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Highly Efficient, Flexible Wireless-Powered Circuit Printed on a
Moist, Soft Contact Lens
Taiki Takamatsu, Yunhan Chen, Toshihiko Yoshimasu, Matsuhiko
Nishizawa, and Takeo Miyake*
DOI: 10.1002/admt.201800671
substantially greater functionality than an electrical
eyeglass.[2,3]
Numerous technological advances have been reported for
sensors,[4–6] displays,[7–10] and microchips[11] with wired[12–14]
or wire-less power supply systems[15–17] to produce smart contact
lenses. In 2014, Google demonstrated a proof-of-concept electrical
contact lens that assists diabetics by moni-toring the glucose
level in their tears and transferring related information
wire-lessly if the glucose concentration remains high after the
wearer has eaten a meal.[18] Other research efforts include a
readout integrated circuit (IC) chip for wireless communication
from a contact lens,[19] a graphene lens coating for
electromagnetic interference shielding,[20] and sensors to monitor
intraocular pressure[21–23] and biochemical changes[24–26] for
human diag-nostics. Separately, Minteer and co-workers developed
chemical sensors[27] and biofuel cell systems[28] on contact
lenses. However,
all such attempts have involved dry lithography on hard/soft
contact lenses, or an electronics sandwich structure between two
contact lens layers. Most electrical circuit printing techniques
are difficult to apply to soft, moist surfaces, but most people
prefer wearing moist and oxygen-permeable soft contact lenses.
Here, we demonstrate an electrochemical (EC) direct printing of
integrated wireless-powered circuits onto a moist, soft con-tact
lens (Figure 1a). EC printing is based on the polymeriza-tion of
3,4-ethylenedioxythiophene (EDOT) glue at the interface between the
circuit and the hydrogel-based substrate.[29,30] The
wireless-powered system consists of an in-parallel connection with
a loop antenna inductor (L) and a miniaturized ceramic capacitor
(C) for an eyeglass and a contact lens; it is designed for power
transfer at a resonant frequency of 13.56 MHz. The frequency is an
industrial science medical (ISM) band suitable for receiving power
without energy loss when the antenna is positioned near an aqueous
medium. This band is also suitable for designing a small loop
antenna mounted on the contact lens. We optimized the antenna
design and the coupling capac-itor for electrical eyeglass and
contact lenses to achieve a high power transfer efficiency (η) at
an appropriate radiation dis-tance. The system is combined with an
AC/DC rectifier circuit and a single light-emitting diode (LED) to
demonstrate wireless LED lighting on a pig eye even when the eye is
rotated to the maximum angle for human eye rotation (Figure
1b).
Contact lens with built-in electronics is a next-generation
wearable product with potential applications such as biomedical
sensing and wearable dis-plays. However, fabricating a
wireless-powered circuit on a moist, soft contact lens, via common
dry lithography, makes producing smart contact lenses challenging.
Here, electrochemically (EC) printing a wireless-powered circuit
onto a moist, soft contact lens is demonstrated. EC printing
involves adding a conductive polymer at the interface between a
metal contact and a hydrogel-based contact lens, resulting in
strong adhesion of the circuit to the lens without losing high
power transfer efficiency (50%) from an eyeglass trans-mitter to
the printed receiver lens. The energy transfer characteristics
during eye movement are modeled using the Neumann equation and
Kirchhoff ’s voltage law for wireless power transfer. The energy
transfer efficiency between the eyeglass transmitter and the
printed receiver lens is derived, and illumina-tion of a
wireless-powered single light-emitting diode display as a function
of eye rotation angle is demonstrated. This work opens the door to
integrating more complex circuits at soft contact lens interface to
produce smart contact lens with increased functionality.
T. Takamatsu, Y. Chen, Prof. T. Yoshimasu, Prof. T.
MiyakeGraduate School of InformationProduction and SystemsWaseda
UniversityKitakyushu, Fukuoka 808-0135, JapanE-mail:
[email protected]. M. NishizawaDepartment of
FinemechanicsGraduate School of EngineeringTohoku UniversitySendai
980-8579, Japan
The ORCID identification number(s) for the author(s) of this
article can be found under
https://doi.org/10.1002/admt.201800671.
Bioelectronics
1. Introduction
Smart contact lenses—contact lenses with built-in
elec-tronics—are a next-generation wearable product with
capa-bilities beyond simple vision correction.[1] Since the
electrical lenses are in continuous contact with the eyeball
surface, they have three main applications: (i) biomedical sensing
of tears to monitor health conditions, (ii) wearable displays for
aug-mented reality (AR), and (iii) actively regulating eye
accommo-dation to ensure perfect vision. Thus, a smart contact lens
has
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2. Results and Discussion
2.1. Wireless Power Transfer (WPT) between an Eyeglass and a
Soft Contact Lens
To calibrate the WPT from the “input” power-transmitting antenna
on the eyeglass to the “output” power-receiving antenna on the
contact lens, we first investigated magnetic res-onance coupling
WPT circuits consisting of two LC resonators (Figure 2a). For these
measurements, we prepared the transmit-ting LT1CT1CT2 resonator
(Figure 2a and Figure S1a, Supporting Information) using a
five-turn copper coil (wire diameter: 0.238 mm and coil diameter:
35 mm) and two ceramic capaci-tors (500 and 68 pF) on the eyeglass
and the receiving LR1CR1 resonator (Figure 2a and Figure S1b,
Supporting Informa-tion) using a single-loop gold coil (wire
diameter: 0.1 mm and loop diameter: 12 mm) and a ceramic capacitor
(4700 pF) on the contact lens. We designed a loop-type, simple
geometry for the antenna to avoid vision blockage and mount it on
the restricted area of a contact lens surface. When we apply
alter-nating current (AC) voltage to the transmitting coil (LT1) in
the eyeglass resonator circuit, the current flows through the LT1
coil to create an oscillating magnetic field. The magnetic field
passes through the receiving coil (LR1), inducing an
alternating voltage and current in the receiver circuit on a pig
eye. To tune the resonant frequency to 13.56 MHz, the induc-tive
coils, in both the transmitter and receiver, are connected to the
matching capacitor in parallel. The resonant frequency is defined
as f L C L C1/2 1/ 1/T T1 T1 T1 T2π= ⋅ + for the eyeglass and f L
C1/2 1/R R1 R1π= ⋅ for the contact lens, where fT and fR are the
resonant frequency at the transmitter and the receiver,
respectively, LT1 and LR1 are the coil inductance in the
trans-mitter and the receiver, respectively, CT1 and CT2 are the
capaci-tances in the transmitter, and CR1 is the receiver
capacitance. We modeled our WPT system as an equivalent circuit
(Figure S2a, Supporting Information) based on circuit
theory.[31–34] Its power transfer efficiency (η) is defined as (see
the equation derivation in the Supporting Information)
Z
f LZ R Z R Z R
Re[ ]1
2Re Re Re
L
0 M2 S T1 L R1
2L R1
η
π( )( ) ( )( )
[ ] [ ] [ ]=
+ + + + (1)
where f0 = 13.56 MHz, LM is mutual inductance between two LC
resonators, Re[ZL] and Re[ZS] are the real parts of the load and
source impedance, respectively, for the transmitter and receiver
circuits at the given resonant frequency of 13.56 MHz, RT1 and RR1
are the parasitic resistance of the transmitter and
Adv. Mater. Technol. 2019, 1800671
Figure 1. Wireless power transfer (WPT) system from the
transmitter on the eyeglass to the receiver on the contact lens. a)
The integrated receiver is mounted on a moist, soft contact lens,
via the electrochemical bonding method, and wirelessly receives
power from the transmitter at a resonant frequency of 13.56 MHz. b)
Illumination of an LED on the lens when an AC voltage is applied to
the eyeglass transmitter at 13.56 MHz.
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receiver coils, respectively, and LM is derived from the
equation L k L LM T1 R1= , where k is the coupling coefficient.
When we set two parallel coils at radiation distance d (Figure 2a),
k can be defined as[35,36]
k
d r r
1
1 2 /23
1 2
2
32
( )=
+
(2)
where r1 and r2 are the radii of the transmitter and receiver
coils, respectively. Parameters LT1 and LR1 are defined as[37]
L KNA
lT1 or R1 0
2µ= (3)
where µ0, K, N, A, and l represent the permeability of free
space, Nagaoka coefficient, number of turns in the coil,
cross-sectional area of the coil, and the length of the coil,
respectively.
According to Equation (1), the power is transferred from the
transmitting eyeglass to the receiving lens as a function of η. To
demonstrate our WPT system in air and on a pig eye, we con-nected
each antenna to a vector network analyzer at a radiation distance
of 10 mm. The antennas at the transmitter (Figure S1a, Supporting
Information) and the receiver 1 (Figure S1b, Sup-porting
Information) were resonated at the fT of 13.56 MHz and the fR of
13.52 MHz, respectively. Due to high performance of the
transmitting resonator, the WPT system was resonated at
13.56 MHz and transferred the power with η = 10% (Figure 2b and
Figure S1f, Supporting Information). Importantly, the η between the
transmitter and receiver was the same value even when the receiver
was placed on the moist pig eye surface. This similarity is
attributed to the negligible influence of the contact between the
antenna and a capacitive, moist eyeball in our WPT system. Further
η improvement was achieved by increasing the number of turns in the
receiving antenna and decreasing the radiation distance (Figure
2c). According to Equations (1)–(3), an increase in the number of
turns N enhances mutual induct-ance as well as the relative η. We
prepared a double-turned gold coil connected to a 1500 pF capacitor
in parallel (Figure S1c, Supporting Information, receiver 2), which
is the optimum operation frequency of 13.64 MHz, and a
triple-turned gold coil connected with a 680 pF capacitor in
parallel (Figure S1d, Supporting Information, receiver 3), which is
the optimum operation frequency of 13.60 MHz. The reflection
coefficient (s22) was enhanced to −12 dB at the double-turned
antenna (receiver 2, Figure S1g, Supporting Information), and to
−24 dB at the triple-turned antenna (receiver 3, Figure S1h,
Supporting Information), compared to −5 dB at the single-turned
antenna (Figure S1f, Supporting Information). When we modified the
antenna geometry, its η was 1.9 times greater (19%) at receiver 2
and 3.3 times greater (33%) at receiver 3 than the η obtained with
the single-turned coil receiver at a 10 mm radiation dis-tance.
When the radiation distance was reduced to 5 mm,
Adv. Mater. Technol. 2019, 1800671
Figure 2. Characterization of the wireless power transfer. a)
Photograph and schematic of VNA measurement images for wireless
power transfer between two resonant circuits on the input
transmitter and the output receiver mounted on a pig eye. b)
Transmission coefficient s21 in the air and on the pig eye, plotted
as a function of frequency. c) Power transfer efficiency η in our
WPT system, with different numbers of turns N in the receiving
antenna, plotted as a function of the radiation distance. The power
transfer efficiency is derived from Equation (1). The experimental
data were fitted with Equation (1), as described in the main
text.
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receiver 3 received one-half of the power (η = 50%) from the
transmitter. Thus, these results indicate that the receiver antenna
mounted on a pig eye surface can receive power from the transmitter
antenna on the eyeglass with a high η.
2.2. Electrochemical Bonding of the Receiver Circuit onto a
Moist, Soft Contact Lens
We investigated bonding the receiver circuit onto a contact lens
with an electrochemically cured adhesive,
poly(3,4-ethylenedi-oxythiophene) (PEDOT) (Figure 3a). Before the
EDOT polymer-ization, we immersed the contact lens in an
electrolyte solution containing 50 × 10−3 m EDOT monomer and 100 ×
10−3 m LiClO4 dopant. Then we stored it at 4 °C overnight to
exchange the lens solution with the electrolyte solution. After the
Au loop coil antenna was positioned on the monomer-containing lens,
an electrochemical potential of 1.0 V versus Ag/AgCl was applied to
the Au antenna to polymerize the EDOT at the receiver–lens
interface. The electrolyte solution in the
integrated lens was subsequently replaced with the artificial
tear solution. During polymerization, we observed a change from the
gold color of the Au antenna (Figure 3b) to the dark blue of the
PEDOT/Au antenna (Figure 3c). The PEDOT grew from the metallic loop
antenna into the contact lens, thus adhering the receiver to the
lens. The cross-sectional images of the PEDOT/Au/contact lens
(Figure 3d) confirm two significant PEDOT growth features: (1) The
PEDOT grew around the gold wire and extended horizontally on the
contact lens surface, and (2) the horizontally extended PEDOT
growth penetrated into the con-tact lens. Since the resonate
circuit was bonded electrochemi-cally onto top surface of the
contact lens and outside the area of eye iris, the bonded circuits
and PEDOT layer do not contact with eye surface and also avoid
vision blockage. When we man-ually compressed the bonded
PEDOT/Au/contact lens by 50% (Figure 3e, Movies S1 and S2,
Supporting Information), the compressed PEDOT/Au loop antenna lens
rapidly recovered to its original shape, without the antenna
peeling from the contact lens surface. This recovery behavior was
observed for several repeated cycles.
Adv. Mater. Technol. 2019, 1800671
Figure 3. EC bonding of the receiver to the lens. a) EC
polymerization of EDOT monomer glue at the interface between the Au
loop antenna circuit and a moist, soft contact lens; polymerization
was induced at 1.0 V versus Ag/AgCl. b,c) Photographs of the
circuits mounted on the lens before and after EC bonding. d)
Schematic and photograph of the cross section of the polymerized Au
wire on the lens surface. e) Pictures of a PEDOT/Au/contact lens
during repeated compression–recovery cycles. f) Real and imaginary
parts of the receiver antenna’s impedance (z) measured before and
after EC bonding.
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Soft contact lenses are classified into four groups: Group I
lenses have a low water content and are composed of nonionic
hydrogels; group II lenses have a high water content and are
composed of nonionic hydrogels; group III lenses have a low water
content and are composed of ionic hydrogels; and group IV lenses
have a high water content and are composed of ionic hydrogels.
Therefore, we tested the EC bonding of the receiver to contact
lenses of each group (Figure S3, Supporting Informa-tion). The
receiver successfully bonded to all of the contact lens surfaces,
although some contact lenses in group IV deformed at their edge
region because the lenses were soft and thin. Fur-thermore, we
confirmed the receiver performance before and after the PEDOT
coating was applied (Figure 3f). After the PEDOT coating was
applied to the Au antenna, the receiver resonated at 13.56 MHz; its
electromagnetic characteristics were not adversely affected. This
result is attributed to the PEDOT layer’s additional coating, which
has a parasitic ionic capacitance and an electrical resistance,
does not interfere with the LR1CR1 resonator at 13.56 MHz. This is
the first dem-onstration of such a circuit adhering to a moist,
soft contact lens using EC bonding. Compared to other assembly
tech-niques, such as the fabrication of a sandwich structure with
two contact lens layers[38] and a cast molding process for
embedding the receiver into medical-grade polymers,[5,23] EC
bonding enables direct mounting of a circuit onto a commer-cially
available, soft contact lens. The bonded circuit on the lens can
contact tear fluid, which is an advantage for biosen-sory
applications.
2.3. Wireless-Powered LED Lighting on the Eye
We demonstrate the LED illumination of a pig eye with the
receiver printed on a contact lens. The printed receiver was
integrated with a half-wave rectifier circuit using a Schottky
barrier diode and a 47 nF smoothing capacitor (CS) to convert power
from AC to DC (Figure 4a). When we applied an AC voltage of 40 Vpp
at 13.56 MHz to the eyeglass transmitter, a DC voltage of 1.7 V was
detected in the integrated receiver lens circuit (Figure S4b,
Supporting Information); in contrast, an AC voltage of 4 Vpp was
measured without the rectifier circuit. Because the 1.7 V input
voltage exceeds the operating voltage of the red LED, we could
illuminate the LED via wireless power from the transmitter to the
receiver lens on the pig eye (Figure 4c). During On/Off lighting of
the LED on the pig eye, we measured the eye temperature with a
thermal imaging camera (Figure 4b–d). The temperature measurement
is impor-tant for evaluating the safety level because the body
fluid in the eye can adsorb an electromagnetic radiation at 13.56
MHz and then may cause tissue heating. In practical use, the
resonator circuits mounted on the eye surface create heat when the
reso-nator received power wirelessly. In the LED’s On state, the
inte-grated circuit temperature increased from 25 °C (Figure 4b) to
31 °C (Figure 4c) over a period of 5 min. When the LED was turned
off, the circuit temperature decreased to ≈26 °C within 1 min.
Although the surface temperature of the eye changed by as much as 6
°C during this cycle, the temperature in the eye increased less
than 1 °C, which is within the safety level described in previous
papers.[39] Additionally, thermal damage
to corneal collagen on the eye surface does not occur until the
tissue temperatures exceeds 60 °C.[40,41] Nevertheless, when we
demonstrate our WPT system on human eyes, we will take the
temperature increase into account.[42]
In practical applications, eye movement is one of the most
important factors affecting WPT between the eyeglass and contact
lens in our system; the magnetic coupling between the antennas on
the eyeglass/contact lens during eye rotation changes as the
function of vertical distance, lateral displace-ment, and the
tilt-angle displacement (Figure 4e and Figure S2b, Supporting
Information). In general, a human eye can rotate from its central
axis to a maximum angle of 15° in all direc-tions.[43] In
horizontal eye movement to the right or left and vertical movement
upward or downward, the maximum angles are 35° and 25°,
respectively. When we applied AC power to the transmitting antenna
(LT1), the power measured at the receiving antenna (LR1) with the
vector network analyzer indicated η = 10% at the central position
(α = 0°) (Figure 4f). The 10% η value was measured until the eye
movement of 15° (α = 15°). Afterward, η decreased dramatically to
3.7% at α = 35°. Thus, the receiver coil could receive constant
power at the optimum eye movement angle.
To better understand the misalignment in the WPT system during
eye movement, we modeled the WPT characteristics between the
eyeglass and the contact lens based on Equation (1) to fit our η
data (Figure 4f). The misalignment between the transmitter and the
receiver during eye movement varies as a function of the coupling
coefficient k in Equation (1). When we set the eye rotation angle
α, vertical distance d, and the lateral distance l (Figure 4e), the
coupling coefficient is defined as a function of the eye rotation
angle[33,34,44]
cos
cos sind deye 1
2 3
4 5 6RXTX
k aa a
a a aII�� ∫∫
θθ θ
φ ϕ= ⋅ ++ + (4)
where aN N
L L41
TX RX 0
TX RX
µπ
= , a2 = r1r2 sin φ sin ϕ, a3 = r1r2 cos φ
cos ϕ, a4 = r12 + r22 + d2 + l2 − 2r1l sin φ − 2r1r2 cos φ cos
ϕ, a5 = 2r2l sin ϕ − 2r1r2 sin φ sin ϕ, and a6 = 2r2d sin ϕ.
Parameters NTX and NRX are the number of turns in the transmitter
and receiver coils (NTX = 5 and NRX = 1 in this work),
respectively, µ0 is the vacuum magnetic permeability, θ is the tilt
angle between the two coils, and φ and ϕ are the angles (0 ≤ φ, ϕ ≤
2π). The vertical distance d and lateral distance l are derived
from the equations d = dinit + r3(1 − cos α) and l = linit + r3 sin
α, where dinit and linit are 10 mm and 0 mm at the initial
position. The model fits the experimental data well using the eye
rota-tion angle from 0° to 35° (Figure 4f). According to Equation
(4), the vertical distance and lateral distance change from the
initial positions to d = 10.4 mm and l = 3 mm, respectively, at 15°
eye rotation and to d = 12.1 mm and l = 6.6 mm, respectively, at
35° eye rotation. To confirm these behaviors, we observed the
emitted LED light during eye movement (Figure 4g). When we applied
an AC 40 Vpp at 13.56 MHz to the transmitter, we con-firmed LED
light was emitted from the integrated lens receiver on a doll eye
until 20° eye rotation. Thus, the model based on our WPT system
provides information about the η during eye movement. This model
should help characterize the η behavior of other eye-worn WPT
systems.
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3. Conclusion
We demonstrated EC direct printing of a wireless-powered
res-onant circuit on a moist, soft contact lens. EC printing, based
on the polymerization of EDOT glue at the interface between the
circuit and the lens, provides strong adhesion of the loop antenna
receiver to the lens. After bonding, the printed receiver on the
lens resonates at 13.56 MHz and wirelessly receives half-power (η =
50%) from the transmitter at 5 mm radiation dis-tance. To
demonstrate LED lighting on the eye with our WPT system, we
integrated it with a rectifier circuit and a single LED chip to
generate DC output voltage to illuminate an LED. The LED was
illuminated with the same output voltage received wirelessly from
the eyeglass transmitter even when the eye was rotated to the
maximum angle of 15°. We used the Neumann equation and Kirchhoff’s
voltage law to model the η between the eyeglass and the contact
lens during eye movement and predicted the power wirelessly
transmitted to the integrated lens on a human eye for future
practical applications. During LED illumination for 5 min, the
temperature of the receiver circuit on the pig eye surface
increased by 6 °C, but the tem-perature in the eye increased by
less than 1 °C when we turned
off the LED light and waited for 1 min. This work opens the door
to integrating electronics with moist, soft contact lens to produce
smart lens with increased functionality.
4. Experimental Section
Fabrication and Characterization of the Transmitter and Receiver
Circuits: To form the transmitting resonator, a five-turn copper
wire coil (wire diameter: 0.238 mm and coil diameter: 35 mm) and
connected chip capacitors (500 and 68 pF) in parallel with Ag paste
were fabricated, which was cured at 120 °C for 30 min. For the
receiving resonator circuit, a gold wire coil (wire diameter: 0.1
mm and loop diameter: 12 mm) was connected to a chip capacitor
(4700 pF) with Ag paste, which was subsequently cured at 120 °C for
30 min. For the measurement, each resonator was connected with SMA
connectors (Orient Microwave BL52-5636-00). Both resonators were
connected to a vector network analyzer (Anritsu-MS46122B), and
their performances were characterized via two-port S-parameter
measurements.
EC Bonding of the Electronic Circuits on the Lens: A
commercially available soft contact lens was immersed in a solution
containing 50 × 10−3 m EDOT and 100 × 10−3 m LiClO4 overnight at 4
°C. Before EC bonding, the matching capacitor connected to the
receiver was isolated from the ionic solution with instant glue.
The receiver was then mounted
Adv. Mater. Technol. 2019, 1800671
Figure 4. Wirelessly powered LED light on the eye. a) Schematic
equivalent circuits of our WPT system for illuminating an LED.
Optical and thermal images of the wirelessly powered LED on a pig
eye during On/Off cycling of the power supply: b) initial Off state
at t = 0 min, c) On state at t = 5 min, and d) Off state at t = 6
min. e) Eye movement schematic with our WPT system in 3D
coordinates. f) Power transfer efficiency between the transmitter
and receiver during eye rotation. The experimental data were fitted
by Equations (1) and (4) described in the main text. g) The
wirelessly powered LED at different eye angles of 5°, 10°, 15°,
20°, 25°, and 30°.
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onto the EDOT monomer-containing lens and polymerized the EDOT
with a three-electrode system using an Ag/AgCl reference electrode
and a Pt counter electrode. The PEDOT glue was electropolymerized
at a charge of 500 mC (applied voltage 1.0 V). After
polymerization, the electric lens was immersed in deionized water
for 3 d to remove the EDOT monomer from the contact lens.
LED Lighting on the Eyeball: For the LED lighting demonstration,
the LC resonator was connected with a single LED chip and a
half-wave rectifier circuit consisting of a Schottky barrier diode
and 47 nF smoothing capacitor. The transmitter on the eyeglass was
connected to a multifunctional signal generator (NF, WF1974), and
AC 40 Vpp was applied at 13.56 MHz. At a radiation distance of 10
mm, the receiver was placed on an artificial eyeball made of
polydimethylsiloxane (PDMS) or on a pig eye. Optical and thermal
images were captured with a thermal imaging camera (FLIR, C2).
Simulation: Iterative simulations were carried out using MATLAB
software. Equivalent circuits of the wireless power system were
constructed between the transmitter on the eyeglass and the
receiver on the contact lens based on circuit theory. The numerical
values associated with each coil’s properties (i.e., their
parasitic resistances and self-inductances) were obtained from
impedance measurements performed with a vector network analyzer
(Anritsu, MS46122B). The results are summarized in Table S1
(Supporting Information).
Supporting InformationSupporting Information is available from
the Wiley Online Library or from the author.
AcknowledgementsThe research presented in this article was
supported by the Tateisi Science and Technology Foundation and
partly by the Okawa Foundation. Part of this work was conducted at
the Nanotechnology Platform Kitakyushu User Facility. T.M.
conceived the research. T.M and T.T. designed the experiments.
T.T., Y.C., and T.M. performed the experiments. T.M, T.T, T.Y, and
Y.C. analyzed the data. T.T and Y.C. fabricated the devices. T.T.
and T.M. wrote the manuscript with input from all authors. T.T.
carried out the theoretical analysis. Y.C. conducted the design
simulation. All authors revised the manuscript.
Conflict of InterestThe authors declare no conflict of
interest.
Keywordsconducting polymers, electrochemical bonding, soft
contact lenses, wireless power transfer systems
Received: December 4, 2018Revised: February 22, 2019
Published online:
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