Jurgen Daniel discusses the growing importance and applications of flexible sensors. Some key points: 1) Sensors are increasingly being incorporated into mobile devices and vehicles to monitor various environmental factors and enable new functionality. Flexible sensors could further this growth by allowing integration into curved and flexible surfaces. 2) Flexible sensors offer potential cost advantages through roll-to-roll manufacturing and new form factor opportunities through their flexibility. This could enable applications like sensors integrated into clothing. 3) Flexible sensors can be fabricated through methods like pick-and-place of chips, photolithography, or various printing techniques depending on the needed performance and scale of production. Research is advancing the capabilities of printed flexible sensor systems.

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The Importance of Sensors for Flexible Electronics
by Jurgen Daniel
Jurgen Daniel is principal at the technology and R&D consultancy Teclination Consulting.
He was previously with the Palo Alto Research Center where his research included MEMS,
displays, printing technologies, flexible/printed electronics and photovoltaics. Earlier, he had
been with Oxford Instruments and FEI Europe and he was visiting scientist at the Matsushita
Research Institute, Tokyo. He holds a degree in physics from the University of Erlangen-
Nuremberg, Germany, and a Ph.D. in electrical engineering from the University of
Cambridge, UK.
In our daily lives, we are increasingly surrounded by sensing devices. Mobile phones
feature accelerometers, gyroscopes, digital compasses, microphones and sensors for touch,
radio frequency and light/image. The trend is going even further. At the recent CEATEC 2011 tradeshow in Japan,
NTT Docomo showed off a prototype smartphone sleeve to measure ultraviolet light, bad breath, gamma radiation
and body fat. Vehicles are another example in which the number of sensors steadily increases. Today’s cars
incorporate about 20-50 MEMS (Micro-Electro-Mechanical-Systems) sensors for vehicle safety, engine
management, driver support and occupant comfort1
.
Much of this growth in the number of sensors is due to their miniaturization via MEMS technology, their reduced
cost (because of micro-fabrication technologies) and, as in the case of the automotive industry, government
mandates. Flexible sensors may contribute to further growth and to new application areas.
Motivation for flexible sensors: In the past, the interest in flexible electronics has been most notably stimulated by
the notion of flexible, conformal, or rollup displays. However, recently flexible sensors have been getting
increasing attention. The key motivation factors for flexible electronics are the potentially lower cost and the
flexible form factor of resulting products. The low cost may be enabled by roll-to-roll (R2R) processing on
inexpensive substrate materials, a process similar to printing newspaper or fabricating potato chip bags, and it is
essential for disposable, ubiquitous and large-area sensors.
A flexible, thin and light-weight form factor allows new designs or applications that were not viable with rigid
devices. Flexible sensors may be attached to curved surfaces or structures such as airplane wings to monitor stress
or temperature. They may form the touch sensors in future flexible displays and be integrated in clothes or bedding
material. Philips Electronics, for example, has studied mats for baby position monitoring in cribs and others have
explored solutions for patient monitoring blankets that detect pressure points of patients in hospital beds2,3
. X-ray
image sensor arrays for medical or security imaging will benefit from a flexible form factor. For example, curved
image sensors may give improved imaging results and greater patient comfort in dental intra-oral radiography.
Other areas in which flexible or inexpensive disposable sensors may find applications are listed in Figure 1a.
Sensor categories: In most flexible electronic devices such as active-matrix displays or circuits the transistor
serves as the fundamental electronic component. Therefore, much research has gone into developing new thin film
transistor types based on materials such as low temperature (amorphous) silicon, organic semiconductors,
nanotubes or nanowires and transparent metal oxides. Because of the transistor’s central function in electronic
circuits, well performing p and n-type transistors are critical.
With regard to sensors, a fundamental element does not exist. The sensing field is very fragmented with a large
variety of quantities to be measured (Figure. 1b) and measurements done under many different conditions. In
addition, the definition of a ‘sensor’ is often vague. It is not clear what is included in ‘the sensor’. For example, the
readout or signal conditioning electronics may be an integral part of the sensor, determining its sensitivity and
accuracy.

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Figure 1: A selection of areas in which flexible sensors might have a significant impact (a). The sensor market is
rather fragmented with sensor measurements including a wide variety of quantities (b).
In general, sensors can be divided into electronic or non-electronic ones. Amongst simple flexible non-electronic
sensors are pH strips that indicate the acidity of a liquid via a color change, disposable thermometer strips such as
the NexTemp thermometer by Medical Indicators, Inc., or large-area pressure indicating films such as the Fujifilm
Prescale. For many applications these kinds of sensors may be sufficient, but if higher accuracy or recording of
measurement data is desired, sensors will presumably have to be of the electronic type. Two examples of already
commercially existing flexible electronic sensors are shown in Figure. 2. The sensor A) is a force sensitive resistor.
Similar sensors or sensor arrays have been developed by Tekscan, Inc., to capture in-shoe pressure information or,
in form of floor mats, for gait analysis. Sensor B) is a bend sensor which changes its resistance when bending stress
is applied. Such sensors have been used for robotics or for virtual reality applications.
The focus here is on electronic sensors and before glancing at recent flexible sensor developments, it is worth
mentioning some of the fabrication options.
Figure 2: Examples of commercially available flexible sensors. The sensor A) is a force sensitive resistor. Sensor B) is
a bend sensor patented by Abrams Gentile Entertainment and fabricated by printing. Such sensor technology was
used in a Nintendo virtual reality glove as input device to recreate human hand motion.
Fabrication Technologies on Flexible Substrates: Flexible sensors or sensor arrays may be fabricated in several
different ways. The substrate material may include plastic foil, paper, thin flexible glass, metal foil, stretchable
polymers or even woven fabrics.

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Pick-and-place technology may be the choice if high performance (silicon) sensors are needed. Muehlbauer, Inc., is
leading in the field with machines that place silicon chips onto antennas for radio-frequency identification (RFID)
tags. The cost of placing a chip lies currently around 1 cent and the typical placement speed is approximnately 2-3
chips per second. With this method, chips as small as ~300-400 microns can be attached to a flexible substrate in a
roll-to-roll process (for example with anisotropically conductive adhesive). Using a pick-and place method, MEMS
sensor chips may be combined with microcontroller chips, memory or communication circuits as well as power
sources such as batteries. The overall system will still remain flexible because each (rigid) chip is typically small in
size. In Figure 3a, an example of a sensor wrist band is shown which was made with a combination of different
R2R fabrication methods including pick-and-place (Fraunhofer, EMFT).
For large arrays of sensors with densely arranged sensors, other fabrication methods such as photolithographic
patterning or printing may be preferred. Cost, minimum feature size and required performance eventually will
determine which method to choose. Figure 3b) shows a prototype flexible x-ray image sensor backplane by the Palo
Alto Research Center (PARC)4
. This sensor array was made using a low-temperature amorphous silicon process
together with a jet-printing method (‘digital lithography’) to pattern the pixel circuit. Printing methods are often
considered because of their potential for extremely low fabrication cost. However, the term ‘printing’ encompasses
a wide spectrum of technologies. Transfer printing as for example used by the Univ. of Illinois resembles a parallel
pick-and place approach in which devices are lifted off a rigid wafer and transferred to a flexible substrate5
. High
performance devices in silicon or III-V semiconductors have been placed onto flexible substrates to fabricate image
sensors that mimic the human eye or sensor arrays for catheters as shown in Figure. 3c5,6
.
More conventional printing techniques, including screen printing, inkjet, gravure, flexographic or aerosol printing,
have been pursued in order to achieve lower cost. Screen printed sensors have been commercially available for
some time. Examples are simple sensors to measure pressure, touch or strain (Figure 2). Another example are
printed glucose sensor labels which are sold in large quantities (multi-billions, yearly)7
. The disposable sensor strip
consists of printed electrodes and a printed enzyme layer. However, all the readout electronics is based on
conventional electronics which is packaged in a reader device that is not disposed of.
Current research is attempting to fabricate more complex sensor systems using printing methods, often including at
least some transistors or diodes. Figure 3d is an example of an all-printed sensor system developed by the European
3PLAST (Printable, pyroelectrical and piezoelectrical large-area sensor technology) consortium8
.
Figure 3: Examples of novel flexible sensors and sensing systems. In a), a sensor wristband is shown which was fabricated
using a combination of technologies including pick-and-place (Fraunhofer, EMFT). The x-ray image sensor array in b) was
made using a low-temperature amorphous silicon process and digital lithography (Palo Alto Research Center). In c), a
catheter balloon with embedded sensors is shown (U. Illinois / MC10 Inc.). The photo in d) demonstrates an all-printed
pressure/infrared sensor array monolithically integrated with transistors and a display (3PLAST project).

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Recent flexible sensor developments: A wide variety of research and development is currently addressing flexible
sensors and only a small selection will be highlighted here. As mentioned above, Figure 3 shows a few examples of
recent flexible prototype sensor devices.
In the US, the work at the University of Illinois (Prof. J. Rogers) has been pioneering and the startup company
MC10 is developing sensor products around the group’s technology5,6
. Sensor concepts include hemispherical
retina-like cameras, smart medical catheters or electronic tattoos to monitor muscle activity or heart rate. The
sensor systems are not only flexible but stretchable (which is achieved by transfer of circuits to a silicone
elastomer).
In Japan, at the University of Tokyo (Prof. T. Someya), new flexible and stretchable sensor concepts for sensor skin
have been developed and a recently reported flexible pressure sensor array had an embedded non-volatile memory
based on floating gate organic transistors9
.
In Taiwan, ITRI has demonstrated pressure sensitive plastic sheets which function as flexible drums (e-drums) and
National Taiwan University has reported interesting work on flexible thermal sensor arrays10, 11
.
Within the European Union (EU) several projects are being funded to explore flexible sensors, including and
3PLAST, LabOnFoil and FlexSMELL12
. The sensor system shown in Figure 3d, (3PLAST consortium) includes an
active-matrix pressure or infrared sensor array with electrochemical transistors and electrochemical display
monolithically integrated on a flexible substrate8
. The project FlexSMELL is targeting a hybrid (organic-
inorganic), very low-cost, ultra low-power olfaction system based on bio-receptors for flexible smart chemical
sensing tags13
.
There is also substantial interest in integrating sensors into textiles (e-textiles) as shown in the European MyHeart
project to monitor a person’s heart condition14
. Such sensors could also be embedded in pillow cases or blankets to
monitor sleep conditions. Other examples of EU funded efforts to foster e-textiles are STELLA (stretchable
electronics for large area applications), PROETEX or the SYSTEX coordination initiative15
.
Another important development is the advancement of flexible thin-film electronic circuits. Memory circuits to
store sensor data, analog to digital converters or amplifier circuits are all essential parts of a sensor system. The
University of Leuven (Belgium) in collaboration with the organizations IMEC and Polymer Vision recently
reported new results on flexible analog circuits that are suitable for sensor readout16
. There are still big challenges
ahead in directly competing with conventional silicon circuits but for some applications thin film alternatives may
soon become viable.
The retrieval of sensor data has been addressed in a variety of ways and radio frequency readout is a promising
method. For example, moisture sensing tags which are embedded in or attached to walls for preventing moisture
damage to structures17
. Wireless monitoring of food quality has been developed at the VTT Technical Research
Center of Finland by combining a meat spoilage sensor with RFID readout18
.
Autonomous sensor systems will need a power source and flexible solar cells, printed batteries or energy
scavenging solutions have been developed to be combined with flexible sensors.
Challenges for flexible sensors: Although flexible sensors have many promising properties, there are also
significant challenges. In general, a sensor’s performance is determined by characteristics such as sensitivity,
accuracy, reproducibility, specificity (e.g. cross-sensitivity), drift, operating range and speed or dynamic behavior.
The flexible form factor or the fabrication process will affect these properties.
Mechanical flexibility in sensor systems can be detrimental because of the stress variation upon bending which may
cause sensor drift and cross-sensitivity (e.g. due to piezoelectric or piezoresistive effects upon bending). Therefore,

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the packaging of flexible sensors is particularly critical. For example, it is advantageous to have the sensor layer
located in the neutral plane of a materials stack. The mounting or attachment of a sensor is also important. This
becomes evident in the example of the ‘disposable pressure sensor’ as part of the T-line continuous blood pressure
monitoring system by Tensys Medical, Inc19
. While the sensor element is disposable, a quite elaborate bracelet is
required to accurately position the sensor and to hold it in place.
Moreover, in applications such as structural monitoring, the coupling (e.g. thermal or mechanical) between the
sensor and the structure affects the sensor signal. For example, unintended damping between the sensor (e.g. an
accelerometer) and the structure to be measured must be avoided. If low cost is the motivator for the flexible form
factor, these surrounding circumstances related to sensor packaging and sensor attachment have to be carefully
contemplated and a tradeoff between sensor performance and sensor cost must be found.
Another challenge for inexpensive sensors, particularly those that are printed, relates to the device to device
reproducibility. Using conventional printing technologies the fabrication tolerances will not be as good as when
using photolithography to form devices. Therefore, the variation between devices will likely be higher. If a sensor
is matched to a readout circuit the electronics would have to be recalibrated or adjusted for each individual sensor.
This may be particularly cumbersome when the same readout electronics is used with exchangeable (disposable)
sensors. Calibration or trimming of each sensor would also be possible but would add extra cost and turn the device
more expensive.
Some of the described challenges may limit the scope of flexible sensors but for many applications their
performance will be sufficient and by careful engineering most of the challenges will likely be resolved.
Conclusion: Sensors are an important driver for flexible electronics and they offer many opportunities. A
combination of technologies will be required to cover the wide variety of applications and sensor needs. Due to the
breadth and the fragmentation of the sensor market there will always be opportunities for novel types of sensors. It
will be exciting to see in which areas and in which form flexible sensors will be able to capture a significant market
share. The intention of this article was to give a taste of the field of flexible sensors. There are numerous other
ongoing developments and many of them are regularly summarized by Veritas et Visus.
References:
1
P. Doe, “Bosch tops MEMS sensor ranking”, MEMS’ Trends, Issue 3, July 2010, p.4-5
2
Philips Research: BabyMat
3
M. Yip, et al., “A Flexible Pressure Monitoring System for Pressure Ulcer Prevention”, 31st Annual International
Conference of the IEEE EMBS, Minneapolis, Minnesota, USA, September 2-6, 2009, pp.1212
4
W.S. Wong, et al., Ch. 6 in “Flexible Electronics: Materials and Applications”, W.S. Wong, A. Salleo, eds.,
Springer, 2009
5
I. Jung, et al., “Dynamically tunable hemispherical electronic eye camera system with adjustable zoom
capability”, PNAS, February 1, 2011, vol. 108, no. 5, 1788–1793
6
D.-H. Kim, et al., “Materials for multifunctional balloon catheters with capabilities in cardiac electrophysiological
mapping and ablation therapy”, Nature Materials10, 316–323(2011) and MC10 Inc.: http://mc10inc.com/
7
GSI Technologies, LLC: http://www.gsitech.com
8
M. Zirkl, et al., “An All-Printed Ferroelectric Active Matrix Sensor Network Based on Only Five Functional
Materials Forming a Touchless Control Interface”, Adv. Mater. 2011, 23, 2069–2074 and http://www.3plast-
sensor.eu/
9
T. Sekitani, et al., “Organic Nonvolatile Memory Transistors for Flexible Sensor Arrays”, Science, Vol. 326, 11
December 2009, 1516
10
C.M. Lo, “A Portable Multi-Pitch e-Drum Based on Printed Flexible Pressure Sensors”, DATE 2010
Proceedings, 8.4.5, p. 1082

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11
W.P. Shih, “Flexible Temperature Sensor Array Based on a Graphite-Polydimethylsiloxane Composite”, Sensors
2010, 10, 3597-3610;
12
http://www.3plast-sensor.eu/; http://www.labonfoil.eu; http://www.flexsmell.eu
13
D. Briand, et al., “Making environmental sensors on plastic foil”, Materialstoday, Vol. 14, No 9, September 2011,
416 and http://www.flexsmell.eu
14
M. Harris, “The MyHeart Project: A Framework for Personal Health Care Applications”, Computers in
Cardiology 2007;34:137−140.
15
http://www.stella-project.de, http://www.systex.org, http://www.proetex.org
16
H. Marien, et al., “An Analog Organic First-Order CT __ ADC on a Flexible Plastic Substrate with 26.5dB
Precision”, ISSCC 2010,7.2, 136-138
17
T. Unander, “Characterization of Low Cost Printed Sensors for Smart Packaging”, Thesis, 2008, MID Sweden
University
18
K. Nummila, “Wireless sensing and RFID”: www.vtt.fi/files/research/mel/wireless_sensing_slides_short.pdf
19
http://www.tensysmedical.com/
http://www.veritasetvisus.com
Volume 5: February 28, 2010 – 86 pages
Volume 6: November 14, 2010 – 112 pages
Volume 7: September 30, 2011 – 127 pages