMS coalescence, is a PDMS−LM
compositeis nonconductive because the encapsulating oxideshells and now-cured PDMS layers around the LM particles
both prevent spontaneous particle coalescence. We used afocused laser beam (wavelength 355 nm) to transform thenonconductive PDMS−LM composite into a conductive
material (Figure 3a,b, Supporting Information Figure S4) withhigh electrical conductivity (7.7 × 105 S m−1). The laser ablates
some of the PDMS and induces large thermal stresses in theparticles through rapid expansion of the LM cores.42 The sudden
expansion of the LM cores ruptures the gallium oxide shells,
allowing the liquid to escape and coalesce into a continuous
network.43 Additional PDMS interfacing layers not directly
ablated by the laser may also be ruptured, facilitating theformation of conductive structures. The hollow spaces (Figure3b) within the network are presumably attributed to thevaporization of particles induced by laser ablation.44 EDS
mappings of the laser-activated PDMS−LM composite reveal
silicon, gallium, and indium all well-distributed over the entire
surface (Figure 3c), whereas silicone was previously isolated
exclusively to the interfaces between particles. Cross-sectional
SEM images of the laser-activated PDMS−LM composite(Figure 3d) show that, compared to the nonactivated PDMS−
LM, the activated composite has a distinct, coalesced region on
top (false colored). The PDMS acts as an adhesive, linking the
top to the bottom, as evidenced by the blurry boundaries of theparticles that are in contact with the underlying silicone
substrate and the EDS Si mapping of the cross-sectional surface(Supporting Information Figure S5).
We varied the volume fractions of PDMS in the EPL double
emulsion to investigate its role in the electrical conductivity of
PDMS−LM composites (Figure 4a). The measured resistanceof the composites is dependent on the PDMS concentration (2−12 vol %). Lower PDMS content in the EPL double emulsion
leads to higher electrical resistance in the activated composite
due to crack formation resulting from particle aggregation
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loading (Supporting Information Figure S9). In contrast, after
moderate settling during the initial cycles, the PDMS−LM
composite with 10 vol % PDMS shows consistent electricalresponses up to 10,000 cycles (Figure 4c,d). We suspect that theinitial strain cycle ruptures the largest particles, which thenconsolidate into the existing conductive paths, leading to adecrease in the electrical resistance. Thereafter, the PDMS
encasing the nonactivated particles bonds them to each otherand to the underlying substrate, preventing crack formation(and thus loss of conductivity) and inadvertent particle
coalescence that might occur during cyclic testing or othermechanical eﬀects. In sum, the small amount of PDMS added tothe initial emulsion ink creates a ﬁnal conductor with favorable
electrical performance and reliability.
The printable and laser-activated PDMS−LM (or LMEE)
composite exhibits a superior balance of strain insensitivity,
electrical conductivity, and ease of fabrication compared toother LM-based conductors in the literature (Supporting
Information Table S1).29,31,47 We demonstrate a complex
maze pattern of PDMS−LM composite connected as a part of anelectric circuit with a LED. We twisted, bent, and stretched thecircuit to 80% strain (Figure 5a) and 50% for 100 cycles(Supporting Information Video S1), with no evident reductionof the LED brightness. The optical microscope image shows thatthe single conductive trace width created by laser activation is as
small as 37 μm. The capability to print and laser-activate LMEEs
represents an advance toward scalable manufacturing of highdensity and high-performance stretchable electronics.

Research Article

EXPERIMENTAL SECTION

Preparation of EPL Double Emulsions. The EL single emulsion
was created by adding 2 ± 0.02 g of eutectic gallium−indium alloy(75.5% Ga, 24.5% In, 495425, Sigma-Aldrich) into 10 mL of ethanol(V1001, Koptec) and then sonicating at an amplitude of 36 μm (30%setting) for 5 min using a tip sonicator (1/16″ microtip probe,
QSonica). Following sonication, the single emulsion was stored at room
temperature for an hour, allowing the heavy particles to settle undergravity. The supernatant was then removed, and the remaining heavy
particle emulsion was mixed vigorously with a vortex mixer(VortexGenie) for 30 s. Two vials of the settled and vortexed emulsion
were then combined and mixed again with the vortex mixer. Last, theEPL double emulsion was created by adding PDMS precursors (2−12
vol %, SEMICOSIL 964 UV/CLEAR, Wacker) into the heavy particle
single emulsion and subsequently sonicating for 3 min at an amplitudeof 60 μm (50% setting) pulsed for 1 s on, 1 s oﬀ. Prior to printing, theseparated top layer was removed, and the double emulsion at thebottom was mixed again thoroughly with a pipette tip.
Preparation of Substrates. The substrates onto which thePDMS−LM emulsions were printed were made of PDMS (poly(dimethylsiloxane), Sylgard 184, Dow Corning). The elastomer baseand the curing agent were mixed in a 10:1 ratio for 30 s and thendefoamed for 30 s by the use of a planetary centrifugal mixer (ARE-310,
Thinky). The PDMS was rod-coated onto a ﬂat acrylic sheet using theapproach described by White et al.48 and cured in an incubator at 60 °C
for 3 h.
Preparation of Laser-Activated PDMS−LM Composites. The
EPL double emulsion was poured at one end of the cured PDMS
substrate and then printed over the substrate using a custom madedrawbar of 200 μm thickness. The thin composite was left at room
temperature to dry and cure in a few minutes. Then, the composite wasactivated using an UV laser micromachining system (Protolaser U4,
LPKF) with a wavelength of 355 nm, a pulse duration of 900 ns, and abeam diameter of 15 μm operated in an ambient environment. The
average power used was 1.4 W with a frequency of 180 kHz and a speedof 1620 mm/s. The activated patterns were designed in CorelDraw,
exported as DXF ﬁles, and then imported into the laser system software.
Electrical and Mechanical Characterization. Resistance measurements were made by laser-activating patterns with multiple
connection points for performing four-terminal resistance measurements with a digital multimeter (5492B, BK Precision). The designed
patterns had an aspect ratio (length/width) of 5:1 (7.5 × 1.5 mm). The
values reported in Figure 4a were averaged from 10 measurements ateach PDMS concentration (2−12 vol %). The measured resistance ofthe laser-activated PDMS−LM composite with 10 vol % PDMS was
0.68 ± 0.21 Ω and was used for conductivity calculation. Thicknesses ofthe laser-activated composite ﬁlms were measured from Figure 3d andaveraged from 10 measurements (9.69 ± 0.59 μm). The conductivity of
the laser-activated PDMS−LM composite was calculated as σ = l/Rwt =7.7 ± 0.5 × 105 S m−1.
For electromechanical characterization, rectangular patterns (length:
20 mm; width: 5 mm) were lasered on the PDMS−LM composite ﬁlms.
Electrical connections were made by adhering copper tape on each endof the trace and then securing using silver conductive epoxy (Part#
8331-14G, MGchemicals). Fabric strips were placed at the interfaces toprevent stretching. The ﬁlms were then encapsulated with PDMS. The
samples were subjected to uniaixal tensile loading at 75 mm min−1 using
a customized tensile stage. Simultaneously, the resistance was measured
using a Wheatstone bridge, and the resistance of the alligator clips (0.85
Ω) was subtracted from the measurements to reduce experimental
error.
Material Characterization. SEM images were taken by a Hitachi
SU8230 UHR cold ﬁeld emission scanning electron microscope.
Samples for cross-sectional imaging were made by cleaving in liquid
nitrogen. Elemental composition and mappings were obtained using
EDS (Flat-Quad, Bruker) at 5 kV. Optical imaging was performed usinga Zeiss Smartzoom 5 microscope. Rheological characterization was
performed using a rotational rheometer (Anton Paar MCR 302) withparallel plate geometry at 20 °C (disk diameter 16 mm, gap size 1 mm).

■

CONCLUSIONS
We have presented a EPL double emulsion that can be easily
printed onto silicone substrates and then selectively laseractivated with intricate patterns to form stretchable conductors
in a scalable fashion. EtOH, the carrier solvent, is leveraged forits low viscosity to facilitate printing. PDMS encases the LM
particles in the emulsion and acts as an adhesive between the LM
particles after cure, bonding the particles to each other and tosilicone substrates. The strong bonding induced by the PDMS
and the conductive, complex, and microporous structure
induced by laser-activation enable a PDMS−LM composite
with comparable behaviors to previously reported LMEEs. The
composite material shows high electrical conductivity (7.7 × 105
S m−1), small changes in resistance (R/R0 ≈ 1.5) over uniaxial
tensile strains at 100%, and consistent electrical responses whenstretched over 10,000 cycles. The advantageous electromechanical properties and manufacturability, combined withthe prospects of generalization via diﬀerent polymers andsubstrate materials, open up possibilities for scalable manufacturing of stretchable, multilayer, and high-density electronic
circuits for various applications including soft robotics andwearables.
The ability of the material to be laser-activated illustrates thepotential for this technique in making small-scale, high-density
stretchable electronic circuits. In future studies, the eﬀect oftrace width on the electromechanical properties of the materials
will be investigated. Additionally, it is currently unclear how theparticle size and distribution aﬀect both performance andminimum trace widths that can be reliably connected via a laser.
We expect that with improved control over the particle size,
distribution, and laser parameters (spot size, frequency, power,
and speed), smaller trace widths and control over the path
tortuosity (and therefore likely conductivity36) could beobtained.
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The apparent viscosity was measured at shear rates from 0.01 to 100 1/
s. Strain sweep test was performed with increasing oscillation amplitude
up to 500% at a frequency of 1 rad/s. Frequency sweep test (0.01−100
rad/s) was performed at 0.5% strain.
Fabrication of Stretchable Circuit Demonstrations. Patterns
were designed and then imported into the laser system software. Afterlaser activation, electrical connections were made with copper tapes andconnected to an LED using biphasic gallium−indium to make reliableinterfaces.49 The circuit was encapsulated by pouring liquid PDMS over
the top surface (PDMS, Sylgard 184, Dow Corning). With voltage held
constant at 2.9 V, the circuit was stretched by hand or by using acustomized tensile stage at 300 mm min−1.

■

ACKNOWLEDGMENTS■

REFERENCES

Research Article

The authors thank Dr. Lina Sanchez Botero for performing
rheology tests and related data analysis and Dr. R. Adam
Bilodeau and Dr. Michelle C. Yuen for their valuable commentson the manuscript. This material is based upon work supportedby the National Science Foundation under grant no. (CMMI1812948). D.S.S. was supported by a NASA Space Technology
Research Fellowship (80NSSC17K0164).

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ASSOCIATED CONTENT

sı Supporting Information*

The Supporting Information is available free of charge athttps://pubs.acs.org/doi/10.1021/acsami.0c23108.
Circuit twisted, bent, and stretched to 80% strain and 50%
strain with no evident reduction of the LED brightness(MP4)
Optical images of EtOH/PDMS/LM double emulsion
with 10 and 14 vol % of PDMS precursors; optical imagesof EPL double emulsion with 10 vol % PDMS precursors
before and after 5 h; rheological properties of EPL double
emulsion with 10 vol % PDMS; optical image of aPDMS−LM composite ﬁlm; EDS Si mapping of thecross-sectional surface of a laser-activated PDMS−LM
composite; optical images of an as-printed EPL double
emulsion with diﬀerent PDMS contents; optical image ofan as-printed EPL double emulsion with 10 vol % PDMS
precursors; analytical estimation of the capillary force;
relative resistance change of the laser-activated PDMS−
LM composites with diﬀerent PDMS contents subjected
to uniaxial tensile cyclic loading to 100%; and electricalconductivity and relative resistance change over strain of
the PDMS−LM composite and other LM-based conductors in the literature (PDF)■■

www.acsami.org

AUTHOR INFORMATION

Corresponding Author

Rebecca Kramer-Bottiglio − School of Engineering andApplied Science, Yale University, New Haven, Connecticut
06520, United States; Email: rebecca.kramer@yale.edu
Authors

Shanliangzi Liu − School of Mechanical Engineering, Purdue
University, West Lafayette, Indiana 47907, United States;
School of Engineering and Applied Science, Yale University,
New Haven, Connecticut 06520, United States; orcid.org/
0000-0002-6362-801X
Sang Yup Kim − School of Engineering and Applied Science,
Yale University, New Haven, Connecticut 06520, United States
Kristen E. Henry − School of Engineering and Applied Science,
Yale University, New Haven, Connecticut 06520, United States
Dylan S. Shah − School of Engineering and Applied Science, Yale
University, New Haven, Connecticut 06520, United States
Complete contact information is available at:https://pubs.acs.org/10.1021/acsami.0c23108
Notes

The authors declare no competing ﬁnancial interest.
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ACS Appl. Mater. Interfaces 2021, 13, 28729−28736