LETTERS
PUBLISHED ONLINE: 21 JULY 2013 | DOI: 10.1038/NMAT3711
User-interactive electronic skin for instantaneous
pressure visualization
Chuan Wang
1,2,3†
, David Hwang
1,2,3
, Zhibin Yu
1,2,3
, Kuniharu Takei
1,2,3
, Junwoo Park
4
, Teresa Chen
4
,
Biwu Ma
3,4
and Ali Javey
1,2,3
*
Electronic skin (e-skin) presents a network of mechanically
flexible sensors that can conformally wrap irregular surfaces
and spatially map and quantify various stimuli
1–12
. Previous
works on e-skin have focused on the optimization of pressure
sensors interfaced with an electronic readout, whereas user
interfaces based on a human-readable output were not ex-
plored. Here, we report the first user-interactive e-skin that not
only spatially maps the applied pressure but also provides an
instantaneous visual response through a built-in active-matrix
organic light-emitting diode display with red, green and blue
pixels. In this system, organic light-emitting diodes (OLEDs)
are turned on locally where the surface is touched, and the in-
tensity of the emitted light quantifies the magnitude of the ap-
plied pressure. This work represents a system-on-plastic
4,13–17
demonstration where three distinct electronic components—
thin-film transistor, pressure sensor and OLED arrays—are
monolithically integrated over large areas on a single plastic
substrate. The reported e-skin may find a wide range of appli-
cations in interactive input/control devices, smart wallpapers,
robotics and medical/health monitoring devices.
Although both passive
6,8,12
and active-matrix
1,2,5,9
designs can
be used for enabling the predicted user-interactive e-skins, the
active-matrix design is advantageous as it minimizes signal crosstalk
and thereby offers better spatial resolution and contrast, and a
faster response. In the active-matrix backplane circuitry, each pixel
is controlled by a thin-film transistor (TFT) that acts as a switch
for addressing either current- or voltage-driven devices. Here,
we incorporate the active-matrix design into the e-skin by using
semiconductor-enriched nanotubes
18
as the channel material of the
TFTs. Carbon nanotube networks are proven to be a promising
material platform for high-performance TFTs (refs 9,17,19–21)
with high current drives needed for switching OLEDs (ref. 22). A
schematic structure of a pixel of the user-interactive e-skin with an
integrated TFT, OLED and pressure sensor is depicted in Fig. 1a.
Each pixel in the active-matrix consists of a nanotube TFT with
the drain connected to the anode of an OLED. The OLED uses
a simple bilayer structure
23
and the colour of the emitted light is
controlled by using different emissive layer materials (details in
the Methods). In this work, red, green, blue and yellow colours
are demonstrated. On top of the OLEDs, a pressure-sensitive
rubber
1,5,24,25
(PSR) is laminated, which is in electrical contact
with the cathode (that is, top contact) of the OLED at each pixel.
The top surface of the PSR is coated with a conductive silver ink
to act as the ground contact. Here, the conductivity of the PSR
increases by an applied pressure
1,5,24,25
that subsequently results
1
Department of Electrical Engineering and Computer Sciences, University of California, Berkeley, California 94720, USA,
2
Berkeley Sensor and Actuator
Center, University of California, Berkeley, California 94720, USA,
3
Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California
94720, USA,
4
The Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA.
†
Present address: Department of Electrical
and Computer Engineering, Michigan State University, East Lansing, Michigan 48824, USA. *e-mail: ajavey@eecs.berkeley.edu
in the underlying OLED turning on. As illustrated in Fig. 1b, the
single-pixel circuitry is integrated into an active-matrix array. The
resulting system-on-plastic provides a touch user interface, allowing
the pressure profile to be both spatially mapped and visually seen.
Optical micrographs of one pixel and the entire array (16×16 pixels
with a size of ∼3 × 3.5 cm
2
) of the fabricated device are shown
in Fig. 1c,d, respectively, with the latter depicting the mechanical
flexibility of the system.
Given the large number of active components needed for the
user-interactive e-skin, it is essential to use a process scheme that
leads to a high yield of uniform devices on a plastic substrate.
The active-matrix back-plane was fabricated on a polyimide-coated
Si wafer followed by OLED processing. First, a thin (∼24 µm)
polyimide layer is spin-coated and cured on a Si handling wafer.
Next, the wafer is processed by conventional microfabrication
steps, such as photolithography and metallization, to pattern and
deposit the various layers. After the completion of fabrication, the
polyimide layer with the devices on top was delaminated from
the handling wafer
5,9,17
, resulting in a highly bendable system-
on-plastic. The step-by-step details of the fabrication process
are presented in the Methods and Supplementary Fig. S1. This
fabrication approach results in highly uniform devices by using
the well-established microfabrication process technology. Although
the sample size is limited to that of the handing wafer, the
entire fabrication process is compatible with liquid-crystal-display
fabrication processes, which could allow metre-scale processing of
the proposed system in the future.
We first characterize the electrical performance of the flexible
carbon nanotube TFT active-matrix backplane. The transfer char-
acteristics (I
DS
− V
GS
) of 20 representative TFTs (L = 20 µm,
W = 2,000 µm) measured at V
DS
= −5 V are presented in Fig. 2a.
The transistors exhibit a uniform performance in terms of
on-current (I
on
), transconductance (g
m
) and mobility (Supplemen-
tary Fig. S2), which is attributed to the uniform nanotube networks
obtained using the assembly method applied in this work
9,17
. The
mean and standard deviation values for I
on
, g
m
and field-effect
mobility are 3.6 ± 0.3 mA, 1.0 ± 0.1 mS and 20 ± 2 cm
2
V
−1
s
−1
,
respectively. On the other hand, the off-state current (and thereby
the I
on
/I
off
) of the transistors exhibits a larger device-to-device
variation as depicted in Fig. 2a, ranging from ∼0.1 to 3 µA, with
an average value of 0.84 ± 0.80 µA. Although this variation is
acceptable for the system demonstration of this work, it may be
further improved in the future by minimizing contaminants in
the devices by using higher-purity nanotube samples, and per-
forming the entire device processing in a cleanroom environment.
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© 2013 Macmillan Publishers Limited. All rights reserved.
LETTERS
NATURE MATERIALS DOI: 10.1038/NMAT3711
PSR
Scan line
Scan line
PSR
Data line
Data line
OLED
ITO electrode
200 µm
5 µm
1 cm
20 µm
Laminated PSR
Data line
Al
2
O
3
/SiO
x
dielectric
Polyimide substrate
Passivation
layer
Backside of the
polyimide substrate
Flexible TFT
backplane
Pixels with
pressure applied
Emitted light
LiF/Al
ITO
Electrode
NPD
Nanotube TFT
Emissive layer
Scan line
Source
Drain
Source
Source
Source
Drain
Drain
Nanotube TFT
a
b
c d
Figure 1 | Concept and structure of the user-interactive e-skin. a, Schematic layout of a single pixel, consisting of a nanotube TFT, an OLED and a pressure
sensor (PSR) integrated vertically on a polyimide substrate. b, Schematic diagram of an array of pixels (16 × 16) functioning as an interactive e-skin,
capable of spatially mapping and visually displaying an applied pressure profile. c, Optical micrograph of a fabricated pixel before the integration of the
OLED and PSR. The drain of the TFT is connected to an ITO pad, which serves as the anode electrode for the corresponding OLED. Scanning electron
micrographs of the active area of a nanotube TFT are also shown. d, Optical photograph of a fully fabricated interactive e-skin containing 16× 16 pixels with
a size of ∼3× 3.5 cm
2
.
Furthermore, the off-state current values may be further reduced
by using higher enriched semiconducting carbon nanotube samples
(99% semiconductor-enriched used in this work). Notably, the high
current-drive of nanotube TFTs is essential for driving OLED pixels
(see Supplementary Information for more detailed analysis) and the
uniformity is critical for achieving high-yield macroscale electronic
systems. In addition, the active-matrix backplane also offers excel-
lent flexibility without degradation to the device performance as
shown in the Supplementary Fig. S3) and our previous studies
9,17
.
The characteristics of the standalone OLEDs are also charac-
terized. The electroluminescence spectra of the OLEDs with four
different colours (red, yellow, green, blue) are measured using a
spectrofluorometer and the results are plotted in Fig. 2b. By simply
changing the emissive layer material, the emitted peak wavelength
can be adjusted to 489, 523, 562 or 601 nm. The exact device
structure and materials, and the corresponding photographs of the
OLEDs with different colours, are presented in Supplementary Fig.
S4. The OLEDs’ current and brightness as a function of the applied
voltage are also measured and plotted in Fig. 2c,d, respectively.
All 4 types of OLED exhibit clear rectifying I –V characteristics
with noticeable light output at an applied bias of >3 V (current
∼10–20 µA, area = 4 mm
2
). By interconnecting the anode of an
OLED to the drain of a carbon nanotube TFT, a single-pixel
OLED control circuit is constructed. Figure 2e,f shows the transfer
and output characteristics of a single-pixel circuitry, respectively,
with the circuit schematics shown in the insets. By sweeping the
gate voltage (V
G
) or the power supply (V
DD
) of the control TFT,
the current flowing through the OLED (I
DD
) can be effectively
controlled, which translates into the modulation of the OLED
brightness, allowing the OLEDs to be turned on and off. The
transfer and output characteristics of a single-pixel circuit resemble
the ones for the nanotube transistors except that the threshold
voltage of the diode (V
th_OLED
) is now introduced into the output
characteristics (Fig. 2f) and thereby a V
DD
above 3 V is needed
to turn on the circuit.
Arranging the above-described single-pixel OLED control cir-
cuitry into a matrix results in a monolithically integrated active-
matrix OLED (AMOLED) display (Fig. 3a), which is subsequently
integrated with pressure sensors for the interactive e-skin. A photo-
graph of a flexible AMOLED display is shown in Fig. 3b. Note that
the light is emitted through the semi-transparent polyimide sub-
strate, as the top surface of the sample is covered by the aluminium
cathodes of the OLEDs. Figure 3c shows a single-colour (green)
flexible AMOLED display with −5 V and 10 V applied to all of the
scan and data lines, respectively. Such a configuration allows all of
the pixels (16×16) to be turned on, and the measured pixel yield is
more than 97%. Figure 3c also confirms that the evaporated organic
and metallic thin films in the display are durable on bending and the
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© 2013 Macmillan Publishers Limited. All rights reserved.
NATURE MATERIALS DOI: 10.1038/NMAT3711
LETTERS
10
0
10
1
L = 20 µm, W = 2,000 µm
10
¬1
10
¬2
10
¬3
10
¬4
¬5.0 ¬2.5 0.0
V
GS
(V)
V
G
(V)
¬V
DD
(V)
V
G
¬V
DD
I
DD
V
DS
= ¬5 V
10 V in 2 V step
V
DD
from 2 to
5 V in 1 V steps
V
G
from ¬5 to
¬I
DS
(mA)
2.5 5.0
1.0
0.8
0.6
0.4
Electroluminescence intensity (a.u.)
0.2
0.0
Current (mA mm
¬2
)
Wavelength (nm)
Red
Yellow
Green
Blue
Red
Yellow
Green
Blue
Red
Yellow
Green
Blue
15
12
9
6
3
0
024
Voltage (V)
6810
Voltage (V)
10
4
10
2
10
0
Brightness (Cd m
¬2
)
10
¬2
10
¬4
0246810
0
¬100
¬200
¬300
¬400
I
DD
(µA)
¬500
I
DD
(µA)
¬600
¬700
¬800
¬100
0
¬200
¬300
¬400
¬500
¬10 ¬8 ¬6
OLED on OLED offOLED on OLED off
¬4 ¬2
V
th_OLED
0
abc
def
¬5.0 ¬2.5 0.0 2.5 5.0
300 400 500 600 700 800
Figure 2 | Electrical characterization of carbon nanotube TFTs and OLEDs. a, Transfer characteristics of 20 different nanotube TFTs in the active-matrix
backplane (L = 20 µm, W = 2 mm), showing uniform device properties. b, Electroluminescence spectra of red, yellow, green and blue OLEDs (measured at
∼7 V) obtained by using (pq)
2
Ir(acac), rubrene, Alq
3
and B(Alq) as the emissive layer, respectively. c, I–V characteristics of the fabricated OLEDs with
different colours. d, Brightness as a function of applied voltage for different colour OLEDs. e,f, Transfer (e) and output characteristics (f) of a green OLED
integrated with a control nanotube TFT. The inset in e shows the circuit schematic of the pixel. The modulation of I
DD
translates into the change of the
OLED brightness so that the pixel can be turned on and off.
display remains able to function properly under various bending
conditions. Furthermore, electrical measurements of an individual
pixel under various bending radii are shown in Supplementary
Fig. S3a, depicting minimal change up to a radius of ∼4 mm.
A full-colour flexible AMOLED display (that is red, green, blue
OLED pixels monolithically integrated on the same substrate) is
also demonstrated through a multi-step evaporation of the various
emissive layers. Figure 3d,e shows the full-colour display being fully
turned on in the relaxed and bent states, respectively, with a pixel
yield of ∼85%. Each of the pixels in the display can be individually
addressed using the nanotube TFTs as depicted in Fig. 3f. Although
single-colour AMOLED displays have been demonstrated previ-
ously on rigid glass substrates using carbon nanotube transistors
22
,
this paper is the first report of a carbon nanotube-based AMOLED
display with red, green and blue colours integrated on a flexible
substrate. The work shows the utility of semiconductor-enriched
nanotube TFTs for high-performance and bendable displays.
The flexible display is subsequently used in the realization of
an interactive e-skin that is capable of spatially mapping and
simultaneously responding to the applied pressure. To make the
pixels pressure responsive, a PSR is laminated on top of the
OLEDs. In this design, the cathode of each OLED is connected
to the ground through the PSR. When pressure is applied on
the PSR, the tunnelling path between the conductive carbon
nanoparticles embedded in the rubber is shortened, resulting
in reduced resistance
1,5,24,25
. The resistance change of the PSR
modulates the current flowing through the OLED and changes the
brightness of the output light. The pressure response of a standalone
OLED with a laminated PSR is depicted in Fig. 4 and Supplementary
Fig. S5. As pressure is applied, the I–V characteristics of the device
undertake pronounced changes (Fig. 4b and Supplementary Fig.
S5b). Figure 4c and Supplementary Fig. S5c show the current (red
trace) and brightness (blue trace) of the OLEDs as a function of the
applied pressure. The results indicate that light visible by the naked
eye (>1 Cd m
−2
) is emitted when the pressure is above ∼10 kPa.
Beyond this point, both the current and brightness increase linearly
as a function of the applied pressure up to 100 kPa. From the
slope of the linear response, the sensitivity is extracted to be
∼42.7 Cd m
−2
kPa
−1
for a device area of ∼4 mm
2
. The change in
the brightness can be visually seen from the inset photographs of
an OLED in Fig. 4c.
Next, the full system is fabricated consisting of 16 × 16 pixel
arrays of nanotube TFTs, OLEDs and pressure sensors heteroge-
neously integrated on a polyimide substrate. Cross-sectional and
circuit schematics are shown in Fig. 5a,b, respectively. The process
involves the fabrication of the nanotube active-matrix backplane,
followed by OLEDs, and subsequently the lamination of the PSR on
top. The PSR is connected to each pixel through the LiF/Al cathode
of the underlying AMOLED. The top surface of the PSR is coated
with conductive silver ink as the ground electrode. During the mea-
surements, the scan and data lines are connected to −5 and 10 V,
respectively. When no pressure is applied, the resistance of the PSR
prevents the current from flowing, thereby all OLEDs are off. The
current flow increases and light is emitted locally from the OLEDs
by applying pressure. The interactive e-skin can be used to spatially
map and visually display the applied pressure profile (Fig. 5c–e and
Supplementary Movie). Here, transparent polydimethylsiloxane
(PDMS) slabs cut into different shapes are used to apply pressure
on the sensor array. In Fig. 5e, e-skin devices with green, blue
and red colour OLEDs are used to spatially map and display the
pressure applied by C-, A- and L-shaped PDMS slabs, respectively.
Applying pressure through the letter-shaped PDMS slabs produces
recognizable letters with good spatial resolution, limited by the size
of the pixels used here. As shown in Fig. 4, the brightness of each
OLED represents the magnitude of the local pressure. In addition
to the optical visualization, electrical readout can be performed by
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LETTERS
NATURE MATERIALS DOI: 10.1038/NMAT3711
Scan line
Scan line
Data line
Data lines Data lines Data lines Data lines
Data line
Data line
1 cm
Scan lines Scan lines Scan lines Scan lines
C5
¬C7 C12, C13 C12¬C15C9
a
c
f
de
b
1 cm 1 cm 1 cm
Figure 3 | Flexible full-colour AMOLED display using carbon nanotube TFTs. a, Schematic of the AMOLED display circuitry. b, Photograph of a
multi-colour AMOLED display before connecting to the supply voltage. c, Photo of a single-colour (green) AMOLED being fully turned on and bent.
Voltages of −5 and 10 V are applied to all of the scan and data lines, respectively. The pixel yield, as defined by the percentage of OLEDs in the matrix that
emit light, is higher than 97%. d, Photograph of a full-colour (red, green, blue) AMOLED display with all pixels being turned on. e, Photo of the same
full-colour display shown in d being bent. f, Photos showing different columns of a single-colour (yellow) AMOLED display being selectively turned on.
PSR
PSR
LiF/aluminium
B(Alq)
NPD
ITO
Substrate
12
1 kPa
2 kPa
5 kPa
10 kPa
20 kPa
49 kPa
98 kPa
10
6,000
5,000
4,000
3,000
2,000
Brightness (Cd m
¬2
)
1,000
0
At a bias of 10 V
Current (mA)
8
Current (mA)
6
4
2
0
1086
Voltage (V)
420
Pressure (kPa)
5 kPa 10 kPa 20 kPa 49 kPa 98 kPa
0 20 40 60 80 100
15
12
9
6
3
0
Current
Brightness
abc
Figure 4 | Pressure response of a standalone OLED with laminated PSR. a, Circuit and cross-sectional schematics of an OLED vertically integrated with a
PSR. b, I–V characteristics of the OLED and PSR combination under various applied pressures. c, The current (red trace) and brightness (blue trace) of the
OLED as a function of applied pressure at a voltage of 10 V. Inset: photographs of an OLED under various applied pressures.
monitoring the current (that is, resistance) of each pixel as shown in
Supplementary Fig. S6. A good correlation between the optical and
electrical readout of an applied pressure profile is obtained.
We have characterized the response time of a single-
pixel circuit consisting of a nanotube TFT and an OLED
(Supplementary Fig. S7). The measured response time of the
circuit is ∼1 ms, which is limited by the parasitic capacitance from
the metal interconnections and large size OLEDs used in this work.
This response time is ∼100× faster than the response time of the
PSR (∼0.1 s) as reported in our previous publication
5
. As a result,
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NATURE MATERIALS DOI: 10.1038/NMAT3711
LETTERS
Silver ink coating
V
DATA
= 10 V
V
SCAN
= ¬5 V
Polyimide substrate
1 cm
Emitted light
Pressure appli
ed
Cured photoresist
Source Drain
PSR PSR PSR
PSR
PSR
PSR
CNTs
1 cm
a
b
de
c
PSR
LiF/Al
Emissive layer
NPD
ITO
Dielectric
Gate
1 cm
Figure 5 | User-interactive e-skin. a, Cross-sectional schematic showing
one pixel of the interactive e-skin device, consisting of a nanotube TFT, an
OLED and a pressure sensor. Light from the OLED is emitted through the
substrate and the brightness of the OLED is determined by the magnitude
of the applied pressure. b, Circuit schematic of the e-skin matrix.
c, Photograph of a fabricated device (16× 16 pixels), showing that light is
locally emitted where the surface is touched. Only the pixels being pressed
are turned on. d, PDMS slabs with C, A, and L shapes are prepared and
used to apply pressure onto the sensor array. e, Green, blue and red colour
interactive e-skins are used to spatially map and display the pressure
applied with C- (left), A- (centre) and L- (right) shaped PDMS slabs,
respectively.
for the user-interactive e-skin system, the maximum operating
speed is limited by the PSR. This response time is fast enough for
most practical sensing and mapping applications. Supplementary
Fig. S8 and Movie depict the response time of the full-system under
operation by a user.
The work here presents a practical technology platform
involving the heterogeneous integration of various electronic,
sensor and light-emitting components based on both organic and
inorganic materials at a system level on thin plastic substrates.
This is the first demonstration of a user-interactive flexible system
that can not only detect and spatially map external stimuli (in
this case pressure) but also respond with a seamlessly integrated
display. This system-on-plastic demonstration enables the sensed
pressure profile to be instantaneously visible without the need
for sophisticated data acquisition circuits and electronic boards.
The pressure sensing used here presents just one example system.
In the future, integration of other sensor components
26–30
can be
predicted using a similar platform to enable more sophisticated
human–surface interfacing. We predict that the presented platform
may find a wide range of applications in automotive control
panels, interactive input devices, robotics, and medical and health
monitoring devices. Future work along this direction could involve
further improving the pixel yield and resolution, incorporation of
an encapsulation layer for the OLEDs to enhance their lifetime
(Supplementary Fig. S9), and integration of multiple types of
sensor element for multifunctional sensing
2,7
. Fully printed TFT
backplane, sensor element and polymer OLED arrays could also be
explored for covering large areas, such as walls of buildings with
interactive wallpapers using the presented e-skin concept.
Methods
Fabrication of carbon nanotube TFT active-matrix backplane. Polyimide (HD
MicroSystems, polyimide-2525) is spin-coated and cured on a silicon handling
wafer to serve as the flexible substrate. On top of the polyimide substrate, Ti/Au
(5/35 nm) back-gate electrodes (scan lines) are defined by photolithography and
lift-off. The gate dielectric consists of 60 nm of Al
2
O
3
deposited using atomic
layer deposition and 5 nm of SiO
x
deposited using electron-beam evaporation.
The wafer is then immersed into poly-l-lysine (0.1% wt in water from Sigma
Aldrich) for 5 min to functionalize the SiO
x
surface to form an amine-terminated
adhesion layer, and the sample is subsequently immersed into the commercially
available 0.01 mg ml
−1
99% semiconducting nanotube solution (NanoIntegris) for
15 min followed by deionized water and an isopropanol rinse, and blown dry with
nitrogen. After the uniform semiconducting nanotube networks are deposited, the
sample is annealed in a vacuum oven at 200
◦
C for 1 h to further clean the surface
and remove surfactant residues. Ti/Pd (0.5/40 nm) source–drain contacts (data
lines) are then formed by photolithography and a lift-off process, and the carbon
nanotube networks are confined to the channel region by patterned etching of all
other areas using oxygen plasma. Finally, 50-nm-thick indium tin oxide (ITO)
pads connected to the drain of each TFT are defined by photolithography, d.c.
sputtering and a lift-off process. The ITO pads are used as the anode electrodes for
the subsequently fabricated OLEDs.
Integration with OLEDs and PSR. After the backplane is fabricated, a photoresist
layer (thickness ∼1 µm) is coated on the surface and rectangular vias with a size
of 0.91 × 0.67 mm
2
are patterned on top of the ITO pads. The sample is then
heated to 250
◦
C in air for 30 min to anneal the sputtered ITO (to reduce the sheet
resistance) and to hard bake the photoresist to serve as an electrical insulation
layer. OLEDs with 50 nm of 4-4
0
-bis[N -(1-naphthyl)-N -phenyl-amino]biphenyl
(NPD) as the hole transport layer, 40 nm of emissive layer, 1 nm of lithium
fluoride (LiF; used to adjust the workfunction) and 100 nm of aluminium are
deposited through a shadow mask by thermal evaporation. OLED fabrication
is performed in a nitrogen-filled glovebox with oxygen and moisture levels
below 1 ppm. Different emissive layer materials are used to obtain OLEDs
with different colours: iridium(iii)bis(2-phenylquinoline-N,C2’)acetylacetonate
((pq)
2
Ir(acac)) for red, tris(8-hydroxyquinoline)aluminium (Alq
3
) for green,
aluminium(iii)bis(2-methyl-8-quinolinato)4-phenylphenolate (B(Alq)) for blue,
and rubrene for yellow. As a final step, a PSR (PCR Technical) is laminated on the
top surface to enable pressure sensing.
Received 7 January 2013; accepted 11 June 2013; published online
21 July 2013
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