Poling-Free Hydroxyapatite/Polylactide Nanogenerator with Improved Piezoelectricity for Energy Harvesting - MDPI
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micromachines
Article
Poling-Free Hydroxyapatite/Polylactide Nanogenerator with
Improved Piezoelectricity for Energy Harvesting
Wei Liu, Yunlai Shi * , Zhijun Sun and Li Zhang
State Key Laboratory of Mechanics and Control of Mechanical Structures, Nanjing University of Aeronautics and
Astronautics, Nanjing 210016, China; bx2001039@nuaa.edu.cn (W.L.); meezjsun@nuaa.edu.cn (Z.S.);
zhangli240@uestc.edu.cn (L.Z.)
* Correspondence: shiyunlai950438@nuaa.edu.cn
Abstract: Polylactide-based piezoelectric nanogenerators were designed and fabricated with im-
proved piezoelectric performances by blending polylactide with hydroxyapatite. The addition of
hydroxyapatite significantly improves the crystallinity of polylactide and helps to form hydrogen
bonds, which further improved the piezoelectric output performance of these piezoelectric nanogen-
erators with over three times the open circuit voltage compared with that of pure-polylactide-based
devices. Such excellent piezoelectricity of hydroxyapatite/polylactide-based nanogenerators give
them great potential for energy harvesting fields.
Keywords: polylactide; hydroxyapatite; nanogenerators; energy harvesting
Citation: Liu, W.; Shi, Y.; Sun, Z.; 1. Introduction
Zhang, L. Poling-Free
With the energy crisis and environmental pollution become more and more serious,
Hydroxyapatite/Polylactide
people have demanded and explored renewable energy technology to support our soci-
Nanogenerator with Improved
ety [1–3]. One possible approach is using solar energy, biofuel, water, and wind energy
Piezoelectricity for Energy
to replace the traditional petrochemical resources. Another possible way is to develop
Harvesting. Micromachines 2022, 13,
889. https://doi.org/10.3390/
ambient energy harvesting devices to satisfy the requirements of microwatts for low-power
mi13060889
electronics. Large amounts of ambient mechanical energy exist in our living surroundings,
such as vibration, noise, human breath, walking, and hand waving. Although the devel-
Academic Editors: Yongteng Qian, opment of mechanical energy is valuable, such kinds of mechanical energy were not fully
Viyada Harnchana, Hui Mao
used until the invention of piezoelectric nanogenerators (PENGs) [4–7].
and Wen He
PENGs can convert ambient mechanical energy to electricity based on piezoelectric ma-
Received: 11 May 2022 terials. In 2008, Wang and his colleagues developed the first PENG using a ZNO nanowire
Accepted: 26 May 2022 array [8]. Since then, many researchers have developed different kinds of nanogenerators
Published: 31 May 2022 based on piezoelectric materials. Piezoelectric materials primarily include inorganic and
organic materials. Generally, the inorganic materials, such as lead zirconate titanate, possess
Publisher’s Note: MDPI stays neutral
higher piezoelectric constants, but are rigid and easy to break. The organic materials, such
with regard to jurisdictional claims in
published maps and institutional affil-
as Poly (vinylidene fluoride) (PVDF) and its copolymer, have lower piezoelectric constants,
iations.
but are flexible, allowing large deformation. With the rapid development of wearable
and implantable devices, the requirements for flexibility and biocompatibility in PENGs
have been raised [9,10]. Thus, organic piezoelectric materials have attracted attention in
recent years.
Copyright: © 2022 by the authors. Compared with the most used and mature organic piezoelectric materials, PVDF,
Licensee MDPI, Basel, Switzerland. polylactide (PLLA) possesses several advantages, such as high biocompatibility, no require-
This article is an open access article ment of poling to achieve piezoelectric performances, high heat-resistant properties, and
distributed under the terms and renewable origins [11–13]. However, its relatively low piezoelectric constant has seriously
conditions of the Creative Commons limited its application in PENGs. Researchers have made efforts to improve the piezo-
Attribution (CC BY) license (https:// electric constants. By using different fabricated methods, including electro-spun or cast
creativecommons.org/licenses/by/ coating, or forming different structures, such as cantilevers or films, several PLLA-based
4.0/).
Micromachines 2022, 13, 889. https://doi.org/10.3390/mi13060889 https://www.mdpi.com/journal/micromachines
Micromachines 2022, 13, 889 2 of 6
Micromachines 2022, 13, 889 2 of 6
based nanogenerators have been made [14–18]. However, the piezoelectric constant im-
provement of PLLA
nanogenerators have is stillmade
been limited.
[14–18]. However, the piezoelectric constant improvement
Blending with a
of PLLA is still limited. functional filler is an easy and effective way to improve the perfor-
mances of polymers. However,
Blending with a functional filler little iswork has and
an easy usedeffective
fillers toway
improve the piezoelectric
to improve the perfor-
performances of PLLA. Meanwhile, a filler should not decrease the
mances of polymers. However, little work has used fillers to improve the piezoelectricbiocompatibility of
PLLA. Thus, in this work, the biocompatible filler hydroxyapatite (HA)
performances of PLLA. Meanwhile, a filler should not decrease the biocompatibility of was blended with
PLLA Thus,
PLLA. to improve
in this its piezoelectric
work, performance.
the biocompatible The addition of
filler hydroxyapatite HAwas
(HA) significantly im-
blended with
proved the β-phase crystal crystallinity of PLLA and further increased
PLLA to improve its piezoelectric performance. The addition of HA significantly improved the piezoelectric
output
the of the
β-phase HA/PLLA
crystal PENG. of PLLA and further increased the piezoelectric output of
crystallinity
the HA/PLLA PENG.
2. Experiment
2. Experiment
2.1. Preparation of HA/PLLA Composite Films
2.1. Preparation of HA/PLLA Composite Films
HA (Chengdu Organic Chemicals Co., Ltd., Chengdu, China) and PLLA (Nature-
HA2003D)
Works, (ChengduwereOrganic Chemicals
dispersed Co., Ltd., Chengdu,
in trichloromethane (CHClChina)
3). The and
mixedPLLA (NatureWorks,
solution was mag-
2003D) were dispersed in trichloromethane (CHCl ). The mixed solution
netic-stirred and scrap-coated onto a silver nanowire with polyethylene terephthalate
3 was magnetic-
stirred and scrap-coated
(PET) film. Then, the wetonto
filmawas
silver nanowire
placed with polyethylene
in a vacuum terephthalate
oven to evaporate (PET) film.
the solution and
Then, the wet film was placed
form a 10 μm-thick composite film. in a vacuum oven to evaporate the solution and form a
10 µm-thick composite
The composite film.
film was annealed at 140 °C for 30 min. The pure PLLA film and com-
The composite film was annealed ◦ C for 30 min. The pure PLLA film and
posite films with HA contents of 10% wt,at20%
140 wt, and 30% wt were prepared and named
composite films with HA contents of 10%
PLLA, 10%-HA/PLLA, 20%-HA/PLLA, and 30%-HA/PLLA, wt, 20% wt, and respectively.
30% wt were prepared and
named PLLA, 10%-HA/PLLA, 20%-HA/PLLA, and 30%-HA/PLLA, respectively.
2.2. Preparation of HA/PLLA PENG
2.2. Preparation of HA/PLLA PENG
Copper electrodes were magnetron-sputtered onto the prepared HA/PLLA compo-
Copper electrodes were magnetron-sputtered onto the prepared HA/PLLA composite
site film with silver nanowire on the PET film. The copper electrodes and the silver nan-
film with silver nanowire on the PET film. The copper electrodes and the silver nanowire
owire on the PET film serve as top and bottom electrodes, respectively. Then, a plastic
on the PET film serve as top and bottom electrodes, respectively. Then, a plastic cover
cover
was was attached
attached as packaging.
as packaging. The preparation
The preparation process ofprocess of the HA/PLLA
the HA/PLLA PENGs is PENGs is
shown in
shown1.in Figure 1.
Figure
Figure 1. Optical
Figure 1. Optical images
images of
of rGO/PVDF-TrFE
rGO/PVDF-TrFE WNGs.
2.3.
2.3. Characterization
Characterization and
and Measurements
Measurements
The
The FTIR spectra of the
FTIR spectra of the composite
composite films
films were
were obtained
obtained using
using aa Bruker
Bruker Tensor
Tensor 27 spec-
27 spec-
trometer. The spectra were obtained in the range from 4000 cm−1−1to 500 cm−−1 1 . The X-ray
trometer. The spectra were obtained in the range from 4000 cm to 500 cm . The X-ray
diffraction (XRD) measurements were performed on a D/Max2500 VB2t/PC X-ray diffrac-
diffraction (XRD) measurements were performed on a D/Max2500 VB2t/PC X-ray diffrac-
tometer (Rigaku, Japan) for a 2θ range of 5–50◦ . The energy harvesting performance, including
tometer (Rigaku, Japan) for a 2θ range of 5–50°. The energy harvesting performance, in-
the output voltage and load resistance behavior, of PLLA and HA/PLLA PENG was assessed
cluding the output voltage and load resistance behavior, of PLLA and HA/PLLA PENG
using a high-speed-acquired card (NI 9308), as shown in Figure 2.
was assessed using a high-speed-acquired card (NI 9308), as shown in Figure 2.
Micromachines
Micromachines2022,
2022,13,
13,889
889 3 3ofof66
Theevaluating
Figure2.2.The
Figure evaluatingsystem
systemof
ofPENG
PENGoutput
outputperformances.
performances.
3. Results and Discussion
3. Results and Discussion
The crystalline behavior of the HA/PLLA composite films were explored using XRD.
The crystalline behavior of the HA/PLLA composite films were explored using XRD.
In Figure 3a, diffraction peaks of PLLA were found at 2θ = 17.1◦ and 19.3◦ , corresponding
In Figure 3a, diffraction peaks of PLLA were found at 2θ = 17.1°and 19.3°, corresponding
to the (200) and (203) planes of the α-phase crystal, respectively. Meanwhile, with the
to the (200)
addition of and
HA,(203) planes ofofthe
the intensity α-phase
these crystal,
two peaks respectively.
became weak, and Meanwhile, with thepeak
a new diffraction ad-
dition of HA, the intensity of these two peaks became weak, and a new
was found at 2θ = 31.2◦ for the HA/PLLA composite films, which corresponded to the (003) diffraction peak
was
plane found
of theatβ-phase
2θ = 31.2° for the
crystal. WithHA/PLLA composite
the content films, which
of HA increasing corresponded
to 30 to the
wt%, the intensity
(003) plane of the β-phase crystal. With the content of HA increasing to 30
of the diffraction peaks of the α-phase crystal continued to decrease, while the intensity wt%, the inten-
sity of the
of the diffraction
diffraction peakspeaks of the
of the α-phase
β-phase crystal
crystal continued
increased. Suchto phenomena
decrease, while the inten-
indicated that
sity of the diffraction peaks of the β-phase crystal increased.
the addition of HA to PLLA may help to enhance the transformation of β-phaseSuch phenomena indicated
crystal
that the addition
to α-phase of [19,20].
crystal HA to PLLA may known
It is well help to enhance
that the the transformation
piezoelectricity of β-phase
of PLLA crys-
originates
tal
fromto α-phase crystal of
the orientation [19,20]. It is wellSince
C=O dipoles. known thethat the piezoelectricity
β-phase crystal in PLLA ofoften
PLLAforms
originates
when
Micromachines 2022, 13, 889
from the orientation of C=O dipoles. Since the β-phase crystal in PLLA often
the PLLA backbones are in a parallel state, the C=O dipoles are in a high-orientation state. forms when
4 of 6
the
Thus,PLLAthebackbones are inof
higher content a parallel
β-phasestate, the C=O
crystals dipoles
in PLLA mayarehelp
in a to
high-orientation state.
enhance the output
Thus, the higher
performances ofcontent of β-phase
the HA/PLLA crystals in PLLA may help to enhance the output per-
PENG.
formances of the HA/PLLA PENG.
FTIR was used to investigate the interactions between HA and PLLA. In Figure 3b,
the characteristic peak at 2875 cm−1 and 2985 cm−1 is due to the symmetric and asymmetric
stretching of methylene groups in PLLA. The characteristic peak around 1700 cm −1 was
attributed to the carbonyl groups in the ester bonds of PLLA. In addition, two new char-
acteristic peaks were found around at 3400 cm−1 and 960 cm−1, respectively, due to the ad-
dition of HA to PLLA. Among them, the characteristic peak at 955 cm −1 belongs to the β-
phase crystal in PLLA. The intensity of such a characteristic peak increased with higher
HA content, implying that the β-phase crystal content increased with increasing HA con-
tent. The characteristic peak at 3400 cm−1 was attributed to hydrogen bonds between car-
bonyl in PLLA and the hydroxy in HA. Meanwhile, with higher HA content, the charac-
teristic peak at 3400 cm−1 gradually moved to a lower wave number area, which confirmed
the formation of stronger hydrogen bonds. Such interfacial interactions between PLLA
and HA may help to improve the piezoelectric output of the HA/PLLA PENG.
Figure 3. The (a) XRD and (b) FTIR spectra of PLLA and HA/PLLA composite films.
Figure 3. The (a) XRD and (b) FTIR spectra of PLLA and HA/PLLA composite films.
FTIR was used to investigate the interactions between HA and PLLA. In Figure 3b, the
The interfacial
characteristic peak atinteractions
2875 cm−1 and and 2985
cross-section morphology
cm−1 is due of the HA/PLLA
to the symmetric compo-
and asymmetric
stretching of methylene groups in PLLA. The characteristic peak around 1700 cmand
sites were further investigated by SEM. In Figure 4, the light phase is the HA filler the
−1 was
dark phasetoisthe
attributed thecarbonyl
PLLA matrix.
groups The
in SEM images
the ester with
bonds of different
PLLA. Inmagnifications
addition, two new showchar-
that
the HA dispersed
acteristic uniformly
peaks were in the PLLA
found around matrix.
at 3400 cm−1Atandhigher magnification,
960 cm we can
−1 , respectively, duesee
to that
the
the HA bonded well with the PLLA matrix with indistinct edges, indicating
− 1
addition of HA to PLLA. Among them, the characteristic peak at 955 cm belongs to the β- interfacial
interactions
phase crystal between
in PLLA.HATheand PLLA,ofwhich
intensity such aischaracteristic
accordance with
peakthe FTIR analysis.
increased with higher HA
Micromachines 2022, 13, 889 4 of 6
content, implying that the β-phase crystal content increased with increasing HA content.
The characteristic peak at 3400 cm−1 was attributed to hydrogen bonds between carbonyl
in PLLA and the hydroxy in HA. Meanwhile, with higher HA content, the characteristic
peak at 3400 cm−1 gradually moved to a lower wave number area, which confirmed the
Figure 3. The
formation of(a) XRD and
stronger (b) FTIR spectra
hydrogen bonds.ofSuch
PLLAinterfacial
and HA/PLLA compositebetween
interactions films. PLLA and
HA may help to improve the piezoelectric output of the HA/PLLA PENG.
The interfacialinteractions
The interfacial interactionsand andcross-section
cross-section morphology
morphology of the
of the HA/PLLA
HA/PLLA compo-
composites
sites were
were further
further investigated
investigated by SEM.
by SEM. In Figure
In Figure 4, the4,light
the light
phasephase
is theisHA
thefiller
HA filler anddark
and the the
dark phase is the PLLA matrix. The SEM images with different magnifications
phase is the PLLA matrix. The SEM images with different magnifications show that the HA show that
the HA dispersed
dispersed uniformly uniformly in thematrix.
in the PLLA PLLA matrix.
At higherAtmagnification,
higher magnification,
we can seewe that
can see
the that
HA
the HA bonded well with the PLLA matrix with indistinct edges, indicating
bonded well with the PLLA matrix with indistinct edges, indicating interfacial interactions interfacial
interactions
between HAbetweenand PLLA,HAwhich
and PLLA, which iswith
is accordance accordance
the FTIR with the FTIR analysis.
analysis.
HA/PLLAcomposite
Figure 4. Cross-section SEM images of HA/PLLA compositefilm
film(20
(20wt%).
wt%).
The output
The output performances
performancesofofthetheHA/PLLA
HA/PLLAPENGsPENGsare provided
are providedin Figure 5. Under
in Figure the
5. Under
same impact conditions, the PLLA PENG output only 1 mV voltage. With the
the same impact conditions, the PLLA PENG output only 1 mV voltage. With the addition addition of
HA to PLLA, the output voltage improved significantly, reaching the maximum
of HA to PLLA, the output voltage improved significantly, reaching the maximum voltage voltage of
~3.4 mVmV
of ~3.4 when thethe
when HAHA content waswas
content 20%,20%,
which was was
which overover
threethree
timestimes
that of theofpure-PLLA
that the pure-
PLLA PENG. According to the XRD and FTIR analysis, the addition of HA tomay
PENG. According to the XRD and FTIR analysis, the addition of HA to PLLA PLLA help to
may
form β-phase crystals and hydrogen bonds between PLLA and HA, which
help to form β-phase crystals and hydrogen bonds between PLLA and HA, which may may improve
the piezoelectric
improve output ofoutput
the piezoelectric the HA/PLLA PENGs.PENGs.
of the HA/PLLA However, with further
However, increase
with further in HA
increase
content, the output voltage of HA/PLLA PENG decreased to 0.7 mV, which was possibly
due to the aggregation of the HA filler in the PLLA matrix.
As the HA-20/PLLA showed the optimum piezoelectric output, the impedance prop-
erties of HA-20/PLLA PENG were further investigated. With the increase in load resistance
from 0 to 120 MΩ, the output voltage increased from 0 to 4 V. Meanwhile, the output current
decreased from 0.35 mA to 0.04 mA. According to the test results, the maximum power of
HA-20/PLLA PENG was 5 µW when the load resistance was 1.3 MΩ. The durability of
HA-20/PLLA PENG is provided in Figure 5. After 5000 impact cycles, the output voltage
of HA-20/PLLA PENG remained constant, implying its good durability.
As the HA-20/PLLA showed the optimum piezoelectric output, the impedance prop-
sistance from 0 to 120 MΩ, the output voltage increased from 0 to 4 V. Meanwhile, the
erties of HA-20/PLLA PENG were further investigated. With the increase in load re-
output current decreased from 0.35 mA to 0.04 mA. According to the test results, the max-
sistance from 0 to 120 MΩ, the output voltage increased from 0 to 4 V. Meanwhile, the
imum
output currentofdecreased
power HA-20/PLLA PENG
from 0.35 mAwas 5 μW
to 0.04 when
mA. the load
According to resistance was the
the test results, 1.3 MΩ.
max- The
durability
imum power of HA-20/PLLA PENG was 5 μW when the load resistance was 1.3 MΩ. The the
of HA-20/PLLA PENG is provided in Figure 5. After 5000 impact cycles,
Micromachines 2022, 13, 889 output voltage
durability of HA-20/PLLA
of HA-20/PLLA PENGPENG remained
is provided constant,
in Figure implying
5. After its good
5000 impact durability.
cycles, the5 of 6
output voltage of HA-20/PLLA PENG remained constant, implying its good durability.
Figure 5. The output performances and durability of PLLA and HA/PLLA PENG.
Figure 5. The
Figure5. The output performancesand
output performances anddurability
durabilityofof PLLA
PLLA and
and HA/PLLA
HA/PLLA PENG.
PENG.
Asnumerous
As numerous typesofofmechanical
mechanicalenergy
energyexist
existinin our daily life, such as human walk-
As numerous typestypes mechanical energy exist in our
our daily
daily life,
life, such
such asashuman
human walking
walk-
ing
oring
or
fingerfinger
or finger
beating,
beating, the conversion
the conversion
beating,
of
of such
the conversion
such mechanical
mechanical
of such mechanical energy energy to
to electric
energy
electric
power
to electric power
power
using using
PENG
using
PENG
isPENG is
meaningful.meaningful.
To verify
is meaningful. To verify the practical
the practical
To verify application
application
the practical application of
of HA/PLLAHA/PLLA
of HA/PLLAPENGs, PENGs,
PENGs,we we
westudy study
study the
the
the output
output output performance
performance
performance ofHA/PLLA
of HA/PLLA
of HA/PLLA PENGsPENGs
PENGs under
under
under human human
human motion
motion
motion conditions.
conditions.
conditions. In Figure
Figure
In Figure 6b,
6b,
the the stress–strain
6b, stress–strain
the stress–strain curves
curves
curves of
of of HA/PLLA
HA/PLLA PENG
HA/PLLA PENG
PENG are are provided,
areprovided, indicating
provided,indicating
indicating its
itsits brittle
brittle fracture
fracture
brittle fracture
behavior
behavior and
behavior and poor
and poor strain.
strain.The
strain. TheHA/PLLA
The HA/PLLAPENG
HA/PLLA PENG
PENG cancan
can output
output
output about
about
about 0.2 0.2 V and
V and
0.2 V and
6 V66voltage
V voltage
V voltage
under
under beating
underbeating
beating andandtreading
and treadingconditions,
treading conditions,respectively,
conditions, respectively,
respectively, which
which
which areare
are higher
higher
higher valuesvalues
values thanthan
than thosethose
those of
of
of the
the PLLA PENG.
PENG. Such
Such output
output voltage
voltage can
can supply
supply
the PLLA PENG. Such output voltage can supply some low-power some
some low-power
low-power electric
electric devices.
devices.
devices.
Figure 6. (a1,a2,c1,c2) Output performances of PLLA and HA/PLLA PENG under beating and
treading conditions; (b) stress–strain curves of HA/PLLA PENG.
4. Conclusions
In this paper, a novel PENG was prepared based on HA/PLLA composite films. The
addition of HA to PLLA significantly improves the crystallinity of PLLA, and helps to form
hydrogen bonds between PLLA and HA, both of which may increase the piezoelectric
output of HA/PLLA PENGs. The HA/PLLA PENG with 20 wt% HA showed over three
times the output voltage of neat PLLA PENG. The practical energy harvesting applicationMicromachines 2022, 13, 889 6 of 6
of HA/PLLA for low-power electric devices was confirmed by finger beating and foot
treading conditions.
Author Contributions: Conceptualization, W.L.; validation, Z.S.; resources, W.L.; data curation,
W.L.; writing—original draft preparation, W.L.; writing—review and editing, Y.S.; visualization, L.Z.;
supervision, Y.S.; funding acquisition, Y.S. All authors have read and agreed to the published version
of the manuscript.
Funding: This work was supported by the National Natural Science Foundation of China (Grant
No. 51975282).
Conflicts of Interest: The authors declare no conflict of interest.
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