Highly reproducible printable graphite strain gauges for flexible devices

Sensors and Actuators A 206 (2014) 75–80

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Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna

Highly reproducible printable graphite strain gauges for flexible devices Alexander Bessonov a,∗ , Marina Kirikova a , Samiul Haque b , Ilya Gartseev a , Marc J.A. Bailey a a b

Nokia Research Center, Skolkovo, Moscow region, Russian Federation Nokia Research Center, Cambridge, United Kingdom

a r t i c l e

i n f o

Article history: Received 3 October 2013 Received in revised form 26 November 2013 Accepted 26 November 2013 Available online 4 December 2013 Keywords: Strain gauge Graphite ink Xenon flash lamp Flexible sensor Temperature compensation Printed electronics

a b s t r a c t A growing area for the electronics industry is the development of flexible components for novel devices. Controlling the flexibility of such devices requires the precise and reliable measurement of strains in a manner compatible with the form and function of the device. In this article, we demonstrate the fabrication and characterization of printed strain gauges with a gauge factor as high as 19.3 ± 1.4, fast signal response and high reproducibility. The device is made of graphite ink deposited by screen printing on a plastic substrate. The flexible printed sensor is capable of precisely measuring repetitive tensile and compressive bending strain changes. An approach for eliminating the temperature-induced errors of strain gauges based on neutral axis engineering is also described. © 2013 Published by Elsevier B.V.

1. Introduction Recent developments in organic and printed electronics are paving the path for the creation of commercially viable flexible devices [1,2]. Such devices will need a number of sensors with quick signal response and reliability for the precise measurement of their flexibility. In particular, strain gauges systems, whose electrical resistance changes in proportion to the amount of strain, are necessary for measuring mechanical deformations in a precise manner [3]. Strain measurement supports determining the state of the device and accomplishing certain operations that are associated with flexibility, for instance, new possibilities of input and output interactions between the user and the device [4]. Printable and flexible strain gauges offer distinct advantages over conventional rigid sensors, such as mechanical flexibility, smaller dimensions, and higher sensitivity [5,6]. Wearable computing requires multiple sensors integrated into small devices and the manufacture of multiple sensors in layers and arrays will be enabled by the development

∗ Corresponding author at: Nokia Research Center, 100, Novaya Str., Skolkovo, Moscow region 143025, Russia. Tel.: +74956603208. E-mail address: [email protected] (A. Bessonov). 0924-4247/$ – see front matter © 2013 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.sna.2013.11.034

of new printable electronic circuits and manufacturing techniques. Conventional strain gauges made of metal foils and semiconductor slabs are not suitable for use in system-in-foil technologies due to their inability to conform to curvilinear surfaces and accommodate large deformations. A number of advanced materials for manufacturing highly flexible and stretchable strain sensors have been previously reported, including silicon nanomembranes [7], silver nanoparticle ink [8,9], thin films of carbon nanotubes [6,10], graphene films [11,12], PEDOT:PSS [13,14], BEDT-TTF salts [15], and electrically conductive composites mainly based on PDMS with different fillers such as carbon black [5], graphite [16], carbon nanotubes [17], or metallic nanoparticles [18,19]. Such materials are attractive because they are capable of measuring deformations as large as 100–150% with a gauge factor ranging from 2 to 30. However, only a few of these can be utilized in printing processes that allow affordable mass-production, furthermore, reliability and robustness of such printed strain gauges is still an issue. This article reports the design and demonstration of a bidirectional strain gauge that is suitable for manufacture at high scale and consists of graphite conducting ink printed on a flexible plastic substrate. The strain gauge can be manufactured by screen printing and then thermal curing. To improve the speed of

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manufacture, a photonic flash curing of printed graphite ink was developed as an alternative to conventional thermal processes. The printed strain measurement device with large gauge factor exhibits quick response and high reproducibility to both tensile and compressive bending strains. The sensor can be reversibly bent resulting in about 0.7% strain for more than 100,000 cycles without performance degradation. Temperature-induced errors are eliminated by employing a second compensating strain gauge made of identical material and located at the strain-free neutral axis of the mechanical plane. 2. Experimental 2.1. Printing and curing processes Strain gauges were fabricated using screen printing. The graphite ink 26-8203, a black high viscosity paste, was supplied by Sun Chemical and used as described by the manufacturer. The substrate was 125 ␮m thick poly (ethylene naphthalate) (PEN) foil that lacks surface features (Teonex Q65FA, DuPont). The hand screen printer was purchased from EuroPrint and used according to the manufacturer’s instructions. The ink was deposited using a polyester screen mesh of 70 threads/cm with the thread diameter of 31 ␮m, and a squeegee with the hardness of 75 Shore A. Feature printing was followed by a curing step. The wet graphite ink was initially cured on a hot plate at 120 ◦ C for 30 min according to the supplier recommendation. For high volume production faster post-processing is advantageous so an alternative technique was developed. Photonic flash curing was performed using a xenon flash lamp tool UIS-100 (Aeromed LLC). The apparatus consisted of a power supply, a discharge unit, a controller, and a lamp housing which included a xenon flash lamp with forced air cooling, an elliptical reflector with reflectivity of >98%, and an adjustable table for samples. A linear xenon flash lamp was placed in one of the focus lines of an elliptical reflector. The reflector projected an image of the lamp on the second focus line, resulting in an illumination area of about 20 × 250 mm. An adjustable table allowed setting a projected image with desirable light intensity. The lamp had an emission spectrum ranging from 200 to 900 nm. The system had an average electric power of about 1000 W with a pulsed peak power of about 2.2 MW. The pulse energy was adjustable in the range of 12–108 J, the pulse frequency varied from 1 to 50 Hz, and the pulse duration was 50 ␮s. The interconnections were fabricated over the printed and cured graphite track by screen printing with a mesh count of 165 threads/cm using a silver ink (26–8204, Sun Chemical). The silver ink was sintered using a hot plate at 120 ◦ C for 30 min. For testing the performance of the strain sensors, two copper wires were soldered at the end pads of printed structures by means of thermosetting conductive adhesive based on silver (Elecolit 3653, Eurobond Adhesives). The lamination of two graphite tracks on plastic foils stacked one on top another for the temperature compensation measurements was performed using UV curable adhesive (9008, Dymax). 2.2. Characterization techniques The printed samples were inspected by optical microscopy. The thickness was measured by means of white-light interferometer MicroXAM-100 from KLA-Tencor. The morphology of graphite film was studied by a LEO 1530VP field-emission scanning electron microscope (SEM). The adhesion was evaluated by scratch tests and cross-cut tape tests based on the standard ASTM D 3359. The electrical resistivity and the sheet resistance values were calculated based on standard 2-point and 4-point probe measurements,

Fig. 1. Graphite strain gauge printed on PEN.

respectively, using the Agilent 34410A digital multimeter. Mechanical properties of printed tracks were studied by applying a tensile and compressive bending stress with the radius of curvature from 120 to 10 mm. Repeated mechanical deformation tests were conducted in 2-point and 4-point bending modes with the maximum bending radius of 25 mm using a custom-made apparatus [20]. More than 100,000 bending cycles were performed for each sample. Temperature-induced change of resistance in heating and cooling modes was studied by means of the hot plate IKA S7 and the Agilent 34410A multimeter. 3. Results and discussion 3.1. Fabrication Fabrication techniques were developed to improve the ability to print graphite-based inks on plastic substrates to obtain thin, flexible conducting surface features. A graphite track with strong adhesion to PEN had a linear geometry with a length of 20–30 mm, a width of 0.5–1.5 mm, and an average thickness of 6.6 ± 0.4 ␮m (Fig. 1). Screen printing supports reproducible fabrication of the patterns with a spacing of approximately 200 ␮m between lines. Conventional thermal curing of the ink resulted in an electrical resistivity of 60 m cm. However, the thermal process is incompatible with large scale production such as roll-to-roll manufacturing, which demands fast and efficient low-temperature techniques. An alternative method known as a photonic flash sintering based on xenon flash lamp irradiation was investigated due to its simplicity and high efficiency. Metal nanoparticle inks such as silver and copper inks printed on plastic substrates can be photo-cured into high conductive structures within seconds, in contrast to minutes and even hours needed for conventional thermal processes. The principle of this technique is the selective heating of the ink by the absorption of xenon high-intensity pulsed light for which the substrate is transparent [21,22]. This paper is the first report that this photonic flash curing is also an efficient method for curing printed structures made from graphite inks. The graphite material strongly absorbs light in the range from 200 to 900 nm where the xenon flash lamp usually emits [23]. The exposure to the ink causes a significant local temperature rise in the wet graphite layer that inevitably leads to rapid curing. The curing process involves a cross-linking of polymeric matrix concomitant with the evaporation of solvent. The benefit of photonic curing approach is similar to that for the sintering of metal nanoparticles in that it allows effective curing of the carbon ink on low-melting-point plastic substrates at room temperature within seconds. Fig. 2 shows that the complete curing of relatively thick graphite layer indicating the sheet resistance of about 50 / can be achieved within 20 s at the average power of 918 W (the pulse energy of 27 J, the frequency of 34 Hz) instead of 30 min at 120 ◦ C on the hot plate.

Sheet resistance (

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120 Photonic flash curing

100 80 60

Thermal curing

40 20 0 0

20

40

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Irradiaon me (s) Fig. 2. Sheet resistance of printed graphite pattern versus irradiation time of photonic flash curing compared to thermal curing. The light intensity and the distance between source and printed structure are constant for all data points. The dash line indicates the sheet resistance of thermally cured graphite ink.

15 tension compression

R/R0 (%)

10 5 0 -5 -10 flat

100 80

60

40

20

flat

Bending radius (mm) Fig. 3. Resistance-bending radius relationship for linear graphite-based strain gauge printed on PEN. The standard error was obtained by measuring 4 devices.

3.2. Sensor characterization Electrical resistance of the printed strain gauge varies depending on the amount of axial bending strain in the device (Fig. 3). The conductivity changes as the bending stress changes due to variations in length and cross-sectional area as well as the distance between conductive particles. The relative change in electrical resistance (R/R0 ) is related to the mechanical strain (ε) by the gauge factor (GF) that can be calculated by the following formula [3]: GF =

R/R0 ε

(1)

The GF of the graphite-based strain gauge appeared to be 19.3 ± 1.4. The maximum bending radius under investigation was 10 mm, which corresponds to a strain of 0.66% at the upper graphite surface. The resulting tensile strain was calculated according to the bending strain theory: ε = y/, where  is the bending radius, y is the coordinate assumed upward from the neutral axis (NA), which is also known as neutral mechanical plane [3]. However, here two assumptions should be made. First, if the thickness of the graphite layer is much less than that of the substrate, it is reasonable to assume that the NA is located approximately at the centroid of the cross-sectional area. Secondly, we can speculate that the stress components in z and y direction can be assumed to be zero.

Fig. 5. Top-view SEM images of as-deposited graphite film (a) and after 1000 cycles of repeated bending (b).

The relatively high GF of the graphite ink can be explained by the unique property of graphite sheets which can slide one onto another and still maintain the conductivity. Furthermore, a polymer matrix enables reversible sliding of the particles and serves as an elastic medium maintaining inter-particle contacts. Previously, scientists reported a GF of 5 at about 1.5% strain for a graphitebased strain gauge printed on textile [24]. Lithographically printed graphite-silver strain gauges revealed a GF as high as 31–46 [25]. Note that the GFs for ordinary strain gauges based on metallic foils and grid patterns are typically between 2 and 5, while piezoresistive semiconductor gauges can exhibit much larger GF up to 200 [5], however the low strain limits and inability to wrap complex curved objects restrict their application in flexible devices. A repeated mechanical deformation test was used to measure reversibility, hysteresis, and recovery performance of the sensor. The graphite strain gauge printed on PEN showed sufficient repeatability in more than 100,000 cycles. We observed a highly repeatable resistance change with bending while rising and falling times were consistent (Fig. 4). As shown in Fig. 4a, the resistance of the printed strain gauge changes positively with tensile strain and the change becomes negative when compressive strain is applied. Interestingly, a bending induced tensile strain produces a slightly higher resistance response than a compressive strain. Perhaps, when the resistor is elongated, the conducting graphite particles can easily separate, resulting in a decrease in the number of electron pathways in the printed structure and thus, the observed significant increase in resistance. In contrast, the compressive stress leads to an increase in the number of inter-particle contacts promoting a higher conductivity of graphite material. Note that the SEM images recorded after 1000 cycles of bending to a radius of 25 mm reveal no aging of the printed graphite layer (Fig. 5). 3.3. Temperature-compensated strain gauge The electrical resistance of strain gauges including printed ones varies not only with strain, but also with temperature, while the GF is also temperature dependent [26]. These deviations from

Fig. 4. Resistance change of graphite sensor with repeated bending and relaxing to flat for 10 cycles (a), a single cycle in detail (b), and for another sample after 100,000 cycles (c).

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R/R0 (%)

20 15 10 5 0 20

40

60

80

100

120

Temperature (°C) Fig. 6. Resistance-temperature relationship of printed graphite ink. The standard error was obtained by measuring 4 devices.

normal behavior need to be measured to give a quantifiable error. The variation of resistance with temperature was studied for graphite inks over a temperature range of 20–110 ◦ C in heating and cooling modes (Fig. 6). The ink showed the stable and predictable resistance-temperature relationship: at temperatures below 50 ◦ C the resistance change is minimal with the temperature coefficient of resistance of about 0.04%/◦ C, but above 50 ◦ C the curve slope rises and the resistance significantly increases resulting in about 20% gain at 110 ◦ C compared to the room temperature value. A small hysteresis was observed at high temperatures, as it can be seen from Fig. 6. The thermal response of graphite strain gauge is affected by at least two algebraically additive effects: temperature-induced change of carbon resistivity and thermal expansion of polymer matrix. On the basis of the observed data, we assume that in the range of 20–50 ◦ C the graphite strain gauge can be used without any significant need to correct for temperature-induced errors. For instance, such strain gauges can be applied to the measurement of strain changes at ambient temperatures or, with suitable deployment, the measurement of mechanical deformations at human body temperatures. However, to measure strain in applications involving higher temperatures, the thermal output errors have to be controlled by compensation or correction. Typically this control is achieved by employing an additional compensating strain gauge connected in an adjacent arm of the Wheatstone bridge circuit and mounted on an unstrained region or on the bottom of the beam directly opposite to the active gauge [26]. Self-temperature compensation by introducing a material with negative temperature coefficient for canceling a temperature effect or correction by simply algebraic subtracting the thermal output from the indicated strain also can be applied [26]. Next we utilized neutral axis engineering for manufacturing a temperature-compensated strain gauge. It is known from bending strain theory that the mechanical stress in the NA is approximately zero [3,27]. This suggests that two identical strain gauges can be integrated such that the compensating gauge is located in the

middle of stack while the active sensor is on the upper surface, as shown in Fig. 7. Although a typical method of first order temperature compensation with strain gauges is to mount one sensor on top of the substrate and another on the bottom of the substrate so that one is in tension and the other in compression when the substrate is bent, we opted for another solution with placing a compensating gauge in the NA. This brings additional benefit to the system when the two sensors exhibit different sensitivity in tension and in compression. Assuming that the temperature gradient over the thin plastic foil is negligible and that the temperature coefficient of resistance is not strain dependent while the GF is not affected by temperature changes at ambient conditions, the temperature compensation can be made by simple algebraic calculations using the resistance measurement of both active and compensating gauges, which can be simply expressed by the following equations: Ra = GFεa + ˛T Ra0

(2)

Rc = GFεc + ˛T Rc0

(3)

where Ra is the change in active gauge resistance, Ra0 is the unstrained resistance of active gauge, Rc is the change in compensating gauge resistance, Rc0 is the unstrained resistance of compensating gauge, εa is the strain of active gauge, εc is the strain of compensating gauge, ˛ and T are the temperature coefficient of gauge resistance and the temperature change, respectively, assuming these as well as the GF are equal for both gauges. Note that the Eqs. (2) and (3) are valid only for small temperature variations because of non-linearity of resistance-temperature dependence (Fig. 6). Taking that εc ∼ 0 due to unstrained location at the NA, the measuring strain can be obtained by algebraic subtraction of the relative resistance changes of active and compensating gauges, resulting in εa =

1 GF

 R

a

Ra0

−

Rc Rc0



(4)

In order to prove this assumption we manufactured a dual strain gauge by integrating two printed graphite strain gauges, compensating and active, directly opposite each other in a stack of polymer foils. In such a structure the sensors are under the same temperature conditions and located at normal to the plane of bending. First, the sensors and electrodes were deposited on the two different substrates. Then an adhesive was coated on the second substrate which was subsequently glued to the bottom of the first substrate, as shown in Fig. 7a. Thus, the compensating strain gauge, which is materially and geometrically identical to the active strain sensor, was put in the NA between the two PEN sheets. The overall thickness of the stack was approximately 270 ␮m. Note that the covering of graphite layer with an adhesive layer had to be avoided, otherwise the resistance change of the compensating gauge is altered

Fig. 7. General schematic (a) and cross section (b) of temperature-compensated strain gauge consisted of two graphite layers and two polymer layers. The y coordinate is assumed upward from the NA and to be half of the overall thickness h in this particular case.

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Time (s)

Fig. 8. Resistance change with heating (a), tensile bending strain (b), and repeated mechanical deformation (c) for active strain gauge (red curve) and compensating strain gauge (black curve). The standard error was obtained by measuring 3 devices. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).

when compared to that of the active gauge, leading to significant measurement errors. Importantly, the thickness of adhesive layer should be similar to that of printed strain gauge. As shown in Fig. 7b, the resulting structure is a composite beam composed of two thin graphite layers and two thick polymer layers. The NA is located at the interface between the two substrates. Further, we can conclude that the maximum bending strain occurs at the surface of upper graphite layer where y ∼ h/2 while the layer in the middle remains unstrained, as experimentally shown by resistance measurement (Fig. 8). The printed silver interconnects appeared to have comparatively low resistivity of 41 ␮ cm and weak dependence on strain, therefore the overall resistance change of the circuit was close to the local resistance change of the strain gauge. Both gauges showed similar changes in resistance as a function of temperature variation, indicating that the thermal response of the two gauges is almost identical (Fig. 8a). The temperature dependence of dual gauge is not linear but is predictable and reproducible, as it is analogous to the temperature dependence observed for a single graphite gauge (Fig. 6). As expected, the compensating strain gauge, in contrast to the active gauge, showed very little resistance change with bending at least up to 20 mm of the radius of curvature (Fig. 8b). Cyclic mechanical deformation measurements also confirmed the difference in behavior of the two sensors (Fig. 8c). 4. Conclusion In summary, highly sensitive and reproducible graphite strain gauges can be printed on a plastic substrate and integrated into system-in-foil devices, for instance, flexible display or electronic skin. Printing and photonic flash curing experiments show great potential for these strain sensors to be implemented in R2R processes at high throughput and with low cost. When combined with other classes of sensors such as temperature sensors the range of function of strain gauges can be essentially expanded. Thus, as shown in this work, temperature compensation within the flexible system can be achieved. Due to the fact that the transparency may increase the applications for strain gauges in printed and organic electronics other relative materials such as pristine graphene and graphene-based inks can be further considered for this application. Acknowledgement We thank Dr. Jani Kivioja for providing helpful comments. References [1] A. Nathan, A. Ahnood, M.T. Cole, S. Lee, Y. Suzuki, P. Hiralal, F. Bonaccorso, T. Hasan, L. Garcia-Gancedo, A. Dyadyusha, S. Haque, P. Andrew, S. Hofmann, J. Moultrie, D. Chu, A.J. Flewitt, A.C. Ferrari, M.J. Kelly, J. Robertson, G. Amaratunga, W.I. Milne, Flexible electronics: the next ubiquitous platform, in: Proceedings of the IEEE, vol. 100, 2012 13 May, Special Centennial Issue, 1486-1517.

[2] T. Sekitani, T. Someya, Stretchable, large-area organic electronics, Adv. Mater. 22 (2010) 2228–2246. [3] K. Hoffmann, An Introduction to Measurements Using Strain Gages, Hottinger Baldwin Messtechnik GmbH, Darmstadt, 1989. [4] R. Vertegaal, I. Poupyrev, Organic user interfaces: introduction to special issue, Commun. ACM 51 (2008) 26–30. [5] N. Lu, C. Lu, S. Yang, J. Rogers, Highly sensitive skin-mountable strain gauges based entirely on elastomers, Adv. Funct. Mater. 22 (2012) 4044– 4050. [6] D.J. Lipomi, M. Vosgueritchian, B.C-K. Tee, S.L. Hellstrom, J.A. Lee, C.H. Fox, Z. Bao, Skin-like pressure and strain sensors based on transparent elastic films of carbon nanotubes, Nat. Nanotechol. 6 (2011) 788–792. [7] M. Ying, A.P. Bonifas, N. Lu, Y. Su, R. Li, H. Cheng, A. Ameen, Y. Huang, J.A. Rogers, Silicon nanomembranes for fingertip, Nanotechnology 23 (2012) 3440047pp. [8] M. Maiwald, C. Werner, V. Zoellmer, M. Busse, INKtelligent printed strain gauges, Sens. Actuators A 162 (2010) 198–201. [9] J. Rausch, L. Salun, S. Griesheimer, M. Ibis, R. Werthschützky, Printed piezoresistive strain sensors for monitoring of light-weight structures, in: Proceeding of SPIE 7982, Smart Sensor Phenomena Technology, Networks, and Systems Integration, 15 April 2011, 79820H. [10] B. Thompson, H.-S. Yoon, Aerosol printed carbon nanotube strain sensor, in: Proceeding of SPIE 8346, Smart Sensor Phenomena Technology, Networks, and Systems Integration, 26 April 2012, 83461C. [11] S.-H. Bae, Y. Lee, B.K. Sharma, H.-J. Lee, J.-H. Kim, J.-H. Ahn, Graphene-based transparent strain sensor, Carbon 51 (2013) 236–242. [12] C. Lee, X. Wei, J.W. Kysar, J. Hone, Measurement of the elastic properties and intrinsic strength of monolayer graphene, Science 321 (2008) 385–388. [13] G. Latessa, F. Brunetti, A. Reale, G. Saggio, A. Di Carlo, Piezoresistive behaviour of flexible PEDOT: PSS based sensors, Sens. Actuators B 139 (2009) 304–309. [14] P. Calvert, D. Duggal, P. Patra, A. Agrawal, A. Sawhney, Conducting polymer and conducting composite strain sensors on textiles, Mol. Cryst. Liq. Cryst. 484 (2008) 291–302. [15] E. Laukhina, R. Pfattner, L.R. Ferreras, S. Galli, M. Mas-Torrent, N. Masciocchi, V. Laukhin, C. Rovira, J. Veciana, Ultrasensitive piezoresistive all-organic flexible thin films, Adv. Mater. 22 (2010) 977–981. [16] S. Moshfegh, N.G. Ebrahimi, Strain sensors based on graphite fillers, Iran. Polym. J. 13 (2004) 113–119. [17] G. Yin, N. Hu, Y. Karube, Y. Liu, Y. Li, H. Fukunaga, A carbon nanotube/polymer strain sensor with linear and anti-symmetric piezoresistivity, J. Compos. Mat. 45 (2011) 1315–1323. [18] K.-Y. Chun, Y. Oh, J. Rho, J.-H. Ahn, Y.-J. Kim, H.R. Choi, S. Baik, Highly conductive, printable and stretchable composite films of carbon nanotubes and silver, Nat. Nanotechol. 5 (2010) 853–857. [19] A. Nocke, S. Richter, M. Wolf, G. Gerlach, Polymer composite strain sensor based on dielectrophoretically aligned tellurium nanorods, Procedia Chem. 1 (2009) 1151–1154. [20] I.M. Graz, D.P.J. Cotton, S.P. Lacour, Extended cyclic uniaxial loading of stretchable gold thin-films on elastomeric substrates, Appl. Phys. Lett. 94 (2009) 071902. [21] R. Abbel, T. van Lammeren, R. Hendriks, J. Ploegmakers, E.J. Rubingh, E.R. Meinders, W.A. Groen, Photonic flash sintering of silver nanoparticle inks: a fast and convenient method for the preparation of highly conductive structures on foil, MRS Commun. 2 (2012) 145–150. [22] J. Perelaer, R. Abbel, S. Wünscher, R. Jani, T. van Lammeren, U.S. Schubert, Roll-to-Roll compatible sintering of inkjet printed features by photonic and microwave exposure: from non-conductive ink to 40% bulk silver conductivity in less than 15 seconds, Adv. Mater. 24 (2012) 2620–2625. [23] V. Chabot, B. Kim, B. Sloper, C. Tzoganakis, A. Yu, High yield production and purification of few layer graphene by Gum Arabic assisted physical sonication, Sci. Rep. 3 (2013) 1378. [24] B. Perc, D. Kuˇscˇ er, J. Holc, D. Belaviˇc, M. Jerlah, D.G. Svetec, M. Kosec, Thickfilm strain sensor on textile, in: Proceeding of 45th International Conference on Microelectronics, Devices and Materials, MIDEM, Sept 2009, pp. 9–11. [25] G.I. Hay, D.J. Southee, P.S.A. Evans, D.J. Harrison, G. Simpson, B.J. Ramsey, Examination of silver–graphite lithographically printed resistive strain sensors, Sens. Actuators A 135 (2007) 534–546.

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A. Bessonov et al. / Sensors and Actuators A 206 (2014) 75–80

[26] Strain gage thermal output and gage factor variation with temperature, MicroMeasurements, USA, Tech. Note TN-504-1, Document number: 11054, July 2012. [27] S. Lee, J.-Y. Kwon, D. Yoon, H. Cho, J. You, Y.T. Kang, D. Choi, W. Hwang, Bendability optimization of flexible optical nanoelectronics via neutral axis engineering, Nanoscale Res. Lett. 7 (2012) 256–263.

Biographies Alexander Bessonov received a degree in Chemistry from the Novosibirsk State University, Russia, in 2005. The Ph.D. in Inorganic Chemistry and Physical Chemistry was obtained at the Nikolaev Institute of Inorganic Chemistry, Siberian Branch of Russian Academy of Sciences, in 2008. From October 2008 to March 2012 he worked at the Patterning Group of Manufacturing Technology Center, Samsung Electronics Co. Ltd., with focus on printed electronics, flexible displays, thin film solar cells and roll-to-roll technologies. Since April 2012, he is a Senior Researcher in the Sensors and Materials Laboratory, Nokia Research Center. His research activities cover the development of advanced materials for printed electronics, nanotechnology, printing technologies, and the fabrication of sensor devices. Marina Kirikova obtained a degree in Chemistry from the Moscow State University, Chemistry Department, in 2005 and the Ph.D. in Physical Chemistry at the same University in 2009. From November 2010 to August 2011 she worked as a Post-Doc at the Institute for Solid State Chemistry, Bordeaux, France, on a project concerning the synthesis and investigation of hydrogen storage materials and their application in accumulator batteries. Since January 2012 she occupies a position of thin film energy and electronics researcher in Nokia Research Center Skolkovo. Her research activity mainly concerns the new materials for printed electronics and the development of printing technologies.

Samiul Haque graduated from Warwick University, Coventry, U.K., in 2003 and received a Ph.D. degree in carbon-nanotube-based sensors from the University of Cambridge, Cambridge, U.K., in 2008. He joined Nokia Research Center, Finland, in March 2008, in Helsinki Nanosystems Division to work on graphene and CNT-based devices. In February 2009, he started working at Nokia Cambridge U.K. labs as a Senior Researcher on Stretchable Electronics for future applications. His focus is on new sensors and materials i.e., graphene based Flexible Electronics. He is interested in integrated nanosystems, flexible, and stretchable electronics, printed electronics and high speed RF devices. Ilya Gartseev obtained a degree in Applied Mathematics and Computer Science cum laude at the Moscow State University, Russia. His Ph.D. in Robotics was received from the Moscow State Technical University MIREA, Russia. Since 2007, he is Associate Professor in the MIREA. During 2009–2010 he worked as Visiting Researcher in the State University of New York, USA. In January 2013, he joined the Nokia Research Center as a Senior Researcher. His research activity mainly concerns the lifecycle treatment of autonomous intellectual robotic and mechatronic systems. Marc Bailey is the Research Leader for the Nokia Research Center Sensors and Materials Laboratory at Skolkovo. He received his degree and Ph.D. from the University of Cambridge and in 2001 he joined the UK’s Metrology Institute, the National Physical Laboratory, where he established one of the first metrological research teams in the world designed to support the biotechnology industry, working with leading multinationals, start-ups and academia. In 2008 Dr. Bailey joined Nokia Research Center and initiated research into the development of new chemo- and biosensors for mobile communications devices and their application to the healthcare and wellness sectors. He currently leads two research programs on the analysis of mobile sensing data and the development of new sensors using printed electronics.