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J. William Boley, Edward L. White, George T.-C. Chiu, and Rebecca K. Kramer*

FULL PAPER

Direct Writing of Gallium-Indium Alloy for Stretchable
Electronics

properties that make it difficult for additive, high throughput, scalable fabrication
processes. For example, in the context of
inkjet printing, the combination of viscosity, surface tension, and density alone
suggest Ga–In alloy to be a non-printablefluid without modifying either the material or the atmosphere (Supporting Information). Additionally, the fast-forming
oxide layer can easily clog a nozzle orifice,further increasing the difficulty of inkjet
printing Ga–In alloys.
Although inkjet printing of Ga–In
alloys has yet to come, there have been
recent advancements made in printing
Ga–In alloy-based structures. Tabatabaiet al. applied micro contact printing to
Ga–In alloys and successfully patterned
dot arrays, lines, pads and a functioning
comb capacitor on elastomer substrates.[11]Ladd et. al. pumped Ga–In alloy through a capillary in order to
3D print vertical columns and arbitrarily stackable connectedpendant drops of conductive liquid.[12] Other direct writing
methods such as dip-pen nanolithography[13] and functional
paste extrusion[14] have been successfully employed for othertypes of functional materials.
Here, we propose a novel direct writing approach which
complements the previously explored printing methods. Thismethod is achieved by flowing eutectic Ga–In (EGaIn) (SigmaAldrich, 75% Ga, 25% In) through a blunt syringe needle inclose proximity to a moving substrate. As shown in Figure 1, theflow rate Q is controlled through a syringe pump while the stage
maps out a prescribed geometry (for example, the serpentine
pattern given in Figure 1b) at a controlled velocity v under a stationary syringe needle of inner diameter ID held at a controlled
standoff distance h0 above the substrate. We experimented withtwo particular silicon-based substrates: glass and Polydimethylsiloxane (PDMS, Slygard R 184, 10:1 weight ratio). This method
exploits the strong adhesion between EGaIn and the two typesof silicon-based substrates, evident from the near zero receding
contact angle observed herein, which facilitates the pinningof the written trace to the surface. The formation of the gallium-oxide skin during the direct writing process allows thewritten pattern to maintain its printed shape, even throughthe elastomer encapsulation process. Moreover, trace geometry
characterization measurements are used to provide a direct
writing design strategy to inform the selection of ID, Q, v, andh0 to obtain a desired trace geometry. This additive method of
fabrication mitigates the need for micro-machining, masks, orIn this paper, a direct writing method for gallium-indium alloys is presented.
The relationships between nozzle inner diameter, standoff distance, flow rate,
and the resulting trace geometry are demonstrated. The interaction between
the gallium oxide layer and the substrate is critically important in understanding the printing behavior of the liquid metal. The difference between
receding and advancing contact angles demonstrates that the adhesion of theoxide layer to the substrate surface is stronger than the wetting of the surface
by the gallium-indium alloy. This further demonstrates why free-standing
structures such as the traces described herein can be realized. In addition to
the basic characterization of the direct writing process, a design algorithm
that is generalizable to a range of trace geometries is developed. This method
is applied to the fabrication of an elastomer-encapsulated strain gauge that
displays an approximately linear behavior through 50% strain with a gauge
factor of 1.5.

1. Introduction
Recent progress in biocompatible and biomimetic devices has
seen an influx of gallium-indium (Ga–In) alloys for stretchable
conductors and sensors. Attractive attributes of these types ofmaterials, such as low melting temperatures, low resistivity(≈29 × 10−6 Ω cm)[1] and low viscosity (≈2.4 mPa s),[2] allow
them to conform and stretch to many different configurations
while maintaining an electrical connection. Additionally, thepresence of a naturally formed oxide skin on the exposed surface of Ga–In alloy in standard conditions induces a high surface tension (≈0.6 N m−1),[1] facilitating the formation of stablefree-standing structures.[3] Recent applications include reversibly deformable and mechanically tunable fluidic antennas,[4]hyperelastic pressure sensors,[5] embedded elastomer conductors,[2] 3D highly stretchable conductors,[6] a stretchable radio
frequency (RF) radiation sensor,[7] wearable tactile interfaces,[8,9]and soft curvature sensors for joint angle proprioception.[10]In contrast, some of the properties of Ga–In alloys that
attract its use as a functional material are the very same

J. W. Boley, E. L. White, Prof. G. T.-C. Chiu,
Prof. R. K. Kramer
School of Mechanical Engineering
Purdue University
585 Purdue Mall, West, Lafayette, IN 47907, USA
E-mail: rebeccakramer@purdue.edu

DOI: 10.1002/adfm.201303220

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approaching 0°. Comparing these results to aprevious study including measurements of θaand θr indicates similar surface interactions
between Galinstan and PDMS (θa ≈ 155° andθr ≈ 6° for Galinstan on PDMS).[16]Assuming that EGaIn behaves as a liquid,
the energy Er associated with removing
EGaIn from each of the surfaces can be
expressed using the equation for the work ofadhesion.[17]E r = γ (1 + cosθ r )(1)

where γ ≈ 0.6 N m−1 is the surface tensionof EGaIn. This yields Er ≈ 2γ ≈ 1.2 N m−1,
indicating that the cohesion (≈0.5 N m−1)[3]of EGaIn is smaller than its adhesion toeither of the substrates. This finding further
explains why pump-filled microchannelsof EGaIn maintained their shape without
exiting the channel upon release of the pressure, as observed in previous studies;[3,10]
Figure 1. Direct writing system. a) Photograph of direct writing system. b) Schematic of direct reflow was prevented by the adhesion of thewriting system writing a serpentine pattern of length L, center-to-center line spacing p with a EGaIn to the PMDS microchannel walls.
writing speed v, flow rate Q, and a needle standoff distance h0. c) Detail cross-sectional view
Another finding by Dickey et al.[3] is thatof a written trace of width W, height H, and contact angle θ.
EGaIn without the oxide layer will indeed
reflow when the pump pressure is released.
Corroborating this result with ours indicates that the oxide
molding processes while limiting the amount of waste matelayer formed on the surface of the EGaIn maintains a strong
rial. Furthermore, this one-step fabrication method can be usedadhesion to the two substrates, facilitating the direct writing ofin automated, high-volume applications while alleviating geomtraces presented herein.
etry limitations and pressure requirements previously encountered with filling microchannels. This work can be used in conjunction with other types of fabrication processes developed for
Ga–In alloy as well, including masked deposition[2] and vacuum
2.2. Characterization of Written Tracesinduced patterning.[6,15]
Figure 2b shows optical micrographs of EGaIn traces realized
by the direct writing system shown in Figure 1a with different
nozzle inner diameters on glass and PDMS. In order to assess
2. Results
the quality of the written traces, line raggedness defined as thestandard deviation of the residuals from a line fitted to the edgeof the written trace,[18] was employed for each written trace in
2.1. Contact Angle Characterization
this study. The resulting line raggedness for each trace was
Figure 2a shows a summary of the contact angle measurements
normalized by its corresponding trace width for cross-comparof EGaIn on glass and PDMS. In order to differentiate between
ison. This resulted in an average normalized line raggedness
the different contact angles within this study we introduce subof 3% with a minimum value of 1% and a maximum value ofscripts so that θa is the advancing contact angle, θs is the static
7%. Additionally, EGaIn trace height H and width W measurements were obtained in order to characterize effects of thecontact angle, and θr is the receding contact angle. For thesystem inputs (flow rate Q, needle inner diameter ID, standoff
remainder of this paper, the contact angle of the written trace
distance h0, writing speed v, and substrate type) on the written
will simply be referred to as θ. The similarity in θa between
glass (153.4° ± 3°) and PDMS (157.6° ± 3°) indicates similar
trace geometry. The results from these measurements are sumspreading dynamics when EGaIn initially comes into contact
marized in Figures 2c–f.
with each of the surfaces. On the other hand, the large differFigure 2c shows representative trace widths for lines writtenence in θs between glass (128.6° ± 1.3°) and PDMS (150.0° ± 2°)
onto glass and PDMS using needles with different inner diameter values. The relatively small 95% confidence intervals forindicates that EGaIn exhibits a higher level of wetting with glass
the slopes (0.72 ± 0.06 for glass and 0.66 ± 0.06 for PDMS)
compared to PDMS. In general, the results suggest low wetand intercepts (−1.0 µm ± 10 µm for glass and −29 µm ± 9 µm
tablility between EGaIn and both substrates, since θs > 90°. The
for PDMS) indicate a linear scaling of the trace width withresulting θr was found to be <10° for both glass and PDMS. In
respect to ID. Although the slopes are similar, it is clear that
fact, a post-hoc visual inspection of the substrate revealed a thin
the trace width for PDMS is consistently smaller than that oflayer of EGaIn remaining on the substrate, suggesting θr to be3502

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AdvancingGlass

153.4º± 3º

PDMS

128.6º± 1.3º379 µm169 µm119 µm

83 µm

PDMS

< ~10º

(c)

(d)
500

1
Glass
PDMS

0.8
W/ID
[μm/μm]

300
200

0.4

100

0.4

0.6
-1

100

200
300
400
500
Inner Nozzle Diameter (ID) [μm]Aspect Ratio (H/W)
Contact Angle (θ)

0.8

175

1
0.9

160

0.8

0.7

145

0.6

130

0.5

115

0.4

100

0.3
85

0.2
0.1

70

0
0

55

0.1

0.2
0.3
0.4
-1
-1
ν* [mm s /mm s ]

0.5

0.8

1

-1

ν* [mm s /mm s ]

600

(f)

1
0.9

H/W [μm/μm]

0.2

0.7

1
0.9
W/ID
H/ID

0.8
0.7

0.6

0.6

0.5
0.4

0.5
0.4

0.3

0.3

0.2

0.2

0.1

0.1

0
0

0.1

0.2

H/ID [μm/μm]

(e)

0.2
0

Contact Angle ( θ) [°]

0
0

0.6

W/ID [μm/μm]

400
Representative
Trace Width [μm]535 µmGlass< ~10º150.0º± 2º157.6º± 3º(b)

Receding

StaticFULL PAPER(a)0h0/ID [μm/μm]

Figure 2. Characterization of directly written traces. a) Representative optical images extracted from contact angle videos (Supporting Information,
Videos M1–M6) of sessile drops used to characterize the advancing, static and receding contact angle of EGaIn on glass and PDMS. Contact angle
values imposed onto advancing and static images represent average measured contact angles and corresponding 95% confidence intervals. Lighter
colored (red) outline of advancing and static sessile drop images are fitted profiles using the corresponding average contact angle (Equations (3),(al),21
Srinivasan et al.).[21] Lighter colored (red) angle outline on receding images is present to illustrate the resulting approximated receding contact angle.
Scale bars are 500 µm in length. b) Optical microscope images of directly written traces resulting from nozzles of varied inner diameters (ID) writingon glass and PDMS. Scale bars are 100 µm in length. c) Representative trace widths as a function of ID on glass and PDMS substrates. Dotted line
represents linear least squares fit to the data. Errorbars represent 95% confidence intervals. d) Trace widths normalized by ID as a function of v* forh0/ID ≈ 0.18 on glass. First dotted line represents linear least squares fit to the first 11 data points. Second dotted line represents the mean value ofthe last 14 data points (≈0.6). Error bars represent 95% confidence intervals. e) Aspect ratio and corresponding trace contact angle as functions ofnormalized exit velocity on glass. Dotted lines represent the average aspect ratio and corresponding contact angle over all of the data. Errorbars represent 95% confidence intervals. f) Normalized trace width and height as functions of h0/ID for v* ≈ 0.99 on glass. Dotted lines represent linear leastsquares fits to the data. Errorbars represent 95% confidence intervals.

glass, displaying the effect of θs on the trace width. In general,
for a given trace cross-sectional area, higher wetting (lower θs)
results in larger trace widths compared to lower wetting (higherθs), which is consistent with the measurements shown in
Figure 2c.

Adv. Funct. Mater. 2014, 24, 3501–3507

Before further discussions on the written trace geometry, it
is instructive to introduce the different parameters plotted in
Figure 2d–f. The normalized trace width W/ID is the ratio of
the measured trace width to the measured needle inner diameter (see Supporting Information, Figure S1 for details on© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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3504

angle. For a fixed v* and Q, cross-sectional
area of the printed trace must be conserved,
which implies a change in trace profile thatis dependent on h0. We believe that at low
values of h0 (when the needle is close to thesubstrate), the presence of the needle may
distort the shape of the depositing EGaIn.
Given the strong adhesion between EGaIn
and a glass substrate (shown in Figure 2a),
Figure 3. a) Stability region for written traces. Horizontal dotted lines represent upper and and previous studies showing that the oxide
lower stable bounds for v*. Vertical dotted line represents upper bound for h0/ID, above which layer on the exposed EGaIn surface can resist
surface stresses up to ≈0.5 N m-1 (Dickey
the lines become disconnected upon motion of the stage. b) Three dimensional surface used
for design guidelines (constructed from Figures 2d–f). Bold black curves correspond to the et al.),[3] we infer that traces deposited atdata in Figures 2d,f.
low h0 are “pinned” with θ ≤ 90°. However,increasing h0 reduces the amount of distortion from the needle, allowing the depositing trace to minimize
measuring ID). Similarly, the normalized trace height H/ID
its surface energy and approach the profile of a static sessile
is the ratio of the measured trace height to the measured
drop (with θ approaching θs).
needle inner diameter. Assuming no change in fluid density
throughout the tubing and needle and equivalence between the
Although there is a clear trend in Figure 2f, the measurecross-section of the needle and that of the EGaIn at the needle
ment uncertainty associated with h0 (≈ ±3 µm) results in large
exit, the ratio of the velocity of the EGaIn exiting the needle to95% confidence intervals for the slopes (2.43 ± 1.02 for W/ID
the stage velocity v* is defined as
and 1.99 ± 0.77 for H/ID) and intercepts (0.08 ± 0.14 for
W/ID and 0.01 ± 0.10 for H/ID) of the fitted linear models.
4Q
Decreasing the uncertainty associated with h0 would need to be
*
ν =
(2)
realized in order to address this caveat. However, h0 << ID in
π ID 2v
our experiments, which indicates that the cross-sectional areaof the printed trace is primarily governed by ID.
The aspect ratio (H/W) is simply the ratio of the trace heightto the trace width. Finally, the normalized standoff distanceh0/ID is the ratio of the standoff distance to the needle innerdiameter.
2.3. Stability of Written Traces
Figure 2d shows normalized trace width as a function of
normalized exit velocity for EGaIn traces written on glass
Stability limits for h0/ID and v* were observed during this study
with h0/ID ≈ 0.18. The relatively small 95% confidence interand are summarized in Figure 3a. To begin, for h0/ID > ≈ 0.21,
vals on the slope (0.7 ± 0.2) and intercept (0.42 ± 0.05) of thethe needle is too high off the substrate for stable traces to be
fit indicates a linear effect of v* on W/ID over the range ofachieved, indicating this value for h0/ID as an upper bound.
0.05 ≤ v* ≤ 0.31. Interestingly, the increasing trend appears to
Although no lower limit on h0/ID was observed, it is expectedsaturate around v* ≈ 0.31, corresponding to W/ID ≈ 0.6. Thisthat stability is an issue as h0/ID approaches 0, at which point
indicates an upper bound on W/ID with respect to v*. As thepressure spikes could occur in addition to the mechanical limit.
In addition, traces written with v* < 0.05 exhibited breaks/
height of the needle from the substrate is fixed in this experidiscontinuities, suggesting a lower limit on the normalized
ment, it is expected that the presence of the needle is thetrace width. This occurs because the flow rate is not highsource for the observed saturation (i.e. the constriction fromenough to maintain stable EGaIn deposition, causing thethe needle causes the contact area between the trace and subexiting EGaIn to stretch and fracture. On the other hand, traces
strate to increase so that θ approaches 90°, as illustrated in Supwritten with v* > 1 are likely to exhibit abrupt bulges during the
porting Information, Figure S2).
writing process as a result of the exit velocity being larger thanThe effect of v* on H/W and θ are summarized in Figure 2e.
the writing speed.
These values are relatively constant (95% confidence intervalsof H/W = 0.57 ± 0.05 and θ = 99° ± 5°). An interesting result
shown in Figure 2e is θ < θs; in fact, θ ≈ 0.77θs. This indicates
2.4. General Guidelines for Written Trace Designthat the internal pressure of the exiting EGaIn in this range
of v* is not large enough to cause the EGaIn to expand to theAs summarized in Figure 3a, the results from this study indiprofile of a static sessile drop.
cate a region within the space of input parameters v* and h0/ID
The above result is corroborated by the observed effect of
h0 on the profile of the printed EGaIn trace. Figure 2f showsthat should be maintained for stable directly written traces. Any
combination lying well within this region (0 < h0/ID < 0.21 andthe resulting normalized width and height measurements oftraces written on glass with ID = 169 µm and v* = 0.99. As
0.05 < v* < 1) should result in process stability.
expected, height increases linearly with respect to h0. UnexThe results summarized in Figures 2d–f are presented indimensionless form so that they may be used as a guide forpectedly, we also observe an increase in width with increasing
input parameter selection to achieve a desired trace geomh0. We hypothesize this trend to be caused by the proximity
etry. Adopting a first order approximation and using the dataof the needle to the substrate and its effect on trace contact

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2.4.1. 0.05 ≤ v* ≤ 0.31
Knowing the desired width and height of the trace to be printed,
an appropriate ID can be selected given that W/ID can take on
values between ≈ 0.1 (corresponding to v* ≈ 0.05 and h0/ID ≈0.03 in Figure 3b) and ≈ 0.7 (v* ≈ 0.99 and h0/ID ≈ 0.2). In addition, Figure 3b denotes that H/ID can take on values between≈ 0.06 (v* ≈ 0.05 and h0/ID ≈ 0.03) and ≈ 0.42 (v* ≈ 0.99 andh0/ID ≈ 0.2). Once ID is chosen, h0/ID and v* can be found using
⎡ h0
⎢
⎢ ID
*
⎢⎣ v

3. Discussion⎤⎥ = 1 ⎡ −73.89 133.33 ⎤ ⎡ H ⎤⎥ ID ⎢⎣ 18.89 −33.33 ⎥⎦ ⎢⎣ W ⎥⎦⎥⎦(3)

which represents the plane corresponding to the range ofv* included in the set [0.05, 0.31] in Figure 3b. If v* is indeed
found to be within this range, then the design is complete andthe ratio Q/v may be found via substitution with Equation (2).
2.4.2. 0.32 ≤ v* ≤ 1
If v* > 0.31, the solution must fall within the plane corresponding to the range of 0.32 ≤ v* ≤ 1, hence a v* value within
this plane must be selected. As trace width and height arecoupled (linearly dependent) in this regime, only one criticaldimension can be selected in conjunction with v*, while thecorresponding h0 and H or W can be derived from Figure 2f.
Assuming the width to be the critical dimension, h0/ID isgiven as:h0 W / ID − 0.17
=
ID
2.4

(4)

and the resulting height can be computed by:
H = 2.1h0 − 0.03ID(5)

Alternatively, if height is the preferred critical dimension,
h0 /ID can be found using:h0 H / ID + 0.03
=
ID
2.1

(6)

and W is calculated with Equation (4).
Finally, v and Q can be tuned to satisfy Equation (2), which
gives some design freedom to adapt the process to throughput
requirements.

2.5. Stretchable Strain Gauge Via Direct Writing
Figure 4 highlights an application of this direct writing
method to strain gauge devices. The main image in Figure 4aAdv. Funct. Mater. 2014, 24, 3501–3507

demonstrates repeatability of the process. To demonstratedevice scalability, the inset is included in Figure 4a showing
a larger scale device with accompanying contact pads. The
featured contact pads were also written by the direct writing
process with the line spacing p set sufficiently small so thatconsecutive traces coalesce. Strain gauges written directly onto
PDMS were then encapsulated in PDMS for performance characterization, summarized in Figure 4b. The resistance increaseis roughly linear with strains up to 50%, with an approximate
slope (gauge factor) of 1.5. However, resistance dependency
deviates from the linear trend with strains greater than 50%,
which is in agreement with previous studies.[19,20]

FULL PAPER

from Figures 2d–f, two intersecting planes can be formed (asshown in Figure 3b) to map out all achievable values for tracegeometry.

The direct writing method and results reported in this work
demonstrate a rapid, repeatable, scalable, maskless, one-step
additive method for fabricating uniform continuous films.
Another key feature of this method is its ability to directly write aliquid such as EGaIn that exhibits properties indicative of a nonprintable fluid with respect to inkjet technology, indicating extension of this method to the direct writing of other liquids thatare problematic for other patterning methods, provided θr << θsfor the liquid/substrate interface. Other functional liquids ofpotential interest include suspensions with high concentrationsof nano-particles, polymers, and ionic liquids. Furthermore, thescalability observed within this method suggests possible extension to scales ranging from nanometer to millimeter, provided
appropriate needle sizes are used (see Supporting Information
Figure S1 for ID measurements pertaining to this study).
The restricted amount of control on h0 limits the current
system to a single layer. However, this limitation is currently
being addressed by incorporating a motorized z-stage and feedback controller, which will facilitate 3D writing via stacking ofadditional layers structurally enabled by the inter-layer oxideskin, as previously demonstrated with spheres of EGaIn.[12] The
results presented are provided to serve as a guide for tuning
the input parameters in order to print a single-layer trace. However, the actual outcome may vary depending on the substrate,writing material, and system specifications. Furthermore,
future work can be done in order to verify the constant flowrate of the extruder. We expect that further optimization of theprocess will facilitate the ability to rapidly create novel devices
and electronics, such as conformable inductors and capacitors, with applications in conformable sensing/actuating skins,
robotics, medical devices, conformable orthotics, and stretchable electronics.

4. Experimental Section
Preparation of PDMS: The Slygard 184 two-part PDMS was received
from Dow Corning. The monomer and hardener were mixed together
at a 10:1 weight ratio in a THINKY ARE-310 planetary centrifugal mixer.
The mixture was spun in the mixer at 2000 rpm for 30 s (mixing),
immediately followed by 2200 rpm for 30 s (degassing).
Preparation of Substrates Used: The glass substrates employed for
direct writing were Gold Seal pre-cleaned 75 mm × 25 mm × 1 mm glass
slides and were used as received. The PDMS substrates were made by© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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Figure 4. Application of direct writing to strain gauge devices. a) (main image) Photograph of multiple strain gauge devices on glass. Scale bar is 5 mm
in length. Patterns were written using the 119 µm inner diameter nozzle. (inset) Photograph of a larger scale directly written strain gauge on glass.
Scale bar is 5 mm in length. Device and pads were written using a 379 µm inner diameter nozzle. b) Performance characterization of directly writtenstrain gauges encapsulated in PDMS. Both loops (50% strain and 100% strain) were repeated for five cycles. Dashed line is a linear least squares fit
through the 50% strain data. Double dashed line is a quadratic least squares fit through the 100% strain data. R0 = 50 Ω for the 50% trials, with a corresponding unstretched length of 20 mm. R0 = 64 Ω for the 100% strain trials, with a corresponding unstretched length of 21 mm. c) Photographs ofcharacterized strain gauge stretched at different strains throughout experiment. Smallest deviations on ruler represent one millimeter.
spin coating the PDMS mixture onto Corning 75 mm × 50 mm × 1 mm
glass slides. The glass slides were pre-treated with Mann Ease Release
200 to better facilitate removal of devices post encapsulation. The
PDMS mixture was then spin coated onto the pre-treated glass slides at
400 rpm for 50 s with 10 s ramp up and ramp down times. This resulted ina PDMS layer of approximately 50 µm according to theory (Equation 3.18
of Krishnan).[21] The PDMS coated glass slides were then placed in an
incubator (Shell Lab GI2) at 60 °C for two hours to cure.
Experimental Details of Direct Writing System: The system used for the
direct writing experiments is featured in Figure 1. Lines of EGaIn weredirectly written using a stationary blunt syringe needle (details of syringetips can be found in Supporting Information Figure S1) and pump
(Harvard Apparatus PHD Ultra 4400) system coupled with a motorized
XY stage (Anorad WKY-150) and an optical imaging camera. Details of
the motorized XY stage are reported elsewhere.[22] For each experiment,
EGaIn was supplied to the syringe needle at a constant flow rate Q bymeans of the syringe pump and tubing as shown in Figure 1. The heighth0 between the syringe needle and the substrate was held constant(≈ ±3 µm) for each experiment and controlled using a manual z-stage(Edmund Optics 56–335) for height adjustment and CCD camera forvisually detecting when the needle comes in contact with the substrate(i.e., h0 = 0). To initiate direct writing the stage was moved to the starting
point of the pattern. Upon reaching the starting position, h0 was set
to zero in order to make contact between the liquid meniscus and thesubstrate. Next, h0 was set to its desired value. Finally, the stage motion
was activated by means of a pre-programmed motion profile dynamicallycontrolled via a motion controller (Anorad CM-2). The preset flow rate
Q from the pump began immediately after the start of the stage motion.
The pump was stopped upon the end of the stage motion. Detachmentof the EGaIn within the needle from the written pattern was realized bysimultaneously increasing h0 and moving the pattern from the needle

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via XY stage at a high velocity in order to overcome the surface tensionof the EGaIn (see Supporting Information Video M7 for a direct writing
example of a serpentine pattern).
Encapsulating and Wiring Devices: The written devices were encasedby spin coating PDMS over the devices at 400 rpm for 50 s with 10 s
ramp up and ramp down times. Post-hoc inspection showed that theadded mechanical strength of the Ga oxide layer facilitated the structural
integrity of the device during the spin encasing process, as seen withother studies.[3,11,12] The encased devices were then placed in an
incubator at 60°C for two hours to cure. After curing, the bottom edgesof the device contact pads were removed from each device by meansof an X-acto knife. Copper wires were then inserted into the remaining
portion of the contact pads and the incisions were covered with a
few additional drops of PDMS. Finally, the devices were placed in an
incubator again at 60 °C for two hours in order for the additional dropsof PDMS to cure.
Characterization: The trace height and widths were measured using alaser confocal microscope (LEXT 3000) with a 50X microscope objective.
Contact angle measurements were obtained using the sessile drop
method. Advancing sessile drops were produced by pushing EGaIn
out of a needle in close proximity to the substrate at a controlled flowrate (≈100 µL min−1). Static sessile drops were made by stopping theflow rate of the advancing sessile drops and allowing for relaxation of
the drop to equilibrium. The advancing and static contact angles wereobtained by applying the perturbation solution of the Bashforth-Adams
equation presented by Srinivasan et al.[23] The sessile drop height andwidth data used in the perturbation solution was obtained by analyzing
the advancing and static contact angle videos (Supporting Information,
Videos M1,M2,M4,M5) via MATLAB’s image processing toolbox.
The receding contact angles were estimated visually from the video(Supporting Information, Videos M4,M6) images, since the embedded© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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

Acknowledgements
The authors would like to Dr. Peroulis and his research group, specifically
Colby Barlett, Juan Zeng, Nithin Raghunathan, and Anurag Garg, foraccess to and training on equipment used for this study.
Received: September 17, 2013
Revised: December 30, 2013
Published online: February 18, 2014
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needle made it difficult to extract height data for the receding cases.
It should be noted that post-hoc visual inspection of the substrates
revealed EGaIn films remained on the surface, indicating a near-zero θr
for both substrates. For the strain gauge characterization, one side of
the strain gauge was mounted in a vice fixed to the XY stage while theother side was mounted to a vice fixed to the stationary optical table asshown in Figure 4. Upon measuring the unstretched length with a ruler,
the strain gauge underwent stretching cycles with 1 mm steps of the
XY stage. The corresponding resistance was measured using a digital
multimeter (Fluke 87V) directly after each step.

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