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3D-printed microelectronics for integrated circuitry and passive wireless sensors

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Three-dimensional (3D) additive manufacturing techniques have been utilized to make 3D electrical components, such as resistors, capacitors, and inductors, as well as circuits and passive wireless sensors. Using the fused deposition modeling technology and a multiple-nozzle system with a printing resolution of 30 μm, 3D structures with both supporting and sacrificial structures are constructed. After removing the sacrificial materials, suspensions with silver particles are injected subsequently solidified to form metallic elements/interconnects. The prototype results show good characteristics of fabricated 3D microelectronics components, including an inductor–capacitor-resonant tank circuitry with a resonance frequency at 0.53 GHz. A 3D “smart cap” with an embedded inductor–capacitor tank as the wireless passive sensor was demonstrated to monitor the quality of liquid food (e.g., milk and juice) wirelessly. The result shows a 4.3% resonance frequency shift from milk stored in the room temperature environment for 36 h. This work establishes an innovative approach to construct arbitrary 3D systems with embedded electrical structures as integrated circuitry for various applications, including the demonstrated passive wireless sensors.
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OPEN
ARTICLE
3D-printed microelectronics for integrated circuitry and
passive wireless sensors
Sung-Yueh Wu
1,2,3,
*
, Chen Yang
1,3,
*
, Wensyang Hsu
2
and Liwei Lin
1,3
Three-dimensional (3D) additive manufacturing techniques have been utilized to make 3D electrical components, such as resistors,
capacitors, and inductors, as well as circuits and passive wireless sensors. Using the fused deposition modeling technology and a
multiple-nozzle system with a printing resolution of 30 μm, 3D structures with both supporting and sacrificial structures are
constructed. After removing the sacrificial materials, suspensions with silver particles are injected subsequently solidified to form
metallic elements/interconnects. The prototype results show good characteristics of fabricated 3D microelectronics components,
including an inductorcapacitor-resonant tank circuitry with a resonance frequency at 0.53 GHz. A 3D smart cap with an
embedded inductorcapacitor tank as the wireless passive sensor was demonstrated to monitor the quality of liquid food
(e.g., milk and juice) wirelessly. The result shows a 4.3% resonance frequency shift from milk stored in the room temperature
environment for 36 h. This work establishes an innovative approach to construct arbitrary 3D systems with embedded electrical
structures as integrated circuitry for various applications, including the demonstrated passive wireless sensors.
Keywords: additive manufacturing; radio-frequency passive sensors; 3D inductors and capacitors; three-dimensional printing;
wireless sensing
Microsystems & Nanoengineering (2015) 1, 2015013; doi:10.1038/micronano.2015.13; Published online: 20 July 2015
INTRODUCTION
The three-dimensional (3D) additive printing process has attracted
great interest in the field of rapid prototyping for a variety of
applications due to its flexibility in geometrical designs and
manufacturing
1
compared to subtractive manufacturing methods,
such as mechanical machining and laser cutting
24
.Inrecentyears,
this technique has been applied to the field of microsystems due
to improved capabilities in making structures with smaller feature
sizes and better accuracies, including microfluidic systems
511
.
Although arbitrary polymeric microfluidic systems can be readily
constructed, the 3D printing process has not been widely applied
to the general field of Microelectromechanical Systems (MEMS).
One reason is the difficulty in producing good 3D conductive
layers, which are essential in most functional devices, as special
equipment and techniques are required
3,4,1215
.
Here, we propose to use regular 3D printing equipment to
construct 3D microstructures with embedded metallic elements by
means of fillings of liquid metal paste to produce a variety of basic
microelectronics components, such as resistors, capacitors, and
inductors. This work is the first demonstration of the combined
process of 3D additive polymer printing combined with liquid
metal paste filling for use in potential practical applications. By
connecting these basic components, one can build more compli-
cated circuitries as well as numerous functional systems. As a
proof-of-concept, we constructed a 3D radio-frequency (RF)
passive circuit and further enhance the passive wireless sensing
system for a practical demonstrationa polymeric smart cap
with an embedded and wirelessly readable inductorcapacitor
(LC)-resonant circuit, which could enable the rapid and built-in
sensing for food safety detection in fluidic packages.
DESIGN AND FABRICATION
Figure 1 illustrates the proposed 3D design and the fabrication
steps. First, functional 3D structures are designed and constructed
using the 3D printing technique. The hollow microchannels and
cavities are designed in the 3D structures to be filled later with
liquid metal paste. A hollow solenoid-shaped channel is formed
as shown in Figure 1a1. To facilitate the liquid metal paste filling
step, injecting holes are designed as the inlet/outlet ports for the
solenoid channels, as shown in Figure 1a2. For direct frequency
characterizations of the designed RLC circuitry, the solenoid-
inductor structure has designated cavities as the ground-signal-
ground (G-S-G) pads on the top surface for contact pads. After
the 3D printing process, liquid metal paste is injected to form
conductive electrical structures, as shown in Figure 1a3. The
overflow of the liquid metal paste at the outlets on the top
surface are flattened and used as the contact pads. The final
solidification process cures the liquid metal paste to form solid
structures, while the top surface of the device is planarized to
remove the injecting holes.
Several basic microelectronic components were designed using
the proposed approach, as shown in Figure 1b, including
resistors, inductors, and capacitors. Specifically, the resistors are
made of meander-shaped conductive wires embedded in the 3D
structures, and the resistance of each wire is determined by the
resistivity of the material of the wire as well as the cross-sectional
area and the length of the wire. The inductors are designed to
have the shape of a spiral coil, and the inductance can be
estimated by the enclosed area of the coils and the number of
turns of the coils. The capacitors are constructed in the form of
two parallel-plates, and the capacitance is determined by the area
1
Department of Mechanical Engineering, University of California, Berkeley, CA 94720, USA;
2
Department of Mechanical Engineering, National Chiao Tung University, Hsinchu
30010 and
3
Berkeley Sensor and Actuator Center (BSAC), University of California, Berkeley, CA 94720, USA
*Both Sung-Yueh Wu and Chen Yang contributed equally to this work.
Correspondence: Liwei Lin (lwlin@me.berkeley.edu)
Received: 21 April 2015; revised: 9 June 2015; accepted: 9 June 2015
Mi c r o s y s t e m s & Na n o e n g i n e e r ing (2015) 1, 15013; doi:10.1038/micronano.2015.13
www.nature.com/micronano
of the plate, the permittivity of the polymeric material and the
distance between the two plates. Functional electrical circuits can
be made by further integration, e.g., connecting a solenoid
inductor and a parallel-plate capacitor to form a LC-resonant
tank, as shown in Figure 1c. Due to the geometric flexibility of the
3D printing, the 3D metal-embedded micro-systems provide
great flexibility for possible practical applications. For example,
the design of a smart cap is proposed as shown in Figure 1d
for use in liquid food package applications. The cap has an
embedded LC-resonant circuit to detect the quality of the liquid
Dual 3D
printing
nozzles
a1
b
c
d
a2 a3 a4
z
x
y
Inductors
Resistors
LC tank
Spiral inductor
Top
electrode
Bottom electrode
Via
Capacitors
Injection hole
Injecton
Liquid
metal
G-S-G
pads
Figure 1 Schematic diagram of the additive 3D manufacturing process including the lling of liquid metal paste for producing basic
microelectronic components, integrated circuitries, and a passive wireless sensor. (a) The 3D fabrication process with embedded and
electrically conductive structures. (b) 3D microelectronics components, including parallel-plate capacitors, solenoid-type inductors, and
meandering-shape resistors. ( c) A 3D LC tank, which is formed by combining a solenoid-type inductor and a parallel-plate capacitor. (d)A
wireless passive sensor demonstration of a smart cap, containing the 3D-printed LC-resonant circuit. The degradation of the liquid food
inside the liquid package can cause the changes of the dielectric constant and the shift of the resonance frequency of the LC circuity. A
wireless inductive reader is used to monitor the signals in real time.
Microsystems & Nanoengineering doi:10.1038/micronano.2015.13
3D-printed electrical circuits and wireless sensors
SY Wu et al
2
food inside the package. The sensing principle is based on the
capacitance changes of the liquid food due to its deterioration
over time. This information is monitored wirelessly in real time by
observing the resonance frequency shifts of the LC tank via an
inductive reader. As such, this smart cap could enable a passive,
wireless sensing scheme without the need to open the packages
for food safety inspection.
The prototype fabrication process uses the 3D printing
machine, ProJet HD 3000, based on the fused deposition
modeling technology
16
with a printing resolution of 30 μm.
During the printing process, polymer materials are heated and
ejected from the nozzles of the inkjet printer. Building (VisiJet EX
200, 3D Systems Inc., Rock Hill, SC, USA)
17
and sacrificial materials
(VisiJet S100, 3D Systems Inc., Rock Hill, SC , USA)
18
are deposited
alternatively from the dual nozzles to form the printed samples, in
which the building material defines the molding structure, while
the sacrificial material occupies the hollow channels
19
. After-
wards, a post-printing process is conducted to remove the
sacrificial materials. First, the whole 3D-printed sample is
immersed in a mineral oil bath at 80 °C to dissolve the sacrificial
material. Second, the residual mineral oil is removed by thor-
oughly washing with detergent and water in sequence. The liquid
metal paste comprised of a silver suspension (Pelco 16040-30,
Ted Pella Inc., Redding, CA, USA)
20
is then injected into the
channels and cavities. Next, the as-filled sample is kept at room
temperature for 2 h for the solidification process. The detailed
fabrication method flow can be found in the Supplementary
Information. These components can be scaled up or down based
on the capabilities of the specific type of 3D printer. However, the
liquid metal paste filling process has practical limits. Specifically,
smaller channels (diameter of 400 μm or smaller in our experi-
ments) have large flow resistance that prevents the filling
process, and the 600-μm diameter design is the optimal channel
size in this work using the ProJet HD 3000 printer, Hewlett-
Packard Company, Palo Alto, CA, USA.
RESULTS
The prototype 3D-printed microelectronics components without
the liquid metal paste are fabricated as shown in the optical
photo of Figure 2a, with a one-cent US coin shown for reference.
After removing the sacrificial materials and injecting the liquid
metal paste, the resulting functional components and the LC
circuitry are shown in Figure 2b. Note that the volume of the
silver suspension shrank after the solidification process, which
could leave voids inside the metal traces. By repeating the filling
operations, these voids are minimized, thereby improving the
electrical conductivity. With five times repeated filling, the
measured average volume-filling ratio reaches 68.7%, as char-
acterized in the cross-section views of these components shown
in Figure 2c. The electrical performances of the fabricated passive
components were characterized as follows. The DC I-V curves of
the resistors are measured using a semiconductor parameter
analyzer (HP 4145B, Hewlett-Packard Company, Palo Alto, CA,
USA). The two-port RF S-parameter spectra of the inductors,
capacitors, and LC tank are measured using Cascade Microtech
ACP40-GSG-200, Cascade Microtech Inc., Beaverton, OR, USA,
probes and a network analyzer (Agilent E5071B, Agilent Techno-
logies, Santa Clara, CA, USA). The parasitic effects of G-S-G pads
are de-embedded accordingly.
Inductor
b
a
c
Capacitor
LC tank
Cured liquid metal
Resistor
Figure 2 (a) An optical image showing fabricated microelectronics components produced using the 3D printing process without the
embedded conductive structure compared with a one-cent US coin. (b) Fabricated 3D components, including resistors, inductors, and
capacitors, and an LC tank after the liquid metal paste lling and curing process. (c) The cross-sectional view of a 4-turn solenoid coil. The
overall size of the prototype resistor, inductor, and capacitor are 10 10 2.4, 10 10 6.4, and 10 10 6.4 mm
3
, respectively, whereas
the size of the LC tank is 10 20 6.4 mm
3
.
doi:10.1038/micronano.2015.13 Microsystems & Nanoengineering
3D-printed electrical circuits and wireless sensors
SY Wu et al
3
Resistors
The DC I-V curves of two resistors with different designs are
shown in Figure 3a. The equivalent conductivity σ of the filled
metal is calculated as:
r ¼
1
R
:
l
S
ð1Þ
where R is the total resistance, l is the length of the conductor,
and S is the cross-sectional area of the conductor structure based
on the real volume-filling ratio. The cross-sectional shape of the
metal traces is circular, with a diameter of 600 μm, and length is
36 mm and 47 mm for resistor R1 and R2, respectively. The overall
resistances of R1 and R2 are measured as 0.68 Ω and 0.85 Ω,
80
ab
cd
ef
100
200
300
400
0
40
–80
25
20
15
10
5
0
20
16
12
8
4
0
14
C
s
L
p
R
p
R
sub
R
sub
C
sub
C
sub
16
12
10
8
6
4
2
0
–60 –40 –20 60 0.0
15
10
5
0
0.5 1.0 1.54020
Metal line diameter
= 600 μm
Metal line length
= 36 mm (R1),
47 mm (R2)
R1 = 0.68 Ω
R2 = 0.85 Ω
0
Voltage (mV) Frequency (GHz)
0.0 0.5 1.0 1.5
Frequency (GHz)
0.0 0.5 1.0 1.5
Frequency (GHz)
0.0 0.5 1.0 1.5
Frequency (GHz)
0.0 0.20.1 0.3 0.4
Frequency (GHz)
6-turn
6-turn
4-turn
2-turn
4-turn
Total impedance
= (Y
11
)
–1
Total impedance
= (Y
11
)
–1
RC
C1
C2
C3
C1
C2
C3
C1
C2
C3
R
L
2-turn
Current (mA)
Quality factor of inductor Q
L
Quality factor of capacitor Q
C
Relative permittivity ε
r
Total capacitance C (pF) Total inductance L (nH)
–40
0
Figure 3 Measurement results of 3D microelectronic components and an LC tank. (a) DC I-V curves of 3D-printed resistors. (b) Total
inductance L and (c ) quality factor Q
L
of 3D-printed RF inductors with different numbers of turns. (d) Total capacitance C,(e) quality factor Q
C
and (f ) calculated relative permittivity ε
r
of 3D-printed RF capacitors with different overlapping areas.
Microsystems & Nanoengineering doi:10.1038/micronano.2015.13
3D-printed electrical circuits and wireless sensors
SY Wu et al
4
respectively. The average σ is found to be as high as 2.8 10
5
Sm
1
, which is approximately 10.5% of the value for ideal silver
paste
19
. The difference comes from the poor packing density of
the silver particles inside the channel after the solidification
process (Supplementary Figure S1).
Inductors
The measured two-port S-parameters of the fabricated inductors
are converted to Y-parameters, and then the inductor perfor-
mances are extracted as
21
:
L ¼
Im
1
Y
11

2πf
; ð2Þ
Q
L
¼
Im
1
Y
11

Re
1
Y
11

; ð3Þ
where L is the total inductance, Q
L
is the quality factor of the
inductor, and f is the frequency.
Figure 3b and 3c show the measured inductance and quality factor
of the inductors with different numbers of coil turns, N. These
solenoid-shaped inductors have a designed diameter of 4 mm.
The cross-sectional shape of the metal traces is circular with a
diameter of 600 μm. The line spacing between adjacent windings
is 400 μm. In Figure 3b, the measured total inductance L increases
as N increases. For example, the inductances at 0.4 GHz are 23 nH,
51 nH, and 92 nH for inductors with 2, 4, and 6 turns, respectively.
For each inductor, the L increases first as the frequency increases
and then reaches a maximum value due to self-resonance. For
example, the inductance of the 6-turn inductor increases from
92 nH at 0.4 GHz to over 350 nH approximately 0.67 GHz. For
frequencies above the resonance, the inductance rapidly
decreases. The frequency at which the L drops to zero, i.e., the
self-resonance frequency f
0
is 1.49 GHz, 0.93 GHz, and 0.71 GHz
for the inductors with 2, 4, and 6 turns, respectively. Note that
larger N corresponds to smaller f
0
due to the larger inductance.
Figure 3c shows the measured quality factors. The quality factor
first increases as the frequency increases and then decreases to
zero due to the high loss at the self-resonance frequency. Note
that higher inductance leads to higher quality factor, which
proves that 3D inductors are helpful in reducing the energy
losses. For example, the 6-turn inductor has the highest Q
L
of
approximately 24 at 0.19 GHz, whereas the 2-turn inductor shows
a maximum Q
L
of 5.6 at 0.70 GHz. During the magnetic energy
storage cycles in the inductors, the energy loss mechanisms
mainly include the skin-effect induced ohmic losses in the
conductor and the electric field energy losses due to parasitic
capacitance.
Capacitors
The measured S-parameters of the capacitors are converted to
Y-parameters, and then the capacitor performances are extracted as
22
:
C ¼
ImðY
11
Þ
2πf
; ð4Þ
Q
C
¼
ImðY
11
Þ
ReðY
11
Þ
; ð5Þ
where C is the total capacitance, Q
C
is the quality factor of the
capacitor, and f is the frequency.
The measured total capacitances and quality factors of three
parallel-plate capacitors are shown in Figure 3d and e, respect-
ively. The rectangular-shaped parallel-plate type capacitors (C1,
C2, and C3) have different overlapping areas (11.4, 14.08, and
16.72 mm
2
, respectively) with the same gap of 400 μm. For each
capacitor, the capacitance C initially increases as the frequency
increases and reaches up to maximum to form the self-resonance
(>1 GHz) due to the parasitic inductance. For example, the
capacitance of capacitor C3 increases from 3 pF at 0.1 GHz to over
14.3 pF at approximately 1.31 GHz. After the peak, the capacit-
ance rapidly decreases. The frequency at which the C decreases
to zero, i.e., the self-resonance frequency f
0
is 1.53 GHz, 1.43 GHz,
and 1.36 GHz for C1, C2, and C3, respectively. Note that a larger
overlapping area corresponds to a smaller f
0
due to the higher
capacitance. The extracted quality factors Q
C
of three capacitors
at low frequency are approximately 1820, which are values
comparable with those of general metalinsulatormetal (MIM)
capacitors
22
; these quality factors can be further increased by
decreasing the electrode gap d. As the frequency increases, the
quality factors of three capacitors reduce gradually and then
reach zero at approximately 1.4 GHz.
Because the building material (VisiJet EX 200)
17
serves as the
dielectric material for the capacitors, its dielectric constant (ε
r
)
determines the performance of the capacitors; however, there are
no prior reported data on the dielectric constant of VisiJet EX 200.
To study ε
r
, a conventional Π-type lumped element equivalent
circuit
23
is designed, as shown in the inset of Figure 3f. The
impedance of the parallel-plates is modeled as the serial
combination of a capacitor (C
s
), a parasitic inductor (L
p
) and a
parasitic resistor (R
p
). The substrate parasitic effects are modeled
as C
sub
and R
sub
at each symmetric port, which represent the
effective capacitance and resistance, respectively, between the
signal port and ground. In the series branch of parallel-plates, C
s
will be the dominant element of the branch impedance (1/Y
12
)in
the lower frequency range, which is far below the self-resonance
frequency. Thus, the value of C
s
could be extracted as:
C
s
¼
ImðY
12
Þ
2πf
: ð6Þ
Next, the dielectric constant of building material is calculated as:
ε
r
¼
C
s
d
ε
0
A
; ð7Þ
where A is the overlapping area and d is the gap. The extracted relative
dielectric constant ε
r
values from all the three devices are shown in
Figure 3f, which are approximately 6, with very good consistency.
Improved inductors and an LC tank
Magnetic materials can be easily integrated into the prototype 3D
solenoid inductors by designing openings inside the coils to allow
for the placements of magnetic materials to enhance the
inductance. For example, one can insert a magnetic bar (Fair-
Rite® 3061990861, Fair-Rite Products Corp., Wallkill, NY, USA)
24
into a 3D solenoid inductor manually. Figure 4a shows the
preliminary results on the prototype 3D-printed inductors with
inserted magnetic bars. The inductance is found to increase due
to the addition of the magnetic core, and larger number of coil
turns is found to lead to higher increases of the inductance.
Specifically, the 6-turn coil device exhibits inductance enhance-
ments over the reference air-core inductor are in the range of
doi:10.1038/micronano.2015.13 Microsystems & Nanoengineering
3D-printed electrical circuits and wireless sensors
SY Wu et al
5
approximately 550% to 734% over the frequency range of 0.01
GHz to 0.2 GHz.
Based on the same fabrication process, prototype LC tank
circuits were constructed. The resonance frequency f
res
of LC tank
is derived as:
f
res
¼
1
2π
ffiffiffiffiffi
LC
p
ð8Þ
The measured frequency responses of the prototype device
consisting of a 4-turn solenoid and a parallel-plate capacitor are
shown in Figure 4b and c. In this case, the cross-sectional shape
of the metal traces is circular, with a diameter of 600 μm, and the
line spacing between the adjacent winding wires of the solenoid
is 400 μm. The rectangular-shaped parallel-plate capacitor has an
area of 8.36 mm
2
, with a gap of 400 μm. As the frequency
increases, the magnitude of the impedance increases and reaches
its maximum at 1.75 kΩ at the resonant frequency of 0.53 GHz, as
shown in Figure 4b. The bandwidth is extracted as 40.72 MHz,
based on 1=
ffiffi
2
p
of the peak impedance value. The calculated
quality factor Q is 13, which could be further increased by
improving the material conductivity. Figure 4c shows the phase
versus frequency plot.
BUILT-IN LC-RESONANT SENSOR
The 3D-printed LC tanks are further demonstrated as passive,
wireless-resonant sensors that can be used in applications in food
quality monitoring. Specifically, the LC sensor features passive
operation without power consumption and wireless reading
capability.
2532
In the prototype demonstration, the LC sensor is
embedded in the design of a smart cap, which has the required
3D geometry similar to a typical cap used in liquid food packages.
The whole structure is fabricated using the 3D additive manufac-
turing process.
Design
Figure 5a illustrates the sensing architecture of the proposed cap
with the embedded LC tank sensor. In this design, the circuit is
composed of an inverted-cone-shape capacitor and a planar
spiral-shaped inductor to form the LC-resonant circuit. By flipping
800
a
700
6-turn
4-turn
2-turn
Magnetic bar
(Fair-Rite 3061990861)
600
500
400
300
200
100
0.0 0.2 0.4 0.6
Frequency (GHz)
Inductance enhancement (%)
0.8 1.0
b
800
600
400
200
0
1800
2000
L of 4-turn
inductor = 60.81 nH
C of parallel-plate
capacitor = 1.49 pF
│Impedance│(Ω)
1600
1400
1200
1000
Frequency (GHz)
0.1
0.2 0.40.3 0.60.5 0.80.7
c
Resonance
frequency
f
res
= 0.53 GHz
Phase (°)
–90
90
–60
60
–30
30
0
Frequency (GHz)
0.1
0.2 0.40.3 0.60.5 0.80.7
Figure 4 (a) Measured inductance enhancements of 3D solenoid inductors with magnetic cores compared to the reference air-core
inductors. (b) Impedance magnitude vs. frequency and (c) phase vs. frequency of a 3D-printed RF LC-resonant circuit with the inductor and
capacitor connected in parallel.
Microsystems & Nanoengineering doi:10.1038/micronano.2015.13
3D-printed electrical circuits and wireless sensors
SY Wu et al
6
the food package upside down, the liquid food is trapped inside
the capacitor gap of the LC tank and acts as the dielectric
material. The LC tanks f
res
, is determined by the dielectric
constant of the liquid food. When the liquid deteriorates, the
value of f
res
can shift as a result of the change of dielectric
constant. The value of f
res
can be detected wirelessly using an RF
reader, as shown in Figure 5b. By imposing a frequency-swept
electrical field in the reader coil, the LC tank stores energy due to
near-field inductive coupling and exhibits electrical oscillation.
The most pronounced oscillation occurs when the driving
frequency matches the LC tanks resonance frequency because,
at this point, the LC tank absorbs the most electromagnetic
energy. This resonance induces a negative peak in reader coils
reflection coefficient |S
11
| spectrum, as measured by a network
analyzer
33
. By recording this peak, f
res
is tracked wirelessly, and
the quality of the liquid food is detected in real time without the
need to open the package.
Test results and discussion
Figure 6ad show the fabricated cap structures, which are
compatible with a half-gallon milk package. The outer diameter
of the cap is 32 mm. The pitch and number of turns of the spiral
inductor are 980 μm and 12.5, respectively. The via-channel
diameter is 600 μm. Other detailed dimensions are shown in
Supplementary Figure S2. Caps with the inverted-cone-shape
capacitor gap as well as flat gap designs are fabricated and
tested. The inverted-cone-shape design is used to eliminate air
bubbles during the operations because liquid food can be
trapped in the cavity by flipping the package while the air
bubbles escape from the cavity to the top due to their own
buoyant force (Supplementary Figure S3).
For wirelessly measuring the electrical resonance frequency of
the smart cap, an RF reader coil with outer diameter of 30 mm
and 13 turns was designed and constructed by manually winding
an enameled insulated wire (diameter 1.15 mm). The reader coil
was connected to the network analyzer and inductively coupled
to the cap, with a distance of 3 mm. The S
11
spectrum of reader
coil was measured, from which the f
res
of the cap was tracked
and characterized. Experimentally, fresh milk was stored under
the room temperature (22 °C) environment to accelerate the
degradation process, and the value of f
res
of the LC tank was
recorded after 0, 12, 24, and 36 h, as shown in Figure 6e. The
value of f
res
is observed to gradually decrease, indicating the
increase of the dielectric constant of the degrading milk. Figure 6f
shows the results from the milk samples at 22 °C and 4 °C versus
time, with average measurement error of ±0.10 MHz (three
different tests). Under the storage condition at 4 °C, the value of
f
res
was found to remain at approximately 51.65 MHz, with a small
shift of 0.12% after 36 h. In contrast, under the storage condition
at 22 °C, f
res
decreased by 4.3% after 36 h. As such, the frequency
shifts as a result of the capacitance changes were validated here
via wireless sensing. The corresponding dielectric constant
changes can be extracted from these data as 9.2% under the
storage condition at 22 °C, with a corresponding resolution
of ±0.19% in frequency, corresponding to ±0.39% in dielectric
constant in this prototype demonstration. Previously, various
impedance devices have been used by the food and manufac-
turing industries and public health agencies to estimate the
product shelf-life and the level of microbial contaminations
27,34,35
.
However, these measurements are often converted to bacterial
counts to satisfy a specific FDA ordinance (see the example for
milk
36
) with further characterizations. Nevertheless, these prior
reports have shown that changes in the ionic composition of a
culture medium can either increase or decrease its electrical
conductivity and capacitance by factors such as temperature,
microbial cell density, microbial growth, and medium conductiv-
ity. The advantage of the demonstrated smart cap here is to
reduce the possibility of contamination and provide the capability
of wireless remote sensing.
With the flexibility of the geometry design of 3D printing
and the demonstra ted metalliz ation method, other meaningful
designs are expected to be produced. For example, in-vivo
implants could further evolve to smart implants (e.g., smart
spinal implants) with embedded transducer circuits to wirelessly
transmit the local information, such as pressures and drug
concentrations.
CONCLUSION
Design, fabrication, and characterization processes for 3D-printed
microelectronics components and circuitry by the combination of
3D printing and liquid metal paste filling techniques were
developed. These components include various resistors, induc-
tors, and capacitors, and circuits include LC-resonant tanks. The
preliminary results demonstrate the good consistency of these
3D-printed devices with the analytical expectations and indicate
the possible performance enhancements and system integration
based on the 3D structures. As a demonstration example, a 3D
smart cap with embedded LC tank as the passive wireless
sensor was constructed for the application of a monitoring the
quality of liquid food (e.g., milk and juice). In this application, as
the liquid food deteriorates, the dielectric constant of the liquid
changes and the shift in the resonance frequency of the
embedded 3D LC tank can be detected wirelessly by an
inductively coupled reader in real time. The results showed a
4.3% frequency shift for a milk package stored under the room
temperature environment for 36 h. The positive results indicate
that 3D devices with embedded metallic components can open
up a new class of applications in devices (beyond the passive
wireless sensors) that benefit from 3D structures with embedded
metallic conductors.
Smart cap
a
b
Spiral inductor
Via
Top electrode
Bottom electrode
Liquid food
Bottle
Reader
Frequency response
Magnitudeof S
11
f
res
f
res
f
Energy
RF
coil
Information
Liquid
food
qualit
y
R
CL
Smart cap
Figure 5 The proposed smart cap for rapid detection of liquid
food quality featuring wireless readout: (a) the smart cap with a
half-gallon milk package, and the cross-sectional schematic dia-
gram; (b) sensing principle with the equivalent circuit diagram.
doi:10.1038/micronano.2015.13 Microsystems & Nanoengineering
3D-printed electrical circuits and wireless sensors
SY Wu et al
7
ab
d
c
Bottom
cavity
Top
cavity
Via
Spiral trench
Spiral inductor
012
Time (h)
24
Milk at 22 °C
Milk at 4 °C
Wireless distance = 3 mm
30 mm
52
f
51
50
49
13 turns
Agilent
E5071B
Wirelessly measured
resonance frequency f
res
(MHz)
RF coil
36
44
0.7
0.8
Initial state
12 h
24 h
36 h
0.9
e
46 48 50
Frequency (MHz)
52 54 56
Magnitude of S
11
Figure 6 Fabricated devices. (a) Cross-sectional view of a fabricated smart cap; (b) optical image of a completed 3D cap structure with a one-
cent coin; (c) the fabricated cap after the liquid metal lling process; and (d) magnied optical image showing the spiral induc tor around the
top surface of the cap. Test results of wireless LC tank sensors from the RF reader: (e) magnitude versus frequenc y cur ves with milk at 22 °C
after 0, 12, 24, and 36 h; (f ) resonance frequency versus time for a milk sample at 4 °C and a milk sample at 22 °C.
Microsystems & Nanoengineering doi:10.1038/micronano.2015.13
3D-printed electrical circuits and wireless sensors
SY Wu et al
8
ACKNOWLEDGEMENTS
Mr. Sung-Yueh Wu is supported by the Ministry of Science and Technology of
Taiwan (Grant No. 103-2917-I-009-192). The authors also thank Prof. Albert P.
Pisano for his help with the measurement equipment.
COMPETING INTERESTS
The authors declare no competing financial interest.
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