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Mohammed G. Mohammed and Rebecca Kramer*
Soft, flexible and stretchable electronics will impact andadvance diverse fields including wearables, robotics, and biocompatible implantable devices. Many current examples offlexible and stretchable electronics are one-off prototypes,
whereas future implementation will require mass production.
However, scalable manufacturing with soft materials still presents many unsolved challenges. In this paper, we propose anddemonstrate an additive manufacturing process that produces
all-printed flexible and stretchable electronics comprised of soft
silicone elastomers and liquid metal. We show that this fully
automated approach can be used to create complex conductive
geometries on a single printing platform, therefore increasing
process scalability and repeatability.
All-printed flexible and stretchable electronics haveremained elusive, despite the emergence of diverse additive
manufacturing processes. The underlying challenge is rooted
in the materials and/or geometries required for flexibility andstretchability. There are three basic approaches to achieving
flexibility and/or stretchability in conductive materials: conductive poly­mer composites, geometrically soft conductors, andconductive liquids.
Conductive polymer composites generally consist of apolymer mixed with a tightly packed conductive material.[1–3]
While increasing the volume fraction of the conductive material leads to higher conductivity, it also detrimentally alters themechanical properties of the composite. Geometrically soft
conductors refer to thin rigid conductors patterned into uniquewavy and curvy structures to achieve stretchability.[4–7] However,the use of inherently rigid materials ultimately limits the strainof these devices and can lead to the formation of cracks andfractures during long-term use.[8,9]While we believe that each approach offers its own advantages
and disadvantages for specific applications, in this work we takethe approach of conductive liquid electronics, which are inherently flexible and stretchable, thus bypassing the mechanicallimitations of conductive composites and geometrically soft conductors. One common choice of conductive liquid in recent literature is liquid metal. In particular, eutectic gallium–indium alloy(eGaIn; 78 wt% Ga, 22 wt% In[10]) exhibits favorable properties
such as a melting point below room temperature,[11,12] low viscosity, high electric conductivity,[13,14] low toxicity, and self-healing
capabilities.[15] Liquid metals have been employed in a variety of
applications such as flexible and stretchable antennas,[16–18] interconnects,[19,20] wires,[21] electrodes,[22,23] and sensors.[24–26]

Dr. M. G. Mohammed, Prof. R. Kramer
School of Mechanical Engineering
Purdue University
585 Purdue Mall, West Lafayette, IN 47907, USA
E-mail: rebeccakramer@purdue.edu

DOI: 10.1002/adma.201604965

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Although gallium–indium alloys have properties that makethem desirable for flexible electronics, they can be challenging
to process. Gallium–indium alloys spontaneously form a thinoxide layer in the presence of atmospheric oxygen, which is
the mechanism behind the liquid’s useful capability to form
freestanding structures,[27,28] but also produces a high surface
tension that makes the liquid incompatible with most standard
liquid processing techniques.[13,29] The challenges associatedwith processing liquid metal have led researchers to seek new
methods for patterning the material.[30,31] Several emerging
methods include injection into elastomeric microchannels,[32,33]
imprinting,[34] masked deposition,[35–37] and direct writing.[38–41]
The goal behind all of these approaches is to improve therepeatability and scalability of liquid metal processing.
We have previously demonstrated the ability to create printable liquid metal nanoparticle inks by sonicating bulk liquid
metal in a carrier solvent.[42] These inks can be patterned using
automated printing processes such as inkjet printing, andare mechanically activated by applying pressure to sinter thenanoparticles. However, small particles are required for inkjet
printing, while larger particles contain more liquid metal andrequire lower pressures to sinter. In this paper, we introduce
a liquid metal slurry, which is comprised of densely packed
microparticles. We further introduce an automated multimode printing process for flexible and stretchable electronics,
which includes extrusion printing of the elastomer layers, spray
printing of the liquid metal slurry, and mechanical sinteringof the slurry particles, all on the same high-precision printing
platform. We mechanically activate (i.e., sinter) the particles atroom temperature to create conductive paths via automated tapping of a soft tip against the stage, and have found that verysmall forces are required to activate the larger liquid metal
microparticles. The multimaterial automated printing processdemonstrated here is capable of manufacturing flexible andstretchable electronics with extremely complex patterns that are
not possible using previously demonstrated fabrication techniques. Furthermore, the electrical resistance of the printed
devices is fairly low and comparable to the resistance of similar
devices fabricated using bulk liquid metal.
The manufacturing process is shown in Figure 1. The process combines soft silicone elastomer printing and liquid metal
processing on a single high-precision 3D stage using four
additive manufacturing steps: extrusion printing of a baseelastomer layer, spray printing of liquid metal slurry, selective
activation of an electrical path via stage tapping, and extrusion
printing of an encapsulating elastomer layer. The extrusion
printed and spray-printed materials are fed to the printer nozzle
at a constant flow rate using a syringe pump mounted on the
3D stage. Video S1 (Supporting Information) demonstrates allof the fabrication steps.
Preparing Liquid Metal Ink: Before initiating the printing
process, we first synthesize the liquid metal ink, which is the© 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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Communicationwww.advmat.deFigure 1. Procedure for printing flexible and stretchable liquid metal electronics: (i) Extrusion printing of a base elastomer layer. (ii) Spray printing of
liquid metal slurry. The slurry is produced by sonicating liquid metal in a carrier solvent. (iii) Selective activation of the electrical path by operating thestage in tapping mode against a soft nozzle tip. (iv) Extrusion printing of an encapsulation elastomer to seal the device.

conductive component of the resulting printed electronics. The
liquid metal ink we employ is a suspension of liquid metal particles in a carrier solvent achieved via rigorous sonication, as
detailed by Hohman, et. al.[43] as well as our previous work.[42]We prepare the liquid metal ink by sonicating 2 g of the bulk
liquid metal in 10 mL of isopropanol for 5 min (see Section S1,
Supporting Information, for more details about liquid metal
ink preparation). An oxide shell spontaneously forms aroundthe particles, which electrically insulates the particles and prevents spontaneous coalescence. Therefore, pressure is required
to later sinter the particles by rupturing the oxide shells andmerging their liquid cores to create an electrically conductive
path, which we refer to as “activation” of the conductive path.
In general, large particle films require less pressure to sinterthan small particle films.[42] For this reason, we sonicate theliquid metal for only 5 min to produce large particles and further allow the particles to settle under gravity, which separates
the ink into a heavy slurry and a suspension of lighter particles
in the carrier solvent (see Figure 2a). In this work, we employ
the liquid metal slurry to ensure easy and reliable particle coalescence in later sintering steps.
Figure 2b shows an example scanning electron microscopy(SEM) image of a deposited particle film, which was analyzed
using ImageJ software to determine the distribution of particle
sizes in the liquid metal slurry ink. The particle size distribution shows positive skewness around 1 µm, where particles
with a diameter greater than 2 µm represent more than 90%of the total mass as shown in Figure 2c. Furthermore, quantitative testing of the pressure required to activate the particle
films show that pressures on the order of 1 N mm−2 are enoughto achieve conductivity across the liquid metal slurry films (seeFigure S1, Supporting Information). This agrees with our qualitative observation of the instant conversion of the dull particle
film to a conductive shiny liquid film with light pressure using
tweezers or a fingertip, as shown in Figure 2d. This “activation
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pressure” is a function of the mean particle size, the particle
size distribution, and the packing density of the particles.[42] We
did not find any detectable correlation between the activationpressure and particle film thickness, as shown in Figure S1d(Supporting Information). With the liquid metal slurry ink
synthesized and characterized, we now detail the steps of theprinting process.
Extrusion Printing of a Base Elastomer Layer: The base elastomer layer is the foundation on which the liquid metal slurry
is printed, activated, and sealed. Therefore, it is crucial to use
an elastomer that can be easily printed using the current setup
and has no negative impact on subsequent pressure activation
or device sealing. We found that very soft elastomers absorb
most of the applied pressure during particle sintering, whichleads to unachievable or inefficient activation. Therefore, thechoice of the base elastomer should be optimized based on thehardness, which impacts activation of the electronics, and elasticity, which contributes to device flexibility and stretchability.
After comparing the physical and flow properties of different
commercially available elastomers, we chose Smooth-Sil 950(Smooth-on Inc.) as the base elastomer. The viscosity and pot
life of Smooth-Sil 950 allow relatively easy printing with minimal spreading after printing. Moreover, the hardness of cured
Smooth-Sil 950 allows easy and efficient activation of the slurry
layer printed over it. More details about elastomer choice areavailable in Section S3 (Supporting Information).
We print the base elastomer layer by feeding the uncured
elastomer to a nozzle that is kept at a controlled clearance above
the stage and moves horizontally in x and y-directions basedon the designed geometry. We can control the thickness of thebase layer by manipulating the elastomer flow rate, the stage
speed, and the clearance between the nozzle and the stage. The
elastomer fully cures in ≈30 min at room temperature and in
shorter times by heating the stage. After the elastomer fullycures, liquid metal slurry can be spray printed over it.

© 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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Communication
Figure 2. a) Images of liquid metal particles in isopropanol directly after sonication and after 20 min. b) SEM image of a spray-printed film of settled
liquid metal particles (slurry). c) Particle size distribution in the liquid metal slurry considering both number and mass of particles. d) Film of liquid
metal particles on silicon with a portion partially sintered using tweezers. The inset shows a SEM image of the sintered region.

Spray Printing of Liquid Metal Slurry: We initially sought toprint the slurry by direct writing using the same setup that wasused to extrude elastomer, however the printed films formed
cracks when the solvent evaporated as a result of aggregationof particles, which is a common phenomenon in similar drying
films.[44,45] Therefore, we used spray printing to achieve consistent and uniform deposition of crack-free films of liquid metal
particles. We installed a simple custom-made spray printing
nozzle 10 cm above the stage. The slurry is injected into thenozzle and pressurized air is blown perpendicular to the nozzle
to spray the slurry on the substrate. Slurry deposition by spray
printing is more time efficient than deposition by direct writing,as the slurry covers wider areas in shorter periods of time.
We control the thickness of the printed film by manipulating
the stage speed and the flow rate of the slurry. Figure 3a showsthat the spray-printed film thickness decreases as the speed of
the stage increases at constant slurry flow rate. Thicker films
may be achieved using higher slurry flow rates or lower stage
speeds. A predesigned mask can be used to control the areas
covered with the liquid metal slurry, which is ready to activate
directly after it is printed.

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Selective Activation of the Electrical Path via Stage Tapping:
We activate the deposited particle film by operating the stage
in a tapping mode, where the stage moves up and down at a
set frequency and simultaneously in x and y-directions. The
pressure applied by the nozzle onto the particle film when
operating the stage in tapping mode mechanically activates thedesired conductive path. The nozzle tip used for activation isa syringe needle with a soft elastomer cap (inset in Figure 1),
which results in circular activated areas. The soft cap is necessary to ensure activation without scraping the particles off thesubstrate. Figure 3b shows the repetitive step motion sequenceof the stage relative to the nozzle to form an activated path.
Section S4 (Supporting Information) includes more about thetapping mode parameters and their effect on the continuity of
the activated trace.
The automated tapping mechanism yields activated traces
with rough edges, which can be observed from the opticalimage in Figure 3c. The optical image also shows texture on
the oxide skin formed on the surface of the activated trace. We
suspect that the texture is a result of oxide skin wrinkling thattakes place during activation, as liquid metal adhesion to the© 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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Communicationwww.advmat.deFigure 3. a) Thickness of spray-printed slurry films as a function of stage speed at a slurry flow rate of 0.2 mL min−1. b) Schematic of stage motion
relative to the activation tip during the activation step. c) Optical image of an activated trace. The inset SEM image shows the edge of the trace.
d) Resistance of activated traces as a function of the normalized center-to-center distance between activated circles. The schematics on the plot show
two consecutive activated circles with the highlighted region representing the effective electrically conductive path.

soft activation tip causes the surface oxide to fracture, reform,and compress during the dynamic process. More details and
SEM images of the activated trace surface texture are available
in Section S5 (Supporting Information).
The resistance of the continuous trace is determined by theminimum width of that trace, which in our case is the widthof the intersecting area between two consecutive activated circles. Figure 3d shows the resistance of activated traces against
the normalized center-to-center distance between the activatedcircles (the ratio between the center-to-center distance and thediameter of the activated area). Note that for ratios greater than
1 the trace is discontinuous and not conductive. The schematics
on the plot show two consecutive activated circles with the highlighted region representing the effective electrically conductive
path. We developed a simple model alongside the experimental
results, which predicts the trace resistance based on the tapping
parameters. The resistance R can be calculated from the formulaρL d  (1)T × 2r cos sin −1  
 2r 
where ρ is the resistivity of eGaIn; A, T, L, and W are the crosssectional area, thickness, length, and width of the activated
R=

ρL ρL
=
=
A TW

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trace, respectively; r is the radius of one activated circle; andd is the center-to-center distance of two successively activatedcircles. More details about the model and its derivation areavailable in Section S6 (Supporting Information). The modelshows very good agreement with the experimental data andconfirms the effect of the center-to-center distance on the resistance of the final activated trace, as shown in Figure 3d.
Extrusion Printing of an Encapsulation Elastomer: The last stepin the printing process is sealing the device after connecting
wires to the terminals of the activated pattern (see Section S7,
Supporting Information). We use Sylgard 184 (Dow Corning)as a sealing elastomer since its relatively low viscosity allows it
to infiltrate between the unactivated particles and prevent further activation. It readily bonds with the Smooth-Sil 950 baselayer because it is also a silicon-based elastomer. The diffusionof Sylgard-184 through the particles and its strong bonding
with Smooth-Sil 950 help the devices to behave as a single unit
without any delamination through long-term use.
Figure 4 shows examples of printed functional devices.
Figures 4a-b show very complex patterns in the form of theFaboratory lab logo, the Purdue University train, and a world
map representation. These patterns are almost impossible tocreate using previous liquid metal-embedded device fabrication© 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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Communication
Figure 4. a–b) Demonstration of fully printed complex geometries: Faboratory gear, Purdue train, and world map. c) Resistance measurement of a
printed coil 268 mm long, 500 μm wide, and 90 μm thick. d) NASA logo patterned on eGaIn slurry and acting as an electric circuit.

methods, such as injection into microchannels. The low electric resistivity of liquid metal (29.4 × 10−6 Ω cm[13]) makes theresistance of printed circuits using the printing process fairly
low; an example is shown in Figure 4c, where the resistance ofa seven-turn coil pattern is 1.6 Ω, which is in the same orderof magnitude of the resistances of devices with similar dimensions that use bulk liquid metal as the conductive material.
Figure 4d shows a printed circuit with a complex geometry (theNASA logo) connected as a part of an electric circuit without
affecting the continuity of the circuit.
The printing procedure not only allows printing of functional
devices with extremely complex geometries, but also provides
the robustness, flexibility, and stretchability inherent to liquid
metal-embedded devices (Figure 5a). To demonstrate this point,
we used the printing process to fabricate sensors based on
previously developed designs for liquid metal strain and pressure sensors.[24] Figure 5b shows a pressure sensor that changes
resistance as a function of the pressure applied by a fingertip. We
also fabricated a strain sensor and measured its output resistanceas a function of strain between 0% and 30% for 500 continuous
cycles. We found the sensor response to be stable, linear, andsimilar to the responses of similar sensors made using previous
fabrication approaches. Further details about strain sensor
testing are available in Section S8 (Supporting Information).
We note that we observed a reduction in the resistanceacross the printed pressure sensors with long-term use. We
hypothesized that this drift was a result of further activationof the particles surrounding the activated trace, which leads
to a wider conductive trace and therefore decreased resistance. We confirmed this hypothesis by comparing microscope
images of activated paths before and after applying a uniform pressure and found that the width of paths consistently
increased by approximately 140 µm (70 µm on each side). For
example, the width of an activated path increased from 800 toAdv. Mater. 2017, 29, 1604965

940 µm after applying the uniform pressure. Therefore, thesignal drift may be avoided by applying substantial and uniform pressure to the sensors prior to use, which coalesces any
remaining liquid metal particles near the traces and stabilizes
the sensor signal.
The process described in this paper enables printing functional electronics on different substrates and makes the processcompatible with several applications. We show here two examples of potential applications: wearable sensors and stretchable
wires.
To demonstrate wearable sensing, we printed strain andpressure sensors onto muslin fabric, as shown in Figure 5c.
The base elastomer layer mechanically infiltrates and bonds
to the fabric, and the full devices reliably sense strain or pressure. The stretchability of the elastomers used to fabricate thesesensors is higher than that of the fabric, resulting in sensory
fabrics that will minimally impact the natural range of motion of
the wearer in potential wearable electronic applications, relative
to wearable systems using rigid and unstretchable components.
To demonstrate stretchable wires, we printed a coil-shaped
wire that can be stretched to a straight wire longer than thelength of the printing stage, as shown in Figure 5d. It is possible to create longer wires by increasing the number of turns
in the coil. The wire maintains its electric conductivity at thecoiled and stretched states (Video S2, Supporting Information).
Flexible and stretchable conductive wires have been previously
used to make stretchable headphones and chargers.[21]Finally, we evaluate the process reliability and throughput,
which are critical for any large-scale production processes. The
yield of working fully printed devices is 75% based on 20 samples(sensors, wires, and complex geometry patterns) with a low rateof failure. The main source of device failure is poor interfacing
between the copper wire and the activated path (60% of failures), which we discuss in Section S7 (Supporting Information).

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Communicationwww.advmat.deFigure 5. a) Demonstration of the printed electronics being bent, squeezed, and released with no electrical or mechanical damage. b) Demonstrationof a printed pressure sensor changing resistance in response to applied pressure. The activated trace is 268 mm long, 500 µm wide, and 40 µm thick.
c) Pressure and strain sensors printed on fabric. d) A printed wire in both a coiled and stretched position.

The process throughput depends on the time consumed by all
involved fabrication steps. For relatively simple geometries, thetime needed to print the elastomers, as well as spray print andactivate the liquid metal slurry does not exceed 15 min. However, the time required for the elastomers to fully cure makes
the actual process take more than 6 h. Heating the stage to 60°C reduces the elastomers curing time to 2 h and enhances theprocess throughput.
In conclusion, we have demonstrated a fully automated printing process for liquid metal-based flexible andstretchable electronics. To enable this process, we have constructed a custom multimaterial additive printer that combines multiple printing processes on a single platform. We
have demonstrated all-printed flexible and stretchable electronics via extrusion printing of elastomers, spray printing ofa liquid metal microparticle ink, and automated liquid metal
microparticle sintering enabled by tapping a soft tip against
printed particle films. The automation of liquid metal-based
electronics enables improvements in device reliability, reproducibility, and throughput. We have demonstrated that theprinting process can produce liquid metal-based sensors witha similar design to those already described in the literature.
Furthermore, we have shown that this printing process can

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produce complex conductive patterns that cannot be achievedby previous and non-automated fabrication approaches. Further optimization of the multimaterial printing process will
enable the transition of this technology from the laboratory
into commercial applications such as soft robotics, sensor
skins, and wearables.

Supporting Information
Supporting Information is available from the Wiley Online Library orfrom the author.

Acknowledgements
The authors thank Jianyi Du for his help in developing and updating the
3D stage software, Edward White and Michelle Yuen for their help insetting up the spray printing system, and Jennifer Case for helping with
the interface connection. This work was supported by an Early Career
Faculty grant from NASA’s Space Technology Research Grants Program(NNX14AO52G). The sensor testing portion of this work was supported
by a NASA STTR Phase 1 contract (NNX16CA54P).

© 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Received: September 14, 2016
Published online: March 1, 2017

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