driven by wireless power supply

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Artificial Retina Using Thin-Film Transistors
Driven by Wireless Power Supply
Article in IEEE Sensors Journal · August 2011
DOI: 10.1109/JSEN.2010.2096807 · Source: IEEE Xplore
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Ryukoku University
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2. IEEE SENSORS JOURNAL 1

Abstract—We have evaluated an artificial retina using thin-film
transistors driven by wireless power supply. It is found that the
illumination profile can be correctly detected as the output
voltage profile even if it is driven using unstable power source
generated by inductive coupling, diode bridge, and Zener diodes.
This means the feasibility to implant the artificial retina into
human eyeballs.
Index Terms—Artificial retina, thin-film transistor (TFT),
wireless poser supply, implant.
I. INTRODUCTION
rtificial retinas have been ardently desired to recover the
sight sense for sight-handicapped people [1]. Recently,
artificial retinas using external cameras, stimulus electrodes,
and three-dimensional LSIs have been actively developed for
patients suffering from retinitis pigmentosa and age-related
macular degeneration [2]-[8]. In these cases, electronic
photodevices and circuits substitute for deteriorated
photoreceptor cells. The implant methods can be classified to
four types: epiretinal implant, subretinal implant,
suprachoroidal stimulation, and transretinal stimulation.
Among these implant methods, the epiretinal implant has
features that the image resolution can be high because the
stimulus signal can be directly conducted to neuron cells and
that living retinas are not seriously damaged.
In our research, we have proposed an artificial retina using
thin-film transistors (TFTs) [9],[10], which can be fabricated
on transparent and flexible substrates. The concept model of
the artificial retina fabricated on a transparent and flexible
substrate and implanted using epiretinal implant is shown in
Manuscript received Month Date, Year. This work was supported in part by
a collaborative research with Seiko Epson Corporation, research project of the
Joint Research Center for Science and Technology of Ryukoku University,
grant for research facility equipment for private universities from the Ministry
of Education, Culture, Sports, Science and Technology (MEXT), grant for
special research facilities from the Faculty of Science and Technology of
Ryukoku University, and Grant-in-Aid for Scientific Research from the Japan
Society for the Promotion of Science (JSPS).
Y. Miura and T. Hachida were with the Department of Electronics and
Informatics, Ryukoku University, Seta, Otsu 520-2194, Japan. They graduated
from Ryukoku University.
M. Kimura is with the Department of Electronics and Informatics, Ryukoku
University, Seta, Otsu 520-2194, Japan, Joint Research Center for Science and
Technology, Ryukoku University, Seta, Otsu 520-2194, Japan, and Innovative
Materials and Processing Research Center, High-Tech Research Center, Seta,
Otsu 520-2194, Japan (phone: +81-77-543-5111; fax: +81-77-543-7794;
e-mail: mutsu@rins.ryukoku.ac.jp).
Fig. 1. Electronic photodevices and circuits are integrated on
the artificial retina, which is implanted on the inside surface of
the living retina at the back part of the human eyeballs. Since
the irradiated light comes from one side of the artificial retina
and the stimulus signal goes out of the other side, the
transparent substrate is preferable. Moreover, since the human
eyeballs are curved, the flexible substrate is also preferable. It is
possible to make spherical shape by designing a petal-like
pattern. As a result, the artificial retina using TFTs are suitable
for the epiretinal implant on the curved human eyeballs.
Until now, wired power supply has been used to drive the
artificial retina using TFTs to ensure reliable operations.
However, the wired power supply harms quality of life of the
sight-handicapped people because of bothersome connection
wires between the artificial retina and external equipments.
Therefore, wireless power supply is requisite to eliminate the
connection wires and to realize complete artificial internal
organs to improve the quality of life. In this paper, we have
evaluated an artificial retina using TFTs driven by wireless
power supply. It is found that the illumination profile can be
correctly detected as the output voltage profile even if it is
driven using unstable wireless power supply.
II. ARTIFICIAL RETINA USING THIN-FILM TRANSISTORS
The artificial retina using TFTs is fabricated using the same
fabrication processes as conventional poly-Si TFTs [11]-[13]
and encapsulated using SiO2 in order to perform in corrosive
enviroments. Although the artificial retina is fabricated on the
glass substrate here to confirm the elementary functions, it can
be fabricated on the plastic substrate [14]. The artificial retina
using TFTs is shown in Fig. 2. The retina array includes
matrix-like multiple retina pixels. Although large contact pads
are located for fundamental evaluation, a principal part is
27300 m2
, which corresponds to 154 ppi. The retina pixel
consists of a photo transistor, current mirror, and load
resistance. The photo transistor is optimized to achieve high
efficiency [15],[16], and the current mirror and load resistance
are designed by considering the transistor characteristic of
TFTs [17]. The photosensitivity of the reverse-biased p/i/n
poly-Si phototransistor is 150 pA at 1000 lx for white light and
proper values for all visible color lights [18]. The field effect
mobility and the threshold voltage of the n-type and p-type
poly-Si TFT were 93 cm2
V-1
s-1
, 3.6 V, 47 cm2
V-1
s-1
and -2.9 V,
respectively. First, the photo transistor perceives the irradiated
light (Lphoto) and induce the photo-induced current (Iphoto).
Artificial Retina using Thin-Film Transistors
driven by Wireless Power Supply
Yuta Miura, Tomohisa Hachida, and Mutsumi Kimura, Member, IEEE
A

3. IEEE SENSORS JOURNAL 2
Next, the current mirror amplifies Iphoto to the mirror current
(Imirror). Finally, the load resistance converts Imirror to the
output voltage (Vout). Consequently, the retina pixels
irradiated with bright light output a higher Vout, whereas the
retina pixels irradiated with darker light output a lower Vout.
III. WIRELESS POWER SUPPLY USING INDUCTIVE COUPLING
The wireless power supply using inductive coupling is
shown in Fig. 3. The right graph in Fig. 3 is a measured stability
of the supply voltage. This system includes a power transmitter,
power receiver, diode bridge, and Zener diodes. The power
transmitter consists of an AC voltage source and induction coil.
The Vpp of the AC voltage source is 10V, and the frequency is
34kHz, which is a resonance frequency of this system. The
material of the induction coil is an enameled copper wire, the
diameter is 1.8 cm, and the winding number is 370 times. The
power receiver also consists of an induction coil, which is the
same as the power transmitter and located face to face. The
diode bridge rectifies the AC voltage to the DC voltage, and the
Zener diodes regulate the voltage value. The diode bridge and
Zener diodes are discrete devices and encapsulated in epoxy
resin. Although the current system should be downsized and
bio-compatibility has to be inspected, the supply system is in
principle very simple to implant it into human eyeballs. As a
result, the generated power is not so stable as shown in Fig. 3,
which may be because the artificial retina is fabricated on a
insulator substrates, has little parasitic capacitance, and is
subject to the influence of noise. Therefore, it is necessary to
confirm whether the artificial retina can be correctly operated
even using the unstable power source.
IV. DETECTED RESULT OF ILLUMINATION PROFILE
The artificial retina with the wireless power supply system is
located in a light-shield chamber, and Vout in each retina pixel
is probed by a manual prober and voltage meter. White light
from a metal halide lamp is diaphragmmed by a pinhole slit,
focused through a convex lens, reflected by a triangular prism
and irradiated through the glass substrate to the back surfaces
of the artificial retina on a rubber spacer. The real image of the
pinhole slit is reproduced on the back surface. The detected
result of Lphoto profile versus Vout profile is shown in Fig. 4.
It is found that the Lphoto profile can be correctly detected as
the Vout profile even if it is driven using the unstable power
source, although shape distortion is slightly observed, which is
due to the misalignment of the optical system or characteristic
variation of TFTs.
V. CONCLUSION
We have evaluated an artificial retina using TFTs driven by
wireless power supply. It was found that the Lphoto profile can
be correctly detected as the Vout profile even if it is driven
using unstable power source generated by inductive coupling,
v
Power
transmitter
Power
receiver
Artificial
retina
Human
eyeball
Wireless
power
supply
Irradiated
light
Transparent and flexible substrate
Stimulus
signal
Retina
pixel
Fig. 1. Concept model of the artificial retina fabricated on a transparent and flexible substrate and implanted using epiretinal implant.
Retina array Retina pixel
Vdd
Vbias
Vout
Vadjust
Current
mirror
Load
resistance
Iphoto Imirror
Photo
transistor
Lphoto
Fig. 2. Artificial retina using thin-film transistors.

4. IEEE SENSORS JOURNAL 3
diode bridge, and Zener diodes. In order to apply the artificial
retina to an actual artificial internal organ, we should further
develop a pulse signal generator appropriate as photorecepter
cells, consider the interface between the stimulus electrodes
and neuron cells, investigate the dependence of Vout on Lphoto,
which realizes grayscale sensing, etc. However, we think that
the above result means the feasibility to implant the artificial
retina into human eyeballs.
ACKNOWLEDGMENT
The authors thank Drs. Hiroyuki Hara, Satoshi Inoue,
Hitoshi Fukushima, and Tomoyuki Kamakura of Seiko Epson,
Drs. Shin Koide, Yutaka Kobashi, and Tomoyuki Ito of Epson
Imaging Devices, Dr. Tsuneo Munakata of Jedat, and some
members in Mutsu laboratory of Ryukoku University.
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Vdd
GND
4.5
5
5.5
0 5 10
Vdd(V)
t (s)
Power
transmitter
Power
receiver
Inductive
coupling
Artificial
retina
Diode
bridge
Zener
diode
Vpp=10V
34kHz 1cm
Power
receiver
Diode
bridge
Zener
diode
Artificial
retina
5.0140.012
Fig. 3. Wireless power supply using inductive coupling.
0V
4V
1V
2V
3V
Irradiated light Output voltage
Metal
halide
lamp
Manual prober
Triangular
prism
Convex
lens Voltage
meter
Artificial
retina
Vibration isolator Vibration isolator
xyz
stage
Rubber
spacer
Optical
microscope
White Light
Shield
chamber
xy table
Illuminance
controller
Pinhole
slit
Fig. 4. Detected result of the illumination profile versus the output voltage profile.

5. IEEE SENSORS JOURNAL 4
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Yuta Miura received his B. E. degree in Electronics and Informatics from
Ryukoku University in 2010. He had been working on research and
development of artificial retinas using TFTs. He is currently a graduate student
at Nara Institute of Science and Technology.
Tomohisa Hachida received his B. E. degree in Electronics and Informatics
from Ryukoku University in 2009. He had been working on research and
development of artificial retinas using TFTs. He is currently a graduate student
at Nara Institute of Science and Technology.
Mutsumi Kimura (M’10) received his B. E. and M. E. degrees in Physical
Engineering from Kyoto University in 1989 and 1991, respectively. He joined
Matsushita Electric Industrial Co., Ltd. in 1991 and Seiko Epson Corp. in 1995.
He received his Ph. D. degree in Electrical and Electric Engineering from
Tokyo University of Agriculture and Technology in 2001. He joined Ryukoku
University in 2003. He has been working on TFT characteristic analysis, TFT
simulator development, TFT-OLED development, and their advanced
applications.
He is a member of Society for Information Display (SID), Japan Society of
Applied Physics (JSAP), and Institute of Electronics, Information and
Communication Engineers (EIC). He is also a chair or member of the technical
committee of IEEE Electron Devices Society Kansai Chapter, the steering and
program committee of AM-FPD, AMD workshop of IDW, and organizing
committee of Thin Film Materials and Devices Meeting. He received
Outstanding Poster Paper Award of Asia Display / IDW ’01, Best Paper Award
of AM-LCD ’05, Best Paper Award of 4th Thin Film Materials and Devices
Meeting, Outstanding Poster Paper Award of IDW ’07, Outstanding Poster
Paper Award of IDW ’09, and 2010th Materials and Structures Laboratory
Director's Award.