36 IEEE Instrumentation & Measurement Magazine October20111094-6969/11/$25.00©2011IEEE

Bruno Andò

O ver the past ten years, the development of low cost graphic technology-based sensors has been pro-ceeding rapidly. The use of innovative materials

and substrates has also picked up momentum. The interest in such sensors is justified by the need for both low cost, rapid prototyping techniques for research laboratories and mass-production processes for the realization of very low cost devices. Examples of addressed devices are RFID tags, antennas, keyboards, displays and especially sensors. The rapid prototyping of inexpensive devices and sensors by printing technologies is of great importance for the ev-eryday activities of the scientific community including research laboratories and academia.

The availability of novel technologies for the development of low cost sensors would

move market interest towards new applications, previously not very attractive because of the costs of traditional silicon electronics [1]. Printed sensors could be the “right answer” to such needs. Screen printing and inkjet printing have received more attention for the realization of printed sensors than other printing techniques. In this paper, we provide brief explana-tions of these technologies.

Screen Printing on SensorsScreen printing is a technique which requires the use of a mask that acts as a stencil. The stencil delimits areas where ink must be deposited on the substrate by the mechanical pressure ex-erted, typically through a roller. Examples of sensors realized by screen printing are: gas detectors exploiting conductive pat-terns realized by screen printing with nanoparticle inks [2], humidity sensors for smart packaging applications [3], im-pedance sensors applied to biosensors [4], and resistive force sensors [5]. The rapid widespread use of this technology led to the availability of a good choice of conductive, insulating and functional materials compatible with screen printing. The lat-ter reduces the need for custom formulations.

Furthermore, screen printing allows for the deposition of thick layers of material, thus increasing the track conductiv-ity and the device reliability. Drawbacks of screen printing

techniques are related to the development process which re-quires masks and physical contact with the substrate. These processes are based on photolithography and, in general, tech-niques requiring masks.

Inkjet printing does not require masks or micromachining which reduces processing time and costs (see Fig. 1).

Inkjet PrintingInkjet printing of polymers and materials is a fairly new tech-nique which could, in specific contexts, replace traditional techniques (e.g. sputtering, lithography, and post-process-ing). By using this ‘drop-on-demand technique,’ a layer of functional ink can easily be deposited on the substrate in well defined patterns without the need of patterning techniques and thus reducing waste of materials. Small volumes of ma-terial, in the range of 1-30 picoliters, may be used, resulting in high spatial resolution and good reproducibility (see Fig. 1). Moreover, inkjet printing is a contactless deposition technique which makes it applicable to many different sub-strates. Inkjet-based sensors offer the possibility to combine the performance of flexible substrates and functional inks with a very low-cost prototyping technique (see Fig. 2).

Examples of devices realized by this technique are avail-able in the literature, such as strain gauges and capacitors

Inkjet-Printed Sensors: A Useful Approach for Low Cost, Rapid Prototyping

Bruno Andò and Salvatore Baglio

instrumentationnotes

Fig. 1. Layout of inkjet printable sensors.

October2011 IEEE Instrumentation & Measurement Magazine 37

exploiting conductive polymers on paper [6], biosensors for glucose on carbon electrodes, RFID tags, and antennas [7], [8]. Ink-jet printing techniques are also used to realize gas-sens-ing films exploiting ZnO thin films for carbon monoxide and methane sensing.

Traditional materials adopted to realize sensors by ink-jet systems are electrically conducting polymers (such as PEDOT-PSS) and functionalized polymers (such as poliani-line, PANI). PEDOT-PSS is a conductive polymer (3, 4-ethylen dioxythiophene) oxidized with polystyrene sulfonated acid. It is a p-doped material with good thermal stability and rela-tively high electrical conductivity. The use of PEDOT-PSS to

Fig. 2. Scanning electron microscope (SEM) images of (a) a silver ink layer and (b) a contact structure.

Fig. 3. The resistive pressure sensor using a PEDOT-PSS sensing element: (a) the membrane with the printed resistive sensor, and (b) the assembled device.

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realize conductive patterns requires several printing cycles to reduce the electric resistivity of the pattern deposited. Poly-aniline (PANI) is a conducting polymer. Many applications of this material are related to gas sensors. Its physical properties are suitable for inkjet printing: it can be dissolved in organic solvents or in aqueous solutions.

Printing EquipmentThere are various printers available on the market that al-low inkjet printing of functional materials. Usually, a printer with a drop-on-demand mode is used, where the ink drop-lets are ejected by using a pulse that is generated by thermal or piezoelectric strategies. Many printing systems for rapid

prototyping are piezoelec-tric due to their ability to be used with different sol-vents . Systems based on piezoelectric heads produce resolutions on the order of tens of micrometers.

One of the main reasons one is drawn to inkjet printing is because of the nature and physical properties of inks. Their viscosity and electri-cal properties are particularly important. Many inks and so-lutions are being made with suitable physical properties and are compatible with com-

mon inkjet apparatus. Recently, inkjet printing has been shown to be a versatile technique for depositing a range of different materials for a variety of different applications (see Fig. 3).

Printing HeadsThe use of high-quality inkjet printing systems allows one to produce a wide variety of pieces of equipment. High quality inkjet printing systems are expensive due to the need for print-ing heads that are compatible with different kinds of inks and that can be used in repeated printing cycles, e.g. as in produc-ing PEDOT-PSS electrodes.

As stated in the literature, due to the requirement to pro-duce conductive patterns, the development of flexible electric components by using low-cost inkjet printing systems usu-ally requires a different technology for the realization of metal electrodes. Conductive structures such as wires, coils, and ca-pacitor electrodes are usually implemented by screen printing technology and metal-based solutions. After screen printing the conductive layers, the polymer layers (using PEDOT-PSS) can be added by using low-cost inkjet printers to produce sen-sors for piezo-resistors, resistive devices and functional layers (such as PANI) for gas sensors (see Fig. 4 and Fig. 5).

Combined Techniques In the literature, there are many reports of devices realized by the use of very low-cost desktop inkjet printers. Resistors im-plemented by PEDOT: PSS on PET substrate, an all-polymer RC filter [9], deposition of PANI for ammonia detection [2], and complex MEMS structures with silver nanoparticles [10] have all been implemented in this way. Where metal struc-tures, contacts or electrodes are required, they are realized by screen printing [11].

The possibility of using very low-cost printing systems to produce devices (including both electrodes and other func-tional layers) would be most welcome especially in research

Fig. 4. (a) The output of the reference pressure sensor, and (b) the output of the PEDOT-PSS resistive pressure sensor.

Fig. 5. Layout of the ammonia sensor.

October2011 IEEE Instrumentation & Measurement Magazine 39

laboratories and academia. In this context, metal based inks (e.g. silver based solutions) could play a strategic role for the realization of electrodes and conductive patterns due to their extremely low resistance and mechanical properties. More-over, the sensing properties (piezoresistive, thermoresistive) of metal inks could also be exploited. However, common sil-ver-based solutions are difficult to use with inkjet systems due to nozzle occlusion problems.

Recently, this possibility, of using metal based inks with inexpensive printing systems, has been investigated as a con-venient solution for research laboratories and educational institutions. It would allow for the realization of metal struc-tures (e.g. electrodes, wires, capacitor plates, IDT), polymer layers (e.g. resistor, piezo-resistor), and functional layers (ad-sorbing layers for applications in the bio-chemical contexts). Specifications and features of the printed components would be defined by adjusting their geometries.

Devices Realized by Low-Cost Inkjet TechnologyThe layout of a component printed at the DIEEI labo-ratories of the University of Catania, Italy is given in Fig. 1. These and other kinds of devices and topologies, such as resistors, strain gauges, capacitors, inductances, Interdigi-tated Devices (IDT), and their combinations, can be realized. Thinking of actual applications – IDT and also coils can be used in the field of bio-chemical investigations. In particu-lar, a functional layer could be deposited on the sensing area to bind the bio-target entity to the sensor by using assay tech-niques. The variation of the readout quantity (R, L, and C) would be strictly related to the concentration of the bio-tar-get in the sensing area. Combinations of components can be used to realize an electrical network for the implementation of resonant readout strategies, as well as remote addressable sensing systems.

An important aspect to be taken into account is the mor-phology of the deposited layers which is directly correlated to the performance of the device in terms of conductivity,

reliability and robustness against mechanical stress. Fig. 2 shows SEM images of a sil-ver ink layer and the detail of a contact structure.

Fig. 3 shows a resistive pressure sensor developed at the DIEES laboratory. The device consists of a flexible substrate with a printed re-sistive sensor, a protective membrane and the structure housing the transducer ele-ment. The device response

has been investigated by a dedicated set-up using a reference pressure sensor to obtain an independent observation of the applied stimulus. An example of the experimental results is shown in Fig. 4, which demonstrates the coherence between the behavior of the inkjet sensor developed and the response of the reference pressure sensor.

The device schematic in Fig. 5 is a chemical sensor for ammonia detection based on IDT electrodes realized in PEDOT-PSS on a PET substrate and exploiting a function-alized layer of PANI. The device is based on the idea that different concentrations of ammonia produce variations in the conductivity of the PANI layer. Such variations are sensed by the IDT device connected to dedicated electron-ics. The device response to ammonia stimulation cycles is shown in Fig. 6. Results have been obtained by increasing the ammonia concentration with a uniform step and observ-ing the corresponding output voltage from the conditioning electronics. As can be observed and is consistent with similar results in the literature, the device output response to am-monia solicitation is very sharp. Time required to partially restore the sensor output after the gas has been removed is quite long compared to the rise time. The restoring time de-pends strictly on the physical and geometric properties of the sensor, such as its dimensions, IDT topology, and the thickness of the PANI layer. Actually, the restoring time could be reduced by using forced air convection at the ex-pense of noise induced on the system output as shown by spiking shots in Fig. 6.

Considerations outlined through this column, as well as results available in the literature, demonstrate the concrete possibility of using relatively inexpensive, direct printing pro-cesses for the rapid prototyping of sensors.

References[1] M. Mäntysalo, V. Pekkanen, K. Kaija, J. Niittynen, S. Koskinen,

E. Halonen, P. Mansikkamäki, and O. Hämeenoja, “Capability of

inkjet technology in electronics manufacturing,” Proc. Electronic

Components and Technology Conf. 2009, pp.1330-1336.

Fig. 6. The ammonia sensor’s response to different ammonia concentrations.

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[2] K. Crowley, A. Morrin, A. Hernandez, E. O’Malley, P. G. Whitten,

G. G. Wallace, M. R. Smyth, and A. J. Killard, “Fabrication

of an ammonia gas sensor using inkjet-printed polyaniline

nanoparticles,” The Int. J. of Pure and Appl. Analytical Chem. 2008,

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[3] T. Unander and H. E. Nilsson, “Characterization of printed

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[4] M. Brischwein, S. Herrmann, W. Vonau, F. Berthold, H. Grothe,

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for the sensing of cell responses,” Proc. IEEE AFRICON 2007, pp. 1-5.

[5] R. Lakhmi, H. Debeda, I. Dufour, and C. Lucat, “Force sensors

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[7] Y. Amin, S. Prokkola, S. Botao, J. Hallstedt, H. Tenhunen, and L.

Zheng, “Inkjet printed paper based quadrate bowtie antennas for

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[10] S. B. Fuller, E. J. Wilhelm, and J. M. Jacobson, “Ink-jet

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Bruno Andò (bruno.ando@ieee.org) is a regular Instrumen-tation Notes columnist. His photo appears on the first page of the column. He received his M.S. and Ph.D. in EE at the University of Catania, Italy, in 1994 and 1999 respectively. From 1999-2001, he worked as a researcher with the Depart-ment of Electrical and Electronic Measurement (DIEES) of the University of Catania, and in 2002, he became an assis-tant professor. His main research interests are sensors design and optimization including advanced multi-sensor archi-tectures for visually impaired people, characterization of new materials for sensors, nonlinear techniques for signal processing with particular interest in stochastic resonance and dithering applications; characterization and condition-ing; and distributed measurement systems. Dr. Andò has co-authored scientific papers, presented in international conferences and published in international journals and books.

Salvatore Baglio received the “Laurea” and Ph.D. degrees from the University of Catania, Catania, Italy, in 1990 and 1994, respectively. Since 1996 he is with the Dipartimento di Ingeg-neria Elettrica Elettronica e dei Sistemi, University of Catania, where he is currently Associate Professor. Prof. Baglio teaches courses in “measurement theory,” “electronic instrumenta-tions,” and “integrated micro-sensors”. He is a coauthor of more than 250 scientific publications, which include books, chapters in books, international journals and proceedings of international conferences. He also holds several U.S. pat-ents. His research interests are mainly focused on micro and nanosensors, hysteretic materials for sensors, and nonlinear dynamics for transducers. Prof. Baglio is senior member of the IEEE, has served as an associate editor for the IEEE Trans-actions on Circuits and Systems and as Distinguished Lecturer for the IEEE Circuits and Systems Society, and is currently as-sociate editor of the IEEE Transactions on Instrumentation and Measurement.