Author et al./ Zulfaqar Int. J. Def. Sci. Eng. Tech.

Zulfaqar J. Def. Sci. Eng. Tech. Vol. 6 Issue 4 (2023) 34-45

ZULFAQAR Journal of
Defence Science, Engineering & Technology
Journal homepage: zulfaqar.upnm.edu.my

IMPACT OF NANOFILLERS ON DIELECTRIC PROPERTIES OF POLYURETHANE ELASTOMERS:

FROM SYNTHESIS TO FUTURE PROSPECTS

Muhammad Naiem Naqiuddin Zaharina,*, Ku Zarina Ku Ahmada, Ridhwan Yahayab

aDepartment of Mechanical Engineering, Faculty of Engineering, National Defense University of Malaysia, Sg. Besi Camp, 57000
Kuala Lumpur, Malaysia
bScience & Technology Research Institute For Defence (STRIDE), STRIDE Main Complex, Taman Bukit Mewah Fasa 9, 43000 Kajang,

Selangor Darul Ehsan, Malaysia

*Corresponding author: naiemnaqiuddin@gmail.com

ARTICLE INFO ABSTRACT

Article history: Dielectric elastomer (DE) devices are soft or flexible capacitors, composed of a thin
Received elastomeric membrane sandwiched between two compliant electrodes, that can
13-10-2021 transduce electrical to mechanical energy, and vice versa. Polyurethane possesses
Received in revised
outstanding dielectric properties which can be applied in developing DE with
02-09-2022
Accepted
excellent performance. Nevertheless, it still needs some modification to improve the
07-02-2023 dielectric properties to ensure their practicality in the industry. Researchers
Available online throughout the years had worked on several nanofillers such as graphene and
30-06-2023 barium titrate to enhance the dielectric performance of polyurethane elastomer to
ensure their applicability in the field. This paper discusses the basic principle of
Keywords: dielectric elastomer as generator and actuator, the popular nanofillers used to
Dielectric elastomer, enhance polyurethane dielectric constant, and the suitable application for
polyurethane, graphene, polyurethane elastomer.
barium titrate, generator,
actuator. © 2023 UPNM Press. All rights reserved.

e-ISSN: 2773-5281
Type: Article

Introduction

Dielectric elastomer (DE) was discovered since the earliest day of electricity when James Maxwell
conceived the effect on dielectric materials known as Maxwell stress in his work on the foundation of
electromagnetic theory [1]. Back in the 1990s, DE started to attract researchers’ attention when several
research papers and discoveries for potential applications were blooming. This may be due to the unique
characteristic of DEs that can convert electrical energy to mechanical energy, and vice versa, making them
versatile to be applied in various industries. There are various suitable polymers to act as a dielectric
elastomer such as acrylic [2–5], natural rubber [6], silicone rubber [7–9], poly (vinylidene fluoride) (PVDF)
[10–11] and polyurethane.
Polyurethane has become a promising material to be adapted as a dielectric elastomer due to its large
force outputs and high dielectric constant, allowing them to be actuated at the lower electric field. Paul and
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Pei [12] in their literature investigated materials that are suitable to be applied as DE in terms of their
mechanical and electrical properties. Some of the material is pre-strained to obtain their best performance.
Figure 1 indicates the attributes of the best state of acrylic, silicone rubber and polyurethane elastomer. As
compared to acrylic and silicone, polyurethane is still an atrocious elastomer material with poor actuation
behavior and low energy density. Besides, it has a high Young’s modulus which negatively affects the
electromechanical properties of the material. The microstructure of polyurethane, especially the hydrogen
bonds between hard segments leading to a high number of Young’s modulus [13]. Other than that, there
are many N-H/C=O hydrogen bonds in polyurethane limiting the mobility of the polarized groups and the
dipole polarizability of polyurethane which also restricts the dielectric constant of polyurethane [14].
Besides, polyurethane also possesses low breakdown strength which limits its allowable working electric
field. Despite that, polyurethane possesses the highest dielectric constant with the lowest dielectric loss,
making it a reasonable material to be applied as a dielectric elastomer. However, enhancement is still
necessary to make polyurethane a favorable dielectric elastomer. Fig. 1 shows the comparison of
polyurethane, silicone, and acrylic attributes.

Actuation pressure

Thickness strain

Young's modulus

Breakdown strength

Dielectric constant

Dielectric loss factor

Energy density

Polyurethane Silicone Rubber Acrylic

Fig. 1: Comparison of polyurethane, silicone, and acrylic attributes [12]

This paper will briefly explain the principle of dielectric elastomer, while further explanation on the
nanofillers used in the enhancement of polyurethane elastomer’s dielectric properties. Discuss on the
potential application of the enhanced polyurethane elastomer will also be done afterward.

Dielectric Elastomer Theory

Dielectric elastomer (DE) is an interesting mechanism that can act as both actuator and generator. DE
applies the basic transduction principle in which the electrical energy stored in capacitors, 𝑈𝑈, increases
with the material’s capacitance, 𝐶𝐶. Given by:

1
𝑈𝑈 = 𝐶𝐶𝑉𝑉 2 (1)
2

and

𝐴𝐴
𝐶𝐶 = 𝜀𝜀𝑜𝑜 𝜀𝜀𝑟𝑟 (2)
𝑧𝑧

where 𝑉𝑉 is the voltage, 𝜀𝜀𝑜𝑜 is the permittivity of free space, 𝜀𝜀𝑟𝑟 is the relative permittivity, 𝐴𝐴 is the surface
area and 𝑧𝑧 is the distance of the electrodes or the thickness of the membrane. Dielectric elastomer
generators (DEG) are variable capacitors in which the elastomer is sandwiched between two electrodes as
illustrated in Fig. 2. Electrical energy can be produced from a stretched, charged DEG by releasing its
mechanical deformation while maintaining the charge on its electrodes [15]. Specifically, the operating
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principle of a DEG in terms of a cyclic sequence of electromechanical transformations can be illustrated in

Fig. 3. The operating principle of DEG subjected to uniform stretches on the plane of the electrodes can be
explained through the following phases:

Phase 1 (P1) - As the DEG initially dwells in a relaxed state, where its capacitance is minimum, external
loads make it expand and lead it to a stretched state, where the capacitance is maximum. No charge is
present on the DEG during this phase.

Phase 2 (P2) - Charge Q is deposited on the electrodes, leading the DEG to a stretched and charged state,
where the capacitance is constant. This phase is called “priming”, which utilises an amount of electrical
energy being spent to charge the device.

Phase 3 (P3) - As the charge on the DEG is held constant, the external loads and the DE elastic stresses
work against the electrostatic charge, taking the DEG back to a state with minimum capacitance, Cmin.
During this generation phase, the external forces are converted into electrostatic energy and stored in
the DEG electric field.

Phase (P4) - The DEG is finally held in the minimum capacitance configuration and discharged, and the
stored electrostatic energy is harvested. The net amount of generated electrical energy is the difference
between the energy recovered during the discharging phase (P4) and spent during priming (P2).

Fig. 2: Dielectric elastomer generator (DEG) illustration

Fig. 3: Operating principle of dielectric elastomer generator (DEG)

On the other hand, the dielectric elastomer actuator (DEA) working principle is just the opposite of
the DEG. Instead of stretching and relaxing to produce electricity, DEA is the polymer that stretches and
relaxes when being electrified. The basic principle of DEA is shown in Fig. 4.

Fig. 4: Basic principle of dielectric elastomer actuator (DEA)

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 Author et al./ Zulfaqar Int. J. Def. Sci. Eng. Tech.

When the voltage is applied to the electrode, the charge will be built at each side, causing attraction force
in between the opposite charges which induces compression of elastomer film in the thickness direction.
However, the incompressible nature of elastomers resulting in increases in the area in other planes,
facilitated by the repulsion of the same charges. As the thickness of the elastomer films reduces, the
conversion of electrical energy into mechanical energy in the form of electrostatic pressure given by the
Maxwell pressure (𝜎𝜎) Eqn. (3):

𝑉𝑉
𝜎𝜎 = 𝜀𝜀𝑜𝑜 𝜀𝜀𝑟𝑟 � � 2 (3)
𝑑𝑑

where V is the applied voltage, 𝜀𝜀𝑜𝑜 is the permittivity of free space, 𝜀𝜀𝑟𝑟 is the relative permittivity, and 𝑑𝑑 is
the distance of the electrodes or the thickness of elastomer film.

As for DEG, the dielectric permittivity of the polymer is very significant in DEA performance. It should
be able to endure large voltages, for the actuation to be as large as possible. In addition, they should have
high dielectric strength and low dielectric losses to maximize efficiency and avoid premature failures [16].
Eqn. (4) represents the electro-mechanical sensitivity of the DE and as it presented, a polymer with high
dielectric permittivity and low Young’s modulus, Y is desired to ensure the high electro-mechanical
sensitivity.

𝜀𝜀𝑟𝑟
𝛽𝛽 = (4)
𝑌𝑌

Nanofillers Used to Enhanced Polyurethane Elastomer Dielectric Properties

Increasing dielectric permittivity of polyurethane elastomer produces a direct increase in its capacitance,
which helps in developing superior DEG and DEA. A conventional approach to do so is by creating elastomer
composites. The idea is to combine high dielectric permittivity and excellent mechanical values of certain
nanofillers with polyurethane. Several types of nanofillers have been proven to enhance polyurethane
elastomer dielectric performance over the years, which are the carbon-based nanofillers, ceramic-based
nanofillers, and hybrid nanofillers.

Carbon-based nanofillers

Due to its extraordinary properties, graphene has attracted tremendous attention in the field of polymer
nanocomposites. Graphene can induce a considerable improvement in mechanical, thermal, and electrical
properties of the resulting graphene–polymer nanocomposites at very low loading contents. It possesses
high electrical and thermal conductivities, a high surface-to-volume ratio, and excellent mechanical
properties [17–18]. Other than graphene, carbon nanotube (CNT) is another ideal material in polymer
nanocomposites due to its high flexibility, high aspect ratio, and low mass density. However, the limitation
of CNT is its uniformed dispersion because of the high surface energy and tendency to agglomerate in the
bulk of the polymer and elastomer matrix [19]. Despite that, it is still one of the most favorable fillers for
researchers over the years. Table 1 shows the dielectric constant of polyurethane composite with graphene
and carbon nanotube fillers at various loadings.

Table 1: Dielectric constant of polyurethane composite with carbon-based nanofillers at various

loadings
Carbon-based filler Filler loadings Dielectric constant Reference
(wt%) (at 1 kHz)
0.5 6.0 [20]
Pure Graphene 1.5 9.0
3.0 13
0.5 10
Graphene Oxide 1.5 12
3.0 16
0.5 16 [21]
Thermally Reduced Graphene Oxide 1.0 209
2.0 1875
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Carbon-based filler Filler loadings Dielectric constant Reference

(wt%) (at 1 kHz)
Thermally exfoliated and annealed graphene [22]
5.5 377
(TRG)/polyurethane composites Filler
0.5 8.0 [23]
1.0 8.3
Carbon Nanotube
1.5 9.2
2.0 9.5
0.5 8.7
1.0 9.0
Nitric Acid-treated Carbon Nanotube
1.5 9.3
2.0 9.4
0.5 8.8
1.0 8.8
Silane-treated Carbon Nanotube
1.5 9.1
2.0 9.4

Polyurethane possesses many hydrogen bonds, in which the NH group acts as the donor, and either
the carbonyl group from the hard segment or the ester oxygen group from the soft segment acts as the
acceptor. These hydrogen bonds limit the mobility of the polarized groups of PU chains, thus limiting the
dipole orientation polarization ability of PU, and restricting the increase in dielectric permittivity of PU. By
adding functionalized graphene, the hydrogen can be disrupted which results in an increase in the dipole
orientation polarization ability of PU chains, thus the dielectric performance of PU. As shown in Table 1,
the dielectric constant of polyurethane increases with the loading of the graphene fillers. In comparison to
pure graphene, graphene oxide can produce polyurethane elastomer with a higher dielectric constant [20].
This is due to many oxygens’ functional groups (C-O-C, C-OH, and C=O) that enable graphene oxide to form
homogenous colloidal suspensions in organic or aqueous solvents. The strong interaction between
graphene oxide and polyurethane could prevent the direct connection of graphene oxide, subsequently
resulting in a low dielectric loss. However, the oxidation of graphite could also lead to severe disruption of
the graphite structure, whereby the dielectric constant will decrease. Thus, researchers were moving
toward reducing the graphene oxide by thermally or chemically treating the graphene oxide.

Liu et al [21] thermally reduced the graphene oxide by in situ partial thermal reductions of graphene
nanosheet-polyurethane composites at 180ᵒC. The result from the X-ray photoelectron spectroscope (XPS)
shows that the C/O ratio of the thermally reduced graphene oxide is higher than the pure graphene oxide,
indicating the removal of the oxygen-containing group from the composite. Therefore, the graphite
network is restored, which increases the interfacial polarization of the reduced graphene oxide and thus
increases the number of charge carriers accumulated at the interface between the nanofillers and the PU
matrix. The remaining oxygen functional group helps in disrupting the hydrogen bonding between the PU
chain and enhances the interaction between the nanofillers with PU through hydrogen bonding, leading to
the coating of the PU on reduced graphene oxide, thus creating suppression of the leakage current. All these
factors are responsible for creating the large increases of dielectric constant in very low loadings (2 wt%)
which is up to 1875 at 10kHz. Another method was used by Bansala et al [22] in the effort to reduce the
oxygen content within the graphene oxide. They exfoliated the graphene oxide thermally at 200ᵒC and
annealed them at 800ᵒC. The XRD confirmed the reduction of graphene oxide through this method. The
polyurethane-reduced graphene oxide elastomer was fabricated, and it resulted in the improvement of the
dielectric constant of the polyurethane elastomer. At 1 kHz, the fabricated elastomer recorded a dielectric
constant of 377 at 5.5 wt% loadings.

Other than graphene, the carbon nanotube is another popular carbon-based nanofillers used to
enhance the dielectric properties of polyurethane elastomer. Tayfun et al [23] synthesized carbon
nanotube with polyurethane and did some surface modification towards the carbon nanotube using nitric
acid and also silane treatment. The results show the increment of the dielectric constant for all types of
carbon nanotube fillers, with the dielectric constant increasing with the content of the filler. The addition
of carbon nanotube filler increases the free volume of the polymer structure, creating easier orientation of
dipole groups along the field which causes an increase in the dielectric constant. However, there is no
significant difference in terms of a dielectric constant between the pristine carbon nanotube filler and the
modified carbon nanotube filler.

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Ceramic-based nanofillers

Inorganic filler such as ceramic possesses high permittivity values which, in principle, should be able to
increase the dielectric constant of a given elastomer while maintaining its dielectric nature when
synthesized together. Several research incorporate other inorganic fillers such as titanium oxide (TiO)
[24–26], barium titrate (BT) [27–28], zinc oxide [29–30] and strontium titanate (ST) [31] to enhance the
dielectric of the elastomer.

Calcium cooper titanate (CCTO) provides interesting features when being incorporated with
polyurethane matrix. Wan et al [32] investigated the dielectric properties of CCTO filled polyurethane
composites. The dielectric permittivity values of CCTO with 10, 20, 30, and 40 were determined as ~13, 17,
26, and 33, respectively, which correspond to 63%, 113%, 225%, and 313% higher than that of neat TPU
respectively at 1 kHz. Increasing the CCTO content in the composite caused a smaller distance between
inclusions that led to more effective dipole interactions and higher charge storage capability. Other than
that, zinc oxide (ZnO) is also a promising nanofiller that can be used to enhance polyurethane dielectric
properties. Kaur et al [33] reported the dielectric constant of the polyurethane increased with the ZnO
doping concentration. At 1 kHz, the dielectric constant of ZnO-polyurethane elastomer can reach up to 7 at
a 15% doping concentration of ZnO. Another ceramic-based filler that provides excellent dielectric constant
improvement is calcium copper titanate (CCTO).

Hybrid nanofillers

A hybrid nanofiller means a combination of two or more types of nanofillers. As discussed earlier, both
carbon-based and ceramic-based nanofillers can enhance the dielectric performance of polyurethane
elastomer. However, both nanofillers have their disadvantages. The main drawback with carbon-based
nanofiller is the high energy dissipation associated within the nanofillers due to high current leakage
resulting from direct contact of nanofillers and high mobility of charge carriers [34]. Nanofillers such as
graphene, carbon black and carbon nanotube tend to easily stack and agglomerate, forming conductive
pathways, and thus diminishing the dielectric strength [35–37]. On the other hand, ceramic nanofillers
usually require high loading fractions to effectively improve ε and thus produce a large increase in elastic
modulus. Hence, the resulting composites usually suffer from a loss of flexibility and processability. Hybrid
nanofillers such as a combination of carbon-based and ceramic-based nanofillers can overcome these
problems as each nanofillers can complement each other.

Chen et al [38] took an initiative by fabricating a hybrid polyurethane-elastomer with reduced

graphene oxide-titanium dioxide (RGO-TiO2) functionalized filler and polyurethane matrix and comparing
them with the reduced graphene oxide-polyurethane composite (RGO-PU). Under SEM characterization,
phase separation was observed in RGO-PU, attributed to the poor dispersion and serious graphene
aggregation. On the contrary, hybrid nanocomposites seem to be homogeneously distributed in the
polymer matrix. Thus, the dielectric constant of the hybrid-polyurethane elastomer is higher than the RGO-
PU elastomer at the same weight percentage. Figure 5 shows the dielectric constant of RGO-PU elastomer
and RGO-TiO2-PU elastomer at a 1:1 ratio (1G1T-PU) at various temperatures and 3 wt%.

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 Author et al./ Zulfaqar Int. J. Def. Sci. Eng. Tech.

180161.3 166.4
160
140
Dielectric constant
116.6
120 101.9
99.2
100
75.1
80 RGO-PU

60 1G1T-PU

40
20
0
-60 -40 -20 0 20 40 60
Temperature (°C)

Fig. 5: Dielectric constant of RGO-PU elastomer and RGO-TiO2-PU elastomer at various

temperatures (at 3 wt%) [38]

Chen et al [39] investigated the dielectric properties of PU dielectric elastomer with carbon nanotube
(CNT) and graphene (GRN) nanofillers. CNT-PU, GRN-PU, 1C1G-PU (1CNT: 1GRN), 1C2G-PU (1CNT: 2GRN),
and 2C1G-PU (2CNT: 1GRN) at different weight percentages (0.5, 1.0, 1.5, 2.0 and 2.5 wt%) were prepared
and examined. The results from the scanning electron microscope (SEM) showed that the fracture surface
of the hybrid composites is very smooth, indicating the homogenous dispersion of the nanofillers.
Consequently, hybrid composite possesses a higher dielectric constant. 1C1G-PU composites possess the
highest dielectric constant (≈ 275 at 1 kHz) compared to other composites. For the CNT-PU, the aggregation
of CNT still serves as the electrode material to form a micro-capacitor network in the matrix for increasing
the dielectric constant. However, the number of micro-capacitors is restricted because of the aggregation
of CNT. As to GRN-PU composites, there is an increment of dielectric constant as well, but not as significant.
As to hybrid filler PU composites, the mixture of hybrids improves the dispersibility of GRN and CNT in the
polymeric matrix which allows the construction of more micro-capacitors in GRN-CNT@PU composites,
leading to a higher dielectric constant.

Potential application of polyurethane DE

High dielectric performance of PU elastomer makes it very reliable to be used as both DEG and DEA. This
part will underline a few applications that are suitable for the excellence of PU elastomer.

Energy Harvesting

DE are highly versatile and can work as both actuators and generator, due to its properties of high energy
density, large deformation and good electromechanical conversion efficiency that can open various
possible applications in the industry and one of them is the energy harvesting technologies [40], [41].
Pioneered by Pelrine et al. in 2001 [42], basic principles of using DEs for-energy harvesting are set and
since then, numerous methods have been investigated. Moretti et al [43] investigated the application of DE
in wave-energy converter (WEC) technologies with an oscillating water column (OWC) system. The
structure of the OWC system consists of a partially submerged hollow structure, with an upper part forming
an air chamber and an immersed part opened to the sea action as illustrated in Figure 6. OWC system
utilizes the wave-induced pressure oscillations to induce the reciprocating motion of the water column
inside the chamber, causing compression and expansion of the air entrapped in the chamber.

DE is suggested to be used as the power take-off (PTO) system that converts the pneumatic power into
beneficial electrical energy. This paper reported the result of the generation test on the prototype that
shows the application of DE in the OWC system provides encouraging results for possible larger-scale
energy harvesting applications. However, further fundamental steps are required for DEGs to become a
viable technology for wave energy PTO systems including the synthesis of advanced dielectric materials
with enhanced dielectric strength in which PU elastomer gives a very brilliant performance.

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Fig. 6: Illustration of oscillation water column (OWC) system [43]

Other than a wave-energy converter, another potential energy harvesting system that can apply DEs in its
system is a wind-power generator or wind turbine. Zhang et al [44] utilized Vibro-impact DEG (VI DEG)
system in wind energy harvester by proposing a model with VI DEG. Acrylic membranes were embedded
into a two-blade turbine to harvest wind-induced rotational energy from low-speed wind environments.
The result is outstanding compared to other unconventional wind energy harvesters. It is found that the
proposed system can achieve better maximal output power up to 0.7125 mW under a wind speed of 3.99
ms-1. This proves the suitability and workability of DE in energy harvesting applications.

Zhang et al [45] proposed a novel contact-type DEG which can harvest energy from two dielectric
elastomer membranes (DEMs) under a regular contact-type displacement excitation. The energy
harvesting performance of the CT DEG in simulated contact environments is investigated and within the
correct excitation, system structure, pre-stretched ratio and input voltage, the system can produce an
output voltage 10 times higher than the input voltage, and the system output power can be achieved as high
as 22.94 mW under a given displacement excitation.

Artificial and Robotic Muscle

Another application that suits polyurethane elastomer attributes as a DEA is the artificial and robotic
muscle which has been investigated and explored in a few works of literature [46–49]. In the marine robotic
industry, DEA has been vastly investigated to be used in soft robotic field for marine application. According
to Christianson et al, traditional underwater robots are usually driven by propellers or jet thrusters, which
generate considerable noise and vibration. This additional noise is especially problematic when studying
elusive animals or when studying underwater acoustics. Nevertheless, they consume a large amount of
power [50]. Shintake et al. [51] stated that DEAs are compliant, highly responsive, efficient and exhibit large
actuation strokes, which make them suitable to be applied for biomimetic underwater robots. Gu et al. [52]
also stated that DEAs usually have the approximate density to the water, which makes it easier to operate
with natural buoyancy.

Otherwise, a significant amount of power will be consumed by maintaining the equilibrium of the
robots underwater. Godaba et al [53] in their paper discussed regarding marine jellyfish robots as
unmanned underwater vehicles for valuable functions, such as study of marine life, exploration of sea and
seabed conditions, monitoring of ocean currents, detection of ocean intruders, etc. Traditional actuators
such as electric motors and electromagnetic actuators have been popularly employed to achieve jellyfish-
like morphology and movement for easier navigation. Although traditional actuators can achieve jellyfish-
like morphology and movement, there are still weaknesses such as small deformation, consumption of large
electric energy, low energy density, high noise, and slow response. This is something that can be overcome
with dielectric elastomers actuators (DEAs) and has been agreed upon in other literature as well.

Soft sensors

As a polymer that possesses a high dielectric constant even at a pristine state, polyurethane elastomer can
be developed into a high sensitivity soft sensor with some enhancement such as adding functionalized
fillers. One of the applications of soft sensors is to safely monitor soft movements or interactions with
humans. It is used to measure strain, pressure, force, light, humidity, and temperature like human skin,
which has advantageous properties such as flexibility, stretchability, high sensitivity, and technological
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 Author et al./ Zulfaqar Int. J. Def. Sci. Eng. Tech.

compatibility with a large area. The sensitivity of the elastomer sensors highly depends on the dielectric
constant. The higher the dielectric constant, the higher the sensitivity of the dielectric elastomer sensor
[54–56]. Ren et al [57] proposed a facile, high-performance, and cost-effective strain sensor using the
carbon nanotubes (CNTs)/thermoplastic polyurethane (TPU) with an aligned wave-like structure. The
strain sensor is prepared through the electrospinning technique and the sensing properties of CNTs/TPU
mats in vertical and parallel directions were investigated. The results show that the aligned CNTs/TPU
fibrous mats in the vertical direction possessed an ultra-high stretchability (900%) and excellent durability
(10,000 cycles at the strain of 200%). An ultra-low detection limit (0.5%) and fast response time of 70 ms
were also achieved, exhibiting a favorable sensitivity. In this study, the strain sensor was proposed to be
applied in monitoring human motion such as cheek bulging and phonation and even vigorous motion like
leg squatting and elbow bending.

Zhuang et al [58] proposed a flexible thermoplastic polyurethane (TPU)/CNT polymer composite for
selective laser sintering (SLS) processing pressure sensors. TPU/CNT composites were prepared with
different conductivity and piezoresistive properties. Piezoresistive performance and percolation theory
results prove that the composite shows the best pressure sensing ability of 0.549 kPa-1 for 17-240 kPa
pressure at 0.25 wt% CNT-containing TPU/CNT composite, and it was successfully used as a sensor to
detect plantar pressure distribution in a human foot. Ke and his team [59] fabricated pressure sensors using
TPU matrix and hybrid-nanofillers of CNT and graphene nanoplatelets (GNP) at various compositions to
tune the composite dielectric properties. In a composition of CNS and GNP at a mass ratio of 3:1, the
composite sensor shows the highest pressure-sensitivity of 2.05 MPa-1 for 0-1.2 MPa pressure, compared
with 0.18 MPa-1 for neat TPU, enabling potential wearable pressure sensor applications.

Conclusion

Polyurethane is an interesting dielectric material that possesses both excellent dielectric and mechanical
properties. With a proper enhancement method, the polyurethane-based dielectric elastomer can achieve
even better properties that can fulfill the demands of the industry. Functionalized fillers such as graphene,
carbon nanotubes and barium titrate are favorable nanofillers to enhance dielectric elastomer
performance. Future study on improving the polyurethane matrix should also be established. Although the
nanofillers are proven to be effective in improving the DEs dielectric performance, there are still few
drawbacks from its application in terms of electrical conductivity and mechanical performance. Thus, the
enhancement of the matrix itself may help in improving the DE performance, reducing on the reliability
towards nanofillers properties.

Acknowledgment

The authors fully acknowledged Ministry of Higher Education (MOHE) and Universiti Pertahanan Nasional
Malaysia (UPNM) for this research feasible. The authors also would like to thank the technical staff in every
institution involved for their assistant.

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