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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


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.




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,


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.




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.


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.




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.





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


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°.




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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