Piezoelectric polymer composites for sensors and actuators

E. Carvalho1,2, L. Fernandes1,3, C. M. Costa1,2, S. Lanceros-Méndez3,4

1
Centro de Física, Universidade do Minho, 4710-057 Braga, Portugal
2
Institute of Science and Innovation for Bio-Sustainability (IB-S), University of Minho,
Portugal
3
BCMaterials, Basque Center for Materials, Applications and Nanostructures, UPV/EHU
Science Park, 48940 Leioa, Spain
4
IKERBASQUE, Basque Foundation for Science, 48013, Bilbao, Spain

Abstract:
As a result of the Internet of Things (IoT) and Industry 4.0 paradigms, based on increasing
interconnectivity, the development of advanced high-performance materials for sensor
and actuator applications are increasingly required. In particular, piezoelectric composites
are of large scientific and technological interest from fundamental and applied point of
views. Piezoelectric composites are applied in a wide range of applications as they
combine the excellent properties of polymers and ceramics. The definition and properties
of piezoelectric materials and composites are presented as well as the recent applications
in areas such as electronics, energy harvesting, environmental sensors and biomedical
applications. The outlook and future trends for piezoelectric composites are also
provided.

Keywords: piezoelectric; sensors; actuators; biomedical applications; dielectric constant;

composites; multifunctional materials; smart materials.

1
 1. Introduction
Currently, with the Internet of Things (IoT) and Industry 4.0 paradigms, increasingly
requiring smart and multifunctional materials with higher performance, piezoelectric
composites are gathering particular attention, as they can be applied in a wide range of
applications from sensors and actuators to biomedical applications, being processable by
conventional and additive manufacturing techniques [1].
Polymer composites result from the combination of a polymeric matrix and different
fillers (one or two different fillers with complementary properties), gathering the
advantages of the polymeric matrix (low density and flexibility) and the fillers
(mechanical and thermal properties, or increased functional response) [2].
In relation to fillers, they can be conductive [3], magnetic [4] and ceramic [5], ceramic
fillers having as main advantages the possibility of being piezoelectric with high
piezoelectric coefficients, low dielectric and mechanical losses, and wide variety of
dielectric constants [6]. It is important to notice that the manufacturing method, the
particle size and the dispersion method play an essential role in the final properties of
piezoelectric polymer composites [7].
In addition, piezoelectric polymer composites can be particulate [8] and/or laminate [9]
composites and the dispersion of each component is defined by the connectivity, that
designates the interconnection of the different phases of the composite materials[10].
The connectivity influences the final structure of the piezoelectric polymer composite,
which in turn influences the macroscopic response and, therefore, the application
possibilities [11].
One of the most recent trends in piezoelectric composites is the production of these
materials with two different fillers, such as ceramic and conductive fillers [12], ceramic
and magnetic fillers [13] and combinations thereof, such as core-shell fillers [14], in order
to improve performance or to provide multifunctionality for applications in areas such as
dielectric-based capacitors, batteries, electronic devices and microwave absorption
devices.
In the following, the main definitions and properties in relation to polymer composites
will be presented, as well as recent advances divided by application. In addition, the main
materials for polymers and fillers will be presented.

2
 2. Piezoelectric Sensors and actuators: definition and properties

The name Piezoelectricity, was proposed by Hankel in 1881, and signifies “electricity by
pressure” being derived from the Greek word piezo which means pressure [15]. However,
the concept of piezoelectricity was discovered a year before, in 1880, by the Curie
brothers where it was found that mechanical stresses induced macroscopic polarization,
i.e. the generation of electric surface charges, in several crystals such as zincblende, topaz
and quartz [16]. The converse piezoelectric effect was predicted only a year later by
Lippmann, derived from the thermodynamic theory, where an external electric potential
is capable of producing mechanical deformations/strains to the materials [17]. With these
initial discoveries, a large interest was paid in this class of materials due to their
applicability in areas ranging from sonars to microphones, accelerometers and pressure
transducers, among others [18].
Another breakthrough was achieved in 1969 by Kawai with the discovery of a strong
piezoelectric effect in poly(vinylidene fluoride) (PVDF) adding mechanically flexible
materials to the list of piezoelectric materials [19]. The discovery of these flexible
piezoelectric materials extended the range of applications to flexible electronics, large
area sensors, flexible energy harvesters and biomedicine.
By definition, piezoelectric materials, a class of dielectric materials, are a family of both
inorganic and organic materials upon which polarization can be varied by the application
of a mechanical stress, or vice versa, as represented in Figure 1. They can be divided into
two classes, namely polar and non-polar piezoelectric materials, depending on the
existence of a net dipole moment or a null total dipole moment respectively.

3
Figure 1 – Schematic representation of the sources of the piezoelectric phenomena.
Adapted from [20].

In Figure 2 it is shown a simplified molecular model explaining the behavior of a

piezoelectric material. Prior to exerting an external stress to the material, in each
molecule, the centers of the positive and negative charges coincide (Figure 2a)), i.e. the
external effects of the charges are equally null, and the molecule is electrically neutral.
Upon application of an external mechanical stress to the material, deformation of the
internal structure of the molecule occurs and, as a result of the separation of the positive
and negative gravity centers, dipoles are generated (Figure 2b)). The opposite facing poles
in the material are mutually canceled and fixed charges appear on its surface (Figure 2c))
[21]. This is known as the direct piezoelectric effect, schematized in Figure 1 [20].
A year after the discovery of the piezoelectric effect, in 1881, the Curie brothers proved
the theory developed by Lippmann showing that piezoelectric materials may also have
the reverse behavior, i.e. when an electric potential is applied across the electrodes, a
mechanical deformation/strain occurs [17]. For this case it is said that the materials have
a reverse piezoelectric effect [21]. Materials that present this behavior can be
implemented in several applications as actuators or positioning devices [20].

4
Figure 2 – Schematic representation of the piezoelectric effect: a) electrically polarized
unperturbed molecule, b) application of an external force (Fk) and induced polarization
(Pk) and c) polarizing effect on the surface of the piezoelectric material [20].

A schematic representation of the behavior of piezoelectric materials containing two

metal electrodes deposited on the surfaces where the opposite surface charges are formed
is represented in Figure 3. If the electrodes are short circuited with a galvanometer
connected to the wire, when pressure is applied to the piezoelectric material a charge
density appears on the surfaces of the crystal in contact with the electrodes. The free
charges will move until they neutralize the polarization effects as presented in Figure 3a).
Once this external force is removed, the polarization disappears with the flow of the free
charges reversing and the material returning to its original state (Figure 3b)) [21]. On the
other hand, by replacing the short-circuiting wire with a resistance, the current would be
capable of flowing through, thus converting mechanical energy into electrical energy,
which is the basis of energy harvesting applications [22].

5
Figure 3 – Representation of the piezoelectric phenomena: a) neutralizing current flow
of a piezoelectric material with two short circuited terminal subjected to an external force
and b) material in its original state with the absence of current in the short-circuit [20].

Being anisotropic by nature, piezoelectric materials mechanical, electrical and

electrochemical properties vary depending on the direction of the mechanical and/or
electrical stimuli and thus the implementation of such materials in the sensing and
actuating areas require an in-depth evaluation of the magnitude of the various properties
in the different directions [23].
In most of the cases, the material must undergo a poling process in order to properly orient
the dipoles and, thus, maximize the piezoelectric response. This poling is typically
achieved by the application of and electrical field along a specific direction. Posteriorly,
a smaller electric potential can be applied to the materials inducing changes in its
dimensions. If this electric potential has the same direction as the poling field, additional
expansion on the poling direction and contraction perpendicularly occurs [20].

6
Figure 4 – Behavior of a piezoelectric material as sensor and actuator. (a) Typical P-E
hysteresis and S-E plots. (b) The piezoelectric material before (dotted) and after poling,
the polarity of poling field is indicated. (c) Mechanical deformation when the applied
electric potential has polarity similar to the poling field and (d) when the applied electric
potential has opposite polarity to the poling field. (e) Generated electric potential with
polarity similar to poling field when compressive force is applied in the same direction
and (f) with polarity opposite to poling field when tensile force is applied in poling
direction [20].

In Figure 4a are represented the typical polarization vs electric field (P-E) hysteresis and
strain vs electric field (S-E) plots of a piezoelectric material. With the application of an
electric field across a piezoelectric material, both the polarization and the strain curves,
in the P-E and S-E plots respectively, follow the path (i) to (ii). Once this field is removed,
a remnant polarization (Pr) and a permanent change in the dimensions is experience by
the material, shown in the curves as the path (ii) to (iii), and a working point shift from
the material occurs the plots in Figure 4a). After this, two situations can occur. First, if an
electric potential with the same polarity as the poling field is applied, the plots if Figure
4a) follow the path (iii) to (ii). Meaning that the piezoelectric material will experience
and expansion along the poling axis (Figure 4c)). On the other hand, if the applied electric
potential has the opposite polarity as the poling field, the P-E and S-E plots will follow
the path (iii) to (iv) and the material experiences a contraction along the poling field axis
and expansion perpendicularly to it (Figure 4d)). For both situations, the piezoelectric

7
material, when the electric potential is removed, returns to the poling dimensions (iii) in
the plots [20].

Similarly, for the reverse piezoelectric effect, an electric potential is generated when a
tensile force is applied (Figure 4e) and f)). In Figure 4e) it is shown that if a compressive
force is applied along or tensile force perpendicular to the poling axis, the generated
electric potential will have the same polarity as the poling axis. Nevertheless, if a
compressive force is applied perpendicularly or tensile force is applied parallel to the
poling axis, the generated electric potential will have and opposite polarity to the poling
axis (Figure 4f)) [20].

From a mathematical point of view, when low electric fields and/or low mechanical stress
are applied, piezoelectric materials show a linear response [24]. When stress is applied to
a piezoelectric material, there will be a variation of the electrical polarization and, as a
consequence, electric charge will be produced on the materials surface. Thus,

𝑃𝑝𝑒 = 𝑑. 𝑇 (1)
where 𝑃𝑝𝑒 is the piezoelectric polarization vector, 𝑑 is the piezoelectric strain coefficient
and 𝑇 is the stress subjected to the material. Similarly, the reverse piezoelectric effect can
be expressed by means of

𝑆𝑝𝑒 = 𝑑. 𝐸 (2)
with 𝑆𝑝𝑒 being the produced mechanical strain and E the magnitude of the applied electric
field. Taking into account the piezoelectric materials elastic properties, the piezoelectric
effect can be formulated as

𝑃𝑝𝑒 = 𝑑. 𝑇 = 𝑑. 𝑠. 𝑇 = 𝑒. 𝑆 (3)
𝑇𝑝𝑒 = 𝑐. 𝑆𝑝𝑒 = 𝑑. 𝑐. 𝐸 = 𝑒. 𝐸 (4)
with 𝑐 being the elastic constant (which relates the generated stress and applied strain as
𝑇 = 𝑐 × 𝑆), 𝑠 being the compliance coefficient (relating the produced deformation with
the applied stress as 𝑆 = 𝑠 × 𝑇) and 𝑒 being the piezoelectric stress constant.

When the piezoelectric material is subjected to a strain, this has two implications. First,
an electric polarization variation is generated and, for the other, an elastic stress 𝑇𝑒 occurs.
Additionally, the generated electrical polarization variation leads to an internal electric
field 𝐸𝑝𝑒 variation which can be written as

8
 𝑃𝑝𝑒 𝑒. 𝑆
𝐸𝑝𝑒 = = (5)
𝜀 𝜀
with 𝜀 being the materials dielectric constant. The application of a compressive stress in
the same direction of the polarization direction will induce an electric field variation with
the same polarity. Moreover, the presence of an electric field in polarization direction
results in an expansion of the piezoelectric material in the same direction (Figure 4d)).
This means that the directions of the produced and applied stresses are opposite, which is
equivalent if the nature of the applied stress is tensile. This means that the produced stress
𝑇𝑝𝑒 is opposite to the piezoelectric material’s deformation and, by consequence the stress
generated can be written as

𝑒2 𝑒2
𝑇 = 𝑇𝑒 + 𝑇𝑝𝑒 = 𝑐. 𝑆 + . 𝑆 = (𝑐 + ) . 𝑆 = 𝑐̅. 𝑆 (6)
𝜀 𝜀
with 𝑐̅ being the piezoelectric stiffened constant. Hence, in the presence of the
piezoelectric effect the material becomes more rigid.

In a similar way, the materials dielectric response is also affected by the piezoelectric
effect [21]. Considering that the material, with dielectric constant 𝜀, is placed between
two electrodes and an external electric field is applied, a surface charge density 𝜎 will be
generated due to the displacement of the electric charges towards the electrodes, with a
magnitude 𝐷 = 𝜀. 𝐸. In the case that the material is piezoelectric, the external electric
field will also produce a strain, represented in eq.(2). The produced strain can be positive
or negative considering the direction of the external electric field with respect to the
polarization direction. As mentioned before, an external electric field with the same
direction of the polarization direction generates a positive strain, meaning the material
expands in this direction. This expansion results in an electric potential with opposite
polarity to the polarization direction, meaning that the surface charge density increases
and the polarization increases as well. Consequently, by maintaining constant the electric
field, the additional polarization increases the displacement of the free charges by a
magnitude of 𝜎𝑝𝑒 = 𝑃𝑝𝑒 and the total electric displacement can be written as

𝐷 = 𝜀. 𝐸 + 𝑃𝑝𝑒 = 𝜀. 𝐸 + 𝑒. 𝑑. 𝐸 = 𝜀̅. 𝐸 (7)

with 𝜀̅ being the effective dielectric constant.

9
The piezoelectric effect is a coupling between the elastic variables, T and S, and the
dielectric ones, D and E [23]. The linear tensor relations between these variables can be
given as

𝐸
𝑆𝑝 = 𝑠𝑝𝑞 𝑇𝑞 + 𝑑𝑝𝑘 𝐸𝑘 (8)
𝑇
𝐷𝑖 = 𝑑𝑖𝑞 𝑇𝑞 + 𝜀𝑖𝑘 𝐸𝑘 (9)
𝐸 𝑇
With 𝑠𝑝𝑞 being the elastic compliance tensor at a constant electric field, 𝜀𝑖𝑘 being the
dielectric constant tensor at a constant stress, 𝑑𝑘𝑝 being the piezoelectric constant tensor,
𝑆𝑝 the mechanical strain in 𝑝 direction, 𝐷𝑖 the electric displacement in the 𝑖 direction, 𝑇𝑞
the mechanical stress in the 𝑞 direction and 𝐸𝑘 the electric field in the k direction. In the
cases of semicrystalline and amorphous polymers and polymer composites [25], the
directions are commonly labelled as shown in Figure 5.

Figure 5 – Tensor directions for mechanical and elastic relations in semicrystalline and
amorphous piezoelectric polymer and polymer composites.

For the specific case of PVDF, the most investigated and used piezoelectric polymer, the
axis 1 corresponds to the draw or stretch direction, axis 2 corresponds to the transverse
direction and axis 3 to the thickness or polarization axis [26]. Using this relation in eqs.
8 and 9 they can be given by

10
 𝐸 𝐸 𝐸 𝐸 𝐸 𝐸
𝑆1 𝑠11 𝑠12 𝑠13 𝑠14 𝑠15 𝑠16 𝑇1 𝑑11 𝑑12 𝑑13
𝐸 𝐸 𝐸 𝐸 𝐸 𝐸
𝑆2 𝑠21 𝑠22 𝑠23 𝑠24 𝑠25 𝑠26 𝑇2 𝑑21 𝑑22 𝑑23
𝐸 𝐸 𝐸 𝐸 𝐸 𝐸 𝐸1
𝑆3 𝑠31 𝑠32 𝑠33 𝑠34 𝑠35 𝑠36 𝑇3 𝑑 𝑑32 𝑑33
= + 31 [𝐸2 ] (10)
𝑆4 𝐸
𝑠41 𝐸
𝑠42 𝐸
𝑠43 𝑠44 𝑠45 𝑠46 𝑇4
𝐸 𝐸 𝐸 𝑑41 𝑑42 𝑑43
𝐸3
𝑆5 𝐸
𝑠51 𝐸
𝑠52 𝐸
𝑠53 𝐸
𝑠54 𝐸
𝑠55 𝐸
𝑠56 𝑇5 𝑑51 𝑑52 𝑑53
[𝑆6 ] 𝐸 𝐸 𝐸 𝐸 𝐸 𝐸 [ 𝑇 ] [𝑑61 𝑑62 𝑑63 ]
[𝑠61 𝑠62 𝑠63 𝑠64 𝑠65 𝑠66 ] 6
𝑇1
𝑇2 𝑇 𝑇 𝑇
𝐷1 𝑑11 𝑑12 𝑑13 𝑑14 𝑑15 𝑑16 𝜀11 𝜀12 𝜀13 𝐸1
𝑇3 𝑇 𝑇 𝑇
[𝐷2 ] = [𝑑21 𝑑22 𝑑23 𝑑24 𝑑25 𝑑26 ] + [𝜀21 𝜀22 𝜀23 ] [𝐸2 ] (11)
𝑇4
𝐷3 𝑑31 𝑑32 𝑑33 𝑑34 𝑑35 𝑑36 𝑇
𝜀31 𝑇
𝜀32 𝑇
𝜀33 𝐸3
𝑇5
[𝑇6 ]
According to eqs. 8 and 9, there are18 possibilities to couple the electrical and mechanical
components of a piezoelectric material with each possibility belonging to one of four
possible operating modes. The four main operating modes are known as longitudinal (L),
longitudinal shear (SL), transverse (T) and transverse shear (ST), as shown in Figure 6.

Figure 6 – Schematic representation of four operating modes of the piezoelectric

effect. ∆𝑃 is the macroscopic change of the electric polarization [23].

As indicated before and considering the direct piezoelectric effect, a mechanical stress
will lead to an electric flux density and by consequence a macroscopic change in the
material’s polarization in a particular direction will occur [23]. These operating modes
and equivalent changes in polarization are described in Table1.

11
 Table 1 – Direct piezoelectric operating modes and brief description.

Operating mode Description

Longitudinal mode, 𝐿 (𝑑11 , 𝑑22 and 𝑑33 ) Application of a normal stress
accompanied by a change in the same
direction of the electric polarization
Transverse mode, 𝑇 (𝑑12 , 𝑑13 , 𝑑21 , 𝑑23 , Change in electric polarization
𝑑31 and 𝑑32 ) perpendicular to the mechanical load
Longitudinal shear mode, 𝑆𝐿 (𝑑14 , 𝑑35 and Application of a shear stress accompanied
𝑑36 ) by an electric polarization change
perpendicular to the plane of the shearing
stress.
Transverse shear mode, 𝑆𝑇 (𝑑15 , 𝑑16 , 𝑑24 , Change in electric polarization in the
𝑑26 , 𝑑34 and 𝑑35 ) plane of the shearing stress

Finally, the electromechanical coupling factor is an essential parameter, quantifying the

capacity of a material to convert mechanical into electrical energy and vice-versa [23]
and can be expressed as

Converted electrical energy Converted mechanical energy

𝑘2 = = (12)
Input mechanical energy Input electrical energy

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 3. Piezoelectric composites: definition and types
Piezoelectric composites belong to the class of smart materials and typically consist of a
piezoelectric ceramic filler incorporated in a piezoelectric polymer matrix. Other fillers,
such as conductive and magnetic fillers, are also added to these composites, the former
just for small amounts of filler as it is shown in figure 7. Together with those particulate
composites, laminated composites, in which the materials are prepared in a layered
assembly, are also often presented in the literature and implemented into applications.
Figure 7 shows a schematic representation of a piezoelectric composite with fillers
dispersed within the polymer matrix.

Polymer
matrix

Piezelectric
polymer
composites
One or
more
differents
fillers

Figure 7 – Schematic representation of a particulate piezoelectric composites.

Polymer composite materials are characterized by low density, flexibility, excellent

mechanical properties and thermal stability and higher dielectric and piezoelectric
coefficient values than the pristine polymers, which result from the combination of the
properties of the polymer matrix and ceramic filler [11, 23].

3.1. Polymer matrix

Several piezoelectric polymer matrices are reported in the literature, as shown in Table 2,
together with the dielectric value and a relevant piezoelectric coefficient, , and are divided
into amorphous and semicrystalline polymers [27].
In the case of semicrystalline polymers, the piezoelectricity is related with crystalline
phase in which the dipolar moments can be oriented by the application of electric field
[28].

13
 Poly (vinylidene fluoride) (PVDF) is a semi-crystalline polymer that stands out in
comparison to other semi-crystalline piezoelectric polymers, such as nylon-9, polyureas,
poly-L-lactic acid (PLLA), poly (b-hydroxybutyrate) (PHB) , among others, due to the
high value of the piezoelectric coefficient (d33 ~ -30 pC / N) [21, 27].
PVDF can crystallize in at least four polymorphs known as α, β, δ and γ-phases, but the
crystalline phase with best ferroelectric and piezoelectric properties is the β-phase.
Commonly, β-phase films are obtained by stretching α-phase films at temperatures
between 70 ºC and 100 ºC and for stretch ratios from 2 until 5 and also through the
addition of various fillers (magnetic: CoFe2O4, ceramic: BaTiO3, ionic
liquids:[EMIM][TFSI]) [29-33]. PVDF electroactive phase content and degree of
crystallininty are influenced by the processing conditions, inclusing stretching ratio and
temperature, as well as filler type and content [29, 31], which in turn will affect the
electroactive properties of the polymer.

Table 2 – Dielectric constant and piezoelectric coefficient for representative

semicrystalline and amorphous polymers, respectively.

Polymer Dielectric constant Piezoelectric coefficient at room temperature Ref

PVDF 8 - 12 d31 = 16 pC/N [28]
d32 = 3 pC/N
d33 = -20 - -23 pC/N
Nylon-9 ~3.5 d31 = 1.1 pC/N [34]
Polyureas 2-4 d31 = 10 pC/N [35]
PLLA d14 = -10 pC/N [36]
2.8 - 3.5
d31 = 1.58 pC/N
PHB 2-3 d14 = 1.3 pC/N [37]
Polyimide 4 d33 = 0.091 – 0.168 pC/N [38]
PVDC 3.4 d31 = 0.5 – 1.3 pC/N [39]

As in the case of amorphous polymer there are no crystalline phases, polarization results
in an almost stable state due to the freezing of molecular dipoles. The amorphous
polymers most reported in the literature are polyimide [40], polyvinylidene chloride
(PVDC) [41], poly(arylene ether nitrile (PAEN) [42], among others.

14
 3.2. Ceramic and other filler types

There are several piezoelectric ceramic materials with high piezoelectric coefficients as
shown in table 3, which also are mechanically strong, chemically inert and also show a
high dielectric constant [43]. The most used ceramic materials in polymer composites
include lead zirconate titanate (PZT), barium titanate (BaTiO3), zinc oxide (ZnO), and
lead-free as potassium niobate (KNN, K0.5Na0.5NbO3), among others [44].
Lead zirconate titanate (PZT) is a ceramic material with chemical formula Pb(ZrxTi1-x)O3
and perovskite crystalline structure [45]. The phase diagram is complex, but one of the
most interesting issues is the existence of the so called morphotropic phase boundary
(MPB) dividing the ferroelectric region in two parts: rhombohedral crystalline phase
region, rich in Zr atoms and a tetragonal crystalline phase region rich in Ti atoms. At
room temperature, the MPB is placed in the region Zr/Ti = 52/48 [46, 47]. At the MPB
the dielectric and piezoelectric response of the ceramic material is the largest.
Considering its high dielectric constant, BaTiO3 is a very used ceramic material in
piezoelectric polymer composites, also crystallizing in a perovskite structure [48].

Table 3 – Dielectric constant and piezoelectric coefficient for some representative

ceramic materials.

Filler Dielectric constant Piezoelectric coefficient Ref

PZT 200 - 5000 d33 = 100 - 1000 pC/N [28]
d31 = -78 pC/N
BaTiO3 1260 - 1700 [49]
d33 = 190 pC/N
ZnO 11 d31 = -5.0 pC/N [50]
d33 = 5.9 pC/N
KNN 250 d33 ~ 63 pC/N [51]

In addition to improved dielectric and piezoelectric properties, polymer composites with

ZnO fillers show additional properties such as photocatalytic behavior [52].
Also, niobium-based piezoelectric ceramics are very interesting due to being lead-free,
showing high dielectric and piezoelectric values, crystallizing also as perovskite
structures [53].

15
Generally, the combination of ceramic fillers with piezoelectric polymers leads to
composite materials with improved thermal and electrical properties without losing the
excellent mechanical properties of the polymeric matrix.
Depending on the size of the ceramic particles, it is possible to produce micro or
nanocomposites to develop piezoelectric composite materials with the desired properties
for applications. Finally, the processing conditions affect their morphology, physical
properties, as well as the macro and microscopic response of these materials, printing
technologies allowing to produce large-area composite materials at low processing cost
[54].
Other interesting composites are magnetoelectric ones [55], which are being investigated
for sensors, data memories, energy collectors, antennas or biomedical applications both
as particulate or layered composites. Magnetoelectic materials result from the addition of
magnetostrictive fillers, such as Zn0.2Mn0.8Fe2O4 (ZMFO) or CoFe2O4 (CFO), in a
piezoelectric polymer matrix, such as poly (vinylindene fluoride trifluoroethylene)
(P(VDF-TrFE) [56] in order to obtain magnetic and magnetoelectric response due to the
coupling of the magnetostrictive and piezoelectric phases, allowing manipulation of the
electrical polarization by a magnetic field or the magnetization by a field electric [57].

16
 4. Applications
Piezoelectric composites are optimized for specific applications ranging from
mechanical structures to electronic devices, for areas including automotive and aerospace
to structural health monitoring and biomedicine. Also, prominent applications of these
materials are already found in high energy storage capacitors [2, 58].

Proper selection of piezoelectric material is an essential parameter, as it directly

influences the device's functionality and performance. The piezoelectric response is
generally higher in piezoceramics and therefore make them the most commonly used
material. However, piezoceramics are naturally brittle, limiting the strain that it can
provide or absorb without being damaged. These materials are susceptible to the growth
of fatigue cracks when subjected to high frequency cyclical loads. One the other hand,
there are piezoelectric polymers which are flexible, acoustically well matched to water
and able to be produced in large areas and in a variety of shapes. Nevertheless, the low
electromechanical coupling and lower dielectric constant, limit their applications. In this
way, composite appears as a way to improve properties when compared to single phase
materials, being also able to produce high piezoelectric output performance [59, 60].

4.1.Electronic applications

Most periods of technological development have been distinguished by specific

materials, such as stone, bronze and iron age. Currently, one of the driving forces of
technology is the use of multifunctional materials, piezoelectric materials being a
paradigmatic example of this demand, in particular in development of electronic devices
[2, 61].
In automotive and aerospace applications there is an increasing need for sensors and
actuators. Composites based in lead zirconate titanate and PZT embedded in PVDF have
been investigated as sensors in application like benders, tire pressure and knock sensor
[62, 63]. Tire pressure sensors based on PZT-PVDF composites are directly bonded to
the inner tire [64].
Protection of spacecraft form radiation used in submarines, seismic and geological
research are areas in which piezoelectric materials have been implemented [65-68],
mainly in acoustic applications. Piezoelectric microphones based on PVDF have been
developed to detect sound inside the cochlea, allowing cochlear implants with normally

17
occurring sound pressures and frequencies (ear canal pressures >50–60 dB SPL and 0.1–
10 kHz) as shown in figure 8 [69]. Piezoelectric Parylene-C (ortho-chloro-p-xylene)
polymer has been also used in microphones and actuators applications [70].

Figure 8 - (a) Schematic representation of fiber optic and PVDF pressure sensor inserted
into the round window of a gerbil cochlea. (b) Plot of output voltage (after a gain of 1,000)
measured with PVDF sensor (red square) and pressure in the scale tympani measured
with fiber optic pressure sensor (black circle). (c) Plot of phase measured with fiber optic
pressure sensor (black circle), PVDF sensor premortem and before disarticulation (red
squares), postmortem (blue triangles), and after disarticulation (greed diamonds). (d) Plot
of output voltage (after a gain of 1,000) measured with PVDF sensor premortem or before
disarticulation (red squares), postmortem (blue triangles), and after disarticulation (green
diamonds) [69].

Currently, with the emergence of mobiles and smart gadgets, as well as wearable
electronics and soft robotics, electronic technology has directed new efforts in the
development of devices and materials compatible with high touch sensibility and flexible
substrates design requirements[71-73]. PVDF and reduced graphene oxide have been
described as capable of producing a flexible film with microstructures to mimic the

18
epidermal and dermal layer of human fingerprint. This sensor can identify and distinguish
between multiple spatiotemporal tactile stimuli including static and dynamic pressure,
temperature and vibration with high sensitivities. Thus, in artery vessels, it is possible a
precision detection of acoustic sounds and the evaluation of different surface textures can
be performed. Wearable devices are an important milestone not only in electronics but
also in the field of biomedicine [74]. For the evaluation of structural dynamic strains,
flexible nanocomposite sensors using carbon black (CB) fillers and polyvinylidene
fluoride (PVDF) matrix was fabricated, the nanocomposite allowing to detect extremely
weak strains associated with sources such as structural damage, high-frequency vibration,
and ultrasonic waves. This nanocomposite is adequate for strain sensor applications such
as advanced bioelectronics, ultrasonic inspection, and in-situ structural health monitoring
[75].
A DC current sensor device based on a laminated PVDF / Metglas magnetoelectric
composite was developed with the ME coefficient (α33) of 34.48 V.cm-1Oe-1, a linear
response (R2 = 0.998) with a sensitivity of 6.7 mV.A-1 [76].

4.2. Energy harvesting

Energy harvesting consists in the process of acquiring the surrounding energy of a

system and translating it into usable electrical energy.
Piezoelectric transduction is an approach that has received attention in the area of
elecromechanical energy harvesting, i.e. the generation of electrical energy from
mechanical vibrations [77, 78]. ZnO nanowires [79], lead zirconate titanite (PZT)
nanofibers [80, 81], barium titanate (BaTiO3) [80, 82, 83] and PVDF [84] are examples
of piezoelectric materials that have been used to construct nanogenerators and to
effectively power small electronic devices, such as lighting up LEDs [85]. In this context,
FAPbBr3 nanoparticles (cubic perovskite structure) uniformly mixed with
Polydimethylsiloxane (PDMS) and then spin‐coated onto an indium tin oxide (ITO)‐
coated polyethylene terephthalate (PET) substrate and integrated with aluminum (Al),
demonstrate a high performance as energy harvesting devices. The FAPbBr3‐PDMS
composite generates electric potential under an external stress with output voltage and
current density of 8.5 V and 3.8 μA.cm−2, respectively, the nanoparticles serving as the
energy generation sources as shown in figure 9. The generated energy can be used to
charge a capacitor and light up a LED through a bridge rectifier [86].

19
Figure 9 - a) COMSOL simulation model of a nanogenerator. The simulated piezoelectric
potential distribution inside the composite between top and bottom electrodes is indicated
by color code. b) Output voltage from nanogenerators with different FAPbBr3
nanoparticles concentration. c) Variation of the output voltage with different FAPbBr3
nanoparticles concentrations. d) COMSOL simulation result of output potential
distribution of the nanogenerator with different FAPbBr3 nanoparticles concentration
[86].

A primary motivation for self-charging structures is to use them for powering small
electronic components. Piezoelectric shoes, electronic skin and other energy harvesting
devices have been developed to take advantage of the produced vibration from human
body activities such as: walking, running, breathing, and dancing to power‐up low power
electronic devices [87].
The possibility of harnessing the energy lost from a biological activity to provide
energy for low-powered electronic devices has been also explored. Cardiac and lung
motions serve as inexhaustible sources of energy during the lifespan. One of the first
highlights in this area [88] is related with the use of an implantable physiological power
supply using PVDF films. The prototype, that used the energy expended for breathing,
was implanted in vivo on a mongrel dog and demonstrated a peak voltage of 18 V, which
corresponds to a power of about 17 mW. Since then, there are several proposals for such

20
devices that translate heartbeat vibrations into electrical energy using piezoelectric
composites also for directly powering a cardiac pacemaker by harvesting the kinetic
energy of heartbeat [89]. Briefly, Pb(Mg1/3Nb2/3)O3-(28%)-PbTiO3 (PMN-PT) was
employed as piezoelectric layer and each side was sputtered with Cr/Au. A
berylliumbronze foil was used to provide uniform stress distribution to the piezoelectric
layer and then a PDMS film was deployed by spin-coating. To further improve the
stability of the device and avoid potential erosion in the in vivo environment, a parylene
film was deposited onto the PDMS film to form a compact and holefree coating layer. In
vivo, a commercial cardiac pacemaker was directly powered by the implantable
piezoelectric energy generator and monitored its behavior. It was concluded that patients
do not need surgical replacement, or at least, the battery replacement will be less frequent.
Currently, one of the most important application of energy harvesters is powering
implantable biomedical devices [90].

4.3.Environmental sensors

Industrialization is causing serious problems in the environment. The accumulation

of contaminants in air, soil and water is a threat that instigates the science attention. A
very important environmental challenge is water remediation [77], knowing this, physical
adsorption, biological methods and chemical oxidation have been applied worldwide [91,
92].
Photocatalytic oxidation has been explored for environmental treatment and
purification and coupled with this, piezocatalyses arises. The electric field generated by
piezoelectric materials separates free electrons-holes pairs, which further react with
dissolved oxygen molecules and water to decompose organic pollutants [93]. ZnO, MoS2,
Pb(Zr0.52Ti0.48)O3, NaNbO3, and BaTiO3 have been extensively investigated. It has been
demonstrated the applicability of MoS2/PDMS nanocomposite as piezocatalyst. The
MoS2/PDMS film was used as negative layer, while a copper thin layer acted as a positive
electrode, the triboelectric nanogenerator was posteriorly fabricated for energy harvesting
by hydropower. The piezoelectric part exhibited catalytically active surface on the active
edge sites forming free radical oxygen to decompose pollutants. Besides the composite
acting as piezocatalyst and energy harvester, it could be utilized as an active sensor for
the monitoring of flowing water and its contamination [94].

21
 Atmospheric monitoring with specific sensors for organic and inorganic pollutants,
potentially toxic elements, and pathogens contributes to the sustainable development of
society. Traditional analytical approaches for pollutants monitoring include various
chromatographic techniques. However, the response times and high mass sensitivity of a
piezoelectric resonator leads to the application of chemical sensors for detecting
components (ions, molecules, their fragments or clusters) [95].

A different type of environmental monitoring, although an application with added

value, is the use of PVDF fibers with potential to detect stress and strain in the fluid flow,
including oceanic current monitoring [96].

4.4. Biomedical applications

One of the main effort in the biomedical area is in the development of measuring
equipment and health monitoring devices that seek to improve the life quality [97].
Electronic skins or e-skin have received considerable attention in recent years for being
a platform for continuous and real-time monitoring of human physiological signals. It
finds potential in prosthetics, robots, wearable devices, artificial intelligence, medical
equipment, and many other areas. Piezoelectric materials play a significant role in this
field [98, 99].
Owing to their properties – flexibility, biosafety, easy process –, ZnO, PVDF and
BaTiO3 are good candidates for electronic skin applications. In a study using composite
nanogenerator based on PVDF fibers, it was verify that the device can be used as a sensor
for real-time monitoring of radial artery pulse and respiratory information [100]. Also,
PVDF-TiO2 nanofibers shown its potential applications in wearable healthcare
monitoring systems and the ability of self-cleaning, since TiO2 can efficiently degrade
organic pollutants[101]. A device for human motion monitoring was developed based on
PVDF nanofibers with tetragonal-phase BaTiO3 NWs and a wireless circuit system [102].
This device allows that signals from human movement are transmitted wirelessly and
displayed in a mobile phone over a long distance (8 m). The results show the potential in
wearable medical electronics in the fields of rehabilitation and sports medicine [102].
When properly processed, piezoelectric materials represent a powerful biomaterial
that, in addition to being used as bioelectronic and biomechanical monitoring devices,
can interact strictly with biological tissues[103]. Tissue engineering, which is an

22
 engineering branch attempting to mimic, in vitro or in vivo, cellular microenvironments
through scaffolds systems, has also taken advantage of the piezoelectric potential.
Piezoelectric materials are used in the production of scaffolds since they reproduce the
electrical and mechanical cues existing in the tissues. Thus, piezoelectric scaffolds act as
actuators in cellular behavior to promote natural tissue formation [104]. PVDF and its
copolymers have already proven their potential in bone [105], skeletal muscle [106] and
neural [107] tissue engineering . Furthermore, it was confirmed that the incorporation of
different fillers, such as cobalt ferrites (Co2FO4), magnetite (Fe3O4) or Terfenol-D, in
PVDF matrix or silk fibroin matrix, ensure the natural regeneration of bones [108, 109].
Another study with a copolymer of PVDF, P(VDF-TrFE) revealed that the incorporation
of ZnO nanoparticles promote blood vessel formation (angiogenesis) – one of the main
problems in tissue engineering approaches. This study also conclude that a composite
scaffold favored its integration into the surrounding tissue when compared with non-
composite scaffold [110].

In summary, taking into account their versatility, piezoelectric polymer composites

are used in several applications from electronics to biomedical applications, table 4
showing representative composites in different application areas.

Table 4 – Representative piezoelectric polymer composites for different application

Composite Functional characteristic Application Ref

PVDF-TrFE/ZnO Pressure sensing Sensors [111]
Polydopamine (PDA)-modified Pressure sensing Sensors [112]
BaTiO3/PVDF
PVDF/polyamide(PA6)/BaTiO3 Triboelectric nano- Energy harvesting [113]
generator
PVDF-TrFE/ single-walled Piezoelectricity Energy harvesting [114]
carbon nanotubes (SWCNTs)
Sn3O4/PVDF Photocatalytic Environmental [115]
Ag–TiO2/PVDF-HFP Photocatalytic and Environmental [116]
antimicrobial
PVDF/Graphene oxide (GO) Electrical stimulation Biomedical [117]
Graphene(G)/ BaTiO3/PMMA Piezoelectricity Biomedical [118]

23
 5. Conclusion

Piezoelectric polymer composite materials are a class of materials that belong to smart
and multifunctional materials. The ability to transduce mechanical to electrical signals
and vice-versa provide their materials with increasing technological interest for the
development of sensors and actuators or energy harvesters in the form of thin, flexible
and potentially large area films. This material class combines the excellent properties of
ceramic fillers and polymeric matrix, allowing high dielectric constant and piezoelectric
coefficient and excellent thermal and mechanical properties, being successfully
implemented in areas such as consumer electronics, aerospace and automotive
applications or biomedicine.
Future trends are the continuous development of multifunctional tricomposites with two
different fillers, the use of environmental friendlier materials, the precise tuning of control
material properties for specific applications and to improve integration into devices by
techniques such as additive manufacturing techniques.

Acknowledgments
The authors thank the FCT (Fundação para a Ciência e Tecnologia) for financial
support under the framework of Strategic Funding grants UID/FIS/04650/2020,
UID/EEA/04436/2020 and UID/QUI/0686/2020; and project no.
PTDC/FISMAC/28157/2017, PTDC/BTM-MAT/28237/2017 and PTDC/EMD-
EMD/28159/2017. The authors also thank the FCT for financial support under grants
SFRH/BD/145455/2019 (E.C.), SFRH/BD/145345/2019 (L.F.) and
SFRH/BPD/112547/2015 (C.M.C.). Financial support from the Basque Government
Industry and Education Departments under the ELKARTEK, HAZITEK and PIBA
(PIBA-2018-06) programs, respectively, is also acknowledged.

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