Human-friendly organic integrated circuits Many electronic systems such as flat-panel displays, optical detectors, and sensor arrays would benefit greatly from mechanical flexibility. Ultraflexible and foldable electronics demonstrate ultimate flexibility, and are highly portable. A major obstacle toward the development of foldable electronics is the fundamental compromise between operation voltage, transistor performance, and mechanical flexibility. This review describes foldable and conformable integrated circuits based on organic thin-film transistors (TFTs) with very high mechanical stability. We review our work on such transistors and integrated circuits, that continue to operate without failure, without detectable degradation during folding of the plastic substrate. Tsuyoshi Sekitani* and Takao Someya Department of Electrical Engineering and Information Systems, School of Engineering, The University of Tokyo, Japan *E-mail:
[email protected] The technological trend in large-area electronics including displays, sensors, and actuators is that the form is thin and lightweight.
developed to reduce the operation voltage of organic transistors.
This trend is currently undergoing further developments to
In particular, Marks et al. reported self-assembled nanometer-thick
make electronics that are also bendable and rollable, i.e., flexible
dielectrics that achieved an operation voltage of 3 V18. Bao et al.
electronics. In fact, flexibility is required for larger electronics to
reported a 10 nm thick PVP dielectric layer that achieved 2 V
realize the simultaneous achievement of portability and mechanical
operation19. However, the use of these materials on glass or rigid
robustness. Utilizing various types of flexible
TFTs1-5,
a number of
substrates has been limited thus far18-21. Essentially, it is very difficult
flexible applications on plastic have been demonstrated, such as
to manufacture dielectric layers with single-nanometer-thick gates and
solar cells6, light emitting diodes7, thin-film transistors8, memory
high performance transistors on plastic because of its intrinsic surface
devices9-13,
roughness.
sensors14,
actuators15,
displays16,
and
transponders17;
however, mechanical flexibility has not been frequently evaluated.
398
Recently, high-capacitive gate dielectric materials have been
One promising approach towards the fabrication of ultraflexible
Although the mechanical flexibility of amorphous-silicone3 and
circuits with high electrical performances is micro-contact printing (such
organic transistors4 on plastic have been reported, the operation
as material transfer methods) that can transfer TFT circuits from rigid
voltages are higher than 40 V. This is because of the rough surface
to flexible and stretchable substrates after circuit fabrication1,22-26.
of the plastic films (typically 2 – 3 nm more in RMS), which
Rogers et al. reported very high-performance inorganic-based TFT
requires thick polymeric gate dielectric layers.
circuits on rubber substrates utilizing a photolithographic process and
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high precision micro-contact printing (so called, soft lithography),
> 2.5 to < 0.3 nm RMS, which is essentially identical to the surface
and then, realized flexible OLED lighting, waterproof optics, and
roughness of a polished single crystalline silicon wafer (see Fig. 1c).
multifunctional balloon catheters for bio-integrated
electronics22-25.
The gate electrodes of the transistors were prepared on the surface
Using a similar method, Ma et al. reported the fabrication of single
of the planarization layer by evaporating a nominally 20 nm thick layer
crystal silicon nanomembrane-based TFTs on a soft plastic substrate,
of aluminum through a shadow mask. A 4 nm thick layer of aluminum
and demonstrated a record frequency response of 12 GHz. Although
oxide (AlOx) was then created on the Al surface by oxygen plasma
they demonstrated simultaneous achievements of excellent electrical
treatment (10 minutes at a plasma power of 100 W). The substrate
and mechanical performances using printing, the scalability is limited
was then immersed in a 2-propanol solution of n-octadecylphosphonic
because photolithographic processes and inorganic materials were used.
acid to create a densely packed organic self assembled monolayer
Organic material based TFTs have an excellent scalability because
(SAM) with a thickness of 2 nm on the oxidized Al surface. The gate
of the low process temperatures, and since organic circuits can be fabricated directly on large scale plastic substrates using several printing technologies, such as inkjet and screen printing13,15.
(a)
(b)
In this review, we demonstrate ultraflexible circuits in the form of 2 V operation organic transistors and complementary circuits that continue to operate without degradation while being folded into a radius of 100 μm5. This exceptional flexibility and bending stability is provided by a very thin plastic substrate (12.5 μm), an atomically smooth planarization coating, and a hybrid encapsulation stack that places the transistors in a neutral strain position, because Young's modules of the bottom layer is the same as the top layer.
Device structure and fabrication A complementary electronic circuit (subsequently referred to as
(c)
CMOS) is an integrated circuit structure comprising p- and n-type transistors that complement each other. In conventional inorganic CMOSs employed in nearly all electronics, the dielectric is a silicon dioxide layer, several nanometers thick; the semiconductors consist of p- and n-type silicon, which can be modulated by doping with accepters or donors. Conventionally, these inorganic CMOSs have been predominantly fabricated by a photolithographic patterning process on rigid substrates using high temperature and high energy manufacturing methods. Owing to the miniaturization from photolithography, the state of the art resolution of CMOSs has achieved a channel length of 65 nm, allowing CMOSs to work as high definition data storage devices, central processing units, and so on; however, because of high temperature and/or high energy manufacturing methods and rigid substrates, conventional inorganic CMOSs are incompatible with large area, cost effective processes on plastic. Fig. 1a shows a photograph of a plastic sheet with organic transistors and CMOS circuits on a 12.5 μm thick polyimide substrate (UPILEX-12.5S, Ube Chemical). The device structure is schematically shown in Fig. 1b. In the first step of the fabrication process, the substrate was coated with a 500 nm thick polyimide planarization layer (selfleveling polyimide-based solutions; KEMITITE CT4112, Kyocera Chemical), which was deposited by spin-coating and cured for 5 hours at a temperature of 180 °C in nitrogen. The planarization layer reduces the surface roughness of the flexible substrate from
Fig. 1 An ultra-flexible organic CMOS circuit. (a) Photograph of an ultraflexible organic CMOS circuit. The array has an effective area of 75 × 75 mm2. The inset shows the circuits bent in half and crumpled by fingers. (b) Schematic crosssection of the transistors. The substrate is a flexible polyimide with a thickness of 12.5 μm. Gate electrodes consist of 20 nm thick evaporated aluminum. The dielectrics are each a combination of 4 nm thick aluminum oxide and a 2 nm thick self-assembled monolayer. The organic semiconductor is 50 nm thick p-type pentacene or n-type F16CuPc, and the source and drain contacts consist of 50 nm thick evaporated gold. (c) Cross-sectional electron microscopy images of a flexible transistor. The specimen was prepared with a focused ion beam and imaged by transmission electron microscopy (300 kV). Adapted from5, © 2010, Macmillan Publishers Ltd.
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Human-friendly organic integrated circuits
dielectric layer is therefore 6 nm thick in total, and has a capacitance per unit area of 0.6 – 0.65 μF/cm2
27,28
R. Schematic illustrations of inward bending are shown in Fig. 3a. Fig. 3b shows a micrograph of an organic transistor bent to a radius
.
Fifty nanometer thick layers of the organic semiconductors
of 0.15 mm. Detailed explanations including the experimental setup
pentacene (for p-channel TFTs) and F16CuPc (for n-channel TFTs)
and analysis methods of strain evaluated from R are provided in our
were then deposited by vacuum sublimation through shadow masks.
previous report30.
The source and drain contacts were prepared on top of the organic semiconductors by evaporating gold Au to a thickness of 50 nm. Finally, a polymer/metal encapsulation stack (300 nm thick parylene,
Fig. 3c shows source-drain currents (IDS) as a function of R where IDS is normalized by measurements before the bending experiment. Pentacene transistors placed at a neutral strain position (manufactured
200 nm thick gold, and 12.5 μm thick parylene) was deposited to
on a 13 μm thick plastic substrate with encapsulation layers)
protect the transistors from air-induced degradation. Consequently,
exhibit no changes in IDS and can be bent down to about 0.2 mm
all measurements reported below were carried out in ambient air.
without mechanical or electrical damage. For comparison, pentacene
Furthermore, the deposition of the 13 μm thick encapsulation stack
transistors on a 75 μm thick plastic substrate characterized in the
places the TFTs and circuits at a neutral strain position because Young's
same experiments also exhibit a decrease in IDS from 4 mm. Fig.
modulus of the bottom layer is the just same as that of the top layer.
3c shows leakage currents (IGS) as a function of R. IGS significantly
Fig. 1c shows a cross-sectional electron microscopy image of a
increases at bending radii where the decrease in IDS begins. This result
completed device. The specimen was prepared by using a focused
suggests that IDS degradation is mainly due to the destruction of gate
ion beam and imaged by transmission electron microscopy. The gate
dielectric layers. Through these bending experiments, we confirmed
electrode and the two dielectric layers (AlOx and SAM) can clearly be
that organic transistors at neutral strain positions can be bent to
distinguished in the TEM image. It should be noted that the 2 nm thick
0.2 mm, which is the smallest critical bending radius for transistors
organic SAM and 4 nm thick aluminum oxide layer are clearly resolved
reported thus far.
and atomically flat. Owing to a spin-coated planarization-polymer
In our previous reports4,30, we suggested that the effective strains
layer, the TEM image shows the surface smoothness of the SAM and
at the channel layer could be reduced by thinning the base films and
the quality of the organic/inorganic interfaces.
creating a sandwich structure between the sealant and the base film; quantitative analysis was also shown in a similar structure. When
Transistor performance
transistors lack encapsulation layers, inward bending of the base film
The transistors exhibit excellent electrical performance, as shown
induces compressive strains at channel positions and outward tensile
in Figs. 2a – d. A p-type pentacene transistor exhibits a fieldeffect
strains. Such strain-induced changes in transistor performance can be
mobility of 0.5 cm2/Vs and an on/off ratio of > 105, whereas an
understood in the context of a carrier-hopping model in polycrystalline
n-type F16CuPc transistor exhibits a fieldeffect mobility of 0.01 cm2/
thin films. On the other hand, encapsulation layers induce opposite
Vs and an on/off ratio of > 104. These characteristics are comparable
strains at channel positions. Therefore, transistors with encapsulation
to those of devices on
glass27.
The electrical characteristics are stable
layers that have precisely the same thickness as the base films should
during long term exposure to air for six months. This stability in air is
not suffer from the bending stress and should show no significant
due to the excellent gas barrier characteristics of the organic/metal
changes in transistor characteristics.
passivation layer29. transistor, a complementary inverter circuit is constructed. This
Bending organic complementary (CMOS) circuits
inverter can operate within 2 V with a gain of more than 65, indicating
Taking advantage of the ultra-flexible organic transistors with p- and
excellent inverter performances in 2 V operation (Figs. 2e and f).
n-type channels, we manufactured organic CMOS inverters and ring-
By combining a p-type pentacene transistor with an n-type F16CuPc
In order to evaluate the electrical characteristics under bending
bending strains. A photograph of the CMOS circuit at a bending radius
where the direction of source-drain current paths is precisely arranged
of 0.3 mm is shown in Fig. 4a, along with magnified pictures of the
parallel and perpendicular to the direction of strain (Fig. 3a). A
CMOS inverter and ring oscillator.
capacitor is simultaneously manufactured on the same film, and
Fig. 4b shows the electrical characteristics of 100 CMOS inverters
its capacitance is measured by varying the bending radius of the
comprising p-type pentacene and n-type F16CuPc transistors. The
base films (R) in order to obtain the precise capacitance of the gate
pentacene and F16CuPc transistors have channel widths of 800 and
dielectric layers as a function of the bending stress. Furthermore, its
4400 μm, respectively, and both transistors have a channel length of
capacitor characteristics can function as a strain
400
oscillators and characterized their performances under very large
stress, bending experiments are performed on the pentacene transistors
gauge30.
We measured
20 μm. A difference in channel length is necessary to achieve similar
the electrical properties of the transistors under various inward bending
drain currents for both transistors despite the significant difference
strains whose magnitudes were systematically controlled by changing
in carrier mobility. The inverter operates with supply voltages
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(a)
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(b)
(c)
(d)
(e)
(f)
Fig. 2 Static electrical performance of organic transistors on plastic. (a) Drain currents of p-type pentacene transistors as a function of drain-source voltage. Gate-source voltage (VGS) is increased in steps of -0.5 V. (b) Drain and gate currents of pentacene transistors as a function of gate-source voltage where the drainsource voltage (VDS) is −2 V. The observed hysteresis and leakage currents are very small. (c) Drain current of n-type F16CuPc transistors as a function of drainsource voltage. (d) Drain and gate currents of F16CuPc transistors as a function of gate-source voltage where the drain-source voltage (VDS) is −2 V. Owing to the very small thickness of SAM gate dielectrics, the operation voltage is about 2 V. (e, f) Output voltage and gain of a complementary inverter comprising an n-type pentacene TFT and n-type F16CuPc TFTs. Adapted from5, © 2010, Macmillan Publishers Ltd.
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(a)
(b)
(c)
(d)
Fig. 3 Bending experiments of organic transistors on plastic. (a) Photograph of the custom-built bending apparatus in action. Magnified pictures show a capacitor and organic transistors with geometry of applied current parallel and perpendicular to the direction of strain, respectively. The channel length and width of pentacene transistors are 50 and 500 μm, respectively. Devices are stressed using a stress apparatus with a precision mechanical stage4,24. (b) The exact bending radius was determined from digital photographs taken along the bending axis. (c) Drain-source currenta (IDS) as a function of inward bending radius where IDS is normalized by the current before bending. Bending experiments were performed on pentacene transistors located at a neutral strain position (manufactured on 12.5 μm thick plastic and encapsulated by 12.5 μm thick parylene). Red and black lines represent IDS from current parallel and perpendicular to the direction of strain, respectively. The inset shows a magnified view around 0 mm in bending radius. Pentacene transistors located at a neutral strain position can be bent down to 0.2 mm without mechanical and electrical damages. (d) Gate-source currents (IGS) as a function of bending radius. The inset shows a magnified view. IGS (leakage current) significantly increases at bending radii where IDS starts to decrease. Adapted from5, © 2010, Macmillan Publishers Ltd.
between 1 and 2 V and a small signal gain of > 40. The variation in
flexible organic CMOS circuits with a 2 V operating voltage.
inversion voltages is less than 14 %, indicating excellent performance
Furthermore, the frequency does not change even under a bending
consistency. Furthermore, the inverter can function even when
stress of 0.3 mm.
wrapped into a cylindrical bar with a 0.3 mm bending radius, as shown in Fig. 4c. A photograph and the electrical transfer characteristics of a five-
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Flexible organic circuits for practical applications
stage ring oscillator are shown in Fig. 4. The oscillation frequency
To demonstrate a potential electronic application that requires
is 22 Hz, and the signal delay is 4.5 ms, which is the fastest among
the operation of high-performance circuits folded into extremely
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(a)
(b)
(c)
(d)
Fig. 4 Ultra-flexible CMOS inverters and ring oscillators. (a) Picture of organic CMOS circuits comprising p-type pentacene and n-type F16CuPc transistors wrapped around a cylindrical bar with a radius of 0.3 mm. (b) Inverter characteristics taken from 100 inverter circuits to evaluate performance variation. This inverter can be functional for operation voltages within 2 V. In addition, the gain achieved is more than 40, indicating excellent inverter characteristics. (c) Electrical transfer characteristics of the CMOS inverter. Performance is unchanged even with a 0.3 mm bending radius. (d) Dynamic characteristics of the CMOS five-stage ring oscillators. The oscillation frequency is 22 Hz and the signal delay is 4.5 ms, even with a bending radius below 0.3 mm. Adapted from5, © 2010, Macmillan Publishers Ltd.
small bending radii, we have manufactured a very thin catheter
circuit fabrication, the membrane is wrapped around a rigid object into
that measures the spatial distribution of pressure by wrapping a
a helical structure and heated to a temperature of 150 °C; this causes
foldable transistor and sensor matrix around its surface in a helical
the membrane to permanently adopt the helical shape. The diameter of
structure.
the helix can be systematically controlled from 1 to 20 mm or larger.
First, we fabricated ultra-flexible organic transistors on a 150 μm thick liquid-crystalline polymer shape-memory membrane, as shown in Fig. 5a. The membrane is initially flat; this facilitates the fabrication of high-performance organic transistors and circuits on its surface. After
We have found that this process is compatible with the fabrication of organic transistor ICs. Second, by using a helical structure, we successfully demonstrated the implementation of organic ICs on the inner and outer surfaces of
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(a)
(b)
(c)
(d)
(e)
Fig. 5 Tightly wound transistor helix. (a) Photograph of a tightly wound transistor active-matrix array in the shape of a tightly wound helix. A TFT array fabricated on a shape-memory polymer film (Nippon Mektron, Ltd.) and permanently transformed into a helix. (b) Transfer curves of individual transistors with and without 80 % stretching. (c) An ultra-flexible active-matrix pressure sensor array as a concept of a catheter that measures the spatial distribution of pressure along its length and circumference by means of an active-matrix sensor helix. (d) Circuit diagram of the pressure sensor cell. The array was fabricated by laminating three sheets: (1) a foldable 4 × 36 array of pentacene TFTs, (2) a pressure-sensitive rubber sheet, and (3) a 12.5 μm thick polyimide sheet with an Au counter electrode. (e) Transfer characteristics of an individual sensor cell measured at two different pressures. (Note that applying pressure to the pressure-sensitive rubber sheet creates a conducting path between the source of the TFT and the counter electrode. Thus, a potential of −3 V is present on the source of the TFT only when pressure is applied, allowing the array to measure the spatial distribution of pressure.) Adapted from5, © 2010, Macmillan Publishers Ltd.
404
ultrafine rubber tubes with a diameter of 1 mm. Owing to its novel
manufactured a thin catheter that measures the spatial distribution
helical structure and great flexibility, the electrically functionalized
of mechanical pressure. The sensor was fabricated by laminating three
tube can bend around turns or corners and hence be wrapped, just
sheets: a foldable 4 × 36 array of pentacene TFTs, a pressure-sensitive
like a tube without transistors. In fact, to demonstrate a possible
rubber sheet, and a 12.5 μm thick polyimide sheet with a gold
application for organic TFTs that operate in the bent state, we
counter electrode. The picture is shown in Fig. 5. The source contacts
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(b)
(c)
Fig. 6 Organic circuits on banknotes. (a) Picture of organic transistors fabricated on a €5 note. (b) Magnified picture of an individual transistor on a banknote. (c) Performances of 92 organic transistors on banknotes, indicating excellent uniformity. Adapted from figure reproduced from35 with permission from Wiley-VCH. © 2011.
of all 144 transistors are connected to the rubber sheet, whereas
TFTs exhibit a mobility of 0.005 cm2/Vs, which is within five times that of
the counter electrode is in contact with the opposite surface of the
F16CuPc TFTs on glass27. These circuits can realize anti-counterfeiting and
rubber sheet. When mechanical pressure is exerted on the catheter,
tracking features in future banknotes.
the electrical resistance between the top and bottom surfaces of the rubber decreases. A potential of −3 V applied to the counter
Discussion
electrode is supplied to the TFTs in those positions where pressure is
We have demonstrated the first transistors that operate while folded
applied, and thus the spatial distribution of pressure can be obtained
into an extremely small bending radius of 100 μm, and the first
by interrogating the TFTs in the active-matrix array. We believe that
integrated circuits that continue to operate without any change in
this approach will lead to various kinds of new applications ranging
performance while folded into a radius of 300 μm. Although there are
from functionalization of catheters to artificial blood tubes with
many previous reports of flexible organic and inorganic transistors,
pressure sensors. To show the feasibility of the new concept, we also
the transistors in all of these studies were degraded or destroyed
demonstrated the measurement of mechanical pressure using the
when bent into a radius smaller than a few millimeters, because of
organic-transistor-based pressure sensors in this configuration.
damage by the bend-induced mechanical strain. Although transistors
Organic TFTs with a SAM gate dielectric layer are fabricated not only
that survive bending to a radius of 500 μm have been reported by
on plastics but also on fibers31 and paper32-34. Klauk et al. have fabricated
the Princeton University group9, they require relatively high operating
high-performance organic transistors and circuits on banknotes owing to
voltages (15 V), have not been tested during bending (only before
very low-temperature processes for TFT
fabrication35.
Fig. 6 shows organic
and after); and have not been incorporated into flexible circuits.
circuits on €5 banknotes and a cross-sectional TEM image. The p-type
To simultaneously achieve extreme bending stability (100 μm) and
DNTT TFTs exhibit a mobility of 0.2 cm2/Vs, whereas the n-type F16CuPc
low-voltage operation (2 V), we fabricated high-performance organic
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transistors and circuits on a very thin plastic substrate (12.5 μm thick),
the transistors in a neutral strain position where the bending-induced
dramatically reduced the substrate’s surface roughness with a 500 nm
compressive and tensile strains cancel each other out. The current-
thick planarization layer that provided an almost atomically smooth
voltage characteristics of the organic transistors and circuits do not
surface, and employed a very thin gate dielectric based on an organic
change while they are folded into a radius as small as 100 μm.
SEM. Ultimately, we deposited a 13 μm thick encapsulation layer that not only protects the devices from oxygen and humidity, but places
Although the surface smoothness at channels is crucial for achieving high-performance transistors and CMOS circuits, almost all
(a)
(b)
Fig. 7 Future concept of human-friendly organic circuits. (a) Schematic illustration of ultra-flexible large-area electronics that spread over arbitrary curved surfaces and movable parts. (b) Schematic illustration of ultraflexible sensors and lighting for medical applications. Medical sensing and treatments can be performed in blood vessels and unconventional surfaces in the human body.
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plastics have rough surfaces, leading to poor electrical performance. In the present work, transistors without the flat-polymer layealso had very rough interfaces and exhibited a mobility of 0.01
cm2/Vs
and
demonstrated large hysteresis. The atomically flat polymer formed a very smooth surface (0.3 nm in RMS) and gave high-mobility organic
Instrument citation Focused ion beam, FB-2100, Hitachi High-Technologies Corp. Transmission electron microscopy, HF-3300 Cold-FE TEM, 300 kV, Hitachi High-Technologies Corp.
transistors and high-speed organic CMOS circuits. A SAM gate dielectric is also necessary for simultaneously realizing
vibration and photo detectors could work as an electrically powered
foldability (bending radius ~0.3 mm), low-voltage operation (2 V),
artificial auris interna and retina, respectively. Given the excellent
and high electrical performance (mobility > 0.5 cm2/Vs on pentacene)
mechanical flexibility and softness, users may experience a lower
because it enables us to form a very thin and uniform single-
resistance to electronic objects in their bodies. Furthermore, if the
monolayer gate dielectric on plastics without controlling thickness.
flexible organic circuits comprise biocompatible materials, electronic
Furthermore, the molecules are packed with a sufficient density
implants could be used in the body without experiencing the rejection
(4.6 × 1014 molecules/cm2) such that SAM could be an ideal ultra-
phenomenon.
flexible insulating material. From the perspective of circuit design, CMOS circuits have
In addition to healthcare, flexible electronics will be able to expand into daily life. The flexible organic circuits fabricated onto banknotes
numerous advantages over circuits based on a single carrier type,
described above could be used as the ultimate anti-counterfeit
including greater noise margin, lower power consumption, and faster
technology and tracking system.
switching speed36. Our foldable organic CMOS can provide new
Organic integrated circuits with exceptional flexibility and bending
electronic devices with functionalities that are more resistant to shock.
stability will be used to realize new concepts in electronics and
This will allow users to carry small devices in pockets and fold up large
introduce a new era of human-friendly electronic systems.
devices to make them more convenient.
Acknowledgements Future prospects
This study was partially supported by JST/CREST, the GrantinAid for
One of the most attractive features of flexible electronics is their
Scientific Research (KAKENHI; WAKATE S), NEDO, and the Special
compatibility with living bodies. In addition to wearable electronics
Coordination Funds for Promoting and Technology. We thank Hagen
including sensors and actuators that function from outside the body,
Klauk, Ute Zschieschang (Max Planck Institute for Solid State Research),
implantable electronics that function inside the body have been
Takayasu Sakurai, and Makoto Takamiya (University of Tokyo) for
expected to play important roles in safety and securing life, especially
technical supports, sample preparation, and valuable discussion. We also
in healthcare. For example, flexible pressure and thermal sensors
thank Athene Co., Ltd for manufacturing very fine shadow masks and
could be used to realize electrically powered artificial skin, and flexible
Daisankasei Co., Ltd. for highpurity parylene (diX-SR).
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