IEEE SENSORS JOURNAL, VOL. 13, NO. 12, DECEMBER 2013 4743

Piezoelectric Force Response of Novel 2D TextileBased PVDF Sensors

Andrzej S. Krajewski, Member, IEEE, Kevin Magniez, Richard J. N. Helmer, and Viktoria Schrank

Abstract— This paper describes the development of 2D flexiblesensors designed by integration of conductive fibrous electrodesand piezoelectric polyvinylidene fluoride (PVDF) fibers into aconventional plain woven polyester fabric. The piezoelectric prop-erties and electrical response to the mechanical deformation ofthe sensors were tested using an electromechanical device built in-house. Both the amplitude of movement and the frequency of thesensors were controlled using this device and the signal efficiencyof these sensors was tested for maximum signal response to thesine frequencies between 80 and 1000 Hz. The electrical signalgenerated by the sensors was correlated to the fineness of thePVDF fibers used, the distance between the electrodes and thenature of the electrodes. Relationships between sensor outputsignal under load and the type of structure were thus established.

Index Terms— Piezoelectric fiber, piezoelectric measurement,woven sensor.

I. INTRODUCTION

THE conversion of mechanical, thermal or chemical energyinto electrical energy using piezoelectric materials has

received great attention in the past few years. Recently, aconsiderable amount of research has been focused on thedevelopment of textile based piezoelectric materials which har-bor potential in various applications such as energy harvesting,sensing and actuation. However the primary challenge in thedesign of these smart textiles is to find a suitable piezo-electricsubstrate material which can provide the desired functionalityand which can be easily meshed into a hybrid textile structure.

Much of the previous reported work in this field has shownthat ceramic based piezoelectric devices perform better thantheir polymer based counterparts. However the brittleness ofceramic materials has hindered their implementation in textile.As a result there has been a shift to exploring polymerbased piezoelectric technologies which offer more flexibilityin design and processing.

Manuscript received March 5, 2013; revised May 14, 2013; acceptedJuly 16, 2013. Date of publication July 19, 2013; date of current version Octo-ber 9, 2013. This work was supported in part by the Commonwealth Scientificand Industrial Research Organization, Materials Science and Engineering, andthe Institute for Frontier Materials, Deakin University. The associate editorcoordinating the review of this paper and approving it for publication wasProf. Sang-Seok Lee.

A. Krajewski and R. J. N. Helmer are with the Materials Science andEngineering Division, Commonwealth Scientific and Industrial ResearchOrganization, Geelong 3216, Australia (e-mail: andrew.krajewski@csiro.au;richard.helmer@csiro.au).

K. Magniez is with Deakin University, Geelong 3216, Australia (e-mail:kevin.magniez@deakin.edu.au).

V. Schrank is with the Institute for Textile Technology, RWTH AachenUniversity, Aachen 52074, Germany (e-mail: viktoria.schrank@gmail.com).

Color versions of one or more of the figures in this paper are availableonline at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JSEN.2013.2274151

The surface area offered by piezoelectric fibres is greaterthan that offered by a film; therefore one would expect animprovement in piezoelectric performance when using piezo-electric fibres rather than a film in piezoelectric clothing.

Most of the initial scientific work evolved around thecoating of the textile fabric with a thin layer of piezo-resistivepolymers such as polypyrrole (PPy, a π-electron conjugatedconducting polymer) [1] or carbon-loaded rubber [2], or theinsertion of piezoelectric films into textile fabric [3]. The longterm performance of these systems after mechanical friction orrepetitive washing is however questionable. Other researchershave focused on the development of piezoelectric fibrous sub-strates based on piezoelectric zirconate titanate (PZT) ceramicand poly (vinylidene fluoride) (PVDF) polymer. For instance,Guillot investigated the energy harvesting properties of wovenfabrics containing PZT ceramic fibers coated with an acrylateoligomer [1]. Swallow et al. designed a series of micro-composite materials by embedding unidirectionally alignedPZT and PVDF piezoelectric fibers into a polymer matrixfor energy harvesting in glove structures. The performanceof the micro-composites with different fiber diameters andmaterial thicknesses were investigated and showed somesuccess. Other similar approaches using micro-compositeshave been reported [4], [5]. In a more novel publication,Laxminarayana et al. [6] demonstrated the direct and reverseconversion of mechanical energy into electrical energy usingelectrospun poly (vinylidene difluoride) PVDF/carbon nan-otube nanofibrous membranes. However the preparation of theelectrospun membranes is not efficient and requires the use oftoxic chemicals.

Recent developments in the processing of piezoelectricpolymers such as PVDF have enabled the scientific communityto progress further in this field. The work on PVDF fibers isvery valuable for the scientific and industrial community sincepolymeric fibers can offer improved flexibility compared totheir inorganic counterparts. The other advantage in producingflexible piezoelectric fibers is the ability to produce a largesurface area in wearable technologies. The evolution of thepiezo β crystal phase content in PVDF films [7], [8] and morerecently on fibers [9] has been addressed in several papersfocusing on the effect of both unidirectional stretching ratiosand temperatures. Siores et al. [10] was the first to patenta continuous melt-spinning process for the development ofpiezoelectric PVDF fibers. The authors showed that by control-ling the cold drawing temperature, drawing ratio and appliedelectric field, the piezoelectric properties of the fibers canbe optimized. Their combination with conventional fibers andconductive electrodes into a weave textile has been described

1530-437X © 2013 IEEE

4744 IEEE SENSORS JOURNAL, VOL. 13, NO. 12, DECEMBER 2013

as the ideal way in making smart piezoelectric materials whichcan be used for many smart wearable applications. This isbecause in woven structures, the piezoelectric fibers (actingas charge generator) can inter-connect with the conductiveelectrodes in a number of ways. However, careful choicesof design and conductive charge carrier have to be made foroptimum performance.

In this work, we designed a number of 2D flexible sensorsby integration of various types of conductive fibrous electrodesand piezoelectric PVDF fibers into a plain woven polyesterfabric. The aim of this research was to comparatively andconsistently test the piezoelectric properties of these sensors.The testing of piezo materials is typically achieved usingcantilever oscillators, four point bending or compression tests.Nonetheless, textile based piezo sensors are more flexibleand their testing requires adapted methodologies. Herein, wedescribe the design and principle of an electromechanicaldevice suitable for testing the piezoelectric properties offlexible textiles. The results produced in this work capturedthe magnitude of signal response of 2D flexible PVDF fiberbased sensors to the various frequencies of the stimulus signalthat was required to deform the fabric. The piezoelectricproperties are correlated to the fineness of the PVDF fibersused, the distance between the electrodes and the nature of theelectrodes. Relationships between sensor output signal underload and the type of structure have been established.

II. MATERIALS AND METHODS

A. Preparation and Structure of the Flexible Sensors

1) Preparation of the Piezoelectric Fibers: Piezo polymerpolyvinylidene fluoride (PVDF) was converted into bulk con-tinuous fiber of 20 single filaments using a single screwBusschaert Fiber extruder. PVDF was supplied by ArkemaGroup (grade Kynar 710). The extruder was heated from240 to 280 degrees, and the speed of the single screwwas set to 20 rpm. The polymer flow rate varied between50 cubic centimeters and 80 cubic centimeters per minute.By adjusting the speeds of the cold and heated rolls on theextruder, the fibers were stretched by 75% of their originallength. This process produced PVDF single filaments of37 and 50 microns in diameter, respectively. The piezoelec-tric β crystal phase in these filaments accounted for approxi-mately 60% content (not shown here).

2) Design and Preparation of Flexible Woven Sensors:The PVDF fibers were used to produce flexible woven textilesensors. The design and preparation of woven sample is animportant step in order to achieve meaningful data. The sam-ples were prepared under the same weaving conditions (Fig. 1)in order to have consistent tension and mechanical properties(in terms of material flexibility and thickness).

The PVDF fibers and conductive fibrous electrodes wereintegrated into plain polyester weave in order to producea two-dimensional (2D) flexible piezoelectric woven sensor.The woven structure contains non-conductive nylon spaceryarn, separating the conductive yarn from one another inorder to avoid shorting. The overall structure is shown inthe following Fig. 1. The conductive fibrous electrodes were

(a)

(b)

(c)

Fig. 1. Process manufacturing of 2D flexible PVDF fiber based sensors.(a) Weaving process. (b) Plain weave fragment of the sensing area. (c) Fabriclayout.

connected to a copper strip using conductive epoxy glue.Each electrode was connected to a metallic snap-button viainsulated electrical wire. The snap-button performed as aphysical connector between the sensor and the measuringinstrument.

In the first set of the experiments, we looked at the effect ofthe PVDF fiber diameter on the efficiency of the sensors. Silvercoated nylon yarns (containing 20 filaments of 50 microns)were used as the electrodes.

In the second set of experiments, the piezoelectric per-formances of the sensor were investigated as influenced bythe distance between the electrodes. Sensors were constructedusing PVDF fibers having fiber diameters of 50 microns.Two silver coated nylon yarns (containing 20 filaments of50 microns) were used as the electrodes. The distance between

KRAJEWSKI et al.: PIEZOELECTRIC FORCE RESPONSE OF NOVEL 2D TEXTILE BASED PVDF SENSORS 4745

TABLE I

TYPE OF CONDUCTIVE ELECTRODES

Fig. 2. Concept of the electromechanical device used for testing of 2Dflexible PVDF fiber based sensors.

electrodes varied from one to two spacers, using either PVDFor polyester yarns.

In the third part of the experiments, various types ofelectrodes (Table I) were used and the efficiency of the cor-responding sensors was measured. Sensors were constructedusing the PVDF fibers having fiber diameters of 50 microns.

B. Design of the Electromechanical Device and Testing of theSensors

1) Design and Signal Processing: Fig. 2 shows a conceptualrepresentation of the electromechanical device. Similar appa-ratus have been reported for energy harvesting purposes [11].A 100 VA audio speaker, for which the amplitude of movementand the frequency can be easily controlled, was used as adeformation driver. Three wires were glued on the membranetop surface of the speaker, their top ends being solderedtogether. A rare earth magnet (of approximately 3 mm indiameter and 2 mm thickness) sourced from Alpha MagneticsPty Ltd was embedded in a non-conductive casing and gluedon top of the wires. The corresponding rare earth magnetwith the same diameter and thickness was positioned on theopposite side of the fabric. The top magnet was coveredby a thin layer of resin in order to prevent shorting theelectrodes built into the sensor. The piezoelectric fabric to betested was inserted between two acrylic plates (10 mm thick).One end of the fabric was clamped at one end of the plate

Fig. 3. Signal processing system – block diagram.

whilst the other end was put in tension using a weight of100 grams (preventing any eventual buckling of the fabricduring movement of the speaker up or down). A hole of25 mm in diameter (corresponding to the sensing area) wascut out of the acrylic plate. In order to shield the internalelectronic circuitry from any external noise sources, the devicewas placed in an earthed galvanized steel metal container.

The block diagram of the signal processing system is shownin Fig. 3.

The electromagnetic device is driven by the sine wavefunction generator. The speaker was energized by the 40 VAuniversal amplifier, Module M034. Oscillations of the speakerwere controlled at various frequencies and amplitudes using aXR2206 function generator. The signal from the piezoelectricfabric sensor was fed into differential amplifier. The inputresistance of this circuit is 4 M�. The total amplification ofthe signal conditioning circuit together with the buffer is 1.The signal from the buffer was fed into a National InstrumentUSB-6210 card connected to a PC. The software is designedto collect the data from all sensors using Lab View NationalInstrument software package. A digital noise filtering wasperformed using the National Instrument software.

2) Operation and Testing: The device was designed torelatively compare the piezo-electric properties of the 2Dtextile sensors. Both the variation in voltage signal providedto the mechanical movement generator and the piezo-electricsignal generated by the PVDF sensor were monitored andrecorded by the National Instrument Data Acquisition cardsimultaneously.

The peak-to-peak values of the signals generated by thesensor as well as signals provided to the mechanical movementgenerator were calculated and averaged for the time of testduration.

The average voltage Ux is calculated according toUx =

∑n0 abs(Uxi)

n (equation 1) where n is the number ofsamples acquired during the test and Uxi is the peak-to-peak voltage sample acquired by the NI data acquisition card(10 kHz sampling frequency, 200000 collected samples).

Each sensor was tested for maximum voltage response to thesine frequencies by sweeping the vibration frequency between80 Hz and 200 Hz. The samples of sensing responses in termsof voltage of the sensors subjected to the sine wave shapedstimulus at frequencies of 80 Hz, 120 Hz and 200 Hz areshown in Fig. 4.

The stimulus sine wave amplitude was set to ±0.4 V.Amplitudes higher than ±0.4 V caused the magnets used

4746 IEEE SENSORS JOURNAL, VOL. 13, NO. 12, DECEMBER 2013

Fig. 4. Samples of sensing response of 2D flexible PVDF fiber based sensorsto sine wave shaped stimulus.

Fig. 5. Sensitivity of the of 2D flexible PVDF fiber based sensors to thesine wave shaped stimulus.

for clamping the sensor to the membrane of the speakerto dislodge causing false reading of the sensor’s generatedresponse.

The maximum signal response for all tested sensors waswithin the range of 128 Hz ± 2 Hz. The sensing signalsensitivity under frequencies of 80 Hz, 120 Hz and 200 Hz isshown in Fig. 5.

There was a rapid decline in sensitivity of the sensor whenthe stimulus frequency was set to below 80 Hz. At the 40 Hzthe sensitivity was around 60 mV. However the signal wasvery noisy and therefore unreliable.

III. RESULTS AND DISCUSSION

The 2D piezoelectric sensors were prepared with two poly-ester yarn spacers between the silver coated nylon electrodesin the weft direction and the PVDF fibers in the warp direction(Fig. 1). The changes in sensor voltage response to vibrationswhen using PVDF fibre diameters of 37 and 50 microns aredepicted in Fig. 6.

Fig. 6. Response to the sine wave shaped stimulus for 37 μm and 50 μmPVDF fibre diameters.

Fig. 7. Impact of the distance between the electrodes on the sensitivity ofthe PVDF based sensor.

It can be noted that the overall voltage response of the sensorconstructed using this particular configuration is relatively low.In addition, the diameter of the PVDF filament did not seemto have a significant effect on the response of the sensor. Fromour perspective however the handling of the thicker 50 micronPVDF fibers is easier and it leads to fewer fiber breakagesduring the weaving process.

We then tried to understand the effect of the distancebetween electrodes. The number of spacer yarn was reducedto one and consequently as it can be seen from Fig. 7 that thevoltage response of the sensor to vibrations was significantlyimproved.

The difference in voltage level may also be associated withfiber compaction during the weaving process. The length ofthe sensing area in the warp direction was found to be 33 and41 mm using PVDF spacer yarn (single or double, respec-tively) whilst 29 and 43 mm was measured with polyester(single or double yarn, respectively). Therefore, it is clearthat the distance between the electrodes, which is influencedby both the type of spacer and the fiber compaction, have asignificant effect on the overall voltage signal generated bythe piezo-sensor.

In the last set of experiments, the sensors were constructedusing various types of electrodes. The fabrics were woven

KRAJEWSKI et al.: PIEZOELECTRIC FORCE RESPONSE OF NOVEL 2D TEXTILE BASED PVDF SENSORS 4747

Fig. 8. Impact of the electrode material in the sensitivity of the PVDF basedsensor.

in such a way that the distance between the electrodes waskept the same for all tested sensors (in order to minimize anyeffect of the distance on the signal generated by the sensor).The results displayed in Fig. 8 show that the best voltageresponse to sine wave shaped vibrations was achieved whenusing the silver coated copper (SCC) electrodes.

This result can be attributed to the large area of contactoffered by these electrodes (a single electrode consist of20 individual filaments of 40 microns in diameter each). On theother hand, the SCN (silver coated nylon) electrodes, forwhich the area of contact was larger than any other electrodes(a single SCN electrode consists of 20 individual filamentsof 50 microns each in diameter) did not perform overlywell. During the weaving process, the weft electrode mostprobably underwent small surface damages due to frictionwith the warp. This would have induced a non-continuoussilver surface coating which in turn would reduce the electricalconductivity of the electrode (considering that its nylon coreis non-conductive).

The sensor constructed using either the steel or titaniumelectrodes performed reasonably well (second best). This couldbe a result of tight fit achieved during the weaving with theelectrodes. However these electrodes (which consisted of asingle wire of 10 and 20 microns in diameter, respectivelysee Table I) were quite delicate to work with. Therefore,coupling of multiple wires into a thicker and stronger yarnwould facilitate their weaving, and would also in turn increasethe contact surface area. Bundling wires would also decreaseoverall resistance of the electrode which may also have animpact on the generated signal.

The worst signal response was noted when using the alu-minum electrodes. We believe that the electrical dischargewhich occurred during testing might have induced someoxidation at the contact point. Metallic aluminum is veryreactive with atmospheric oxygen, and a thin passivationlayer of aluminum oxide (electrical insulator) can form veryrapidly upon discharge on any exposed surface. Also becausethe fabric is periodically stretched during testing, expansionand contraction of the more ductile aluminum might havecaused some significant changes in electrical connectivity tothe PVDF fibre.

IV. CONCLUSION

This work showed that it is possible to design flexibletextile-based sensor using unidirectional drawn piezoelectricPVDF fibers. The purposely built device which was used toexamine the signal level response generated by the sensors tothe sine wave shaped vibrations provided meaningful results,indicating some relationship between the output signal underload and the configuration of the sensor. Changes in the levelof response to the mechanical deformation of the fabric werenoted depending on the type of conductive electrodes used anddepending on the distance between the electrodes. When thedistance is reduced to one the level of signal generated by thesensor was significantly improved, but one has to bear in mindthe electrical and shorting issues that can be associated withshort distances. Both the large area of contact offered by thesilver coated copper yarn (SCC) and their low resistance hada positive impact on the performance of the sensor. Furtherwork will be carried out by varying the type of weave and thetype of conductive electrodes in an attempt to optimize thepiezoelectric output for sensing applications.

ACKNOWLEDGMENT

We gratefully acknowledge Mr. M. Neuenhofer from theTextile Institute RWTH Aachen Germany for all the help theyprovided during the duration of this project. We also grate-fully acknowledge CSIRO Materials Science and Engineeringweaving and workshop staff without whom, this device wouldbe just another idea.

REFERENCES

[1] S. Dong, Z. Sun, and Z. Lu, “Chloride chemical sensor based on anorganic conducting polypyrrole polymer,” Analyst, vol. 113, no. 10,pp. 1525–1528, 1988.

[2] D. De Rossi, F. Lorussi, A. Mazzoldi, P. Orsini, and E. P. Scilingo,“Monitoring body kinematics and gesture through sensing fabrics,” inProc. 1st Annu. Conf. Microtechnol. Med. Biol., 2000, pp. 587–592.

[3] J. Edmison, M. Jones, Z. Nakad, and T. Martin, “Using piezoelectricmaterials for wearable electronic textiles,” in Proc. 6th Int. Symp.Wearable Comput., 2002, pp. 41–48.

[4] X. Chen, S. Xu, Y. Shiyou, and S. Nan, “Nanogenerator for mechanicalenergy harvesting using PZT nanofibers,” Nano Lett., vol. 10, no. 6,pp. 2133–2137, 2010.

[5] C. E. Seeley, E. Delgado, J. Kunzman, and D. Bellamy, “Miniaturepiezo composite bimorph actuator for elevated temperature operation,”in Proc. ASME Conf., 2007, pp. 405–415.

[6] K. Laxminarayana and N. Jalili, “Functional nanotube-based textiles:Pathway to next generation fabrics with enhanced sensing capabilities,”Textile Res. J., vol. 75, no. 9, pp. 670–680, 2005.

[7] P. Sajkiewicz, A. Wasiak, and Z. Gocłowski, “Phase transitions duringstretching of poly(vinylidene fluoride),” Eur. Polymer J., vol. 35, no. 3,pp. 423–429, 1999.

[8] A. Salimi and A. A. Yousefi, “Analysis method: FTIR studies of β-phasecrystal formation in stretched PVDF films,” Polymer Test., vol. 22, no. 6,pp. 699–704, 2003.

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Andrzej S. Krajewski (M’10) received the M.Sc. degree in electronicengineering from the Department of Electric and Electronics, University ofMining and Metallurgy, Krakow, Poland, in 1979, and the Ph.D. degree ininstrumentation in 2007 after working on improving nonwoven web qualitywith a new web scanning device. He is currently a Research Project Scientistwith expertise in the area of flexible electronic and electronic textiles. Hiscurrent research interests include the development of novel textile impactsensors and other smart textile devices.

Kevin Magniez received the M.Eng. degree in textile engineering fromENSAIT, Roubaix, France, the leading French school of textile engineeringin 2001, and the Ph.D. degree in materials science in 2006 after working onthe structure-property relationships in polymer blends and nanocomposites.He is currently a Researcher with extensive expertise in the area of melt-processing and nanocomposites. His current research interests include thedevelopment of novel composite materials including self-healing, toughenedand nanocomposite systems as well as smart textile materials.

Richard J. N. Helmer is actively involved in developing and applyingwearable technologies for sports, health, and defense applications that com-bine smart materials with ICT infrastructure. His current research interestsinclude fiber based sensors and energy devices. He has published over tenpatent filings, a number of which have proceeded to grant and license, andover 50 scientific publications. He led the Advancing Human PerformanceResearch Theme, CSIRO, from July 2008 to June 2009 and is a member ofthe Australian Institute of Sport and CSIRO Research Steering Committee,whilst leading multidisciplinary projects and fostering new research activityacross Australia.

Viktoria Schrank, photograph and biography not available at the time ofpublication.