Tunable Buckled Beams with Mesoporous PVDF-TrFE/SWCNTComposite Film for Energy HarvestingZhe Xu,† Yin Liu,† Lin Dong,† Andrew B. Closson,† Nanjing Hao,† Meagan Oglesby,‡

Gladys Patricia Escobar,‡ Sidan Fu,† Xiaomin Han,† Chunsheng Wen,† Jifeng Liu,†

Marc David Feldman,‡ Zi Chen,*,† and John X.J. Zhang*,†

†Thayer School of Engineering, Dartmouth College, Hanover, New Hampshire 03755, United States‡Schools of Medicine, University of Texas Health Science Center, San Antonio, Texas 78229, United States

*S Supporting Information

ABSTRACT: By incorporating mesoporous piezoelectric materials and tuningmechanical boundary conditions a simple beam structure can significantly takeadvantage of limited mechanical displacements for energy harvesting.Specifically, we employed a mesoporous PVDF-TrFE composite thin filmmixed with single-wall carbon nanotubes to improve the formation of thecrystalline phase in this piezoelectric polymer. The film was then patterned on athin buckled beam to form a compact energy harvester, which was used to studythe effects of two boundary conditions, including the end rotation angle and thelocation of a mechanical stop along the beam. We carefully designed controlledexperiments using mesoporous PVDF-TrFE film and PVDF-TrFE/SWCNTcomposite films, both of which were tested under two cases of boundaryconditions, namely, the rotation of the end angle and the addition of amechanical stop. The voltage and current of the energy harvester under thesetwo boundary conditions were, respectively, increased by approximately 160.1%and 200.5% compared to the results of its counterpart without imposing any boundary conditions. Thereby, our study offers apromising platform for efficiently powering implantable and wearable devices for harnessing energy from the human body whichwould otherwise have been wasted.

KEYWORDS: piezoelectricity, composite material, beam structure, energy harvesting, low mechanical inputs

■ INTRODUCTION

In recent years, with the rapidly increasing demand ofimplantable medical devices, a number of researchers havebeen concentrating on developing new sustainable powersystems in order to increase the longevity of biomedicaldevices such as implantable cardioverter defibrillators(ACIDs). This popular topic mainly stems from the fact thatthe capacity of currently used batteries cannot suffice the totalpower consumption necessity of these devices, therebyresulting in the need for additional surgery to replace thebattery every 5−7 years. Also, the components of batteries arenormally hazardous and thus inhospitable to biologicalsystems. The concept of implantable energy harvesting hasthus attracted significant attention because it can scavengemechanical energy, e.g., physical displacements from vitalorgans (i.e., the heart), and then convert it to useful electricalpower. Hence, developing battery recharging mechanisms isdesirable for these life-saving biomedical devices.1−3 Eventhough a wide range of materials can be employed in theimplantable energy harvesting field,4−7 piezoelectric polymers,especially PVDF and its copolymer PVDF-TrFE, stand outamong others because of their excellent energy conversion

efficiency, fabrication with minimum technical difficulty, and,most importantly, superior biocompatibility.8−13

Recently, driven by the prominent potentials in biomedicalapplications, the study of piezoelectric polymers has madesignificant progress.1415 For example, Wang et al. fabricated thesponge-like piezoelectric polymer film by the etching of zincoxide (ZnO) nanoparticles. When the film was excited by theacoustic wave at 40 Hz it produced an open-circuit voltage andshort-circuit current up to 11 V and 9.8 μA, respectively.16

With the effort of the Khatua group, the highest voltage andcurrent from the self-poled PVDF/AlO-rGO flexible nano-composite can, respectively, reach 36.03 V and 0.79 μA whentriggered with approximately 24.39 N contact force.17 Withoutany doubt, the piezoelectric polymer does have a strongcapability to generate a high level of electric power.Unfortunately, based on the current research progress, if

greater voltage and current outputs from the piezoelectricpolymer or its composite materials are expected, highermechanical inputs, such as larger contact force and higher

Received: June 5, 2018Accepted: September 10, 2018Published: September 10, 2018

Research Article

www.acsami.orgCite This: ACS Appl. Mater. Interfaces 2018, 10, 33516−33522

© 2018 American Chemical Society 33516 DOI: 10.1021/acsami.8b09310ACS Appl. Mater. Interfaces 2018, 10, 33516−33522

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working frequency, must be adopted. It is not evident toconclude that aforementioned types of materials can guaranteetheir electrical output when the working frequency is relativelylow (less than 2 Hz) and the physical displacement from thevital organ, such as the heart, is not large enough

(approximately 3−5 mm). In this work, we are focusing on

developing a novel mesoporous PVDF-TrFE composite

material mixed with the single-wall carbon nanotube and

further integrating and modifying the boundary conditions

Figure 1. (A) Fabrication process of the mesoporous PVDF-TrFE/SWCNT composite film. Central image is the exploded view of the final beam-structure energy harvester. (B and C) Top and cross-section view of the mesoporous PVDF-TrFE/SWCNT composite film. (D) Zoomed-in imageof the cross-section view showing the presence of the single-wall carbon nanotube (with white arrows) inside the film.

Figure 2. Results of the angle effect on the beam-structure energy harvester. (A) Schematics of testing situations with various angles (0°, 20°, and45°). (B and C) Voltage and current output acquired from the mesoporous PVDF-TrFE/SWCNT composite film, respectively. (D and E)Comparisons of voltage and current between the normal mesoporous PVDF-TrFE film and the mesoporous PVDF-TrFE/SWCNT composite film,respectively.

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imposed on the conventional beam structure energy harvesterto increase its strain deformation along the beam.As shown in Figure 1A, one of key processes for the thin film

formation is spin coating.18 Prior to this process, the as-prepared PVDF-TrFE solution was placed on the 50 μm thickKapton film which has a predeposited 20 nm gold layer as thebottom electrode. Then it is spun for 30 s at 1000 rpm. Thesolution can be fully covered on the Kapton substrate. Toprevent short circuiting in the device, after the top electrodewas coated, the first layer of spin-coated PVDF was designed tobe the solid film, which stayed in an isothermal oven at 60°Cfor 10 min in order to accelerate its solidification process. It isworth noting that even though the pure PVDF-TrFE layer wasthe solid layer at the beginning of the fabrication process,eventually it became the porous layer because the DMF solventin the composite solution can dissolve the solid PVDF-TrFEfilm inevitably. Repeating the whole procedure elaboratedabove, the layer of PVDF-TrFE/SWCNT was spin coated onthe top of solid PVDF-TrFE film. Following the spin-coatingprocess, the liquid-phase PVDF-TrFE/SWCNT layer was thenimmediately transferred into the humidity chamber where therelative humility was maintained at a constant 90% (at 37°C)for more than 4 h.1920 Throughout this step the water dropletswere able to enter the polymer-rich matrix and form themesoporous structures in Figure 1B and 1C. Due to theprinciple of pore formation in the PVDF-TrFE film, themajority of single-wall carbon nanotubes (SWCNTs) wereembedded in the wall of mesoporous structure, but fewSWCNT bundles can still be observed as shown in Figure 1D.In Figure S1A in the Supporting Information, someundispersed SWCNT chunks can be observed as well. Thebundles of SWCNT were observed with transmission electronmicroscopy as shown in Figure S1B. The top electrode wasdeposited on the surface of mesoporous film after the film wasfully dried. The thickness of the gold top electrode is 20 nm.Finally, the whole film was cut into a number of beamstructures, 50 mm in length and 5 mm in width. In order tofurther improve the crystallinity of the sample, beams werepostprocessed by annealing at 135°C for 2 h and followed bypoling.2122 In the poling process, the sample was placed in thehomemade poling apparatus and applied with about 8 kVexternal electrical voltage at 100°C on a hot plate for 30 min.After the samples were cooled down to room temperature,they were ready for electrical characterization.Here we look into the possibility of employing this energy

harvester on a pacemaker lead by using the same level of themechanical input generated from the deformed lead in a dog’sheart. We conducted X-ray image analysis of the lead motionthat was provided by our collaborative cardiology team at theUniversity of Texas at San Antonio. The maximum bendingdisplacement of the lead is in a range between 3 and 5 mm, asseen in Figure S2 in the Supporting Information. It isnoteworthy that the amount of lead bending displacementand the bending location highly depend on the slack of thelead in the heart. In our study, a shaker (Modal Shop Inc.)sharing the same function of the lead was calibrated to providea similar level of mechanical displacement input (3 mm in thiscase).The schematics shown in Figure 2A of the testing strategy

involve the change of angle. The beam structure energyharvester was installed on a 3D-printed testing platform, andthe distance between both ends of the frame was reduced to80% of the beam’s original length. In the first situation, no

relative angle formed between two ends of the beam, as shownin Figure 2A-I. Next, the beam structure energy harvester wasexamined when the right end of the post was rotated clockwiseby 20° and 45° separately, as shown in Figure 2A-II and A-III.It is worth mentioning that 45° is the angle boundary of thebeam’s phase transition from its bistability to monostability.23

As demonstrated in Figure 2B and 2C, with angle changing,the three segments of curves obtained from the PVDF-TrFE/SWCNT composite film correspond well with the three testingsituations. It obviously reveals the increasing trend with theincrement of the angle at the left side of the beam. Specifically,the peak to peak voltage value of the composite film increasesfrom 1.4 to 1.78 V, and the current increases from 12.8 to 17.3nA. In order to provide more evident and meaningfulcomparisons, the normal mesoporous PVDF-TrFE film wasalso tested under the same boundary conditions. Results aregiven in Figure 2D and 2E. By comparing each column set ofresults between normal mesoporous film and PVDF-TrFE/SWCNT composite film, it can be seen that both the voltageand the current obtained from the PVDF-TrFE/SWCNTcomposite film are higher than those from the normalmesoporous film. Taking one situation as an example, thevoltage and current of the composite film at 45° are,respectively, ∼46% and ∼49% higher than those of the normalfilm.From the material point of view, the superiority of the

composite film greatly benefits from the application ofSWCNT which can serve as the nuclear sites once uniformallydistributed in the composite film, thereby enhancing theformation of piezoelectric β-phase of PVDF-TrFE.24−28 InFigure S3A in the Supporting Information, the enthalpy at themelting point, which is an indication of the crystalline phase(β-phase), was integrated with the melting peak in differentialscanning calorimetry (DSC) thermograms. Specifically, theobtained melting enthalpies of normal film and composite filmwere, respectively, 15.6 and 23.0 J/g, as shown in Figure S3B inthe Supporting Information. With the contribution ofSWCNT, formation of β-phase was enhanced approximatelyby 47% in the composite film compared with that in thePVDF-TrFE film without SWCNT. The FTIR spectrum inFigure S4 of the Supporting Information also confirms theenhancement of β phase in the PVDF-TrFE/SWCNT porouscomposite film due to the presence of SWCNT. Moreover, theSWCNT can act as an internal electrode that can transfer theelectrons generated inside the film to the measuring circuit.According to previous studies,24 the open-circuit voltage

output, V, for a piezoelectric patch attached to a buckled beamis proportional to the local curvature κ, i.e., V ∝ k. In thecurrent situation, the PVDF layer is distributed across thewhole Kapton film, so in addition to the local curvature, theoutput voltage should be proportional to its integration overthe beam, i.e.

∫ ∫ θ θ θ∝ = = = −V k k ss

sddd

dL L

L0 0

0 (1)

where s is the arc length coordinate of the beam, L is its totallength, and θ0 and θL are the rotation angles at the left andright support of the beam, respectively. According to eq 1, theoverall curvature can be determined once θ0 and θL are given.To verify the theoretical calculation of κ and obtain theevolution of local curvature, we construct a finite element (FE)model in Abaqus to simulate the shape evolution of the beaminduced by the compression at both its ends and subsequent

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contact by the shaker. Three rotation angles are exerted at theright support end, as done in our experiment, and thedistributions of curvature along the arc length s are given inFigure S5A in the Supporting Information. It can be seen thatthe rotation of the support end significantly changes the localcurvature in the regions ∈ [2.5,5], while the curvature in theleft region shows small changes for different rotation angles.Such local curvature change causes an increase of the overallcurvature κ with the rotation angle (Figure S5B). It can befound that the calculated κ in the FE analysis is almost equal tothe rotation angle θL, assigned at the right support end.Because the voltage output is proportional to κ, the case with arotation angle of 45° will correspond to the largest voltageoutput, which is consistent with the experimental results inFigure 2B and 2C.Fundamentally, the overall electrical outputs generated from

the copolymer PVDF-TrFE highly depend on the level of thestrain deformation imposed on the dipoles which arecomposed of the fluorine and hydrogen atoms. This inspiredus to design an optimized structure that can maximize theoverall strain along the beam coupled with the limiteddisplacement from the heart to the piezoelectric polymer inorder to generate sufficient energy for charging the pacemaker.Although the electrical output has been improved by adoptingthe angle rotation scheme at one end of the beam, it is stillcritical to further explore new strategies to increase the straindeformation of the whole energy harvester and finally improveits power output.

To further improve the energy harvesting performance, amechanical stop, also be referred to as a mechanical pin, wasintroduced as an extra boundary condition and imposed on thebeam simultaneously with the angle rotation scheme, as shownin Figure 3A. In order to figure out the best location of the pinsuch that the beam can generate the highest electrical output,the pin was first engaged along the beam transversely; after thevoltage profile reached its highest point, the pin was movedperpendicularly until its voltage climbed to the peak point.Here, it is noteworthy that different types of samples, e.g.,normal porous film and composite film, have different optimallocations of the mechanical pin. In Figure 3B and 3C, whenboth types of boundary conditions were applied, both thevoltage and the current outputs were significantly improvedcompared with those from the cases where only the anglerotation scheme was implemented. In particular, taking the 0°situation as an example, it can be seen that the voltageobtained from the composite film increased from 1.4 V withoutthe pin effect to 2.2 V with the pin effect, and the current wasalso improved from 12.9 to 23.3 nA. Next, comparing theresults from the normal mesoporous PVDF-TrFE film withthose from the case where neither angle rotation normechanical pin effect was adopted, the voltage acquired fromthe composite film that was affected with a 45° end-rotationangle and the mechanical pin was improved by approximately161%, and the current was enhanced by about 200%. From theformer works, it has been demonstrated that the electricaloutput increases with the local curvature of the beam,2425 i.e., alarger degree of bending leads to larger voltage or current

Figure 3. Results of both the angle effect and the mechanical pin effect on the beam-structure energy harvester. (A) Schematics of testing situationswith various angles (0°, 20°, and 45°) in the presence of a mechanical stop, colored green. (B and C) Voltage and current output acquired from themesoporous PVDF-TrFE/SWCNT composite film, respectively. (D and E) Comparisons of voltage and current between the normal mesoporousPVDF-TrFE film and the mesoporous PVDF-TrFE/SWCNT composite film, respectively.

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output. In our case, the location of the mechanical pin plays arole of tuning the curvature of the beam and thus can beoptimized to enhance the output. As shown in Figure S6, thelocal curvature at the left support end of the beam with a 45°end-rotation angle shows a significant increase with thelocation of the pin. Comparing the cases with 45° endrotation in Figures S6 and S5A in the Supporting Information,we can also find a significant increase of the local curvature inthe area around the tip, which should be the principal reasonfor the enhanced output for the energy harvester.By far, we have presented and analyzed the comprehensive

results of electrical characterizations acquired from the beam-structure energy harvesters composed of either mesoporousPVDF-TrFE film or mesoporous PVDF-TrFE/SWCNTcomposite film. Here we further evaluate the theoreticalpower output of the harvesters working in different boundaryconditions since this property closely relates to the powerrequirement of pacemakers in the current medical devicemarket.1429 Specifically, the instantaneous power output ofthese two devices was approximately calculated by usingequation P = U2/R, where U is the measured voltage and R isthe load resistance in SR560.30 As shown in Figure 4, with the

influence of the same boundary conditions, the theoreticalpower output from the beam-structure energy harvestercoupled with mesoporous PVDF-TrFE/SWCNT compositefilm is at least 62% higher than that from the identical structureenergy harvester with the regular PVDF-TrFE film. Inparticular, when the device with the PVDF-TrFE/SWCNTcomposite film operates with both the 45° end angle rotationeffect and the mechanical pin, the estimated power output is∼7.53 times of that from the device made of pure PVDF-TrFEwithout the contribution of boundary conditions. From thepractical application point of view, the power outputperformance of these devices under different conditions candirectly impact their charging period on a pacemaker battery.On the basis of the power requirement of one pacemaker pulsein the range of 25 μJ,29 the charging period of the PVDF-TrFE/SWCNT energy harvester imposed with 45° endrotation and the mechanical stop will be about 6 h, whilealmost 15.4 h is needed to charge up the pacemaker to thesame power level by using the normal mesoporous PVDF-TrFE film without any boundary conditions. In order to

provide a meaningful calculation, the root-mean-square valueof the power output is used instead of the instantaneous one.

■ CONCLUSIONIn summary, this work has made two major advances on thedesign of high-efficacy energy harvesters. On the other hand,with the addition of SWCNT, both the current and the voltageoutput from the beam-structure energy harvester are improved.For example, in the situation of 0° end rotation angle, thevoltage and the current output of the PVDF-TrFE/SWCNTcomposite film are improved 21.7% and 18.1%, respectively,compared with the outputs of the normal mesoporous film.The increase in the crystalline phase in the PVDF-TrFE filmdue to the presence of SWCNT is proved by the DSC andFTIR. On the other hand, the end-angle rotation and themechanical pin can be applied in the energy harvesting systemto tune either overall or local curvature of the deformed beam,leading to further enhancement of voltage and current output.To conduct meaningful and evident performance estimations,the theoretical power output of energy harvesters composingmesoporous PVDF-TrFE film or mesoporous PVDF-TrFE/SWCNT composite film under six different boundarycondition effects is analyzed, and their charging performanceunder various situations is also evaluated. The voltage andcurrent of the energy harvester made of composite piezo-electric film under these two boundary conditions were,respectively, increased by approximately 160.1% and 200.5%compared to the results of its counterpart without imposingany boundary conditions. The estimated power output fromthe beam structure energy harvester with PVDF-TrFE/SWCNT film increases by 68−122% compared with thatfrom the identical structure energy harvester with PVDF-TrFEfilm corresponding to different boundary condition circum-stances. In particular, the estimated power output of thePVDF-TrFE/SWCNT composite device with a 45° end anglerotation and the mechanical pin is ∼7.53 times of that from thedevice made of pure PVDF-TrFE equipped with no rotationalboundary conditions applied. Therefore, our study has pavedthe way for new energy harvesters with higher electricaloutputs by utilizing the relatively small mechanical inputsprovided by the human body.

■ EXPERIMENTAL SECTIONPreparation of PVDF-TrFE with SWCNT Solution. In order to

prepare 10 g of 15% PVDF-TrFE with 0.1 wt % single-wall carbonnanotube, first, 10 mg of SWCNT (from SUNano group) was mixedinto 8.5 g of N,N-dimethylformamide organic solvent (DMF,purchased from Sigma-Aldrich Inc., US). Then the mixture washandled with a sonication process for about 20 min to ensure that theSWCNT disperses well in the DMF solution.31 Second, 1.5 g ofPVDF-TrFE powder (Piezotech Inc., France) was blended into theDMF/SWCNT mixture, and the whole solution was vigorously stirredunder 60°C temperature for at least 4 h.11 During this period thePVDF-TrFE power was fully dissolved, and the SWCNT can beuniformly distributed into the viscous PVDF-TrFE solution.

Electrical Characterizations. The 3D-printed frame was used toaccommodate the beam structure energy harvesting unit; meanwhile,it can also provide the angle rotation boundary conditions at bothends of the beam. The mechanical pin was placed on the XYZ-direction moveable stage and can be moved along with the beam inorder to find the optimized position which can produce themaximized electrical outputs. The whole experimental apparatus isshown in Figure S7 in the Supporting Information. The voltage andcurrent are measured by using a low-noise preamplifier (SR 560,Stanford Research Systems) and low-noise current preamplifier

Figure 4. Estimation of power performance from the beam structureenergy harvester and theoretical power evolution profiles from thebeam structure energy harvesters composed of the normalmesoporous PVDF-TrFE film and the mesoporous PVDF-TrFE/SWCNT composite film, separately, associating with 6 different typesof boundary conditions.

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(SR570, Stanford Research Systems). The self-built LabVIEWprogram served for data acquisition.Differential Scanning Calorimetry (DSC). DSC was performed

using a Q2000 DSC (TA Instruments) with RCS-80 and autosampler.The testing procedure includes a heating process and a coolingprocess. It started from a heating process that increased thetemperature from −40 to 200 °C at a heating rate of 10 °C/min.The sample stayed at 200 °C for 5 min, and then the sample wascooled at a rate of 10 °C/min.Fourier-Transform Infrared Spectroscopy (FT-IR). FT-IR

results were measured by using a Jasco 4100 spectrophotometer.Transmittance curve, from 2000 to 600 cm−1, was obtained byintegration of 32 scans.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsami.8b09310.

SEM image of undispersed single-wall carbon nanotube(SWCNT) in the mesoporous PVDF-TrFE/CNTcomposite film and transmission electron microscopy(TEM) image of the SWCNT bundle; X-ray image ofthe lead deformation pattern in dog’s heart; thermalproperties of PVDF-TrFE/CNT composite material;FTIR spectra acquired from porous PVDF-TrFE filmand porous PVDF-TrFE/SWCNT composite film; strainanalysis from theoretical simulation in the angle rotationsituation; curvature profile of the deformed beams fordifferent pin locations, d obtained from the FEsimulation; basic components of the experimentalapparatus (PDF)

■ AUTHOR INFORMATIONCorresponding Authors*E-mail: Zi.Chen@dartmouth.edu.*E-mail: John.Zhang@dartmouth.edu.ORCIDZhe Xu: 0000-0002-9495-685XLin Dong: 0000-0002-1498-2792Nanjing Hao: 0000-0003-3808-7941Jifeng Liu: 0000-0003-4379-2928Zi Chen: 0000-0001-5927-0249NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThe authors acknowledge financial support from the NationalInstitute of Health (NIH) Director’s Transformative ResearchAward (R01HL137157, PI: J.Z.), the National ScienceFoundation award (ECCS1509369, PI: J.Z.), and the startupfund from the Thayer School of Engineering at Dartmouth.Z.C. also acknowledges support from the Branco Weiss-Societyin Science fellowship, administered by ETH Zurich. Theauthor is thankful for the help of DSC characterization fromProfessor Douglas Van Citters in the Thayer School ofEngineering of Dartmouth College.

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ACS Applied Materials & Interfaces Research Article

DOI: 10.1021/acsami.8b09310ACS Appl. Mater. Interfaces 2018, 10, 33516−33522

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