Flexible and Printed Electronics

PAPER

Reconfigurable electronic devices enabled by laser-sintered liquid metal
nanoparticles
To cite this article: Shanliangzi Liu et al 2019 Flex. Print. Electron. 4 015004

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Flex. Print. Electron. 4 (2019) 015004

https://doi.org/10.1088/2058-8585/aafa3b

PAPER

RECEIVED

23 October 2018

Reconﬁgurable electronic devices enabled by laser-sintered liquid
metal nanoparticles

REVISED

10 December 2018
ACCEPTED FOR PUBLICATION

20 December 2018
PUBLISHED

4 February 2019

Shanliangzi Liu1,2 , Michelle C Yuen1,2 and Rebecca Kramer-Bottiglio2
12

School of Mechanical Engineering, Purdue University, West Lafayette, IN 47907, United States of America
School of Engineering and Applied Science, Yale University, New Haven, CT 06510, United States of America

E-mail: rebecca.kramer@yale.edu
Keywords: eutectic gallium-indium, liquid metal, laser sintering, reconﬁgurable electronics, ﬂexible and stretchable electronics

Abstract
Liquid metals are promising conductive materials for creating ﬂexible and stretchable electronic
devices. However, methods to pattern liquid metal remain limited in scope: fabrication processes
often require substantial manual intervention and the resulting devices are often patterned indelibly
into the substrate. In this paper, we demonstrate the creation of electrical traces and components fromlaser-sintered liquid metal nanoparticles, a digital fabrication approach that creates devices without
permanently patterning the materials. As deposited, the liquid metal nanoparticles form a nonconductive ﬁlm because the conductive liquid metal cores of each particle are surrounded by a nonconductive metal oxide shell. Conductive traces are patterned into the ﬁlm using an infrared laser tosinter and coalesce the nanoparticles. We demonstrate that we can create resistive and capacitiveelectrical elements by tailoring the geometry of the laser-sintered patterns. Furthermore, the patterned
capacitors can be used to sense the proximity of an object and the patterned resistors can be used inconjunction with analog integrated circuit components to create analog circuits. We show that the
fabricated analog circuits can be reconﬁgured and reused to attain different output responses. The
laser processing technique also offers the opportunity to create stretchable conductive traces. We showthat compound laser processing can be used to both sinter the nanoparticle ﬁlm and cut the underlying
substrate in the same processing area to achieve in-plane stretching via out-of-plane ﬂexing of liquid
metal ﬁlms.

1. Introduction
Flexible and stretchable electronics are essential forsoft robotics and wearable systems where electronic
devices must be mechanically compatible with highly
deformable structures while maintaining functionality[1–5]. Metals that are liquid at or near room temperature have emerged as appealing materials for theseapplications [6, 7]. Gallium-based liquid metal alloys,
such as eutectic gallium–indium, (EGaIn; 75% Ga,
25% In) are inherently conformable conductive materials that are particularly appealing because of their
low toxicity [8], low viscosity (2 mPa s) [9], lowmelting point (15.5 ◦C), negligible vapor pressure[10] and high electrical conductivity (EGaIn 3.4×
106 S m−1; copper 5.96×107 S m−1) [11]. Galliumbased liquid metals have been used for stretchable
and self-healing wires [12–14], stretchable antennas© 2019 IOP Publishing Ltd[15–17], actuators [18, 19], electrical interconnects[20, 21], sensors [22–24] and superelastic conductors [25].
Gallium-based liquid metals rapidly form a thin passivating oxide skin (∼1–3 nm in ambient environment[26];∼0.7 nm under vacuum conditions [27, 28])
primarily composed of Ga2O3 when exposed to an oxygen concentration above 1 ppm [29, 30]. The oxide
growth has also been found to be accelerated under elevated temperature [31]. The oxide skin allows liquid
metal to adhere to surfaces and to form stable non-spherical shapes [6, 32], but also imparts high surface tension(∼624 mN m−1 [11, 33]), making the material challenging to process [34]. Existing methods to pattern liquid
metals include injection into microchannels [29],masked deposition [35, 36], 3D printing [32, 37], direct
writing [38, 39], vacuum-induced patterning [40],embedding into elastomers [41, 42] and subtractive laser

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S Liu et al

ablation [24, 43]. However, these proposed techniques
often require premade masks or microchannels formed
irrevocably in bulk polymer, making fabrication processes less amenable to rapid prototyping and challenging to automate. In particular, for liquid metal devices
that require microchannels or bulk embedding in polymer, the fabrication processes often necessitate manualintervention which renders the process to be inefﬁcientand not scalable, and the devices to be permanent (i.e.
non-reconﬁgurable) becuase the polymer cannot bereformed to create alternate conductive pathways.
Previously, we demonstrated the ability to makeliquid metal particle inks and to print them onto various substrates using inkjet [44] and spray printing
techniques [45], which are widely known scalable
manufacturing methods that are highly compatible
with roll-to-roll production [46–49]. The oxide layer
on each of the particles insulates them from theirneighbors, making the deposited ﬁlm non-conductive. To coalesce the particles into conductive patterns, the oxide layer must be removed or ruptured,
allowing their metal cores to reﬂow and merge. The
simplest method is mechanical sintering—employing
pressure to rupture the oxide skin of the particles,
without any specialized equipment required [44, 50].
Recently, we showed that a focused laser beam can also
be used to coalesce liquid metal cores into electricallyconductive patterns. The observed coalescing behavior can be attributed to the rupture of the metal oxideshell due to thermal expansion of the heated liquid
metal core [51]. The resistance values of the laser-sintered patterns are signiﬁcantly affected by laser power.
We demonstrated that this method can be used to produce soft multilayer circuits [51] and ﬂexible digital
circuits [52] rapidly and efﬁciently.
In the present work, we utilize the ﬁnite resistanceof laser-sintered liquid metal particles to program analog circuits by selectively patterning discrete electricalcomponents. Our non-prescriptive approach utilizes
scalable deposition and patterning methods suitable
for large-area fabrication of arbitrary patterns. We
demonstrate that we can use laser sintering to create
capacitors and resistors with customized electricalproperties. This allows us to create analog sensors andcircuits, which we demonstrate by using the sinteredcapacitor to detect proximity of a ﬁnger and by sintering resistors of different values to tune the gains of an
opamp circuit, respectively. Furthermore, we showthat by encapsulating part of the circuit in polymer, we
can selectively reconﬁgure the fabricated analog circuit by chemically etching away the exposed regions of
the circuit, re-printing a layer of liquid metal nanoparticles, and sintering new conductive patterns into
the ﬁlm to attain different output responses. Lastly, weshow that we can make stretchable circuits by simply
spray printing liquid metal nanoparticles onto polyimide substrates, and then simultaneously laser sintering the particles and cutting tunable kirigami
structures. Figure 1 shows a ﬂexible and stretchable
2

Figure 1. Photo of a ﬂexible and stretchable electronic device
composed of a laser-sintered liquid metal nanoparticle ﬁlm
deposited onto a ﬂexible printed circuit board (PCB) substrate (copper-clad polyimide ﬁlm). The laser-sintered patterns (light gray) are used to create conductive traces,
resistors, and capacitors between copper traces and integrated
circuits. The appendage on the bottom right of the device
contains slits to enable stretching of the conductive trace
without loss in conductivity. The SEM images show representative areas of the as-printed and laser-sintered particle ﬁlm.
The scale bars are 1 μm in length. The length of the integrated
circuit (black) is 3.15 mm.

device consisting of patterned resistors, capacitors andanalog circuits.

2. Results and discussion
2.1. Analog components
Liquid metal nanoparticle ink was prepared by sonicating bulk eutectic gallium–indium alloy (eGaIn,
75.5% Ga, 24.5% In; 362±5 mg) in ethanol (4 ml)
for 2 h with a tip sonicator (QSonica Q700), as
previously presented by Boley et al [44]. Ultrasonicwaves induce strong shear forces that break up bulk
liquid metal into particles. The oxide layer residing on
the surface of the liquid metal is continuously broken
and rapidly reformed, creating smaller particles.
As characterized by Lear et al [53] using scanningelectron microscopy (SEM), the resulting particle
size is 220±10 nm (mean±standard deviation).
We etched desired copper patterns into copper-cladpolyimide ﬁlms (Pyralux) and then used a customized
spray printer to spray the nanoparticle ink onto thepatterned ﬂexible substrate. The spray-printing setup
consisted of three micropositioning stages for x-, y-,and z-axis motion (PhysikInstrumente), a compressed
air delivery nozzle, and a ink delivery nozzle attached
to a syringe pump (883015, Harvard Apparatus).
Compressed air (3 psi) was blown over a syringe needle
while ink was dispensed at a ﬁxed rate (0.22 ml min−1),
creating a thin, uniform particle ﬁlm (20 μm thickness)on the polyimide substrate. The velocity of the printerstage was 0.7 mm min−1. In order to activate thedeposited non-conductive nanoparticle ﬁlm, we usedan IR laser (Universal Laser Systems VLS 2.30, 30 W,
10.6 μm CO2 laser) to sinter the particle ﬁlm into
conductive patterns by scanning a focused laser beamover an area (raster mode, 14% power, 100% speed).

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S Liu et al

Figure 2. Photos and measurements of laser-patterned
resistors and capacitors. (a) Interdigitated capacitors where
capacitance was modulated by the number and proximity ofﬁngers comprising the device. The mean±95% conﬁdence
intervals are listed for each device design. The insets show
detail views of the patterned capacitors. (b) Demonstration of
an interdigitated capacitor being used as a proximity sensor.
Each data point was captured by taking a photo of the LCR
meter screen and the ﬁnger in proximity of the sensor. The
red line indicates the mean baseline capacitance of the sensor.

Our previous work demonstrated the ability tocoalesce liquid metal nanoparticles into conductive
patterns using laser sintering technique and adjusting
sheet resistance by tuning laser power [51]. Here, we
extend the work beyond simply patterning conductive
traces and use laser sintering to create analog electricalcomponents. To demonstrate this, we used ﬁxed laserparameters and varied the geometry of the laser-sintered patterns to attain analog components with different values. Before spray printing and laser sintering,copper electrodes were etched into copper-clad polyimide ﬁlm to serve as measurement pads. Various
laser-sintered capacitors and resistors are shown inﬁgures 2 and 3. The components were sintered using
the raster mode in the laser etcher with the long3

dimension of the component (horizontal direction inﬁgures 2 and 3) oriented parallel to the rastering axis,
rather than the scanning direction. This ensured amore repeatable continuous electrical path by avoiding positional error from steps in the scanning direction. In general, the width of a horizontal trace wasmore consistent along its length as compared to thewidth of a vertical trace, as seen in the insets ofﬁgure 2(a).
Capacitors were sintered in interdigitated capacitor patterns, varying the number of ﬁngers (sinteredconductive traces) and the gap between the ﬁngers(unsintered regions) to adjust capacitance values(ﬁgure 2(a)). The capacitors were measured using an
LCR meter (BK Precision 880) at a 1 kHz interrogation
frequency. The mean and 95% conﬁdence intervals of
the measured capacitances are reported in ﬁgure 2(a).
As the number of ﬁngers increase and the gap between
the ﬁngers decrease, the capacitance increases.
Additionally, we demonstrated that these planarinterdigitated capacitors can be used to sense theproximity of an approaching ﬁnger (ﬁgure 2(b)). We
used an LCR meter (Keysight E4980AL) at a 1 kHz
interrogation frequency and 1 V excitation voltage tomeasure the capacitance while hovering a ﬁnger
within 1 mm of the surface. The distance between theﬁnger and the surface of the capacitor was measured
via optical image analysis where a scale bar of known
length was marked onto the support stand and wasused to scale the image accordingly. Without a ﬁnger
nearby, the capacitors had a mean capacitance of
8.48 pF. As the ﬁnger approaches the capacitor, thecapacitance increases monotonically. When the ﬁnger
is in contact with the sensor, the capacitance ﬂuctuates
at values from approximately 9 to 30 pF due to slightdifference in contact conditions (contact area, surface
moisture). These preliminary results suggest that laser
sintering may be used to create planar capacitive
proximity sensors.
Simple resistor patterns with varying trace width(0.25, 0.5, 1, 2, 3, 4 mm) and more intricate serpentine
patterns with varying length (one to three segments)
were sintered as shown in ﬁgures 3(a) and (b), respectively. Despite the feasibility of obtaining patterned
traces with distinct resistance values by tuning laser
power as indicated previously [51], in this work, weused ﬁxed laser processing parameters for all samples
and only varied pattern geometries. We ﬁrst measured
the sheet resistance of sintered dogbone patterns on
spray-printed nanoparticle ﬁlms, all processed withthe same laser parameters (14% power, 100% speed)using the four-probe resistance measurement method.
The sheet resistance of the sintered patterns was relatively consistent (1.94±0.15Ω/sq, mean ±95% conﬁdence interval, 12 measurements) across samples.
Using this mean sheet resistance value, we calculated
the expected resistance of the various resistors withdifferent geometries and compared it to the measured
values (ﬁgures 3(d), (e)). The calculated values tended

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S Liu et al

Figure 3. (a) Simple resistor patterns with varying trace width (0.25, 0.5, 1, 2, 3, 4 mm). (b) Serpentine resistor patterns with varying
length (one to three segments). (c) SEM images of the interface between the sintered trace and a copper pad. (d)–(e) Resistance valuesof the components shown in (a)–(b) as measured and as calculated from sheet resistance values. The error bars represent 95%
conﬁdence intervals of the measured resistance and capacitance values.

to underestimate the measured value by ∼5%–15%.
We hypothesize that this is primarily due to contact
resistance between the laser-sintered trace and thecopper measurement pad.
While the particles on top of the copper pads andthe polyimide substrate form fully-coalesced networks, the particles connecting them are not fully-coalesced which contributes to an increased resistance, as
evidenced by the larger but discontinuous particles
seen at the edge of the copper pad in ﬁgure 3(c). We
hypothesize that this is due to the steep, cliff-like edgeof the copper pad traversed by the nanoparticle ﬁlm.
Since the spray printing distributes nanoparticles in acone rather than a downward jet, nanoparticles aredeposited on all surfaces regardless of their angle. In
contrast, laser sintering acts only in a downward direction. Thus, the particles residing on the angled edge ofthe copper pad are sintered with less energy per unit
area, resulting in a partially coalesced network that
only tenuously spans the region between the sinteredpattern on the polyimide ﬁlm and on the copper pad.
Future work can mitigate this issue by using thermal
evaporation or other approaches to deposit thinnercopper layers onto the substrate. These improvements
would enable creation of patterns with resistancevalues that more closely match the theoretical calculations based on constant sheet resistance and the pattern geometry.
4

2.2. Analog circuits
We utilized the ability to pattern conductive traces at
speciﬁc resistances to create analog circuits. Here, weused an operational ampliﬁer (opamp) integrated
circuit (MCP6001, Microchip Technology) to create a
non-inverting amplifying circuit with tunable gains
using laser-sintered resistors (ﬁgure 4(a)). The opamp
was ﬁrst soldered to the copper traces of an IC
breakout substrate. The liquid metal ink was thendeposited and patterned onto the substrate, overcopper traces where necessary (ex. R1 lies on top of thecopper trace leading from the IC to Vout). Here, thesprayed ﬁlm was deposited to a thickness of 30 μm tobetter ensure electrical isolation between the sinteredliquid metal trace and the underlying copper trace.
The nominal output of the non-inverting ampliﬁer
circuit is the input voltage signal multiplied by thegain, calculated as Gain=Vout/Vin=1+R1/R2.
We varied the dimensions of the sintered traces toobtain different resistance values and nominal gains,
as tabulated in ﬁgure 4(b). To determine the actual
gain of the circuits, we supplied a 50 mVpp sinusoidal
signal with a 50 mV offset and measured the output
signal with an oscilloscope. Representative responses
are shown in ﬁgure 4(c). The mean measured gains arelisted in the table in ﬁgure 4(b).
Although the output signal of the opamps clearly
demonstrate voltage gain, the measured gain is lessthan that of the nominal value. We hypothesizeFlex. Print. Electron. 4 (2019) 015004

S Liu et allaser-sintered to create more complex analog circuits,
in particular, with the addition of other analog components or ICs. Furthermore, while this device is a
two-layer electronic circuit, the concept of spray printing and laser sintering over existing conductive traces
may be extended to create multilayer circuits.

Figure 4. Design and characterization of opamp circuits withgain programmed by laser-sintered resistive traces. (a) Schematic of the implemented circuit. The image on the rightshows the laser-sintered liquid metal nanoparticle ﬁlm
deposited over an opamp IC soldered to copper traces on apolyimide substrate. The length of the IC is 3.15 mm. (b)Table of resistance values, nominal gain, and actual gain of
circuits with three different gains. (c) Plot of the input andoutput voltage signals.

that this is due to slightly conductive (∼400 kΩ) connections between the coalesced liquid metal patterns
as they are sintered over copper traces, i.e. between R1
and the VDD copper trace. As demonstrated in our previous work [51], due to exposure to high temperature,
the uncoalesced particles at the bottom of the sinteredﬁlm are likely to become conductive because (1) gallium oxides become semiconductive at high temperatures [54, 55], and (2) particles coarsen and fuse
together at high temperatures, as seen from thermal
treatment of solid metal particles [56, 57]. Therefore,
the thermally treated but uncoalesced particles
become electrically connected to the underlying copper traces. The effective sintering depth (the distanceat which uncoalesced particles become conductive) is
dependent on laser parameters. Future work will aim
to identify the appropriate laser sintering parameters
so that conductive patterns may be sintered to a speciﬁc depth, without inadvertently connecting tounderlying copper traces. Work in this area will also
aid in repeatably fabricating the conductive interface
between the liquid metal ﬁlm and a copper pad. Whilethe contact resistance is not as signiﬁcant of an issue
for logic devices, in an analog circuit, contact resistance may not be so easily discounted.
Alongside the resistors (ﬁgures 3(a)–(b)) shown
here, capacitive components (ﬁgure 2(a)) may also be
5

2.3. Circuit reconﬁgurability
Unlike traditional electronic circuits, the demonstrated liquid metal patterned analog circuits arereusable. They can be rapidly reconﬁgured multiple
times to achieve different output responses via theprocess of chemical etching, re-printing and repatterning. Figure 5 shows the fabrication process ofan reconﬁgurable analog circuit.
The substrate was ﬁrst prepared prior to deposition of liquid metal. Flexible copper-clad polyimide
ﬁlm (Pyralux, Adafruit) was covered with Kapton tapeas an etching mask. The Kapton tape was cut to mask
the copper pattern using an IR laser (Universal Laser
Systems VLS 2.30, 30 W, 10.6 μm CO2 laser; vector
mode, 25% power, 20% speed). The undesired copper
was then etched off the polyimide ﬁlm using hydrochloric acid and hydrogen peroxide. Standard electrical components (integrated circuits (ICs)) were
soldered to the copper traces (ﬁgure 5(a)). Small pieces
of masking tape were applied to each of the ICs andalong the edges of the substrate to mask the components and copper interface pads, respectively(ﬁgure 5(b)). The nanoparticle ink was then sprayedonto the circuit using a customized spray printer(ﬁgure 5(c)). Next, the laser was used to sinter the particle ﬁlm into conductive patterns by scanning a
focused laser beam over an area (raster mode, 14%
power, 100% speed). As observed in ﬁgure 5(d), thesintered, conductive traces appear white. The SEM
images in ﬁgure 1 show the intact particles in theunsintered regions and the coalesced morphology ofthe sintered region.
Because the oxide skin of liquid metal is amphoteric, it can be removed by exposure to hydrochloric
acid (HCl, pH<3) [58] or sodium hydroxide(NaOH, pH>10) [59]. In the absence of the oxideskin, liquid metal that adheres to the surfaces can beadup due to high surface tension of the bare metal [11].
Therefore, chemical treatment can be used to remove
existing liquid metal ﬁlms, allowing the device to bereconﬁgured. Here, we used NaOH due to its provenfast dissolution of oxide compared to HCl [59].
In order to selectively reconﬁgure the device, partof the circuit was encapsulated by casting a thin layerof PDMS (Sylgard 184, Dow Corning) over the nanoparticle ﬁlm, appearing as a dark gray region(ﬁgure 5(e)). After the PDMS cured, the circuit wasrinsed with 1.0 M sodium hydroxide (NaOH) for a few
seconds. As shown in ﬁgure 5(f), the exposed particle
ﬁlm was etched away, while the encapsulated region
was protected by the PDMS. The circuit was thenFlex. Print. Electron. 4 (2019) 015004

S Liu et al

Figure 5. Fabrication process of analog circuits. (a) Flexible PCB on a copper-clad polyimide ﬁlm (Pyralux) with integrated circuit (IC)components soldered to copper traces. (b) Tape applied to mask ICs before spray printing. (c) Spray-printed liquid metal nanoparticle
ﬁlm. (d) Analog circuits created by laser sintering liquid metal traces patterned to interface with the ICs. (e) Partial encapsulation of thedevice using silicone elastomer (polydimethylsiloxane (PDMS)). (f) Unencapsulated region exposed after rinsing with NaOH. (g) Respray-printed liquid metal nanoparticle ﬁlm. (h) Final analog circuit device with particle ﬁlm re-sintered into zigzag patterns. The
steps shown in (f)–(h) may be repeated to reconﬁgure the circuit multiple times. The close-up views in (d) and (h) highlight thesintered traces with different geometries. The length of the IC is 3.15 mm.

wiped with NaOH using a paper towel rinsed
with water, and dried. Finally, the partially etchedcircuit was reprinted (ﬁgure 5(g)) and re-sintered into
different geometries (zigzag patterns, ﬁgure 5(h)). This
selective reconﬁguration sequence can be repeatedmultiple times to reconﬁgure the circuit to attain different output responses. Additionally, since the PDMS
does not bond to the Pyralux substrate, the PDMS
encapsulation layer may be peeled away to expose theliquid metal ﬁlm underneath for full device reconﬁguration. The PDMS encapsulation also serves tomaintain the integrity of the ﬁlm by preventing cracking and disruption of both the sintered and unsinteredregions for more stable electrical properties as the circuit is bent or ﬂexed.
2.4. Stretchable circuits
Taking inspiration from kirigami—the art of paper
cutting—we created stretchable conductive traces by
cutting through the polyimide substrate after sintering
the liquid metal ﬁlm. Researchers have shown thatmodulating the geometry of the cutting pattern canyield substrates with different degrees of stretchability[60, 61], and have begun to create stretchable conductors using this approach [62, 63]. The specimens
shown in ﬁgures 6(a)–(d) consist of copper electrodes
on either end of a polyimide substrate with a large
sintered region connecting the electrodes. Overlapping slits were cut through the polyimide substrateunderneath the sintered region to enable stretching.
To tune the stretchability of the traces, we varied thehorizontal distance between parallel vertical slits (i.e.
slit density) (1 mm; 2 mm) and the vertical gap
distance (1 mm; 2 mm) between collinear slits. The
6probe interface shown in ﬁgure 6(c) shows a droplet ofbulk liquid metal enhancing the electrical contact
between the probe and the sintered pattern. The SEM
image in ﬁgure 6(d) shows the coalescence of particles
around the slit cut through the polyimide ﬁlm.
Because both the sintering and kirigami processes canbe performed in the same laser patterning step,
stretchable circuits with tunable electrical andmechanical properties can be rapidly manufactured.
We characterized the stretchable circuits byrecording the resistance between the copper electrodes
as strain was applied cyclically (ex. 0%, 10%, 0%, 20%,etc.) (ﬁgure 6(e)). The normalized change in resistance(DR R 0 = (R - R 0) R 0 ) effectively does not changewith applied strain until electrical and/or mechanicalfailure suddenly occurs. We classiﬁed electrical failure
as a normalized change in resistance >0.5. The 2×2(ﬁgure 6(a)) and 2×1 (ﬁgure 6(b)) designs exhibitednear-simultaneous (within 10% strain) mechanicaland electrical failure between 50% and 60% strain.
Increasing the slit density (1×2 (Figure 6(c)), 1×1(ﬁgure 6(d))) allowed the devices to stretch further andto electrically fail at strains signiﬁcantly less than theyield strain for mechanical failure. After reaching electrical failure, we restarted the strain increment at 0%
strain and repeated the same test (ﬁgure 6(f)). After theﬁrst electrical failure, the resistance did not return to
its initial value when the strain was released; the resistance of the sintered pattern was irreversibly increased.
During the second strain sequence, electrical failure
occurred at the same strain value. These results indicate that laser-sintered liquid metal ﬁlms deposited
onto polymer substrates may be used as stretchable
interconnects where strain (up to a limit) does notFlex. Print. Electron. 4 (2019) 015004

S Liu et al

Figure 6. Stretchable conductive liquid metal ﬁlms enabled by kirigami. The four slit patterns are shown in (a)–(d). In each subﬁgure,the top photo shows the unstretched state of the sample, with the inset being the slit pattern labeled as (distance between slits) × (gap
between collinear slits) in mm. The bottom photo shows the maximum strain achieved prior to electrical and/or mechanical failure.
The spacing between each line is 3 mm (10% strain). The probe interface shown in (c) shows a droplet of bulk liquid metal enhancing
the electrical contact between the probe and the sintered pattern. The SEM image in (d) shows the coalescence of particles around theslit cut through the polyimide ﬁlm. (e) Plots of the normalized resistance as strain was applied in 10% increments from 0% strain toelectrical and/or mechanical failure. Three specimens of each slit pattern were measured. (f) Plot of the resistance of three samples cut
with a 1×2 slit pattern shown in (c). The solid line corresponds to the ﬁrst strain sequence wherein strain was applied incrementally
until there was a signiﬁcant change in the measured resistance (electrical failure. The strain sequence was then re-initiated at 0% strain
and the experiment was repeated (dashed line).

affect the conductivity of the trace. In contrast, theresistance of interconnects fabricated on inherently
stretchable substrates would change as strain is applied
due to a change in the geometry of the trace or cracking of the liquid metal nanoparticle ﬁlm. Future workin this area will investigate alternate kirigami designs
to attain different levels of stretch and the effects ofrepeated cycles of stretching on electromechanicalbehavior of the interconnect.

3. Conclusion
Laser sintering of liquid metal nanoparticle ﬁlms
enables the rapid fabrication of arbitrary conductive
patterns. In this paper, we demonstrated that this7

method may be used to create analog electricalcomponents, hybrid ﬂexible circuits, and stretchable
circuits. We explored the fabrication of resistors andcapacitors where electrical properties were modulatedby the geometric design of the laser sintering pattern.
Using these elemental components, we demonstrated
an opamp circuit by sintering resistors of different
values to tune the gains, and a proximity sensor todetect an approaching ﬁnger. We showed that thesecircuits may be selectively reconﬁgured via encapsulation, chemical etching, and re-patterning. Finally, we
discussed how the materials and methods used here
(i.e. selective laser processing of conductive ﬁlms on
ﬂexible polymer substrates) can be leveraged tocreate stretchable conductive circuits using kirigami
methods.

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S Liu et al

Future work will further characterize the applications demonstrated here, translate the laser sintering
approach to inherently stretchable substrates, and tocombine the various methods presented here to create
more complex, ﬂexible, and stretchable electronic
devices. Another area of future work is to create moresophisticated components, namely those with semiconductive materials. Because the oxide skin has semiconductive properties, further investigation intotuning the electrical properties of the nanoparticles
and designing more intricate methods of patterning
the liquid metal ink may yield liquid metal-enabled
diodes and transistors.

Acknowledgments
Shanliangzi Liu and Michelle C Yuen contributed
equally to this work. The authors would like to thankJennifer C Case and Edward L White for theircomments and insight on this project. This work was
supported by the National Science Foundation(CAREER Award 1454284). Michelle C Yuen was
supported by a National Science Foundation Graduate
Research Fellowship (Grant DGE-1333468).

ORCID iDs
Shanliangzi Liu
6362-801X

https://orcid.org/0000-0002-

References[1] Lu N and Kim D H 2013 Soft Robot. 1 53–62[2] Rich S I, Wood R J and Majidi C 2018 Nat. Electron. 1 102–12[3] Manti M, Cacucciolo V and Cianchetti M 2016 IEEE Robot.
Autom. Mag. 23 93–106[4] Stoppa M and Chiolerio A 2014 Sensors 14 11957–92[5] Liu Y, Pharr M and Salvatore G A 2017 ACS Nano 11 9614–35[6] Dickey M D 2017 Adv. Mater. 29 1606425[7] Wang Q, Yu Y and Liu J 2017 Adv. Eng. Mater. 20 1700781[8] Hallfors N, Khan A, D Dickey M and Marion Taylor A 2013
Lab Chip 13 522–6[9] Spells K E 1936 Proc. Phys. Soc. 48 299[10] Geiger F, Busse C A and Loehrke R I 1987 Int. J. Thermophys. 8425–36[11] Zrnic D and Swatik D S 1969 J. Less-Common Met. 18 67–8
[12] Zhu S, So J H, Mays R, Desai S, Barnes W R,
Pourdeyhimi B and Dickey M D 2013 Adv. Funct. Mater. 23
2308–14[13] Park J, Wang S, Li M, Ahn C, Hyun J K, Kim D S, Kim D K,
Rogers J A, Huang Y and Jeon S 2012 Nat. Commun. 3 916[14] Palleau E, Reece S, Desai S C, Smith M E and Dickey M D 2013
Adv. Mater. 25 1589–92[15] Zandvakili M, Honari M M, Mousavi P and Sameoto D 2017
Adv. Mater. Technol. 2 1700144[16] Kubo M, Li X, Kim C, Hashimoto M, Wiley B J, Ham D andWhitesides G M 2010 Adv. Mater. 22 2749–52[17] Qusba A, RamRakhyani A K, So J H, Hayes G J,
Dickey M D and Lazzi G 2014 IEEE Sens. J. 14 1074–80[18] Russell L, Wissman J and Majidi C 2017 Appl. Phys. Lett. 111
254101[19] Tang S-Y, Sivan V, Petersen P, Zhang W, Morrison P D,
Kalantar-Zadeh K, Mitchell A and Khoshmanesh K 2014 Adv.
Funct. Mater. 24 5851–88[20] Hirsch A, Michaud H O, Gerratt A P, de Mulatier S and
Lacour S P 2016 Adv. Mater. 28 4507–12[21] Yoon J, Hong S Y, Lim Y, Lee S-J, Zi G and Ha J S 2014 Adv.
Mater. 26 6580–6[22] Park Y L, Majidi C, Kramer R, Bérard P and Wood R J 2010
J. Micromech. Microeng. 20 125029[23] Sahlberg A, Nilsson F, Berglund A, Nguyen H, Hjort K andJeong S H 2018 Adv. Mater. Technol. 3 1700330[24] Pan C, Kumar K, Li J, Markvicka E J, Herman P R and Majidi C
2018 Adv. Mater. 30 1706937[25] Wang J, Cai G, Li S, Gao D, Xiong J and Lee P S 2018 Adv.
Mater. 0 1706157[26] Scharmann F, Cherkashinin G, Breternitz V, Knedlik C,
Hartung G, Weber T and Schaefer J A 2004 Surf. Interface Anal.
36 981–5[27] Regan M J, Tostmann H, Pershan P S, Magnussen O M,
DiMasi E, Ocko B M and Deutsch M 1997 Phys. Rev. B 55
10786–90[28] Cademartiri L et al 2012 J. Phys. Chem. C 116 10848–60[29] Dickey M D, Chiechi R C, Larsen R J, Weiss E A, Weitz D A andWhitesides G M 2008 Adv. Funct. Mater. 18 1097–104[30] Liu T, Sen P and Kim C J 2012 J. Microelectromech. Syst. 21
443–50[31] Cutinho J, Chang B S, Oyola-Reynoso S, Chen J, Akhter S S,
Tevis I D, Bello N J, Martin A, Foster M C and Thuo M M 2018
ACS Nano 12 4744–53[32] Ladd C, So J H, Muth J and Dickey M D 2013 Adv. Mater. 25
5081–5[33] Larsen R J, Dickey M D, Whitesides G M and Weitz D A 2009
J. Rheol. 53 1305–26[34] Khondoker M A H and Sameoto D 2016 Smart Mater. Struct.
25 093001[35] Kramer R K, Majidi C and Wood R J 2013 Adv. Funct. Mater.
23 5292–6[36] Gozen B A, Tabatabai A, Ozdoganlar O B and Majidi C 2014
Adv. Mater. 26 5211–6[37] Daalkhaijav U, Yirmibesoglu O D, Walker S and Mengüç Y
2018 Adv. Mater. Technol. 3 1700351[38] Boley J W, White E L, Chiu G T C and Kramer R K 2014 Adv.
Funct. Mater. 24 3501–7[39] Gao Y, Li H and Liu J 2012 PLoS ONE 7 e45485[40] Lin Y, Gordon O, Rashed Khan M, Vasquez N, Genzer J and
D Dickey M 2017 Lab Chip 17 3043–50[41] Kazem N, Hellebrekers T and Majidi C 2017 Adv. Mater. 29
1605985[42] Fassler A and Majidi C 2015 Adv. Mater. 27 1928–32[43] Lu T, Markvicka E J, Jin Y and Majidi C 2017 ACS Appl. Mater.
Interfaces 9 22055–62[44] Boley J W, White E L and Kramer R K 2015 Adv. Mater. 27
2355–60[45] Mohammed M G and Kramer R 2017 Adv. Mater. 29 1604965[46] Ko S H, Pan H, Grigoropoulos C P, Luscombe C K,
Fréchet J M J and Poulikakos D 2007 Nanotechnology 18
345202[47] Li Z, Klein T R, Kim D H, Yang M, Berry J J, Hest M F A M V and
Zhu K 2018 Nat. Rev. Mater. 3 18017[48] Angmo D, Larsen-Olsen T T, Jørgensen M,
Søndergaard R R and Krebs F C 2013 Adv. Energy Mater. 3172–5[49] Morton S W, Herlihy K P, Shopsowitz K E, Deng J, Chu K S,
Bowerman C J, DeSimone J M and Hammond P T 2013 Adv.
Mater. 25 4707–13[50] Lin Y, Cooper C, Wang M, Adams J J, Genzer J and Dickey M D
2015 Small 11 6397–403[51] Liu S, Yuen M, White E L, Boley J W, Deng B, Cheng G J andKramer-Bottiglio R 2018 ACS Appl. Mater. Interfaces 1028232–41[52] White E L, Case J C and Kramer R K 2017 2017 IEEE SENSORS
pp 1–3[53] Lear T R, Hyun S H, Boley J W, White E L, Thompson D H andKramer R K 2017 Extreme Mech. Lett. 13 126–34[54] Ortiz A, Alonso J C, Andrade E and Urbiola C 2001
J. Electrochem. Soc. 148 F26–9

Flex. Print. Electron. 4 (2019) 015004

S Liu et al[55] Basharat S, Carmalt C J, King S J, Peters E S and Tocher D A
2004 Dalton Trans. 3475–80[56] Ermak O, Zenou M, Toker G B, Ankri J, Shacham-Diamand Y andKotler Z 2016 Nanotechnology 27 385201[57] Pan H, Ko S H and Grigoropoulos C P 2008 J. Heat Transfer
130 092404[58] Doudrick K, Liu S, Mutunga E M, Klein K L, Damle V,
Varanasi K K and Rykaczewski K 2014 Langmuir 30 6867–77

9[59] Bilodeau R A, Zemlyanov D Y and Kramer R K 2017 Adv.
Mater. Interfaces 4 1600913[60] Rafsanjani A, Zhang Y, Liu B, Rubinstein S M and Bertoldi K
2018 Sci. Robot. 3 eaar7555[61] Tang Y, Lin G, Yang S, Yi Y K, Kamien R D and Yin J 2016 Adv.
Mater. 29 1604262[62] Blees M K et al 2015 Nature 524 204–7[63] Hwang D G and Bartlett M D 2018 Sci. Rep. 8 3378