Ferroelectric materials, one of the current research focus with a number of fascinating properties such as ferroelectricity, piezoelectricity, pyroelectricity and dielectricity, has extensive application value in biomedical systems. Here we review the biomedical applications of ferroelectric materials, including ferroelectric ceramics, ferroelectric thin films, and ferroelectric polymers. Some unresolved problems are summarized and the future directions of applications in biomedicine are prospected as well.
Currently, researchers are mainly focusing the phase transitions, domain structures, domain wall functionalities, magneto electric coupling, and the potential applications in high-density ferroelectric non-volatile memories of ferroelectric materials [1-5]. However, there are relatively less research interests in the biomedical applications of ferroelectric materials which is very important to human life, such as the lead zirconate titanate (PZT) ceramics used in ultrasonic transducers for medical ultrasound systems and ferroelectric thin film capacitors implanted to the body as energy storage supply power for medical micro-devices. In this mini-review, we will introduce the biomedical applications of ferroelectric ceramics, ferroelectric thin films and ferroelectric polymers.
Ferroelectric ceramics, due to its piezoelectricity, pyroelectricity and dielectricity, are developing rapidly recently. The research directions of ferroelectric ceramics are wide and we focus on the introduction of its applications in biomedicine here.
Since lead zirconate titanate (PZT) of perovskite structure was discovered, it has been the most widely used ferroelectric ceramics because of its strong and stable piezoelectricity. In recent years, the PZT ceramics have a greater potential in
application of ultrasonic transducers for medical ultrasound systems which are one of the basic imaging modalities covering cardiology, radiology, obstetrics, gynecology and general abdominal imaging. Piezoelectric materials are the core part of the ultrasonic transducer. They can generate a certain frequency of ultrasonic signals to detect some characteristics of a target media. Then they receive the signals containing information of the media and convert ultrasound signals into electrical signals. Piezoelectric phenomenon is that when a voltage is applied on the piezoelectric materials, the materials will elongate or contract. Reciprocally, they will generate voltages when an external mechanical force presses or elongates on them. Based on the high electro-mechanical conversion efficiency, PZT ceramics are extensively used in biomedical systems.
Recently, many other advanced ceramics have been developed rapidly. Chen et al.  reported the use of extrusion technique to fabricate transparent lead lanthanum zirconate titanate (PLZT) ceramic fibers. The final transducer was housed in an SMA connector for poling and pulse-echo measurement. The ceramic fibers can been used to create the small aperture ultrasonic transducer. And it has a great potential in photoacoustic imaging by using the transparent fiber as the path of light propagation and ultrasonic transducer materials.
Researchers also  investigated the design, fabrication, and experimental evaluation of a prototype PZT matrix transducer with an integrated receive Application Specific Integrated Circuit (ASIC) on what consists of an array of 9 × 12 piezoelectric elements mounted via an integration scheme, as a proof of
concept for a miniature three-dimensional transesophageal
echocardiography probe. The designs are employed to meet
the strict space and power constraints, and demonstrated the
effectiveness of the proposed techniques.
As the piezoelectric coefficient and electromechanical
coupling coefficient of relaxor ferroelectric single crystal (such
as lead magnesium niobate–leadtitanate (PMN-PT) and lead zinc
niobate–leadtitanate (PZN-PT)) are more excellent than PZT,
DeAngelis et al.  described a new effective method to explore
lead indium niobate–lead magnesium niobate–leadtitanate (PINPMN-
PT) transducer designs. The major advantage for PIN-PMNPT
is that it can minimize the volume of piezoelectric material
for the same impedance, and also allow larger transducers for
a given operating frequency due to lower wave speed c and the
handling durability or crack resistance was found to be similar
to PZT8 with transducer build and rebuild.
However, there are still some problems to be solved, such
as lead will cause harm to human and environment. It is crucial
to look for the right elements to replace lead. Therefore, it
is necessary to study the lead-free materials for biomedical
applications such as BiFeO3, BaTiO3, LiNbO3 and relax or
ferroelectric single crystals, etc [9-11].
Bismuth-layered ferroelectric ceramics (BLF) have aroused
much attention as a lead-free ferroelectric ceramics. Usually,
BLF can be expressed as (Bi2O2)2+(An-1BnO3n+1)2- , wherein A
represents +1, +2 or +3 valence ions and B represents +3, +4 or
+5 valence ions. A (An-1BnO3n+1)2- layer of a pseudo-perovskite
structure is inserted between the (Bi2O2)2+ layers. Strontium
bismuth tantalate (SBT), a bismuth-layered ferroelectric
material, has been found that it has a larger remaining
polarization, good thermal stability performance and high Curie
temperatures and it is widely used for sensors and memories
. However, it contains the volatile element, bismuth, which
is easy to cause the formation of oxygen vacancies to deteriorate
the anti-fatigue performance and ferroelectricity.
Much effort has been made to improve the related properties
for the realization of biomedical applications. Zhou et al. 
described an approach by adding Al2O3 to Na0.5Bi2.5Nb2O9-
based bismuth layered piezoceramics to improve ferroelectric,
piezoelectric and high temperature resistance. The bismuthlayered
piezoceramic Na0.5Bi2.5Nb2O9-Al2O3shows large
piezoelectric constant (d33=15.2pC/N), a relatively large
remnant polarization (~11.6 μ C/cm2) and good temperature
stability (600 ℃), promising for future sensor applications and
high temperature applications.
Sam et al.  used the solid-state reaction method to
prepare intergrowth bismuth-layered ferroelectric(BLF)-type
BiT-CaBi4Ti4O15 (CBTO), BLaT-CBTO, and BNdT-CBTO ceramics. The remanent polarization and piezoelectric coefficient values
of the intergrowth ceramics are slightly larger than the BLF
ceramics. After doping with rare-earth ions La and Nd, i.e., the
dielectric constants and the piezoelectric coefficients were
improved to 146 and 15.4 pC/N, respectively. On this basis, novel
ferroelectric capacitor will be exploited for non-volatile memory
storage and biomedical tactile sensor applications.
With the continuous development of system integration
and device miniaturization, ferroelectric thin films are gaining
increasing attention for biomedical systems. For instance,
ferroelectric thin film capacitor as an energy storage supply
power for medical micro-device implanting into the body. Its
high energy storage density, small volume and long service life
make a great significant. Compared with ferroelectric ceramics,
the ferroelectric thin films is more sensitive to pressure and
the ferroelectric thin film technologies are a breakthrough
technology for a new class of thin film ultrasonic transducers
. Seiya Ozeki et al.  synthesized PZT polycrystalline film
via a hydrothermal method which is used for transducers to
replace the conventional piezoelectric ceramic transducers, and
succeed to reduce the size of PZT polycrystalline film vibrator
for Coiled Stator Ultra-Sound Motor (CS-USM) with similar size
as the piezoelectric ceramic resonator for CS-USM.
The elements of array of transducers can be defined only by
the top electrode pattern, since the low lateral coupling of the
printed PZT leads to a low cross-talk between the elements. It
is necessary to apply PZT thick film technology to manufacture
multi-element transducers enabling a novel method for costeffective
fabrication of imaging arrays for medical applications.
Bierregaard et al.  fabricated the transducers using screen
printing technique, where the gold bottom electrode has been
deposited first followed by the deposition of the PZT thick film;
then the top electrodes define the elements of the array. The
measured properties indicate very good reproducibility and
repeatability within as well as between the devices. In addition,
Lee et al.  made a diaphragm-type piezoelectric resonator
for an operating flexure mode fabricated with thinned bulk PZT
of a 50-μm thickness bonded onto a silicon plate that was then
strongly bonded onto a PDMS substrate using an oxygen-plasma
treatment. This Flexible piezoelectric micro machined ultrasonic
transducer (pMUT) array can be applied to study utilizing
ultrasound brain stimulation.
Ferroelectric polymers can not only turn mechanical,
heat, sound vibration energies into electricity to harvest
human mechanical energy, but also be used for pressure and
temperature sensors due to their piezoelectricity, pyroelectricity,
ferroelectricity and electrostrictive effect. Polyvinylidene
fluoride (PVDF), polyvinyl fluoride and several copolymers are
ferroelectric polymers with strong piezoelectric and pyroelectric properties. However, only cold drawing or electric polarization
can enable PVDF show strong piezoelectric and pyroelectric
performances. Otherwise, PVDF will miss these physical
properties. Unlikely, the copolymers behave strong piezoelectric
and pyroelectric as steric factors result in the mechanism of
Fortunately, PVDF is a flexible materials and has been
used as the medical ultrasonic transducers because of smaller
decay time and more depth resolution than piezoelectric
ceramics sensors. Salvatore A. Pullano et al.  presented a
ferroelectric polymer-based temperature sensor by using a
uniaxially stretched 28 μm thick sheet of PVDF. The sensor made
into a chip for micro fluidic devices can monitor the localized
temperature of a biological fluid quickly. In terms of energyharvesting
devices, their performances are relevant to the types
of nanostructures of ferroelectric polymers.
Wang et al.  used a PVDF nanowire-PDMS composite
film as the turboelectric layer, a polarized PVDF film as both
the piezoelectric and pyroelectric layers to form a hybridized
nanogenerator, which can harvest mechanical and thermal
energies by turboelectric–piezoelectric–pyroelectric effects.
These works extend the energy sources of the ferroelectric
polymers and is attractive for biomedical applications.
In this mini-review, we give a brief summary of ferroelectric
ceramics, ferroelectric thin films and ferroelectric polymers
for the biomedical applications, including PZT ceramics used
in ultrasonic transducers and ferroelectric thin film capacitors
used as energy storage supply power for medical micro-devices.
Besides, we propose possible future directions of this field, such
as the study of lead-free materials for biomedical applications.
We hope this review will arouse the readers’ interest in this field.