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ARTICLE
Cite this: DOI: 10.1039/x0xx00000x
N. Lazarus,a C. D. Meyer,a and W. J. Turnerb
Electrical traces made using liquid metal can survive strains of tens or hundreds of percent
without damage. Liquid metal is promising for creating the thick cross sections and low
Received 00th January 2015,
resistance necessary for power devices, while maintaining high stretchability. In this article,
Accepted 00th January 2015
a stretchable wireless power receiver system is demonstrated featuring a liquid metal circuit
board (galinstan traces embedded in silicone) to connect individual discrete components as
DOI: 10.1039/x0xx00000x
well as to create the receiver inductor coil. The mechanical effects of embedding parts were
www.rsc.org/
also simulated using a numerical solver. The fluidic channels defining the liquid metal
traces are built using a silicone molding and bonding process based on 3D printed molds.
The system consists of an inductor-capacitor resonant tank, voltage-doubling diode rectifier
for AC to DC conversion, and a representative load. Circuit operation was demonstrated up
to 80% uniaxial mechanical strain.
Introduction
result in poor electrical performance. This has led to interest in
the use of liquid metal inductors for wireless power delivery to
The use of liquid metal in electrical systems dates to the very improve efficiency.19,20
beginning of computing, from the drops of mercury used as
Although wireless power transfer can be demonstrated
inputs in punch card readers1 to the fluidic delay line memories using an inductively coupled coil pair, most applications
of early digital computers such as UNIVAC.2 However, the require additional components such as power conditioning
toxicity of mercury has minimized widespread use in more circuitry as well as the desired load. A stand-alone stretchable
recent years. Recently, liquid metals have made a resurgence in system was recently demonstrated using a centimeter-scale
highly deformable devices for applications such as soft robotics flexible polyimide-based printed circuit board (PCB) embedded
and biomedical sensors3, attracting interest due to their ability within an elastomeric substrate containing a fluidic inductor.21
to stretch and bend without damage. One factor has been a shift The inductor alone was shown to maintain functionality up to a
toward the use of less toxic gallium alloys such as galinstan and uniaxial strain of 50%, but the addition of the polyimide PCB
eutectic gallium indium,4 more suitable for use on or near a resulted in system failure above 25% strain due to stress
user’s skin. These alloys have the further advantage that the concentrations at the interface between the two materials. In
gallium forms a thin layer of oxide on its surface that resists this work, a self-contained wirelessly powered receiver is
leakage and creates a self-healing effect.5 From the initial demonstrated using liquid metals for both the receiver coil and
demonstrations as electrical interconnects,6 the field has interconnects between embedded electrical components.
expanded to include pressure sensors,7 capacitors,8,9 Mechanical simulations were also performed to verify the stress
inductors,9,10 resistive heaters11 and tunable antennas.12
concentrations of embedding piece parts with liquid metal
Liquid-metal-based systems, known as microfluidic traces. The system consists of a resonant inductor-capacitor
electronics, are promising for stretchable devices because very (LC) tank, a single-ended diode rectifier and a representative
large cross sections, on the order of millimeters, can be used load, consisting in total up to seven individual components. The
while retaining deformability. This allows resistance to be kept system is able to operate for strains up to 80% uniaxial
far lower than stretchable alternatives such as serpentine mechanical strain, more than a factor of three larger than
traces13 and conductor loaded polymers.14 Fluidic devices are of previous work.
particular interest for power devices such as inductors where
increased resistance causes power losses and degraded Fabrication
efficiency. Stretchable electronics are commonly powered
wirelessly to eliminate the need for fragile physical connections Forming electrical connections to discrete components was one
to the outside world. Although stretchable coils for wireless of the earliest applications for liquid metal within stretchable
power have been demonstrated using thin serpentine traces,15-17 electronics, and is typically done using one of two approaches
the added length and small cross section, along with negative distinguished by the timing of the liquid metal addition. In the
mutual coupling resulting from the interconnect geometry,18 first, liquid metal is patterned on a surface by a method such as
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Microfluidic wireless power system
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DOI: 10.1039/C5RA17479A
tape transfer21 or screen printing22. The components are
arranged on the surface and then encapsulated by the addition
of liquid elastomer precursors. Although this approach allows
for high resolution patterning of parallel traces, issues with
smearing or shifting of the liquid metal can cause shorting
during the rigid component placement.22 Alternatively,
components can be first embedded into an elastomer that
contains pre-defined microchannels.23 Liquid metal is then
injected into the channels using a syringe, which eliminates the
risk of shifting the liquid metal during the addition of parts.
This method is serial in nature and therefore likely slower, and
also requires additional area overhead for inclusion of the
fluidic inlets and outlets. The latter method was used for this
work but similar results could be obtained with either method.
Sockets are defined within a fluidic network to orient and hold
the components during the bonding process (Fig. 1 (a)).
Through-hole components are used with trimmed leads, which
extend into the channels to make robust electrical contact after
the addition of the liquid metal (Fig. 1 (b)).
(a)
injection of the liquid metal galinstan to form the inductor and
interconnect traces (Fig. 2(f)). Drops of liquid silicone are then
used to seal the injection holes and prevent leakage.
(a)
(b)
(c)
(d)
(e)
(f)
Fig. 2 Fabrication process for wireless power system: (a) 3D
printed mold, (b) pouring of silicone precursors, (c)
removing cured silicone from mold, (d) adding rigid
components, (e) bonding to partially cured silicone and (f)
injection of galinstan
Electrical Design
(b)
Fig. 1 (a) Addition of components to pre-defined channel
network and (b) structure after filling with liquid metal
Molds are first fabricated in acrylonitrile butadiene styrene
(ABS) using a commercial 3D printer (Replicator 2X,
Makerbot) (Fig. 2(a)). Liquid silicone precursors (Ecoflex 0030, Smooth-on.com) are poured over the mold (Fig. 2(b)) and,
after vacuum degassing, are allowed to cure completely at 85˚
C on a hotplate. The cured silicone is peeled from the mold
(Fig. 2(c)) and the components are arranged in the pre-defined
sockets and glued into place using drops of uncured silicone
(Fig. 2(d)). The channels are sealed by bonding to Ecoflex 0030 that has been allowed to gel and partially solidify but not
cure completely (Fig. 2(e)). This partial cure was done by
curing for an hour and ten minutes at room temperature before
bonding, rather than the four hours necessary for a complete
cure. The bonded piece is then placed on a hotplate at 85˚ C
and baked until completely cured. Inlet and outlet holes are
cored using narrow gauge syringe tips, followed by the
2 | RSC Advances, 2012, 00, 1-3
The wirelessly powered receiver developed in this work
contains a total of seven embedded components along with a
liquid metal inductor for use in an inductive power link (Fig.
3(a)). The circuit (Fig. 3(b), with list of component values
given in Table 1) consists of three major blocks: a resonant LC
tank, rectification circuitry and a representative load. While
inductive wireless power transfer can operate over a broad
frequency range, resonance improves energy transfer efficiency
within the inductive link. In the LC tank, the capacitor C1 is
therefore shunted across the secondary coil terminals to
resonate the receiver and allow improved coupling. The
stretchable inductor was implemented as a single-turn circular
planar coil (20 mm radius, 1 mm trace width and 1.5 mm trace
thickness) with an approximate inductance and resistance of
101 nH and 79 mΩ respectively. A complete model of the
effects of mechanical deformation on a similar type of coil was
previously developed.20 A capacitance of 0.22 µF resonates the
stretchable coil at 1.24 MHz.
A one-turn coil was chosen primarily for fabrication
reasons. Creating a multi-turn inductor requires a more
complicated fluidic network, since a second routing layer and
vias between the two layers are necessary to create an overpass
from the center of the coil. Creating and aligning these
additional channel layers, although possible, add significantly
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Journal Name
to the fabrication complexity. Single turn samples can also be
made thinner, again due to the elimination of the second routing
layer. This does however come at a cost: multiple-turn
inductors couple more strongly, allowing better energy transfer
between the transmitter and receiver coils. To help quantify the
tradeoff in using the single turn receiving coil, the field solver
FastHenry was used to model the inductive coupling
coefficients for both the single-turn as fabricated and an eightturn coil with the same outer diameter and 1 mm line width and
spacing, the resolution of the fabrication process used here. In
both cases, the transmitter coil was modeled to simulate the one
used in the experimental section of this work. The drop in
coupling coefficient to the transmitter coil geometry by
choosing a single turn coil was modest, approximately 12.5%.
Although the multiple turn coil was found to couple more
strongly, the inner turns contributed less strongly to the mutual
coupling due to the increasingly smaller diameters. Therefore
the modestly reduced coupling coefficient of the single turn coil
was found to be an acceptable tradeoff in return for the
significantly simplified fabrication process.
Applying a sinusoidal current to the transmission coil
produces time varying magnetic fields that in turn induce a
sinusoidal voltage at the receiver input. The induced AC
voltage can be used to power a system directly, but in many
cases rectification of the sinusoidal signal is required to provide
a steady DC voltage. A single-ended voltage doubler is used as
the rectifier structure to rectify and boost the AC signal at the
input into a larger DC voltage. While multiple stages can be
cascaded to increase the rectified voltage, a single stage is
implemented for simplicity within the stretchable receiver. The
load consists of a light emitting diode (LED) (1.6 V turn-on
voltage) with current-limiting resistor to allow for visual
verification of the circuit operation without a need for direct
electrical connection.
ARTICLE
DOI: 10.1039/C5RA17479A
Doubler
LR
C1
Load
(a)
Stretchable
C2
+
Vin
- LT
D2
LR
C1
LC Tank
D1
D3
C3
Voltage
Doubler
Rload
+
VLoad
-
Load
(b)
Fig. 3 (a) Wireless power system and (b) circuit schematic
Component
C1
C2
C3
D1
D2
D3
Rload
Value/Type
0.22 µF
0.22 µF
0.22 µF
HP2826
HP2826
Avago HLMP-P156-EGO31
10 Ω
Table 1: Embedded Components
A commercial inductive coil with magnetic backplane
(Wurth Electronics 760308111, 6.3 µH nominal inductance)
was used to wirelessly transmit energy to the stretchable system
and turn on the LED (Fig. 4(a)). The transmitter coil was driven
at the measured unstrained resonance of the system (1.29 MHz,
4% higher than calculated due to component tolerances),
requiring a minimum of 7 V peak-to-peak sinusoidal signal on
the transmitter to turn on the light emitting diode (Fig. 4 (b)).
During electrical testing, the voltages across the receiver coil
and load were measured using copper wires that were
connected to the liquid metal traces at each node. Due to the
turns ratio between the transmitter and receiver coil (eight
turns, as opposed to one turn in the receiver), the voltage drops
to 2.4 V peak-to-peak across the liquid metal coil. A singleturn inductor could be used for transmission to minimize this
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DOI: 10.1039/C5RA17479A
used to make electrical contact to the liquid metal traces. After
subtracting away the analytically estimated the inductance of
the copper lead wires, the inductance of the liquid metal coil
was found to vary from 75 nH at 0% strain to 124 nH at 80%
strain (Fig. 5 (a)), the maximum strain reached by the full
system before failure. The resistance varied from 62 mΩ to 323
mΩ for 0% and 80% respectively (Fig. 5 (b)).
In addition to the change in the electrical properties of the
inductor itself, the electromagnetic coupling to the rigid coil is
also affected by the change in geometry. At 0% strain, the
liquid metal coil is similar in size to the rigid transmitter coil, as
seen in Fig. 4(a)). During stretching, the inductor is elongated
along the axis of mechanical strain, becoming elliptical in
shape. A portion of the trace will extend past the outer
dimensions of the rigid coil, resulting in weaker coupling
between the two coils. In a two-coil system, the strength of the
magnetic coupling can be characterized by the coupling
coefficient k:
k=
M
L1 L2
(1)
(a)
Transmit Coil
Receive Coil
Load
4
3
2
Voltage (V)
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reduction in voltage, but a larger, multi-turn inductor was
chosen to simplify the needed power circuitry on the transmitter
side. This rectifier topology has an expected output voltage of
2Vin-2Vdiode, where Vin is the single sided amplitude applied to
the input of the rectifier and Vdiode is the forward conducting
drop across an individual rectifier diode. The receiver circuitry
rectifies the signal to 1.65 V with 100mV (6%) ripple,
consistent with the measured forward conduction drop on each
rectifier diode of 0.35 V.
1
0
-1
-2
-3
-4
0.0
0.5
1.0
Time (µs)
(b)
1.5
Fig. 4 (a) Wireless power demonstration and (b) measured
transient voltages
Mechanical Characterization
where L1 and L2 are the inductance of the two individual coils
and M is the mutual inductance resulting from the coupling
between them. The coupling coefficient varies from negative
one for perfect negative coupling to positive one indicating
perfect positive coupling, with zero for no coupling at all
between the coils. The coupling coefficient between the liquid
metal coil and the rigid transmitter coil was measured across
strain (Fig. 5 (c)). During testing, the rigid coil was placed in
contact with the surface of the encapsulating silicone, leading to
a separation of approximately 1.5 mm between the two coils.
The coupling was found to drop with higher strain, from 0.37
for 0% strain to 0.31 for 80%, due to the change in the
stretchable coil geometry. The inductor was also curved across
3D printed pieces with different radii of curvature to measure
the change in coupling to a flat rigid inductor as the stretchable
inductor bends (Fig. 5 (d)). Since the rigid transmission coil is
unable to conform to the curved silicone surface, the curvature
of the liquid metal inductor results in a modest reduction of the
coupling between the two.15 The coupling drops from 0.37
unbent to 0.32 for a 30 mm radii of curvature.
Unlike a rigid inductor, a stretchable system is dynamic and
able to deform freely during operation. However, the
inductance of a coil is geometry dependent, with wire self and
mutual inductance varying with wire length and spacing
between neighboring wires respectively. The resistance also
varies based on the length and cross section dimensions. This
effect is typically undesirable in wireless power transfer,
although both resistance and inductance variation of liquid
metal traces has been used for hyperelastic strain sensing.7,9 The
inductor used in this work was characterized using a custom
uniaxial strain testing setup10 to extract the deformed electrical
behavior. Testing of the inductor was performed without the
additional embedded power circuitry, and copper wires were
4 | RSC Advances, 2012, 00, 1-3
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DOI: 10.1039/C5RA17479A
Inductance (nH)
130
120
110
100
90
80
0
20
40
60
80
Strain (%)
(a)
Resistance (mΩ)
350
300
250
200
Principal
Strain (%)
150
100
70
60
50
0
20
40
60
80
50
Strain (%)
40
Coupling Coefficient
(b)
0.4
30
0.38
20
0.36
10
0.34
0.32
0
0.3
0
20
40
60
Strain(%)
80
(c)
0.4
Coupling Coefficient
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70
silicone results in local stress concentrations within the
elastomer and earlier system failure. The larger the embedded
parts, the greater the effect the parts will have in concentrating
stress and affecting the rigidity of the overall system. The
through-hole components used in this work have dimensions
ranging from two to four millimeters on a side. To examine the
effects on the overall system performance of parts on this size
scale, a uniaxial strain of 30% was modeled using the finite
element solver COMSOL (Fig. 6). The position of the parts
within the simulation was estimated using the slots in the 3D
printed mold defining the channels. Locally, the maximum
principal strain within the silicone was found to reach 70% near
the components, more than twice the strain on the overall
system. Although liquid metal inductors have been
demonstrated to survive up to 200% uniaxial mechanical
strain,9 with the rigid components the system is likely to fail at
a lower overall strain, likely less than half this value.
0.38
0.36
0.34
0.32
0.3
20
40
60
80
100 120
Radius of Curvature (mm)
(d)
Fig. 5 Liquid metal coil (a) inductance (b) resistance and (c)
coupling to rigid transmit coil during (c) stretching and (d)
bending across 3D printed pieces with different radii of
curvature
Fig. 6 Mechanical simulation for 30% applied uniaxial strain; the
black outlines show the original positions of the components
prior to stretching
To test the failure point, mechanical testing was performed
on the full system after the addition of the rigid components. As
long as sufficient power is coupled to the inductor and the
rectifier is providing at least 1.6 V of DC voltage, the red LED
will turn on. The system was first tested by bending the
stretchable sample across 3D printed pieces of different radii of
curvature down to 30 mm (Fig. 7 (a)). The minimum spacing
set by the silicone thickness results only at the point of actual
contact, the center of the inductor, while farther from that point
the spacing is significantly wider, resulting in the weaker
coupling. Due to the reduced coupling, the required
transmission voltage to turn on the LED increased to 8.5 V
peak-to-peak at 30 mm radius of curvature from the 7.0 V peakto-peak required for the static case.
The mechanical behavior of the wireless power system was
also analyzed. The addition of rigid components into the soft
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with strain, an increase of about 13% in voltage for 80%
mechanical strain, due both to the higher resistance of the
stretchable inductor and the reduction in coupling between the
two coils. Above 80% strain, voiding within the liquid metal
traces resulted in system failure and loss of connectivity. At
high strains microfluidic channels can partially or completely
collapse24 or become pinched off by protrusions left by
imperfections in the molds used.9 These effects result in redistribution of the liquid conductors and loss of electrical
connection. Once squeezed out of the channel, the viscous
fluid resists flow back to its initial position,24 preventing the
contact from being restored after the applied strain is removed
without re-filling the device.
To verify survivability during cycling, the uniaxial strain
testing setup was also used to repeatedly stretch the system.
The wireless power system was stretched 100 times to 50%
uniaxial strain without loss of electrical connection or
appreciable reduction in LED brightness during wireless power
coupling.
(a)
(b)
Conclusion
In this work, a wireless power system consisting of liquid metal
traces for both an inductor for power coupling and
interconnects between embedded components for signal
conditioning is demonstrated. The system is based on mm-scale
through-hole components embedded with a silicone matrix and
connected by channels filled with galinstan. Using individual
piece parts, rather than an embedded flexPCB as in past work,
allows for greater stretchability due to the smaller individual
rigid part sizes. The system demonstrated here has a maximum
strain of 80%, more than three times the flexPCB method. The
fabrication technique could be easily scaled up to more
complicated circuitry, either by incorporation of additional
circuitry or by using a previously developed multi-layer liquid
metal inductor process in a similar technology.10
(c)
1.3
1.2
1.1
1
Notes and references
0.9
a
0
20
40
60
80
Strain (%)
(d)
Fig. 7 Wireless power system (a) curved to 30 mm radius of
curvature, stretched to (b) 40% and (c) 80% uniaxial mechanical
strain and (d) resonant frequency with strain
A sample was also stretched using the uniaxial strain testing
setup (Fig. 7 (b) and (c)). The system continued to function and
turn on the LED up to 80% strain. Since the inductor
dimensions and resulting inductance change during stretching,
the resonant frequency also changes. The brightness of the
LED with frequency was used to estimate the approximate
resonant frequency of the system. Since the inductance
increases with stretch, the resonant frequency drops to a
minimum of 0.97 MHz for 80% strain. The required input
voltage on the transmitter to turn on the LED also increases
6 | RSC Advances, 2012, 00, 1-3
Sensors and Electron Devices Directorate, US Army Research
Laboratory, Adelphi MD 20783 E-mail: nathan.lazarus2.civ@mail.mil,
christopher.d.meyer1.civ@mail.mil
b
Electrical and Computer Engineering Department, University of Florida,
Gainesville, FL, USA 32611 E-mail: wjturner@ufl.edu
1
2
3
4
5
6
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This journal is © The Royal Society of Chemistry 2012
RSC Advances Accepted Manuscript
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