A Flexible, Lightweight Braille Sheet Display with Plastic Actuators
Driven by An Organic Field-Effect Transistor Active MatrixYusaku Kato1, Shingo Iba1, Tsuyoshi Sekitani1, Yoshiaki Noguchi1, Kazuki Hizu1,
Xizhang Wang1, Kenji Takenoshita1, Yasushi Takamatsu1, Shintaro Nakano1, Kenjiro Fukuda1,
Kosuke Nakamura2, Tatsuya Yamaue2, Masao Doi2, Kinji Asaka3, Hiroshi Kawaguchi4,
Makoto Takamiya4, Takayasu Sakurai4, and Takao Someya1*
1 Quantum-Phase Electronics Center, School of Engineering, the University of Tokyo, Tokyo 113-8656, Japan2 Department of Applied Physics, School of Engineering, the University of Tokyo, Tokyo 113-8656, Japan
3 National Institute of Advanced Industrial Science and Technology (AIST), Osaka 563-8577, Japan
4 Center for Collaborative Research, the University of Tokyo, Tokyo 153-8904, Japan*
Email: someya@ap.t.u-tokyo.ac.jp, Phone: +81(Japan)-3-5841-6820, Fax: +81(Japan)-3-5841-6828
Abstract
A flexible, shock-resistant, and lightweight Braille sheet
display has been successfully manufactured on a plastic film
by integrating a plastic sheet actuator array with ahigh-quality organic transistor active matrix. This is the firstdemonstration, to the best of our knowledge, to integrate
plastic MEMS (microelectromechanical systems) actuatorswith organic transistor active matrices, which opens up newversatile possibilities for flexible, large-area electronic
applications including tactile displays.

1.Frame layer

2.Sheet actuator

3.Transistor film

Introduction
An organic field-effect transistor (FET) [1-6] belongs to a
new class of electronics that can be fabricated directly onplastic films at ambient temperatures; therefore it ismechanically flexible, lightweight, very thin, shock-resistant,and easy to transport. However, such functions cannot beachieved easily by the present silicon-based electronicmaterials.
In this paper, we present a detailed report on a flexible,
shock-resistant, and lightweight Braille sheet display that isFig. 2: Images of a Braille sheet display. A picture of the device
assembly. Some components are intentionally removed to show the innerstructures. A transistor film, sheet actuator, and frame covered by arubber-like surface are laminated together.

(a)Semisphere

PDMS

NafionAgPET

ParyleneS
Via(b)

Polyimide or PEN
GPolyimide
DPentacene

WL1
WL2

WL8BL1

Fig. 1: Images of a Braille sheet display. An image of the world’s firstpocket Braille sheet display. It was successfully manufactured on a plastic
film by integrating the active matrix of high-quality organic transistors with aplastic sheet actuator array based on a perfluorinated polymer electrolyte
membrane. The device is mechanically flexible, very thin, and lightweight.

BL21Fig. 3: The design. (a) A schematic cross-sectional view of the device
structure is shown. An organic transistor active matrix is laminatedwith an array of actuators using a silver paste patterned by amicrodispenser. (b) Circuit diagram of the core part of the Braille sheet
display. Each cell consists of a transistor and a rectangular actuator.
The vertical and the horizontal lines represent bit and word lines,
respectively.

0-7803-9269-8/05/$20.00 (c) 2005 IEEE

(a)

(b)

(c)
One character

Fig. 4: The organic transistor active matrix in the chip. (a) A complete view of the organic transistor active matrix sheet, which switches 12 ×
12 Braille dots. The scale is 2 cm. (b) Magnified image of an active matrix. The dotted line indicates an area of transistors that are required forswitching a Braille character consisting of 3 × 2 dots. The scale is 5 mm. (c) A further magnified image of an organic transistor. The channel length
and width are 20 µm and 49 mm, respectively. The scale is 1 mm.

(a)(b)(c)
Semisphere
Actuator

OnSurface
OffFig. 5: Plastic actuator array. (a) Picture of a 12 × 12 array of rectangular actuators composed of a perfluorinated polymer electrolyte membrane(Nafion), which is processed by a cutting machine. The scale is 2 cm. (b) One character is displayed by a 3 × 2 array of rectangular actuators (4 mmin length and 1 mm in width), which are designed to be placed alternately. A semisphere of radius 0.9 mm is attached to each actuator, which bends
and lifts the semisphere. The scale is 4 mm. (c) Principle of Braille motion.

fabricated on a plastic film for the first time, by integrating
high-quality organic FETs with plastic MEMS actuators. Anarray of rectangular plastic actuators is processed from aperfluorinated polymer electrolyte membrane. A small
semisphere that projects upwards from the rubber-likesurface of the display is attached to the tip of eachrectangular actuator. The effective display size is 4 × 4 cm2.
Each character consists of 3 × 2 Braille dots, and the total
number of dots is 144; thus, 24 characters or 6 characters × 4lines can be displayed. Pentacene FETs with top contact
geometry have a channel length of 20 µm and a mobility of
1 cm2/Vs. The Braille dots on one line are driven for 0.9 s.
The total thickness and weight of the entire device are 1 mm
and 5.3 g, respectively. The present scheme will enablepeople with visual impairments to carry the Braille sheet
display in their pockets, and read Braille e-books at anytime.
Device manufacturing process
As shown in Fig. 1 and Fig. 2, the integrated device formed
on a plastic film is mechanically flexible, very thin, andlightweight. The device structure is schematically illustrated
in Fig. 3(a). A plastic film with an organic FET active matrix

is manufactured in a clean room and then laminated with anarray of rectangular plastic actuators. The circuit diagram of
the core part of the chip is shown in Fig. 3 (b).
A. Organic FET active matrixes
A matrix of pentacene FETs with top contact geometry is
manufactured with an ultrafine shadow mask that has aspatial resolution of 20 µm, as shown in Fig. 4. The basefilm (substrate) is a poly(ethylene naphthalate) (PEN) film
or a polyimide film. Its surface is coated with a 50-nm-thickgold layer and a 5-nm-thick chromium adhesion layer in avacuum evaporator along with the shadow masks. Next, thepolyimide precursors are spin-coated and cured at 180 °C toform 240-nm-thick gate dielectric layers. A 50-nm-thickpentacene layer is deposited to form a channel layer, and a
50-nm-thick gold layer is evaporated through the shadow
asks to form the source and drain electrodes of thetransistors. The channel length L and width W are 20 µmand 49 mm, respectively. The fabricated film is subsequently
transferred to a vacuum chamber without exposure to air,and it is uniformly coated with an 8-µm-thickpoly-chloro-para-xylylene (parylene) passivation layer. Viainterconnections are made using a CO2 laser.

10-4-4V-2V0V00-2-4-6VDS (V)-8-10-IDS (A)

-200

106

10-610-7

104

10-8-1010-910-10

105-20on/off ratio

-6V

107
-30

10-5-8V-400

108(c) -40

IDS (µA)

VGS = -10V-600

IDS (µA)(b) 10-3(a)1050-50-10103

VDS = VGS = -40V
W/L = 10VDS = -10V0200
400Time (min)VGS (V)

102Fig. 6: Organic FETs. (a) A plot of IDS vs VDS of the organic transistors measured by varying the gate bias VGS from 0 V to –10 V with astep of –2 V. (b) The transfer curve measured at VDS = –10 V. The mobility is evaluated to be 1 cm2/Vs. The on/off ratio exceeds 106 if theoff current is defined as the minimum current at positive voltage bias. (c) IDS in the saturation regime of the organic transistors and on/off
ratio with a parylene passivation layer directly immersed in deionized water. The device exhibits no significant degradation over 400 min.

actuator

C. Integration of FETs and actuators

Device characteristics
We characterized organic transistors, stand-alone rectangular
plastic actuators, and integrated devices using asemiconductor parameter analyzer.
A. Organic FETs
Figure 6 (a) and (b) show typical characteristics of themanufactured p-type organic transistor. The measured
mobility is as high as 1 cm2/Vs. The on/off ratio exceeds 106

0-30.50.40.316182022Time (sec)240.5(b)Displacement (mm)The transistor film is laminated with a plastic actuator arrayusing a silver paste patterned by a microdispenser. Theselaminated electronic components were finally packed insidea plastic frame. The surface is covered by a 10-µm-thickpolydimethylsilane (PDMS) layer coated with a lubricating
fluoride to reduce friction between human fingers and therubber surface.

3(V)

(a)

VA 300-µm-thick perfluorinated polymer electrolyte
membrane (Nafion, NE-1110, Dupont), which is chemicallyplated with gold, is used as a sheet-type soft actuator. In
order to enhance displacement, high-frequency response,and actuator force, the electrolyte membrane was immersed
in a LiCl solution to replace the H+ cations in the membrane
to Li+. As shown in Fig. 5 (a) and (b), the electrolyte
membrane is processed using a numerical controlled (NC)cutting machine to achieve a 12 × 12 array of rectangularactuators, whose length is 4 mm and width is 1 mm. A
semisphere of radius 0.9 mm is attached to each rectangularactuator, which bends and lifts a semisphere up by
approximately 0.3 mm with the application of voltage
stimuli. The principle of Braille motion is shown in Fig. 5(c).

if the off current is defined as the minimum current for apositive voltage bias. Figure 6 (c) shows the electric stabilityof the organic transistors with a parylene passivation layer
directly immersed in deionized water. From the figure, it canbe concluded that it is feasible to integrate the present
organic FETs with a soft actuator operating in a wetenvironment. Indeed, this will be proven experimentally
later.

Displacement(mm)B. Plastic actuator array:

0.4
0.3
0.2
0.1
0012V (V)

3Fig. 7: Stand-alone plastic sheet actuators. (a) We characterized astand-alone rectangular actuator of dimensions 4 × 1 mm2. When aseries of rectangular waves of amplitude 3V is input, the frequency
response of the actuators extends up to 2 Hz. The voltage between
two electrodes of an actuator is shown. (b) The displacement isplotted as a function of the input voltage.

Vactuator (V)Displacement
(mm)

(a)

0

VGS = 0V-2

0.2

Displacement (mm)As shown in Fig. 8 (a), the displacement of a Braille cellconsisting of a transistor and an actuator, which isimplemented in the frame, is measured as a function of timeby inputting rectangular waves of variable voltage
amplitudes. Figure 8 (b) shows the initial rise, which isretraced from (a) and presented with a different
magnification of the horizontal axis. With an increase in thegate voltage, the rise time required to displace a surface dot
from 0 to 0.2 mm is reduced drastically, and it becomes 0.9 s
at –30 V. The velocity is limited by magnitude of IDS and
therefore a larger mobility and/or W/L is rather effective inenhancing the operation speed.

-10V

0.10V
00.2(b)C. Braille cells

VGS = -30V
-20V

0.30The present actuators can be driven in wet as well as dryenvironments; however, those in wet environment exhibit
slightly better electric and mechanical performance.

-30V
-20V
-10V

24812 16Time (sec)

-30V

-20V

20

24

0.1
-10V

D. Display operation
VGS = 0V
0012Time (sec)

3Fig. 8: Braille cells. (a) The displacement of a Braille cell consisting ofa transistor and an actuator is measured as a function of time byinputting the rectangular waves. The voltage between two electrodes ofan actuator is also measured. (b) Initial rise retraced from (a). The risetime required to displace a surface dot from 0 to 0.2 mm becomes 0.9 s
at –30 V.

B. Stand-alone plastic sheet actuators
We characterized a stand-alone rectangular actuator ofdimensions 4 × 1 mm2. When a series of rectangular waves
of amplitude 3 V is input, the frequency response of theactuators extends up to 2 Hz, as shown in Fig. 7 (a). Thedisplacement is plotted as a function of the input voltage inFig. 7 (b). In terms of the high-frequency response (~2 Hz),
low operation voltage (~3 V), and large force (~1 gf), as
demonstrated in those experiments, a perfluorinated polymer
possesses suitable electronic properties for its use as a sheet
actuator that is compatible with organic transistors.

(a)

UpFigure 9 shows the characters “l” and “w” on the Brailledisplay in the American Braille style. Magnified pictures of
the activated and the inactivated cells are also shown.
Acknowledgements
This study is partially supported by IT program, TOKUTEI(15073204), MEXT, NEDO, MPHPT, and CREST, JST. Theauthors thank Kyocera Chemical Corporation for thehigh-purity polyimide precursors.
References[1] H. Hagen, et. al., IEDM 15.2 (2004). [2] C-C. Kuo, et. al., IEDM 15.3(2003). [3] I. Kymissis, et al., IEDM 15.4 (2004). [4] A. Wang, et. al.,
IEDM 15.5 (2003). [5] T. Someya, et. al., IEDM 15.1, 580 (2004); H.
Kawaguchi, et. al., ISSCC, 365 (2005); T. Someya, et. al., IEEE Trans.
Electron Devices (the issue of November 2005). [6] T. Someya, et. al.,
IEDM, 203 (2003); PNAS 101, 9966 (2004); PNAS 102 (35) (2005);
ISSCC, 288 (2004); H. Kawaguchi, et. al., IEEE JSSC 40, 177 (2005); Y.
Kato, et. al., APL 84, 3789 (2004).

(b)

(c)
Up

Down

Up

Up

Down

Up

Down

Up

Up

Down

Down

Up

Down

Fig. 9: Display operation. Pictures of the Braille sheet display showing the characters “l” and “w” in the American Braille style. Magnified
pictures of the activated and inactivated cells are also shown. The scale bar is 1 mm.