1564 IEEE SENSORS JOURNAL, VOL. 11, NO. 7, JULY 2011

Artificial Retina Using Thin-Film Transistors Drivenby Wireless Power Supply

Yuta Miura, Tomohisa Hachida, and Mutsumi Kimura, Member, IEEE

Abstract—We have evaluated an artificial retina using thin-filmtransistors driven by wireless power supply. It is found that theillumination profile can be correctly detected as the output voltageprofile even if it is driven using unstable power source generatedby inductive coupling, diode bridge, and Zener diodes. This meansthe feasibility to implant the artificial retina into human eyeballs.

Index Terms—Artificial retina, implant, thin-film tran-sistor (TFT), wireless poser supply.

I. INTRODUCTION

A RTIFICIAL retinas have been ardently desired to re-cover the sight sense for sight-handicapped people [1].

Recently, artificial retinas using external cameras, stimuluselectrodes, and three-dimensional large scale integrations(LSIs) have been actively developed for patients sufferingfrom retinitis pigmentosa and age-related macular degenera-tion [2]–[8]. In these cases, electronic photodevices and circuitssubstitute for deteriorated photoreceptor cells. The implantmethods can be classified to four types: epiretinal implant,subretinal implant, suprachoroidal stimulation, and transretinalstimulation. Among these implant methods, the epiretinal im-plant has features that the image resolution can be high becausethe stimulus signal can be directly conducted to neuron cellsand that living retinas are not seriously damaged.

In our research, we have proposed an artificial retina usingthin-film transistors (TFTs) [9], [10], which can be fabricated

Manuscript received November 14, 2010; accepted November 28, 2010. Dateof publication December 03, 2010; date of current version May 18, 2011. Thiswork was supported in part by a collaborative research with Seiko Epson Corpo-ration, research project of the Joint Research Center for Science and Technologyof Ryukoku University, grant for research facility equipment for private univer-sities from the Ministry of Education, Culture, Sports, Science and Technology(MEXT), grant for special research facilities from the Faculty of Science andTechnology of Ryukoku University, and Grant-in-Aid for Scientific Researchfrom the Japan Society for the Promotion of Science (JSPS). The associate ed-itor coordinating the review of this paper and approving it for publication wasProf. Gerald Gerlach.

Y. Miura was with the Department of Electronics and Informatics, RyukokuUniversity, Seta, Otsu 520-2194, Japan. He is now with the Graduate School ofMaterials Science, Nara Institute of Science and Technology, Takayama, Ikoma,630-0192, Japan (e-mail: t060232@mail.ryukoku.ac.jp).

T. Hachida was with the Department of Electronics and Informatics, RyukokuUniversity, Seta, Otsu 520-2194, Japan. He is now with the Graduate School ofMaterials Science, Nara Institute of Science and Technology, Takayama, Ikoma,630-0192, Japan (e-mail: ukkoke1735@hotmail.co.jp).

M. Kimura is with the Department of Electronics and Informatics, RyukokuUniversity, Seta, Otsu 520-2194, Japan; the Joint Research Center for Scienceand Technology, Ryukoku University, Seta, Otsu 520-2194, Japan; and the Inno-vative Materials and Processing Research Center, High-Tech Research Center,Seta, Otsu 520-2194, Japan (e-mail: mutsu@rins.ryukoku.ac.jp).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JSEN.2010.2096807

on transparent and flexible substrates. The concept model of theartificial retina fabricated on a transparent and flexible substrateand implanted using epiretinal implant is shown in Fig. 1. Elec-tronic photodevices and circuits are integrated on the artificialretina, which is implanted on the inside surface of the livingretina at the back part of the human eyeballs. Since the irradiatedlight comes from one side of the artificial retina and the stim-ulus signal goes out of the other side, the transparent substrateis preferable. Moreover, since the human eyeballs are curved,the flexible substrate is also preferable. It is possible to makespherical shape by designing a petal-like pattern. As a result,the artificial retina using TFTs are suitable for the epiretinal im-plant on the curved human eyeballs.

Until now, wired power supply has been used to drive the arti-ficial retina using TFTs to ensure reliable operations. However,the wired power supply harms quality of life of the sight-hand-icapped people because of bothersome connection wires be-tween the artificial retina and external equipments. Therefore,wireless power supply is requisite to eliminate the connectionwires and to realize complete artificial internal organs to im-prove the quality of life. In this paper, we have evaluated anartificial retina using TFTs driven by wireless power supply. Itis found that the illumination profile can be correctly detectedas the output voltage profile even if it is driven using unstablewireless power supply.

II. ARTIFICIAL RETINA USING THIN-FILM TRANSISTORS

The artificial retina using TFTs is fabricated using the samefabrication processes as conventional poly-Si TFTs [11]–[13]and encapsulated using SiO in order to perform in corrosiveenvironments. Although the artificial retina is fabricated onthe glass substrate here to confirm the elementary functions,it can be fabricated on the plastic substrate [14]. The artifi-cial retina using TFTs is shown in Fig. 2. The retina arrayincludes matrix-like multiple retina pixels. Although largecontact pads are located for fundamental evaluation, a principalpart is 27 300 m , which corresponds to 154 ppi. The retinapixel consists of a photo transistor, current mirror, and loadresistance. The photo transistor is optimized to achieve highefficiency [15], [16], and the current mirror and load resistanceare designed by considering the transistor characteristic ofTFTs [17]. The photosensitivity of the reverse-biased p/i/npoly-Si phototransistor is 150 pA at 1000 lx for white light andproper values for all visible color lights [18]. The field effectmobility and the threshold voltage of the n-type and p-typepoly-Si TFT were 93 cm V s , 3.6 V, 47 cm V s and

2.9 V, respectively. First, the photo transistor perceives theirradiated light (Lphoto) and induce the photo-induced current

1530-437X/$26.00 © 2010 IEEE

MIURA et al.: ARTIFICIAL RETINA USING THIN-FILM TRANSISTORS DRIVEN BY WIRELESS POWER SUPPLY 1565

Fig. 1. Concept model of the artificial retina fabricated on a transparent and flexible substrate and implanted using epiretinal implant.

Fig. 2. Artificial retina using thin-film transistors.

Fig. 3. Wireless power supply using inductive coupling.

(Iphoto). Next, the current mirror amplifies Iphoto to the mirrorcurrent (Imirror). Finally, the load resistance converts Imirrorto the output voltage (Vout). Consequently, the retina pixelsirradiated with bright light output a higher Vout, whereas theretina pixels irradiated with darker light output a lower Vout.

III. WIRELESS POWER SUPPLY USING INDUCTIVE COUPLING

The wireless power supply using inductive coupling is shownin Fig. 3. The right graph in Fig. 3 is a measured stability of thesupply voltage. This system includes a power transmitter, power

receiver, diode bridge, and Zener diodes. The power transmitterconsists of an ac voltage source and induction coil. The Vpp ofthe ac voltage source is 10 V, and the frequency is 34 kHz, whichis a resonance frequency of this system. The material of the in-duction coil is an enameled copper wire, the diameter is 1.8 cm,and the winding number is 370 times. The power receiver alsoconsists of an induction coil, which is the same as the powertransmitter and located face to face. The diode bridge rectifiesthe ac voltage to the dc voltage, and the Zener diodes regulate thevoltage value. The diode bridge and Zener diodes are discretedevices and encapsulated in epoxy resin. Although the current

1566 IEEE SENSORS JOURNAL, VOL. 11, NO. 7, JULY 2011

Fig. 4. Detected result of the illumination profile versus the output voltage profile.

system should be downsized and bio-compatibility has to be in-spected, the supply system is in principle very simple to implantit into human eyeballs. As a result, the generated power is notso stable as shown in Fig. 3, which may be because the artificialretina is fabricated on a insulator substrates, has little parasiticcapacitance, and is subject to the influence of noise. Therefore,it is necessary to confirm whether the artificial retina can be cor-rectly operated even using the unstable power source.

IV. DETECTED RESULT OF ILLUMINATION PROFILE

The artificial retina with the wireless power supply system islocated in a light-shield chamber, and Vout in each retina pixelis probed by a manual prober and voltage meter. White lightfrom a metal halide lamp is diaphragmmed by a pinhole slit, fo-cused through a convex lens, reflected by a triangular prism andirradiated through the glass substrate to the back surfaces of theartificial retina on a rubber spacer. The real image of the pinholeslit is reproduced on the back surface. The detected result of theLphoto profile versus the Vout profile is shown in Fig. 4. It isfound that the Lphoto profile can be correctly detected as theVout profile even if it is driven using the unstable power source,although shape distortion is slightly observed, which is due tothe misalignment of the optical system or characteristic varia-tion of TFTs.

V. CONCLUSION

We have evaluated an artificial retina using TFTs driven bywireless power supply. It was found that the Lphoto profile canbe correctly detected as the Vout profile even if it is driven usingunstable power source generated by inductive coupling, diodebridge, and Zener diodes. In order to apply the artificial retinato an actual artificial internal organ, we should further developa pulse signal generator appropriate as photorecepter cells, con-sider the interface between the stimulus electrodes and neuroncells, investigate the dependence of Vout on Lphoto, which re-alizes grayscale sensing, etc. However, we think that the aboveresult means the feasibility to implant the artificial retina intohuman eyeballs.

ACKNOWLEDGMENT

The authors would like to thank Dr. H. Hara, Dr. S. Inoue,Dr. H. Fukushima, and Dr. T. Kamakura of Seiko Epson;Dr. S. Koide, Dr. Y. Kobashi, and Dr. T. Ito of Epson ImagingDevices; Dr. T. Munakata of Jedat, and some members inMutsu laboratory of Ryukoku University.

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Yuta Miura received the B.E. degree in electronicsand informatics from Ryukoku University, Otsu,Japan, in 2010.

He had been working on research and developmentof artificial retinas using thin-film transistors (TFTs).He is currently a graduate student at Nara Institute ofScience and Technology, Ikoma, Japan.

Tomohisa Hachida received the B.E. degree in elec-tronics and informatics from Ryukoku University,Otsu, Japan, in 2009.

He had been working on research and developmentof artificial retinas using thin-film transistors (TFTs).He is currently a graduate student at Nara Institute ofScience and Technology, Ikoma, Japan.

Mutsumi Kimura (M’10) received the B.E. andM.E. degrees in physical engineering from KyotoUniversity, Japan, in 1989 and 1991, respectively,and the Ph.D. degree in electrical and electric en-gineering from the Tokyo University of Agricultureand Technology, Tokyo, Japan, in 2001.

He joined the Matsushita Electric Industrial Com-pany, Ltd., in 1991 and Seiko Epson Corporation in1995. He joined Ryukoku University, Otsu, Japan,in 2003. He has been working on thin-film tran-sistor (TFT) characteristic analysis, TFT simulator

development, TFT-OLED development, and their advanced applications.Dr. Kimura is a member of the Society for Information Display (SID), the

Japan Society of Applied Physics (JSAP), and the Institute of Electronics, Infor-mation and Communication Engineers (EIC). He is also a Chair or Member ofthe Technical Committee of the IEEE Electron Devices Society Kansai Chapter,the steering and program committee of AM-FPD, the AMD workshop of IDW,and the organizing committee of the Thin Film Materials and Devices Meeting.He received the Outstanding Poster Paper Award of Asia Display/IDW 2001,the Best Paper Award of AM-LCD 2005, the Best Paper Award of the FourthThin Film Materials and Devices Meeting, the Outstanding Poster Paper Awardof IDW 2007, the Outstanding Poster Paper Award of IDW 2009, and the 2010Materials and Structures Laboratory Director’s Award.