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Received 10 Jul 2014 | Accepted 3 Nov 2014 | Published 10 Dec 2014
DOI: 10.1038/ncomms6745
All-organic optoelectronic sensor for pulse
oximetry
Claire M. Lochner1,*, Yasser Khan1,*, Adrien Pierre1,* & Ana C. Arias1
Pulse oximetry is a ubiquitous non-invasive medical sensing method for measuring pulse rate
and arterial blood oxygenation. Conventional pulse oximeters use expensive optoelectronic
components that restrict sensing locations to finger tips or ear lobes due to their rigid form
and area-scaling complexity. In this work, we report a pulse oximeter sensor based on organic
materials, which are compatible with flexible substrates. Green (532 nm) and red (626 nm)
organic light-emitting diodes (OLEDs) are used with an organic photodiode (OPD) sensitive
at the aforementioned wavelengths. The sensor’s active layers are deposited from solutionprocessed materials via spin-coating and printing techniques. The all-organic optoelectronic
oximeter sensor is interfaced with conventional electronics at 1 kHz and the acquired pulse
rate and oxygenation are calibrated and compared with a commercially available oximeter.
The organic sensor accurately measures pulse rate and oxygenation with errors of 1% and
2%, respectively.
1 Department
of Electrical Engineering and Computer Sciences, University of California, Berkeley, California 94720, USA. * These authors contributed equally
to this work. Correspondence and requests for materials should be addressed to A.C.A. (email: acarias@eecs.berkeley.edu).
NATURE COMMUNICATIONS | 5:5745 | DOI: 10.1038/ncomms6745 | www.nature.com/naturecommunications
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ARTICLE
NATURE COMMUNICATIONS | DOI: 10.1038/ncomms6745
C
onventional pulse oximeters non-invasively measure
human pulse rate and arterial blood oxygen saturation
with an optoelectronic sensor composed of two inorganic
light-emitting diodes (LEDs) with different peak emission
wavelengths and a single inorganic photodiode1,2. The LEDs
are placed on one side of a finger and the light transmitted
through the tissue is subsequently sensed by the photodiode that
is placed on the opposite side of the finger. Sequential sampling of
the transmitted light provides information on the ratio of
oxygenated and deoxygenated haemoglobin in the blood. This
ratio and a calibration curve are used to compute arterial blood
oxygen saturation. Currently, the application of commercially
available pulse oximeters is limited by the bulk, rigidity and
high large-area scaling cost of conventional inorganic-based
optoelectronics. Here we show a pulse oximeter sensor composed
of organic LEDs (OLEDs)3,4 and a flexible organic polymer
photodiode (OPD)5. We successfully demonstrate that the
organic optoelectronic sensor provides accurate measurement
capability and we anticipate that our application of solutionprocessable organic optoelectronics in pulse oximetry will enable
low-cost, disposable and wearable medical devices.
Wearable medical sensors have the potential to play an
essential role in the reduction of health care costs: they encourage
healthy living by providing individuals feedback on personal vital
signs and enable the facile implementation of both in-hospital
and in-home professional health monitoring. Consequently, wide
implementation of these sensors can reduce prolonged hospital
stays and cut avertible costs6. Recent reports show ample
wearable sensors capable of measuring pressure7,8, biopotential
and bioimpedance9,10, pulse rate11 and temperature12,13 in real
time. These sensors are developed in wearable and flexible form
factors using organic8,13,14, inorganic12,15,16 and hybrid organic–
inorganic7,9,15 materials.
Organic semiconductors developed for OLEDs and OPDs have
been primarily applied to display and photovoltaic technologies17,18. This is due to their potential for large-area roll-to-roll
manufacturing at large volumes, which is enabled by solution
processing and the use of flexible substrates19. These attributes
also make organic optoelectronics very attractive for medical
sensors, where flexibility combined with large areas can result in
an improvement of the overall sensor performance. In the past 10
years, a lot of resources were used to improve the stability of
organic semiconductors to meet the lifetime requirements of
displays and photovoltaics20,21. When compared with the above
markets, disposable medical sensors have less-stringent lifetime
requirements on the materials, since these devices would be used
only for a few days as opposed to years. Indeed, organic
optoelectronics have previously been used to perform pulse
measurements22–24.
Here we report a sensor composed solely of organic
optoelectronics that measures both human pulse and arterial
blood oxygenation. We anticipate that our results will inspire
system-level integration of organic–inorganic electronics, where
the large area, low cost and mechanical flexibility of organic
sensors will be combined with the computational efficiency of
inorganic electronics. A schematic view of the sensor is given in
Fig. 1a, where two OLED arrays and two OPDs are placed on
opposite sides of a finger.
Results
Pulse and oxygenation with red and green organic light
emitting diodes. In contrast to commercially available inorganic
oximetry sensors, which use red and near-infrared LEDs, we use
red and green OLEDs. Incident light from the OLEDs is attenuated by pulsating arterial blood, non-pulsating arterial blood,
2
venous blood and other tissue as depicted in Fig. 1b. When
sampled with the OPD, light absorption in the finger peaks in
systole (the heart’s contraction phase) due to large amount of
fresh arterial blood. During diastole (the heart’s relaxation phase),
reverse flow of arterial blood to the heart chambers reduces
blood volume in the sensing location, which results in a minima
in light absorption. This continuous change in arterial blood
volume translates to a pulsating signal—the human pulse.
The d.c. signal resulting from the non-pulsating arterial blood,
venous blood and tissue is subtracted from the pulsating
signal to give the amount of light absorbed by the oxygenated
and deoxygenated haemoglobin in the pulsating arterial blood.
Oxy-haemoglobin (HbO2) and deoxy-haemoglobin (Hb) have
different absorptivities at red and green wavelengths, as highlighted on the absorptivity of oxygenated and deoxygenated
haemoglobin plotted in Fig. 1c. The difference in the molar
extinction coefficient of oxygenated and deoxygenated
haemoglobin at the green wavelength is comparable to the difference at near-infrared wavelengths (800–1,000 nm) used in
conventional pulse oximeters. In addition, solution-processable
near-infrared OLED materials are not stable in air and show
overall lower efficiencies25,26. Thus, we elected to use green
OLEDs instead of near-infrared OLEDs.
Using red and green OLEDs and an OPD sensitive at visible
wavelengths (the OLEDs’ emission spectra and the OPD’s
external quantum efficiency (EQE) as a function of incident light
wavelength are plotted in Fig. 1d), blood oxygen saturation (SO2)
is quantified according to equation 1. Here, CHbO2 and CHb are
the concentrations of oxy-haemoglobin and deoxy-haemoglobin,
respectively.
SO2 ¼
CHbO2
CHbO2 þ CHb
ð1Þ
Ros, the ratio of absorbed red (Ard) and green (Agr) light,
depends on the normalized transmitted red (Tn,rd) and green
(Tn,gr) light intensities according to Beer–Lambert’s law (shown
in equation 2).
Ard ln Tn;rd
Ros ¼
¼
ð2Þ
Agr ln Tn;gr
Finally, arterial oxygen saturation (SaO2) can be calculated
using equation 3. Here, erd,Hb and egr,Hb are the molar absorptivity
of deoxy-haemoglobin at red (l ¼ 626 nm) and green (l ¼ 532
nm) wavelengths, respectively. Similarly, erd;HbO2 and egr;HbO2 are
the molar absorptivity of oxy-haemoglobin at red (l ¼ 626 nm)
and green (l ¼ 532 nm) wavelengths, respectively.
Sa O2 ðRos Þ ¼
erd;Hb egr;Hb Ros
erd;Hb erd;HbO2 þ egr;HbO2 egr;Hb Ros
ð3Þ
Organic optoelectronic oximeter components. OLED and OPD
performances are both paramount to the oximeter measurement
quality. The most important performance parameters are the
irradiance of the OLEDs’ (Fig. 2b) and the EQE at short circuit of
the OPD (Figs 1d and 3b). As the OLEDs operating voltage
increases, irradiance increases at the expense of efficiency27, as
shown by the lower slope of irradiance than current as a function
of applied voltage in Fig. 2b. For a pulse oximeter, this is an
acceptable trade-off because higher irradiance from the OLEDs
yields a strong measurement signal.
We have selected polyfluorene derivatives as the emissive layer
in our OLEDs due to their environmental stability, relatively high
efficiencies and self-assembling bulk heterojunctions that can be
tuned to emit at different wavelengths of the light spectrum4.
The green OLEDs were fabricated from a blend of poly(9,9-
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ARTICLE
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Flexible
plastic
substrate
Absorptivity (l mmol–1 cm–1)
Deoxy-hemoglobin (Hb)
Oxy-hemoglobin (HbO2)
5
0.5
0.05
1
Red OLED
Green OLED
OPD
50
0.9
Red OLED EL
0.8
OPD EQE
45
Incident light
Tsystolic
Tdiastolic
Non-pulsating
arterial blood
AC
Absorbed
light
Pulsating
arterial blood
Venous blood
DC
One cardiac cycle
Other tissue
Systole: heart muscles
contract, and pump
blood to body.
Tdiastolic
Diastol: heart muscles
relax, and blood flows
into heart chambers.
t
40
0.7
35
0.6
30
0.5
25
0.4
20
0.3
15
0.2
10
0.1
5
0
450
EQE (%)
Tsystolic
Transmitted
light
Normalized EL (a.u.)
Green OLED EL
0
550
650
750
850
Wavelength (nm)
Figure 1 | Pulse oximetry with an organic optoelectronic sensor. (a) Pulse oximetry sensor composed of two OLED arrays and two OPDs. (b) A
schematic illustration of a model for the pulse oximeter’s light transmission path through pulsating arterial blood, non-pulsating arterial
blood, venous blood and other tissues over several cardiac cycles. The a.c. and d.c. components of the blood and tissue are designated, as well as the peak
and trough of transmitted light during diastole (Tdiastolic) and systole (Tsystolic), respectively. (c) Absorptivity of oxygenated (orange solid line) and
deoxygenated (blue dashed line) haemoglobin in arterial blood as a function of wavelength. The wavelengths corresponding to the peak OLED
electroluminescence (EL) spectra are highlighted to show that there is a difference in deoxy- and oxy-haemoglobin absorptivity at the wavelengths of
interest. (d) OPD EQE (black dashed line) at short circuit, and EL spectra of red (red solid line) and green (green dashed line) OLEDs.
dioctylfluorene-co-n-(4-butylphenyl)-diphenylamine) (TFB) and
poly((9,9-dioctylfluorene-2,7-diyl)-alt-(2,1,3-benzothiadiazole-4,
8-diyl)) (F8BT). In these devices, electrons are injected into the
F8BT phase of phase-separated bulk-heterojunction active layer
while holes are injected into the TFB phase, forming excitons at
the interfaces between the two phases and recombining in the
lower energy F8BT phase for green emission28. The emission
spectrum of a representative device is shown in Fig. 1d. The red
OLED was fabricated from a tri-blend blend of TFB, F8BT and
poly((9,9-dioctylfluorene-2,7-diyl)-alt-(4,7-bis(3-hexylthiophene5-yl)-2,1,3-benzothiadiazole)-20 ,20 -diyl) (TBT) with an emission
peak of 626 nm as shown in Fig. 1d. The energy structure of the
full stack used in the fabrication of OLEDs, where ITO/
PEDOT:PSS is used as the anode, TFB as an electron-blocking
layer29 and LiF/Al as the cathode, is shown in Fig. 2a. The
physical structure of the device is provided in Supplementary
Fig. 2b. The red OLED operates similarly to the green, with the
additional step of excitonic transfer via Förster energy transfer30
to the semiconductor with the lowest energy gap in the tri-blend,
TBT, where radiative recombination occurs. The irradiance at 9 V
for both types of OLEDs, green and red, was measured to be 20.1
and 5.83 mW cm 2, respectively.
The ideal OPD for oximetry should exhibit stable operation
under ambient conditions with high EQE at the peak OLED
emission wavelengths (532 and 626 nm). A high EQE ensures the
highest possible short-circuit current, from which the pulse and
oxygenation values are derived. Poly({4,8-bis[(2-ethylhexyl)oxy]
benzo[1,2-b:4,5-b0 ]dithiophene-2,6-diyl}{3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4–b]thiophenediyl}) (PTB7) mixed with
[6,6]-phenyl C71-butyric acid methyl ester (PC71BM) is a stable
donor:acceptor bulk-heterojunction OPD system, which yields
EQE as high as 80% for spin-coated devices5. The transparent
electrode and active layer of the OPD are printed on a plastic
substrate using a surface tension-assisted blade-coating technique
recently developed and reported by Pierre et al.31 Figure 3a shows
the energy band structure of our device including the transparent
electrode (a high-conductivity/high-work-function PEDOT:PSS
bilayer) and an Al cathode. The physical device structure of the
OPD is shown in Supplementary Fig. 2d. The EQE at 532 and
626 nm is 38 and 47%, respectively, at short-circuit condition, as
shown in Fig. 1d, and the leakage current of about 1 nA cm 2 at
2 V applied reverse bias is shown in Fig 3b together with the
photocurrent when the device is illuminated with a
355 mW cm 2 light source at 640 nm.
Despite the low reverse bias leakage current shown in Fig 3b,
we chose to bias the OPD at 0 V, the short-circuit condition, to
sense low photocurrent levels. The frequency response of both the
OPD and OLEDs was also characterized, since oximetry is usually
performed at 1 kHz. The 3 dB cut-off was found to be at
frequencies higher than 10 kHz for the all-organic optoelectronic
sensor, which is significantly higher than the operational
frequency required for oximetry (Supplementary Fig. 4). Notably,
the frequency performance of the OPD is not hampered at
short circuit because the shunt capacitance of organic photodiodes decreases negligibly with reverse bias, unlike inorganic
photodiodes32.
We observed that the OLED irradiance for both red and green
wavelengths is sufficient for the transmission of light through the
finger and the signal acquired by the organic photodetector is
sufficiently high for resolving the pulsating photoplethysmogram
(PPG) signal shown in Fig. 1b. The pulse waveforms (two cardiac
cycles) generated with a combination of organic and inorganic
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ARTICLE
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TFB
PTB7
2.3 eV
3.31 eV
TBT
4.08 eV
3.4 eV
3.5 eV
Al
PC71BM
LiF/AI
3.15 eV
F8BT
4.3 eV
Conductive
PEDOT:PSS
PTB7
PEDOT:PSS
ITO
4.8 eV
5.15 eV
5.2 eV
PEDOT:PSS
5.2 eV
TFB
TBT
5.37 eV
5.3 eV
PC71BM
F8BT
6 eV
5.9 eV
1E–03
0.45
1.5E–2
0.3
0.25
1.0E–2
0.2
0.15
5.0E–3
0.1
Current density (A cm–2)
0.35
1E–04
Irradiance (W cm–2)
Green OLED current density
Reg OLED current density
Green OLED irradiance
Red OLED irradianc
0.4
Current density (A cm–2)
2.0E–2
1E–05
1E–06
1E–07
1E–08
1E–09
Dark current
1E–10
0.05
0.0E+0
0
0
2
4
6
8
10
Light current
1E–11
–2
Voltage (V)
0
1
2
Voltage (V)
Figure 2 | OLED design and performance. (a) OLED energy structure.
(b) Current density of red (red solid line) and green (green dashed line)
OLEDs and irradiance of red (red squares) and green (green triangles)
OLEDs as a function of applied voltage.
devices are shown in Fig. 4. The PPG obtained when a human
finger is illuminated by inorganic LEDs and the transmitted light
is measured with an OPD is shown in Fig. 4a. When the same
measurement is performed using OLEDs and a conventional Si
photodiode (Fig. 4b), the magnitude of the PPG signal is reduced
from 26 to 16 mVp-p for the green and 16 to 6 mVp-p for the red
due to the lower optical power of the organic LEDs compared
with their inorganic equivalent device. Finally, both OLEDs and
an OPD are used to obtain a PPG under the same experimental
conditions (Fig. 4c), yielding signal magnitudes of 3 mVp-p for
the green and 2.5 mVp-p for the red. It is clear that the magnitude
of the signal is substantially reduced with the introduction of
organic-based devices, but the PPG obtained at red and green
wavelengths yields similar shapes for all device combinations
shown in Fig. 4, which will result in similar pulse and arterial
oxygenation values. The lower signal magnitude shown by the
organic probe is compensated for by increasing the area of
devices, resulting in higher photocurrents that directly translate
into higher PPG signals, as shown in Supplementary Fig. 3a.
System design for an organic optoelectronic pulse oximeter.
The organic pulse oximetry sensor composed of two red and
green OLED arrays and an OPD (Fig. 5a) is interfaced with a
microcontroller that drives the OLEDs, measures the OPD signal
4
–1
Figure 3 | OPD design and performance. (a) OPD energy structure.
(b) Light current (red solid line) with excitation from a 640 nm,
355 mW cm 2 light source and dark current (black dashed line) as a
function of applied voltage.
and transfers the data to a computer for analysis (Fig. 5b).
The obtained signal from the OPD passes through an analogue
front end where the PPG signal is filtered and amplified. The
pulsating part of the signal yields heart rate and oxygenation
according to an empirical correction to equation 3 (details are
provided in Supplementary Methods and Supplementary Fig. 1).
The accuracy of the organic optoelectronic sensor is characterized
and calibrated by comparing pulse and oxygenation measurements taken simultaneously by the organic optoelectronic sensor
and a commercially available pulse oximeter. The resultant pulse
waveforms, pulse value, ratio of absorbed light and arterial blood
oxygen saturation from the red and near-infrared LEDs in the
inorganic oximeter and the red and green OLEDs in the organic
oximeter are shown in Fig. 5c,d, respectively. The OLEDs are
powered by a 9 V battery and the OPD is biased at 0 V. The a.c.
component of the signal (Fig. 1b) is essential for visualizing
cardiac rhythm and computing arterial blood oxygen saturation.
The OPD read-out circuit consists of two internal operational
amplifiers (Fig. 5b) in which the first stage amplifies the whole
PPG signal from the photodiode. The second stage only amplifies
the pulsating part of the signal and is read by an analogue-todigital converter (ADC). With two-stage amplification, we
obtained a 50–60 mVp-p PPG signal for the inorganic probe
(Fig. 5c) and a 3–4 mVp-p PPG signal for the organic probe
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16 mV p-p
Red
10
20
30
40
50
60
70
Signal (mV)
40
35
30
25
20
16 mV p-p
Green
34
Signal (mV)
Signal (mV)
Green
Signal (mV)
Signal (mV)
Signal (mV)
26 mV p-p
0
OLEDs + OPD
OLEDs + inorganic PD
OPD + inorganic LEDs
50
45
40
35
30
25
20
45
40
35
30
25
20
6 mV p-p
32
30
28
Red
10
Sample (count)
20
30
40
50
Sample (count)
60
70
34
33
32
31
30
34
33
32
31
30
3 mV p-p
Green
2.5 mV p-p
Red
10
20
30
40 50 60
Sample (count)
70
80
Figure 4 | PPG acquisition using combinations of inorganic and organic LEDs and photodiodes (PDs). (a) PPG signal acquired using inorganic red and
green LEDs and an OPD. Green and red PPG signal amplitudes of 26 and 16 mVp-p were obtained, respectively. (b) PPG signal acquired using OLEDs
and silicon PD—absence of lensing epoxy and reduced irradiance of the OLEDs bring down signal magnitude to 16 and 6 mVp-p for green and red
excitations. (c) PPG signal acquired using OLEDs and OPD; although signal magnitudes are reduced to 3 and 2.5 mVp-p, the signal is sufficient for resolving
the PPG waveform and provide light absorbance ratio information for arterial blood oxygenation calculation.
LED driver circuit
Red LED intensity (DAC)
UART transmission
Red LED on/off (GPIO)
Microcontroller
Red
OLED
Green
OLED
Red / green signal
(ADC)
Green LED intensity (DAC)
Green LED on/off (GPIO)
PD read circuit
Organic
oximeter probe
Red LED
Green LED
Organic pulse oximeter
probe
Ros
HR (b.p.m.) Red (mV) Green (mV)
80
75
70
65
60
99
96
93
90
0
0
50
50
100
100
150
200
150
Sample (count)
200
250
250
300
80
75
70
65
60
1
0.9
0.8
0.7
0.6
99
96
93
90
300
2nd stage Multiplexer
amplification
(AC)
34
32
30
Ros
60
40
20
0
1st Stage
amplification
(AC + DC)
34
32
30
SaO2 (%)
60
40
20
0
SaO2 (%)
HR (b.p.m.) Red (mV) Infrared (mV)
OPD
1
0.9
0.8
0.7
0.6
Photodetector
0
50
100
150
200
250
300
0
50
100
150
Sample (count)
200
250
300
Figure 5 | Organic optoelectronic pulse oximetry system. (a) Red and green OLEDs are placed on subject’s finger and transmitted light is collected with
one OPD pixel placed below the finger. (b) Hardware block diagram for the system set-up—a microcontroller acts as the data acquisition and processing
unit. OLEDs are triggered and controlled using general-purpose input/output port and DAC pins, and the OPD signal is recorded using the ADC of the
microcontroller. A two-stage amplifier between the OPD and ADC removes the d.c. part from the PPG signal and amplifies the pulsating PPG signal.
(c,d) Simultaneous oximetry measurements with a commercially available inorganic oximeter probe and the organic oximeter probe, respectively. The PPG
signal was obtained using red and infrared light for the commercially available probe (c) and using red and green light for the organic probe (d). Heart rate
(HR; magenta line in c,d) was obtained by timing the systolic peaks in the PPG signals. The ratio of the transmitted light at two wavelengths (Ros; blue
line in c,d) is converted to arterial blood oxygen saturation (SaO2; yellow line in c,d) using Beer–Lambert’s law in conjunction with an empirical correction.
(Fig. 5d). The heart rate and ratio of transmitted light at two
wavelengths (Fig. 5c,d) were calculated directly from the PPG
signals and the arterial blood oxygen saturation was derived from
the ratio of transmitted light, as discussed previously and in the
Supplementary Methods. The calculated heart rate and oxygenation derived from the PPG signals from the inorganic and
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organic probes are both 65–70 beats per minute and 94–96%,
respectively (Fig. 5c,d). We observed 1% error for pulse rate and
2% error for oxygenation when comparing the organic optoelectronic sensor with the inorganic sensor.
Motion artefacts are one possible source of error in pulse
oximetry measurements. Motion-induced errors can be minimized with signal-processing algorithms that can be found in
literature33,34. In this work, we focus mainly on organic
optoelectronic probe design and development; motion artefact
characterization and mitigation algorithms can be implemented
to further improve sensor performance.
Discussion
The novel combination of red and green OLEDs, as opposed to a
red and near-infrared LED pair, is successfully implemented in
pulse oximetry because the difference in the absorptivity of
oxygenated and deoxygenated haemoglobin at the green wavelength
is comparable to the difference at near-infrared wavelengths35 as
seen in Fig. 1c. Green LEDs have not been used conventionally in
transmission oximetry because shorter wavelengths are more
efficiently absorbed by the blood. However, the higher irradiance
of the green OLEDs (Fig. 2b) compensates for any absorption losses
in non-pulsating blood and tissue, as can be inferred from the
higher green signal amplitudes in Fig. 4 compared with the red
signal amplitudes. We employed an empirical correction to
calculate arterial blood oxygenation from the ratio of transmitted
green and red light, a scheme widely used for correcting for the
deviation from Beer–Lambert’s law (which does not account for the
scattering that occurs in human tissue) in red and near-infrared
pulse oximetry measurements.
Aside from maximizing OPD EQE and short-circuit photocurrent and OLED irradiance, the OPD’s short-circuit current
resulting from ambient light should be minimized to achieve the
best pulse oximetry signal, as parasitic photodetector current is a
contributor to conventional pulse oximetry failure36. The effects
of ambient light on the OPD’s short-circuit current were
measured using two-finger phantoms with radii of 9 and 5 mm,
representative of the wide range of human finger sizes. Flexing the
photodiode around the finger phantom, as opposed to taking the
measurement with the photodiode placed flat, non-flexed, against
the phantom, significantly reduces the parasitic short-circuit
current produced by ambient light. Under typical room-lighting
conditions of 72–76 mW cm 2, flexing the OPD around the 9 and
5 mm radii phantoms reduced the parasitic current from 270 to
20 nA and 280 to 60 nA, respectively (Supplementary Fig. 3b).
The ability of the flexible OPD to conform around the human
body therefore improves the pulse oximeter’s versatility.
The long-term stability of the organic optoelectronic pulse
oximeter, like most organic optoelectronics, is limited by the
robustness of the encapsulation technology employed in its
fabrication37,38. It has been shown that lifetime of organic
optoelectronics can be significantly improved using robust
encapsulation and packaging. With our encapsulation process,
we see a 24% signal intensity decrease in the green and a 54%
decrease in the red PPG signal over a 7-day time frame.
Supplementary Figure 5 shows a decline in signal intensity;
however, the PPG signal shapes are intact.
The organic optoelectronic pulse oximetry sensor described
here demonstrates the potential for the application of organic
electronics to thrive in the medical device field. The large-area
scalability, inexpensive processing and flexibility of organic
optoelectronics will allow medical sensors to be made in new
shapes and sizes, diversifying possible sensing locations on the
human body, enabling medical professionals to better monitor
their patients’ care.
6
Methods
OLED fabrication and characterization. The semiconducting polymers used in
the emissive layer of the OLEDs were supplied by Cambridge Display Technology
Limited. The red OLED active layer was made from a 25:70:5 blend of
TFB:F8BT:TBT in a 10 mg ml 1 o-xylene solution. The green OLED active layer
was made from a 1:9 blend of TFB:F8BT in a 10 mg ml 1 o-xylene solution.
Patterned ITO substrates were cleaned via sonication in acetone and then isopropyl
alcohol. The substrate surfaces were made hydrophilic with a 2 min plasma
treatment before spincoating a 40 nm layer of Clevios PEDOT:PSS AI4083. Any
remaining moisture was evaporated in a 10 min annealing step at 120 °C before
moving the samples into a nitrogen glove box for the remainder of the fabrication
procedure. TFB was spin coated from a 10 mg ml 1 o-xylene solution and then
annealed at 180 °C for 45 min before cooling and spin rinsing with o-xylene,
producing a 10–20 nm-thick electron-blocking layer. The active layer was then
spun at 4500 r.p.m. for a 100 nm film thickness. The LiF (1 nm)/Al (100 nm)
cathode was thermally evaporated under vacuum at 4 10 6 Torr. Finished
devices were encapsulated with ultraviolet-curable Delo Katiobond LP612 epoxy
and clean quartz glass. Each OLED pixel had 4 mm2 of active area. OLED current/
voltage characteristics and irradiance measurements were taken with an Orb
Optronix light measurement system complete with an Orb Optronix SP-50
spectrometer, integrating sphere, Keithley 2400 Source Meter and Spectral Suite
3.0 software.
OPD fabrication and characterization. OPDs were printed on top of planarized
polyethylene naphthalate (PEN) substrates (DuPont) using a blade-coating technique previously reported31. A layer of high-conductivity PEDOT:PSS (SigmaAldrich, 739316-25G) was printed by blade coating (200 mm blade height at
1.6 cm s 1) the solution over a large hydrophilic strip in the substrate defined by a
10 s plasma treatment through a stencil. Following a 10 min anneal at 120 °C, a
layer of high-work-function PEDOT:PSS (Clevios Al4083) was coated and
annealed over the previous print using the same process. The active layer ink
comprised of a 1:1 weight ratio of PTB7:PC71BM (Solaris Chem) dissolved to
35 mg ml 1 in chlorobenzene with a 3 vol.% concentration of 1,8-diiodooctane
and was blade coated (350 mm blade height at 1.6 cm s 1) in a glove box with the
substrate heated to 40 °C. The aluminum cathode (100 nm) was thermally
evaporated under vacuum at 4 10 6 Torr to yield an active area of 21 mm2.
Finished devices were encapsulated with ultraviolet-curable Delo Katiobond LP612
epoxy and Saran wrap after being post-annealed at 120 °C for 10 min. All OLED
and OPD layer thicknesses were measured with a Dektak Profilometer.
Electronic hardware and software for data acquisition and processing. The
Texas Instruments MSP430 microcontroller was chosen for data acquisition and
processing because of its built-in ADCs and digital-to-analogue converters (DACs),
which are required for the pulse oximeter. General-purpose input–output pins
from the microcontroller control LED switching, ADCs are utilized to read the
amplified OPD signal from the multiplexer, and DACs are used to control LED
intensity and in the signal amplification stage. The LEDs are operated in a
sequential approach, so that only one of the LEDs is on at a particular moment.
Five hundred and twelve samples are taken from each of the LEDs per second.
A software trigger from the microcontroller controls a PNP bipolar junction
transistor (BJT) switch that triggers the LED on/off. In addition, DACs are used to
control the drive current for the LEDs using a NPN transistor. For ensuring
compatibility with the organic LEDs, signals from the microcontroller are shifted to
9 V using general-purpose operational amplifiers. Finally, universal asynchronous
receiver/transmitter (UART) protocol is used to send processed data to a computer
for visualization. We opted for a modular approach by separating the LED driver
circuit and OPD read circuit, simplifying circuit design and debugging. Hamamatsu Large Area Photodiode S1227-1010BQ (active area of 35 mm2) and 5 mm
red and green round LEDs were used in PPG data comparisons with the organic
devices.
All-pulse oximetry experiments performed on human subjects were carried out
with informed consent under the approval of the University of California, Berkeley
Institutional Review Board, protocol ID number 2013-03-6081.
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NATURE COMMUNICATIONS | 5:5745 | DOI: 10.1038/ncomms6745 | www.nature.com/naturecommunications
& 2014 Macmillan Publishers Limited. All rights reserved.
ARTICLE
NATURE COMMUNICATIONS | DOI: 10.1038/ncomms6745
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Acknowledgements
This work was supported in part by the National Science Foundation under Cooperative
Agreements No. ECCS-1202189 and UTA-12000944, ARL W911NF-09-3-001 under
RFP 12-159 and by the NSF Graduate Fellowship Research Program under Grant No.
DGE-1106400. We thank Cambridge Display Technology Limited (CDT) for supplying
OLED materials, and Dr Mozziyar Etemadi for helpful technical discussions.
Author contributions
A.C.A., C.M.L., A.P. and Y.K. conceptualized the work. C.M.L. and A.P. carried out
device fabrication and characterization of the OLEDs and OPDs, respectively, and
experimental set-ups. Y.K. designed and implemented the oximeter system and worked
on software and hardware programming. C.M.L., A.P. and Y.K. volunteered as subjects
and collected pulse and oxygenation results. All authors discussed the results and
commented on the manuscript.
Additional information
Supplementary Information accompanies this paper at http://www.nature.com/
naturecommunications
Competing financial interests: The authors declare no competing financial interests.
Reprints and permission information is available online at http://npg.nature.com/
reprintsandpermissions/
How to cite this article: Lochner, C. M. et al. All-organic optoelectronic sensor for pulse
oximetry. Nat. Commun. 5:5745 doi: 10.1038/ncomms6745 (2014).
NATURE COMMUNICATIONS | 5:5745 | DOI: 10.1038/ncomms6745 | www.nature.com/naturecommunications
& 2014 Macmillan Publishers Limited. All rights reserved.
7
Task 1 (Read)
Read the Lochner paper about pulse oxymetry and Inkjet Technologies
Read the file called notes_Lit.pdf from the book “Product Design and Development:
Fifth Edition” by Ulrich and Eppinger
Task 2 (Write) (4 pages)
Page 1
Find 5 references each of the following, as most relevant to the above paper (Lochner)
o Scientific literature and journals (5 references)
o patents (5 references)
For each reference, write 1-2 sentence justifying how it is related to the project
Notes:
o
o
Feel free to start by using their references but eventually you MUST search using
Google Scholars, Web of Science, etc.
Look for as relevant of literate based on the following keywords: pulse oxymetry,
inkjet, OLED, plastic substrate
Page 2
Summary of your literature review of the scientific literature and journals (especially look at
inkjet related patents for medical devices and pulse oxymetry)
Page 3
Summary of your literature review of patents on this subject (especially look at inkjet
related patents for medical devices and pulse oxymetry)
Page 4
Discuss the importance of this exercise for this project, for these design constraints and
reference notes_Lit.pdf as much as possible. Apply as much of what is discussed in
notes_Lit.pdf to the design idea of inkjet printed pulse oxymetry
Task 1 (Read)
Read the Lochner paper about pulse oxymetry and Inkjet Technologies
Read the file called notes_interviews.pdf from the book “Product Design and
Development: Fifth Edition” by Ulrich and Eppinger
Task 2:
Write (1 page—350 words)
Write an interview with Lead User (like a consumer or patient) about what they would
want from a printable pulse oxymeter and what would be important to them
Task 3:
Write (1 page—350 words)
Write an interview with expert (like a developer of medical product) about what they
would want from a printable pulse oxymeter and what would be important to them
Task 4:
Write (1 page—350 words)
Discuss the importance of this exercise for this project, for these design constraints and
reference notes_interview.pdf as much as possible. Apply as much of what is discussed in
notes_interview.pdf to the design idea of inkjet printed pulse oxymetry
Task 1 (Read)
Read the Lochner paper about pulse oxymetry and Inkjet Technologies
Study this table:
Start Date
Task
Duration
9/23
Researching components and vendors
3 weeks
10/14
Ordering Items
1 day
10/17
Creating the control circuit
4 days
10/21
Testing the sensors
2 days
11/10
Making a built-in method of calibration.
4 days
11/ 16
Microcontroller selection
1 day
11/19
Power testing and selection
2 days
11/20
building the light transmitter
4 days
11/29
Building the detection circuit
4 days
12/1
Pulse Oximeter Inspection Procedure
1 week
12/4
Presentation
2 days
Visit these 4 links
http://www.freescale.com/applications/medical-healthcare/health-andwellness/pulse-oximetry:APLPOX
http://www.microchip.com/pagehandler/enus/promo/pulseoximeterdemo/home.html
http://www.ti.com/solution/pulse-oximetry-diagram
http://www.sigmaaldrich.com/materials-science/material-scienceproducts.html?TablePage=9548901
Task 2
Write (500 words):
Discussion the current status of your technical efforts based on the outlined
schedule above.
o Difficulties with understanding inkjet technologies
o Not enough experience with material science to know how to create the
product needed
Write (500 words):
Discussion the current status of your project management efforts based on the
outlined schedule above.
o Difficulties in estimating time
o Difficulties in estimating budget
o Discuss that now that a project mentor has been found, it has been much
easier to get through the electronics and data acquisition part of the
project because a microcontroller selection is now a lot easier with a
mentor who is electrical engineering in nature
Task 1 (Read)
Read the Lochner paper about pulse oxymetry and Inkjet Technologies
Read the file called ch7.pdf from the book “Product Design and Development: Fifth
Edition” by Ulrich and Eppinger
Read the file called ch16.pdf from the book “Product Design and Development: Fifth
Edition” by Ulrich and Eppinger
Task 2:
Write (250 words)
Summary of Ch7.pdf
Task 3:
Write (250 words)
Discuss the importance and implications of Ch7.pdf concepts in the future of being able
to print a pulse oxymeter specifically as described in Lochner
Task 4:
Write (250 words)
Summary of Ch16.pdf
Task 5:
Write (250 words)
Discuss the importance and implications of Ch16.pdf concepts in the future of being able
to print a pulse oxymeter specifically as described in Lochner
ARTICLE
Received 10 Jul 2014 | Accepted 3 Nov 2014 | Published 10 Dec 2014
DOI: 10.1038/ncomms6745
All-organic optoelectronic sensor for pulse
oximetry
Claire M. Lochner1,*, Yasser Khan1,*, Adrien Pierre1,* & Ana C. Arias1
Pulse oximetry is a ubiquitous non-invasive medical sensing method for measuring pulse rate
and arterial blood oxygenation. Conventional pulse oximeters use expensive optoelectronic
components that restrict sensing locations to finger tips or ear lobes due to their rigid form
and area-scaling complexity. In this work, we report a pulse oximeter sensor based on organic
materials, which are compatible with flexible substrates. Green (532 nm) and red (626 nm)
organic light-emitting diodes (OLEDs) are used with an organic photodiode (OPD) sensitive
at the aforementioned wavelengths. The sensor’s active layers are deposited from solutionprocessed materials via spin-coating and printing techniques. The all-organic optoelectronic
oximeter sensor is interfaced with conventional electronics at 1 kHz and the acquired pulse
rate and oxygenation are calibrated and compared with a commercially available oximeter.
The organic sensor accurately measures pulse rate and oxygenation with errors of 1% and
2%, respectively.
1 Department
of Electrical Engineering and Computer Sciences, University of California, Berkeley, California 94720, USA. * These authors contributed equally
to this work. Correspondence and requests for materials should be addressed to A.C.A. (email: acarias@eecs.berkeley.edu).
NATURE COMMUNICATIONS | 5:5745 | DOI: 10.1038/ncomms6745 | www.nature.com/naturecommunications
& 2014 Macmillan Publishers Limited. All rights reserved.
1
ARTICLE
NATURE COMMUNICATIONS | DOI: 10.1038/ncomms6745
C
onventional pulse oximeters non-invasively measure
human pulse rate and arterial blood oxygen saturation
with an optoelectronic sensor composed of two inorganic
light-emitting diodes (LEDs) with different peak emission
wavelengths and a single inorganic photodiode1,2. The LEDs
are placed on one side of a finger and the light transmitted
through the tissue is subsequently sensed by the photodiode that
is placed on the opposite side of the finger. Sequential sampling of
the transmitted light provides information on the ratio of
oxygenated and deoxygenated haemoglobin in the blood. This
ratio and a calibration curve are used to compute arterial blood
oxygen saturation. Currently, the application of commercially
available pulse oximeters is limited by the bulk, rigidity and
high large-area scaling cost of conventional inorganic-based
optoelectronics. Here we show a pulse oximeter sensor composed
of organic LEDs (OLEDs)3,4 and a flexible organic polymer
photodiode (OPD)5. We successfully demonstrate that the
organic optoelectronic sensor provides accurate measurement
capability and we anticipate that our application of solutionprocessable organic optoelectronics in pulse oximetry will enable
low-cost, disposable and wearable medical devices.
Wearable medical sensors have the potential to play an
essential role in the reduction of health care costs: they encourage
healthy living by providing individuals feedback on personal vital
signs and enable the facile implementation of both in-hospital
and in-home professional health monitoring. Consequently, wide
implementation of these sensors can reduce prolonged hospital
stays and cut avertible costs6. Recent reports show ample
wearable sensors capable of measuring pressure7,8, biopotential
and bioimpedance9,10, pulse rate11 and temperature12,13 in real
time. These sensors are developed in wearable and flexible form
factors using organic8,13,14, inorganic12,15,16 and hybrid organic–
inorganic7,9,15 materials.
Organic semiconductors developed for OLEDs and OPDs have
been primarily applied to display and photovoltaic technologies17,18. This is due to their potential for large-area roll-to-roll
manufacturing at large volumes, which is enabled by solution
processing and the use of flexible substrates19. These attributes
also make organic optoelectronics very attractive for medical
sensors, where flexibility combined with large areas can result in
an improvement of the overall sensor performance. In the past 10
years, a lot of resources were used to improve the stability of
organic semiconductors to meet the lifetime requirements of
displays and photovoltaics20,21. When compared with the above
markets, disposable medical sensors have less-stringent lifetime
requirements on the materials, since these devices would be used
only for a few days as opposed to years. Indeed, organic
optoelectronics have previously been used to perform pulse
measurements22–24.
Here we report a sensor composed solely of organic
optoelectronics that measures both human pulse and arterial
blood oxygenation. We anticipate that our results will inspire
system-level integration of organic–inorganic electronics, where
the large area, low cost and mechanical flexibility of organic
sensors will be combined with the computational efficiency of
inorganic electronics. A schematic view of the sensor is given in
Fig. 1a, where two OLED arrays and two OPDs are placed on
opposite sides of a finger.
Results
Pulse and oxygenation with red and green organic light
emitting diodes. In contrast to commercially available inorganic
oximetry sensors, which use red and near-infrared LEDs, we use
red and green OLEDs. Incident light from the OLEDs is attenuated by pulsating arterial blood, non-pulsating arterial blood,
2
venous blood and other tissue as depicted in Fig. 1b. When
sampled with the OPD, light absorption in the finger peaks in
systole (the heart’s contraction phase) due to large amount of
fresh arterial blood. During diastole (the heart’s relaxation phase),
reverse flow of arterial blood to the heart chambers reduces
blood volume in the sensing location, which results in a minima
in light absorption. This continuous change in arterial blood
volume translates to a pulsating signal—the human pulse.
The d.c. signal resulting from the non-pulsating arterial blood,
venous blood and tissue is subtracted from the pulsating
signal to give the amount of light absorbed by the oxygenated
and deoxygenated haemoglobin in the pulsating arterial blood.
Oxy-haemoglobin (HbO2) and deoxy-haemoglobin (Hb) have
different absorptivities at red and green wavelengths, as highlighted on the absorptivity of oxygenated and deoxygenated
haemoglobin plotted in Fig. 1c. The difference in the molar
extinction coefficient of oxygenated and deoxygenated
haemoglobin at the green wavelength is comparable to the difference at near-infrared wavelengths (800–1,000 nm) used in
conventional pulse oximeters. In addition, solution-processable
near-infrared OLED materials are not stable in air and show
overall lower efficiencies25,26. Thus, we elected to use green
OLEDs instead of near-infrared OLEDs.
Using red and green OLEDs and an OPD sensitive at visible
wavelengths (the OLEDs’ emission spectra and the OPD’s
external quantum efficiency (EQE) as a function of incident light
wavelength are plotted in Fig. 1d), blood oxygen saturation (SO2)
is quantified according to equation 1. Here, CHbO2 and CHb are
the concentrations of oxy-haemoglobin and deoxy-haemoglobin,
respectively.
SO2 ¼
CHbO2
CHbO2 þ CHb
ð1Þ
Ros, the ratio of absorbed red (Ard) and green (Agr) light,
depends on the normalized transmitted red (Tn,rd) and green
(Tn,gr) light intensities according to Beer–Lambert’s law (shown
in equation 2).
Ard ln Tn;rd
Ros ¼
¼
ð2Þ
Agr ln Tn;gr
Finally, arterial oxygen saturation (SaO2) can be calculated
using equation 3. Here, erd,Hb and egr,Hb are the molar absorptivity
of deoxy-haemoglobin at red (l ¼ 626 nm) and green (l ¼ 532
nm) wavelengths, respectively. Similarly, erd;HbO2 and egr;HbO2 are
the molar absorptivity of oxy-haemoglobin at red (l ¼ 626 nm)
and green (l ¼ 532 nm) wavelengths, respectively.
Sa O2 ðRos Þ ¼
erd;Hb egr;Hb Ros
erd;Hb erd;HbO2 þ egr;HbO2 egr;Hb Ros
ð3Þ
Organic optoelectronic oximeter components. OLED and OPD
performances are both paramount to the oximeter measurement
quality. The most important performance parameters are the
irradiance of the OLEDs’ (Fig. 2b) and the EQE at short circuit of
the OPD (Figs 1d and 3b). As the OLEDs operating voltage
increases, irradiance increases at the expense of efficiency27, as
shown by the lower slope of irradiance than current as a function
of applied voltage in Fig. 2b. For a pulse oximeter, this is an
acceptable trade-off because higher irradiance from the OLEDs
yields a strong measurement signal.
We have selected polyfluorene derivatives as the emissive layer
in our OLEDs due to their environmental stability, relatively high
efficiencies and self-assembling bulk heterojunctions that can be
tuned to emit at different wavelengths of the light spectrum4.
The green OLEDs were fabricated from a blend of poly(9,9-
NATURE COMMUNICATIONS | 5:5745 | DOI: 10.1038/ncomms6745 | www.nature.com/naturecommunications
& 2014 Macmillan Publishers Limited. All rights reserved.
ARTICLE
NATURE COMMUNICATIONS | DOI: 10.1038/ncomms6745
Flexible
plastic
substrate
Absorptivity (l mmol–1 cm–1)
Deoxy-hemoglobin (Hb)
Oxy-hemoglobin (HbO2)
5
0.5
0.05
1
Red OLED
Green OLED
OPD
50
0.9
Red OLED EL
0.8
OPD EQE
45
Incident light
Tsystolic
Tdiastolic
Non-pulsating
arterial blood
AC
Absorbed
light
Pulsating
arterial blood
Venous blood
DC
One cardiac cycle
Other tissue
Systole: heart muscles
contract, and pump
blood to body.
Tdiastolic
Diastol: heart muscles
relax, and blood flows
into heart chambers.
t
40
0.7
35
0.6
30
0.5
25
0.4
20
0.3
15
0.2
10
0.1
5
0
450
EQE (%)
Tsystolic
Transmitted
light
Normalized EL (a.u.)
Green OLED EL
0
550
650
750
850
Wavelength (nm)
Figure 1 | Pulse oximetry with an organic optoelectronic sensor. (a) Pulse oximetry sensor composed of two OLED arrays and two OPDs. (b) A
schematic illustration of a model for the pulse oximeter’s light transmission path through pulsating arterial blood, non-pulsating arterial
blood, venous blood and other tissues over several cardiac cycles. The a.c. and d.c. components of the blood and tissue are designated, as well as the peak
and trough of transmitted light during diastole (Tdiastolic) and systole (Tsystolic), respectively. (c) Absorptivity of oxygenated (orange solid line) and
deoxygenated (blue dashed line) haemoglobin in arterial blood as a function of wavelength. The wavelengths corresponding to the peak OLED
electroluminescence (EL) spectra are highlighted to show that there is a difference in deoxy- and oxy-haemoglobin absorptivity at the wavelengths of
interest. (d) OPD EQE (black dashed line) at short circuit, and EL spectra of red (red solid line) and green (green dashed line) OLEDs.
dioctylfluorene-co-n-(4-butylphenyl)-diphenylamine) (TFB) and
poly((9,9-dioctylfluorene-2,7-diyl)-alt-(2,1,3-benzothiadiazole-4,
8-diyl)) (F8BT). In these devices, electrons are injected into the
F8BT phase of phase-separated bulk-heterojunction active layer
while holes are injected into the TFB phase, forming excitons at
the interfaces between the two phases and recombining in the
lower energy F8BT phase for green emission28. The emission
spectrum of a representative device is shown in Fig. 1d. The red
OLED was fabricated from a tri-blend blend of TFB, F8BT and
poly((9,9-dioctylfluorene-2,7-diyl)-alt-(4,7-bis(3-hexylthiophene5-yl)-2,1,3-benzothiadiazole)-20 ,20 -diyl) (TBT) with an emission
peak of 626 nm as shown in Fig. 1d. The energy structure of the
full stack used in the fabrication of OLEDs, where ITO/
PEDOT:PSS is used as the anode, TFB as an electron-blocking
layer29 and LiF/Al as the cathode, is shown in Fig. 2a. The
physical structure of the device is provided in Supplementary
Fig. 2b. The red OLED operates similarly to the green, with the
additional step of excitonic transfer via Förster energy transfer30
to the semiconductor with the lowest energy gap in the tri-blend,
TBT, where radiative recombination occurs. The irradiance at 9 V
for both types of OLEDs, green and red, was measured to be 20.1
and 5.83 mW cm 2, respectively.
The ideal OPD for oximetry should exhibit stable operation
under ambient conditions with high EQE at the peak OLED
emission wavelengths (532 and 626 nm). A high EQE ensures the
highest possible short-circuit current, from which the pulse and
oxygenation values are derived. Poly({4,8-bis[(2-ethylhexyl)oxy]
benzo[1,2-b:4,5-b0 ]dithiophene-2,6-diyl}{3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4–b]thiophenediyl}) (PTB7) mixed with
[6,6]-phenyl C71-butyric acid methyl ester (PC71BM) is a stable
donor:acceptor bulk-heterojunction OPD system, which yields
EQE as high as 80% for spin-coated devices5. The transparent
electrode and active layer of the OPD are printed on a plastic
substrate using a surface tension-assisted blade-coating technique
recently developed and reported by Pierre et al.31 Figure 3a shows
the energy band structure of our device including the transparent
electrode (a high-conductivity/high-work-function PEDOT:PSS
bilayer) and an Al cathode. The physical device structure of the
OPD is shown in Supplementary Fig. 2d. The EQE at 532 and
626 nm is 38 and 47%, respectively, at short-circuit condition, as
shown in Fig. 1d, and the leakage current of about 1 nA cm 2 at
2 V applied reverse bias is shown in Fig 3b together with the
photocurrent when the device is illuminated with a
355 mW cm 2 light source at 640 nm.
Despite the low reverse bias leakage current shown in Fig 3b,
we chose to bias the OPD at 0 V, the short-circuit condition, to
sense low photocurrent levels. The frequency response of both the
OPD and OLEDs was also characterized, since oximetry is usually
performed at 1 kHz. The 3 dB cut-off was found to be at
frequencies higher than 10 kHz for the all-organic optoelectronic
sensor, which is significantly higher than the operational
frequency required for oximetry (Supplementary Fig. 4). Notably,
the frequency performance of the OPD is not hampered at
short circuit because the shunt capacitance of organic photodiodes decreases negligibly with reverse bias, unlike inorganic
photodiodes32.
We observed that the OLED irradiance for both red and green
wavelengths is sufficient for the transmission of light through the
finger and the signal acquired by the organic photodetector is
sufficiently high for resolving the pulsating photoplethysmogram
(PPG) signal shown in Fig. 1b. The pulse waveforms (two cardiac
cycles) generated with a combination of organic and inorganic
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NATURE COMMUNICATIONS | DOI: 10.1038/ncomms6745
TFB
PTB7
2.3 eV
3.31 eV
TBT
4.08 eV
3.4 eV
3.5 eV
Al
PC71BM
LiF/AI
3.15 eV
F8BT
4.3 eV
Conductive
PEDOT:PSS
PTB7
PEDOT:PSS
ITO
4.8 eV
5.15 eV
5.2 eV
PEDOT:PSS
5.2 eV
TFB
TBT
5.37 eV
5.3 eV
PC71BM
F8BT
6 eV
5.9 eV
1E–03
0.45
1.5E–2
0.3
0.25
1.0E–2
0.2
0.15
5.0E–3
0.1
Current density (A cm–2)
0.35
1E–04
Irradiance (W cm–2)
Green OLED current density
Reg OLED current density
Green OLED irradiance
Red OLED irradianc
0.4
Current density (A cm–2)
2.0E–2
1E–05
1E–06
1E–07
1E–08
1E–09
Dark current
1E–10
0.05
0.0E+0
0
0
2
4
6
8
10
Light current
1E–11
–2
Voltage (V)
0
1
2
Voltage (V)
Figure 2 | OLED design and performance. (a) OLED energy structure.
(b) Current density of red (red solid line) and green (green dashed line)
OLEDs and irradiance of red (red squares) and green (green triangles)
OLEDs as a function of applied voltage.
devices are shown in Fig. 4. The PPG obtained when a human
finger is illuminated by inorganic LEDs and the transmitted light
is measured with an OPD is shown in Fig. 4a. When the same
measurement is performed using OLEDs and a conventional Si
photodiode (Fig. 4b), the magnitude of the PPG signal is reduced
from 26 to 16 mVp-p for the green and 16 to 6 mVp-p for the red
due to the lower optical power of the organic LEDs compared
with their inorganic equivalent device. Finally, both OLEDs and
an OPD are used to obtain a PPG under the same experimental
conditions (Fig. 4c), yielding signal magnitudes of 3 mVp-p for
the green and 2.5 mVp-p for the red. It is clear that the magnitude
of the signal is substantially reduced with the introduction of
organic-based devices, but the PPG obtained at red and green
wavelengths yields similar shapes for all device combinations
shown in Fig. 4, which will result in similar pulse and arterial
oxygenation values. The lower signal magnitude shown by the
organic probe is compensated for by increasing the area of
devices, resulting in higher photocurrents that directly translate
into higher PPG signals, as shown in Supplementary Fig. 3a.
System design for an organic optoelectronic pulse oximeter.
The organic pulse oximetry sensor composed of two red and
green OLED arrays and an OPD (Fig. 5a) is interfaced with a
microcontroller that drives the OLEDs, measures the OPD signal
4
–1
Figure 3 | OPD design and performance. (a) OPD energy structure.
(b) Light current (red solid line) with excitation from a 640 nm,
355 mW cm 2 light source and dark current (black dashed line) as a
function of applied voltage.
and transfers the data to a computer for analysis (Fig. 5b).
The obtained signal from the OPD passes through an analogue
front end where the PPG signal is filtered and amplified. The
pulsating part of the signal yields heart rate and oxygenation
according to an empirical correction to equation 3 (details are
provided in Supplementary Methods and Supplementary Fig. 1).
The accuracy of the organic optoelectronic sensor is characterized
and calibrated by comparing pulse and oxygenation measurements taken simultaneously by the organic optoelectronic sensor
and a commercially available pulse oximeter. The resultant pulse
waveforms, pulse value, ratio of absorbed light and arterial blood
oxygen saturation from the red and near-infrared LEDs in the
inorganic oximeter and the red and green OLEDs in the organic
oximeter are shown in Fig. 5c,d, respectively. The OLEDs are
powered by a 9 V battery and the OPD is biased at 0 V. The a.c.
component of the signal (Fig. 1b) is essential for visualizing
cardiac rhythm and computing arterial blood oxygen saturation.
The OPD read-out circuit consists of two internal operational
amplifiers (Fig. 5b) in which the first stage amplifies the whole
PPG signal from the photodiode. The second stage only amplifies
the pulsating part of the signal and is read by an analogue-todigital converter (ADC). With two-stage amplification, we
obtained a 50–60 mVp-p PPG signal for the inorganic probe
(Fig. 5c) and a 3–4 mVp-p PPG signal for the organic probe
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16 mV p-p
Red
10
20
30
40
50
60
70
Signal (mV)
40
35
30
25
20
16 mV p-p
Green
34
Signal (mV)
Signal (mV)
Green
Signal (mV)
Signal (mV)
Signal (mV)
26 mV p-p
0
OLEDs + OPD
OLEDs + inorganic PD
OPD + inorganic LEDs
50
45
40
35
30
25
20
45
40
35
30
25
20
6 mV p-p
32
30
28
Red
10
Sample (count)
20
30
40
50
Sample (count)
60
70
34
33
32
31
30
34
33
32
31
30
3 mV p-p
Green
2.5 mV p-p
Red
10
20
30
40 50 60
Sample (count)
70
80
Figure 4 | PPG acquisition using combinations of inorganic and organic LEDs and photodiodes (PDs). (a) PPG signal acquired using inorganic red and
green LEDs and an OPD. Green and red PPG signal amplitudes of 26 and 16 mVp-p were obtained, respectively. (b) PPG signal acquired using OLEDs
and silicon PD—absence of lensing epoxy and reduced irradiance of the OLEDs bring down signal magnitude to 16 and 6 mVp-p for green and red
excitations. (c) PPG signal acquired using OLEDs and OPD; although signal magnitudes are reduced to 3 and 2.5 mVp-p, the signal is sufficient for resolving
the PPG waveform and provide light absorbance ratio information for arterial blood oxygenation calculation.
LED driver circuit
Red LED intensity (DAC)
UART transmission
Red LED on/off (GPIO)
Microcontroller
Red
OLED
Green
OLED
Red / green signal
(ADC)
Green LED intensity (DAC)
Green LED on/off (GPIO)
PD read circuit
Organic
oximeter probe
Red LED
Green LED
Organic pulse oximeter
probe
Ros
HR (b.p.m.) Red (mV) Green (mV)
80
75
70
65
60
99
96
93
90
0
0
50
50
100
100
150
200
150
Sample (count)
200
250
250
300
80
75
70
65
60
1
0.9
0.8
0.7
0.6
99
96
93
90
300
2nd stage Multiplexer
amplification
(AC)
34
32
30
Ros
60
40
20
0
1st Stage
amplification
(AC + DC)
34
32
30
SaO2 (%)
60
40
20
0
SaO2 (%)
HR (b.p.m.) Red (mV) Infrared (mV)
OPD
1
0.9
0.8
0.7
0.6
Photodetector
0
50
100
150
200
250
300
0
50
100
150
Sample (count)
200
250
300
Figure 5 | Organic optoelectronic pulse oximetry system. (a) Red and green OLEDs are placed on subject’s finger and transmitted light is collected with
one OPD pixel placed below the finger. (b) Hardware block diagram for the system set-up—a microcontroller acts as the data acquisition and processing
unit. OLEDs are triggered and controlled using general-purpose input/output port and DAC pins, and the OPD signal is recorded using the ADC of the
microcontroller. A two-stage amplifier between the OPD and ADC removes the d.c. part from the PPG signal and amplifies the pulsating PPG signal.
(c,d) Simultaneous oximetry measurements with a commercially available inorganic oximeter probe and the organic oximeter probe, respectively. The PPG
signal was obtained using red and infrared light for the commercially available probe (c) and using red and green light for the organic probe (d). Heart rate
(HR; magenta line in c,d) was obtained by timing the systolic peaks in the PPG signals. The ratio of the transmitted light at two wavelengths (Ros; blue
line in c,d) is converted to arterial blood oxygen saturation (SaO2; yellow line in c,d) using Beer–Lambert’s law in conjunction with an empirical correction.
(Fig. 5d). The heart rate and ratio of transmitted light at two
wavelengths (Fig. 5c,d) were calculated directly from the PPG
signals and the arterial blood oxygen saturation was derived from
the ratio of transmitted light, as discussed previously and in the
Supplementary Methods. The calculated heart rate and oxygenation derived from the PPG signals from the inorganic and
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ARTICLE
NATURE COMMUNICATIONS | DOI: 10.1038/ncomms6745
organic probes are both 65–70 beats per minute and 94–96%,
respectively (Fig. 5c,d). We observed 1% error for pulse rate and
2% error for oxygenation when comparing the organic optoelectronic sensor with the inorganic sensor.
Motion artefacts are one possible source of error in pulse
oximetry measurements. Motion-induced errors can be minimized with signal-processing algorithms that can be found in
literature33,34. In this work, we focus mainly on organic
optoelectronic probe design and development; motion artefact
characterization and mitigation algorithms can be implemented
to further improve sensor performance.
Discussion
The novel combination of red and green OLEDs, as opposed to a
red and near-infrared LED pair, is successfully implemented in
pulse oximetry because the difference in the absorptivity of
oxygenated and deoxygenated haemoglobin at the green wavelength
is comparable to the difference at near-infrared wavelengths35 as
seen in Fig. 1c. Green LEDs have not been used conventionally in
transmission oximetry because shorter wavelengths are more
efficiently absorbed by the blood. However, the higher irradiance
of the green OLEDs (Fig. 2b) compensates for any absorption losses
in non-pulsating blood and tissue, as can be inferred from the
higher green signal amplitudes in Fig. 4 compared with the red
signal amplitudes. We employed an empirical correction to
calculate arterial blood oxygenation from the ratio of transmitted
green and red light, a scheme widely used for correcting for the
deviation from Beer–Lambert’s law (which does not account for the
scattering that occurs in human tissue) in red and near-infrared
pulse oximetry measurements.
Aside from maximizing OPD EQE and short-circuit photocurrent and OLED irradiance, the OPD’s short-circuit current
resulting from ambient light should be minimized to achieve the
best pulse oximetry signal, as parasitic photodetector current is a
contributor to conventional pulse oximetry failure36. The effects
of ambient light on the OPD’s short-circuit current were
measured using two-finger phantoms with radii of 9 and 5 mm,
representative of the wide range of human finger sizes. Flexing the
photodiode around the finger phantom, as opposed to taking the
measurement with the photodiode placed flat, non-flexed, against
the phantom, significantly reduces the parasitic short-circuit
current produced by ambient light. Under typical room-lighting
conditions of 72–76 mW cm 2, flexing the OPD around the 9 and
5 mm radii phantoms reduced the parasitic current from 270 to
20 nA and 280 to 60 nA, respectively (Supplementary Fig. 3b).
The ability of the flexible OPD to conform around the human
body therefore improves the pulse oximeter’s versatility.
The long-term stability of the organic optoelectronic pulse
oximeter, like most organic optoelectronics, is limited by the
robustness of the encapsulation technology employed in its
fabrication37,38. It has been shown that lifetime of organic
optoelectronics can be significantly improved using robust
encapsulation and packaging. With our encapsulation process,
we see a 24% signal intensity decrease in the green and a 54%
decrease in the red PPG signal over a 7-day time frame.
Supplementary Figure 5 shows a decline in signal intensity;
however, the PPG signal shapes are intact.
The organic optoelectronic pulse oximetry sensor described
here demonstrates the potential for the application of organic
electronics to thrive in the medical device field. The large-area
scalability, inexpensive processing and flexibility of organic
optoelectronics will allow medical sensors to be made in new
shapes and sizes, diversifying possible sensing locations on the
human body, enabling medical professionals to better monitor
their patients’ care.
6
Methods
OLED fabrication and characterization. The semiconducting polymers used in
the emissive layer of the OLEDs were supplied by Cambridge Display Technology
Limited. The red OLED active layer was made from a 25:70:5 blend of
TFB:F8BT:TBT in a 10 mg ml 1 o-xylene solution. The green OLED active layer
was made from a 1:9 blend of TFB:F8BT in a 10 mg ml 1 o-xylene solution.
Patterned ITO substrates were cleaned via sonication in acetone and then isopropyl
alcohol. The substrate surfaces were made hydrophilic with a 2 min plasma
treatment before spincoating a 40 nm layer of Clevios PEDOT:PSS AI4083. Any
remaining moisture was evaporated in a 10 min annealing step at 120 °C before
moving the samples into a nitrogen glove box for the remainder of the fabrication
procedure. TFB was spin coated from a 10 mg ml 1 o-xylene solution and then
annealed at 180 °C for 45 min before cooling and spin rinsing with o-xylene,
producing a 10–20 nm-thick electron-blocking layer. The active layer was then
spun at 4500 r.p.m. for a 100 nm film thickness. The LiF (1 nm)/Al (100 nm)
cathode was thermally evaporated under vacuum at 4 10 6 Torr. Finished
devices were encapsulated with ultraviolet-curable Delo Katiobond LP612 epoxy
and clean quartz glass. Each OLED pixel had 4 mm2 of active area. OLED current/
voltage characteristics and irradiance measurements were taken with an Orb
Optronix light measurement system complete with an Orb Optronix SP-50
spectrometer, integrating sphere, Keithley 2400 Source Meter and Spectral Suite
3.0 software.
OPD fabrication and characterization. OPDs were printed on top of planarized
polyethylene naphthalate (PEN) substrates (DuPont) using a blade-coating technique previously reported31. A layer of high-conductivity PEDOT:PSS (SigmaAldrich, 739316-25G) was printed by blade coating (200 mm blade height at
1.6 cm s 1) the solution over a large hydrophilic strip in the substrate defined by a
10 s plasma treatment through a stencil. Following a 10 min anneal at 120 °C, a
layer of high-work-function PEDOT:PSS (Clevios Al4083) was coated and
annealed over the previous print using the same process. The active layer ink
comprised of a 1:1 weight ratio of PTB7:PC71BM (Solaris Chem) dissolved to
35 mg ml 1 in chlorobenzene with a 3 vol.% concentration of 1,8-diiodooctane
and was blade coated (350 mm blade height at 1.6 cm s 1) in a glove box with the
substrate heated to 40 °C. The aluminum cathode (100 nm) was thermally
evaporated under vacuum at 4 10 6 Torr to yield an active area of 21 mm2.
Finished devices were encapsulated with ultraviolet-curable Delo Katiobond LP612
epoxy and Saran wrap after being post-annealed at 120 °C for 10 min. All OLED
and OPD layer thicknesses were measured with a Dektak Profilometer.
Electronic hardware and software for data acquisition and processing. The
Texas Instruments MSP430 microcontroller was chosen for data acquisition and
processing because of its built-in ADCs and digital-to-analogue converters (DACs),
which are required for the pulse oximeter. General-purpose input–output pins
from the microcontroller control LED switching, ADCs are utilized to read the
amplified OPD signal from the multiplexer, and DACs are used to control LED
intensity and in the signal amplification stage. The LEDs are operated in a
sequential approach, so that only one of the LEDs is on at a particular moment.
Five hundred and twelve samples are taken from each of the LEDs per second.
A software trigger from the microcontroller controls a PNP bipolar junction
transistor (BJT) switch that triggers the LED on/off. In addition, DACs are used to
control the drive current for the LEDs using a NPN transistor. For ensuring
compatibility with the organic LEDs, signals from the microcontroller are shifted to
9 V using general-purpose operational amplifiers. Finally, universal asynchronous
receiver/transmitter (UART) protocol is used to send processed data to a computer
for visualization. We opted for a modular approach by separating the LED driver
circuit and OPD read circuit, simplifying circuit design and debugging. Hamamatsu Large Area Photodiode S1227-1010BQ (active area of 35 mm2) and 5 mm
red and green round LEDs were used in PPG data comparisons with the organic
devices.
All-pulse oximetry experiments performed on human subjects were carried out
with informed consent under the approval of the University of California, Berkeley
Institutional Review Board, protocol ID number 2013-03-6081.
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Acknowledgements
This work was supported in part by the National Science Foundation under Cooperative
Agreements No. ECCS-1202189 and UTA-12000944, ARL W911NF-09-3-001 under
RFP 12-159 and by the NSF Graduate Fellowship Research Program under Grant No.
DGE-1106400. We thank Cambridge Display Technology Limited (CDT) for supplying
OLED materials, and Dr Mozziyar Etemadi for helpful technical discussions.
Author contributions
A.C.A., C.M.L., A.P. and Y.K. conceptualized the work. C.M.L. and A.P. carried out
device fabrication and characterization of the OLEDs and OPDs, respectively, and
experimental set-ups. Y.K. designed and implemented the oximeter system and worked
on software and hardware programming. C.M.L., A.P. and Y.K. volunteered as subjects
and collected pulse and oxygenation results. All authors discussed the results and
commented on the manuscript.
Additional information
Supplementary Information accompanies this paper at http://www.nature.com/
naturecommunications
Competing financial interests: The authors declare no competing financial interests.
Reprints and permission information is available online at http://npg.nature.com/
reprintsandpermissions/
How to cite this article: Lochner, C. M. et al. All-organic optoelectronic sensor for pulse
oximetry. Nat. Commun. 5:5745 doi: 10.1038/ncomms6745 (2014).
NATURE COMMUNICATIONS | 5:5745 | DOI: 10.1038/ncomms6745 | www.nature.com/naturecommunications
& 2014 Macmillan Publishers Limited. All rights reserved.
7
ARTICLE
Received 10 Jul 2014 | Accepted 3 Nov 2014 | Published 10 Dec 2014
DOI: 10.1038/ncomms6745
All-organic optoelectronic sensor for pulse
oximetry
Claire M. Lochner1,*, Yasser Khan1,*, Adrien Pierre1,* & Ana C. Arias1
Pulse oximetry is a ubiquitous non-invasive medical sensing method for measuring pulse rate
and arterial blood oxygenation. Conventional pulse oximeters use expensive optoelectronic
components that restrict sensing locations to finger tips or ear lobes due to their rigid form
and area-scaling complexity. In this work, we report a pulse oximeter sensor based on organic
materials, which are compatible with flexible substrates. Green (532 nm) and red (626 nm)
organic light-emitting diodes (OLEDs) are used with an organic photodiode (OPD) sensitive
at the aforementioned wavelengths. The sensor’s active layers are deposited from solutionprocessed materials via spin-coating and printing techniques. The all-organic optoelectronic
oximeter sensor is interfaced with conventional electronics at 1 kHz and the acquired pulse
rate and oxygenation are calibrated and compared with a commercially available oximeter.
The organic sensor accurately measures pulse rate and oxygenation with errors of 1% and
2%, respectively.
1 Department
of Electrical Engineering and Computer Sciences, University of California, Berkeley, California 94720, USA. * These authors contributed equally
to this work. Correspondence and requests for materials should be addressed to A.C.A. (email: acarias@eecs.berkeley.edu).
NATURE COMMUNICATIONS | 5:5745 | DOI: 10.1038/ncomms6745 | www.nature.com/naturecommunications
& 2014 Macmillan Publishers Limited. All rights reserved.
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ARTICLE
NATURE COMMUNICATIONS | DOI: 10.1038/ncomms6745
C
onventional pulse oximeters non-invasively measure
human pulse rate and arterial blood oxygen saturation
with an optoelectronic sensor composed of two inorganic
light-emitting diodes (LEDs) with different peak emission
wavelengths and a single inorganic photodiode1,2. The LEDs
are placed on one side of a finger and the light transmitted
through the tissue is subsequently sensed by the photodiode that
is placed on the opposite side of the finger. Sequential sampling of
the transmitted light provides information on the ratio of
oxygenated and deoxygenated haemoglobin in the blood. This
ratio and a calibration curve are used to compute arterial blood
oxygen saturation. Currently, the application of commercially
available pulse oximeters is limited by the bulk, rigidity and
high large-area scaling cost of conventional inorganic-based
optoelectronics. Here we show a pulse oximeter sensor composed
of organic LEDs (OLEDs)3,4 and a flexible organic polymer
photodiode (OPD)5. We successfully demonstrate that the
organic optoelectronic sensor provides accurate measurement
capability and we anticipate that our application of solutionprocessable organic optoelectronics in pulse oximetry will enable
low-cost, disposable and wearable medical devices.
Wearable medical sensors have the potential to play an
essential role in the reduction of health care costs: they encourage
healthy living by providing individuals feedback on personal vital
signs and enable the facile implementation of both in-hospital
and in-home professional health monitoring. Consequently, wide
implementation of these sensors can reduce prolonged hospital
stays and cut avertible costs6. Recent reports show ample
wearable sensors capable of measuring pressure7,8, biopotential
and bioimpedance9,10, pulse rate11 and temperature12,13 in real
time. These sensors are developed in wearable and flexible form
factors using organic8,13,14, inorganic12,15,16 and hybrid organic–
inorganic7,9,15 materials.
Organic semiconductors developed for OLEDs and OPDs have
been primarily applied to display and photovoltaic technologies17,18. This is due to their potential for large-area roll-to-roll
manufacturing at large volumes, which is enabled by solution
processing and the use of flexible substrates19. These attributes
also make organic optoelectronics very attractive for medical
sensors, where flexibility combined with large areas can result in
an improvement of the overall sensor performance. In the past 10
years, a lot of resources were used to improve the stability of
organic semiconductors to meet the lifetime requirements of
displays and photovoltaics20,21. When compared with the above
markets, disposable medical sensors have less-stringent lifetime
requirements on the materials, since these devices would be used
only for a few days as opposed to years. Indeed, organic
optoelectronics have previously been used to perform pulse
measurements22–24.
Here we report a sensor composed solely of organic
optoelectronics that measures both human pulse and arterial
blood oxygenation. We anticipate that our results will inspire
system-level integration of organic–inorganic electronics, where
the large area, low cost and mechanical flexibility of organic
sensors will be combined with the computational efficiency of
inorganic electronics. A schematic view of the sensor is given in
Fig. 1a, where two OLED arrays and two OPDs are placed on
opposite sides of a finger.
Results
Pulse and oxygenation with red and green organic light
emitting diodes. In contrast to commercially available inorganic
oximetry sensors, which use red and near-infrared LEDs, we use
red and green OLEDs. Incident light from the OLEDs is attenuated by pulsating arterial blood, non-pulsating arterial blood,
2
venous blood and other tissue as depicted in Fig. 1b. When
sampled with the OPD, light absorption in the finger peaks in
systole (the heart’s contraction phase) due to large amount of
fresh arterial blood. During diastole (the heart’s relaxation phase),
reverse flow of arterial blood to the heart chambers reduces
blood volume in the sensing location, which results in a minima
in light absorption. This continuous change in arterial blood
volume translates to a pulsating signal—the human pulse.
The d.c. signal resulting from the non-pulsating arterial blood,
venous blood and tissue is subtracted from the pulsating
signal to give the amount of light absorbed by the oxygenated
and deoxygenated haemoglobin in the pulsating arterial blood.
Oxy-haemoglobin (HbO2) and deoxy-haemoglobin (Hb) have
different absorptivities at red and green wavelengths, as highlighted on the absorptivity of oxygenated and deoxygenated
haemoglobin plotted in Fig. 1c. The difference in the molar
extinction coefficient of oxygenated and deoxygenated
haemoglobin at the green wavelength is comparable to the difference at near-infrared wavelengths (800–1,000 nm) used in
conventional pulse oximeters. In addition, solution-processable
near-infrared OLED materials are not stable in air and show
overall lower efficiencies25,26. Thus, we elected to use green
OLEDs instead of near-infrared OLEDs.
Using red and green OLEDs and an OPD sensitive at visible
wavelengths (the OLEDs’ emission spectra and the OPD’s
external quantum efficiency (EQE) as a function of incident light
wavelength are plotted in Fig. 1d), blood oxygen saturation (SO2)
is quantified according to equation 1. Here, CHbO2 and CHb are
the concentrations of oxy-haemoglobin and deoxy-haemoglobin,
respectively.
SO2 ¼
CHbO2
CHbO2 þ CHb
ð1Þ
Ros, the ratio of absorbed red (Ard) and green (Agr) light,
depends on the normalized transmitted red (Tn,rd) and green
(Tn,gr) light intensities according to Beer–Lambert’s law (shown
in equation 2).
Ard ln Tn;rd
Ros ¼
¼
ð2Þ
Agr ln Tn;gr
Finally, arterial oxygen saturation (SaO2) can be calculated
using equation 3. Here, erd,Hb and egr,Hb are the molar absorptivity
of deoxy-haemoglobin at red (l ¼ 626 nm) and green (l ¼ 532
nm) wavelengths, respectively. Similarly, erd;HbO2 and egr;HbO2 are
the molar absorptivity of oxy-haemoglobin at red (l ¼ 626 nm)
and green (l ¼ 532 nm) wavelengths, respectively.
Sa O2 ðRos Þ ¼
erd;Hb egr;Hb Ros
erd;Hb erd;HbO2 þ egr;HbO2 egr;Hb Ros
ð3Þ
Organic optoelectronic oximeter components. OLED and OPD
performances are both paramount to the oximeter measurement
quality. The most important performance parameters are the
irradiance of the OLEDs’ (Fig. 2b) and the EQE at short circuit of
the OPD (Figs 1d and 3b). As the OLEDs operating voltage
increases, irradiance increases at the expense of efficiency27, as
shown by the lower slope of irradiance than current as a function
of applied voltage in Fig. 2b. For a pulse oximeter, this is an
acceptable trade-off because higher irradiance from the OLEDs
yields a strong measurement signal.
We have selected polyfluorene derivatives as the emissive layer
in our OLEDs due to their environmental stability, relatively high
efficiencies and self-assembling bulk heterojunctions that can be
tuned to emit at different wavelengths of the light spectrum4.
The green OLEDs were fabricated from a blend of poly(9,9-
NATURE COMMUNICATIONS | 5:5745 | DOI: 10.1038/ncomms6745 | www.nature.com/naturecommunications
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ARTICLE
NATURE COMMUNICATIONS | DOI: 10.1038/ncomms6745
Flexible
plastic
substrate
Absorptivity (l mmol–1 cm–1)
Deoxy-hemoglobin (Hb)
Oxy-hemoglobin (HbO2)
5
0.5
0.05
1
Red OLED
Green OLED
OPD
50
0.9
Red OLED EL
0.8
OPD EQE
45
Incident light
Tsystolic
Tdiastolic
Non-pulsating
arterial blood
AC
Absorbed
light
Pulsating
arterial blood
Venous blood
DC
One cardiac cycle
Other tissue
Systole: heart muscles
contract, and pump
blood to body.
Tdiastolic
Diastol: heart muscles
relax, and blood flows
into heart chambers.
t
40
0.7
35
0.6
30
0.5
25
0.4
20
0.3
15
0.2
10
0.1
5
0
450
EQE (%)
Tsystolic
Transmitted
light
Normalized EL (a.u.)
Green OLED EL
0
550
650
750
850
Wavelength (nm)
Figure 1 | Pulse oximetry with an organic optoelectronic sensor. (a) Pulse oximetry sensor composed of two OLED arrays and two OPDs. (b) A
schematic illustration of a model for the pulse oximeter’s light transmission path through pulsating arterial blood, non-pulsating arterial
blood, venous blood and other tissues over several cardiac cycles. The a.c. and d.c. components of the blood and tissue are designated, as well as the peak
and trough of transmitted light during diastole (Tdiastolic) and systole (Tsystolic), respectively. (c) Absorptivity of oxygenated (orange solid line) and
deoxygenated (blue dashed line) haemoglobin in arterial blood as a function of wavelength. The wavelengths corresponding to the peak OLED
electroluminescence (EL) spectra are highlighted to show that there is a difference in deoxy- and oxy-haemoglobin absorptivity at the wavelengths of
interest. (d) OPD EQE (black dashed line) at short circuit, and EL spectra of red (red solid line) and green (green dashed line) OLEDs.
dioctylfluorene-co-n-(4-butylphenyl)-diphenylamine) (TFB) and
poly((9,9-dioctylfluorene-2,7-diyl)-alt-(2,1,3-benzothiadiazole-4,
8-diyl)) (F8BT). In these devices, electrons are injected into the
F8BT phase of phase-separated bulk-heterojunction active layer
while holes are injected into the TFB phase, forming excitons at
the interfaces between the two phases and recombining in the
lower energy F8BT phase for green emission28. The emission
spectrum of a representative device is shown in Fig. 1d. The red
OLED was fabricated from a tri-blend blend of TFB, F8BT and
poly((9,9-dioctylfluorene-2,7-diyl)-alt-(4,7-bis(3-hexylthiophene5-yl)-2,1,3-benzothiadiazole)-20 ,20 -diyl) (TBT) with an emission
peak of 626 nm as shown in Fig. 1d. The energy structure of the
full stack used in the fabrication of OLEDs, where ITO/
PEDOT:PSS is used as the anode, TFB as an electron-blocking
layer29 and LiF/Al as the cathode, is shown in Fig. 2a. The
physical structure of the device is provided in Supplementary
Fig. 2b. The red OLED operates similarly to the green, with the
additional step of excitonic transfer via Förster energy transfer30
to the semiconductor with the lowest energy gap in the tri-blend,
TBT, where radiative recombination occurs. The irradiance at 9 V
for both types of OLEDs, green and red, was measured to be 20.1
and 5.83 mW cm 2, respectively.
The ideal OPD for oximetry should exhibit stable operation
under ambient conditions with high EQE at the peak OLED
emission wavelengths (532 and 626 nm). A high EQE ensures the
highest possible short-circuit current, from which the pulse and
oxygenation values are derived. Poly({4,8-bis[(2-ethylhexyl)oxy]
benzo[1,2-b:4,5-b0 ]dithiophene-2,6-diyl}{3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4–b]thiophenediyl}) (PTB7) mixed with
[6,6]-phenyl C71-butyric acid methyl ester (PC71BM) is a stable
donor:acceptor bulk-heterojunction OPD system, which yields
EQE as high as 80% for spin-coated devices5. The transparent
electrode and active layer of the OPD are printed on a plastic
substrate using a surface tension-assisted blade-coating technique
recently developed and reported by Pierre et al.31 Figure 3a shows
the energy band structure of our device including the transparent
electrode (a high-conductivity/high-work-function PEDOT:PSS
bilayer) and an Al cathode. The physical device structure of the
OPD is shown in Supplementary Fig. 2d. The EQE at 532 and
626 nm is 38 and 47%, respectively, at short-circuit condition, as
shown in Fig. 1d, and the leakage current of about 1 nA cm 2 at
2 V applied reverse bias is shown in Fig 3b together with the
photocurrent when the device is illuminated with a
355 mW cm 2 light source at 640 nm.
Despite the low reverse bias leakage current shown in Fig 3b,
we chose to bias the OPD at 0 V, the short-circuit condition, to
sense low photocurrent levels. The frequency response of both the
OPD and OLEDs was also characterized, since oximetry is usually
performed at 1 kHz. The 3 dB cut-off was found to be at
frequencies higher than 10 kHz for the all-organic optoelectronic
sensor, which is significantly higher than the operational
frequency required for oximetry (Supplementary Fig. 4). Notably,
the frequency performance of the OPD is not hampered at
short circuit because the shunt capacitance of organic photodiodes decreases negligibly with reverse bias, unlike inorganic
photodiodes32.
We observed that the OLED irradiance for both red and green
wavelengths is sufficient for the transmission of light through the
finger and the signal acquired by the organic photodetector is
sufficiently high for resolving the pulsating photoplethysmogram
(PPG) signal shown in Fig. 1b. The pulse waveforms (two cardiac
cycles) generated with a combination of organic and inorganic
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ARTICLE
NATURE COMMUNICATIONS | DOI: 10.1038/ncomms6745
TFB
PTB7
2.3 eV
3.31 eV
TBT
4.08 eV
3.4 eV
3.5 eV
Al
PC71BM
LiF/AI
3.15 eV
F8BT
4.3 eV
Conductive
PEDOT:PSS
PTB7
PEDOT:PSS
ITO
4.8 eV
5.15 eV
5.2 eV
PEDOT:PSS
5.2 eV
TFB
TBT
5.37 eV
5.3 eV
PC71BM
F8BT
6 eV
5.9 eV
1E–03
0.45
1.5E–2
0.3
0.25
1.0E–2
0.2
0.15
5.0E–3
0.1
Current density (A cm–2)
0.35
1E–04
Irradiance (W cm–2)
Green OLED current density
Reg OLED current density
Green OLED irradiance
Red OLED irradianc
0.4
Current density (A cm–2)
2.0E–2
1E–05
1E–06
1E–07
1E–08
1E–09
Dark current
1E–10
0.05
0.0E+0
0
0
2
4
6
8
10
Light current
1E–11
–2
Voltage (V)
0
1
2
Voltage (V)
Figure 2 | OLED design and performance. (a) OLED energy structure.
(b) Current density of red (red solid line) and green (green dashed line)
OLEDs and irradiance of red (red squares) and green (green triangles)
OLEDs as a function of applied voltage.
devices are shown in Fig. 4. The PPG obtained when a human
finger is illuminated by inorganic LEDs and the transmitted light
is measured with an OPD is shown in Fig. 4a. When the same
measurement is performed using OLEDs and a conventional Si
photodiode (Fig. 4b), the magnitude of the PPG signal is reduced
from 26 to 16 mVp-p for the green and 16 to 6 mVp-p for the red
due to the lower optical power of the organic LEDs compared
with their inorganic equivalent device. Finally, both OLEDs and
an OPD are used to obtain a PPG under the same experimental
conditions (Fig. 4c), yielding signal magnitudes of 3 mVp-p for
the green and 2.5 mVp-p for the red. It is clear that the magnitude
of the signal is substantially reduced with the introduction of
organic-based devices, but the PPG obtained at red and green
wavelengths yields similar shapes for all device combinations
shown in Fig. 4, which will result in similar pulse and arterial
oxygenation values. The lower signal magnitude shown by the
organic probe is compensated for by increasing the area of
devices, resulting in higher photocurrents that directly translate
into higher PPG signals, as shown in Supplementary Fig. 3a.
System design for an organic optoelectronic pulse oximeter.
The organic pulse oximetry sensor composed of two red and
green OLED arrays and an OPD (Fig. 5a) is interfaced with a
microcontroller that drives the OLEDs, measures the OPD signal
4
–1
Figure 3 | OPD design and performance. (a) OPD energy structure.
(b) Light curre...