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1
Application of Flexible Flat Panel Display
Technology to Wearable Biomedical
Devices
J. Smith, E. Bawolek, YK. Lee, B. O’Brien, M. Marrs,
E. Howard, M. Strnad, J. Blain Christen, and M. Goryll
In this work, we explored how the application of commercial (thin film)
flat panel display technology, used in the production of flexible displays
and flexible digital x-ray detectors, can also be applied to reduce the
manufacturing cost of wearable biomedical devices, as well as
potentially improve their diagnostic functionality. As a technology
platform to evaluate our new concept, we developed a prototype
photoplethysmograph (PPG) biosensor using flexible organic light
emitting diode (OLED) display and PiN photodiode (thin film) sensor
technology for optical heart rate monitoring.
Introduction: The size of today’s flat panel display industry almost
defies comprehension. In 2012, flat panel displays were manufactured
at a rate of 100 square kilometers per year [1], which is enough material
to completely cover one hundred 18-hole golf courses. However, even
with extremely high capital equipment costs that can approach several
billion dollars ($US), commercial flat panel display factories currently
can manufacture displays for less than 10 cents per square centimeter,
based on today’s HDTV pricing. In this work, we explored applying the
immense low cost mass production capabilities of the flat panel display
industry to also manufacture wearable electronics, with a focus on
wearable biomedical devices. We believe leveraging flat panel display
technology offers a huge untapped opportunity to provide a
fundamental breakthrough in reducing the cost to manufacture these
increasingly popular devices as well as improve their functionality,
while also leveraging a well-established industrial base already capable
of supplying an enormous number of consumer electronic products
annually. As an example, if just 1% of existing flat panel display
industrial capacity were diverted to manufacture wearable electronics,
approximately 1 billion (~10 cm2) low cost and ultimately disposable
devices could be manufactured each year.
Wearable biomedical devices hold the promise of early detection,
diagnosis, and treatment of a number of diseases, and have the potential
to help both patients and clinicians by providing non-invasive
continuous physiological monitoring in a non-clinical setting prior to
the onset of more severe symptoms. Wearable biomedical devices are
typically manufactured using silicon wafer-based microelectronic
components bonded to a printed circuit board (PCB). However, these
conventional electronics manufacturing technologies are typically rigid,
while biological surfaces and systems are soft and pliable [2]. As
illustrated in Fig. 1, an alternative approach is to instead manufacture
wearable biomedical devices using an inherently biocompatible flexible
electronics and display substrate.
Flexible displays are fundamentally a very thin and typically
transparent sheet of plastic, approximately the same thickness as a piece
of paper and constructed by sequentially layering and patterning
nanoscale thin films (Fig. 2). This approach allows the electronics
functionality to be built or integrated directly into flexible plastic
substrates using active thin film devices, including the organic light
emitting diodes (OLED), PiN photodiodes, and thin film transistors
(TFTs). To make a display flexible, we essentially use the same process
and tooling currently used to manufacture large commercial flat-panel
displays on glass substrates, but instead replace the starting glass
substrate with a flexible plastic substrate temporarily bonded to a rigid
alumina carrier. After the thin film process steps are completed, the
flexible plastic substrate with the patterned thin film layers on top is
then simply peeled off (Fig. 1a), similar to peeling off a Post-it® brand
note [3-5].
Experimental Device Fabrication and Test: As a technology platform to
evaluate our new concept, we developed a prototype
photoplethysmograph (PPG) biomedical sensor using flexible display
technology for optical heart rate monitoring in a smart bandage-style
configuration (Fig. 1). Unlike previously reported flexible PPG
biosensors, which used experimental printed electronics technology [6],
our new approach is designed for mass production using existing
commercial flat panel (thin film) display tooling.
Photoplethysmography is a technique which detects changes in blood
volume in the peripheral vascular system using either optical
transmission or optical reflection [7]. As illustrated in Fig. 3, the change
in blood volume in our prototype was detected by illuminating the skin
Fig. 1 (a) Flexible displays on Gen2 370 x 470 mm plastic substrate
being debonded from temporary rigid alumina carrier (b) 7.4”
diagonal full-color flexible organic light emitting diode (OLED)
display manufactured at the ASU Flexible Electronics and Display
Center (FEDC) (c) Concept for conformal Bluetooth® connected
disposable optical heart rate monitor smart bandage manufactured
using flexible OLED display technology with alternating red and
green OLED and a-Si PiN photodiode pixels arranged in a two
dimensional photoplethysmograph (PPG) sensor array.
Fig. 2 (a) Cross-section of active pixel for the flexible OLED display,
including structure for both bottom emitting OLED and bottom-gate,
inverted-staggered TFT device structure (b) Red and Green OLED
light intensity as a function of forward bias voltage for our 5 mm2
flexible OLED test structures (c) Detailed phosphorescent green
OLED device structure, with scaled organic layer thicknesses, HIL
(hole injection layer), HTL (hole transport layer), phosphorescent host
dopant system for (red or green) emissive layer, HBL (hole blocking
layer), ETL (electron transport layer), and EIL (electron injection
layer).
2
with light from flexible OLED test structures (red and green) biased at
+6 volts DC (Fig. 2b), and then measuring the amount of AC modulated
light either transmitted or reflected, respectively, to a paired 6 x 7 mm
amorphous silicon (a-Si) flexible PiN photodiode test structure that we
originally manufactured for use in our flexible digital x-ray detectors.
Additional details on our flexible a-Si PiN photodiode device properties
and the thin film flexible display-based manufacturing process used to
manufacture our flexible digital x-ray detectors can be found in Smith,
et. al., and Marrs, et. al. [5, 8].
Results and Discussion: As illustrated in Fig. 4, recorded transmission-
mode and reflected mode PPG waveforms clearly show both the
systolic and diastolic peaks, and the dicrotic notch, illustrating the
successful capture of PPG physiological parameters using flexible
display and PiN photodiode thin film technology in our prototype test
configuration. However during test, we observed that the intensity of
the detected PPG signal was position dependent, especially during
reflected-mode testing. The maximum signal level was found by
positioning the 5 mm2 green OLED emitter directly over an artery in the
wrist where the strongest pulse pressure waves could be felt. This
observed position dependence is one of the key issues limiting the
widespread adoption of PPG measurements in determining
physiological parameters [7, 9]. As a solution to avoid the difficulties
associated with having to precisely position a single (OLED) light
emitter in the optimum location to detect the strongest PPG signal, we
envision that a more efficient wearable PPG sensor would instead use a
2-D checkerboard-style addressed array of alternating OLED and PiN
photodiode pixels connected to an external controller (Fig. 1). Upon
start up, an optical search pattern would be activated and the 2-D pixel
array would cycle through a series of patterns (i.e., activated OLED and
PiN photodiode pixels) while simultaneously monitoring the detected
PPG signal to identify and lock in the optimal pattern (directly over the
artery) to maximize the detected PPG signal, while simultaneously
minimizing the background un-modulated DC signal level by turning
off neighbouring OLED pixels in the 2-D array.
Conclusion: In this work, we discussed how the application of
commercial (thin film) flat panel display technology, used in the
production of our flexible displays and digital x-ray detectors, can also
be applied to reduce the manufacturing cost of wearable biomedical
devices, as well as potentially improve their diagnostic functionality. A
prototype PPG sensor for optical heart rate monitoring using flexible
OLED display and PiN photodiode sensor technology was developed
and then used as a test case to illustrate our new concept.
Acknowledgments: The authors would like to thank the Flexible
Electronics and Display Center at ASU for manufacturing the
prototypes devices. The research was sponsored in part by the Army
Research Lab (ARL) and was accomplished under Cooperative
Agreement W911NG-04-2-005.
J. Smith, E. Bawolek, YK. Lee, B. O’Brien, M. Marrs, E. Howard, and
M. Strnad (Flexible Electronics and Display Center at Arizona State
University, Tempe, AZ 85284 USA)
E-mail: joseph.t.smith@asu.edu
J. Blain Christen, and M. Goryll (School of Electrical, Computer and
Energy Engineering, Arizona State University, Tempe, AZ 85281 USA)
References
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Fig. 3 Experimental optical heart rate monitoring test platform
using 5 mm2 flexible green OLED to illuminate the skin surface to
detect the reflected PPG signal using 6 x 7 mm thin-film amorphous
silicon (a-Si:H) flexible PiN photodiode connected to LMC6041 op
amp configured as
high gain transimpedance amplifier. PPG
waveforms were then recorded using AC coupled digital
oscilloscope. The capacitor blocks the un-modulated background
DC signal
Fig. 4 (a) Transmission-mode PPG captured by placing index finger
placed between 620 nm 5 mm2 red OLED and 6 x 7 mm a-Si PiN
photodiode (b) Reflection-mode PPG captured by placing 515 nm
green OLED and 6 x 7 mm a-Si PiN photodiode against surface of
wrist. Immediately below both photos
are the captured PPG
waveforms with physiological parameters annotated in (b)
