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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 28232−28241

Laser Sintering of Liquid Metal Nanoparticles for Scalable
Manufacturing of Soft and Flexible Electronics
Shanliangzi Liu,†,‡ Michelle C. Yuen,†,‡ Edward L. White,† J. William Boley,†,∥ Biwei Deng,§
Gary J. Cheng,§ and Rebecca Kramer-Bottiglio*,‡
School of Mechanical Engineering and §School of Industrial Engineering, Purdue University, West Lafayette, Indiana 47907, UnitedStates‡Department of Mechanical Engineering and Materials Science, Yale University, New Haven, Connecticut 06511, United States
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S Supporting Information*

ABSTRACT: Soft, ﬂexible, and stretchable electronics areneeded to transmit power and information, and track dynamic
poses in next-generation wearables, soft robots, andbiocompatible devices. Liquid metal has emerged as apromising material for these applications due to its highconductivity and liquid phase state at room temperature;
however, surface oxidation of liquid metal gives it uniquebehaviors that are often incompatible with scalable
manufacturing techniques. This paper reports a rapid andscalable approach to fabricate soft and ﬂexible electronics
composed of liquid metal. Compared to other liquid metal
patterning approaches, this approach has the advantages ofcompatibility with a variety of substrates, ease of scalability, and eﬃciency through automated processes. Nonconductive liquid
metal nanoparticle ﬁlms are sintered into electrically conductive patterns by use of a focused laser beam to rupture and ablate
particle oxide shells, and allow their liquid metal cores to escape and coalesce. The laser sintering phenomenon is investigatedthrough comparison with focused ion beam sintering and by studying the eﬀects of thermal propagation in sintered ﬁlms. The
eﬀects of laser ﬂuence, nanoparticle size, ﬁlm thickness, and substrate material on resistance of the sintered ﬁlms are evaluated.
Several devices are fabricated to demonstrate the electrical stability of laser-patterned liquid metal traces under ﬂexing,
multilayer circuits, and intricately patterned circuits. This work merges the precision, consistency, and speed of laser
manufacturing with the material beneﬁts of liquid conductors on elastic substrates to demonstrate decisive progress towardcommercial-scale manufacturing of soft electronics.
KEYWORDS: eutectic gallium−indium, liquid metal, nanoparticles, laser sintering, soft electronics, scalable manufacturing

1. INTRODUCTION

neously form a thin oxide skin when exposed to air (∼1−3 nm
thick in ambient environment),19 the resulting high surface
tension makes bulk liquid metal incompatible with common
scalable manufacturing techniques, such as inkjet printing.9,19,20 To overcome the limitations posed by high surface
tension, we have previously demonstrated the creation ofliquid metal nanoparticle inks, by sonicating bulk eGaIn in acarrier solvent, and deposition of the particle-ﬁlled inks, byinkjet printing12,21 and spray-printing.22
Sintering is an essential postdeposition process to treat thesurface oxide surrounding each nanoparticle so as to form a
conductive pathway, enabling functionality in printed
electronic devices.23−25 Sintering methods for ordinary solid
metallic particles include thermal sintering,26 microwave
sintering,27 photonic sintering,28 and plasma sintering.29

Soft electronics have been demonstrated in applications
requiring low elastic modulus, such as in biomedical devices,1,2
soft robotics,3−6 and wearable technologies.7,8 A limitation in
the current state-of-the-art is a lack of scalable manufacturingmethods for soft electronics. A promising approach to create
soft electronics is to use liquid metals as a conductive medium.
Liquid metals are intrinsically soft, and eutectic gallium−
indium alloys (eGaIn) in particular have high conductivity (3.4
× 106 S·m−1; copper 5.96 × 107 S·m−1).9 Several approaches
for processing gallium-based liquid metals have been
demonstrated, including injection into microchannels,10
vacuum-ﬁlling microchannels,11 extrusion-based direct writing,12 hand writing,13 masked deposition,14,15 three-dimensional (3D) printing,16 embedding microscopic inclusions in
an elastomer matrix,17 and subtractive laser-based fabrication.18
However, the proposed processing techniques can bechallenging to automate, often requiring manual intervention.
Furthermore, because gallium-based liquid metals sponta© 2018 American Chemical Society

Received: May 26, 2018
Accepted: July 24, 2018
Published: July 25, 2018
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Figure 1. Laser-sintered eGaIn nanoparticle ﬁlm, with sintered regions appearing as a lighter gray. (A) Image of a laser-sintered Faboratory gear.
Scale bar is 2 mm in length. Faboratory logo reprinted with permission. (B) SEM image of a sintered-to-unsintered transition region across theedge of the pattern. Scale bar is 10 μm in length. (C, D) Detailed views of (C) uncoalesced eGaIn nanoparticles and (D) coalesced eGaIn
nanoparticle ﬁlm after laser sintering. Scale bars are 1 μm in length. The nanoparticles have an average diameter of 58 nm, and the nanoparticle ﬁlm
was sintered at a laser ﬂuence of 2.86 J/cm2.

inks onto various substrates, as shown by Mohammed andKramer.22 The spray-printing simultaneously aerosolized anddeposited the ink by blowing compressed air through an ink
droplet fed by a syringe pump at a constant rate down onto thesubstrate, resulting in a nanoparticle ﬁlm on the substrate.
Three milliliters of nanoparticle ink, infused into the spraynozzle at a rate of 0.7 mL/min and printed over an area ofapproximately 13 cm × 10 cm, resulted in a 10 μm thick ﬁlm(details in Experimental Section). While bulk eGaIn is highly
conductive, eGaIn nanoparticles are electrically insulated bytheir oxide skins, rendering the ﬁlm as-deposited to be
ultimately nonconductive. Spontaneous particle coalescenceis prevented by the stability of the oxide layer residing on thesurface of each particle. We used an ytterbium pulsed ﬁber
laser with a wavelength of 1065 nm, a pulse duration of 400 ns,
and a repetition rate of 2 kHz, operated in ambientenvironment, as a localized laser source to selectively rupture
and ablate the oxide skins, and coalesce the eGaIn nanoparticles into conductive ﬁlms. The estimated focused laser
spot size was 180 μm, as seen from the calculation in theExperimental Section. The scanning speed of the laser beam
was 125 mm/s. We selected a pulsed laser rather than acontinuous-wave laser to reduce the average temperature of thepatterned material during laser sintering, thus avoiding thermal
damage to the substrate and suppressing unfavorable
oxidation.37 The fabrication process is described in further
detail in the Experimental Section and in Figure S1. A
schematic of the laser sintering system is included in Figure S2.
Conductive patterns of arbitrary shape can be created via
laser sintering of eGaIn nanoparticles. Figure 1A demonstrates
a laser-patterned Faboratory lab logo on a soft elastomer
substrate. The ﬁlm is composed of liquid metal nanoparticles
with an average diameter of 58 nm. The laser-sintered patternconsists of a raster-processed region achieved by scanning apulsed laser spot over the area and a more intricate vectorbased region achieved by sintering conductive lines. The lasersintered area (light gray) becomes conductive, whereas theunsintered region (dark gray) remains insulating. A comparison of the surface morphology of the unsintered and sinteredregions of the nanoparticle ﬁlm is presented in Figure 1B. Prior
to sintering, the particles remain intact (Figure 1C); after
sintering, the particles coalesce into a connected ﬁlm network,

EGaIn nanoparticles can be sintered mechanically at room
temperature, wherein mechanical pressure is applied to rupture
the oxide shells that electrically insulate the nanoparticles,
releasing their liquid metal cores to coalesce into electricalpaths. Mechanical sintering, however, is limited to substrates
that are stiﬀ enough to ensure that suﬃcient force is transferredto the nanoparticles to cause the oxide layers to rupture.22
Additionally, when liquid metal nanoparticle sizes are smaller
than 70 nm, the particles’ cores are solid because they become
indium-enriched, preventing coalescence through liquid ﬂow atroom temperature by mechanical sintering.30
Recently, laser sintering has emerged as an attractive
technique for sintering metal nanoparticles such as Cu,31
Zn,24 or Ag,32−34 due to advantages including direct
patterning, rapid processing, multisubstrate compatibility, andease of scalability through automated processes. Although thefeasibility of laser sintering eGaIn nanoparticles has been
mentioned,35,36 sintering principles and parameters have yet tobe fully investigated and harnessed to produce functional
devices.
In this paper, we introduce an automated approach to
rapidly fabricate ﬂexible electronics made of eGaIn nanoparticles by combining spray-printing with direct laser-sintering
processing. By using a pulsed laser beam as a localized heatsource, sintered eGaIn nanoparticle ﬁlms with high conductivity (from 3.625 × 105 to 3 × 106 S·m−1) are achieved;
resistance values are easily tuned with laser parameters. Thisapproach is compatible with a variety of substrates withoutcausing any thermal damage. Moreover, laser sintering enables
coalescence of smaller solid-core eGaIn nanoparticles,30 paving
the way for higher-resolution deposition techniques. Finally,
ﬂexible and wearable electronic devices with intricate patterns
and soft, multilayer circuits are demonstrated.

2. RESULTS AND DISCUSSION
2.1. Fabrication and Surface Morphology of LaserSintered Liquid Metal Films. Using the methods presented
by Boley et al.,21 we created liquid metal nanoparticle inks bysonicating bulk eutectic gallium−indium alloy (eGaIn, 75.5%
Ga/24.5% In) in ethanol for times ranging from 120 to 720
min, resulting in average particle diameters ranging from 220
to 58 nm as characterized by Lear et al.30 We spray-printed the28233

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Figure 2. Investigation of laser sintering phenomenon through comparison with focused ion beam sintering and studying the eﬀects of thermal
propagation in sintered ﬁlms. (A) EGaIn nanoparticles coalescing due to oxide ablation via focused ion beam (FIB). Scale bars are 5 μm in length.
(B) Fabrication steps for measuring conductivity of uncoalesced particles at the bottom of the ﬁlm through copper electrodes: (i) copper electrodes
are etched into a copper-clad polyimide ﬁlm (Pyralux), (ii) eGaIn nanoparticle inks (∼10 or ∼20 μm) are deposited on the ﬁlm, and (iii) a laser isused to coalesce the nanoparticles from the top. The two SEM images on the right are cross-section views of as-printed and laser-treated ﬁlms. Scalebars are 5 μm in length. Insets show a more zoomed-in view; scale bars are 1 μm in length. (C) Top view of thermally sintered ﬁlm (550 °C, 0.5 h).
Scale bar is 5 μm in length. (D) Resistance values of as-printed, laser-treated (ﬁlm thickness ∼10 or ∼20 μm) and thermally sintered ﬁlm over thesame area (50 × 5 mm). (E) Survey spectra from XPS analysis of as-printed, sintered, and unsintered (adjacent to sintered region) nanoparticle
ﬁlms. (F) High-resolution spectrum over the Ga 3d/In 4d region, highlighting the presence of gallium oxide and gallium metal.

instantly transformed into heat).38,39 In our case, the gallium
oxide shell is transparent to light at wavelengths greater than
300 nm because of its wide band gap (∼5 eV),40−42 but thegallium−indium core will heat up due to the photothermal
eﬀect. We therefore hypothesize that the observed coalescing
phenomenon can be attributed to ablation (i.e., vaporization)of the metal oxide shell, induced by the heated liquid metalcore, and to the rupture of the metal oxide shell due to thermal
expansion of the liquid metal core relative to the shell.43−45
When excessive laser energy is applied, the nanoparticles arecompletely ablated from the surface by vaporization rather
than coalescing into a conductive ﬁlm. The morphology
observed in Figure 1D suggests that (1) the surface of theescaped liquid metal core stays unoxidized for long enough toallow the liquid metal to ﬂow and coalesce and (2) despite theoverall coalescence of the particles, some particles are still
vaporized during sintering, which is likely a consequence of theGaussian proﬁle of the laser beam39 and results in a network
structure instead of a continuous ﬁlm. In addition to oxide
ablation, further thermal diﬀusion causes a temperature rise in
the ﬁlm and results in particle densiﬁcation and changes to theforming electrically conductive paths (Figure 1D). We further
note the small size of the laser-sintered particles in Figure 1,
where previous work has indicated that small (<70 nm)
particles cannot be mechanically sintered.30 We attribute thisresult to laser-induced temperature rise, which is high enoughto initiate melting of small, solid-core particles while
simultaneously rupturing and ablating their oxide shells,
allowing the melted metal to ﬂow and form conductive
patterns.
2.2. Laser Sintering Mechanism. To gain a better
understanding of the laser sintering phenomenon, we isolated
the oxide ablation and thermal eﬀects by studying focused ion
beam (FIB) sintering and the eﬀects of thermal propagation insintered ﬁlms, respectively. For typical metals, the thermalization time τ (the time it takes for excited electronic states toreach equilibrium during laser light interaction with thematter) is on the order of 1 ps to hundreds of picoseconds.38
When metals are laser-processed at a pulse width of 400 ns ≫τ, the laser-induced excitation rate is not high enough to cause
photochemical reactions, such as electron repulsion and bondbreaking, etc., and laser sintering instead typically manifests asa photothermal process (i.e., the absorbed laser energy is
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properties of gallium oxide, inducing conductive ﬁlms
underneath the coalesced region.
The success of particle coalescence by mechanical sintering
methods has proven that the oxide skins of liquid metal
nanoparticles can be ruptured at room temperature without
any thermal contribution to allow the liquid cores inside ﬂow
out and merge.21,22,30 As a comparison to laser and mechanicalsintering, FIB was used to demonstrate nanometer-scaleparticle coalescence under high vacuum at room temperature.
As shown in Figure 2A, we precisely coalesced particles over asmall area (10 × 10 μm) using a focused ion beam with agallium ion source (Ga+) at various system settings. The oxideshells of eGaIn nanoparticles were sputtered away physically by
the bombardment of incident ions, and their liquid metal cores
merged spontaneously.46 FIB produces a visually similar eﬀect
to laser sintering with the exception that the images in Figure
2A and Figure S3A appear smoother because oxide regrowthcannot occur in the vacuum of the FIB system. When theenergy of the FIB beam was too high, the eGaIn nanoparticles
were ablated oﬀ the substrate rather than coalescing into
connected ﬁlms, as shown in Figure 2A. The incident beamenergy parameters used to remove the oxide skin wereconsistent with values reported in the literature.47 Local
temperature rise induced by ion bombardment can be
estimated to be less than 40 °C by existing methods, and the
FIB sintering process can be considered nonthermal.48,49 While
this method is slower for sintering particles over a large area
and is far from applicable to scalable manufacturing, itdemonstrates that oxide ablation in a highly focused region
without extensive heating is an eﬀective method for sinteringliquid metal nanoparticles. Additional FIB images of highresolution (∼1 μm) sintered patterns are included in Figure
S3B. A time lapse presentation of eGaIn nanoparticles
coalescing via FIB is included in Figure S4 and Movie S1.
Following melting and coalescence of particles on the top
surface of the ﬁlm, further thermal propagation raises
temperatures in the ﬁlm to the point where the underlying
uncoalesced particles become conductive up to an eﬀective
sintering depth (teff). The particles underneath the coalesced
patterns become electrically conductive because (1) gallium
oxide shows prominent semiconducting behavior at temperatures higher than 500 °C50,51 and (2) particles display
coarsening and fusing eﬀects at elevated temperature, as
commonly seen from sintering ordinary solid metal nanoparticles.52,53 We examined the eﬀective sintering depth bymeasuring conductivity of the uncoalesced particles at thebottom of the ﬁlm through copper electrodes in three steps. As
shown in Figure 2B, (i) copper electrodes were patterned byetching into a copper-clad polyimide ﬁlm, (ii) we deposited∼10 and ∼20 μm thick eGaIn nanoparticle ﬁlms (d ≈ 220 nm)
by spray-printing while masking the ends of the copper
electrodes, and (iii) we used the laser to sinter a pattern (50 ×5 mm) over the top of the ﬁlm across the electrodes with alaser ﬂuence of 2.86 J/cm2 and measured the resistance values
between the copper electrodes. For a ∼10 μm thick ﬁlm, theelectrical resistance across the copper electrodes was only 9 Ω.
For a ∼20 μm thick ﬁlm, the resistance increased to 870 kΩ,
indicating that the particles near the bottom of the ﬁlm werebarely conductive because much less heat was transferred tothe bottom of the ﬁlm from the top surface. The results revealthat the uncoalesced particles became conductive due toexposure to high temperature, and the eﬀective sintering depth,
at a laser ﬂuence of 2.86 J/cm2, was between 10 and 20 μm.

Photographs of the experimental specimens and setup can beseen in Figure S5. In Figure 2B, the two scanning electron
microscopic (SEM) images on the right show representative
cross-section views of as-printed and laser-sintered eGaIn
nanoparticle ﬁlms corresponding to panels ii and iii. The ﬁlms
were encapsulated with a poly(dimethylsiloxane) (PDMS)
layer on the top and cleaved after freezing in liquid nitrogen.
The laser-sintered ﬁlm has a distinct coalesced region on top
with a thickness of approximately 800 nm. As shown in moredetail in the insets, the as-printed particles are easily
distinguished from one another, whereas the uncoalesced
particles underneath the sintered region display much moreblurred boundaries between each other, indicating that hightemperature from the laser beam fuses particles together andbuilds electrical connections.
As a comparison to laser sintering, an eGaIn nanoparticle
ﬁlm was spray-deposited onto a ceramic substrate, thermallysintered in a furnace at 550 °C for 0.5 h, and then cooled in an
ambient environment.31,45 The resistance value over the same
area (50 × 5 mm) was measured to be 167 Ω. As shown in
Figure 2C, the thermally sintered ﬁlm shows densiﬁcation andcoarsening rather than particle coalescence. Resistance values
in the diﬀerent cases are summarized in Figure 2D. The higher
resistance obtained from thermal sintering with respect to laser
sintering is likely due to more severe oxidation and contact
resistance among the particles.
We performed an X-ray photoelectron spectroscopic (XPS)
analysis to characterize the surface composition of as-printed,
laser-sintered, and unsintered (adjacent to the sintered region)eGaIn nanoparticle ﬁlms. Since XPS is a highly sensitive
surface technique, all the detected signals are typically from the
top 10 nm of the ﬁlm (within a single particle diameter).54
Survey spectra for the three diﬀerent conditions are shown in
Figure 2E. In every case, the spectrum shows gallium peaks(Ga 2p, Ga 3d) and indium peaks (In 3p, In 3d). The distinct
oxygen peaks indicate that the nanoparticle ﬁlms are oxidized
in all cases. Figure 2F shows high-resolution spectra over the
Ga 3d/In 4d region (shaded yellow box in Figure 2E).
Notably, the laser-sintered nanoparticle ﬁlm shows a higher Ga
3d (Ga2O3)/Ga 3d (metal) ratio in comparison to the othertwo cases (black line). Additionally, the as-printed nanoparticle
ﬁlm exhibits a slightly lower Ga 3d (Ga2O3)/Ga 3d (metal)
ratio than the unsintered ﬁlm. These results indicate that thelaser-induced high-temperature environment leads to a highdegree of oxidation.54 For ordinary metal nanoparticles such ascopper or zinc, sintering in ambient environment often
requires additional synthetic processes to minimize oxidation
issues.24,55 For eGaIn nanoparticles, due to the nearinstantaneous ablation of gallium oxide and particle
coalescence during laser sintering, high conductivity is stillachieved despite the oxide growth at high temperatures.
Furthermore, the XPS results in Figure 2F show that the ratios
between the pure gallium and indium peaks do notsubstantially diﬀer between sintered and unsintered regions,indicating that the oxide growth negligibly changes thecomposition of the ﬁlm.
2.3. Characterization of Laser-Sintered Liquid Metal
Films. To characterize the laser sintering manufacturingmethod, we investigated the eﬀects of various processing
parameters on the resistance of laser-sintered eGaIn ﬁlms
(statistical analysis results in Tables S1 and S2). Resistancevalues were measured via four-terminal resistance sensing on
laser-sintered dogbone patterns (7 × 1.4 mm), as shown in the
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ﬁlms (d ≈ 220 nm) deposited on glass and PDMS substrates,
as plotted in Figure 3C. The sintered line width increases from∼200 to ∼330 μm as laser ﬂuence increases from to 2.86 to
9.55 J/cm2. The inset in Figure 3C shows optical images of thelaser-sintered lines corresponding to data points in the plot.
The minimum line width here is limited by the laser beamitself; ﬁner patterns can be achieved by a laser system with asmaller spot size. The measured resistance of the sintered ﬁlms
is typically between 2 and 20 Ω. From the approximate
sintered thickness from the cross-section images (∼800 nm,
Figure 2B) and measured dimension of the dogbone pattern(length/width), the volumetric conductivity of the sinteredeGaIn ﬁlm is calculated to be in the range from 3.625 × 105 to
3 × 106 S·m−1. In comparison to the conductivity of bulk
eGaIn (3.4 × 106 S·m−1), we ﬁnd that, despite oxide growthfrom laser-induced high temperatures, the electrical properties
are negligibly compromised because particle coalescence yields
well-connected electrical paths.
In general, particle size has a minimal eﬀect on resistance ofthe sintered ﬁlms (Figure 3A,B). However, the sintered ﬁlms
composed of ∼58 nm particles on PDMS substrate (green
symbols) are notably not as conductive as the liquid core
particle ﬁlms and the ﬁlms on glass substrates, as seen in Figure
3B. We reduced the infusion rate of the ink from 0.7 to 0.34
mL/min while maintaining the same total deposited ink
volume when spray-printing onto a PDMS substrate, and weevaluated the resistance values after laser sintering. As shown in
Figure 3D, the resistance of the sintered ﬁlm printed at a
reduced infusion rate is much lower. We suspect that surface
roughness of the deposited ﬁlm is critical for nanoparticles witha diameter of ∼58 nm. When the ﬁlm is not smooth enough,the volume of liquid metal contained in the coalesced particles
is too small to bridge the diﬀerent heights of the ﬁlm, resulting
in discontinuous regions within a sintered pattern, an eﬀect
that is magniﬁed as particle size decreases. Figure 3E reveals
the surface roughness and morphology of nanoparticle ﬁlms
deposited at two diﬀerent infusion rates on PDMS substrates.
Average arithmetical mean height (Sa) and root-mean-square
height (Sq) values, listed in Figure 3E, show that the surface
roughness of the ﬁlms is reduced with a slower infusion rate.
This result suggests that, to achieve better sintering results forﬁlms composed of small particles (d ≈ 58 nm) on PDMS
substrates, the surface roughness of the spray-printed nanoparticle ﬁlm needs to be controlled.
Furthermore, we studied the change in resistance of sinteredﬁlms composed of particles with an average diameter of 220
nm on PDMS substrates by varying ﬁlm thickness (∼10−40μm). As shown in Figure 4A, we observed that, at laser
ﬂuences ranging from 2.86 to 10.5 J/cm2, the eﬀect of ﬁlm
thickness on resistance is negligible, indicating that thecontribution from heated but uncoalesced particles beyond a
depth of 10 μm is minimal. The same study on glass substrates
and ﬁlms composed of particles with d ≈ 58 nm indicate thesame result, as shown in Figure S6. Because the eﬀect of ﬁlm
thickness on resistance is negligible, the amount of ink used
may be minimized to a known degree with no eﬀect on the
conductivity of the ﬁlm, allowing more eﬃcient material use
without sacriﬁcing electrical properties.
With the mechanical sintering approach, substrate softness
has a detrimental eﬀect on the eﬃcacy of particle sintering.22
Laser sintering is a noncontact method that is less aﬀected bysubstrate properties, particularly when larger particles (d ≈ 220nm) are sintered. We spray-printed eGaIn nanoparticle inks

inset of Figure 3A. To avoid disrupting and puncturing the ﬁlm
with rigid measurement probes, we used bulk eGaIn droplets

Figure 3. Electrical properties of laser-sintered eGaIn nanoparticle
ﬁlms. (A, B) Eﬀects of nanoparticle size and laser ﬂuence on theresistance of laser-sintered eGaIn nanoparticle ﬁlms on (A) glass and(B) PDMS substrates. The y-axes are log scales. Error bars represent 1
standard deviation. All the particles were spray-printed at 0.7 mL/
min. (Inset) Four-terminal resistance sensing on laser-sintereddogbone patterns. The designed dimensions of the dogbone patterns
are 7.5 × 1.5 mm. (C) Variation in single-scan (vector) line widths byadjusting laser ﬂuences on nanoparticle ﬁlms deposited onto glass and
PDMS substrates. (Insets) Corresponding optical images of single
lines sintered at 2.86 to 9.55 J/cm2 (left to right). Scale bar is 500 μm
in length. (D) Eﬀect of infusion rate of eGaIn nanoparticle inks on theresistance of laser-sintered ∼58 nm nanoparticle ﬁlms. Error bars
represent 1 standard deviation. The y-axis is log scale. (E) Twodimensional (2D) surface proﬁle images (Leica DCM8 and SEM) andvalues of surface roughness parameters (average arithmetic meanheight Sa and root-mean-square height Sq) of nanoparticle ﬁlms
deposited at two diﬀerent infusion rates, 0.7 and 0.34 mL/min, on
PDMS substrate. The color bar of the 2D surface proﬁles represents
the scale for the z-range. Scale bars are 500 μm in length.

to bridge the contact between the probe tips and the laserpatterned dispersion.56 We varied the nanoparticle size
(average diameter ± standard deviation 58 ± 7.0, 75 ± 5.8,
90 ± 6.4, or 220 ± 10.0 nm) and laser ﬂuence (energy per unit
area 2.86, 3.82, 4.77, 5.73, 6.68, 7.64, 8.60, or 9.55 J/cm2).
Figure 3A,B presents the measured resistance of sintered ﬁlms
as a function of laser ﬂuence for liquid metal nanoparticles withvarious average sizes deposited on glass and PDMS substrates.
Across diﬀerent particle sizes, the resistance of the sinteredﬁlms decreases with increasing laser ﬂuence. As more energy isdelivered during sintering, more particles are coalesced laterally
across the ﬁlm, resulting in a more conductive ﬁlm. To verify
this statement, we measured the line widths achieved byscanning a single line at diﬀerent laser ﬂuences on the particle
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Figure 4. Eﬀects of ﬁlm thickness and substrate materials on laser-sintered eGaIn nanoparticle ﬁlms. (A) Eﬀect of ﬁlm thickness on resistance oflaser-sintered eGaIn nanoparticle ﬁlms printed on PDMS. Error bars represent 1 standard deviation. (B) Resistance of liquid metal eGaIn ﬁlms
printed on various substrates and sintered at various laser ﬂuences. Error bars represent 1 standard deviation. The y-axes are log scale. (C) SEM
images of both laser-sintered and unsintered regions of eGaIn nanoparticle inks printed onto epoxy (laser ﬂuence 3.82 J/cm2). Scale bars are 4 μm
in length. (D) SEM image of a laser-sintered region of eGaIn nanoparticle inks printed onto PDMS (laser ﬂuence 4.77 J/cm2). Scale bar is 10 μm in
length.

(∼20 μm thick) with an average particle diameter of 220 nm
on various substrates: PDMS, epoxy, glass, and ceramic(alumina), with Young’s modulus ranging from ∼2 MPa to∼300 GPa.57 The plot in Figure 4B shows that resistance ofthe sintered ﬁlms on diﬀerent substrates is indistinguishable.
Meanwhile, a surface morphology comparison of nanoparticle
ﬁlms on representative substrates is shown in Figure 4C,D.
The SEM images of the nanoparticle ﬁlm printed on epoxy(Figure 4C) display a relatively uniform surface with a smoothtransition from the sintered to unsintered region, and no cracks
are observed across the entire surface. The inset images showthat sintered and unsintered regions are signiﬁcantly diﬀerent
in morphology. In contrast, the laser-sintered eGaIn ﬁlm on
PDMS shows a more porous appearance with distributed
cracking, although electrical conductivity was still achieved bycoalesced particles bridging across cracks throughout the ﬁlm(Figure 4D). Surface proﬁles and roughness parameters of the
as-printed ﬁlms on various substrates are presented in Figure
S7. The ﬁlm printed on PDMS substrate has the roughest
surface, with cracks observed, while no cracks are observed onother substrates. We attribute the diﬀerences in surface
roughness of the spray-printed ﬁlms to diﬀerences in wetting
between the inks and the various substrates. The eGaIn inks
fully wet on glass, ceramic, and epoxy, as indicated by a contact
angle <5°, whereas the contact angle on PDMS isapproximately 30° (see Experimental Section for details).
Crack formation is related to ﬁlm compaction, particle size,
and thermal properties of the substrate, etc., during both ﬁlm
drying and laser sintering.58−60 The results indicate thatelectrical properties of the sintered ﬁlms composed of larger
particles are less dependent on substrate properties, but further
investigation of crack morphology and ﬁlm roughness areimportant in creating uniform nanoparticle ﬁlms with highperformance.61

2.4. Applications toward Flexible Electronics. Towardapplications in ﬂexible electronics, we fabricated a ﬂexible
circuit using the laser sintering approach and conductedbending tests to evaluate the electrical stability. As shown in
Figure 5A, the circuit was bent upward and downward withbending radii from 8 to 180 mm, and the relative resistanceonly changed slightly in the most extreme case (0.2%). After
cyclic bending tests up to 1000 cycles at a bending radius of 8
mm (sample size 3 × 3 cm), the resistance increased relative tothe original value (R0 ≈ 272.2 Ω) by less than 3%, as displayedin Figure 5B. Laser sintering also provides an eﬃcient means offabricating multilayer soft circuits. As illustrated in Figure 5C,
we ﬁrst spray-printed a ∼10 μm thick ﬁlm (d ≈ 220 nm) ontoa PDMS substrate (2 × 3 in.) and then sintered a serpentine
pattern (shown in blue) at a laser ﬂuence of 2.86 J/cm2. Given
the previous investigation about sintering eﬀective depth, we
spray-printed another ∼20 μm ﬁlm over the bottom pattern toobtain electrical insulation between the two layers. Then we
sintered another serpentine pattern (shown in pink) at thesame laser ﬂuence onto the top layer of dispersion,
perpendicular to the bottom one, while masking the contact
pads of the sintered pattern with paper. Finally, we placed
drops of bulk eGaIn onto the contact pads to interface withcopper wires and then poured a thin layer of liquid PDMS over
the top of the dispersion for encapsulation. As demonstrated in
Figure 5D, we connected light-emitting diodes (LEDs) andsintered patterns in series in a circuit and applied a powersource (2 V). The top and bottom patterns are bothconductive; however, the two patterns are electrically isolated
from each other. This proof-of-concept can be further
expanded to create multilayer soft circuits with more than
two layers. The laser sintering approach is also easily adopted
for fabricating wearable electronics. As shown in Figure 5E, asoft electronic device can be fabricated by directly laser28237

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Figure 5. Soft and ﬂexible electronics. (A) Normalized resistance of a ﬂexible circuit under bending, both upward and downward, at various radii ofcurvature. Three measurements were averaged to obtain the reported values. (B) Normalized resistance of a ﬂexible circuit under bending for up to
1000 cycles at a radius of 8 mm. The initial resistance R0 value is ∼272.2 Ω. (C) Schematic of multilayer soft circuit fabrication. (D) Illustration of amultilayer soft circuit consisting of two electrically isolated, orthogonal serpentine patterns. (E) Soft circuit board composed of laser-sintered eGaIn
nanoparticles on a biocompatible adhesive substrate adhered to a human wrist, demonstrating applications in wearable electronics. Faboratory logo
reprinted with permission.

by previously reported approaches and liquid metal circuits forapplication in ﬂexible and wearable electronics. Possible
improvements include employing a laser system with a small
spot size to make high-resolution patterns. Further enhancement of ﬁlm uniformity and integrity would be beneﬁcial for
manufacturing highly deformable electronic devices. This
process can be easily extended to a large area, with no needfor premade masks, and integrated into scalable manufacturing
processes.

sintering intricate circuit patterns onto elastomer substrates
and adhering onto a biocompatible silicone-based adhesive forapplication onto a human wrist. These applications represent
the potential of using laser sintering as a scalable, fully
automated process for production of soft electronics.

3. CONCLUSIONSIn summary, we have demonstrated laser sintering as a method
to selectively coalesce liquid metal nanoparticle ﬁlms into
conductive patterns with high intricacy and high repeatability
for soft and ﬂexible electronics. To better understand theunderlying process, we compared laser sintering with oxide
ablation by performing FIB sintering and studied the depth ofthermal eﬀects in laser-sintered ﬁlms. We also performed aparametric study investigating the eﬀects of laser ﬂuence,
nanoparticle size, and ﬁlm thickness on the resistance of thesintered ﬁlms. Additionally, we compared electrical performance and surface morphology of the particle ﬁlm on various
substrates. Finally, we demonstrated that this method can
produce soft multilayer devices that are diﬃcult to manufacture

4. EXPERIMENTAL SECTION
4.1. Preparation of eGaIn Dispersion. The eGaIn dispersion
was made by depositing 362 ± 5 mg of eutectic gallium−indium alloy(495425, Sigma−Aldrich) into the bottom of a 3-dram glass vial (03339-10C, Fisher Scientiﬁc). Ethanol (4 mL; V1001, Koptec) was thenadded to the vial by a micropipette (BPP1000, Lagnet BioPette Plus).
The top of the vial was covered with Paraﬁlm (52858-076, VWR) to
prevent entry of foreign particles into the sample. The tip of thesonicator probe (Q700 with 1/8-in. microtip probe, QSonica) was
then punched through the Paraﬁlm and positioned approximately 1
mm from the bottom of the glass vial. A water bath held at 6 °C was
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raised to immerse the vial for keeping the sample cool. The sample
was sonicated at an amplitude of 36 μm (30% setting) for the desired
amount of time to control the nanoparticle size distribution.
Following sonication, the vial was capped and stored at room
temperature. Just prior to spray-printing, each sample was mixed
vigorously with a vortex mixer (VortexGenie) for 3 min to ensure
uniform dispersion.
4.2. Preparation of Substrates. All the substrates had
dimensions of 3 × 2 in. Glass (48382-180, VWR) and ceramic(nonporous alumina, 8462K25, McMaster-Carr) substrates werecleaned by rinsing with ethanol, 2-propanol, and distilled water.
PDMS [poly(dimethylsiloxane), Sylgard 184, Dow Corning]
substrates were made by mixing and then defoaming the elastomer
base and the curing agent in a 10:1 ratio by use of a planetarycentrifugal mixer (ARE-310, Thinky) for 30 s each. These were spincoated onto a glass slide at 200 rpm for 60 s by use of a spin coater(G3-8, SCS) and cured in an incubator at 60 °C for 3 h. Epoxy(Gorilla Epoxy) substrates were made by mixing and defoaming theresin and the hardener in a 1:1 ratio by use of the mixer for 30 s each.
These were spin-coated onto a glass slide at 500 rpm for 60 s, andcured at room temperature for 1 h.
4.3. Spray-Printing of eGaIn Dispersion. The spray-printing
setup consisted of three micropositioning stages for x-, y-, and z-axis
motion (PhysikInstrumente, two M-531.DD linear stages and an M501.1PD precision vertical stage controlled by a DC servo-motor
controller C-843.41), the spraying assembly, and the enclosure. The
spraying assembly consisted of the ink delivery nozzle attached to asyringe pump (883015, Harvard Apparatus) and the compressed air
delivery nozzle, and this assembly was ﬁxed in position to the top of
the enclosure. The ink nozzle was located 3 mm below the opening
for the air nozzle. The syringe pump was positioned vertically suchthat the tubing connecting the syringe containing the ink to the ink
delivery nozzle was minimal in length. The infusion rate of the syringe
pump was set at 0.7 mL/min. The air pressure was regulated to 3 psi.
The substrate was positioned 10 cm below the air nozzle by use of thez-stage. The ink was injected into the nozzle through the syringe
pump, and pressurized air was blown perpendicular to the nozzle tospray the ink on the substrate. The enclosure consisted of a cage
constructed from aluminum extrusion to bound the full width anddepth of the x- and y-stages and was of suﬃcient height to hold thespraying apparatus above the stages. Poly(vinyl chloride) (PVC) ﬁlm
was aﬃxed to the cage to contain the sprayed material. The ink from asingle vial was drawn into a 3 mL syringe and installed into the syringe
pump. The air ﬂow was opened, and the infusion of the ink wasstarted. Once the ink began to spray onto the printing surface, thestages began to move at 7 mm/s in a serpentine pattern to ensure aneven coat of the dispersion across the entire substrate. The serpentine
pattern was repeated a total of three times for the infusion of 3 mL of
ink at a rate of 0.7 mL/min, resulting in a ∼10 μm thick ﬁlm over an
area of approximately 13 cm × 10 cm. The thickness of the ﬁlm wasestimated from the cross-section image of the ﬁlm (Figure 2B). All thesubstrates were 2 × 3 in. and placed in the middle of the print stage.
The overspray was to ensure that the spray over the substrate was
uniform. Thicker ﬁlms were printed by cleaning the ink nozzle andtubing with ethanol, installing a new 3 mL syringe of ink, andrepeating the printing process. For the additional low-infusion-rate
tests on elastomer substrate, the rate of the syringe pump was reduced
to 0.34 mL/min and the same serpentine pattern was repeated 6 times
to print a ﬁlm ∼10 μm thick.
4.4. Focused Ion Beam Manipulation Procedure. For the FIB
images shown in Figure 2A and Figure S3A, eGaIn nanoparticle
deposits were produced by pipetting ∼50 μL of 1 mM thiolated
eGaIn nanoparticle inks (d ≈ 180 ± 32 nm), which were created by
the method described by Boley et al.,21 onto a Si wafer (0.0035−
0.0038 Ω·cm, Silicon Sense, Inc.). The four images from left to rightin Figure 2A are eGaIn nanoparticle deposits sintered at various
currents, voltage settings, and cut depths as follows: (1) 10 kV, 1.1
nA, 50 nm; (2) 10 kV, 1.1 nA, 500 nm; (3) 30 kV, 1.0 nA, 50 nm; and(4) 30 kV, 1.0 nA, 500 nm. Cut depth in nanometers is predeﬁned in
the system by selecting speciﬁc material type. For the FIB images

shown in Figure S3B, eGaIn nanoparticle monolayer deposits were
produced by a hybrid self-assembly drop-casting process described by
Boley et al.61 EGaIn nanoparticles (d ≈ 103 ± 29 nm) were dispersed
into a cosolvent system of ethanol and water (∼1:3 ratio of ethanol towater by volume) at a concentration of ∼4 mg/L. A drop of ∼1 μL
was then deposited onto a Si substrate [0.01−0.02 Ω·cm Si wafers(Silicon Quest International, Inc.)] where a monolayer deposit wasformed upon drying. All the deposits were etched by use of a focusedion beam (FEI Nova 200 dual-beam TM-SEM/FIB) with a gallium
ion source.
4.5. Resistance Measurements. Resistance measurements weremade by laser-sintering a pattern 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). Drops of eGaIn
were used to interface between the measurement probes and thesurface of the sintered pattern. This method enabled good electricalcontact with the ﬁlm without physically contacting or disturbing thesurface. Because of the textured surface of the sintered ﬁlm, eGaIn did
not wet nor adhere to the sintered ﬁlm, allowing the droplets to belifted and moved to a new sample. The values reported in this paper
were calculated from triplicate measurements.
4.6. Laser Sintering Parameters. The samples were sinteredwith an ytterbium pulsed ﬁber laser (YLP-HP-10-400-20-200, IPG
Photonics Corp.) with a wavelength of λ = 1065 nm. The output
beam diameter (1/e2) was D = 6 mm, the beam quality factor was M2= 8, and the focal length was f = 100 nm. The estimated focused laser
spot size was 180 μm, calculated by spot size = 1.27f λM2/D. The scan
speed of the laser head was ﬁxed at 125 mm/s, and the laser output
power was set between 1.8 and 6.6 W (9−30% of the total 20 W).
The calculated laser ﬂuence was between 2.86 and 9.55 J/cm2.
Patterns were drawn in CorelDraw and exported as an HPGL Plotterﬁle. The pulse repetition rate of the laser was 2 kHz, with a pulse
duration of 400 ns. No further processing was done to the samples
before resistance measurements were taken.
4.7. Contact-Angle Measurements. An ink droplet of 8 μL (d≈ 220 nm) was placed on the substrate with a micropipette. Droplet
proﬁles were captured from the side by a camera lens (InﬁniProbe
TS-160), and the contact angle was measured from the images by useof ImageJ. Values were averaged from ﬁve measurements.
4.8. XPS and Sample Preparation. XPS spectra were collectedby use of a monochromatic 1486.7 eV Al Kα X-ray source on PHI
VersaProbe II X-ray photoelectron spectrometer with 0.47 eV system
resolution. The energy scale has been calibrated by use of Cu 2p3/2(932.67 eV) and Au 4f7/2 (84.00 eV) peaks on a clean copper plate
and a clean gold foil. Survey spectra were collected at a constant passenergy of 187.85 eV. The peak positions were corrected with carbon
peak C 1s at 284.8 eV. The high-resolution spectra C 1s, O 1s, Ga 3d,
Ga 2p2/3, and In 3d2/5 were collected at a constant pass energy of 23.5
eV. Samples for XPS measurements sat in a desiccator under vacuum
overnight and then were blown with dry nitrogen before being loaded
into the XPS chamber.
4.9. Flexible Circuit Fabrication. The ﬂexible circuit for bending
tests was fabricated by ﬁrst spraying 6 mL of eGaIn nanoparticle inks(d ≈ 220 nm) on a PDMS substrate. After laser sintering of theserpentine pattern that was drawn in CorelDraw, drops of bulk eGaIn
were placed onto the contact pads to interface with copper wires.
Finally, the circuit was encapsulated by pouring liquid PDMS over the
top surface. The bending test was conducted by bending the circuit atvarious radii and measuring resistance values from a multimeter (5492B, BK Precision).
4.10. Soft Circuit Board Fabrication. The soft circuit board
with the intricate pattern was fabricated by ﬁrst spraying 6 mL of
eGaIn nanoparticle ink (d ≈ 220 nm) on a PDMS substrate. Afterlaser sintering of the pattern that was drawn in CorelDraw, the circuitboard was encapsulated by spin-coating liquid PDMS over the top
surface. After this layer of PDMS cured, Silbione skin adhesive (RTGel 4717, Bluestar Silicones) was prepared by mixing and defoaming
the two components in a 1:1 ratio by use of a Thinky Mixer for 3 0s
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each, and these were spin-coated onto the back surface of the soft
circuit. The sample was then cured in an incubator at 60 °C for 3 h.

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ASSOCIATED CONTENT

S Supporting Information*

The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acsami.8b08722.
One movie showing time sequence of sintering eGaIn
nanoparticles via FIB treatment MPG)
Seven ﬁgures showing fabrication process, schematic oflaser sintering system, sintering eGaIn nanoparticles tovarious patterns via FIB, time sequence of sintering
eGaIn nanoparticles via FIB treatment, photographs andschematics of measuring conductivities of uncoalesced
particles under coalesced patterns using copper electrodes, changes of resistance with varying ﬁlm thickness,and 2D surface proﬁle images and surface roughness
parameters of nanoparticle ﬁlms deposited on various
substrates; two tables listing parameter levels, F-value, pvalue, and eﬀect sizes from ANOVA of main eﬀects ofthe fractional-factorial study (PDF)■

AUTHOR INFORMATION

Corresponding Author*E-mail rebecca.kramer@yale.edu.
ORCID

Shanliangzi Liu: 0000-0002-6362-801X
Gary J. Cheng: 0000-0002-1184-2946
Present Address∥

Department of Mechanical Engineering, Boston University,
Boston, Massachusetts 02215, United States
Notes

The authors declare no competing ﬁnancial interest.

■

ACKNOWLEDGMENTS
We thank R. Adam Bilodeau for help with SEM and opticalimages, Shaotang Wang for help with laser sinteringexperimental samples, Dr. Mohammed G. Mohammed andTrevor R. Lear for initial experimental work, Dr. Sang Yup Kim
and Dr. Olivier Cyr-Choiniere for their valuable comments on
earlier versions of the paper, Dr. Min Li for access to andtraining on the XPS at Yale West Campus Materials
Characterization Core, and Dr. Michael Rooks for access toand training on the SEM at Yale Institute for Nanoscience andQuantum Engineering. This work was supported by the
National Science Foundation (Career Award 1454284). E.L.W.
and M.C.Y. were supported by National Science Foundation
graduate research fellowships (Grant DGE-1333468).

■

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DOI: 10.1021/acsami.8b08722
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