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Composites, Fabrication and Application of Polyvinylidene Fluoride for Flexible Electromechanical Devices: A Review

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The technological development of piezoelectric materials is crucial for developing wearable and flexible electromechanical devices. There are many inorganic materials with piezoelectric effects, such as piezoelectric ceramics, aluminum nitride and zinc oxide. They all have very high piezoelectric coefficients and large piezoelectric response ranges. The characteristics of high hardness and low tenacity make inorganic piezoelectric materials unsuitable for flexible devices that require frequent bending. Polyvinylidene fluoride (PVDF) and its derivatives are the most popular materials used in flexible electromechanical devices in recent years and have high flexibility, high sensitivity, high ductility and a certain piezoelectric coefficient. Owing to increasing the piezoelectric coefficient of PVDF, researchers are committed to optimizing PVDF materials and enhancing their polarity by a series of means to further improve their mechanical–electrical conversion efficiency. This paper reviews the latest PVDF-related optimization-based materials, related processing and polarization methods and the applications of these materials in, e.g., wearable functional devices, chemical sensors, biosensors and flexible actuator devices for flexible micro-electromechanical devices. We also discuss the challenges of wearable devices based on flexible piezoelectric polymer, considering where further practical applications could be.
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micromachines
Review
Composites, Fabrication and Application of
Polyvinylidene Fluoride for Flexible
Electromechanical Devices: A Review
Shuaibing Guo 1, Xuexin Duan 1, Mengying Xie 1, Kean Chin Aw 2and Qiannan Xue 1,*
1State Key Laboratory of Precision Measuring Technology & Instruments,
College of Precision Instrument and Opto-Electronics Engineering, Tianjin University, Tianjin 300072, China;
2019202091@tju.edu.cn (S.G.); xduan@tju.edu.cn (X.D.); mengying_xie@tju.edu.cn (M.X.)
2Department Mechanical Engineering, University of Auckland, Auckland 1023, New Zealand;
k.aw@auckland.ac.nz
*Correspondence: qiannanxue@tju.edu.cn
Received: 6 November 2020; Accepted: 29 November 2020; Published: 3 December 2020


Abstract:
The technological development of piezoelectric materials is crucial for developing wearable
and flexible electromechanical devices. There are many inorganic materials with piezoelectric
eects, such as piezoelectric ceramics, aluminum nitride and zinc oxide. They all have very high
piezoelectric coecients and large piezoelectric response ranges. The characteristics of high hardness
and low tenacity make inorganic piezoelectric materials unsuitable for flexible devices that require
frequent bending. Polyvinylidene fluoride (PVDF) and its derivatives are the most popular materials
used in flexible electromechanical devices in recent years and have high flexibility, high sensitivity,
high ductility and a certain piezoelectric coecient. Owing to increasing the piezoelectric coecient
of PVDF, researchers are committed to optimizing PVDF materials and enhancing their polarity by
a series of means to further improve their mechanical–electrical conversion eciency. This paper
reviews the latest PVDF-related optimization-based materials, related processing and polarization
methods and the applications of these materials in, e.g., wearable functional devices, chemical sensors,
biosensors and flexible actuator devices for flexible micro-electromechanical devices. We also discuss
the challenges of wearable devices based on flexible piezoelectric polymer, considering where further
practical applications could be.
Keywords: PVDF; piezoelectric polymer; wearable device; flexible sensor; electromechanical
1. Introduction
Piezoelectric materials have been in development for 140 years [
1
]. These materials are widely used
in electromechanical devices because they can convert mechanical energy to electrical energy under
pressure or generate mechanical motion by electricity [
2
,
3
]. The use of piezoelectric materials for the
preparation of micro-electromechanical devices makes it possible to develop miniature resonators [
4
8
],
frequency meters [
9
] and micro energy supply chips [
10
14
]. Piezoelectric crystals have a single crystal
structure and natural piezoelectricity, such as
α
-quartz [
15
,
16
]. In addition to natural piezoelectric
materials that can adapt to dierent application scenarios, researchers have also manufactured many
high-performance artificial piezoelectric materials. Typical piezoelectric-crystal materials include
aluminum nitride (AlN) and zinc oxide (ZnO) [
17
,
18
], which have a variety of advantages, such as
very high strength, large piezoelectric coecients and good applicability to micro devices [
19
,
20
].
Piezoelectric ceramics consist of many small crystals and the crystals are randomly oriented. The crystal
orientation can be changed through a polarization process to exhibit piezoelectricity. The polarization
process is usually performed by applying a high electric field. PZT-series materials such as PMN-PZT
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Micromachines 2020,11, 1076 2 of 29
and PMMPT are typical representatives of piezoelectric ceramics; they not only have high piezoelectric
coecients but also good ductility. However, the bending property of PZT-series materials is still poor,
and they pollute the environment because of their lead content. Generally, these inorganic piezoelectric
materials have the features of high hardness and brittleness, which are not suitable for flexible devices
that require high material flexibility.
With the rapid development of wearable devices, there is an increasing demand for device flexibility.
Compared with inorganic and ceramic materials, piezoelectric polymers have more remarkable bending
and tensile properties. Polymer piezoelectric materials have a carbon chain as the backbone, and
their flexibility is higher than those of single crystals and ceramics [
21
23
]. This high flexibility
allows them to withstand a greater amount of strain, thus making them more suitable for application
scenarios with large bending and twisting requirements. Some suitable piezoelectric polymers, such as
polyvinylidene fluoride (PVDF), polyvinylidene fluoride-trifluoro ethylene (P(VDF-TrFE)), cellulose
and its derivatives, polyamides and polylactic acids, are widely used in pressure sensors, vibrometers,
audio sensors, ultrasonic sensors, impact sensors, display equipment, piezoelectric catalysis for
wastewater treatment [
24
,
25
], chemical sensors and medical sensors [
26
]. Among flexible piezoelectric
materials, PVDF has a high piezoelectric coecient, good flexibility and excellent biocompatibility;
therefore, PVDF is widely used to develop flexible electromechanical devices.
However, improvement of the polarization characteristics of flexible piezoelectric materials
continues to be challenging. The piezoelectric coecient of PVDF can be eectively improved by
doping with inorganic piezoelectric materials, such as PZT and piezoelectric ceramics. In addition,
nano-clay, BTO, Ag, graphene and other nanomaterials can change the crystal structure of PVDF
and increase the
β
phase proportion. Moreover, some new fabrication methods can also improve
the polarity of flexible piezoelectric materials. In this paper, the recent progress in material science
of PVDF and PVDF composites, aiming for micro fabrication techniques, is reviewed. We discuss
the latest technological scheme and processing methods and compare their merits and demerits.
The improvement in the performance of flexible piezoelectric materials has overcome the limitations of
wearable electromechanical devices. The applications of these materials in electromechanical devices
are discussed in detail. Finally, the development trend of flexible electromechanical devices based
on PVDF is prospected in the future, such as using the help of piezoelectric properties of biocrystals,
improving the biocompatibility for the development of bionic devices.
2. Polyvinylidene Fluoride-Based-Series Piezoelectric Material
2.1. The Piezoelectric Eect Principle of PVDF
The piezoelectric eect [
27
,
28
] refers to the polarization of certain dielectrics in a specific direction
under the action of an external force. A dielectric material has positive and negative charges on
two opposite surfaces. When the external force is eliminated, the material returns to a neutral state.
This phenomenon is called the positive piezoelectric eect. As the direction of the force changes,
the polarity of the charge also changes. Conversely, when an electric field is applied in the polarization
direction of the dielectric, the material will deform. When the electric field is removed, the deformation
disappears. This phenomenon is called the reverse piezoelectric eect. The fundamental reason for the
piezoelectric eect is that the mechanical deformation caused by the polarization intensity is linear
with respect to the electric field strength in crystal materials without a symmetry center.
Piezoelectric materials have unique mechanical and electrical coupling characteristics, including
the direct piezoelectric eect of charge generated under the action of external mechanical stress and the
converse piezoelectric eect of mechanical strain caused by an external electric field. They are usually
expressed by the following piezoelectric coupling equation [29]:
"δ
D#="sEd
dεσ#" σ
E#(1)
Micromachines 2020,11, 1076 3 of 29
For the direct piezoelectric eect, electric displacement Dcaused by stress
σ
based on piezoelectric
eect and the internal electric field E based on material’s dielectric permittivity. Here,
D=dσ+εσE
,
where dis the piezoelectric coecient and
εσ
is the dielectric permittivity for constant stress. For the
converse piezoelectric eect, strain
δ
is caused by stress
σ
and the internal electric field based on
converse piezoelectric eect. Here,
δ=sEσ+dE
, where
sE
is the elastic compliance for constant
electric field. The direct piezoelectric eect is very important for the sensing and energy collection of
surface charge generated by applied stress on piezoelectric materials. The electromechanical actuator
of electro-deformation is based on the inverse piezoelectric eect.
Many piezoelectric materials have a very clear polar axis. Energy harvesting performance is
related to the angle between the stress and polar axis. The direction of the polar axis is Direction 3,
and the direction perpendicular to the polar axis is Direction 1. The direction of stress application can
be along either Direction 1 or 3 [30].
The piezoelectric properties of PVDF are directly related to its structure [
31
]. PVDF is a
semi-crystalline polymer polymerized by H
2
C=CF
2
monomer, whose properties depend on the
phase due to the alignment of crystals (Figure 1a). When two CF
2
or CH
2
groups are connected, a
defect occurs in the connected polymer chain. Defects in the polymeric chain influence the polarity
of the PVDF film, thereby increasing the piezoelectric response. The polymer has
α
,
β
,
γ
,бand
ε
phases, which are five semi-crystalline polymorphs. The
α
and
β
phases are the most common phases.
The
γ
phase, which is not common, is a transitional state between
α
and
β
.бand
ε
are dicult to
isolate. The
α
phase of PVDF is non-polar [
32
], because it contains two molecular chains and the dipole
moments are reversed, forming a trans-gauche conformation (TGTG
0
) (Figure 1b). The
β
phase of
PVDF has an all-trans conformation (TTTT) (Figure 1c). Most hydrogen atoms and fluorine atoms are
separated; therefore, it has a dipole moment perpendicular to the polymer chain, which shows the
highest net dipole moment. The
γ
phase of PVDF is a transitional state between
α
and
β
(Figure 1d),
and it has a smaller dipole moment than the
β
phase [
33
]. It is necessary to increase the proportion of
β
phase in PVDF to maximize its electromechanical conversion eciency [
34
]. PVDF has two transition
mechanisms: Mode 31 is perpendicular to the direction of stress applied to the material, and Mode 33
is parallel to the direction of stress [26]. These two transition mechanisms are shown in Figure 1e,f.
Micromachines 2020, 11, x 3 of 30
E
d
ds
D
E
(1)
For the direct piezoelectric effect, electric displacement D caused by stress
based on
piezoelectric effect and the internal electric field E based on material’s dielectric permittivity. Here,
EdD
, where d is the piezoelectric coefficient and
is the dielectric permittivity for
constant stress. For the converse piezoelectric effect, strain
is caused by stress
and the
internal electric field based on converse piezoelectric effect. Here, dEs
E
, where
E
s is the
elastic compliance for constant electric field. The direct piezoelectric effect is very important for the
sensing and energy collection of surface charge generated by applied stress on piezoelectric materials.
The electromechanical actuator of electro-deformation is based on the inverse piezoelectric effect.
Many piezoelectric materials have a very clear polar axis. Energy harvesting performance is
related to the angle between the stress and polar axis. The direction of the polar axis is Direction 3,
and the direction perpendicular to the polar axis is Direction 1. The direction of stress application can
be along either Direction 1 or 3 [30].
The piezoelectric properties of PVDF are directly related to its structure [31]. PVDF is a semi-
crystalline polymer polymerized by H2C =CF2 monomer, whose properties depend on the phase due
to the alignment of crystals (Figure 1a). When two CF2 or CH2 groups are connected, a defect occurs
in the connected polymer chain. Defects in the polymeric chain influence the polarity of the PVDF
film, thereby increasing the piezoelectric response. The polymer has α, β, γ, б and ε phases, which
are five semi-crystalline polymorphs. The α and β phases are the most common phases. The γ phase,
which is not common, is a transitional state between α and β. б and ε are difficult to isolate. The α
phase of PVDF is non-polar [32], because it contains two molecular chains and the dipole moments
are reversed, forming a trans-gauche conformation (TGTG′) (Figure 1b). The β phase of PVDF has an
all-trans conformation (TTTT) (Figure 1c). Most hydrogen atoms and fluorine atoms are separated;
therefore, it has a dipole moment perpendicular to the polymer chain, which shows the highest net
dipole moment. The γ phase of PVDF is a transitional state between α and β (Figure 1d), and it has a
smaller dipole moment than the β phase [33]. It is necessary to increase the proportion of β phase in
PVDF to maximize its electromechanical conversion efficiency [34]. PVDF has two transition
mechanisms: Mode 31 is perpendicular to the direction of stress applied to the material, and Mode
33 is parallel to the direction of stress [26]. These two transition mechanisms are shown in Figure 1e,f.
Figure 1. (a) Structure of PVDF; (b) space structure of α-phase PVDF; (c) space structure of β-phase
PVDF; (d) space structure of γ-phase PVDF; and the two transition mechanisms of PVDF (e) Mode 33
and (f) Mode 31.
2.2. Piezoelectric Materials based on PVDF and Its Derivatives
Polymer piezoelectric material is a material with a carbon chain as the basic skeleton, and its
flexibility is higher than that of single crystals and ceramics [21,22]. This high flexibility allows it to
withstand a greater amount of strain, thereby making it more suitable for application scenarios with
large bending and twisting requirements. Piezoelectric polymer PVDF and its copolymers have the
Figure 1.
(
a
) Structure of PVDF; (
b
) space structure of
α
-phase PVDF; (
c
) space structure of
β
-phase
PVDF; (
d
) space structure of
γ
-phase PVDF; and the two transition mechanisms of PVDF (
e
) Mode 33
and (f) Mode 31.
2.2. Piezoelectric Materials based on PVDF and Its Derivatives
Polymer piezoelectric material is a material with a carbon chain as the basic skeleton, and its
flexibility is higher than that of single crystals and ceramics [
21
,
22
]. This high flexibility allows it to
withstand a greater amount of strain, thereby making it more suitable for application scenarios with
large bending and twisting requirements. Piezoelectric polymer PVDF and its copolymers have the
advantages of natural flexibility, easy processing and sucient mechanical strength, which make them
more suitable for flexible sensors than inorganic piezoelectric materials [3541].
Thus far, PVDF has been found to exist as five semi-crystalline polymorphs:
α
,
β
,
γ
,бand
ε
. Among these,
α
phase is not electroactive, and
β
phase has the strongest piezoelectricity.
PVDF with a low component of
β
phase has a d33 value of approximately 20–30 pC/N [
42
49
].
Micromachines 2020,11, 1076 4 of 29
Therefore, increasing the
β
-phase PVDF composition is vital to enlarge the mechanical–electrical energy
conversion performance of PVDF. Therefore, the piezoelectric coecient of PVDF is lower than that of
common inorganic piezoelectric materials. Chang et al. [
41
] prepared PVDF nanofibers with a high
β
-phase PVDF component by near-field electrospinning (Figure 2a). The fiber-based generator could
provide a maximum output voltage of 30 mV and maximum output current of 3 nA (Figure 2c). It has
a much higher energy conversion eciency than generators made of PVDF thin films [41].
Micromachines 2020, 11, x 4 of 30
advantages of natural flexibility, easy processing and sufficient mechanical strength, which make
them more suitable for flexible sensors than inorganic piezoelectric materials [35–41].
Thus far, PVDF has been found to exist as five semi-crystalline polymorphs: α, β, γ, б and ε.
Among these, α phase is not electroactive, and β phase has the strongest piezoelectricity. PVDF with
a low component of β phase has a d33 value of approximately 20–30 pC/N [42–49]. Therefore,
increasing the β-phase PVDF composition is vital to enlarge the mechanical–electrical energy
conversion performance of PVDF. Therefore, the piezoelectric coefficient of PVDF is lower than that
of common inorganic piezoelectric materials. Chang et al. [41] prepared PVDF nanofibers with a high
β-phase PVDF component by near-field electrospinning (Figure 2a). The fiber-based generator could
provide a maximum output voltage of 30 mV and maximum output current of 3 nA (Figure 2c). It
has a much higher energy conversion efficiency than generators made of PVDF thin films [41].
Figure 2. (a) Structure of P(VDF-TrFE); (b) PVDF nanofibers with the high component of β-phase
PVDF by near-field electrospinning [41]; (c) the generator based on fiber could provide a maximum
output power is 5–30 mV and 0.5–3 nA with different feature sizes [41]; (d) a free-standing film’s
photograph and SEM image [42]; (e) the output power of the generators based on textile [42]; (f) the
Figure 2.
(
a
) Structure of P(VDF-TrFE); (
b
) PVDF nanofibers with the high component of
β
-phase PVDF
by near-field electrospinning [
41
]; (
c
) the generator based on fiber could provide a maximum output
power is 5–30 mV and 0.5–3 nA with dierent feature sizes [
41
]; (
d
) a free-standing film’s photograph
and SEM image [
42
]; (
e
) the output power of the generators based on textile [
42
]; (
f
) the “3D spacer”
cross-sectional SEM image [50]; and (g) 2D and 3D piezoelectric textile’s output power [50].
The P(VDF-TrFE) copolymer has high crystallinity and piezoelectric properties, thus it has a high
piezoelectric response [
51
53
]. Persano et al. [
42
] proposed a piezoelectric textile based on highly
aligned electrospun P(VDF-TrFE) fibers. The proposed piezoelectric textile has a large area, and it
Micromachines 2020,11, 1076 5 of 29
is flexible and freestanding (Figure 2d) [
42
]. The maximum open-circuit voltage of the generators
based on textiles can reach 1.5 V (Figure 2e) [
42
], thus showing excellent flexibility and mechanical
strength. Many new designs have emerged in the practice field to produce all-fiber piezoelectric textiles.
A “3D spacer” based on all-fiber piezoelectric textiles was reported [
50
], consisting of top and bottom
electrodes and a spacer yarn. The spacer yarn is composed of high concentration of
β
-phase PVDF
monofilaments. The top and bottom electrodes are composed of silver-coated polyamide multifilament
yarn layers (Figure 2f) [
50
]. The new type of 3D textile-based PVDF generator had a much larger
output power density than the traditional 2D piezoelectric textile, up to 5.07
µ
W/cm
2
(Figure 2g) [
50
].
2.3. Piezoelectric Materials Doped in PVDF
To balance the contradiction between flexibility and higher voltage coecient, researchers have
tried to mix polymer piezoelectric materials with inorganic piezoelectric materials. Because of the good
piezoelectric properties and relatively soft properties of PZT and the tensile and flexural properties of
PVDF, the combination of the two will make sense. According to the above ideas, Tiwari et al. [
54
]
prepared PZT and PVDF piezoelectric composite films using solution casting technology. This method
can achieve both electric field polarization and mechanical stretching during the electrospinning
process, making it very suitable for manufacturing piezoelectric nanofibers. Similarly, the dielectric
properties of PZT/P (VDF-TrFE) composite films [
55
] were also studied. In 2020, Pal et al. [
56
] used
lanthanum-doped lead zirconate titanate (PLZT), PVDF and multi-walled carbon nanotubes (MWCNTs)
as supplementary fillers to create a three-phase hybrid piezoelectric nanogenerator. Figure 3a shows
the variation of stored power with time and SEM of PLZT particles dispersed in the PVDF matrix.
Raad et al. [
57
] fabricated nanogenerator devices on the basis of composite structure of PVDF, ZnO,
nanorods and BaTiO
3
. The voltage output of these composite structures is up to 12 V under the force
of 1.5 N (Figure 3b).
Considering the compatibility with the human body, lead-free materials are more popular.
Titanate or bismuth [
58
] are mixed with PVDF to develop lead-free piezoelectric nanofiber composites.
The piezoelectric properties of PVDF can be changed by adding calcined BaTiO
3
ceramic powder [
59
,
60
].
Micromachines 2020, 11, x 5 of 30
“3D spacer” cross-sectional SEM image [50]; and (g) 2D and 3D piezoelectric textile’s output power
[50].
The P(VDF-TrFE) copolymer has high crystallinity and piezoelectric properties, thus it has a
high piezoelectric response [51–53]. Persano et al. [42] proposed a piezoelectric textile based on highly
aligned electrospun P(VDF-TrFE) fibers. The proposed piezoelectric textile has a large area, and it is
flexible and freestanding (Figure 2d) [42]. The maximum open-circuit voltage of the generators based
on textiles can reach 1.5 V (Figure 2e) [42], thus showing excellent flexibility and mechanical strength.
Many new designs have emerged in the practice field to produce all-fiber piezoelectric textiles. A “3D
spacer” based on all-fiber piezoelectric textiles was reported [50], consisting of top and bottom
electrodes and a spacer yarn. The spacer yarn is composed of high concentration of β-phase PVDF
monofilaments. The top and bottom electrodes are composed of silver-coated polyamide
multifilament yarn layers (Figure 2f) [50]. The new type of 3D textile-based PVDF generator had a
much larger output power density than the traditional 2D piezoelectric textile, up to 5.07 μW/cm2
(Figure 2g) [50].
2.3. Piezoelectric Materials Doped in PVDF
To balance the contradiction between flexibility and higher voltage coefficient, researchers have
tried to mix polymer piezoelectric materials with inorganic piezoelectric materials. Because of the
good piezoelectric properties and relatively soft properties of PZT and the tensile and flexural
properties of PVDF, the combination of the two will make sense. According to the above ideas, Tiwari
et al. [54] prepared PZT and PVDF piezoelectric composite films using solution casting technology.
This method can achieve both electric field polarization and mechanical stretching during the
electrospinning process, making it very suitable for manufacturing piezoelectric nanofibers.
Similarly, the dielectric properties of PZT/P (VDF-TrFE) composite films [55] were also studied. In
2020, Pal et al. [56] used lanthanum-doped lead zirconate titanate (PLZT), PVDF and multi-walled
carbon nanotubes (MWCNTs) as supplementary fillers to create a three-phase hybrid piezoelectric
nanogenerator. Figure 3a shows the variation of stored power with time and SEM of PLZT particles
dispersed in the PVDF matrix. Raad et al. [57] fabricated nanogenerator devices on the basis of
composite structure of PVDF, ZnO, nanorods and BaTiO3. The voltage output of these composite
structures is up to 12 V under the force of 1.5 N (Figure 3b).
Considering the compatibility with the human body, lead-free materials are more popular.
Titanate or bismuth [58] are mixed with PVDF to develop lead-free piezoelectric nanofiber
composites. The piezoelectric properties of PVDF can be changed by adding calcined BaTiO3 ceramic
powder [59,60].
Figure 3. (a) Variation of stored power with time and SEM of PLZT particles dispersed in the PVDF
matrix [56]; (b) the relationship between output voltage and applied forces with different frequencies
[57]; (c) the relationship between output frequency and voltage of PVDF nanofiber composite
Figure 3.
(
a
) Variation of stored power with time and SEM of PLZT particles dispersed in the PVDF
matrix [
56
]; (
b
) the relationship between output voltage and applied forces with dierent frequencies [
57
];
(
c
) the relationship between output frequency and voltage of PVDF nanofiber composite containing
BNT-ST [
9
]; (
d
) optical diagram PVDF/BTZO hybrid film [
61
]; and (
e
) SEM of PVDF/BTZO hybrid
film [61].
Researchers at Kyungnam University synthesized an excellent lead-free piezoelectric material [
9
].
It is a kind of flexible lead-free piezoelectric nanofiber composite of BNT-ST((1
x)Bi0.5Na0.5TiO
3
xSrTiO) ceramics and PVDF polymer, which was fabricated by electrospinning. By measuring the
functional relationship between the output frequency and voltage of the PVDF nanofiber composite
Micromachines 2020,11, 1076 6 of 29
containing BNT-ST (Figure 3c) [
9
], the piezoelectric properties of the composite and the sensitivity of
the piezoelectric output to frequency are substantially improved because BNT-series materials can
overcome large coercive field defects and exhibit strong piezoelectric properties [6266].
In 2015, Alluri et al. [
61
] developed a flexible hybrid film of PVDF doped with highly
crystalline BaTi(1
x)ZrxO
3
(x=0, 0.05, 0.1, 0.15 and 0.2) nanocomposites (BTZO). During preparation,
the PVDF matrix solution was embedded in BTZO using a molten-salt process under ultrasonication
(Figure 3d) [
61
]. The SEM images of the PVDF/BTZO hybrid film are shown in Figure 3e. It can be
concluded that, by the molten salt method, replacing the Ti4+(0.605 Å) site with a Zr4+atom (0.72 Å)
can modify the high-purity bulk BaTiO
3
nanocomposites and enhance the piezoelectric coecient
(d33) from 100 pC/N (BaTiO
3
) [
67
71
] to 174–236 pC/N (BaTi(1
x)ZrxO
3
) [
72
76
]. The properties of
BTZO/PVDF hybrid films have been substantially improved compared to those of pure piezoelectric
films and have great potential in manufacturing environmental protection devices, active sensors and
flexible nanogenerators.
2.4. Conductive Nanomaterials Doped in PVDF
Although polymer materials have the advantages of high flexibility and high mechanical strength
compared to inorganic materials, the piezoelectric coecient of piezoelectric materials is relatively small;
moreover, their piezoelectric conversion eciency is low, and they do not easily conduct electricity.
The introduction of nanomaterials may change the crystal structure of polymer piezoelectric polymers
and improve the piezoelectric properties [
77
82
]. Zirconate titanate, barium titanate and zinc oxide have
high piezoelectric constants; therefore, polymer piezoelectric matrices usually use them as piezoelectric
fillers [
83
88
]. Compared with those of the pure piezoelectric polymer, the piezoelectric properties
of the nanocomposites are improved, dielectric constant is significantly increased, crystallinity of
the phase is increased and local stress is increased. Adding nanoclay [
89
] to PVDF can prompt the
formation of the
β
-phase in the spinning process, which is an ideal method to prepare piezoelectric
nanofibers. BTO nanoparticles were used to improve the P(VDF-TrFE) nanofibers’ piezoelectric sensing
properties (Figure 4a) [
90
]. BTO/P(VDF-TrFE) nanofibers doped with BTO nanoparticles have the
largest β-phase crystallinity and the best piezoelectric properties.
Wu et al. [
91
] studied the piezoelectric properties, crystal structure and sound absorption properties
of PVDF nanofibers doped with carbon nanotubes prepared by electrospinning. The results show that
adding CNTs further improves the piezoelectric properties and the ability to absorb sound waves at
low frequencies. Hosseini et al. [
92
] studied the potential synergistic eect of Cloisite 30B (OMMT)
nanoclay and multi-walled carbon nanotube (MWCNT) nanofillers on PVDF crystal structure and
piezoelectric device performance. They evaluated PVDF fiber mats’ sound-absorbing and piezoelectric
properties, and the results show that, compared with OMMT, the MWCNT could decrease PVDF
impedance and increase the dielectric constant. MWCNT/OMMT hybrid nanocomposites have good
sound absorption properties, which might be caused by the enhanced interaction at the polymer-filled
interface between the acoustic wave and OMMT plates and MWCNT nanotubes [
93
97
]. They found
that the sound absorption eciency of PVDF/MWCNT/OMMT hybrid nanocomposites was higher
than that of pure PVDF fibers and film. Negar et al. [
11
] fabricated PVDF nanocomposites by doping
with PZT particles. The PVDF-PZT nanocomposite fiber, the crystallization mechanism and
β
-phase
formation based on the addition of PZT are shown in Figure 4b.
PVDF and graphene-silver (GAg) nanocomposites have plasma-coupled piezoelectric properties,
and the piezoelectric energy conversion eciency reaches 15% [
98
]. Using the unique interface structure
of silver nanoparticles, silver-doped oriented PVDF nanofibers with a high content of
β
phase were
prepared by electrospinning [
99
], and the crystallinity of the
β
phase was 44.5%. Silver nanoparticles
in graphene dispersion form n-type doping on graphene owing to electrostatic eects [
100
104
].
The self-polarization of silver nanoparticles in PVDF is beneficial to the nucleation of the electroactive
β
phase. PVDF nanocomposites are environmentally friendly and low-cost nanocomposites, having
Micromachines 2020,11, 1076 7 of 29
high piezoelectric coecients and dierent piezoelectric coecients under dierent light conditions
(Figure 4c) [98].
Gan et al. [
105
] developed composite materials using TiO
2
nanoparticles and PVDF. The increase
in conductivity makes the polarization process easier, and the required electric field was reduced
from 260 to 120 MV/m. In 2017, Al-Saygh et al. [
106
] proposed a flexible pressure sensor based on
PVDF doped with rGO and TiO
2
nanolayers (TNL). The hybrid PVF/rGO-TNL film had a higher
β
-phase content (75.68%) than PVDF (70.37%), PVDF/rGO (72.73%) and PVDF/TNL (73.5%) films.
Similarly, Pusty et al. [
107
] combined iron with carbon nanotubes and graphene nanomaterials and
proposed that adding CNT and Fe-RGO to PVDF can increase the conductivity of nanocomposite
membranes. Mishra et al. [
108
] developed a flexible piezoelectric polymer nanocomposite film to
collect mechanical energy, which is based on PVDF doped with gallium ferrite nanoparticles (GFO).
Under mechanical pressure and release conditions, the output voltage and current are 3.5 V and 4 nA,
respectively (Figure 4d). As a type of piezoelectric ceramic, nano ZnO is also used as a piezoelectric
filler [
109
]. Dodds et al. [
110
] prepared PVDF TrFE/ZnO nanoparticle films by spin coating and studied
their piezoelectric response. The prepared films not only maintain their mechanical flexibility but
also improve their piezoelectric properties. Zinc oxide-PVDF composite membrane [
111
] can also be
prepared by sol-gel technology. The latest report is on the preparation of metal ZnO PVDF composite
films by doping ZnO nanoparticles into metals, which shows the potential for mechanical energy
generation and motion sensing. Figure 4e shows the fabrication process of the device. They studied
and compared the voltage output of pure ZnO-PVDF samples and ZnO-PVDF composites doped with
sodium (Na), cobalt (Co), silver (Ag), ferric oxide (Fe3O4) and lithium (Li) [112].
Micromachines 2020, 11, x 8 of 30
Figure 4. (a) The SEM of BTO/P(VDF-TrFE) nanocomposite fiber [90]; (b) PVDF-PZT nanocomposite
fiber, the crystallization mechanism and β-phase formation based on adding PZT [11]; (c) change of
piezoelectric coefficient in the form of output voltage under different light conditions [98]; (d) output
voltage and current signal of GFO-PVDF composite films [108]; and (e) schematic of the
manufacturing process of the device [112].
2.5. Biomaterials Functional PVDF
Some biomaterials have certain piezoelectric properties naturally. In 2017, Nuraeva et al. [113]
investigated (S)-glutamine and ortho-carboranyl derivatives of (S)-asparagine films and bulk crystal
piezoelectric properties, demonstrating a very high piezoelectric response. The high local transverse
piezoelectric coefficients of these biocrystals indicate that they are promising materials for various
piezoelectric applications. Stapleton et al. [114] provided experimental evidence of the globulin and
lysozyme’s direct piezoelectric effect. They measured the direct piezoelectric effect of the lysozyme
crystal aggregation film and found that the average piezoelectric coefficients of the monoclinic and
tetragonal lysozyme films were 0.94 and 3.16 pC/N, respectively (Figure 5a). Nguyen et al. [115]
reported that diphenylalanine (FF) is a short peptide composed of two natural amino acids, with
special piezoelectric properties and excellent mechanical properties (Figure 5b). Doping with a
piezoelectric polymer can stimulate the piezoelectric properties. Recently, Yang et al. [116] modified
barium titanate (BTO) with polydopamine (PDA). Then, the mixing ratio was adjusted between it
and the PVDF matrix, and they finally formed a uniform and homogeneous PDA@BTO/PVDF
composite material. The facial solution-casting method was used to make the flexible piezoelectric
pressure sensor shown in Figure 5c. PDA can improve the dispersion of BTO in the PVDF matrix and
Figure 4.
(
a
) The SEM of BTO/P(VDF-TrFE) nanocomposite fiber [
90
]; (
b
) PVDF-PZT nanocomposite
fiber, the crystallization mechanism and
β
-phase formation based on adding PZT [
11
]; (
c
) change of
piezoelectric coecient in the form of output voltage under dierent light conditions [
98
]; (
d
) output
voltage and current signal of GFO-PVDF composite films [
108
]; and (
e
) schematic of the manufacturing
process of the device [112].
Micromachines 2020,11, 1076 8 of 29
2.5. Biomaterials Functional PVDF
Some biomaterials have certain piezoelectric properties naturally. In 2017, Nuraeva et al. [
113
]
investigated (S)-glutamine and ortho-carboranyl derivatives of (S)-asparagine films and bulk crystal
piezoelectric properties, demonstrating a very high piezoelectric response. The high local transverse
piezoelectric coecients of these biocrystals indicate that they are promising materials for various
piezoelectric applications. Stapleton et al. [
114
] provided experimental evidence of the globulin and
lysozyme’s direct piezoelectric eect. They measured the direct piezoelectric eect of the lysozyme
crystal aggregation film and found that the average piezoelectric coecients of the monoclinic and
tetragonal lysozyme films were 0.94 and 3.16 pC/N, respectively (Figure 5a). Nguyen et al. [
115
]
reported that diphenylalanine (FF) is a short peptide composed of two natural amino acids, with special
piezoelectric properties and excellent mechanical properties (Figure 5b). Doping with a piezoelectric
polymer can stimulate the piezoelectric properties. Recently, Yang et al. [
116
] modified barium titanate
(BTO) with polydopamine (PDA). Then, the mixing ratio was adjusted between it and the PVDF matrix,
and they finally formed a uniform and homogeneous PDA@BTO/PVDF composite material. The facial
solution-casting method was used to make the flexible piezoelectric pressure sensor shown in Figure 5c.
PDA can improve the dispersion of BTO in the PVDF matrix and reduce interface void defects and
cracks between the two components. The output voltage is about 2 times higher than the unmodified
one and 13.3 times higher than the original PVDF. Tamang et al. [
117
] reported that DNA was used as
a nucleating agent to produce a self-polarized PVDF membrane with higher piezoelectric properties
(Figure 5d). This nucleating agent can achieve the molecular dipole arrangement and the nucleation of
the electroactive
β
phase in PVDF. Phosphate ions interact with hydrogen bonds on the single-strand
deoxyribonucleic acid (ss-DNA) backbone to form a stable polar
β
phase, accounting for more than
80% of polar βphase in the PVDF matrix (Figure 5e).
Micromachines 2020, 11, x 9 of 30
reduce interface void defects and cracks between the two components. The output voltage is about 2
times higher than the unmodified one and 13.3 times higher than the original PVDF. Tamang et al.
[117] reported that DNA was used as a nucleating agent to produce a self-polarized PVDF membrane
with higher piezoelectric properties (Figure 5d). This nucleating agent can achieve the molecular
dipole arrangement and the nucleation of the electroactive β phase in PVDF. Phosphate ions interact
with hydrogen bonds on the single-strand deoxyribonucleic acid (ss-DNA) backbone to form a stable
polar β phase, accounting for more than 80% of polar β phase in the PVDF matrix (Figure 5e).
Figure 5. (a) Piezoelectric voltage generated increases with increasing applied force [114]; (b) open-
circuit voltage of the FF-generator [115]; (c) the fabrication process of PDA@BTO/PVDF composite
membrane on the basis of flexible pressure sensor [116]; (d) DNA was used as a nucleating agent to
produce a self-polarized PVDF membrane to fabricate nanogenerator [117]; and (e) denaturation of
DNA and the forming process of ß-enriched DNA-PVDF membrane [117].
3. Fabrication and Polarization of PVDF
There are many methods to convert PVDF into the β phase, including electrospinning, spin
coating, solvent casting and 3D printing. PVDF processed by spin coating and solvent casting
methods mainly forms the α phase; therefore, a subsequent polarization method is needed to increase
the β phase state of PVDF [118,119]. Polarization can be achieved with a set of widely used techniques
that can be used to reorient polymers to increase the net polarization vector in three directions, which
is an important step during the fabrication process. For the processing methods of electrospinning
and 3D printing, the polarization process can be completed directly in situ under the action of an
electric field and temperature, and no subsequent polarization process is required. The following
sections mainly discuss the preparation and polarization methods of PVDF films, including the
piezoelectric effect principle.
3.1. Fabrication of PVDF by Spin Coating and Solvent Casting
Spin coating and solvent casting mainly form the α phase of PVDF; therefore, a subsequent
polarization process is required. In the spin coating method, the surface stress is increased during the
spinning process. The addition of some compounds helps induce and contribute to the formation of
the β phase state of PVDF, and sometimes it can be formed with a certain piezoelectric coefficient
without subsequent polarization. A schematic diagram of the spin coating and solvent casting is
shown in Figure 6a,b.
Figure 5.
(
a
) Piezoelectric voltage generated increases with increasing applied force [
114
];
(
b
) open-circuit voltage of the FF-generator [
115
]; (
c
) the fabrication process of PDA@BTO/PVDF
composite membrane on the basis of flexible pressure sensor [
116
]; (
d
) DNA was used as a nucleating
agent to produce a self-polarized PVDF membrane to fabricate nanogenerator [
117
]; and (
e
) denaturation
of DNA and the forming process of ß-enriched DNA-PVDF membrane [117].
3. Fabrication and Polarization of PVDF
There are many methods to convert PVDF into the
β
phase, including electrospinning, spin coating,
solvent casting and 3D printing. PVDF processed by spin coating and solvent casting methods mainly
forms the
α
phase; therefore, a subsequent polarization method is needed to increase the
β
phase
Micromachines 2020,11, 1076 9 of 29
state of PVDF [
118
,
119
]. Polarization can be achieved with a set of widely used techniques that can
be used to reorient polymers to increase the net polarization vector in three directions, which is an
important step during the fabrication process. For the processing methods of electrospinning and
3D printing, the polarization process can be completed directly in situ under the action of an electric
field and temperature, and no subsequent polarization process is required. The following sections
mainly discuss the preparation and polarization methods of PVDF films, including the piezoelectric
eect principle.
3.1. Fabrication of PVDF by Spin Coating and Solvent Casting
Spin coating and solvent casting mainly form the
α
phase of PVDF; therefore, a subsequent
polarization process is required. In the spin coating method, the surface stress is increased during the
spinning process. The addition of some compounds helps induce and contribute to the formation of the
β
phase state of PVDF, and sometimes it can be formed with a certain piezoelectric coecient without
subsequent polarization. A schematic diagram of the spin coating and solvent casting is shown in
Figure 6a,b.
Micromachines 2020, 11, x 10 of 30
Figure 6. (a) Schematic of electrode poling method; and (b) corona poling method.
Electrode poling and corona poling are the two most common methods of polarization.
Electrode poling has the advantages of high d33 coefficient and reproducibility. Electrode poling is
the relatively simpler one of these two methods. The PVDF film is sandwiched between two
electrodes and then wrapped by a shell (Figure 6a). In the process of corona poling, there is a heated
substrate at the bottom of the shell and an electrode on the heated substrate. The PVDF film is only
in contact with one of the electrodes. The corona tip is placed on the top and applied with a 10 kV
voltage. There is a grid between tip and PVDF material. The grid is applied a voltage, which is much
lower than 10 kV. Therefore, there is a voltage difference close to 10 kV between corona tip and grid.
It ionizes the gas between the corona tip and the grid. The ionized gas accelerates to the film, and the
PVDF film is polarized (Figure 6b). Corona poling needs to keep the gas in the shell dry. The electron
beam poling method uses a focused electron beam and irradiates the PVDF film to reorient into the
β phase. In addition to the above polarization methods, mechanical drawing and additive
manufacturing can polarize PVDF. As for additive manufacturing, we can induce the formation of
the β phase using composite systems. Table 1 shows the advantages of various polarization methods.
The method of solvent casting mainly relies on the addition of some compounds and application of
high temperature and external electric field, to promote the formation of the β phase state of PVDF.
Chien et al. [120] spin-coated PVDF–TrFE dissolved in MEK and hot embossed the silicon mold on
the film under DC poling (Figure 7c). The electric voltage varied from 30 to 60 V, and the temperature
was 90 or 110 °C. The maximum piezoelectricity of the single PVDF−TrFE nanopillar was 210.4 pm/V,
with an average d33 value of 72.7 pC/N of the developed PVDF−TrFE nanograss structures. Tushar
et al. [121] spin-coated PVDF–TrFE copolymer into thin films to induce the formation of the β phase
with a d33 value varying from 38 to 74 pC/N (Figure 7d). Schulze et al. [122] prepared P(VDF-TrFE)
films for 20 μm thickness by solvent casting under the effect of electrode poling and an electric field
of 75 MV/m. The d33 coefficients were measured to be about 20 pC/N. The spin-coating technique
[123] was used to prepare the PVDF-BaTiO3 nanocomposite films on an interdigital ITO
electrode(Figure 7e). Vineet et al. [54] fabricated PVDF/PZT piezoelectric composite films by a
solution cast technique. The d33 value varied from 60 to 84 pC/ N. The higher is the content of PZT
ceramic, the greater is the proportion of β phase of the PVDF.
After the PVDF film is formed, there are further studies to constrain the nanostructure of the
film, such as the serpentine structure [124–126] and Kirigami structure [127] formed by cutting
plotter, achieving sufficient compliance of mechanical deformation to skin motion. Conversely, the
polymer film is cut into the desired shape and then stacked and laminated to increase the density of
the filler [128].
Figure 6. (a) Schematic of electrode poling method; and (b) corona poling method.
Electrode poling and corona poling are the two most common methods of polarization.
Electrode poling has the advantages of high d33 coecient and reproducibility. Electrode poling is the
relatively simpler one of these two methods. The PVDF film is sandwiched between two electrodes
and then wrapped by a shell (Figure 6a). In the process of corona poling, there is a heated substrate
at the bottom of the shell and an electrode on the heated substrate. The PVDF film is only in contact
with one of the electrodes. The corona tip is placed on the top and applied with a 10 kV voltage.
There is a grid between tip and PVDF material. The grid is applied a voltage, which is much lower
than 10 kV. Therefore, there is a voltage dierence close to 10 kV between corona tip and grid. It ionizes
the gas between the corona tip and the grid. The ionized gas accelerates to the film, and the PVDF
film is polarized (Figure 6b). Corona poling needs to keep the gas in the shell dry. The electron beam
poling method uses a focused electron beam and irradiates the PVDF film to reorient into the
β
phase.
In addition to the above polarization methods, mechanical drawing and additive manufacturing can
polarize PVDF. As for additive manufacturing, we can induce the formation of the
β
phase using
composite systems. Table 1shows the advantages of various polarization methods. The method of
solvent casting mainly relies on the addition of some compounds and application of high temperature
and external electric field, to promote the formation of the
β
phase state of PVDF. Chien et al. [
120
]
spin-coated PVDF–TrFE dissolved in MEK and hot embossed the silicon mold on the film under DC
poling (Figure 7c). The electric voltage varied from 30 to 60 V, and the temperature was 90 or 110
C. The
maximum piezoelectricity of the single PVDF
TrFE nanopillar was 210.4 pm/V, with an average d33
Micromachines 2020,11, 1076 10 of 29
value of 72.7 pC/N of the developed PVDF
TrFE nanograss structures. Tushar et al. [
121
] spin-coated
PVDF–TrFE copolymer into thin films to induce the formation of the
β
phase with a d33 value varying
from 38 to 74 pC/N (Figure 7d). Schulze et al. [
122
] prepared P(VDF-TrFE) films for 20
µ
m thickness
by solvent casting under the eect of electrode poling and an electric field of 75 MV/m. The d33
coecients were measured to be about 20 pC/N. The spin-coating technique [
123
] was used to prepare
the PVDF-BaTiO
3
nanocomposite films on an interdigital ITO electrode(Figure 7e). Vineet et al. [
54
]
fabricated PVDF/PZT piezoelectric composite films by a solution cast technique. The d33 value varied
from 60 to 84 pC/N. The higher is the content of PZT ceramic, the greater is the proportion of
β
phase
of the PVDF.
Table 1. The advantages of various polarization methods.
Polarization Methods Advantages
Electrode poling High d33 coecient and reproducibility
Corona poling
High d33 coecient and No requirement for one end
structure of the material
Additive manufacturing Increased the design scope of the three-dimensional
structure of the material and low temperature
Mechanical drawing High d33 coecient and reproducibility
Electron beam poling Increased the design scope of the three-dimensional
structure of the material
Micromachines 2020, 11, x 11 of 30
Figure 7. (a) Schematic diagram of the spin coating; (b) schematic diagram of the solvent casting; (c)
spin-coated PVDF-TrFE was dissolved in MEK and the silicon mold was hot pressed on the film [120];
(d) SEM of PVDF–TrFE copolymer thin films fabricated by spin coating [121]; and (e) preparing the
PVDF-BaTiO3 nanocomposite films by spin coating [123].
Table 1. The advantages of various polarization methods.
Polarization
Methods Advantages
Electrode poling High d33 coefficient and reproducibility
Corona poling High d33 coefficient and No requirement for one end structure of the
material
Additive
manufacturing
Increased the design scope of the three-dimensional structure of the
material and low temperature
Mechanical drawing High d33 coefficient and reproducibility
Electron beam
poling
Increased the design scope of the three-dimensional structure of the
material
3.2. Fabrication of PVDF by Electrospinning
Electrospinning is a very promising processing technology. The needle tube containing the
PVDF solution sprays the PVDF solution onto a drum through a nozzle, the substrate wraps around
the drum, and a voltage between 10 and 20 kV is usually added between the nozzle and the substrate
[129]. Nano- to micro-scale PVDF fibers are randomly deposited on the substrate, forming a low-
density PVDF film. The electrospinning technique completes the PVDF deposition and polarization
process in one step. The rotation of the drum increases the stress between PVDF and the substrate,
which in turn promotes the formation of the β-phase state of PVDF. A schematic diagram of the
electrospinning technique is shown in Figure 8a. Gong et al. [130] combined the PVDF nanofiber
membrane by electrospinning with PDMS-Ag NWS and PET/ITO electrodes to fabricate sensors. Lu
et al. [99] made a flexible bend sensor to monitor human respiration by electrospinning PVDF and a
silver mixed solution (Figure 8b).
Figure 7.
(
a
) Schematic diagram of the spin coating; (
b
) schematic diagram of the solvent casting;
(
c
) spin-coated PVDF-TrFE was dissolved in MEK and the silicon mold was hot pressed on the film [
120
];
(
d
) SEM of PVDF–TrFE copolymer thin films fabricated by spin coating [
121
]; and (
e
) preparing the
PVDF-BaTiO3nanocomposite films by spin coating [123].
After the PVDF film is formed, there are further studies to constrain the nanostructure of the
film, such as the serpentine structure [
124
126
] and Kirigami structure [
127
] formed by cutting plotter,
achieving sucient compliance of mechanical deformation to skin motion. Conversely, the polymer
film is cut into the desired shape and then stacked and laminated to increase the density of the
filler [128].
3.2. Fabrication of PVDF by Electrospinning
Electrospinning is a very promising processing technology. The needle tube containing the PVDF
solution sprays the PVDF solution onto a drum through a nozzle, the substrate wraps around the
drum, and a voltage between 10 and 20 kV is usually added between the nozzle and the substrate [
129
].
Micromachines 2020,11, 1076 11 of 29
Nano- to micro-scale PVDF fibers are randomly deposited on the substrate, forming a low-density
PVDF film. The electrospinning technique completes the PVDF deposition and polarization process
in one step. The rotation of the drum increases the stress between PVDF and the substrate, which in
turn promotes the formation of the
β
-phase state of PVDF. A schematic diagram of the electrospinning
technique is shown in Figure 8a. Gong et al. [
130
] combined the PVDF nanofiber membrane by
electrospinning with PDMS-Ag NWS and PET/ITO electrodes to fabricate sensors. Lu et al. [
99
] made
a flexible bend sensor to monitor human respiration by electrospinning PVDF and a silver mixed
solution (Figure 8b).
Micromachines 2020, 11, x 12 of 30
Figure 8. (a) Schematic diagram of the Electrospinning technique; (b) flexible bend sensor to monitor
human respiration [99]; (c) optical photo and SEM of (BTO)/P(VDF-TrFE) composite nanofibers [90];
(d) optical photo and SEM of BNT-ST/PVDF nanofiber composite [9]; (e) PVDF strain sensor for
measuring the force of the string [131]; (f) mechanical movement in the primitive phase and the
bending phase [132]; and (g) optical image and output voltage generated by human movement [132].
Xiaohe et al. [90] fabricated (BTO)/P(VDF-TrFE) composite nanofibers by electrospinning and
were able to distinguish and sense the movement of walking ants and the energy of a free-falling ball
as low as 0.6 μJ (Figure 8c). Sang et al. [9] fabricated flexible lead-free piezoelectric nanofibers
composed of PVDF and BNT-ST ceramic by electrospinning. The output voltage measurement
system is a frequency function for the BNT-ST/PVDF nanofiber composite module (Figure 8d). Rahul
et al. [131] fabricated PVDF nanofibers with a high β-phase state to develop a PVDF strain sensor to
measure the string force (Figure 8e). Kunming et al. [132] fabricated nanocomposite fiber mats by
electrospinning graphene nanosheets, barium titanate and PVDF, obtaining as high as 11 V of open-
circuit voltage (Figure 8f,g).
Figure 8.
(
a
) Schematic diagram of the Electrospinning technique; (
b
) flexible bend sensor to monitor
human respiration [
99
]; (
c
) optical photo and SEM of (BTO)/P(VDF-TrFE) composite nanofibers [
90
];
(
d
) optical photo and SEM of BNT-ST/PVDF nanofiber composite [
9
]; (
e
) PVDF strain sensor for
measuring the force of the string [
131
]; (
f
) mechanical movement in the primitive phase and the bending
phase [132]; and (g) optical image and output voltage generated by human movement [132].
Micromachines 2020,11, 1076 12 of 29
Xiaohe et al. [
90
] fabricated (BTO)/P(VDF-TrFE) composite nanofibers by electrospinning and were
able to distinguish and sense the movement of walking ants and the energy of a free-falling ball as low
as 0.6
µ
J (Figure 8c). Sang et al. [
9
] fabricated flexible lead-free piezoelectric nanofibers composed of
PVDF and BNT-ST ceramic by electrospinning. The output voltage measurement system is a frequency
function for the BNT-ST/PVDF nanofiber composite module (Figure 8d). Rahul et al. [
131
] fabricated
PVDF nanofibers with a high
β
-phase state to develop a PVDF strain sensor to measure the string force
(Figure 8e). Kunming et al. [
132
] fabricated nanocomposite fiber mats by electrospinning graphene
nanosheets, barium titanate and PVDF, obtaining as high as 11 V of open-circuit voltage (Figure 8f,g).
3.3. Fabrication of PVDF Devices by 3D Print
The 3D print can deposit PVDF or PVDF compounds on the bottom heating plate under
heating and nozzle extrusion (Figure 9a). Based on the 3D printing principles, we can incorporate
polarizing processes including heat press, electric field poling and mechanical stretching simultaneously.
The polarization process is shown in Figure 9b.
Micromachines 2020, 11, x 13 of 30
3.3. Fabrication of PVDF Devices by 3D Print
The 3D print can deposit PVDF or PVDF compounds on the bottom heating plate under heating
and nozzle extrusion (Figure 9a). Based on the 3D printing principles, we can incorporate polarizing
processes including heat press, electric field poling and mechanical stretching simultaneously. The
polarization process is shown in Figure 9b.
Figure 9. (a) Schematic of 3D printing of PVDF layer; (b) corona poling process; (c) simultaneous 3D
printing and in-situ polarization; (d) 3D printing process and fabricated PVDF film [133]; (e) 3D
cylindrical sensor’s piezoelectric voltage output when five finger taps [134]; and (f) optical diagram
of 3D printing technique and corona poling process [135].
Finally, we completed the integration of 3D printed PVDF and PVDF composites with the in situ
polarized PVDF. The schematic is shown in Figure 9c. The shear force between the nozzle and PVDF,
as well as heating, nucleation on filler surfaces and electric field, all help to promote the formation of
the β-phase state of PVDF. Hoejin et al. [136] fabricated PVDF/BaTiO3 nanocomposites using a 3D
printing technique. Then, the composite material was thermally polarized to improve the
piezoelectric response of the composite. The piezoelectric response of the PVDF/BaTiO3
nanocomposite was three times higher than that of the nanocomposites fabricated by solvent casting.
Using the 3D printing technique, we can obtain a more homogeneous dispersion of PVDF/BaTiO3
nanocomposites, in comparison with nanocomposites fabricated by solvent casting. Chen et al. [133]
fabricated electroactive PVDF thin films of bi-axially orientation using layer-by-layer 3D printing.
Carbon nanotubes were added as a nucleating agent to induce the formation of the β-phase state of
PVDF (Figure 9d).
Sampada et al. [134] fabricated flexible, lightweight and complex-shaped piezoelectric devices
by 3D printing (Figure 9e). The d31 coefficients were measured to be about 18 pC/N. PVDF films with
enhanced β-phase percentage [135] were fabricated by 3D printing (Figure 9f).
This section reviews the common preparation methods for PVDF. The different fabrication
methods, polarization, description and comparison of the performance of PVDF are shown in Table
2. The electrospinning process has the advantages of direct polarization, while spin coating and
pouring methods are simpler and more compatible with other processes. As a new technology, 3D
printing has also attracted the attention of researchers. Table 2 shows that the electrospinning method
still shows higher piezoelectric conversion performance for energy acquisition and strain sensing.
However, for the electro deformation, the spin coating method can produce thinner films with greater
deformation.
Figure 9.
(
a
) Schematic of 3D printing of PVDF layer; (
b
) corona poling process; (
c
) simultaneous
3D printing and in-situ polarization; (
d
) 3D printing process and fabricated PVDF film [
133
]; (
e
) 3D
cylindrical sensor’s piezoelectric voltage output when five finger taps [
134
]; and (
f
) optical diagram of
3D printing technique and corona poling process [135].
Finally, we completed the integration of 3D printed PVDF and PVDF composites with the in situ
polarized PVDF. The schematic is shown in Figure 9c. The shear force between the nozzle and PVDF,
as well as heating, nucleation on filler surfaces and electric field, all help to promote the formation of the
β
-phase state of PVDF. Hoejin et al. [
136
] fabricated PVDF/BaTiO
3
nanocomposites using a 3D printing
technique. Then, the composite material was thermally polarized to improve the piezoelectric response
of the composite. The piezoelectric response of the PVDF/BaTiO
3
nanocomposite was three times
higher than that of the nanocomposites fabricated by solvent casting. Using the 3D printing technique,
we can obtain a more homogeneous dispersion of PVDF/BaTiO
3
nanocomposites, in comparison with
nanocomposites fabricated by solvent casting. Chen et al. [
133
] fabricated electroactive PVDF thin
films of bi-axially orientation using layer-by-layer 3D printing. Carbon nanotubes were added as a
nucleating agent to induce the formation of the β-phase state of PVDF (Figure 9d).
Sampada et al. [
134
] fabricated flexible, lightweight and complex-shaped piezoelectric devices by
3D printing (Figure 9e). The d31 coecients were measured to be about 18 pC/N. PVDF films with
enhanced β-phase percentage [135] were fabricated by 3D printing (Figure 9f).
Micromachines 2020,11, 1076 13 of 29
This section reviews the common preparation methods for PVDF. The dierent fabrication
methods, polarization, description and comparison of the performance of PVDF are shown in Table 2.
The electrospinning process has the advantages of direct polarization, while spin coating and pouring
methods are simpler and more compatible with other processes. As a new technology, 3D printing has
also attracted the attention of researchers. Table 2shows that the electrospinning method still shows
higher piezoelectric conversion performance for energy acquisition and strain sensing. However, for the
electro deformation, the spin coating method can produce thinner films with greater deformation.
Micromachines 2020,11, 1076 14 of 29
Table 2. Fabrication methods, polarization, description and comparison of the performance of PVDF.
Material Fabrication
Method
Poling
Conditions
Piezoelectric
Coecient
β-Phase
Content (%)
Peak to Peak
Voltage Sensitivity
Maximum
Electro
Deformation
Application
BTO/P(VDF-TrFE)
[90]
Electro spinning
No poling d33 50 pC/N 81% - 5 PC/kPa -
sensing strain
PVDF [134] 3D print No poling d31 18 pC/N 64% Vp-p =6.3 V ~1.8 V/N -
PVDF/AG [99]
Electro spinning
No poling - 44.50% Vp-p =4.6 V 2.5 V (slow and
light breathing) -
PVDF [133] 3D print No poling d33 8.7 pC/N 61.52% Vp-p =0.4 V - -
PVDF/PZT [54] Solvent Casting No poling d33 60–84 pC/N 75% - - -
PVDF-TrFE
[122]Solvent Casting
Electrode poling
(75 MV/m) d33 20 pC/N - - 0.57 pC/g -
PVDF-TrFE
[121]Spin coating No poling d33 38–74 pC/Nhigher with thin
film -
2.35 mV /mmHg
-
PVDF-TrFE
[120]Spin coating DC poling
(30~60 V) d33 72.7 pC/N - 526 mV /a
nanopillar - -
PVDF with
PDMS/Ag NWs
[130]
Electro spinning
No poling - 57% Vp-p =2.2 V 0.02 V/kPa -
PVDF /BaTiO3
[136]3D print Thermal poling
(90 °C for 2 h) d31 2.1 ×10E–3
pC/N61.20% - - -
energy
harvesting
PVDF [135] 3D print Corona poling
(280 MV/m) d31 4.9 ×10E–2
pC/N56.83% - - -
PVDF /BT
/Graphene [132]
Electro spinning
No poling - 91.50% Vp-p =11 V - -
PVDF/BNT-ST
[9]
Electro spinning
No Poling - - Vp-p =1.31 V - -
PVDF [137] 3D print No Poling d33 33 pC/N - Vp-p =0.3 V 13.3 mV/N
2.02
µ
m at 860 V
electro
deformation
PVDF/CNT
[138]
Electro spinning
No Poling - 90% Vp-p =5 V 0.9 V/N 18 µm at 300 V
PVDF [139] Spin coating DC poling (70
MV/m) - - - - 170 µm (3 mm,
at 300 V)
Micromachines 2020,11, 1076 15 of 29
4. Flexible Electromechanical Device Made by PVDF
PVDF polymer material has the advantages of low weight, high flexibility, high sensitivity,
good fit and ductility and high piezoelectric coecient. It is suitable for flexible sensors with high
flexibility requirements and is used in physics, chemistry and biology. These fields have a wide range
of applications.
There are many applications based on PVDF materials, such as energy harvesting device, physical
sensors, chemical sensors and biosensors. When PVDF is used for energy harvesting devices and
physical sensors, PVDF is mainly used to convert external mechanical energy into electrical energy.
Methods such as doping inorganic piezoelectric and nanomaterials and improving processes have
been adopted to improve the piezoelectric conversion ability of materials. Piezoelectric polymer-based
chemical sensors and biosensors, such as humidity, gas and DNA sensors, are constructed by
modifying the sensitive layer on piezoelectric materials, which can specifically adsorb humidity, gas and
biomolecules. After specific adsorption of the target, the sensor will undergo a slight volume change,
which will cause deformation of the PVDF material, thereby generating electrical signals, or the quality
change will cause the sensor’s resonance frequency to shift. The materials, fabrication methods and
sensitivity are shown in Table 3.
4.1. Energy Harvesting Device
The main principle of energy harvesting device manufacturing is to use the piezoelectric conversion
eect of piezoelectric materials. Under external pressure, the piezoelectric material deforms and
generates a voltage, thereby converting mechanical to electrical energy. A schematic diagram of the
energy harvesting device is shown in Figure 10a.
Micromachines 2020, 11, x 15 of 30
4. Flexible Electromechanical Device Made by PVDF
PVDF polymer material has the advantages of low weight, high flexibility, high sensitivity, good
fit and ductility and high piezoelectric coefficient. It is suitable for flexible sensors with high flexibility
requirements and is used in physics, chemistry and biology. These fields have a wide range of
applications.
There are many applications based on PVDF materials, such as energy harvesting device,
physical sensors, chemical sensors and biosensors. When PVDF is used for energy harvesting devices
and physical sensors, PVDF is mainly used to convert external mechanical energy into electrical
energy. Methods such as doping inorganic piezoelectric and nanomaterials and improving processes
have been adopted to improve the piezoelectric conversion ability of materials. Piezoelectric
polymer-based chemical sensors and biosensors, such as humidity, gas and DNA sensors, are
constructed by modifying the sensitive layer on piezoelectric materials, which can specifically adsorb
humidity, gas and biomolecules. After specific adsorption of the target, the sensor will undergo a
slight volume change, which will cause deformation of the PVDF material, thereby generating
electrical signals, or the quality change will cause the sensor’s resonance frequency to shift. The
materials, fabrication methods and sensitivity are shown in Table 3.
4.1. Energy Harvesting Device
The main principle of energy harvesting device manufacturing is to use the piezoelectric
conversion effect of piezoelectric materials. Under external pressure, the piezoelectric material
deforms and generates a voltage, thereby converting mechanical to electrical energy. A schematic
diagram of the energy harvesting device is shown in Figure 10a.
Figure 10. (a) Schematic of the energy harvesting device; (b) schematic diagram of woven
nanogenerator production process [140]; (c) nanogenerator harvests energy from human finger
tapping, human arm movement and human footsteps [140]; and (d) the phenomenon of charge
separation during the process of press nanogenerator and release pressure of nanogenerator [141].
In 2020, Mokhtari et al. [142] fabricated a high-performance hybrid piezofiber composed of a
PVDF and barium titanate (BT) nanoparticle (mass ratio 10:1). These fibers are knitted to fabricate a
Figure 10.
(
a
) Schematic of the energy harvesting device; (
b
) schematic diagram of woven nanogenerator
production process [
140
]; (
c
) nanogenerator harvests energy from human finger tapping, human arm
movement and human footsteps [
140
]; and (
d
) the phenomenon of charge separation during the process
of press nanogenerator and release pressure of nanogenerator [141].
Micromachines 2020,11, 1076 16 of 29
In 2020, Mokhtari et al. [
142
] fabricated a high-performance hybrid piezofiber composed of a
PVDF and barium titanate (BT) nanoparticle (mass ratio 10:1). These fibers are knitted to fabricate
a wearable energy generator with a power density of 87
µ
W cm
3
and a maximum voltage output
of 4 V. They confirmed through experiments that the stability performance of the triaxial braided
piezoelectric fibers in the bending test during 1000 cycles to a maximum strain of 50% at 0.6 Hz with
no change in its performance. In 2019, Sang et al. [
143
] prepared a wearable piezoelectric energy
harvester based on core–shell piezoelectric yarns prepared by twining the yarns around a conductive
thread, improving the durability of the energy harvesting process. The yarns were composed of
flexible piezoelectric nanofibers of BNT-ST and PVDF-TrFE by electrospinning. Muhammad et al. [
140
]
fabricated a woven nanogenerator using commercial nylon cloth and PVDF nanofibers, which can
harvest energy from human motions. A woven nanogenerator harvests energy from human movement;
a schematic diagram of the production process of this device is shown in Figure 10b. Figure 10c shows
that the nanogenerator harvests energy from human finger tapping, human arm movement and human
footsteps. Satyaranjan et al. [
141
] fabricated a nanogenerator based on flexible PVDF/SM-KNN, which
has a current density of 5.5 mA/cm
2
and a power density of 115.5 mw/cm
2
. They found that the output
voltage error of repeated detection under the same pressure is about 1%. Figure 10d shows the charge
separation phenomenon during the pressing process and the release pressure of the nanogenerator.
Researchers at the University of Texas at San Antonio have developed a self-charging pacemaker
based on PVDF piezoelectric films [
144
]. The kinetic energy of the heartbeat is converted into electrical
energy to charge the battery. The work is expected to be developed into products on the market within
five years.
4.2. Physical Sensors
In sensing applications, flexible and wearable pressure sensors are made of piezoelectric polymer
materials with flexible characteristics. High-voltage electrospinning can be used to fabricate PVDF
piezoelectric nanofiber films [
49
,
145
149
]. Gong et al. [
130
] combined the PVDF nanofiber membrane
by electrospinning with PDMS-Ag NWS and PET/ITO electrodes to fabricate sensors. When the tester
repeats the word “you”, the detected output signal is also periodic (Figure 11a) [
130
]. This type of
sensor has evident piezoelectric properties and performs well in quantitative pressure measurement.
In addition, because of their shape-preserving function, they can be used as real-time monitors of
people’s activities (Figure 11b). Lee et al. [
87
] proposed a highly sensitive gauge sensor based on PVDF
and ZnO nanostructures on graphene electrodes (Figure 11c) [
87
]. It can detect pressure changes with
the lowest value of 10 Pa, which is 1000 times lower than the minimum required for artificial skin.
The temperature value calculation is based on the signal recovery time (Figure 11d). The PVDF-TrFE
copolymer membrane pressure sensor is manufactured via a standard photolithography process that can
be used for batch processing, thereby reducing costs. The resulting membrane has good uniformity and
high polymer pattern resolution [
150
]. Alluri et al. [
61
] processed piezoelectric films made of PVDF and
BaTi(1
x)ZrxO
3
, which can eectively convert pressure signals into electrical signals, with a maximum
peak power of 15.8 nW. They confirmed superb reproducibility and durability of the PNGs by a stability
test using a cyclic pushing instrument. Sang et al. [
143
] manufactured a wearable piezoelectric energy
harvester by electrospinning PVDF-TrFE and BNT-ST(0.78Bi0.5Na0.5TiO
3
-0.22SrTiO
3
). The generated
output voltages, output currents and output powers by finger bending as well as knee and elbow
movements are shown in Figure 11e. The 3D printed PVDF with BaTiO
3
(BTO) filler [
151
] can also be
used for pressure sensing. Compared with the single PVDF composite of TNL [
152
154
], the sensitivity
was increased by 333.46% at 5 kPa under the eects of the new additives shape, good interaction
and well-distributed hybrid additives in the matrix. This confirmed that it was possible to fabricate
low-cost and lightweight electronic devices with reduced quantities of metal oxides and pressure
sensing devices.
Micromachines 2020,11, 1076 17 of 29
Micromachines 2020, 11, x 17 of 30
Figure 11. (a) Schematic of flexible pressure sensor used to detect pressure signal [130]; (b) device was
used as a motion sensor to monitor knuckle bends and vocal cord vibrates [130]; (c) schematic of
sensor based on PVDF and ZnO nanostructures on graphene electrodes [87]; (d) the relationship
between temperature and recovery time [87]; (e) generated output voltages, output currents and
output powers by finger bending as well as knee and elbow movements [143]; and (f) pressure sensor
embedded in the bottom of the insole or stuck on human arm and voltage generated by jumping,
walking, running and different bending angles’ elbows [116].
Ye et al. [116] fabricated flexible wearable pressure sensors that were sensitive to various human
behaviors based on piezoelectric materials. The pressure sensor was embedded in the bottom of the
insole or stuck on the human arm, and the voltage generated by jumping, walking, running and
elbows at different bending angles is shown in Figure 11f. According the fatigue testing of the sensor
for 1000 cycles at 12 N, the output voltages dropped by 6.7%, demonstrating the excellent stability
and durability of the sensor.
Commercially, many commercial products related to pressure are based on PVDF materials. For
example, based on PVDF, Dymedix Diagnostics, Inc (Minnesota, USA) developed airflow and snore
sensors [155]. Cambridge Touch Technologies (CCT) is a fast-growing high-tech enterprise
originating from the University of Cambridge. Its KF Piezo series PVDF is a well-known piezoelectric
film product in the industry. CCT has developed an UltraTouch Sensor with a simple structure, a low
cost and a high performance [156].
4.3. Chemical Sensor based on Piezoelectric Polymer
As a flexible piezoelectric material, the mechanical deformation state of PVDF electrically driven
is directly related to its mass load. Then, by modifying the sensor surface with adsorbent materials,
it can specifically adsorb gas or water vapor to form a mass sensor. For example, when the sensing
interface contacts the target gas, the physical shape and mass changes caused by the absorption of
gas will affect the resonance frequency and other physical parameters of the piezoelectric polymer-
sensitive film. Zengwei et al. [157] applied the phase separation method to deposit a PVDF
microporous membrane on the surface of a palladium-loaded tin dioxide (Pd-SnO2) gas-sensitive
film. The stability of the sensor under high humidity was evidently improved by the influence of the
PVDF microporous membrane. To detect the hydrogen concentration, a hydrogen sensor [158] can
be made by sticking the Pd film on both sides of the PVDF membrane. Because of the selective
Figure 11.
(
a
) Schematic of flexible pressure sensor used to detect pressure signal [
130
]; (
b
) device was
used as a motion sensor to monitor knuckle bends and vocal cord vibrates [
130
]; (
c
) schematic of sensor
based on PVDF and ZnO nanostructures on graphene electrodes [
87
]; (
d
) the relationship between
temperature and recovery time [
87
]; (
e
) generated output voltages, output currents and output powers
by finger bending as well as knee and elbow movements [
143
]; and (
f
) pressure sensor embedded in
the bottom of the insole or stuck on human arm and voltage generated by jumping, walking, running
and dierent bending angles’ elbows [116].
Ye et al. [
116
] fabricated flexible wearable pressure sensors that were sensitive to various human
behaviors based on piezoelectric materials. The pressure sensor was embedded in the bottom of the
insole or stuck on the human arm, and the voltage generated by jumping, walking, running and elbows
at dierent bending angles is shown in Figure 11f. According the fatigue testing of the sensor for
1000 cycles at 12 N, the output voltages dropped by 6.7%, demonstrating the excellent stability and
durability of the sensor.
Commercially, many commercial products related to pressure are based on PVDF materials.
For example, based on PVDF, Dymedix Diagnostics, Inc (Minnesota, USA) developed airflow and snore
sensors [
155
]. Cambridge Touch Technologies (CCT) is a fast-growing high-tech enterprise originating
from the University of Cambridge. Its KF Piezo series PVDF is a well-known piezoelectric film product
in the industry. CCT has developed an UltraTouch Sensor with a simple structure, a low cost and a
high performance [156].
4.3. Chemical Sensor based on Piezoelectric Polymer
As a flexible piezoelectric material, the mechanical deformation state of PVDF electrically driven is
directly related to its mass load. Then, by modifying the sensor surface with adsorbent materials, it can
specifically adsorb gas or water vapor to form a mass sensor. For example, when the sensing interface
contacts the target gas, the physical shape and mass changes caused by the absorption of gas will aect
the resonance frequency and other physical parameters of the piezoelectric polymer-sensitive film.
Zengwei et al. [
157
] applied the phase separation method to deposit a PVDF microporous membrane on
the surface of a palladium-loaded tin dioxide (Pd-SnO
2
) gas-sensitive film. The stability of the sensor
under high humidity was evidently improved by the influence of the PVDF microporous membrane.
To detect the hydrogen concentration, a hydrogen sensor [
158
] can be made by sticking the Pd film on
Micromachines 2020,11, 1076 18 of 29
both sides of the PVDF membrane. Because of the selective absorption of hydrogen by the palladium
membrane, expansion deformation occurs under the PVDF action, which is converted into voltage
signal output. The output voltage of sensors showed a similar change when the sensor was exposed to
repeated cycles of H2.
The introduction of humidity in sensitive materials can also play a role in humidity sensing.
The spin coating technique was used to prepare the PVDF TiO
2
nanocomposite films on interdigital
ITO electrode [
159
], and the surface morphology was modified by acetone etching. The volume
deformation of PVDF is caused by the hygroscopicity of nano-TiO
2
, resulting in a piezoelectric
eect (Figure 12a) [
159
]. Wang et al. [
160
] proposed a simple and low-cost method to prepare a
moisture-responsive composite membrane composed of PEDOT:PSS and PVDF by spin coating and
thermal evaporation. The bending angles of the composite film in both directions can reach
191
and
225
, respectively, showing good bidirectional bending performance. They also demonstrated that the
bending of the PVDF/PEDOT:PSS strip at dierent RHs is recyclable and repeatable. As shown in
Figure 12b–d, the PVDF film is glued together with the PEDOT:PSS film. PEDOT:PSS easily absorbs
water [
160
163
]. In dierent humidity environments, PEDOT has dierent bending degrees due to
dierent water absorption rates. PVDF films produce voltage signals under the corresponding bending
degrees, representing the specific humidity.
Micromachines 2020, 11, x 18 of 30
absorption of hydrogen by the palladium membrane, expansion deformation occurs under the PVDF
action, which is converted into voltage signal output. The output voltage of sensors showed a similar
change when the sensor was exposed to repeated cycles of H2.
The introduction of humidity in sensitive materials can also play a role in humidity sensing. The
spin coating technique was used to prepare the PVDF TiO2 nanocomposite films on interdigital ITO
electrode [159], and the surface morphology was modified by acetone etching. The volume
deformation of PVDF is caused by the hygroscopicity of nano-TiO2, resulting in a piezoelectric effect
(Figure 12a) [159]. Wang et al. [160] proposed a simple and low-cost method to prepare a moisture-
responsive composite membrane composed of PEDOT:PSS and PVDF by spin coating and thermal
evaporation. The bending angles of the composite film in both directions can reach 191° and 225°,
respectively, showing good bidirectional bending performance. They also demonstrated that the
bending of the PVDF/ PEDOT:PSS strip at different RHs is recyclable and repeatable. As shown in
Figure 12b–d, the PVDF film is glued together with the PEDOT:PSS film. PEDOT:PSS easily absorbs
water [160–163]. In different humidity environments, PEDOT has different bending degrees due to
different water absorption rates. PVDF films produce voltage signals under the corresponding
bending degrees, representing the specific humidity.
Figure 12. (a) Schematic of the spin coated PVDF-TiO2 on interdigital ITO electrode [159]; and (bd)
graphical representation of the PEDOT:PSS/PVDF generator producing a stable DC voltage [160].
4.4. Biosensor and Bionic Actuator Based on Piezoelectric Polymer
Piezoelectric polymers are used as biosensors mainly through biosensor surface modification.
The binding of biological target molecules on the sensing surface changes the mass of the sensing end
of the sensor, thus affecting the resonant output of the piezoelectric polymer. PVDF-TrFE films were
fabricated by electrodeposition, and piezoelectric polymer ultrasonic transceivers were developed,
which proved that they can be used for the ultra-sensitive detection of antibiotics. The diameter of
the entire ultrasonic transceiver was 680 μm. The ultrasonic transducers consist of a receiver and a
transmitter, which is patterned on a gold electrode and integrated with a microfluidic channel. The
biosensor was successfully applied to detect the animals’ doxycycline with a detection limit of 50 ppb
(Figure 13a–c) [164]. In the field of microfluidic channels, the developed on-chip ultrasonic transducer
performed excellently, with the potential to be used for ultra-sensitive detection of DNA, proteins
and food antibiotics. Lin et al. [165] fabricated microporous PVDF membranes with different surface
morphologies from coagulation baths of different strengths by immersion-precipitation. Using the
dual-step procedure, ss-DNA was covalently immobilized on the membranes. The antibody in serum
Figure 12.
(
a
) Schematic of the spin coated PVDF-TiO
2
on interdigital ITO electrode [
159
]; and (
b
d
)
graphical representation of the PEDOT:PSS/PVDF generator producing a stable DC voltage [160].
4.4. Biosensor and Bionic Actuator Based on Piezoelectric Polymer
Piezoelectric polymers are used as biosensors mainly through biosensor surface modification. The
binding of biological target molecules on the sensing surface changes the mass of the sensing end of the
sensor, thus aecting the resonant output of the piezoelectric polymer. PVDF-TrFE films were fabricated
by electrodeposition, and piezoelectric polymer ultrasonic transceivers were developed, which proved
that they can be used for the ultra-sensitive detection of antibiotics. The diameter of the entire ultrasonic
transceiver was 680
µ
m. The ultrasonic transducers consist of a receiver and a transmitter, which is
patterned on a gold electrode and integrated with a microfluidic channel. The biosensor was successfully
applied to detect the animals’ doxycycline with a detection limit of 50 ppb (Figure 13a–c) [
164
]. In
the field of microfluidic channels, the developed on-chip ultrasonic transducer performed excellently,
with the potential to be used for ultra-sensitive detection of DNA, proteins and food antibiotics.
Lin et al. [
165
] fabricated microporous PVDF membranes with dierent surface morphologies from
Micromachines 2020,11, 1076 19 of 29
coagulation baths of dierent strengths by immersion-precipitation. Using the dual-step procedure,
ss-DNA was covalently immobilized on the membranes. The antibody in serum can be eectively
adsorbed by the immobilized DNA, performing antibody detection. Similarly, nucleic acid sensors
based on PVDF [
166
] have been developed. In the experiment, they used the hybridization between
the capture probe and target analyte (Figure 13d) [
166
] and found that the mass load on the membrane
was proportional to the quantity of target nucleic acids (Figure 13e) [166].
Micromachines 2020, 11, x 19 of 30
can be effectively adsorbed by the immobilized DNA, performing antibody detection. Similarly,
nucleic acid sensors based on PVDF [166] have been developed. In the experiment, they used the
hybridization between the capture probe and target analyte (Figure 13d) [166] and found that the
mass load on the membrane was proportional to the quantity of target nucleic acids (Figure 13e) [166].
Figure 13. (a) Schematic of the chip [164]; (b) the experimental setup for characterizing the ultrasonic
transceivers [164]; (c) the plastic microfluidic chip with ultrasonic transceivers [164]; (d) schematic of
the biosensor using PVDF film as piezoelectric layer [166]; and (e) relationship between mass load
and frequency shift (Δf) [166].
In recent years, much attention has been paid to the research of artificial bionics devices. In 2019,
Wu et al. [167] reported a flexible robot based on a piezoelectric single chip bending structure (Figure
14a,b) [167]. Its relative speed is 20 times its body length per second, which is the fastest measurement
speed among the reported artificial robots on an insect scale. The soft robot uses the principle of
several kinds of animal movements, such as carrying load, the firmness of a cockroach and climbing
slopes. Even after bearing the weight of an adult’s footsteps (approximately 1 million times heavier
than the robot), the system can continue to move (Figure 14c) [167].
Piezoelectric polymers also have good performance in the manufacturing of artificial muscles.
Simaite et al. [168] prepared a hybrid membrane with hydrophilic PVDF grafted poly(ethylene
glycol) monomethyl ether methacrylate (PEGMA) outer surface and hydrophobic main body, which
is a model human muscle composition, as shown in Figure 14d. The results are expected to be used
in artificial muscles. Xiao et al. [169] fabricated a fish-like robot to imitate fish swimming. The fish-
like robot is composed of a body and a tail. The body is made of expandable polystyrene and the tail
is made of graphene-PVDF as an actuator. The graphene-PVDF film is connected with a flexible Au
wire. When the power is turned on, the fish’s tail is bent, and, when the power is turned off, the tail
returns to its original position. As the power is continuously turned on and off, the fish-shaped robot
can move forward at a speed of 5.02 mm/s (Figure 14e). Kim et al. [170] fabricated a new IPMC
actuator on the basis of blend membrane made by PVDF/polyvinyl pyrrolidone/polystyrene sulfuric
acid . The robot can support 2-, 4-, or 6-IPMC-leg models and this miniature walking robot has a size
of 18 mm 11 mm 12 mm, a weight of 1.3 g and a maximum speed of 0.58 mm/s.
Figure 13.
(
a
) Schematic of the chip [
164
]; (
b
) the experimental setup for characterizing the ultrasonic
transceivers [
164
]; (
c
) the plastic microfluidic chip with ultrasonic transceivers [
164
]; (
d
) schematic of
the biosensor using PVDF film as piezoelectric layer [
166
]; and (
e
) relationship between mass load and
frequency shift (f) [166].
Table 3. Dierent fabrication methods, fabricated material and sensitivity of various sensors.
Application Material Fabrication Methods Sensitivity
Energy Harvesting
Device
PVDF/BT [142] Electrospinning power density of
87 µW cm3
PVDF-TrFE/BNT-ST
[143]Electrospinning. 0.004 V/kPa
PVDF/SM-KNN [141] Electrospinning power density of 115.5
mW/cm2
Pressure sensor
PVDF/PDMS-Ag NWS
[130]Electrospinning 0.03 V/kPa
PVDF/ZNO [87] Spin-coating and poling Pressure changes with
the lowest value of 10 Pa
PVDF/BTO [116] Spin-coating and poling 0.056 V/N
Gas sensor PVDF/Pd [158]Polymer film is coated
with thin films of Pd on
both sides
0.3 V/1% hydrogen
concentration
Humidity sensor PVDF/TiO2[159]PVDF TiO2
nanocomposite films by
spin coating 0.02 PF/1% RH
PVDF/PEDOT: PSS [160]composite film by spin-
coating and thermal
evaporation 5/1% RH
Biosensor PVDF-TRFE [164]
electrodeposited
PVDF-TrFE on the
electrode, and modified
with chitin on the
biosensor surface
detection limit of 50 ppb
of Antibiotic
PVDF [166]
capture probe
immobilized on the
PVDF film of the
biosensor
0.2 kHz/µg
Micromachines 2020,11, 1076 20 of 29
In recent years, much attention has been paid to the research of artificial bionics devices. In 2019,
Wu et al. [
167
] reported a flexible robot based on a piezoelectric single chip bending structure
(Figure 14a,b) [
167
]. Its relative speed is 20 times its body length per second, which is the fastest
measurement speed among the reported artificial robots on an insect scale. The soft robot uses the
principle of several kinds of animal movements, such as carrying load, the firmness of a cockroach and
climbing slopes. Even after bearing the weight of an adult’s footsteps (approximately 1 million times
heavier than the robot), the system can continue to move (Figure 14c) [167].
Micromachines 2020, 11, x 20 of 30
Figure 14. (a) Optical photo of the fabricated robot and the SEM image of the cross-sectional view of
the prototype robot with different layers of materials [167]; (b) images of prototype robot the
movements in a driving cycled [167]; (c) after bearing the weight of an adult’s footsteps (about one
million times heavier than the robot), the system can continue to move [167] ; (d) the cross-sectional
view of the hydrophilic PVDF-graft-PEGMA [168]; and (e) a fish-like robot to imitate fish
swimming [169].
Table 3. Different fabrication methods, fabricated material and sensitivity of various sensors.
Application Material Fabrication Methods Sensitivity
Energy
Harvesting
Device
PVDF/BT [142] Electrospinning power density of
87 μW cm–3
PVDF-TrFE/BNT-ST
[143] Electrospinning. 0.004 V/kPa
PVDF/SM-KNN
[141] Electrospinning power density of 115.5
mW/cm2
Pressure sensor
PVDF/PDMS-Ag
NWS [130] Electrospinning 0.03 V/kPa
PVDF/ZNO [87] Spin-coating and poling
Pressure changes with
the lowest value of 10
Pa
PVDF/BTO [116] Spin-coating and poling 0.056 V/N
Gas sensor PVDF/Pd [158] Polymer film is coated with thin films
of Pd on both sides
0.3 V/1% hydrogen
concentration
Humidity
sensor
PVDF/TiO2 [159] PVDF TiO2 nanocomposite films by
spin coating 0.02 PF/1% RH
PVDF/PEDOT: PSS
[160]
composite film by spin- coating and
thermal evaporation 5°/1% RH
Biosensor
PVDF-TRFE [164]
electrodeposited PVDF-TrFE on the
electrode, and modified with chitin on
the biosensor surface
detection limit of 50
ppb of Antibiotic
PVDF [166] capture probe immobilized on the
PVDF film of the biosensor 0.2 kHz/μg
Figure 14.
(
a
) Optical photo of the fabricated robot and the SEM image of the cross-sectional view of the
prototype robot with dierent layers of materials [
167
]; (
b
) images of prototype robot the movements
in a driving cycled [
167
]; (
c
) after bearing the weight of an adult’s footsteps (about one million times
heavier than the robot), the system can continue to move [
167
]; (
d
) the cross-sectional view of the
hydrophilic PVDF-graft-PEGMA [168]; and (e) a fish-like robot to imitate fish swimming [169].
Piezoelectric polymers also have good performance in the manufacturing of artificial muscles.
Simaite et al. [
168
] prepared a hybrid membrane with hydrophilic PVDF grafted poly(ethylene glycol)
monomethyl ether methacrylate (PEGMA) outer surface and hydrophobic main body, which is a
model human muscle composition, as shown in Figure 14d. The results are expected to be used in
artificial muscles. Xiao et al. [
169
] fabricated a fish-like robot to imitate fish swimming. The fish-like
robot is composed of a body and a tail. The body is made of expandable polystyrene and the tail is
made of graphene-PVDF as an actuator. The graphene-PVDF film is connected with a flexible Au
wire. When the power is turned on, the fish’s tail is bent, and, when the power is turned o, the
tail returns to its original position. As the power is continuously turned on and o, the fish-shaped
robot can move forward at a speed of 5.02 mm/s (Figure 14e). Kim et al. [
170
] fabricated a new IPMC
actuator on the basis of blend membrane made by PVDF/polyvinyl pyrrolidone/polystyrene sulfuric
acid. The robot can support 2-, 4-, or 6-IPMC-leg models and this miniature walking robot has a size of
18 mm ×11 mm ×12 mm, a weight of 1.3 g and a maximum speed of 0.58 mm/s.
5. Conclusion and Perspectives
Although inorganic piezoelectric materials have the advantages of a large piezoelectric coecient
and high strength, they are fragile and unsuitable for applications with higher flexibility requirements.
Micromachines 2020,11, 1076 21 of 29
As a typical representative of flexible piezoelectric polymers, PVDF has irreplaceable advantages in the
development of piezoelectric materials. It has great flexibility and its piezoelectric properties can also
be improved by doping optimization or other processes. Various doping methods for PVDF polymer
materials have been introduced, and the eects of dierent piezoelectric materials, nanomaterials and
biomaterials are compared. Some preparation methods for improving the polarization properties of
piezoelectric polymers are also introduced in detail. Based on flexible piezoelectric polymer materials,
we can obtain flexible electromechanical devices for dierent purposes, such as energy harvesting
devices, chemical sensor for humidity, pressure, gas and biomolecules.
We can prepare actuators of dierent sizes and functions based on flexible piezoelectric polymer
materials. However, the piezoelectric coecient of flexible piezoelectric polymer materials in general
is still less than that of the inorganic piezoelectric material. In the follow-up work, the optimization
method of the piezoelectric polymer should continue to be studied and applied to wearable devices.
(1) Piezoelectric materials based on biological crystals are one of the materials with great potential.
The modification of the unique functional groups of biological materials can be used to improve the
vertical piezoelectric coecient of PVDF. (2) Aiming at shape retention, the method of preparing
flexible electromechanical devices with high mechanical voltage conversion eciency should be
further explored. (3) With the help of piezocatalysis of PVDF, it can be used for the pretreatment of
chemicals in the development of lab-on-a-chip microsystem. (4) In addition, the use of piezoelectric
polymer materials for wearable flexible devices requires a piezoelectric polymer exhibiting better
skin anity and more compatibility with other flexible materials to be suitable for the applications of
physiological sensors and implantable devices. Biomaterials and flexible conductive polymers can
be introduced to improve its biocompatibility, antifouling ability and the capability of integration
with hydrophilic surface. In summary, giving full play to the flexible characteristics of piezoelectric
polymers and developing wearable piezoelectric sensors and executable devices will have very broad
application prospects.
Author Contributions:
Conceptualization, S.G. and X.D.; investigation, S.G. and Q.X.; writing—original draft
preparation, S.G., Q.X. and X.D.; writing—review and editing, S.G., Q.X., X.D. and M.X.; supervision, Q.X.
and K.C.A.; and funding acquisition, Q.X. and X.D. All authors have read and agreed to the published version of
the manuscript.
Funding:
This research was funded by the National Natural Science Foundation of China, grant numbers
61674114, 91743110 and 21861132001; by National Key R&D Program of China, grant numbers 2017YFF0204604
and 2018YFE0118700; by Tianjin Applied Basic Research and Advanced Technology, grant number 17JCJQJC43600;
by the 111 Project, grant number B07014; and the Initial Scientific Research Fund of State Key Laboratory of
Precision Measuring Technology and Instruments, Tianjin University, grant number Pilq1902. The authors thank
the Foundation for Talent Scientists of Nanchang Institute for Micro-technology of Tianjin University.
Conflicts of Interest: The authors declare no conflict of interest.
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