2012 Sixth International Conference on Sensing Technology (ICST)
A novel flexible strain gauge sensor fabricated using screen printing A. Moorthi1, B. B. Narakathu1, A. S. G. Reddy1, A. Eshkeiti1, H. Bohra2, M. Z. Atashbar1,2 1
Department of Electrical and Computer Engineering Center for the Advancement of Printed Electronics Western Michigan University Kalamazoo, MI, USA *
[email protected]
2
printed sensors such as humidity sensors using rotogravure printing [21], electrochemical sensors using rotogravure printing [22], organic thin film transistors using inkjet printing [23, 24] and gas sensors using screen printing [25]. There have also been reports on strain gauge sensors using non-flexible substrates such as printed circuit boards [26] and glass substrates [27] as well as inkjet printed strain gauges [28] and aerosol jet printed strain sensors [29]. However, there have been little or no reports of strain gauge sensors which are fully fabricated using screen printing.
Abstract—A novel flexible strain gauge sensor was successfully screen printed on polyethylene terephthalate (PET) substrate using silver (Ag) ink as metallization. The electro-mechanical characteristics of the printed strain gauge sensor were investigated by subjecting the sensor to a cyclic 3-point bend fatigue test. The sensors were subjected to an elongation of 1 mm and 2 mm, for 10,000 cycles at 0.5 Hz. Resistance changes of 0.64 % and 1.89 % were observed for the 1 mm and 2 mm elongations, respectively. An average increase of 9 % in the resistance was calculated for every 0.004 mm/mm increase in the strain, during a strain analysis test performed on the sensor. These responses of the fabricated sensor demonstrate its potential to be used in sensing applications for safety measures.
The advantage of screen printing is its versatility for being able to print fully 2-dimensional patterns. It also has an added advantage of producing relatively larger wet film thickness which is difficult to attain using other printing methods [30]. Screen printing is a push through process in which the substrate is not in direct contact with the mask or the image carrier as shown in Fig. 1 (a). The main components of screen printing are a squeegee which is typically made of rubber and a screen printing plate which consists of a frame (steel or aluminum), screen fabric and stencil; the material of which depends on the solvent of the ink and the cleaning agent used. While printing, the ink is applied on top of the screen and then the rubber squeegee sweeps on top of the screen with pressure sufficient enough to make the ink pass through the screen and transfer onto the substrate.
Keywords:-Flexible; Strain gauge; PET; Screen printed
I.
INTRODUCTION
In this era of modern technology, there is a need for high standard safety requirements, specifically in the civil [1, 2] and mechanical [3] engineering industries. The use of sensor technology has played a vital role in keeping up with these safety standards. Strain gauge sensors with a major role in productivity and safety have been developed for applications in medical, automotive and aeronautical engineering [4-6]. Due to the rigid nature of sensors that have been fabricated using conventional methods, they tend to get damaged or broken, which would cause inaccurate results [7]. The use of flexible strain gauge sensors is a promising solution to overcome this limitation [8, 9]. By using printed strain gauge sensors, there is an increase in the flexibility which helps the sensor deform without breaking. Therefore, the need for the development of cost efficient, easy-to-use and flexible strain gauge sensors is of utmost importance.
Movement Squeegee Ink
Over the past decade, traditional printing techniques have captured immense interest in the manufacturing of electronic sensors such as inkjet printed sensors [10- 15] and paper like displays [16, 17]. Low operational temperatures, cost efficiency and reduction of material wastage have aided in focusing the research on traditional printing techniques for the development of electronic devices. Conventional methods for the fabrication of sensors such as photolithography, acid-etching or photo-resist processes are relatively expensive and also result in excessive material wastage which includes impurities and toxic materials [18-20]. Research has demonstrated the practical use of
978-1-4673-2248-5/12/$31.00 ©2012 IEEE
Ink on the substrate
Screen
Substrate
Figure 1. Screen printing technique.
765
Contact pads (5 mm)
2 cm
Grid Trace (1 cm )
1.5 cm Figure 2. Schematic of the strain gauge sensor.
In this work, conventional screen printing technique was used to fabricate a strain gauge sensor on flexible polyethylene terephthalate (PET) substrate, with silver (Ag) ink as metallization. The capability of the sensor to be used as a strain gauge will be demonstrated by investigating the electro-mechanical characteristics based on a cyclic 3-point bend fatigue test, during which the sensor will be subjected to known elongations. This causes the resistance of the sensor to vary and this response is analyzed and presented.
Figure 4. Vertical scanning interferometry of printed Ag ink on PET with a WYKO RST-plus optical profiler.
Sensor Placed on Fixture Test stand
Sensor
II.
Sensor connected via
EXPERIMENTAL
connecting wires
A. Device Fabrication Fig. 2 shows a schematic of the fully printed resistive strain gauge sensor. The fabrication of the sensor was performed at the Center of Advancement of Printed Electronics (CAPE) in Western Michigan University using a screen printer (AMI MSP 485). The sensor consists of a 200 μm wide and 1 cm long grid trace. The overall dimensions of the strain gauge are 2 cm × 1.5 cm. The screen printed sensor device was cured at 90 °C for 20 minutes in a VWR oven (VWR-1320-2). Fig. 3 shows a photograph of the fully printed flexible strain gauge sensor. An average thickness of 15 μm was calculated using vertical scanning interferometry with a WYKO RST-plus optical profiler for the printed silver film as shown in Fig. 4.
Connected Via USB
PC
(a)
LCR Meter
Relaxed Position Elongated Position
B. Experimental Procedure The experimental setup of the 3-point bend fatigue test is shown in Fig. 5 (a). The sensor is placed on the supports of a 3-point bent fixture of a Mark-10 ESM 301 motorized test stand. A wedge, attached to the main fixture and capable of both upward and downward movement is used to
Sensor Relaxed Position
Grips
Wedge (b)
(c)
Figure 5. (a) Experimental setup for 3-point bend fatigue test and strain analysis, (b) Elongation and relaxed position of the sensor during 3-point bend test and (c) Sensor placed between the vertical grips for the strain analysis test.
elongate the sensor by 1 mm and 2 mm, respectively in the y-direction. Connecting wires are attached to the contact pads using silver conductive epoxy paste (Circuit works
200 μm Figure 3. Printed strain gauge sensor.
766
CW2400). These wires are then connected to an Agilent E4980A precision LCR meter. The sensor is subjected to a constant elongation for 10,000 cycles at 0.5 Hz frequency. This elongation and relaxation of the sensor is shown in Fig. 5 (b). The elongation causes the grid traces in the sensor to deform, thus varying the resistance. This change in resistance is measured by the LCR meter which is in turn connected to a PC. The data is recorded on the PC using a custom built LabVIEW program. For the strain analysis test, the sensor was mounted on vertical grips attached to the Mark-10 motorized test stand (Fig. 5 (c)). The sensor was then stretched by 1 mm in steps of 0.1 mm and the corresponding change in resistance was recorded. The strain exerted on the sensor, in the x-direction, was mathematically calculated using (1) [31]:
Strain (ε ) = ΔL / L III.
Figure 7. Percentage change in the resistance of the sensor when subjected to 10,000 cycles of 2 mm elongation at a frequency of 0.5 Hz.
(1)
change is 3 times higher in magnitude than that caused by the 1 mm elongation on the sensor. Thus the strain gauge is capable of detecting different elongations applied.
RESULTS AND DISCUSSION
The purpose of subjecting the sensor to a 3-point bend fatigue test, which induces maximum bending moment at the middle of the printed trace, is to investigate the behavior of the sensor to varying elongations. Fig. 6 shows the response of the sensor towards an elongation of 1 mm, with a frequency of 0.5 Hz and 10.000 cycles. A characteristic importance in this curve is the initial portion of around 1000 cycles which shows a larger change in the resistance when compared to the rest of the graph. During this time, the variation in the electrical resistance was larger, although it maintains the variation at a constant rate later. The change in the resistance is only 0.65 % from its initial value after 10,000 cycles. This shows that the sensor has a long enough life cycle. The complete life cycle of the sensor could not be determined as the test would be very lengthy.
The response of the strain analysis test is shown in Fig. 8. It was observed that the resistance increased from 154 to 331.9 as the strain was increased from 0.008 mm/mm to 0.04 mm/mm. An average increase of 9 % in the resistance was calculated for every 0.004 mm/mm increase in the strain. Thus, for an applied strain of 400 %, the resistance change was calculated to be 115.5 %. These results demonstrate the capability of the sensor to detect high levels of strain. IV.
CONCLUSION
In this work, we have successfully fabricated a flexible strain gauge sensor using screen printing. Ag ink was used as metallization on PET substrate. The capability of the sensor to detect changes in its resistance, caused by the elongation of the sensor, due to an external force was demonstrated. A cyclic 3-point bend fatigue test produced only 0.64 % and 1.89 % variations in resistance from the initial value for 1 mm and 2 mm elongations, respectively thus showing a long enough life cycle for the sensor. A strain analysis test performed on the sensor demonstrated an average increase of
Fig. 7 shows the response of the sensor towards an elongation of 2 mm. The response curves for the 1 mm and 2 mm elongations follow a similar path initially but at different intensities. A change in resistance of 1.89 % from the initial value was observed for the 2 mm elongation. This
Strain vs. Resistance 350
Resistance (ohms)
330 310 290 270 250 230
Resistance
210 190 170 150 0
0.01
0.02
0.03
0.04
0.05
Strain (mm/mm) Figure 6. Percentage change in the resistance of the sensor when subjected to 10,000 cycles of 1 mm elongation at a frequency of 0.5 Hz.
Figure 8. Change in the resistance of the sensor with varying strain.
767
[18] J. C. Hulteen, D. A. Treichel, M. T. Smith, M. L. Duval, T. R. Jensen, R. P. Van Duyne, “Nano sphere lithography: Size-tunable silver nanoparticle and surface cluster arrays”, J. Phys. Chem. B, vol. 103, pp. 3854-3863, 1999. [19] N. P. Pham, E. Boellaard, J. N. Burghartz, P. M. Sarro, “Photoresist coating methods for integration of novel 3-D RF microstructures”, J. of Microele.mech. Sys., vol. 13, pp. 491-499, 2004. [20] E. H Cinradie, D. F. Moore, “SU-8 thick photoresist processing as a functional material for MEMS application”, J. of Micromech. and Microeng., vol. 12, pp 369-372, 2002. [21] A. S. G. Reddy, B. B. Narakathu, M. Z. Atashbar, M. Rebros, E. Rebrosova, B. J. Bazuin, M. K. Joyce, P. D. Fleming, A. Pekarovicova, “Printed Capacitive Based Humidity Sensors on Flexible Substrates”, Sen. Let., vol. 9, pp. 869–871, 2011. [22] A. S. G. Reddy, B. B. Narakathu, M. Z. Atashbar, M. Rebros, E. Rebrosova, M. K. Joyce, “Gravure Printed Electrochemical Biosensor”, Proc. Eng., vol. 25, pp. 956-959, 2011. [23] T. Yamao, K. Juri, A. Kamoi, S. Hottaa, “Field-effect transistors based on organic single crystals grown by an improved vapor phase method”, Org. Electron, vol. 10, pp. 1241-1247, 2009. [24] T. Sekitani, Y. Noguchi, U. Zschieschang, H. Klauk, T. Someya, “Organic transistors manufactured using inkjet technology with subfemtoliter accuracy”, Proc. For Nat. Acad. of Sci., vol. 105, pp. 4976-4980 , 2001. [25] S. Laschi, I. Palchetti and M. Mascini, “Gold-based screen-printed sensor for detection of trace lead”, Sens. and Act. B, vol. 114, pp. 460–465, 2006. [26] H. Shi, S. Ikexawa, T. Ueda, “A novel method for evaluating triaxial strain gages used in printed circuit board assemblies (PCBA) strain monitoring”, Sens. IEEE, pp. 1697–2700, 2011. [27] R. Lakshmi, H. Debeda, I. Defour, C. Lucat, “Force sensor based of screen-printed cantilevers”, Sens. IEEE, vol. 10, pp. 1133–1137, 2010. [28] J. A. Quintero, R. D. Mancosu, A. W. C de Oliveira, D. C. Rolim, O. C. da Silva, J. M.Silva, “Electro-mechanical evaluation of Ag trace patterns by ink-jet printing”, EPTC, pp. 1000-1005, 2009. [29] M. Maiwald, C. Werner, V. Zoellmer, M. Busse, “INKtelligent printed strain gauges”, Proc. Chem., vol. 1, pp. 907-910, 2009. [30] C. Krebs, “Fabrication and processing of polymer solar cells: A review of printing and coating techniques”, Solar Energy Mat. and Solar Cells, vol. 93, pp. 394-412, 2010. [31] T. Yamada, Y. Hayamizu, Y. Yamamoto, Y. Yomogida, A. LzadiNajafabadi, D. N. Futaba, K. Hata, “A stretchable carbon nanotube strain sensor for human-motion detection”, Nat. Nanotech., vol. 6, pp. 296–301, 2011.
9 % in the resistance for every 0.004 mm/mm increase in the strain. Future studies are underway to determine the sensor response with varying grid lengths and to increase its life cycle. ACKNOWLEDGMENT This work has been partially supported by the US Army Grant Nos. WS911QY-07-1-0003 and W911NF-09-C-0135. REFERENCES [1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10] [11]
[12]
[13]
[14]
[15]
[16] [17]
H. N. Li, D. S. Li, G. B. Song, “Recent applications of fiber optic sensors to health monitoring in civil engineering”, Eng. Stru., vol. 26, pp. 1647-1657, 2004. D. Inaudi, A. Elamari, L. Pflug, N. Gisin, J. Breguet, S. Vurpillot, “Low-coherence deformation sensors for the monitoring of civilengineering structures”, Sens. and Act. A: Phy., vol. 44, pp. 125-130, 1994. A. Pohl, G. Ostermayer, L. Reindl, F. Seifert, “Monitoring the tire pressure at cars using passive SAW sensors”, Ultrasonics Symposium, IEEE Proc., vol. 1, pp.471-474, 1997. C. E. Henning, M. A. Lynch, K.R. Glick Jr., “An in vivo strain gage study of elongation of the anterior cruciate ligament”, The Amer. J. of Spo. Med., vol. 13, pp. 22-26, 1985. R. Matsuzaki, A. Todoroki, “Wireless flexible capacitive sensor based on ultra-flexible epoxy resin for strain measurement of automobile tires”, Sens. and Act. A: Phy., vol. 140, pp. 32-42, 2007. E. J. Saltzman, T. G. Ayers, “Selected examples of MACA?NASA supersonic flight research”, NASA special Publication, vol. 513, pp 1-60, 1995. L. Arruda, “On the Simulation of Flexible Circuits Boards”, EMPC2007 6th European Microelectronics and Packaging Conf., pp. 984-989, 2007. R. Silva, W. C Oliveira, L. O. Arruda, R. Bonadiman, J. Quintero, R. Mancosu, J. Neto, “Finite Element simulation of inkjet printed strain gage on polyimide substrates applied to flexible boards”, 11th European Microelectronics and Packaging Conf., pp. 607-611, 2009. C. Cochrane, V. Koncar, M. Lewandowski, C. Dufour, “Design and development of a flexible strain sensor for textile structure based on a conductive polymer composite”, Sens., vol. 7, pp. 473-492, 2007. D. Li, P. Calvert, C. Mello, “Inkjet printing virus-based sensors”, IEEE 35th Annual Northeast Bioengineering Conf., pp 1-2, 2009. 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 using inkjet-printed polyaniline nanoparticles”, Tal., vol. 77, pp. 710-717, 2008. J. Jang, J. Ha, J. Cho, “Fabrication of Water-Dispersible PolyanilinePoly(4-styrenesulfonate) Nanoparticles For Inkjet-Printed ChemicalSensor Applications”, Adv. Mat., vol. 19, pp. 1772–1775, 2007. M. F. Mabrook, C. Pearson, M. C. Petty, “Inkjet-printed polymer films for the detection of organic vapors”, J. Sens. IEEE, vol. 6, pp. 1435-1444, 2006. Y. Li, A. Rida, R. Vyas, M. M. Tentzeris, “RFID tag and RF structure on a paper substrate using inkjet printing technology”, IEEE Micro. Theo. and Tech., vol. 55, pp. 2894-2901, 2007. V. Aua, S. P. Surwade, S. Ammu, S. R. Agnihotra, S. Jain, K. E. Roberts, S. Park, R. S. Ruoff, S. K. Manohar, “All-organic vapor sensor using inkjet printed reduced graphene oxide”, Angew. Chem. Int. Ed., vol. 49, pp. 2154-2157, 2010. J. A. Rogers, Z. Bao, “Printed plastic electronics and paper like displays”, Poly. Chem., vol. 40, pp. 3327–3334, 2002. J. A. Rogers, Z. Bao, K. Baldwin, A. Dodabalapur, B. Crone, V. R. Raju, V. Kuck, H. Katz, K. Amundson, J. Ewing, P. Drzaic, “Paper- like electronic displays” large area rubber-stamped plastic sheets of electrnics and microencapsulated electrophoretic ink”, Proc. of the Nat. Acad. of Sci., vol. 98, pp. 4835-4840, 2001.
768
