Making environmental sensors on plastic foil

Making environmental sensors on plastic foil With the emergence of the printed electronics industry, the development of sensing technologies on non conventional substrates such as plastic foils is on-going. In this article, we review the work performed and the trends in the development of environmental sensors on plastic and flexible foils. Our main focus is on the integration of temperature, humidity, and gas sensors on plastic substrates targeting low-power operation for wireless applications. Some perspectives in this dynamic field are also provided showing the potential for the realization of several types of transducers on substrates of different natures and their combination with other components to realize smart systems. Danick Brianda*, Alexandru Opreab, Jérôme Courbata, and Nicolae Bârsanb a Ecole Polytechnique Fédérale de Lausanne (EPFL), Institute of Microengineering, Sensors, Actuators and Microsystems Laboratory, Rue Jaquet-Droz 1, P.O. Box 526, CH-2002, Neuchâtel, Switzerland b Institute of Physical and Theoretical Chemistry, University of Tübingen, Auf der Morgenstelle 15, D-72076 Tübingen, BW, Germany * E-mail: [email protected] The driving forces for organic and printed electronics are the

health, safety, and security purposes. A lot of work is underway on

display and lightning, solar cell, battery and electronics (e.g., RFID)

the development of smart sensors and wireless sensor networks based

industries1. The complete technology chain is being established in

on silicon technology, targeting different types of application. Printed

the fields of materials preparation, processing and characterization

electronics are becoming a more and more mature technology every

equipment, and production. Over the last decade, there has been

day, and new kinds of product are expected in the near future. Besides

a significant increase in the efforts dedicated to the development

a strong potential for cost-effective production based on additive

and implementation of electronic components on flexible and

processes with a reduced infrastructure, the benefits of printing devices

stretchable substrates for other types of application, such as

on plastic foil include their potential to be light weight, foldable/rollable,

sensing, and notable results have been obtained by different

transparent, thin and conformal, wearable, and produced on a large

research groups2-8. This technology could result in sensors

scale, depending on the processing technology involved.

being introduced to new settings, by significantly reducing their production cost and by adding new functionalities. The monitoring of environmental parameters in a distributed manner is of significant interest in different fields, for comfort, environmental,

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In this review, we report on the recent advances achieved in the development of individual sensors and multi-sensor platforms on plastic foil for environmental monitoring, with a special focus on temperature, humidity, and gases. We will begin by reviewing the

ISSN:1369 7021 © Elsevier Ltd 2011

Making environmental sensors on plastic foil

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exciting work that has been performed in this field in recent years.

range7. However, the gas sensors based on organic transistors require

Secondly, we will introduce some of the work we carried out on this

further development to achieve the required sensing performance and

topic, with the integration of different environmental sensors on a

reliability needed for commercialization, as discussed by the group of V.

single plastic platform. We have integrated different sensing principles

Subramanian at University of California, Berkeley in reference16.

on a polyimide foil, such as capacitive and resistive read-outs for the

The development of other types of gas sensor on plastic-flexible

detection of several types of environmental parameters including

substrates has only begun very recently. Most of the samples are made

temperature, humidity, reducing and oxidizing gases, and volatile

on polyethylene-terephthalate (PET), polyethylene naphthalate (PEN),

organic compounds (VOCs). These sensors on plastic foils are required

and polyimide (PI), and some on parylene substrates. Conventional and

to realize intelligent RFID tags for environmental monitoring9. Such

printed hybrid processes and organic and inorganic hybrid materials

devices may eventually find application in wearable systems, smart

are generally used with the aim of producing fully print compatible

buildings, and in the logistics of perishable products.

devices, as conceptually illustrated in Fig. 1. A major part of the published work consists of single flexible humidity sensors, sometimes

Gas sensors on plastic foil

combined with a temperature sensor on the same platform17-24.

Different transducing principles have been developed for atmospheric

Capacitive and resistive transducers are commonly used as sensor

gas sensing10. These principles include the resistive principle, mainly

architectures. Volatile organic compounds ammonia and hydrogen

based on metal-oxide and polymeric (chemiresistors) gas sensitive

sulphide sensors have also been reported, some of which use printing

films; the capacitive principle, involving a change in the dielectric

technologies to deposit gas-sensitive conducting polymers and silver

constants and/or a swelling of the sensing film; the field-effect

electrodes25-29. Articles have also been published on NOx detection

principle, based on a change of work function and semiconductor

in the sub and low ppm range based on ink-jet printed inorganic or

surface potential; the colorimetric principle, in which the optical

organic polymeric materials, amorphous OTFT operating at room

absorption spectrum is modified by the gaseous analyte; and the

temperature, and resistive PEDOT:PSS; however, the gas sensing

resonating principle, in which an addition of mass modifies the

performance has been poor30,31. The colorimetric detection of gases

resonant frequency of the resonator. Most of the gas responses of

on foil has been also demonstrated with the use of gas sensitive dyes

these devices significantly depend on the temperature and humidity

combined with an optical waveguide on plastic, with the detection

content of the surrounding environment. Moreover, the gas sensors

of concentrations of CO2 below one percent in nitrogen, and the

based on these transducing principles suffer from a lack of selectivity.

sub-ppm detection of NH3 in air32,33. Looking at the different sensing

It is therefore constructive to use an array of gas sensors alongside

principles described above, only resonating type gas sensors have

temperature and humidity sensors to obtain valuable information on

not yet been fabricated directly on plastic foil. A hybrid approach in

the composition of the surrounding atmosphere. Considerable efforts

which a surface acoustic wave (SAW) chip has been transferred onto

have been dedicated to the miniaturization of these transducers

a plastic substrate has been reported, but was applied to light sensing

based on silicon technology. A nice example of their integration into

in that communication34. Another interesting and new approach is

arrays with the electronics interfaced on a single silicon chip has been

the coating of passive (no power source on board) conventional RFID

produced at the ETHZ in Switzerland11.

tags with chemically sensitive films to form a chemical sensor35. The

There are a limited number of publications on gas sensors on

detection of several vapors of industrial, health, law enforcement, and

plastic/flexible foils, but this number is growing. Interest in this area began with the development of organic electronic transistors and the study of issues surrounding their sensitivity to humidity and different gases; this has necessitated investigations on humidity and gas impermeable encapsulation barriers, as well as air stable organic semiconductors. Some groups saw an opportunity to exploit this drawback to make gas and humidity sensitive devices. They have worked on organic thin film transistor (OTFT) based gas sensors: on single devices and arrays, on silicon and plastic substrates, for sensing volatile organic compounds (VOCs)2,7,13-15. One important achievement has been reported by Torsi et al., in the form of a novel chiral bilayer organic thin-film transistor gas sensor, comprising an outermost layer with built-in enantioselective properties that exhibits a field-effect amplified sensitivity that enables differential detection of optical isomers in the tens-of-parts-per-million concentration

Fig. 1 Schematic drawing of a roll to roll production line for chemical gas sensors on plastic foil. The transducers and coating layers are coated using additive printing techniques, such as the gravure printing of interdigitated electrodes and the local ink-jet printing of different sensing layers.

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Making environmental sensors on plastic foil

security interest (ethanol, methanol, acetonitrile, and water vapors)

sensing devices on plastic foils. Meanwhile, the reliability aspects and

was demonstrated with a single 13.56 MHz RFID tag coated with a

the evaluation of their flexibility have yet to be fully addressed, with

solid polymer electrolyte sensing film. For multicomponent detection

studies on reliability only recently being released in the field of flexible

and quantification using a single RFID sensor, multiple parameters

electronics50,51.

from the measured real and imaginary portions of the complex impedance are calculated. Finally, some groups have started to look

Multi-parametric sensing platforms

at the implementation of gas sensitive nanomaterials on plastic foil

The simultaneous detection and quantification of physical, chemical,

with the transfer or self-assembly of nanowires, nanotubes, and

and biological information from the ambient with mobile/autonomous/

nanoparticles on flexible substrates36-42.

remote sensing systems is easier accomplished when using complex

Regarding the integration of temperature sensors on plastic-flexible

platforms that integrate several dissimilar sensors; ideally all those

foil, the conventional platinum resistance temperature detector (RTD)

required by a specific application. The first step towards flexible

has been realized on a flexible polyimide substrate, for operation up

multi-parametric sensing platforms should be, and actually was,

to 400 °C, and resistors made of TaSiN have been shown to exhibit

the development of different kinds of sensors on plastic substrates.

high temperature coefficient of resistance (TCR) values43,44. Some

Successfully attempts have been already made for humidity23, reducing

approaches based on thermo-sensitive polymers have also been

or oxidizing gases48,52,53, and volatile organic compounds (VOCs)49.

evaluated45,46,

In reference52 the integration of metal oxide (MOX) based gas sensors

based on graphite or metallic powders in a PDMS

matrix, which suffered from non-linearity and were limited to 100 °C.

for reducing and oxidizing gases is described (see Fig. 2). The results

Another approach that has been reported is the screen printing

obtained were promising, as the response of the sensor on polyimide

of a polymeric thermo-sensitive material on Kapton for textronic

(PI) foil was, analyte depending, between 40 % and 100 % from that

applications, e.g., measurement of the temperature of the human

of a reference sensor produced on a silicon nitride hotplate using the

body47.

same deposition method (drop coating)54. Principally intended to

In the process of considering the next generation of smart gas sensors (besides silicon based technologies) EPFL-IMT SAMLAB has launched GASID (GAS IDentification, in reference to RFID) to look

demonstrate the sensor concept viability, the hotplates on flexible PI arrays have been coated with only one sensing material: SnO2. A diversification of the sensor types on one PI platform is reported

at the potential integration of micro gas sensors on plastic foil. In

in references48,53. In reference48 two different types of MOX (SnO2

collaboration with the Institute of Physical Chemistry at the University

and WO3) have been deposited on PI platforms containing several

of Tübingen and the Fraunhofer IPM in Germany, this initiative has

transducing areas. Hydrophobic Teflon based filtering layers (see Fig. 3)

led to the proof of concept for capacitive differential VOCs/humidity

have been employed to increase the selectivity. The foil level packaging

sensors, semi-conductor metal-oxide gas sensors, and colorimetric gas

of the chemical gas sensors is described in detail in reference55. In

sensors on plastic foil (PET, PEN, and PI)33,48,49. Surprisingly enough,

order to reduce the readout power, the same contribution proposes

EPFL-IMT SAMLAB and the University of Tuebingen were able to

the direct sensor readout on the sensor system microcontroller using

demonstrate the continuous operation of metal oxide gas sensors

the time constant of an RC circuit that includes the sensor resistor. The

made on polyimide hotplates for several months48. They have also

heating power could be also reduced from 13.7 mW to 340 μW for one

shown, using a simple sensor architecture and making use of the plastic

hotplate through pulse operation53.

substrate as humidity sensing element, that volatile organic compounds and humidity can be measured simultaneously using two capacitive sensors in differential measurement mode49. EPFL-IMT SAMLAB and the University of Tuebingen have produced a multi-parameter sensing platform (for VOCs, temperature, humidity, reducing and oxidizing gases) on plastic foil, based on standard clean room processes48. These devices have great potential but their manufacture has to be rethought since plastic substrates are not welcome in conventional microelectronics foundries. Production at the lowest possible cost is vital in order to open up the market, which is currently inaccessible thanks to silicon sensor technologies. To reach the cost targets, one needs to develop heterogeneous materials, processes, and integration methods to enable the development of multi gas sensor platforms. The work presented in the next section most likely represents the most advanced assessment of the performances of multi-parametric

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Fig. 2 Integrated metal-oxide semiconductor gas sensors on polyimide foil. Reproduced from46 with permission of Elsevier.

Making environmental sensors on plastic foil

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Fig. 3 Metal-oxide gas sensor platform with dry photoresist rims around the transducing areas and gas permeable filters.

in these cases the full platform potential is coming into play. In order to eliminate the effects of the unwished residual sensitivity of the substrates to gases, only one capacitor is covered with functionalized polymers, the other remaining uncovered and delivering a capacitance reference. Using a differential readout, which can be directly implemented at hardware level through a differential capacitance to digital converter, the response of the substrate foil is canceled out from the useful signal. Thus the “smartness” of the sensor system relies directly on the sensors themselves and not on the software driving the system microcontroller. The operation of the platform can be easily understood by analyzing the sensor responses to controlled changes of the ambient atmosphere composition, by using a measuring chamber with a gas mixing system. As depicted in Fig. 5, the standard evaluation and calibration procedure is based on several independent exposures towards test VOCs/gases (n-hexane, n-propanol, ethanol, toluene, ammonia and humidity) diluted in dry synthetic air (80 % N2 + 20 % O2 – carrier gas) or in synthetic Fig. 4 Integrated temperature and capacitive gas sensors on flexible polyimide foil. Reproduced from43 with permission of Elsevier.

air with a certain humidity content (humidity background). The test gas concentrations usually start from the time weighted average (TWA) values. Between exposure sequences recovery times are allowed, when

In the work presented in reference49, a Pt-resistance thermometer

only the carrier gas with background humidity is purged through the

and two additional capacitive interdigital structures have been

measuring chamber. Fig. 6a displays the raw capacitance signals from

patterned in the same processing step (see Fig. 4). The number of

an individual capacitive sensor and reference capacitor. One has to

capacitors is not technologically limited, as shown later on, but two are

first remark on the huge responses to humidity (caused by the high

enough to underline the principle of operation.

dipolar momentum of the water molecules), the large time constants

Coated with suitable polymers the capacitors can independently

associated with the humidity changes, and the apparent lack of response

detect different VOCs, provided the substrate sensitivity towards the

for toluene and ammonia. However, the difference between the sensor

analytes is significantly lower than that of the sensing layers. Often this

capacitance and the reference one (actually the output of the platform

requirement is not satisfied by plain electrotechnical-grade foils and

in the differential operation mode) results in a quite unexpected picture

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Making environmental sensors on plastic foil

Fig. 5 Gas exposure protocol used for the evaluation/calibration sequences of capacitive VOCs/gas sensors. Reproduced from43 with permission of Elsevier.

(see Fig. 6b). The platforms are sensitive enough to all analytes to

as a humidity sensor. The lower graphs dedicated to the MOX devices

allow the extraction of the calibration curves over roughly three orders

point out the respectable sensor functionality, especially with 50 %

of magnitude for the concentration of the target gases49. They are

humidity background that is the normal for environmental applications.

reversible and relatively fast (response and recovery times on the order

One has to remark on the increased sensitivity of the nanogranular WO3

of minutes) but not very selective. The cross sensitivity to humidity,

sensing element for NO2 (~70 ppm-1 @ 1 ppm NO2) and its significantly

obvious in Fig. 6, drastically reduces the performance of single platforms.

reduced sensitivity for the other analyte, ethanol, resulting in a fair

By using the same concept, capacitive sensor arrays have been realized56,

which in conjunction with suitable recognition software

provide reasonable predictions concerning the composition of the gaseous/VOCs mixtures (see Fig. 7). For demonstration proposes the Unscrambler® program has been used, but dedicated software would

selectivity. Using the TWA values for NO2 and ethanol (5 ppm and 500 ppm respectively) as reference concentrations, one obtains a response (R) ratio (quantifying the selectivity) of: ⎡ R TWA ⎤ NO ⎢ TWA ⎥ ~ 15 R . Et ⎥⎦WO ⎣⎢ 2

3

be required for practical applications. The success of linear algebraic

The overall merit figure of the SnO2 device (also a nanogranular

methods, based on linear sensor responses, is often compromised

material) as ethanol sensor is not as good (ethanol sensitivity of

by the cross sensitivity of the devices. By losing the simplicity and

~1 ppm-1 @ 20 ppm ethanol and a response ratio of ~3 at TWA

increasing the cost, it is possible to foresee the implementation of

concentrations). The dissimilarities between the two MOX sensor

more complicated mathematical approaches.

performances are related to the sensing material and analyte

In some cases, the “parasitic” capacitive contributions of the

was not optimized; the communication proposing only a sensor

as a humidity sensor in applications where the response and recovery

concept and proving its feasibility. The influence of the humidity on

times are not

critical49.

A more complex sensor integration approach48 brings together

the MOX sensors is reduced if the variations occur in the middle and upper humidity ranges (30 % to 90 % relative humidity) due to the

temperature, MOX and capacitive gas/humidity/VOCs sensors on

rather high operation temperature (280 °C). However, alternation

the same PI substrate by combining the sensors addressed above.

of the exposure sequences with and without humidity backgrounds

In order to miniaturize the multi-sensor platform the capacitive and

impinges on the sensing layer surface properties (through the superficial

temperature sensors have been scaled down (see Fig. 8).

concentration and type of the OH groups) and results in some baseline

The signals of all gas sensors acquired during a demonstrative

420

characteristics, but also to the fact that the manufacturing technology

substrates (dashed olive curve in Fig. 6a) can play a positive role, acting

(sensor signal in the absence of the main analyte) drifts that can be

exposure to NO2 (oxidizing gas), ethanol (reducing gas) and humidity,

observed in Fig. 9. In spite of these drifts, the response reproducibility

are depicted in Fig. 9. Panel (a) shows the sensor behavior at low ethanol

is good, with mid and long term stability for both sensors types (MOX

concentration while panel (b) refers to a higher ethanol concentration.

and polymer based). A new contribution containing statistical data

In the high ethanol concentration range the capacitive response of the

over several months of operation has been prepared and submitted for

platform to ethanol is visible, in addition to that for humidity, which is

publication elsewhere. The power consumption of the sensor heater in

always present. The middle graph of each panel reassesses the extraction

continuous operation mode was about 18 mW but intermittent or pulse

procedure of the capacitive response in the differential operation regime

operation are also possible as mentioned above. Through the examples

indicating, at the same time, the possibility to use the reference capacitor

given, “flexible” and “on foil” environmental sensor systems have

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(a)

(b)

Fig. 7 PLSR2 prediction for ethanol in humidity and toluene background for a polydimethylsiloxane (PDMS), polyetherurethane (PEUT) and polycyanopropylphenylsiloxane (PCPPS) sensor array.

Fig. 6 (a) Individual responses of the capacitive sensor and reference capacitor provided by a capacitive and temperature sensor platform. (b) The gas response of the platform in differential operation mode. Reproduced from43 with permission of Elsevier.

Fig. 8 Multi-sensor platform micrograph. MOX: nanogranular SnO2 and WO3 metal oxide thick films; CAP: interdigital capacitors, one of them coated with PEUT; Pt Them: Pt -resistance thermometer. Reproduced from42 with permission of Elsevier.

been revelaed as feasible, and are pushing research and development

type of sensing devices addressed here, the ink-jet printing of electrodes

interest/activities/efforts towards cheaper and large scale manufacturing

and sensing layers, organic and inorganic, is underway in different

technologies. This type of approach will require new material structures

groups. The use of plastic substrates is also compatible with the low

and morphologies, compatible with the new production tools and

temperatures required for the preparation of nanostructures and their

conditions, and require continuous feedback from materials science.

functionalization with chemical and biological agents.

Perspectives

& printed electronics is bringing about new opportunities for the

It is foreseeable that the fabrication of physical and chemical sensors

realization of sensors on unconventional substrates that could lead to

on plastic foils will evolve towards all printable technological solutions.

new applications in the near future. On one hand, printed electronics are

For that to happen, one needs the formulation of the appropriate inks,

being considered as a production means for very low-cost RFID tags. On

especially for the chemically sensitive materials. As already seen in

the other hand, there is a need for a variety of cost-effective sensors that

this review, polyimide will only be used for applications with specific

could be manufactured directly on RFID labels to make them smarter;

requirements regarding temperature and the robustness of the substrate:

not only temperature, humidity and gas sensors, but also accelerometers

most devices will be produced on PET and PEN substrates. Regarding the

(vibrations, shock), light and pressure sensors, to name a few.

The emerging industry of large area manufacturing and organic

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Making environmental sensors on plastic foil

(a)

(b)

Fig. 9 Multi-sensor platform signals in response to a demonstrative gas exposure protocol: (a) Low ethanol concentration range. (b) High ethanol concentration range. Reproduced from42 with permission of Elsevier.

The heterogeneous integration of components into smart sensing

the production yield and reliability, especially under mechanical

sensing devices. The development of memories, power sources, and

deformation, of the individual components and the systems.

communication components (e.g., antennas) on plastic foil and their

422

foils “laminated” together, but many issues remain regarding

systems will be a key aspect in the future development of these

These smart labels are expected to have an impact in the logistic

integration is on-going. A System in Foil approach could allow the

sector with the monitoring of goods during their transport. In a longer

integration of all these components on a unique foil or on different

term perspective, cost-effective smart sensing labels on plastic can

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be envisioned as a key enabling technology in the deployment of the

components. At EPFL-IMT SAMLAB, we are also looking at the

“Internet of Things”. We can imagine that these devices could not only

integration of other physical sensors (accelerometers, pressure sensors,

be made on plastic but also on other types of flexible substrates such

resonators) to widen the applications of these smart labels.

as paper, thin metal sheets, textiles, biodegradable materials.

There are surely a wider range of sensors and potential applications that may make use of production on plastic substrates. Besides displays,

Conclusion

lightning panels, photovoltaic cells, batteries, and OTFTs circuitries,

Some examples of sensors made on plastic foil have been reviewed

the coming years will lead to a generation of lightweight, flexible,

in this paper with a specific focus on temperature, humidity, and

conformable and even transparent sensing devices manufactured on

gas sensors. We have also introduced our work on a multi-sensor

compliant substrates of different natures. The direct printing of devices

platform on flexible polyimide foil that has been developed for the

onto a product is even foreseeable for specific applications.

environmental monitoring of different parameters. The characteristics of these platforms are of high interest for the realization of ultra-low

Acknowledgements

power devices that could be processed at low-cost using printing

We are grateful to the Marie-Curie Initial Training Network program

processes. Our next steps are focused on the fabrication of devices

under the FlexSmell project (FP7 - Grant ITN no.238454) and the

using printed processes and their direct integration onto flexible

GOSPEL Network of Excellence on Artificial Olfaction and Gas Sensing

plastic RFID smart labels based on a System in Foil approach,

Technologies (FP6 -Grant IST no.507610) for the partial funding of the

using a combination of organic/printed and inorganic/silicon based

work performed by the authors.

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