All inkjet printed system for strain measurement

Bruno Andò, Salvatore Baglio, Salvatore La Malfa, Gaetano L’Episcopo

D.I.E.E.I.

University of Catania

Catania, Italy

bruno.ando@dieei.unict.it

Abstract— Usually low cost approaches for the direct printing of

flexible electric components require two different technologies

for conductive patterns and functional areas. The realization of

metal structures (wires, coils, capacitor electrodes) requires

screen printing technology while polymer components (piezo-

resistive layer, resistor, functional layers) can be printed by low

cost inkjet systems. In this paper the realization of strain gauge

sensors by inkjet printing of metal based ink is discussed as a

general approach for the rapid prototyping of low cost sensors

by direct inkjet printing techniques.

I. INTRODUCTION

In the past ten years the scientific community has shown a growing interest in the possibility to make very low-cost electronics and sensors which adopts the technologies of the graphic industry and exploiting innovative materials and flexible substrates. This interest is driven by needs for low cost mass-production processes e.g. for RFID tags, antennas, keyboards, display and sensors production [1, 2].

The availability of novel technologies for the development of low cost sensors would move the market interest towards new applications, previously not much attractive because of the costs of traditional silicon electronics.

Moreover, the rapid prototyping of cheap devices and sensors by printing technologies is also of great importance for the scientific community including research laboratories and academy.

Among direct printing techniques, those who have received more attention for the realization of printed sensors are essentially screen printing and inkjet printing.

Screen Printing is a technique which requires the use of a mask that acts as stencil. The stencil delimits the areas where ink is free to adhere to the substrate, following a mechanical pressure exerted typically through a roller. Examples of sensors realized by screen printing are: gas detectors exploiting conductive patterns realized by screen printing nanoparticles inks [3], humidity sensors for smart packaging applications [4], impedance sensors applied to biosensors [5] and resistive force sensors [6].

Given the spread of this technology at a global level, a good range of conductive, insulating and functional materials, compatible with screen printing, are now commercially available. Furthermore, screen printing allows to easily obtain thick layers of material which increases the tracks conductivity.

In spite of processes based on photolithography and in general requiring masks such as screen printing, inkjet printing does not require masks or micromachining.

Inkjet printing of polymers and materials is a fairly new technique which could replace in specific contexts traditional techniques (e.g. sputtering, lithography, and post-processing). By this drop-on-demand technique a layer of functional ink can be easily deposited on the substrate in well defined patterns without the need of patterning techniques and thus reducing time, costs and waste of materials. Small volumes of material, in the range of 1-30 picoliters, may be filed with a high spatial resolution and good reproducibility. Moreover, inkjet printing is a contactless deposition technique which makes it applicable to many different substrates.

Specifically, inkjet-based sensors offer the possibility to combine performances of flexible substrates and functional inks with a cheap rapid prototyping technique.

Traditional materials adopted to realize sensors by inkjet systems are electrical conducting polymers (such as PEDOT-PSS) and functionalized polymers (such as polianiline, PANI).

PEDOT-PSS is a conductive polymer polymerpoly (3,4-ethylen dioxythiophene) oxidized with polystyrene sulfonated acid. It is a p-doped material with a good ambient and thermal stability and a relatively high electrical conductivity. The use of PEDOT-PSS to realize conductive patterns requires several printing cycles in order to reduce the electric resistivity of the pattern deposited.

Polyaniline (PANI) is a conducting polymer. Many applications of this material are related to gas sensors. Concerning its physical properties, it can be dissolved in organic solvents or in aqueous solutions which are suitable to be used by inkjet printing.

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The implementation of resistor, contacts and electrodes by PEDOT-PSS on PET [7], all-polymer RC filter [8], PANI based devices for the detection of ammonia [3] and complex MEMS structures with silver nanoparticles [9] are examples of devices realized through inkjet techniques.

For what concerns printing equipments, there are various solutions available on the market allowing for inkjet printing of functional materials [10-11]. Usually, printer adopting drop-on-demand mode are used where the ink droplets are ejected by the use of a pulse generated by thermal or piezoelectric strategies.

Many printing systems for rapid prototyping are piezoelectric due also their intrinsic suitability to be used with different solvents. Systems based on piezoelectric heads performs a resolutions in the order of the tens of micrometers.

Usually, inkjet printers are expensive equipments due to the need for both printing heads compatible with different kind of inks and the implementation of repeated printing cycles. The latter is mandatory, e.g. for the realization of conductive patterns by PEDOT-PSS. It can be observed that the use of expensive inkjet printing systems allows for the realization of complex devices.

Actually, one of the main deal using low cost inkjet printing is the nature and the physical property of inks with particular regards to their viscosity and electrical properties.

As an example metal based inks (e.g. silver based solutions), which could play a strategic role for electrodes and sensing structures due to the extremely low resistivity and mechanic properties, are difficult to be used with cheap inkjet systems due to nozzle occlusions problems.

Many efforts have been dedicated to make conductive inks compatible to specifications required by common inkjet apparatus.

Many examples in the literature adopt a mixed approach for the realization of low cost devices by direct printing technology. Conductive structures such as wires, coils, capacitor electrodes are usually implemented by screen printing technology. As an example, it must be recalled that the development of conductive layers by polymers such as PEDOT-PSS requires several printing cycle which are not implementable by low-cost inkjet printers. But, once electrodes have been realized, polymer layers (such as PEDOT-PSS) for piezo-resistors, resistive devices and functional layers (such as PANI) for gas sensors are successively deposited by low cost inkjet printers.

The possibility to use cheap printing systems for the realization of all inkjet devices (including both electrodes and other functional layers) would be terrific especially for research laboratories and academy.

The novel approach proposed in this paper for the realization of different kinds of planar electric components (including electrodes and sensing structures) is based on the use of just one silver ink printable by a low cost inkjet system. The adopted metal based inks (a silver based solutions) has a good conductivity while its sensing properties (e.g. piezoresistive) could be conveniently exploited.

Specifications and features of the printed components can be defined by suitably fixing their geometries. In the following sections the possibility to develop strain sensors is investigated and experiments are presented to assess the proposed approach and the behavior of prototype developed.

The approach proposed in this paper for the realization of low cost sensing devices, by an all inkjet printing process adopting silver ink, is strategic for both the above highlighted technological aspects and for research purposes.

II. THE DEVICE PROTOTYPE

Figure 1 shows the layout and a real view of a piezo-resistive strain sensor with the following dimensions: track

width 200 µm, track spacing 200 µm, total length 20 mm. The

device substrate is PET with a thickness of 100µm.

The device belongs to a set of strain sensors with different geometries developed by using a cheap EPSON piezo inkjet printer. The metal ink adopted for the realization of the conductive pattern is the silver nano-particles solution “Metalon® JS-B15P” by Novacentrix.

Figure 2 shows electron microscopy (SEM) images of the silver layer deposited on the device shown in Figure 1b. The structure of the silver tracks is quite homogeneous at the inspected scales and an approximate thickness of 1.90 µm has been estimated.

Figure 3 gives both a schematization and the real view of the clamping system developed to implement the electrical connectivity of the device.

(a)

(b)

Figure 1. (a) Layout of the piezoresistive sensor; (b) A real view of the

printed device.

III. EXPERIMENTAL RESULTS

In order to estimate the device behavior a dedicated experimental set-up has been developed. The system allows to produce a controlled strain on the cantilever beam and to measure the variation of the device resistance. A standard bridge configuration with temperature compensation has been used as conditioning electronics.

(a)

(b)

Figure 2. SEM images of one silver track of the device shown in Figure 1b.

The bottom image was used for estimating the silver thickness (∼1.90 µm).

(a) (b)

Figure 3. Clamping mechanism implementing the electrical contact for the

inkjet printed device.

Several measurement surveys have been accomplished to assess the device reliability. Figure 4 shows experimental results obtained for the sample sketched in Figure 1b. In particular, the calibration diagram in terms of the applied

strain as a function of the relative variation of the strain gauge resistance R, as respect to the nominal resistance R0=67.7Ω, is given. The uncertainty band has a coverage factor of 2.7. The estimated gauge factor is around 3. A complete characterization survey has been implemented for the whole set of the developed devices and coherence between the expected results and the obtained experimental behavior has been assessed.

Figure 4. Calibration diagram of the strain sensor shown in Figure 1b.

Symbols represent the real measurements

REFERENCES

[1] M.Mäntysalo, V.Pekkanen, K.Kaija, J.Niittynen, S.Koskinen, E.Halonen, P.Mansikkamäki, O.Hämeenoja, Capability of Inkjet Technology in ElectronicsManufacturing, Electronic Components and Technology Conference, 2009

[2] B. Andò, S. Baglio, Inkjet-Printed Sensors: A Useful Approach for low cost, rapid prototyping, IEEE Instr. Meas. Magazine, Vol.14-5, 2011.

[3] K. Crowley, A. Morrin, A. Hernandez, E. O'Malley, P. G. Whitten, G. G. Wallace, M.R. Smyth, A.J. Killard, Fabrication of an ammonia gas sensor usinginkjet-printed polyaniline nanoparticles,The International Journal of Pure and Applied Analytical Chemistry 2008

[4] T. Unander, H.E. Nilsson, Characterization of Printed Moisture Sensors in Packaging Surveillance Applications, IEEE Sensors Journal, Vol. 9, No 8, August2009

[5] Brischwein, M.; Herrmann, S.; Vonau, W.; Berthold, F.; Grothe, H.; Motrescu, E.R.; Wolf, B.; The Use of Screen printed Electrodes for the Sensing of Cell Responses, IEEEAFRICON 2007

[6] Lakhmi, R.; Debeda, H.; Dufour, I.; Lucat, C.;Force Sensors Based on Screen-Printed Cantilevers, IEEE Sensors Journal, Vol. 10, No 6, 2010

[7] C. Srichan, T. Saikrajang, T. Lomas, A. Jomphoak, T. Maturos, D. Phokaratkul, T. Kerdcharoen, A. Tuantranont, Inkjet printing PEDOT PSS using desktop inkjetprinter, Electrical Engineering/Electronics, Computer, Telecommunications and Information Technology, 2009. ECTI-CON 2009. 6th International Conference on,2009

[8] Y. Liu, T. Cui, K. Varahramyan, All - Polymer capacitor fabricated with inkjet printing technique, Solid-State Electronics, Volume 47, Issue 9, September 2003

[9] S.B. Fuller, E.J. Wilhelm, J.M. Jacobson, Ink-jet Printed Nanoparticle Microelectromechanical Systems, Journal of Microelectromechanical Systems,vol.11,no.1,February 2002

[10] www.dimatix.com

[11] www.microdrop.de