# Piezoelectric PVDF-TrFE/ PET Energy Harvesters for Structural Health
Monitoring (SHM) Applications

Berkay Kullukçu  Mohammad J. Bathaei  Muhammad Awais  Hadi Mirzajani and
Levent Beker Manuscript received XX, 2022; revised XX, 2022; accepted March
XX, 2022. Date of publication XX, 2022; date of current version XX, 2022. The
associate editor coordinating the review of this paper and approving it for
publication was XXX. (Corresponding author: Levent Beker.)B. Kullukçu is with
the Mechanical Engineering Department, Koç University, Istanbul, Turkey
(e-mail: bkullukcu15@ku.edu.tr). M.Awais is with the Biomedical Engineering
Department, Koç University, Istanbul, Turkey (e-mail:
mawais21@ku.edu.tr).J.Bathaei is with the Biomedical Engineering Department,
Koç University, Istanbul, Turkey, (e-mail: sbathaei20@ku.edu.tr).H.Mirzajani
is with the Mechanical Engineering Department, Koç University, Istanbul,
Turkey, (e-mail: hmirzajani@ku.edu.tr).L.Beker is with the Mechanical
Engineering Department, Koç University, Istanbul, Turkey, (e-mail:
lbeker@ku.edu.tr).Digital Object Identifier: XXXX

###### Abstract

This research describes a piezoelectric Poly(vinylidene fluoride-co-
trifluoroethylene)/ Polyethylene Terephthalate energy harvester for structural
health monitoring of wind turbines. The piezoelectric energy harvester was
made of a polyvinylidene fluoride-trifluoroethylene (PVDF-TrFE) layer. In
addition, PET sheets, double-sided micron-thick tapes, and PVDF sheets were
used for device fabrication. In order to optimize the performance of the
proposed device, the electrical and mechanical characteristics of PVDF-TrFE
material were investigated. Two different piezoelectric energy harvester
devices were designed, fabricated, and compared. A wind test setup was
developed, and the proposed device was characterized by output voltage and
current. Furthermore, a 104 nF capacitance was coupled to the proposed device
to store the generated voltage and run a commercial pressure sensor. The
generated output power of the harvester was sufficient to drive a MEMS
pressure sensor with a maximum current and voltage of 291.34 µA and 937.01 mV,
respectively, under wind pressure giving a power value of 272.99 µW.

Energy harvesting; PET; PVDF; Wind Turbines; Structural Health Monitoring

##  1 Introduction

The field of microelectronics has made tremendous advancements, resulting in
the emergence of various electronic devices that have become commonplace in
daily life. The continual advancement of energy harvesting technologies has
reduced these devices’ cost, size, weight, and power needs therefore, the
formation of distributed environments, has been made feasible as a consequence
of this. The existing energy storage capacity continues to be the major factor
in determining the size, mass, and cost of modern electronic equipment.
Batteries typically lead people to be worried about the inﬂuence that their
disposal has on the environment as well as the feasibility of replacing them
in systems. Because of this problem, there seems to be a rise in interest in
the development of energy harvesting technologies, that are predicted to
resolve the issues that are brought about by the usage of energy sources in
electronic devices [1]. Sensors, actuators, transmitters, receivers, and
processors, all highly integrated intelligent electronic devices, have paved
the way for various new life-improving applications. The development of
technology known as microelectromechanical systems, or MEMS, has led to
improvements in the functionality of electronics along with reductions in both
size, cost, and amount of power they use [2]. Distributed systems were made
possible by the ongoing trend toward smaller, less expensive equipment and
advancements in wireless communication [3]. These networks have the potential
to be utilized for various purposes. Examples of potential applications
include detecting hazardous chemical compounds in high-traffic areas and
measuring tire acceleration and pressure [4, 5]. Various industry
professionals believe that very low wattage embedded electrical equipment will
soon be an essential component of our day-to-day lives, performing various
activities ranging from the automation of factories to our entertainment needs
[6]. Despite this, the eﬀectiveness of these emerging technologies is
dependent on the accessibility of reliable sources of energy. Cost, size, and
duration are issues for energy sources with fixed capacities, such as
batteries. This research is based on energy harvesting, an alternative
approach to addressing these challenges. The most recent iteration of
structural health monitoring (SHM) systems incorporate low power sensors and
wireless communication components fueled by energy harvesters’ outputs [7].
Energy can be extracted from the immediate environment to power autonomous
sensor systems [8]. SHM integration into wind energy systems is a lucrative
application for demonstrating SHM technology in general [9]. The components
that must be inspected are large and difficult to inspect. During operation,
wind turbines are subjected to intense and complex wind loadings; therefore,
evaluating their dynamic responses to wind loads is desirable due to the
possibility of fatigue and the desire to better comprehend the turbine’s
response [10]. In addition to fundamental condition assessment, monitoring may
contribute to more efficient turbine operation. Wind energy harvesting has a
limitation known as the” Betz Limit” (in practice, a multiplier of 0.593
(16/27) is applied to the maximum deliverable power). SHM can be performed
with vibration-activated transducers because vibration is a common occurrence
in nature. In recent years, there has been a great deal of interest in
vibration energy harvesters employing piezoelectric transduction mechanisms
due to their higher power density, ease of application, and scalability [11].
In situations with no fixed vibration, wind vibration powers piezoelectric
harvesters. This thesis describes a piezoelectric energy harvester device
dedicated to SHM in wind turbines by providing the necessary power for
electronic devices through vibrational energy harvesting.

##  2 Modeling of the Harvester Device

In the electrical domain, there is a capacitor; in the mechanical domain,
there are spring and mass elements. Ideal transformer elements are used to
indicate domain coupling. The mechanical domain damping caused by anchor loss
and other factors is insignificant. Hence it is not considered in the model.
Table I contains the model parameters. The capacitance, C, is described in the
electrical domain as a conventional parallel plate capacitor having
capacitance defined by [12]:

Table 1: Values for Equivalent Circuit of PEH Symbol |  Description |  Units
---|---|---
C |  Electrical Capacitance |  F
$r_{pm}$ |  Radius of the top electrode |  m
$t_{pvdf}$ |  PVDF thickness |  m
$\varphi(x_{\theta})$ |  Normal plate deflection shape function |  -
$I_{m}$ |  Piezoelectric coupling integral |  -
$I_{e}$ |  Strain energy integral |  -
M |  Piezoelectric bending moment |  ${CVs^{2}}/{kg}$
$e_{31,f}$ |  Transverse piezoelectric coefficient |  C/N
$V_{in}$ |  Applied voltage |  V
z |  Neutral plane |  m
$t_{k},z_{k},E_{k}$ |  Layer thickness, the center axis of each layer, Youngs modulus |  m,m,Pa
$v_{k}$ |  Poisson’s ratio |  -
D,$h_{k}$ |  Flexural rigidity, the distance to the top of each layer |  $Pam^{4}$,m
$\mu_{eff}$ |  Effective mass per unit |  -
$rho_{k}$ |  Area, density |  $m^{2},kg/m^{3}$
$k_{m}$ |  Stiffness |  $kg/s^{2}$
$\eta$ |  Electromechanical coupling |  Cm/N
$f_{n}$ |  Resonance frequency |  Hz
$\lambda_{01}$ |  Eigenvalue for the first mode vibration |  -
$m_{m}$ |  Total mass |  kg
$m_{d}$ |  Modal mass |  kg

$C={\varepsilon\pi{r_{pm}}^{2}}/{t_{pvdf}}.$ (1)

where $\varepsilon$ the proportion of relative permittivity of PVDF to the
permittivity of a vacuum, $t_{pvdf}$ represents the PVDF thickness, and
$r_{pm}$ represents the radius of the top electrode. With a normal plate
deflection shape function of $\varphi$(x) [13]:

$\varphi(x)={(1-{x_{\theta}}^{2})}^{2}.$ (2)

where $x_{\theta}$ is the radial coordinate in its normalized form. The
axisymmetric plate deflection is w($x_{\theta}$) = $w_{0}\varphi$(x), where
$w_{0}$ is the static plate deflection at the clamped plate’s center. One can
calculate $I_{m}$, the piezoelectric coupling integral, and $I_{e}$, strain
energy integral by which are specified as [12]:

$\begin{multlined}I_{m}=1/{(1-v)}\int_{0}^{\gamma}{((x_{\theta}d^{2}\varphi(x_{\theta})}/{d{x_{\theta}}^{2})}+\\\
{(d\varphi(x_{\theta})}/{dx_{\theta}))dx_{\theta}}.\end{multlined}I_{m}=1/{(1-v)}\int_{0}^{\gamma}{((x_{\theta}d^{2}\varphi(x_{\theta})}/{d{x_{\theta}}^{2})}+\\\
{(d\varphi(x_{\theta})}/{dx_{\theta}))dx_{\theta}}.$ (3)

$\begin{multlined}I_{e}=\int_{0}^{1}((x_{\theta}{{d^{2}\varphi(x_{\theta})}/{d{x_{\theta}}^{2}})^{2}}+\\\
2v{({d\varphi(x_{\theta})}/{x_{\theta}dx_{\theta})}{(d^{2}\varphi(x_{\theta})}/{d{x_{\theta}}^{2}})}+{(d\varphi(x_{\theta})}/{dx_{\theta}))}d{x_{\theta}}.\end{multlined}I_{e}=\int_{0}^{1}((x_{\theta}{{d^{2}\varphi(x_{\theta})}/{d{x_{\theta}}^{2}})^{2}}+\\\
2v{({d\varphi(x_{\theta})}/{x_{\theta}dx_{\theta})}{(d^{2}\varphi(x_{\theta})}/{d{x_{\theta}}^{2}})}+{(d\varphi(x_{\theta})}/{dx_{\theta}))}d{x_{\theta}}.$
(4)

where v is the composite plate’s effective Poisson’s ratio, and M is the
piezoelectric bending moment provided by [14]:

$M=-e_{31,f}V_{in}z.$ (5)

The transverse piezoelectric coefficient is $e_{31,f}$. The applied voltage is
$V_{in}$. The distance between the active PVDF layer’s mid-plane and the
neutral plane is z. It is worth noting that the piezoelectric coupling
integral $I_{m}$ is computed solely over the electrode region, but the strain
energy integral $I_{e}$ is computed across the whole radius. The location of
the composite plate structure’s neutral plane z is defined by:

$z={\sum_{k=1}^{3}{(t_{k}z_{k}E_{k}}/{1-v_{k}^{2})}}/{\sum_{k=1}^{3}(t_{k}E_{k}}/{1-v_{k}^{2})}.$
(6)

Counting from the bottom, the subscripts denote the thin plate layers. The
Young’s modulus is $E_{k}$ the Poisson’s ratio is $v_{k}$, and the center axis
of each layer is $z_{k}$. Similarly, the composite plate’s flexural rigidity,
D [15], and effective mass per unit area, $\mu_{eff}$:

$z=\sum_{k=1}^{3}1/3({{{(h_{k}-z)}^{3}}-{(h_{k-1}-z)}^{3})}/{({1-{v_{k}}^{2}}/E_{k})}.$
(7) $\mu_{eff}=(1/D)\sum_{k=1}^{3}\rho_{k}t_{k}.$ (8)

$\rho_{k}$ is the density and $h_{k}$ represents the height of each layer. is
the distance to the top of each layer. One gets when solving Eq. 11 for the
mechanical compliance $1/k_{m}$:

$1/k_{m}={r^{2}}/{2{\pi}DI_{e}}.$ (9)

The following equation for the electromechanical coupling ratio is found by
solving:

$\eta=2{\pi}I_{m}e_{31,f}z.$ (10)

The circular plate formula is utilized [13] to get the natural frequency:

$f_{n}=({{{\lambda_{01}}/r)}^{2}}\sqrt{D/{\mu_{eff}}}.$ (11)

where $\lambda_{01}$ is the corresponds to the (01) vibration mode eigenvalue.
As the plates will be vibrating in their first eigenfrequencies,
$\lambda_{01}$ was used throughout the equations. The total mass of the
composite disk, $m_{d}$ and the shape function is used to determine the modal
mass:

$m_{d}=({\rho_{\rho\rho}t_{\rho\rho}+\rho_{be}t_{be}+\rho_{a\rho}t_{a\rho})\pi\\\
r^{2}\\\ +{\rho_{te}t_{te}\pi\\\ {r_{te}}^{2}}}.$ (12)
$m_{m}=m_{d}*2\int_{0}^{1}{{\varphi(x_{\theta})}^{2}}x_{\theta}dx_{\theta}.$
(13)

where the densities of the distinct layers are
$\rho_{\rho\rho},\rho_{be},\rho_{a\rho}$ and $\rho_{te}$ respectively. The
finite element model (FEM) was developed to measure the voltage output of the
device. An axisymmetric geometry was run under FEM simulation. Fig. 2 shows
COMSOL® FEM simulation results of the circular plate.

##  3 Experiments

PVDF-TrFe was selected as the actuation material for the piezoelectric
harvester. Table 2 shows the superiority of PVDF-TrFE compared with other
widely used piezoelectric materials in terms of voltage output KPI.

Table 2: Piezoelectric Material Comparison Parameter |  $BaTiO_{3}$ |  Quartz |  PZT4 |  PZT5H |  PVDF-TrFe
---|---|---|---|---|---
$d_{33}$ [$10^{-12}$] |  180 [18] |  2.3 [22] |  289 [17] |  593 [16] |  -38 [17]
$\rho[{kg}/{m^{3}}]$ |  6010 [20] |  2620 [23] |  7500 [24] |  7500 [19] |  1800 [21]
${\varepsilon^{x}}/{\varepsilon_{0}}$ |  1557 [18] |  5 [22] |  1300 [22] |  3400 [21] |  9.4 [21]
$g_{33}$ [$10^{-3}$] |  13.06 |  51.98 |  25.12 |  19.71 |  456.79

PVDF-TrFe material characterizations were determined in terms of thickness,
pyroelectric, and ferroelectric measurements. In order to characterize the
material, it is necessary to first use 2wt$\%$ PVDFTrFE in acetone as solvent
(Piezotech FC25). The specimen is then placed on a magnetic stirrer for 1 hour
at 80°C. The solution was then filtered to make it more homogenous. The
filtered solution was spin-coated on glass substrates sweeping between 750 RPM
to 5000 RPM. After this operation, the solution was annealed for 1 hour at 140
°C. Fig. 1 shows the thickness of the deposited layer for different RPMs.

Figure 1: Profilometer thickness measurements were taken with Dektak XT for
different spin-coated specimens.

Film formation was not desirable in 5000 RPM spin-coated film as short-circuit
tests for this specimen failed for most regions when it was spin-coated on ITO
glass—indicating high nonuniformity or porosity of the surface. Film formation
was better in 2000 RPM than in 5000 RPM spin-coated film. However, the short
circuit test failed on the edges of ITO glass. A 33wt$\%$ PVDF-TrFe (FC25
Arkema Piezotech) solution was too viscous to be squeezed using the pipettes.
In conclusion, lower RPMs ($<$ 2000) resulted in better film uniformity, but
acetone seems not to be a suitable solvent for PVDF-TrFE due to its highly
volatile nature. Poling was done using the high voltage DC power supply. An
additional 100 V was applied for 1 µm thickness. A Sawyer-Tower Circuit was
built for conducting ferroelectric measurements with the specimens. Fig. 2
shows the result of these ferroelectric measurements conducted with the
circuit.

Figure 2: Polarization results for 4 different test specimens are given, and
measurements are done using triangular waves with 20 $V_{pp}$.

PVDF-TrFE has a dual-phase microstructure consisting of alpha and beta phases
where the percentage of the beta phase determines the stiffness and
crystallinity of the film. Fig. 3 represents the test setup used for stress-
strain curve plotting and represents the stress-strain curve in the elastic
region for the semi-crystalline PVDF-TrFE copolymer.

Figure 3: a) Compressive tests were done using a force gauge and PI L-306
Compact Precision Z-Stage b) Metal coated PVDF specimen was used for the
experiment c) Stress-strain plot of the material resembles a linear curve in
strain levels as high as 25$\%$.

According to Fig. 3c, the polymer has a Young Modulus of 0.87 GPa, proving its
robustness. In addition to the high stiffness of the polymer, PVDF-TrFE is a
suitable candidate for energy harvesting applications where flexibility and
mechanical robustness are required [25]. According to equation (1) where
$g_{33}$ is piezoelectric voltage coefficient, d is thickness, and s is
applied stress, the voltage output of a piezoelectric film is directly
proportional to the applied stress on the film. Fig. 4 demonstrates the
flexibility of the film under a microscope and Fig. 5 shows an optical image
of a harvester prototype.

Figure 4: a) Optical image representing the flexibility of the harvester b)
PET layer was used as a substrate for the piezoelectric layer to bend without
going into a tear, permanent deformation c) Optical image of the fabricated
sensor from the top view (First Design) d) Bottom view of the same design.

Figure 5: a) Optical image of the second design for the harvester b) Bottom
view of the same design.

Also, based on the stress correlation with strain, which is realized by the
Young Modulus (equation 2), the higher magnitude for E results in higher
voltage output, which is crucial for energy harvesters.

$V=g_{33}ds.$ (14) $V=g_{33}dE\varepsilon.$ (15)

In this way, tunability of $\beta$ phase percentage by chemical methods or
electrical poling of the film results in higher crystallinity, which
determines the young modulus of PVDF-TrFE films. Thus, we utilized FC20
polymer powders with 20 percent mol content of TrFE ($\beta$ phase) to achieve
proper flexibility and stiffness. This way, various PVDF-TrFE films with
elastic modulus between 0.7-3 GPa can be selected based on the trade-off
between the required flexibility and voltage output in a specific energy
harvesting application.

###  3.1 Fabrication Process

To validate the fabrication flow, a 3 mm diameter hole on a 10 × 10 mm PET
sheet was created using a mechanical cutter. A thin double-sided tape was put
on top of a supportive PET cut into a 7 × 10 mm rectangle. Another PET sheet,
which PVDF will later replace, was then cut according to design constraints.
This layer also had parts shaped as contact pads with 3 × 3 mm dimensions and
was attached to the double-sided tape. To fabricate the device, in the first
step, plain PET sheets were cut in 10 × 10 mm size with a mechanical cutter
device (step 1). The sheets were 140 µm thick and cut with a laser to form 1.5
mm diameter circular cavities (step 2). A 65 µm thick double-sided tape was
then put on top of the PET (step 3). The tape was used to fix the PolyK PVDF
film on the sheet. In addition, the tape was used as the structural layer for
the harvester, moving the piezoelectric layers away from the neutral axis.
Fig. 6 shows the fabrication steps used to fabricate the PEH prototypes.
Before the functionality tests, voltage output tests under high impact force
were done.

Figure 6: (1) Plain PET sheets were cut in 10 × 10 mm size with a mechanical
cutter device (2) Sheets were cut with a laser to form 1.5 mm diameter
circular cavities (3) A thin double-sided tape was put on top of the PET (4)
The tape was used to fix the piezoelectric film on the sheet (5) Bonding
connections were realized.

An 18 µm PVDF-TrFE layer was placed on the double-sided tape (step 4). Two 3 ×
3 mm electrode pads were cut out of the PVDF film to make bonding easier while
getting connections from the device. Connections are made using flat tip
connectors (step 5). The sheet was metal coated on both sides. Using the
described process, circular plate energy harvesters with 1.5 mm diameter were
fabricated. The acoustic resonator tube length was fixed to 140 µm in all
prototypes.

###  3.2 Tests

The experiment platform was built as shown in Fig. 7. The setup consisted of a
pulse width modulation (PWM) 4wire fan, an Arduino Uno, a dry air supply, and
a BMP280 pressure sensor breakout board from Adafruit. The output properties
of the resonant cavity piezoelectric energy harvester were studied
experimentally. A code was developed to measure the pressure dynamically from
serial pins. Serial Peripheral Interface (SPI) protocol was used for
communication. A unique test setup was built using 3D printed parts. Dry air
was flown towards the fan’s turning propeller, creating pressure fluctuations
by creating intervals. The frequency of these intervals was set by changing
the PWM frequency of the fan and operating voltage value. PEH was fixed close
to the fan to be vibrated by these fluctuations, creating alternating voltage.
The maximum wind speed measured was 20 m/s. The rotational speed of the fan
was validated using a tachometer. The current output of the harvester was
measured using an oscilloscope. The output voltage was converted to the
current by dividing the peak-to-peak CH2 output of the oscilloscope with the
resistance value (1.5 k$\Omega$).

Figure 7: a) Pressure fluctuation spikes were plotted for 1680 RPM measurement
showing peak-to-peak 45 Pa pressure changes per hit b) Using the dry air
supply, 400 Pa of pressure could apply to the piezoelectric plates c) Sweeping
through 720 RPM to 1680 RPM tests using the PWM fan, FFT was done to each test
data extracting the dominant fluctuation frequencies. 1680 RPM was chosen for
the application for having the highest fluctuation frequency having dominant
peaks at 72 Hz and 84 Hz d) Testing data was filtered through moving median
filter by taking the average of last 5 values. The 0 Hz noise in the FFT plots
were eliminated using filtering.

Figure 8: Test setup was built using 3D printed parts which have a cavity for
dry air supply and a surface to fix the pressure sensor.

The procedure was applied to both harvester designs.

##  4 Results and Discussion

The research presents two 10 × 10 mm transducers built on the PET layer. The
neutral axis and its distance from the midplanes of piezoelectric layers were
determined using the measured layer thicknesses; z was calculated as 74 µm.
Fig. 8 and Fig. 9 represent the voltage output of the harvester having the
first design and characterization plots for the wind setup respectively.

Figure 9: The maximum peak-to-peak value seen was 1.4 V under impulse (First
Design).

It is also worth noting that the piezoelectric material can withstand
significant degrees of strain. Fig. 10 (a-c) demonstrates the harvester’s
current and voltage output for two different designs, and the tested harvester
is given in Fig. 11.

Figure 10: a) The maximum current was 136.9 µA under impulse when taking the
average of the peak values (First Design) b) The output current from the
sensor under impulse current of $I_{max}$ = 273 µA (Second Design) c) The
output voltage measured from the sensor (Second Design) plotted by taking
average values of 5 different impulse tests reporting in error bars. The
results are not time dependent. It is worth noting that the output voltage
produced is 6.61 $V_{max}$.

Fig. 23 shows the peak-to-peak voltage graph obtained from the second design.
It is worth noting that the output voltage produced is 6.61 $V_{max}$.

Figure 11: Two flat tip crocodile wires were used to get contacts from the
bottom and top electrodes.

Figure 12: a) The maximum voltage obtained from the wind setup was $V_{max}$ =
937.01 mV while the sensor was connected to the current measurement circuit b)
The maximum current obtained from the wind setup was $I_{max}$ = 291.34 $\mu$A
c) The maximum power obtained from the wind setup was $P_{max}$ = 272.99
$\mu$W.

Having higher power output, the second harvester design was evaluated under a
wind test setup. Fig. 12 (a-c) demonstrates the harvester’s voltage, current,
and power output on the same time scale. The power output difference between
the two designs is mainly based on the effective piezoelectric layer area,
where the second design has a 36.2$\%$ larger piezoelectric layer area than
the first design. Impact applied to the sensor for measuring a capacitor
charge discharge curve. A full bridge rectifier circuit was used for
converting the AC output and initiating capacitor charging. The ceramic
capacitor was charged up to 585 mV. The maximum voltage value measured across
the capacitor was 760 mV. The voltage increase for a single impulse was 174
mV. The data for capacitor charging is given in Fig. 13. After the force was
released from the piezoelectric element, the capacitor charge was discharged
due to the 1 M$\Omega$ oscilloscope input impedance. This experiment runs for
10k seconds to show the stability of the capacitor charge.

Figure 13: a) Charge of the capacitor was periodically changed due to impulses
on the piezoelectric element b) The 104 nF capacitor had a stable charge
during the acquisition c) Sensor’s voltage output under wind load without any
external circuit connection to validate the values taken from the current
measurement circuit. $V_{pp}$ = 1.74 V was measured from the wind tests.

The PVDF-TrFe energy harvester can run a MEMS BMP280 (Bosch) pressure sensor
widely used in structural health monitoring applications in wind turbines.
Needing 2.7 µA current and 1.20 V voltage input, a BMP280 sensor can be run
using the PVDF-TrFE energy harvester. Fig. 14 reports the power and current
requirements of two other commercial pressure sensors having operating
voltages of 1.5 V maximum together with BMP280, showing that the PVDF-TrFe
piezoelectric harvester can power these devices.

Figure 14: MS5637C (MEAS Switzerland), DPS310 (Infineon), and BMP280 (Bosch)
are widely used commercial pressure sensors that can be powered with the
harvester output [26-28].

##  5 Conclusion

PVDF-TrFE has significant advantages over other ferroelectric materials due to
its soft and flexible character. This study uses conventional fabrication
techniques to describe the design, construction, and characterization of PEHs
with PVDF-TrFE layers. The proposed PEH structures are thin films with
circular plates of 1.5 mm. The device’s dimensions were chosen to keep the
neutral axis outside the active piezoelectric layers. Two sensor designs were
used in this experiment. The thickness of the PEH designs was 223 µm after
fabrication. On an impulse load, the maximum harvested voltage was measured at
6.61 V and the current measured at 273 µA; on a wind load, the maximum
harvested voltage was measured at 937.01 mV and the current measured at 291.34
µA giving a maximum power value of 272.99 µW for the second energy harvester
design.

## Acknowledgment

The authors thank BMDL lab members at Koç University for their support in
device fabrication and characterization, Muhammad Junaid Akhtar for current
measurement trials, and Berke Ataseven for FFT code development.

## References

  * [1] S. Roundy et al., ”Improving power output for vibration-based energy scavengers,” IEEE Pervasive Comput., vol. 4, no. 1, pp. 28–36, 2005, doi: 10.1109/MPRV.2005.14.
  * [2] K. Chan, M. Patel, N. Barniol, S. Bhowmick, S. Cases, and A. Wong, Three Dimensional Modeling of Piezoelectric Micromachined Transducers Built on Top of Integrated Circuits.
  * [3] A. A. L. I. Alomari, ”Modelling, Fabrication and Characterization for Improved Piezoelectric Energy Harvesters,” no. July, 2016.
  * [4] S. Roundy, P. K. Wright, and K. S. J. Pister, ”Micro-electrostatic vibration-to-electricity converters,” ASME Int. Mech. Eng. Congr. Expo. Proc., pp. 487–496, 2002, doi: 10.1115/IMECE2002-39309.
  * [5] F. Yildiz, J. Zhu, R. Pecen, and L. Guo, ”Energy scavenging for wireless sensor nodes with a focus on rotation to electricity conversion,” 2007.
  * [6] S. J. Roundy, ”Energy scavenging for wireless sensor nodes with a focus on rotation to electricity conversion,” 2003.
  * [7] H. J. Jung, S. Nezami, and S. Lee, ”Power supply switch circuit for intermittent energy harvesting,” Electron., vol. 8, no. 12, 2019, doi: 10.3390/electronics8121446.
  * [8] M. E. Martinez, ”Polymeric Pvdf Fibers for Piezoelectric Applications in Energy Harvesting,” The University of Texas Rio Grande Valley, 2019.
  * [9] C. Boller, ”Structural Health Monitoring – Its Association and Use,” in New Trends in Structural Health Monitoring, Springer, 2013, p. 73.
  * [10] J. P. Lynch and R. A. Swartz, ”Wireless sensors and networks for SHM,” in Structural Health Monitoring of Civil Infrastructure Systems, 2009, p. 94.
  * [11] W. Wang, X. He, X. Wang, M. Wang, and K. Xue, ”A bioinspired structure modification of piezoelectric wind energy harvester based on the prototype of leaf veins,” Sensors Actuators, A Phys., vol. 279, pp. 467–473, 2018, doi: 10.1016/j.sna.2018.06.059.
  * [12] S. E. SHELTON, ”Piezoelectric Micromachined Ultrasound Transducers for Air-coupled Applications,” University of California Davis, 2014.
  * [13] J. N. Reddy, Theory and Analysis of Elastic Plates and Shells vol. 1: Taylor avoıcend Francis, 2007.
  * [14] P. Muralt, N. Ledermann, J. Paborowski, A. Barzegar, S. Gentil, B. Belgacem, S. Petitgrand, A. Bosseboeuf, and N. Setter, ”Piezoelectric micromachined ultrasonic transducers based on PZT thin films,” IEEE Trans. Ultrason., Ferroelectr., Freq. Control, vol. 52, pp. 2276-2288, 2005.
  * [15] Y. LU, ”Piezoelectric Micromachined Ultrasonic Transducers for Fingerprint Sensing,” University of California Davis, 2015.
  * [16] H.A. Kunkel, S. Locke, B. Pikeroen, IEEE Trans. Ultrason. Ferroelect. Freq. Control 37 (1990) 316.
  * [17] Tian, W., Ling, Z., Yu, W., $\&$ Shi, J. (2018). A Review of MEMS Scale Piezoelectric Energy Harvester. Applied Sciences, 8(4), 645. doi:10.3390/app8040645
  * [18] Machado, R.; Santos, V.B.D.; Ochoa, D.A.; Cerdeiras, E.; Mestres, L.; García, J.E. Elastic, dielectric and electromechanical properties of (Bi0.5Na0.5)TiO3-BaTiO3 piezoceramics at the morphotropic phase boundary region. J. Alloys Compd. 2017, 690, 568–574.
  * [19] Sappati, K., $\&$ Bhadra, S. (2018). Piezoelectric Polymer and Paper Substrates: A Review. Sensors, 18(11), 3605. doi:10.3390/s18113605
  * [20] Ueyama, T., Yamana, S., $\&$ Kaneko, N. (1987). Sintering Properties of BaTiO3Green Sheets with High Packing Density. Japanese Journal of Applied Physics, 26(S2), 139. doi:10.7567/jjaps.26s2.139
  * [21] Piezotech, ”Piezoelectric films technical information.”
  * [22] Evans, Jared. (2014). Piezoelectric-Based, Self-Sustaining Artificial Cochlea.
  * [23] Yan, Xikang $\&$ Wen, Jiapeng. (2020). Study on factors influencing the performance of reactive powder concrete. IOP Conference Series: Earth and Environmental Science. 510. 052075. 10.1088/1755-1315/510/5/052075.
  * [24] Binh, Do $\&$ Chebanenko, Valerii $\&$ Duong, Le $\&$ Kirillova, Evgenia $\&$ Thang, Pham $\&$ Solov’ev, A.. (2020). Applied theory of bending vibration of the piezoelectric and piezomagnetic bimorph. Journal of Advanced Dielectrics. 10. 10.1142/S2010135X20500071.
  * [25] Roman A. Surmenev, Roman V. Chernozem, Igor O. Pariy, Maria A. Surmeneva, ”A review on piezo- and pyroelectric responses of flexible nano- and micropatterned polymer surfaces for biomedical sensing and energy harvesting applications,” Nano Energy, Volume 79, 2021, 105442, ISSN 2211-2855, https://doi.org/10.1016/j.nanoen.2020.105442.
  * [26] Bosch, ”BMP280 – Datasheet,” 2021.
  * [27] TE Connectivity, ”MS5637C – Low Voltage Barometric Pressure Sensor,” 2017.
  * [28] Infineon, ”DPS310 Digital XENSIVTM Barometric Pressure Sensor for Portable Devices,” 2020.

[]Berkay Kullukçu is a master’s student in the Mechanical Engineering
Department at Koç University. His research topic focuses on piezoelectric
PVDF-TRFE/ PET energy harvesters for structural health monitoring (SHM)
applications. The aim of his research is to develop a flexible energy
harvester for powering low power electronics. He obtained his BSc in
Mechanical Engineering from Koç University in 2020. []Muhammad J. Bathaei is a
graduate student in Biomedical Engineering and started his PhD in February
2020. He received his BSc degree in Materials Science and Engineering from
Tehran University (UT). His research currently focuses on biodegradable
sensors for medical applications under supervision of Prof. Levent Beker at
Bio-integrated Microdevices laboratory. []Muhammad Awais is a Ph.D. student in
the Biomedical engineering department of Koç University. Currently, he is
working on wearable ultrasonic sensors and implantable electronics for
biomedical purposes. He obtained his bachelor’s degree in Electronics
engineering from Ghulam Ishaq Khan Institute of Engineering Sciences and
Technology Topi, Pakistan. []Hadi Mirzajani is pursuing his research as a
postdoc at the Mechanical Engineering Department of Koç University. His focus
is on wireless sensing patches for noninvasive or minimally invasive diagnosis
of disease. His topics of interest are biosensors and bioelectronics,
microfluidics, wireless power/data transmission, and RF MEMS. He has published
more than 25 scientific papers and served as reviewer for many journals and
conferences. []Levent Beker got undergrad and master’s degrees in mechanical
engineering and micro-nano technology from the Middle East Technical
University (METU), Ankara, Turkey. Then, he got his PhD in mechanical
engineering from the University of California, Berkeley, U.S. In 2017, he
joined Prof. Zhenan Bao’s group at Stanford University and in September 2019,
he joined Koç University as an assistant professor and established the Bio-
integrated microdevices laboratory.