![Different focusing techniques for ultrasonic transducers. (a) The actual picture and concrete steps of surface PFUT preparation by ball pressing method [84]. Reproduced from [84]. CC BY 4.0. (b) 3D printing curved piezoelectric ceramics [88]. Reproduced from [88]. CC BY 4.0. (c) Acoustic lens based on Fresnel principle [89]. Reproduced from [89]. CC BY 4.0. (d) Acoustic lens based on acoustic metamaterials [90]. Reprinted from [90], with the permission of AIP Publishing. (e) 3D printed annular piezoelectric transducer [91]. Reproduced from [91]. CC BY 4.0.](https://www.researchgate.net/publication/382229506/figure/fig5/AS:11431281263073743@1721879765368/Different-focusing-techniques-for-ultrasonic-transducers-a-The-actual-picture-and_Q320.jpg)
Access to this full-text is provided by IOP Publishing.
Content available from International Journal of Extreme Manufacturing
This content is subject to copyright. Terms and conditions apply.
IMMT
International Journal of Extreme Manufacturing
Int. J. Extrem. Manuf. 6(2024) 062001 (25pp) https://doi.org/10.1088/2631-7990/ad62c6
Topical Review
Design and micromanufacturing
technologies of focused piezoelectric
ultrasound transducers for biomedical
applications
Xingyu Bai1,2, Daixu Wang2, Liyun Zhen1,2, Meng Cui1,2, Jingquan Liu1, Ning Zhao3,∗,
Chengkuo Lee4,5and Bin Yang1,∗
1National Key Laboratory of Advanced Micro and Nano Manufacture Technology, Shanghai Jiao Tong
University, Shanghai 200240, People’s Republic of China
2Department of Micro/Nano Electronics, School of Electronic Information and Electrical Engineering,
Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China
3Department of Orthodontics, Shanghai Ninth People’s Hospital, Shanghai Jiao Tong University School
of Medicine, College of Stomatology, Shanghai Jiao Tong University, National Center for Stomatology,
National Clinical Research Center for Oral Diseases, Shanghai Key Laboratory of Stomatology, Shanghai
200011, People’s Republic of China
4Department of Electrical & Computer Engineering, National University of Singapore, 4 Engineering
Drive 3, 117576, Singapore
5Center for Sensors and MEMS, National University of Singapore, 4 Engineering Drive 3, 117576,
Singapore
E-mail: zhaon1995@126.com and binyang@sjtu.edu.cn
Received 28 February 2024, revised 4 April 2024
Accepted for publication 12 July 2024
Published 24 July 2024
Abstract
Piezoelectric ultrasonic transducers have shown great potential in biomedical applications due
to their high acoustic-to-electric conversion efciency and large power capacity. The focusing
technique enables the transducer to produce an extremely narrow beam, greatly improving the
resolution and sensitivity. In this work, we summarize the fundamental properties and biological
effects of the ultrasound eld, aiming to establish a correlation between device design and
application. Focusing techniques for piezoelectric transducers are highlighted, including
material selection and fabrication methods, which determine the nal performance of
piezoelectric transducers. Numerous examples, from ultrasound imaging, neuromodulation,
tumor ablation to ultrasonic wireless energy transfer, are summarized to highlight the great
promise of biomedical applications. Finally, the challenges and opportunities of focused
ultrasound transducers are presented. The aim of this review is to bridge the gap between
focused ultrasound systems and biomedical applications.
∗Authors to whom any correspondence should be addressed.
Original content from this work may be used under the terms
of the Creative Commons Attribution 4.0 licence. Any fur-
ther distribution of this work must maintain attribution to the author(s) and the
title of the work, journal citation and DOI.
© 2024 The Author(s). Published by IOP Publishing Ltd on behalf of the IMMT
2631-7990/24/062001+25$33.00 1
Int. J. Extrem. Manuf. 6(2024) 062001 Topical Review
Keywords: piezoelectric ultrasonic transducers, focusing techniques, preparation method,
biomedical applications
1. Introduction
Ultrasonic transducer is an electronic device that converts
ultrasonic waves into electrical energy and vice versa.
According to the working principle, ultrasonic transducers can
be divided into electric eld types, like capacitive and piezo-
electric, and magnetic eld types, such as magnetostrictive
transducers [1–5]. The earliest notable ultrasonic transducer
was a sandwich transducer designed by Langevin in 1917 [6].
The transducer was made of quartz crystal as a piezoelectric
material. In 1933, the magnetic eld type ultrasonic trans-
ducer, which used magnetostriction, rst appeared [7]. Due
to the advantages of high strength and high power, magneto-
strictive transducers caused a wave of research at that time [8].
From the 1950s, the development of ferroelectric ceramics like
barium titanate (BT) and lead zirconate titanate (PZT) solidi-
ed the dominant role of piezoelectric transducers, a position
they maintains today, continually evolving with new materials
and technologies [9].
Piezoelectric ultrasonic transducers without beam focusing
have a certain degree of natural focusing properties due to their
physical structure, which has been widely used in the medical
eld [10]. However, general piezoelectric transducers can only
produce a wider ultrasound eld, which may limit its effect-
iveness in applications requiring high accuracy. Focused trans-
ducers are capable of producing extremely narrow beams and
are more specialized for tasks requiring high precision, such
as in certain diagnostic and therapeutic procedures [11–15].
In ultrasound imaging, focused beamforming techniques and
methods will affect the resolution of imaging [16–18]. In order
to control smaller cells in ultrasonic tweezers, the sound wave
emitted by the transducer preferably generates a sufciently
high radiation force at the focal point [19,20]. In ultrasound
therapy, low-intensity focused ultrasound (LIFU) and high-
intensity focused ultrasound (HIFU) were used to modulate
nerve and tumor ablation, respectively [21–24].
In recent years, there have been many reports on the
review articles surrounding the application of focusing tech-
nology in the medical eld, summarizing the important role
played by focused ultrasound transducers in various diseases.
Meng et al [25] provided an overview of focused ultra-
sound for the treatment of brain diseases. Bachu et al [26]
highlighted the mechanism and clinical application of HIFU.
Javid et al [27] emphasized the use of focused ultrasound
in transcranial neuromodulation. While the above reviews
all focus on the application of focused ultrasound, there is
also a need for more detailed discussions on the transducer
design and manufacturing processes to facilitate the devel-
opment of better performing transducers for more applica-
tions. The key challenge in focused ultrasound transducers lies
in the selection of appropriate materials and focusing tech-
niques. It is necessary to solve the problem of how to pro-
cess brittle and hard materials such as piezoelectric ceramics
and piezoelectric single crystals [28–31]. In addition, it is also
important to avoid acoustic impedance mismatches between
the sections. It has been reported that focused ultrasound trans-
ducers based on piezoelectric materials have been put into clin-
ical applications and play an important role in nervous sys-
tem diseases, tumors, and chronic diseases. There is an urgent
need to summarize and analyze the preparation and design
of focused ultrasound transducers in different application
scenarios.
In this review, we provide a comprehensive overview of the
latest progress of focused ultrasound transducers, especially
the preparation methods of each part and the latest applica-
tions. Here, we mainly target related applications in the med-
ical eld, including ultrasound diagnosis, ultrasound therapy,
ultrasound wireless energy transfer, and ultrasound acoustic
tweezers. The rst section introduces the basic characterist-
ics of ultrasound and the biological effects of ultrasound on
human body in detail. Subsequently, the second section sum-
marizes the relevant functional materials of the focusing trans-
ducer. The third section critically examines four distinct focus-
ing techniques and discusses their respective merits and limita-
tions. Finally, the applications of piezoelectric focusing trans-
ducer in medical eld are summarized in detail. The summary
and prospect section gives the future development direction of
the focused ultrasound transducer and its application in emer-
ging elds. Figure 1presents a systematic overview of this
review.
2. Device physics
2.1. Ultrasound basics
There are many basic characteristic parameters describing
ultrasonic wave, such as sound speed, wavelength, period, fre-
quency, energy density, sound intensity, and sound pressure.
However, sound intensity is usually used more because of its
temporal and spatial characteristics [32]. First, the spatial dis-
tribution of sound intensity is not uniform. For the sound eld
generated by the planar transducer, the maximum peak intens-
ity of the general central axis is 4 times the average sound
intensity. Second, the sound intensity is also uneven in time.
No matter continuous wave or pulse wave, there are time peak
(TP) and time average (TA). Common sound intensity expres-
sions are discussed below.
Spatial peak time Peak sound intensity ISPTP (also known as
maximum instantaneous sound intensity) is generally used to
describe the sound intensity corresponding to the sound pres-
sure at the spatial peak point of the pulse wave (P2
mas shown
in gure 2). According to the conversion formula of sound
pressure and sound intensity, ISPTP =P2
m/ρccan be obtained
[33], where the density and sound velocity are respectively
represented. The spatial peak pulse means sound intensity
ISPTA is the energy transmitted per unit area of a pulse divided
2
Int. J. Extrem. Manuf. 6(2024) 062001 Topical Review
Figure 1. Systematic outline of this review. We summarize the focusing techniques of piezoelectric ultrasound transducers and their
application.
by the pulse duration (T1in gure 2). Continuous wave is gen-
erally described by the mean value of spatial peak time sound
intensity ISPTA, which is numerically equal to P2
0/2ρc[33], P0
is the sound pressure amplitude.
2.2. Focused ultrasound
As discussed above, the distribution of sound intensity in space
and time is not uniform. In fact, the sound eld generated by
the non-focusing transducer can be divided into near eld and
far eld regions. The sound wave in the near eld region uctu-
ates greatly, and the sound intensity in the far eld region tends
to be stable [34]. The boundary between the two regions is the
natural focus of the non-focusing transducer. For the sound
eld generated by the focusing transducer, there are still far
eld and near eld regions, but the sound beam at the focus
becomes extremely narrow and the sound intensity becomes
extremely strong.
In the focused ultrasound system, a focal spot is often
formed at the focal point, which is due to the diffraction of the
sound wave. In most cases, the center of curvature of the trans-
ducer surface does not coincide with the focus of the wave. For
example, for a curved transducer, except for a spherical surface
or a rectangular cylinder, the geometric focus of other shapes
does not coincide with the wave focus. This results in the dif-
fraction of the sound wave, forming an axisymmetric solano-
shaped sound beam, known as the focal spot. The radius of the
focal spot can be expressed as: r0=1.61λf/R[35], where λ
is the wavelength, Ris the aperture radius of the transducer,
and fis the focal length. The focal spot radius determines the
3
Int. J. Extrem. Manuf. 6(2024) 062001 Topical Review
Figure 2. The curve of sound pressure squared (P2) of a pulse wave
with time.
lateral resolution of the system, and the smaller the focal spot
radius is, the higher the lateral resolution. Axis resolution is
dependent on frequency and bandwidth.
Another distribution characteristic of the focused sound
eld is the occurrence of a maximum value on the sound axis
and many submaximal around it [36]. The acoustic axis direc-
tion lobe is called the main lobe, and the two sides of the main
lobe are sidelobe. This is due to the interference of the dif-
fraction wave generated by the transducer at the output aper-
ture. By optimizing the structural design of the transducer, the
sound eld distribution of the main maximum and sidelobe
can be improved, and the applicability in different scenes can
be improved [37]. For example, in ultrasound ablation, it is
best to use an extremely narrow beam with as little sidelobe
as possible, while in general low-frequency ultrasound ima-
ging, no special focusing design is required in order to reduce
costs [38].
2.3. Ultrasonic biological effect
Since ultrasound is a wave form, it is possible to obtain
physiological and pathological information of the organism
using self-transmitting and receiving transducers, which is the
principle of ultrasonic diagnosis [39]. At the same time, ultra-
sound is a form of energy, when it spreads in the biological
body, it will cause changes in the function, structure and state
of the biological system, which is the ultrasonic biological
effect [40]. This section details several common ultrasonic
biological effects.
Thermal effects. When ultrasonic energy is converted into
heat energy in the organism, it is called the thermal effect.
Thermal effects can cause a rapid rise in local temperature
within an organism. The heat generated by a planar ultra-
sonic wave with an intensity of I(W cm−3)in a unit volume
of biological tissue is: Q=2αIt(J·cm−3)[41], where αis
the sound pressure absorption coefcient of the tissue, and
t(s) is the action time. Considering the contact between the
ultrasonic probe and the human tissue through the coupling
agent, when the ultrasonic radiation to the human tissue,
its acoustic absorption coefcient is related to the ultrasonic
frequency, which can be approximately expressed as: α=
0.026f1·1(cm−1). Assuming that the density of human tis-
sue is ρ=1.00 g ·cm−3, and the specic heat is Cm=4.14 J ·
cm−3·◦C−1, the temperature of the tissue after ultrasonic irra-
diation at t time is as follows [42]:
∆T=2×0.026
ρCm
Itf1·1=0.012Itf1·1(◦C).
The above formula shows that when the ultrasonic wave
with a frequency of 1 MHz and a sound intensity of 1 W cm−2
is irradiated into human tissue, the temperature will rise
at a rate of 0.012 ◦C·s−1. Therefore, in ultrasonic tumor
ablation, the ablation temperature can be controlled accord-
ing to ultrasonic frequency, sound intensity and irradiation
time.
Mechanical mechanism. When the heat generated by ultra-
sound is not enough to signicantly raise the temperature
of the tissue, the mechanical parameters of the sound eld
will play a major role, and this biological effect is the mech-
anical mechanism. A typical mechanical action is a radiant
force, which means that when ultrasound acts on an organ-
ism through a uid with a sound speed of c, it is equivalent to
thinking that a steady-state force acts on the object [43].
Cavitation effect. In a broad sense, cavitation effect refers
to the various forms of movement of bubbles caused by
ultrasound [44]. According to the different dynamic behavior
of bubbles, it can be divided into stable cavitation and transient
cavitation [45]. When there are bubbles in the liquid, the action
of ultrasound will cause the bubbles to enter a resonance state,
and the stable movement of the bubbles caused by ultrasound
is called stable cavitation. When the sound intensity is high,
the movement of the bubble becomes complex and violent, and
the empty core existing in the liquid rapidly expands, contracts
and collapses, which is called transient cavitation. Both types
of cavitation require bubbles, which, when the bubble size
is right, can produce stable cavitation at low pressure. When
the bubble is too small, it can only act as a cavitation core,
which is a higher sound intensity that can produce transient
cavitation [46].
The above biological effects are summarized in gure 3.
When the sound intensity is lower than 10−3W cm−2, no bio-
logical effect of ultrasound occurs. When the sound intensity
is lower than 1 W cm−2, the steady-state cavitation effect is
dominant. Until the sound is stronger than 1 W cm−2, the ultra-
sound will cause tissue damage. In the range of low intensity
and long irradiation time, the damage of issue is mainly caused
by thermal mechanism. In the range of extremely loud intens-
ity and short irradiation time, the damage mechanism is mainly
the transient cavitation mechanism. When the sound intens-
ity is in the middle range of 700–1 500 W cm−2, the damage
mainly comes from the mechanical mechanism.
4
Int. J. Extrem. Manuf. 6(2024) 062001 Topical Review
T > 60 °C
Figure 3. Biological effects of ultrasound at different intensity ranges. (a) ultrasonic echo. (b) stationary cavitation. (c) thermal effects. (d)
mechanical mechanism. (e) inertial cavitation.
3. Functional materials
Piezoelectric transducers are generally composed of three
parts: piezoelectric layer, backing layer and matching layer.
The piezoelectric layer is located in the middle, which plays
the role of electromechanical conversion. The backing layer is
located behind the piezoelectric layer, which mainly absorbs
ultrasonic waves and prevents the ringing effect caused by
excessive vibration of the piezoelectric material. The match-
ing layer is located in the front, which is used to achieve
acoustic impedance matching and reduce the reection of
ultrasonic waves at the interface [47]. Among them, piezo-
electric materials are indispensable [48]. In this section, we
describe in detail the materials used in the focused ultrasonic
transducer.
3.1. Piezoelectric materials
Piezoelectric materials are a key part of piezoelectric trans-
ducers, and piezoelectric transducers are the core of most
focused ultrasound systems, the selection of suitable piezo-
electric materials is very important for focused ultrasonic
transducers. As shown in gure 4, we present the comparison
of different piezoelectric materials in terms of electromech-
anical coupling coefcient, density, longitudinal piezoelectric
coefcient, Young’s modulus, acoustic impedance and com-
plex shape and matching capability. This section focuses on
the advantages and disadvantages of ve common piezoelec-
tric materials for focused ultrasound applications.
Piezoelectric ceramics. Piezoelectric ceramics are types of
polycrystalline materials that play a crucial role in the eld of
ultrasonic application [49,50]. Common piezoelectric ceram-
ics include PZT, potassium sodium niobate (KNN), BT among
others [51–54]. Zhen et al prepared a piezoelectric trans-
ducer with resonance frequency of 2 MHz using PZT-4 and
assembled it with ultrasonic lens into a focused ultrasonic sys-
tem. Su et al [55] used PZT to prepare a focusing system for
intravascular ultrasound imaging. Although the piezoelectric
properties of piezoelectric ceramics are very good, their mech-
anical strength is low. This limitation becomes particularly
evident when the material is ground to thicknesses compar-
able to the grain size, complicating the design of focused ultra-
sound systems [56]. This limitation has posed challenges to
their broader application in the development of focused ultra-
sound systems.
Piezoelectric single crystal. Quartz single crystal is the earli-
est piezoelectric single crystal discovered by human beings.
It has good mechanical properties and is easy to cut, grind
and polish. The single crystal commonly used in focused
ultrasonic transducers is lithium niobate (LiNbO3). Its piezo-
electric properties are very excellent, which has a great rela-
tionship with the cutting orientation and cutting orientation.
Typically, the LiNbO3for piezoelectric transducers is a 36◦
Y cut. Tiefensee et al [57] prepared a focused ultrasonic
probe with a piezoelectric layer thickness of only 7.1 µms
using LiNbO3. Through the focusing design, it is possible to
emit narrow beam sound waves that can be used to capture
5
Int. J. Extrem. Manuf. 6(2024) 062001 Topical Review
Figure 4. Comparison of the advantages and disadvantages of piezoelectric and physical properties of piezoelectric materials for focused
ultrasound applications.
individual tiny cells. Currently, fabricating LiNbO3layers of
several microns thick remains a challenging and complex task.
Piezoelectric polymer. Piezoelectric polymers represent a
relatively newer class of materials in the eld of piezo-
electricity. The earliest discovered piezoelectric polymer
is polyvinylidene uoride (PVDF) [58], which has the
advantages of stretchability, thermoplasticity and easy fabric-
ation. Although PVDF’s performance is notably inuenced
by temperature variations and its electromechanical coupling
coefcient is relatively low, the outstanding mechanical prop-
erties make it very suitable for making narrow-pulse ultrasonic
transducers with high resolution [59,60]. Furthermore, PVDF
offers potential applications in the development of wearable
focused ultrasound devices [61].
Piezoelectric composites. Piezoelectric composites are engin-
eered materials made by embedding piezoelectric ceramic
phases in a polymer matrix to enhance certain properties bene-
cial for specic applications. Common piezoelectric compos-
ites refer to ferroelectric ceramics and polymer matrix com-
posites, which can have high electromechanical coupling coef-
cient and low acoustic impedance at the same time [62].
This combination allows for efcient conversion of electrical
energy into mechanical energy (and vice versa), making them
highly effective for use in ultrasound imaging where clear,
high-resolution images are required. Xu et al [63–68] used
piezoelectric composites with a 1–3 connectivity pattern (one-
dimensional ceramic rods embedded in a three-dimensional
polymer matrix) in wearable ultrasound devices, enhancing
acoustic matching due to their lower impedance compared to
traditional materials. Although the exibility of piezoelectric
composites is better than that of piezoelectric ceramics and
single crystals, making them more suitable for integration into
wearable devices that conform to the human body, achieving
consistent material properties and performance across pro-
duction batches remains a challenge due to variability in the
embedding process [69].
Piezoelectric lm. The development of Microelectromechanical
Systems (MEMS) technology has spurred advancements in
piezoelectric materials, leading to innovations in thinness,
miniaturization, and complexity of structures. In 1997, Barrow
et al [70] prepared 60 µm PZT lms by the sol-gel method.
One team fabricated MEMS acoustic sensors and ultrasonic
transducers by sputtering PZT on silicon substrates using thin
lm deposition technology [71,72]. This work signicantly
enhances the sensors’ sensitivity and resolution. In addition
to conventional ferroelectric thin lms, ZnO [73] and AlN
[74], which are commonly used in semiconductors, can also
be used as active materials for focused ultrasound transducers.
While ZnO and AlN offer superior consistency and reliability
for semiconductor applications, their piezoelectric perform-
ance is not as strong as that of traditional PZT-based lms.
However, they remain valuable for focused ultrasound trans-
ducers where material properties can be optimized for specic
applications.
3.2. Backing layer
There are two main roles of the backing layer in the focused
ultrasound transducer. The rst function is to suppress the
excessive vibration of the piezoelectric material and achieve
short pulse waves. This process is vital for improving the axial
resolution of the transducer, enabling more precise imaging by
producing clearer images of structures along the beam’saxis.
The second role is to absorb ultrasonic waves to stop the con-
tinued backward propagation of sound waves from the piezo-
electric material. According to the above principle, the acous-
tic impedance of the backing layer needs to be signicantly
6
Int. J. Extrem. Manuf. 6(2024) 062001 Topical Review
different from that of the piezoelectric material to facilitate
the desired attenuation and reection characteristics. Ideally,
the acoustic impedance of the backing material should be care-
fully chosen to be either signicantly lower than the 35 MRayl
typical of piezoelectric ceramics [69], aiming for an imped-
ance mismatch that optimizes sound wave attenuation and
minimizes backward propagation. Typically, a material with
an acoustic impedance of 3 ∼5 MRayl is chosen to fabric-
ate the backing layer. Traditionally, the material commonly
used as a backing layer is a conductive silver epoxy resin [75].
If you want to obtain a backing layer with adjustable imped-
ance, you can choose epoxy resin doped tungsten powder to
obtain different acoustic impedance by changing the content
of tungsten [76]. With the development of transducers in the
direction of high frequency and high resolution, many other
materials have emerged. Nicolaides et al [77] studied the effect
of using materials such as aluminum, brass, and polyvinyl
chloride as backing layers on the performance of underwater
acoustic transducers. Finding different materials will result in
different resolutions.
3.3. Matching layer
Acoustic matching greatly increases the coupling between the
focusing transducer and the propagating medium and reduces
the energy loss. There are many theories on how to design
acoustic matching layers. The most widely used design prin-
ciple for acoustic matching layers is the 1/4 wavelength the-
ory. According to this theory, the optimal thickness of the
matching layer is one quarter of the ultrasound wavelength in
the material, allowing for maximal energy transmission into
the target medium by minimizing the reection at the inter-
face. For the focused ultrasound transducer, the matching layer
needs to be extremely thin and the acoustic impedance of the
matching layer needs to be well matched with the medium in
order to make the ultrasonic wave form a very narrow beam.
Therefore, a variety of active and passive materials have been
developed, such as nanocomposites [78–80], metamaterials
[81] and metasurfaces [82]. Fang et al [83] used anodized alu-
minum oxide doped with epoxy resin for acoustic matching.
The composite was able to establish a link between PZT with
an acoustic impedance of 35 MRayl and human skin with an
acoustic impedance of 1.5 MRayl. To obtain better perform-
ance, they fabricated the matching layer using a deep reactive
ion etching method. Tiefensee et al [57] fabricated a matching
layer with an acoustic impedance of 7 MRayl using a com-
posite material containing cerium oxide nanoparticles. This
choice was guided by the nanoparticles’ ability to ne-tune
the acoustic impedance of the layer, optimizing it for specic
medical ultrasound applications.
4. Manufacturing approaches for PFUT
In the focused ultrasonic system, the piezoelectric transducer
is the most important part. Unlike non-focusing transducers,
which emit waves that naturally spread out, focusing trans-
ducers employ physical or electronic methods to concentrate
the sound eld, thereby forming an extremely narrow beam for
enhanced precision and depth control. Single-element trans-
ducers, in contrast to array system, can only use physical
focusing. Both linear and planar arrays utilize electronic focus-
ing, which adjusts the phase of the signals emitted by each ele-
ment in the array to precisely shape and direct the sound beam.
Physical focusing methods include: surface focusing and addi-
tional acoustic lens. In this section, the content and develop-
ment status of these focusing methods are introduced in detail.
4.1. Curved surface focusing
Surface focusing involvesthe surface of a piezoelectric mater-
ial forming a curved shape, with the aim of directing ultrasonic
waves to a focal point, thereby enhancing the intensity and pre-
cision of the ultrasound beam. Common bending methods for
piezoelectric materials are summarized below.
One of the most commonly used methods is the ball com-
pression method, in which pressure is applied to the surface
of the piezoelectric material through a steel ball to create a
depression on the surface of the material. Figure 5(a) shows
the process of preparing the LiNbO3single crystal transducer
by ball pressing [84]. First, the piezoelectric single crystal or
ceramic is mechanically thinned to an ideal thickness. A layer
of Cr/Au electrodes is then sputtered on the surface, and the
backing material is cast on one side of the piezoelectric mater-
ial. Finally, the ball pressing process is pressed and focused
at 90 ◦C using highly polished chromium/steel ball bearings.
A special clamp is usually used to achieve pressure focus-
ing. This method is generally used for piezoelectric ceram-
ics and piezoelectric single crystals, especially after mechan-
ical thinning to tens of micrometres. The advantage of the ball
pressing method is that the process is simple, but due to the
brittle nature of ceramic and single crystal materials [85], it
often leads to the fragmentation of piezoelectric materials. To
address limitations of the ball pressing method, the mechanical
grinding technique was developed, utilizing a high-strength
object to gradually wearing down the piezoelectric material
until the desired curved shape is achieved. That is, a high-
strength object is used to grind the surface of the piezoelectric
material until a curved shape is formed [86,87]. For piezoelec-
tric ceramics and piezoelectric single crystals of tens of micro-
meters, this method has absolute advantages, but for ultra-thin
piezoelectric ceramics with thickness less than 10 micromet-
ers, this method will not be applicable.
Both of the above methods directly transform the surface of
the at piezoelectric material into a curved surface, and since
most of the piezoelectric materials are brittle and hard mater-
ials, the preparation process faces great risks, which may lead
to a signicant decline in the performance of the piezoelec-
tric material or even failure. A better scheme is to prepare a
backing layer with a curved surface structure [92]. Some stud-
ies have used aluminum (Al) [93] as a bending backing layer
because it is soft and easy to process into a focused structure.
In addition, aluminum has good electrical conductivity and
suitable acoustic impedance. Zhang et al [94] loaded P(VDF-
TrFE) lms on the surface of an ellipsoidal backing layer for
photoacoustic microscopy.
7
Int. J. Extrem. Manuf. 6(2024) 062001 Topical Review
Figure 5. Different focusing techniques for ultrasonic transducers. (a) The actual picture and concrete steps of surface PFUT preparation by
ball pressing method [84]. Reproduced from [84]. CC BY 4.0. (b) 3D printing curved piezoelectric ceramics [88]. Reproduced from [88].
CC BY 4.0. (c) Acoustic lens based on Fresnel principle [89]. Reproduced from [89]. CC BY 4.0. (d) Acoustic lens based on acoustic
metamaterials [90]. Reprinted from [90], with the permission of AIP Publishing. (e) 3D printed annular piezoelectric transducer [91].
Reproduced from [91]. CC BY 4.0.
Advances in additive manufacturing technology have
promoted the fabrication of curved piezoelectric ceramics
[95]. The use of 3D printing technology to fabricate
various geometries of piezoelectric ceramics has attracted
more and more attention. Due to the problems of poor
compactness and low piezoelectric coefcient of ceram-
ics produced by this technology, many studies are mak-
ing efforts to improve the processing process [96]. As
shown in gure 5(b), Lu et al [88] proposed a new addit-
ive manufacturing technology with an excellent sintering
process. They fabricated PZT elements with good per-
formance and corresponding matching layers and backing
layers.
4.2. Acoustic lens
Acoustic lenses are a form of passive focusing that can change
the spatiotemporal distribution of ultrasonic waves. Compared
with curved surface transducers, more complex sound elds
can be formed by means of acoustic lenses. When designing
acoustic lenses, various equivalent parameters of each lens
are usually determined according to the desired sound eld.
Common acoustic lenses include: curved lens [97], Fresnel
lens [89,98], metamaterial [99] and phonon crystal [90], etc.
Curved lens uses materials of different thicknesses to
focus sound waves [100] and is the simplest type of acous-
tic lens. The Fresnel lens is characterized by concentric cir-
cular sections, each designed to phase-shift the sound waves
in a manner that they constructively interfered at the focal
point. As shown in gure 5(c), Tang and Kim [89] used the
MEMS process to fabricate three kinds of Fresnier acoustic
lenses with air cavities, which are not only very thin, but also
able to reduce impedance mismatch and achieve good acous-
tic focusing. Acoustic metamaterials are materials with sub-
wavelength periodic articial structures, including local res-
onant structures [101], gradient index planes [102], Helmholtz
cavities [103], and others. Li et al [90] proposed a metasurface
lens for underwater focusing, but its integration with an ultra-
sonic transducer is difcult. Phononic crystals can also be used
for sound eld modulation. An example is the gradient index
phononic crystal, as shown in gure 5(d), adept at acoustic
focusing by manipulating wave paths through spatially vary-
ing material properties.
In fact, the role of acoustic lenses is not limited to ultra-
sound focusing, it can achieve more complex sound eld mod-
ulation. However, because the lens is often located between
the external medium and the piezoelectric material, it will
lead to impedance mismatch and cause problems such as
8
Int. J. Extrem. Manuf. 6(2024) 062001 Topical Review
low energy transmission efciency. Recently, the develop-
ment of 3D printing technology has promoted the applica-
tion of various shapes of acoustic lenses in focused ultrasound
transducers.
4.3. Phased array focusing
With the development of electronic technology and micro
and nano machining technology, phased array based focusing
transducer has appeared. Unlike single-element transducers,
which require mechanical scanning, phased array probes can
control the size and position of the focus using electronic
techniques [104,105]. In a phased array transducer, the size
of each array element is no larger than half of the wavelength
in the target medium, and several small elements are arranged
according to certain rules. By controlling the time at which
each element emits a spherical wave, a dynamically focused
sound eld is formed.
A common phased array is based on a ring array that is
capable of generating a focus that moves axially by apply-
ing an electrical signal to each ring piezoelectric element. The
earliest use of concentric ring arrays was in 1989 [106], and
Foster et al used 12 ring chips for focused ultrasound ima-
ging. Chen et al [107] used laser cutting to process 66µm lead
magnesium niobate (PMN-PT) single crystals to form a ring
array with a radius of 8 mm. Each element of the transducer
was connected to the pad by a lead bonding process. Snook
et al [108] developed ring arrays based on ne-grained lead
titanate ceramics for the crosstalk problem. In the above stud-
ies, laser cutting technology is used to process piezoelectric
materials. Compared with traditional cutting technology, laser
technology has less inuence on the properties of piezoelec-
tric materials [109]. Femtosecond laser and picosecond laser
technology, as ultra-short pulse technology, are widely used
in micro and nano processing technology [29,110]. In 2004,
Brown et al [111] proposed an alternative to mechanically cut-
ting by fabricating arrays by deposition of ring electrode pat-
terns onto the surface of piezoelectric materials. In this work,
the PZT sample is rst bonded to a glass plate and then mech-
anically thinned. The electrodes were then sputtered on the
PZT and patterned using a lithography process to form eight
separate elements. Finally, the copper-covered printed circuit
board is bonded to the PZT. Sammoura et al [112] designed
a multi-electrode ring PMUT with a PZT thickness of 2 µm
and a radius of 135 µm. The resulting device has an excel-
lent electromechanical coupling factor. Not only piezoelectric
ceramics and single crystals, but also piezoelectric polymers
have been used to fabricate ring arrays. Ketterling et al [113]
bonded annular PVDF on a polyimide lm covering a cop-
per electrode and lled the gap using epoxy resin. Gottlieb
et al [114] fabricated a high-frequency ring transducer using
P(VDF-TrFE) and a double-sided polyimide circuit. Since the
acoustic impedance of the polymer itself is low, no matching
layer is required. In recent reports [91], attempts have been
made to fabricate annular arrays using 3D printing and pro-
gress has been made (gure 5(e)).
A more complex array is a two-dimensional (2D) planar
array. It not only needs to consider the structure design of
the transducer, but also needs to consider the electronic con-
trol technology. Here only the device fabrication is discussed,
excluding the electronics. Conventional 2D arrays use cutting
techniques to determine the number of elements by varying
numbers of cuts. MEMS process-based arrays directly utilize
patterning processes, such as dry etching, wet etching, etc, to
form small components. Due to the complexity of 2D phased
array technology, it has not been widely used in the eld of
ultrasound therapy.
In recent years, exible ultrasonic transducers have
received extensive attention. The wearable ultrasonic trans-
ducer array can be worn directly on the skin, enabling real-time
and continuous blood pressure monitoring [66]. Due to the use
of elastic electrodes, the array is conformal with the skin. In
order to be widely used in medical applications, more research
needs to be done in electronic control, signal processing and
phased array algorithms. In addition, researchers have used
exible ultrasound arrays for neural regulation [115], and even
use it for brain stimulation in conjunction with brain-computer
interface (BCI) technology [116].
4.4. Multiple self-focusing
The multi-element self-focusing transducer makes a special
structure on the surface of the piezoelectric material, so that
each region of the piezoelectric material emits different phases
of ultrasonic waves, so as to realize the regulation of sound
waves [117]. This method is very similar to acoustic lenses,
especially through the special structural design of the material
to achieve focusing [118]. The difference is that the acous-
tic lens is a passive material and does not change the struc-
ture of the piezoelectric material itself, while the multivari-
ate self-focusing directly changes the piezoelectric mater-
ial. Therefore, although the multivariate self-focusing method
does not produce complex acoustic elds like acoustic lenses,
it can greatly reduce the energy loss caused by impedance mis-
match for focused ultrasound transducers.
The rst common self-focusing method is based on the
Fresnel half-band. Spaced rings are formed on the surface
of the piezoelectric material by micromachining techniques,
and the sound waves generated by each sound source are
delayed by an integer multiple of the wavelength, then inter-
ference occurs at the focal point [119]. The specic prepara-
tion process mainly includes: laser cutting, lling epoxy resin,
and sputtering electrode, making matching layer and back-
ing layer. Compared with the ring phased array, this method
requires only one wire to be connected to each ring separ-
ately, reducing the difculty caused by electron focusing. An
alternative approach to self-focusing is to fabricate piezoelec-
tric materials with acoustic metas faces. Acoustic metamater-
ial can be designed to modulate the phase and amplitude of the
acoustic wave by designing the structure of different units to
enable acoustic focusing. For example, Li et al [120] fabric-
ated an active gradient metamaterial that achieves broadband
self-focusing of ultrasonic waves under water.
Another self-focusing method is to use type 1–3 piezoelec-
tric composites. In the type 1–3 piezoelectric composite mater-
ial, the piezoelectric material is connected in 1 dimension and
9
Int. J. Extrem. Manuf. 6(2024) 062001 Topical Review
the polymer is connected in 3 dimensions. With the help of the
curved surface forming process, each piezoelectric column can
be located in different planes and emit different phase sound
waves. The rst type of focusing transducer is made of 1–3
type piezoelectric composite material with uneven thickness
[121]. The traditional 1–3 piezoelectric composites can form
many longitudinal vibrating piezoelectric columns by cutting
and lling process. Further, through mechanical grinding, a
surface of uneven thickness can be formed to improve focusing
performance. The second type of focused transducers made of
type 1–3 piezoelectric composites is based on exible elec-
tronic technology [122]. The composite structure of elastic
material and piezoelectric ceramics is prepared to make the
whole device exible.
5. Applications of PFUT
The biomedical applications of focused ultrasonic transducers
can be divided into two broad categories. The rst type is used
in ultrasonic medical diagnosis, ultrasonic solid detection and
other elds. In this kind of focused ultrasound system, the
intensity of the ultrasonic wave is not large, and the focusing
technology is mainly used to improve the sensitivity and res-
olution. The second type of application is to focus ultrasound
energy to a sufcient intensity to produce corresponding bio-
logical effects, such as ultrasonic tumor ablation, ultrasonic
nerve stimulation, ultrasonic acoustic tweezers, etc. Table 1
summarizes several properties of the focused ultrasound trans-
ducers in biomedical applications. This section mainly intro-
duces the development status of focused ultrasound in these
elds, including process progress and application direction.
5.1. Ultrasonic imaging
With the development of piezoelectric materials and prepar-
ation technology, piezoelectric ultrasonic transducers have
been widely used in medical diagnostic imaging. Generally
speaking, he image quality of ultrasonic imaging is mainly
affected by the spatial resolution and sensitivity of the
transducer [48]. Spatial resolution determines the degree of
differentiation between the imaging target and other objects,
and the higher the center frequency and bandwidth of the trans-
ducer are, the better the axial resolution. The transverse resol-
ution is determined by the focusing parameters of the trans-
ducer, which can be expressed as the ratio of the focal length
of the transducer to the aperture size. Sensitivity is a phys-
ical quantity that represents the efciency of electroacous-
tic energy conversion. High sensitivity means that the trans-
ducer can concentrate more emission energy at the focal
point, thereby improving the contrast of the ultrasonic image
and producing a brighter target image. When the frequency
is higher than 20 MHz, focusing design is often required
to improve imaging resolution and sensitivity due to energy
attenuation. In addition, in photoacoustic imaging technology,
the acoustic focusing technology of piezoelectric transducers
can provide high reception sensitivity to the photoacoustic sig-
nals generated in the target region.
In medical imaging, intravascular ultrasound (IVUS) ima-
ging is an important tool in the diagnosis of cardiovascular
diseases. IVUS imaging is to obtain information such as vessel
wall thickness and lesion location by forming high-resolution
ultrasound images in arteries with the help of tiny ultrasonic
transducers. Ultrasound plays a pivotal role in assessing blood
ow and detecting abnormalities such as blockages, clots, and
aneurysms within the vascular system. The center frequency
of the general medical IVUS imaging transducer is between
20 and 60 MHz. Further extending its application, endoscopic
ultrasound combines the advantages of endoscopy and ultra-
sound, allowing for high-resolution images of the digestive
tract and its adjacent structures. This technique is particularly
useful in evaluating submucosal lesions, staging cancer, and
guiding biopsies, offering a level of detail that surpasses tra-
ditional imaging methods. Additionally, ultrasound imaging is
integral in obstetrics for monitoring fetal development, in mus-
culoskeletal assessments to visualize ligaments and tendons,
and in guiding needle placement during biopsies or injec-
tions. Advances in ultrasound technology, such as 3D imaging
and focused ultrasound for therapeutic purposes, continue to
expand its applications, underscoring its vital role in the dia-
gnosis, treatment, and management of various medical condi-
tions. The following introduces some specic work in recent
years.
Unit transducers usually use a curved surface structure to
focus. Yoon et al [128] proposed an angle-focused single crys-
tal probe to address the problem of small lumen area dur-
ing intravascular obstruction (gure 6(a)). The transducer con-
sists of a curved single crystal material combined with an
aperture with a 60◦inclination angle, a center frequency of
45 MHz, and an axial and transverse resolution of 25 and
120 µm, respectively. It can achieve high resolution imaging
of intravascular narrow areas. Fei et al [129] prepared an IVUS
transducer with a single-sided concave structure by ball pres-
sure method. Compared to planar transducers, half-concave
transducer has a higher center frequency (35 MHz), wider
bandwidth (54%), and higher resolution (34.5 µm axial resol-
ution, 392 µm transverse resolution). The above studies have
all used piezoelectric single crystal PMN-PT due to its out-
standing piezoelectric properties and mature preparation pro-
cess. However, considering its brittleness, other piezoelectric
materials are also used, such as PZT [123], KNN [84], LiNbO3
[130], PVDF [131], ZnO [132] and so on. Lee et al [130]
developed a dual-frequency ultrasonic transducer prepared by
a 50 µm thick PZT and 28 µm thick LiNbO3, operating at
35 MHz and 105 MHz, respectively, as shown in gure 6(b).
The simulation results show that the geometric focal length
produced by the spherical pressure method is shorter than the
natural focal depth, and the DOF regions of the two trans-
ducers overlap to work together. As an important supplement
to ultrasound imaging, photoacoustic imaging has attracted
much attention due to its combination of high sensitivity of
optical imaging and low attenuation of acoustic imaging [16].
Focusing technique plays an important role in photoacoustic
transducers because it can improve the receiving sensitivity
of piezoelectric transducers at the focal point. Nguyen et al
[133] fabricated a photoacoustic imaging system based on a
10
Int. J. Extrem. Manuf. 6(2024) 062001 Topical Review
Table 1. Summary of several properties of focused ultrasound transducers in biomedical applications.
Application Focusing technique
Piezoelectric
materials
Center
frequency/MHz
Spatial Res./mm
Lateral Axial
Intravascular ultrasound imaging Curved surface focusing PMN-PT 35 2.3 5.5 [123]
Acoustic lens PZT-5H 52.5 0.046 8 0.183 [56]
Integration of ultrasound imaging
and therapy
Phased array focusing PZT-5H 1 1.9 12.6 [124]
Ultrasonic tumor ablation Spherical transducer — 0.43 2.2 2.7 [38]
Ultrasonic neuroregulation Phased array focusing PZT 8.4 0.215 1.68 [125]
Phased array focusing PZT 0.215 0.215 1.68 [126]
Cell manipulation Curved surface focusing LNO 30 0.03 0.2 [127]
multifocal piezoelectric ultrasound transducer. The designed
multi-spherical PVDF transducer has seven focal points, and
the depth of eld (from 0.4 to 10 mm) is extended by the super-
position of focal elds (gure 6(c)). Fang et al [134] developed
a transparent photoacoustic transducer for optical microscopy,
made of PVDF laminated on a concave glass sheet. The exper-
imental results show that the transducer has a center frequency
of 24 MHz, a focal length of 1.3 mm, a focal depth of 1.6 mm,
and 60% optical transmittance. Zhang et al [94] fabricated an
ellipsoidal transducer using a piezoelectric polymer, which,
in contrast to the spherical shape, has a continuous multi-
focal and is able to extend the depth of eld. To fabricate
the transducer, polyvinylidene diuoride triuoroethylene P
(VDF-TrFE) was pressed onto the surface of a bending die to
achieve self-focusing.
Most high-frequency ultrasound transducers are too small
and the lenses are not easy to fabricate, so improving the
imaging resolution through acoustic lenses is not optimal.
But efforts have been made. Su et al [55] stacked Fresnel
band plates onto the surface of a single-plane transducer, and
the sound waves were diffused and converged into the focal
region, both increasing the intensity of the sound waves and
reducing the focal size. Park et al [135] developed an integ-
rated imaging system by fusing four techniques: ultrasound
imaging, photoacoustic imaging, optical coherence tomo-
graphy, and uorescence imaging (gure 6(d)). The spherical
acoustic lens is placed in the front to improve the lateral res-
olution and image contrast.
Currently, there is increasing interest in array transducers
in the eld of ultrasound imaging because they can improve
imaging performance using electronic focusing. The develop-
ment of semiconductor processes has promoted the application
of MEMS technology in piezoelectric transducers. But limited
by cost and yield, there is still a long way to go.
5.2. Focused ultrasound ablation
Unlike ultrasonic diagnosis, which uses transducers to receive
ultrasonic signals reected by human tissues for imaging,
ultrasonic therapy uses various biological effects of ultra-
sound on human tissues to treat focal areas. Therefore, the
transducers used for ultrasound therapy need to focus the
sound waves in a small area, so that the energy can be applied
to the body’s lesions with the least possible damage to the
surrounding tissue. HIFU was rst proposed in 1932, when
Ter Haar and Coussios found that focused ultrasound could
heat tissue [136]. In 1942, Lynn et al [137] pointed out that
focused ultrasound could cause drastic changes in energy at
the focal point without damaging the tissue in the beam path.
So far, HIFU transducers have been successfully used in the
treatment of various tumors, such as liver tumors [138], breast
cancer [139], brain cancer [140], uterine broids [141] and
bone cancer [142].
In tumor ablation, ultrasonic waves emitted by the trans-
ducer pass through the skin and human tissues, accompanied
by energy attenuation, and eventually reach the lesions in the
human body. Through the focusing design of the transducer,
the acoustic beam can form a high-energy focus at the lesion
and use the biological effect with the tissue to achieve the
destruction of the disease cells. Generally speaking, the max-
imum diameter of the lesion area is 3 ∼4 cm, and the bound-
ary width cannot exceed 50 µm to ensure that the surrounding
tissue is not destroyed. In order to control the focused sound
beam accurately, efforts have been made in theoretical sim-
ulation and structural design [143–147]. Choi and Roh [36]
designed a novel toroidal concave transducer through theor-
etical simulation (gure 7(a)). The transducer can effectively
focus the ultrasound beam while suppressing the sidelobe and
avoiding damage to the surrounding tissue. Zhang et al [148]
numerically investigated the phased array algorithm of a ring
array transducer to create a more precise focus for the treat-
ment of breast cancer (gure 7(b)). Li et al [38] designed a
spherical cavity transducer with two ends open. 64 concave
piezoelectric ceramics emit 430 kHz ultrasonic waves and
form a focused sound eld after many reections. Figure 7(c)
shows the regular sound eld generated by the spherical cav-
ity transducer. Liu and Ren [37] designed an acoustic focus-
ing lens with a periodic groove structure. Using this cor-
rugated lens can reduce the relative sidelobe amplitude by
about 3 dB.
Due to the small scope of each ultrasound examination,
ultrasound tumor ablation still faces the problem of long
11
Int. J. Extrem. Manuf. 6(2024) 062001 Topical Review
Figure 6. A focused transducer for ultrasound imaging. (a) Angle-focused single-crystal probe for intravascular imaging [128]. Reprinted
from [128], Copyright © 2015 Elsevier B.V. All rights reserved. (b) Dual-frequency focused ultrasound transducer [130]. Reproduced from
[130]. CC BY 4.0. (c) Multifocal piezoelectric ultrasonic transducer [133]. Reproduced from [133]. CC BY 4.0. (d) Four-mode ultrasound
imaging system [135]. Reproduced from [135]. CC BY 4.0.
surgical time. In order to enlarge the ablation area, a com-
mon method is to increase the focus area with a dual-frequency
focusing transducer. As shown in gure 7(d), Jeong et al [149]
prepared an ultrasonic probe consisting of two frequencies
of 4.1 and 2.7 MHz. Through sound eld and temperature
eld tests, it was proved that the transducer could increase
the range of axial ablation and greatly shorten the treatment
time. Ma et al [150] designed and prepared a dual-frequency
focused ultrasonic transducer with the same focal length of
15 mm at both 3 and 1.5 MHz operating frequencies. The tis-
sue ablation experiment observed that dual-frequency ultra-
sound could produce higher ablation temperature. Park et al
[151] combined two PZTS with opposite polarization direc-
tions and different thicknesses, and by lling epoxy resin, pre-
pared focusing transducers with two frequencies, with a cen-
ter frequency of 1.2 MHz and a second harmonic frequency of
2.4 MHz.
One of the key advantages of HIFU therapy is its non-
invasive nature, signicantly reducing the risk of complica-
tions associated with traditional surgical interventions, such as
infections and prolonged recovery times. Additionally, HIFU
offers a therapeutic option for patients who may not be can-
didates for surgery due to various reasons, including the loca-
tion of the tumor or underlying health conditions. As research
and technology in the eld of focused ultrasound continue to
advance, the scope of HIFU therapy’s applications is expected
to widen, further establishing its role as a vital tool in modern,
minimally invasive medical treatments. In short, since HIFU
transducers usually require higher sound intensity for use in
biological tissues, it still needs to be improved considering the
issue of safety.
5.3. Ultrasonic neuroregulation
Neuroregulation is a new technology that transfers energy into
the body to regulate the nervous system, and it has a wide
range of applications in human brain, ophthalmology and other
diseases. LIFU is a non-invasive mode of nerve stimulation.
Compared with transcranial magnetic stimulation, transcra-
nial direct electrical stimulation, and transcranial alternating
12
Int. J. Extrem. Manuf. 6(2024) 062001 Topical Review
Figure 7. A focused transducer for ultrasound ablation. (a) Theoretical model of an annular focusing transducer [36]. Reprinted from [36],©
2017 Elsevier B.V. All rights reserved. (b) An annular ultrasound transducer for breast cancer treatment [148]. Reprinted from [148],© 2021
Elsevier B.V. All rights reserved. (c) A spherical cavity transducer with a periodic structure with reduced side lobe [38]. Reprinted from
[38], with the permission of AIP Publishing. (d) A dual-frequency ultrasound transducer to extend the ablation range [149]. Reprinted from
[149],© 2010 World Federation for Ultrasound in Medicine & Biology. Published by Elsevier Inc. All rights reserved.
current stimulation [152], LIFU has a higher spatial resolu-
tion at the millimetre scale, especially in the case of penetrat-
ing the skull and deep tissues, ultrasound has great applica-
tion potential. LIFU has shown promise in preliminary studies
for the treatment of conditions such as depression, obsessive-
compulsive disorder, and chronic pain, as well as in enhan-
cing cognitive functions and potentially treating neurodegen-
erative diseases like Alzheimer’s. Moreover, the specicity
of LIFU allows for targeting deep brain structures with min-
imal impact on overlying tissues, offering a level of preci-
sion that is difcult to achieve with other non-invasive mod-
alities. Early LIFU transducers used geometric focusing. Fry
et al [153] rst demonstrated in 1950 that ultrasound modu-
lated neural activity. They utilized a quartz crystal with a dia-
meter of one inch (resonant frequency 980 kHz) and reported
a temporary suppression of spontaneous activity after ultra-
sound was transmitted through the ventral nerve cords of the
craysh. In 1958, they also presented results of in vivo ultra-
sound neuroregulation based on visual pathways [154]. This
study reports that ultrasound stimulation of the lateral gen-
iculate nucleus of cats for 20 ∼120 s was able to signi-
cantly inhibit photo evoked potentials. This inhibition was
fully restored after 30 min. In 2013, Defeux et al [155]
used a single crystal ultrasonic transducer with a diameter of
64 mm to modulate the visual motor behaviour of monkeys, as
shown in gure 8(a), which was capable of generating a cigar-
shaped focus of 5 mm ×5 mm ×33 mm. Legon et al [156]
found in his study that 0.5 MHz focused ultrasound could pass
through the human skull and produce a beam prole with a
lateral resolution of 4.9 mm and an axial resolution of 18 mm.
13
Int. J. Extrem. Manuf. 6(2024) 062001 Topical Review
ch.1 ch.2
(US on)
ch.3 ch.4 ch.5
ch.1 ch.2 (US on)
ch.3ch.4 ch.5
A′ A
A
Figure 8. Focusing transducer for ultrasound neuromodulation. (a) A single-crystal focusing transducer for modulation of the visual nerve
in a monkey [155]. Reprinted from [155],© 2013 Elsevier Ltd. Published by Elsevier Inc. (b) Compare the sound eld characteristics of the
focused and unfocused transducers [157]. Reproduced from [157]. CC BY 4.0. (c) Proof of ultrasound stimulation of the retina [158].
Reproduced from [158]. CC BY 4.0. (d) MEMS ultrasonic stimulation system [159] Reproduced from [159]. CC BY 4.0. (e) Annular
phased array for retinal prosthesis [160]. Reproduced from [160]. CC BY 4.0.
Mehi´
cet al [157] studied the acoustic eld distribution char-
acteristics of single-crystal focused and non-focused trans-
ducers (gure 8(b)), and improved the anatomic specicity of
neuroregulation by modulated focused ultrasound.
In order to realize the application of ultrasonic neuroreg-
ulation in broader neuroscience research, more experimental
exploration has been carried out. Zheng and his team focused
on transcranial wearable ultrasound stimulators. They pre-
pared a bending transducer [161], heated PZT-5H/epoxy resin
piezoelectric composite material in an oven to 65 ◦C, and
focused the transducer through ball pressure. This wear-
able ultrasound device can improve the movement ability of
Parkinson’s patients. Zheng and his team also developed a
head-mounted ultrasonic transducer consisting of a disk piezo-
electric ceramic and a focusing lens to induce neuroregulation
in awake and freely moving mice [162]. Liu et al [163] con-
nected rigid PZT islands with exible Bridges to make them
stretchable and more suitable for complex surfaces. Most stud-
ies have focused on single targets, and phased array trans-
ducers can be used when neural regulation of multiple tar-
gets is required [164,165]. Tipsawat et al [126] fabricated a
32-element phased array PMUT using a MEMS process and
demonstrated how to achieve ultrasonic focusing and steer-
ing. Costa [166] has prepared 26 ×26 2D phased array trans-
ducers with 2D arrays of PZT piezoelectric sensors microma-
chined directly on top of complementary metal oxide semi-
conductor (CMOS) integrated circuits. Lee et al [159] target-
ing the compatibility of PMUT ultrasonic transducers with
cell experiments, proposed a new MEMS ultrasonic stimu-
lation system for modulating neurons or brain slices at high
resolution (gure 8(d)). In the future, ultrasonic neuroregu-
lation will continue to develop into arrays to achieve more
complex application needs. Although the acoustic lens will
cause a certain amount of energy loss, with the help of the
acoustic lens, a diversied ultrasonic eld can be formed in
the skull. Hu et al [167] developed a binary acoustic surface
lens to achieve both the dual function of correcting cranial-
induced beam aberrations and achieving dynamic focusing.
Maimbourg et al [168,169] for the rst time coupled a
single crystal transducer to a thick-controlled acoustic lens,
enabling adaptive transcranial focusing by adjusting the thick-
ness of the crystal. Jimenez–Gambin and his team reported
14
Int. J. Extrem. Manuf. 6(2024) 062001 Topical Review
Table 2. Summary of typical low-intensity focused ultrasound transducers.
Device type
Center freq.
kHz−1Focal distance
F mm−1
Spatial res. mm−1Peak pressure
MPa−1References.
Lateral Axial
Single crystal transducer (Dia =10 cm) 690 6 2.3 5.5 0.38 [177]
Single crystal transducer (Dia =6.4 cm) 320 10 5 33 0.6 [155]
Single crystal transducer (Dia =3 cm) 500 30 4.9 18 2.5 [156]
Type 1–3 piezoelectric composites 800 7 2.2 — 0.151 [161]
Single crystal probe with acoustic lens 2000 7 — — 1.2 [162]
32 element PMUT phased array 1400 20 1 9.2 0.44 [165]
Integrated 2D phased array transmitter in CMOS 8400 5 0.215 1.68 0.1 [166]
that 3D printed acoustic hololens can generate a wide vari-
ety of ultrasonic elds within the skull, such as bifocal beams,
self-bending beams, volumetric focused beams, and ultrasonic
vortices [170,171].
In addition, LIFU has been widely used in retinal stimula-
tion as an alternative to electrode stimulators. In 2012, Naor
et al [172] rst proposed an ultrasonic retinal prosthesis con-
sisting of a multi-element phased array focusing transducer
and a camera with an image processor that enables multi-
focal stimulation. In practical tests, acoustic coupling com-
ponents are connected to the cornea to project complex acous-
tic images onto the retina. A year later, Menz et al [173]
used a higher acoustic frequency (43 MHz) to demonstrate a
stable response induced by ultrasonic stimulation in the ret-
ina of isolated salamanders. Interestingly, ultrasonic stimu-
lation was found not to activate ganglion cells directly, but
interneurons outside the photoreceptors. In a separate study
[174], they further explored the physical mechanisms of ultra-
sonic retinal stimulation, which has three effects that can activ-
ate neurons: cavitation, acoustic radiant force, and thermal
effects. They applied ultrasonic stimulation with a central fre-
quency range of 0.5–43 MHz to the retinas of isolated sala-
manders, demonstrating that the principle of ultrasonic ret-
inal stimulation is radiant force based on changes in neur-
onal strength thresholds. Zheng et al [160,175,176] carried
out a series of studies on the design of the ring transducer,
as shown in gure 8(e). By simulating the sound eld and
optimizing the number of chips, they determined 512 ele-
ments as the best design considering the feasibility of the
process.
Table 2summarizes some typical LIFU transducers and
their associated characteristics, including center frequency,
focal length, resolution and sound intensity. 2D phased array
transducers integrated with CMOS have advantages in per-
formance. In the future, LIFU transducers will be combined
with BCI technology to play an important role in neural reg-
ulation. In addition, ultrasound stimulation of retina is also a
worthy research direction [158].
5.4. Ultrasonic energy transfer
Wireless power transmission plays an important role in
implantable devices and nerve electrical stimulation [178,
179]. Traditional inductive coupling technology cannot meet
the development needs of implantable devices because of ser-
ious energy attenuation and poor safety [180]. Ultrasonic-
induced wireless energy transmission (UWET) is a new tech-
nology emerging in recent years. In UWET technology, mech-
anical energy carried by ultrasound is transmitted from the
transmitting transducer to the receiving transducer to power
other devices. The receiving transducer is generally placed at
the focal point of the transmitting transducer to maximize the
energy transfer efciency. Because of its long transmission
distance and safety for the human body, UWET is widely used
in the implantable device [181]. Therefore, this section will
introduce the latest progress of focusing ultrasonic transducers
in wireless energy transmission systems.
In the past 20 years, ultrasonic wireless energy transmis-
sion technology has developed rapidly. Back in 2007, Maleki
et al proposed an energy collector based on ZnO nanowire, in
which they connected multiple collectors in parallel and series
to increase the output current and voltage [181]. Subsequently,
researchers have proposed ultrasonic energy transfer devices
based on other piezoelectric materials, such as PZT, PMN-
PT, PVDF, and 1–3 piezoelectric composites [182–184]. In
order to improve energy transmission efciency, focused ultra-
sonic transducers are generally used as the energy emission
part. Feng et al developed an ultrasonic transmission and com-
munication system based on exible electronics technology.
For the energy emission transducer, they specially designed
a focusing ultrasonic transducer with a plum structure, as
shown in gure 9(a), which can change the longitudinal posi-
tion of the focus by adjusting the curvature of the base [185].
Compared with conventional planar transducers, the trans-
ducer developed by the team has better exibility and lower
energy attenuation [186]. Jiang et al made use of circular PZT
to prepare a curved structure piezoelectric ultrasonic trans-
ducer with self-focusing function, as shown in gure 9(b). In
this work, they combined acousto-optic converters with piezo-
electric transducers to establish a two-mode ultrasonic energy
transmission system. The test results show that the system can
produce milliwatt energy and is capable of high resolution sig-
nal communication (SNR to 22.5 dB) under 12 mm thick pig
tissue [187].
In theory, the focusing transducer can improve the energy
output by enhancing the sound intensity at the focal point, but
considering the safety of ultrasound in the human body, the
sound intensity must be less than the threshold. Another way
15
Int. J. Extrem. Manuf. 6(2024) 062001 Topical Review
Figure 9. Focusing transducers for ultrasonic wireless energy transfer systems. (a) Ultrasonic transmitter with plum structure [185].
Reproduced from [185]. CC BY 4.0. (b) Photoacoustic and piezoelectric dual-modal ultrasonic energy delivery system [187]. Reproduced
from [187] with permission from the Royal Society of Chemistry. (c) Piezoelectric transducer based on bionic wood structure [188].
Reproduced from [188] with permission from the Royal Society of Chemistry. (d) The rst use of triboelectric sensors to receive ultrasonic
energy [189]. From [189]. Reprinted with permission from AAAS.
to boost output is to improve the performance of the receiv-
ing transducer. For example, Hong et al focused on improving
the acoustic impedance and electromechanical coupling coef-
cient of the ultrasonic transducer, as shown in gure 9(c).
Inspired by the bionic wood structure, they prepared a new
type 1–3 piezoelectric composite material, which was success-
fully used in the wireless energy transmission system across
muscles [188]. In fact, most of the research work on wireless
ultrasonic energy transfer is based on piezoelectric materials
[190–193]. It was not until 2019 that Kim et al rst pro-
posed the use of triboelectric sensors for ultrasonic energy
harvesting. As gure 9(d) shows, They built an ultrasonic
wireless transmission system that emits ultrasonic waves from
piezoelectric ultrasonic probes located outside the body, while
implantable triboelectric sensors integrated with exible prin-
ted circuits collect the energy [189]. In 2022, Liu et al fur-
ther improved the energy transfer efciency by improving the
structure of the triboelectric sensor [194].
In recent years, the research interest in combining wireless
ultrasonic energy transmission systems with electrodes has
been growing rapidly [195,196]. The electrical signal output
of the ultrasonic transducer is applied to the electrode by the
rectier to achieve retinal nerve stimulation and deep brain
stimulation. Zhang et al [195] developed an energy harvest-
ing device that connects a single piezoelectric crystal with a
thickness of 350 µms to a stretchable electrode to form a 6 ×6
microarray. Using a focused transducer probe to apply 1 MHz
ultrasound, the electrodes will output electrical signals to stim-
ulate the brain and achieve analgesic management [195]. Jiang
et al used a wireless ultrasonic energy delivery system for ret-
inal electrical stimulation. Powering the electrodes through
external ultrasonic stimulation could facilitate the treatment
of retinal degenerative diseases [197,198]. This work devel-
ops a new path for the future application of focused ultra-
sound transducers in eye diseases. In the future, with the rapid
development of transparent piezoelectric materials, emission-
type transducers located outside the eyeball can be made into
glasses to provide continuous energy for implanted electrodes.
In summary, the focusing performance of transmitter trans-
ducers will affect the application of wireless energy trans-
mission systems in implantable devices. Especially in some
tiny human organs, the location of the focal point must be
16
Int. J. Extrem. Manuf. 6(2024) 062001 Topical Review
small enough to guarantee the safety of the surrounding tis-
sue, which is often not satised by traditional non-focusing
transducers.
5.5. Ultrasonic tweezers
Sound waves can generate a large acoustic radiation force
and be used to manipulate small substances such as particles
and cells [199]. It has many applications in biomechanics,
chemistry, medicine and other elds. Compared with optical
and magnetic tweezers, acoustic tweezers have the advant-
ages of non-invasive and wide application. A common acous-
tic tweezer is the single-beam acoustic tweezer, which is gen-
erally made of curved piezoelectric material. The particles are
located at the focus of the transducer and are affected by acous-
tic radiation force to change their shape or movement.
Ultrasonic acoustic tweezers can play a role in cell mechan-
ics, such as assessing the mechanical properties of cells. Shung
et al [200,201] fabricated two acoustic tweezers with a cen-
ter frequency of 50 and 30 MHz, respectively, using LNO of
different thicknesses (gure 10(a)). The transverse and axial
resolution of the transducer are increased to 84 and 49 µm,
respectively, by the hot pressing process. As shown in the
mechanical properties of the cell can be obtained by studying
the relationship between the sound pressure and the deform-
ability of the cell when it is trapped at the focal point. Another
application is to capture the cell and make it move following
the moving trajectory of the transducer [202,203]. Jiang et al
[127] fabricated a high-performance curved surface type 1–3
piezoelectric composite with a center frequency of 23.4 MHz.
And as seen in gure 10(b), when the acoustic tweezers move
at a lower speed, the trapped particles also follow the focus of
the acoustic tweezers.
In order to achieve higher frequencies, piezoelectric thin
lm technology has been valued. Chen et al [204] used a mech-
anical thinning process to polish LNO crystals, which can
achieve thicknesses as low as 6 µms and center frequencies
as high as 300 MHz (gure 10(c)). The relationship between
excitation frequency and control particle size is investigated.
It is found that particles of different sizes can be captured
by changing the wavelength of incident wave. Fei et al [205]
used lithography and etching techniques to prepare silicon-
based acoustic lenses that can focus UHF acoustic beams
emitted by ZnO transducers (gure 10(d)). In cell manip-
ulation experiments, the system can stably clamp 5-micron
microspheres. Fei et al [205] fabricated ultrasonic tweezers
with ultra-high center frequency (230 MHz) and ultra-narrow
acoustic microbeam (8.2 µm) by sputtering 10 µm Al0.82Sc0.18
onto a pre-focused metal substrate, as shown in gure 10(e).
The results of acoustic capture experiments showed that the
device was able to capture polystyrene microspheres and epi-
dermoid cancer cells under non-contact conditions.
Ultrasonic tweezers represent a groundbreaking advance-
ment in the eld of biomedical engineering. Unlike traditional
mechanical tweezers, ultrasonic tweezers can handle delic-
ate biological structures without direct physical contact, thus
minimizing the risk of damage and contamination. This fea-
ture is particularly advantageous for applications requiring
sterile conditions and gentle handling, such as in the sort-
ing and assembly of cellular constructs, tissue engineering,
and regenerative medicine. Furthermore, in the realm of tar-
geted drug delivery, ultrasonic tweezers can guide micro-
carriers loaded with therapeutic agents directly to specic
sites within the body, enhancing the efcacy and reducing
the side effects of treatments. Ultrasonic tweezers also nd
applications in the precise manipulation and study of single
cells, enabling researchers to investigate cellular responses
and interactions under controlled conditions. This contributes
to a deeper understanding of cellular mechanics, embryonic
development, and disease progression at the cellular level.
As research progresses, the potential medical applications of
ultrasonic tweezers continue to expand, promising revolution-
ary advancements in cell therapy, regenerative medicine, and
drug delivery. Their ability to manipulate matter at the micro-
scale with high precision and minimal invasiveness positions
ultrasonic tweezers as a key tool in the future of biomedical
research and therapeutic interventions.
Indeed, a predominant trend in the development of ultra-
sonic tweezers involves the utilization of single crystal mater-
ials for their construction. Within the realm of biomedical
applications, these advanced tweezers have emerged as indis-
pensable assets for the manipulation and sorting of cells with
unparalleled precision. Leveraging focused ultrasonic trans-
ducers, these tweezers offer a rened control scheme, enhan-
cing their efcacy in various biomedical contexts. Looking
ahead, the ongoing optimization of acoustic parameters stands
as a key challenge, essential for expanding the scope of
applications while concurrently minimizing the risk of tissue
damage. This concerted effort toward renement promises to
unlock the full potential of ultrasonic tweezers, enabling their
widespread adoption and transformative impact across diverse
biomedical domains.
6. Conclusions and outlook
In the past few decades, advances in piezoelectric materials
and their processing and manufacturing techniques have pro-
moted the development of focused piezoelectric transducers
[206]. Ultrasound transducers with focusing technology have
made great breakthroughs in the elds of medical imaging,
wireless energy transfer, and neuromodulation. Figure 11
gives a summary of the development of focused ultrasound
since 1930. The focused transducer has a higher resolution in
the high-frequency range than the unfocused ultrasound trans-
ducer, and is able to focus the energy over a small range. This
advance has given focused ultrasound an important place in the
eld of medical imaging and therapy. However, many prob-
lems need to be solved to realize the clinical application of
focusing transducers. In the following, prospects are presen-
ted from the aspects of materials, preparation, and application
safety.
(1) Material innovation: the selection of piezoelectric materi-
als plays an important role in the performance and design
of focused ultrasound systems, and new piezoelectric
17
Int. J. Extrem. Manuf. 6(2024) 062001 Topical Review
Figure 10. Focusing transducers for ultrasonic wireless energy transfer systems. (a) Dual-frequency ultrasonic tweezers [201]. Reproduced
from [201]. CC BY 4.0. (b) Acoustic tweezers based on type 1–3 piezoelectric composite materials [127]. Reprinted with permission from
[127]. Copyright (2022) American Chemical Society. (c) 300 MHz acoustic tweezers based on LNO single crystal [204] [204] John Wiley
& Sons. © 2017 Wiley Periodicals, Inc. (d) Pulse echo test and acoustic eld distribution diagram of silicon-based acoustic lens [205].
Reprinted from [205],© 2017 Published by Elsevier B.V. (e) Sound eld diagram of scandium doped aluminum nitride focused ultrasound
transducer [93]. Reprinted with permission from [93]. Copyright (2017) American Chemical Society.
materials will have higher piezoelectric coefcients and
electromechanical conversion efciency, better mechan-
ical properties and thermal stability. One trend is the devel-
opment of high-performance piezoelectric composites that
can provide compatibility with advanced manufacturing
processes and improve the durability of piezoelectric
transducers. In addition, the development of nanotechno-
logy has promoted the applicaetion of nano-scale piezo-
electric ultrasonic transducers.
(2) Fabrication techniques: the design and fabrication of
focused piezoelectric ultrasonic transducers still face
many challenges. The development of physical focusing
18
Int. J. Extrem. Manuf. 6(2024) 062001 Topical Review
Figure 11. Development of focused ultrasound since 1930.
such as surface self-focusing and multicomponent self-
focusing is limited by the properties of materials and
processing techniques. The emergence of 3D printing
technology opens up new ways to design and fabricate
ultrasonic transducers with complex geometry and cent-
ralized functions. For acoustic lens focusing, the devel-
opment of various new materials promotes the integration
of tunable acoustic lenses while reducing impedance mis-
matches. Although electronic focusing technology enables
more precise focusing, it also presents a series of chal-
lenges, requiring complex electronic control systems and
algorithms to achieve sonic manipulation. Despite these
challenges, electronic focusing offers signicant advant-
ages in terms of exibility, precision, and the ability to
achieve complex focusing patterns.
(3) Integration with other technologies: the combination of
focused ultrasonic transducers with other advanced tech-
nologies holds great promise for advances in medicine,
industry and consumer electronics. The combination of
ultrasound imaging technology and MRI technology for
ultrasound therapy can improve the accuracy of treatment.
In the future, the integration of diagnosis and treatment
is an important trend in the medical eld. In addition to
this, focusing technology is combined with nanotechno-
logy for targeted drug delivery. In consumer electronics,
focused ultrasonic transducers are integrated with wear-
able technology and can be used for non-invasive monit-
oring of vital signs and internal conditions. Focused ultra-
sonic transducers can be integrated into combining BCI
systems to deliver non-invasive brain stimulation with
high spatial resolution. This could enable precise tar-
geting of specic brain regions associated with neuro-
logical disorders, cognitive enhancement, or mood reg-
ulation. At the same time, with virtual reality and aug-
mented reality systems, haptic feedback can be provided
without direct contact. In conclusion, the application
prospects of focused ultrasonic transducers are huge,
and future research will focus on the combination with
other technologies to achieve more intelligent adaptive
systems.
Acknowledgments
This work was supported by National Natural Science
Foundation of China (12072189, 82171011), Shanghai Jiao
Tong University ‘Deep Blue Program’ Fund (Grant No.
SL2103), the Project of Biobank (No. YBKB202117 )from
Shanghai Ninth People’s Hospital, Shanghai Jiao Tong
University School of Medicine and Science Foundation
of National Key Laboratory of Science and Technology
on Advanced Composites in Special Environments
(No.6142905223704). The authors are also grateful to the
Center for Advanced Electronic Materials and Devices
(AEMD) of Shanghai Jiao Tong University.
ORCID iD
Bin Yang https://orcid.org/0000-0001-7948-3823
References
[1] Zhao J B, Hu N, Wu J Y, Li W X, Zhu Z J, Wang M F,
Zheng Y J and Dai H J 2022 A review of piezoelectric
metamaterials for underwater equipment Front. Phys.
10 1068838
[2] Birjis Y, Swaminathan S, Nazemi H, Raj G C A,
Munirathinam P, Abu-Libdeh A and Emadi A 2022
Piezoelectric micromachined ultrasonic transducers
(PMUTs): performance metrics, advancements, and
applications Sensors 22 9151
[3] Jung J, Lee W, Kang W, Shin E, Ryu J and Choi H 2017
Review of piezoelectric micromachined ultrasonic
transducers and their applications J. Micromech.
Microeng. 27 113001
[4] Fu Y Q et al 2017 Advances in piezoelectric thin lms for
acoustic biosensors, acoustouidics and lab-on-chip
applications Prog. Mater. Sci. 89 31–91
[5] Murayama R, Wang B J, Shindou K and Katsunaga K 2020
Study of an ultrasonic probe installed into a small
diameter-pipe using an electromagnetic acoustic
transducer Engineering 12 549–62
[6] Allik H, Webman K M and Hunt J T 1974 Vibrational
response of sonar transducers using piezoelectric nite
elements J. Acoust. Soc. Am. 56 1782–91
19
Int. J. Extrem. Manuf. 6(2024) 062001 Topical Review
[7] Thompson R B 1973 A model for the electromagnetic
generation and detection of Rayleigh and lamb waves
IEEE Trans. Sonics Ultrason. 20 340–6
[8] Mikhailov A V, Gobov Y L, Smorodinskii Y G and
Shcherbinin S V 2015 An electromagnetic–acoustic
transducer with pulsed biasing Russ. J. Nondestruct. Test.
51 467–75
[9] Sekhar M C, Veena E, Kumar N S, Naidu K C B,
Mallikarjuna A and Basha D B 2022 A review on
piezoelectric materials and their applications Cryst. Res.
Technol. 58 2200130
[10] Belzberg M et al 2020 Minimally invasive therapeutic
ultrasound: ultrasound-guided ultrasound ablation in
neuro-oncology Ultrasonics 108 106210
[11] Lescrauwaet E, Vonck K, Sprengers M, Raedt R, Klooster D,
Carrette E and Boon P 2022 Recent advances in the use of
focused ultrasound as a treatment for epilepsy Front.
Neurosci. 16 886584
[12] Baek H, Lockwood D, Mason E J, Obusez E, Poturalski M,
Rammo R, Nagel S J and Jones S E 2022 Clinical
intervention using focused ultrasound (FUS) stimulation
of the brain in diverse neurological disorders Front.
Neurol. 13 880814
[13] Gorick C M et al 2022 Applications of focused
ultrasound-mediated blood-brain barrier opening Adv.
Drug. Deliv. Rev. 191 114583
[14] Al-Jumaily A M, Liaquat H and Paul S 2024 Focused
ultrasound for dermal applications Ultrasound Med. Biol.
50 8–17
[15] Elias W J et al 2016 A randomized trial of focused
ultrasound thalamotomy for essential tremor New Engl. J.
Med. 375 730–9
[16] Ren D Y, Li C Y, Shi J H and Chen R M 2022 A review of
high-frequency ultrasonic transducers for photoacoustic
imaging applications IEEE Trans. Ultrason. Ferroelectr.
Freq. Control 69 1848–58
[17] Peng C, Wu H Y, Kim S, Dai X M and Jiang X N 2021
Recent advances in transducers for intravascular
ultrasound (IVUS) imaging Sensors 21 3540
[18] Shung K K, Cannata J M and Zhou Q F 2007 Piezoelectric
materials for high frequency medical imaging
applications: a review J. Electroceram. 19 141–7
[19] Ozcelik A, Rufo J, Guo F, Gu Y Y, Li P, Lata J and
Huang T J 2018 Acoustic tweezers for the life sciences
Nat. Methods 15 1021–8
[20] Cummer S A, Christensen J and Al´
u A 2016 Controlling
sound with acoustic metamaterials Nat. Rev. Mater.
116001
[21] Quadri S A, Waqas M, Khan I, Khan M A, Suriya S S,
Farooqui M and Fiani B 2018 High-intensity focused
ultrasound: past, present, and future in neurosurgery
Neurosurg. Focus 44 E16
[22] Elhelf I A S, Albahar H, Shah U, Oto A, Cressman E and
Almekkawy M 2018 High intensity focused ultrasound:
the fundamentals, clinical applications and research trends
Diagn. Int. Imaging 99 349–59
[23] Jiang X X, Savchenko O, Li Y F, Qi S A, Yang T L, Zhang W
and Chen J 2019 A review of low-intensity pulsed
ultrasound for therapeutic applications IEEE Trans.
Biomed. Eng. 66 2704–18
[24] Yu K, Niu X D and He B 2020 Neuromodulation
management of chronic neuropathic pain in the central
nervous system Adv. Funct. Mater. 30 1908999
[25] Meng Y, Hynynen K and Lipsman N 2021 Applications of
focused ultrasound in the brain: from thermoablation to
drug delivery Nat. Rev. Neurol. 17 7–22
[26] Bachu V S, Kedda J, Suk I, Green J J and Tyler B 2021
High-intensity focused ultrasound: a review of
mechanisms and clinical applications Ann. Biomed. Eng.
49 1975–91
[27] Javid A, Ilham S and Kiani M 2023 A review of ultrasound
neuromodulation technologies IEEE Trans. Biomed.
Circuits Syst. 17 1084–96
[28] Sugioka K 2019 Hybrid femtosecond laser three-dimensional
micro-and nanoprocessing: a review Int. J. Extrem. Manuf.
1012003
[29] Wang J S, Fang F Z, An H J, Wu S, Qi H M, Cai Y and
Guo G Y 2023 Laser machining fundamentals: micro,
nano, atomic and close-to-atomic scales Int. J. Extrem.
Manuf. 5012005
[30] Hasan M, Zhao J W and Jiang Z Y 2019 Micromanufacturing
of composite materials: a review Int. J. Extrem. Manuf.
1012004
[31] Gao W Y, Lei M W, Li B H, Li G, Li K, Feng Q L and
Wang J L 2020 Investigations on the laser cutting of
LiNbO3Optik 201 163508
[32] Retz K, Kotopoulis S, Kiserud T, Matre K, Eide G E and
Sande R 2017 Measured acoustic intensities for clinical
diagnostic ultrasound transducers and correlation
with thermal index Ultrasound Obstet Gynecol.
50 236–41
[33] Rumack C M, Wilson S R and Charboneau J W 2007
Diagnostic Ultrasound (Peoples Military Medical Press)
[34] Redford D T 2006 Understanding ultrasound physics
Anesthesia Analgesia 102 337
[35] von Ramm O T and Smith S W 1983 Beam steering with
linear arrays IEEE Trans. Biomed. Eng. 30 438–52
[36] Choi E and Roh Y 2017 Optimal design of a concave annular
high intensity focused ultrasound transducer for medical
treatment Sens. Actuators A263 91–101
[37] Lin Z, Guo X S, Tu J, Cheng J C, Wu J R, Huang P T and
Zhang D 2015 A collimated focused ultrasound beam of
high acoustic transmission and minimum diffraction
achieved by using a lens with subwavelength structures
Appl. Phys. Lett. 107 113505
[38] Li F Q et al 2013 Sub-wavelength ultrasonic therapy using a
spherical cavity transducer with open ends Appl. Phys.
Lett. 102 204102
[39] Shung K K 2011 Diagnostic ultrasound: past, present, and
future J. Med. Biol. Eng. 31 371–4
[40] Quarato C M I et al 2023 A review on biological effects of
ultrasounds: key messages for clinicians Diagnostics
13 855
[41] Fry W J and Fry R B 1954 Determination of absolute sound
levels and acoustic absorption coefcients by
thermocouple probes—theory J. Acoust. Soc. Am.
26 294–310
[42] Nowicki A 2020 Safety of ultrasonic examinations; thermal
and mechanical indices Med. Ultrasonogr. 22 203–10
[43] Humphrey V F 2007 Ultrasound and matter-physical
interactions Prog. Biophys. Mol. Biol. 93 195–211
[44] Song P F, Andre M, Chitnis P, Xu S, Croy T, Wear K and
Sikdar S 2023 Clinical, safety and engineering
perspectives on wearable ultrasound technology: a review
IEEE Trans. Ultrason. Ferroelectr. Freq. Control
71 730–44
[45] Dalecki D 2004 Mechanical bioeffects of ultrasound Annu.
Rev. Biomed. Eng. 6229–48
[46] Holland C K and Apfel R E 1989 An improved theory for the
prediction of microcavitation thresholds IEEE Trans.
Ultrason. Ferroelectr. Freq. Control 36 204–8
[47] Rathod V T 2020 A review of acoustic impedance matching
techniques for piezoelectric sensors and transducers
Sensors 20 4051
[48] Lee W and Roh Y 2017 Ultrasonic transducers for medical
diagnostic imaging Biomed. Eng. Lett. 791–97
20
Int. J. Extrem. Manuf. 6(2024) 062001 Topical Review
[49] Hao J G, Li W, Zhai J W and Chen H 2019 Progress in
high-strain perovskite piezoelectric ceramics Mater. Sci.
Eng. 135 1–57
[50] Low T S and Guo W 1995 Modeling of a three-layer
piezoelectric bimorph beam with hysteresis J.
Microelectromech. Syst. 4230–7
[51] Rödel J, Jo W, Seifert K T P, Anton E M, Granzow T and
Damjanovic D 2009 Perspective on the development of
lead-free piezoceramics J. Am. Ceram. Soc. 92 1153–77
[52] Saito Y, Takao H, Tani T, Nonoyama T, Takatori K,
Homma T, Nagaya T and Nakamura M 2004 Lead-free
piezoceramics Nature 432 84–87
[53] Karaki T, Yan K, Miyamoto T and Adachi M 2007 Lead-free
piezoelectric ceramics with large dielectric and
piezoelectric constants manufactured from BaTiO3
Nano-powder Jpn. J. Appl. Phys. 46 L97
[54] Liu W F and Ren X B 2009 Large piezoelectric effect in
Pb-free ceramics Phys. Rev. Lett. 103 257602
[55] Su M, Xia X X, Liu B Q, Zhang Z Q, Liu R, Cai F Y,
Qiu W B and Sun L 2021 High frequency focal transducer
with a Fresnel zone plate for intravascular ultrasound
Appl. Phys. Lett. 119 143702
[56] Butt Z, Pasha R A, Qayyum F, Anjum Z, Ahmad N and
Elahi H 2016 Generation of electrical energy using lead
zirconate titanate (PZT-5A) piezoelectric material:
analytical, numerical and experimental verications J.
Mech. Sci. Technol. 30 3553–8
[57] Tiefensee F, Becker-Willinger C, Heppe G, Herbeck-Engel P
and Jakob A 2010 Nanocomposite cerium oxide polymer
matching layers with adjustable acoustic impedance
between 4 MRayl and 7 MRayl Ultrasonics 50 363–6
[58] Kawai H 1969 The piezoelectricity of poly (vinylidene
Fluoride) Jpn. J. Appl. Phys. 8975
[59] Sappati K K and Bhadra S 2018 Piezoelectric polymer and
paper substrates: a review Sensors 18 3605
[60] Gomes J, Serrado Nunes J, Sencadas V and
Lanceros-Méndez S 2010 Inuence of the β-phase content
and degree of crystallinity on the piezo- and ferroelectric
properties of poly(vinylidene uoride) Smart Mater.
Struct. 19 065010
[61] Soin N et al 2014 Novel “3-D spacer” all bre piezoelectric
textiles for energy harvesting applications Energy Environ.
Sci. 71670–9
[62] Newnham R E, Skinner D P and Cross L E 1978
Connectivity and piezoelectric-pyroelectric composites
Mater. Res. Bull. 13 525–36
[63] Hu H J et al 2023 A wearable cardiac ultrasound imager
Nature 613 667–75
[64] Lin M Y et al 2024 A fully integrated wearable ultrasound
system to monitor deep tissues in moving subjects Nat.
Biotechnol. 42 448–57
[65] Hu H J et al 2023 Stretchable ultrasonic arrays for the
three-dimensional mapping of the modulus of deep tissue
Nat. Biomed. Eng. 71321–34
[66] Wang C H et al 2018 Monitoring of the central blood
pressure waveform via a conformal ultrasonic device Nat.
Biomed. Eng. 2687–95
[67] Wang C H et al 2021 Continuous monitoring of deep-tissue
haemodynamics with stretchable ultrasonic phased arrays
Nat. Biomed. Eng. 5749–58
[68] Lin M Y, Hu H J, Zhou S and Xu S 2022 Soft wearable
devices for deep-tissue sensing Nat. Rev. Mater. 7850–69
[69] Kabakov P, Kim T, Cheng Z X, Jiang X N and Zhang S J
2023 The versatility of piezoelectric composites Annu.
Rev. Mater. Res. 53 165–93
[70] Barrow D A, Petroff T E, Tandon R P and Sayer M 1997
Characterization of thick lead zirconate titanate lms
fabricated using a new sol gel based process J. Appl. Phys.
81 876–81
[71] Ruan T, Wang Q, Hu Z Y, Xu Q D, Xi Y, Li L X, Yang B and
Liu J Q 2023 Multifunctional high-density ultrasonic
microsensor for ranging and 2D imaging Appl. Phys. Lett.
123 063501
[72] Wang Q, Ruan T, Xu Q D, Yang B and Liu J Q 2021
Wearable multifunctional piezoelectric MEMS device for
motion monitoring, health warning, and earphone Nano
Energy 89 106324
[73] Ti J M, Li J H, Fan Q Q, Ren W, Yu Q and Wang C H 2023
Magnetron sputtering of ZnO thick lm for high
frequency focused ultrasonic transducer J. Alloys Compd.
933 167764
[74] Wu S, Liu K F, Wang W J, Li W, Wu T, Yang H and Li X X
2023 Aluminum nitride piezoelectric micromachined
ultrasound transducer arrays for non-invasive monitoring
of radial artery stiffness Micromachines 14 539
[75] Cannata J M, Ritter T A, Chen W H, Silverman R H and
Shung K K 2003 Design of efcient, broadband
single-element (20–80 MHz) ultrasonic transducers for
medical imaging applications IEEE Trans. Ultrason.
Ferroelectr. Freq. Control 50 1548–57
[76] Webster R A, Button T W, Meggs C, MacLennan D and
Cochran S 2007. P3K-5 passive materials for high
frequency ultrasound components Proceedings of 2007
IEEE Ultrasonics Symposium Proceedings (IEEE) pp
1925–8
[77] Nicolaides K, Nortman L and Tapson J 2010 The effect of
backing material on the transmitting response level and
bandwidth of a wideband underwater transmitting
transducer using 1–3 piezocomposite material Phys. Proc.
31041–5
[78] Manh T, Nguyen A T T, Johansen T F and Hoff L 2014
Microfabrication of stacks of acoustic matching layers for
15 MHz ultrasonic transducers Ultrasonics 54 614–20
[79] Manh T, Jensen G U, Johansen T F and Hoff L 2013
Microfabricated 1–3 composite acoustic matching layers
for 15 MHz transducers Ultrasonics 53 1141–9
[80] Manh T, Jensen G U, Johansen T F and Hoff L 2012.
Modeling and characterization of a silicon-epoxy 2–2
composite material Proceedings of 2012 IEEE
International Ultrasonics Symposium (IEEE) pp 2234–7
[81] Zhang S, Yin L L and Fang N 2009 Focusing ultrasound with
an acoustic metamaterial network Phys. Rev. Lett.
102 194301
[82] Kaina N, Lemoult F, Fink M and Lerosey G 2015 Negative
refractive index and acoustic superlens from multiple
scattering in single negative metamaterials Nature
525 77–81
[83] Fang H J, Chen Y, Wong C M, Qiu W B, Chan H L W,
Dai J Y, Li Q and Yan Q F 2016 Anodic aluminum
oxide-epoxy composite acoustic matching layers for
ultrasonic transducer application Ultrasonics 70 29–33
[84] Fei C L, Chiu C T, Chen X Y, Chen Z Y, Ma J G, Zhu B P,
Shung K K and Zhou Q F 2016 Ultrahigh frequency
(100 MHz-300 MHz) ultrasonic transducers for optical
resolution medical imagining Sci. Rep. 628360
[85] Goat C A and Whatmore R W 1999 The effect of grinding
conditions on lead zirconate titanate machinability J. Eur.
Ceram. Soc. 19 1311–3
[86] Liu J H, Chen S Y and Li P C 2009 A single-element
transducer with nonuniform thickness for high-frequency
broadband applications IEEE Trans. Ultrason. Ferroelectr.
Freq. Control 56 379–86
[87] Kim J and Kim M 2022 Focal position control of ultrasonic
transducer made of plano-concave form piezoelectric
vibrator Ultrasonics 121 106668
[88] Lu H T et al 2023 3D printing and processing of miniaturized
transducers with near-pristine piezoelectric ceramics for
localized cavitation Nat. Commun. 14 2418
21
Int. J. Extrem. Manuf. 6(2024) 062001 Topical Review
[89] Tang Y K and Kim E S 2022 Simple sacricial-layer-free
microfabrication processes for air-cavity Fresnel acoustic
lenses (ACFALs) with improved focusing performance
Microsyst. Nanoeng. 875
[90] Li Z X, Yang S H, Wang D F, Shan H, Chen D D, Fei C L,
Xiao M and Yang Y T 2021 Focus of ultrasonic
underwater sound with 3D printed phononic crystal Appl.
Phys. Lett. 119 073501
[91] Chen Z Y, Qian X J, Song X, Jiang Q G, Huang R J, Yang Y,
Li R Z, Shung K, Chen Y and Zhou Q F 2019
Three-dimensional printed piezoelectric array for
improving acoustic eld and spatial resolution in medical
ultrasonic imaging Micromachines 10 170
[92] Chen J, Dai J Y, Zhang C, Zhang Z T and Feng G P 2012
Broadband focusing ultrasonic transducers based on
dimpled LiNbO3plate with inversion layer IEEE Trans.
Ultrason. Ferroelectr. Freq. Control 59 2797–802
[93] Zhu B P, Fei C L, Wang C, Zhu Y H, Yang X F, Zheng H R,
Zhou Q F and Shung K K 2017 Self-focused AlScN lm
ultrasound transducer for individual cell manipulation
ACS Sens. 2172–7
[94] Zhang D C, Wang Z Y, Cheng Z W, Zhang W Y, Yang F and
Yang S H 2021 An ellipsoidal focused ultrasound
transducer for extend-focus photoacoustic microscopy
IEEE Trans. Biomed. Eng. 68 3748–52
[95] Chen Z Y et al 2016 3D printing of piezoelectric element for
energy focusing and ultrasonic sensing Nano Energy
27 78–86
[96] Yao D S et al 2019 Achieving the upper bound of
piezoelectric response in tunable, wearable 3D printed
nanocomposites Adv. Funct. Mater. 29 1903866
[97] Melde K, Mark A G, Qiu T and Fischer P 2016 Holograms
for acoustics Nature 537 518–22
[98] Tarrazó-Serrano D, Casti˜
neira-Ibá˜
nez S, Minin O V,
Candelas P, Rubio C and Minin I V 2019 Design of
acoustical Bessel-like beam formation by a pupil masked
soret zone plate lens Sensors 19 378
[99] Liao G X, Luan C C, Wang Z W, Liu J P, Yao X H and Fu J Z
2021 Acoustic metamaterials: a review of theories,
structures, fabrication approaches, and applications Adv.
Mater. Technol. 62000787
[100] Chen J, Xiao J, Lisevych D, Shakouri A and Fan Z 2018
Deep-subwavelength control of acoustic waves
in an ultra-compact metasurface lens Nat. Commun.
94920
[101] Cai Z R, Zhao S D, Huang Z D, Li Z, Su M, Zhang Z Y,
Zhao Z P, Hu X T, Wang Y S and Song Y L 2019 Bubble
architectures for locally resonant acoustic metamaterials
Adv. Funct. Mater. 29 1906984
[102] Datta S, Tamburrino A and Udpa L 2022 Gradient index
metasurface lens for microwave imaging Sensors
22 8319
[103] Liu H J, Zheng Y, Lu Y, Kang Q L, Guo K and Guo Z Y 2021
Helmholtz-resonator metasurface based high-efciency
acoustic focusing lens Ann. Phys. 533 2100218
[104] Liu J F, Foiret J, Stephens D N, Le Baron O and Ferrara K W
2016 Development of a spherically focused phased array
transducer for ultrasonic image-guided hyperthermia
Phys. Med. Biol. 61 5275–96
[105] Ghanem M A, Maxwell A D, Kreider W, Cunitz B W,
Khokhlova V A, Sapozhnikov O A and Bailey M R 2018
Field characterization and compensation of vibrational
nonuniformity for a 256-element focused ultrasound
phased array IEEE Trans. Ultrason. Ferroelectr. Freq.
Control 65 1618–30
[106] Foster F S, Larson J D, Mason M K, Shoup T S, Nelson G
and Yoshida H 1989 Development of a 12 element annular
array transducer for realtime ultrasound imaging
Ultrasound Med. Biol. 15 649–59
[107] Chen J, Lam K H, Dai J Y, Zhang C, Zhang Z T and
Feng G P 2012 35 MHz PMN-PT single crystal annular
array ultrasonic transducer Integr. Ferroelectr. 139 116–22
[108] Snook K A, Hu C H, Shrout T R and Shung K K 2006
High-frequency ultrasound annular-array imaging. Part I:
array design and fabrication IEEE Trans. Ultrason.
Ferroelectr. Freq. Control 53 300–8
[109] Lei Z H, Xie Y J, Chen Y, Yuan M D, Zeng L M, Ji X R and
Wu D W 2020 Fabrication of high-frequency ultrasonic
array transducers with outstanding performance based on
laser techniques Proceedings of 2020 IEEE International
Ultrasonics Symposium (IUS) (IEEE)
[110] Liu X Q, Chen Q D, Guan K M, Ma Z C, Yu Y H, Li Q K,
Tian Z N and Sun H B 2017 Dry-etching-assisted
femtosecond laser machining Laser Photonics Rev.
11 1600115
[111] Brown J A, Demore C E M and Lockwood G R 2004 Design
and fabrication of annular arrays for high-frequency
ultrasound IEEE Trans. Ultrason. Ferroelectr. Freq.
Control 51 1010–7
[112] Sammoura F, Akhari S, Aqab N, Mahmoud M and Lin L W
2014. Multiple electrode piezoelectric micromachined
ultrasonic transducers Proc. 2014 IEEE Int. Ultrasonics
Symposium (IEEE) pp 305–8
[113] Ketterling J A, Aristizabal O, Turnbull D H and Lizzi F L
2005 Design and fabrication of a 40-MHz annular array
transducer IEEE Trans. Ultrason. Ferroelectr. Freq.
Control 52 672–81
[114] Gottlieb E J, Cannata J M, Hu C H and Shung K K 2005
High frequency copolymer annular array ultrasound
transducer fabrication technology Proceedings of 2005
IEEE Ultrasonics Symposium (IEEE) pp 121–4
[115] Pashaei V, Dehghanzadeh P, Enwia G, Bayat M,
Majerus S J A and Mandal S 2020 Flexible
body-conformal ultrasound patches for image-guided
neuromodulation IEEE Trans. Biomed. Circuits Syst.
14 305–18
[116] Ma Y J et al 2020 Flexible hybrid electronics for digital
healthcare Adv. Mater. 32 1902062
[117] Hur S, Choi H, Yoon G H, Kim N W, Lee D G and Kim Y T
2022 Planar ultrasonic transducer based on a metasurface
piezoelectric ring array for subwavelength acoustic
focusing in water Sci. Rep. 12 1485
[118] Wang X Y, Wu H D, Zhang X D, Zhang D, Gong X and
Zhang D 2018 Investigation of a multi-element focused
air-coupled transducer AIP Adv. 8095010
[119] Jian X H, Liu P B, Li Z J, Lv J B, Yang C, Li P Y and
Cui Y Y 2019 Development of self-focusing piezoelectric
composite ultrasound transducer using laser engraving
technology IEEE Trans. Ultrason. Ferroelectr. Freq.
Control 66 1866–73
[120] Li Z X, Zhao J X, Hou C X, Fei C L, Zheng C X, Lou L F,
Chen D D, Li D and Yang Y T 2022 High-frequency
self-focusing ultrasonic transducer with piezoelectric
metamaterial IEEE Electron Device Lett. 43 946–9
[121] Sun Y Q, Gao X M, Wang H, Chen Z G and Yang Z T 2018
A wideband ultrasonic energy harvester using 1–3
piezoelectric composites with non-uniform thickness
Appl. Phys. Lett. 112 043903
[122] Hou C X et al 2023 Active acoustic eld modulation of
ultrasonic transducers with exible composites Commun.
Phys. 6252
[123] Lee J, Jang J H and Chang J H 2017 Oblong-shaped-focused
transducers for intravascular ultrasound imaging IEEE
Trans. Biomed. Eng. 64 671–80
[124] Chen G S, Liu H C, Lin Y C and Lin Y L 2013 Experimental
analysis of 1–3 piezocomposites for high-intensity
focused ultrasound transducer applications IEEE Trans.
Biomed. Eng. 60 128–34
22
Int. J. Extrem. Manuf. 6(2024) 062001 Topical Review
[125] Li G F et al 2018 Imaging-guided dual-target
neuromodulation of the mouse brain using array
ultrasound IEEE Trans. Ultrason. Ferroelectr. Freq.
Control 65 1583–9
[126] Tipsawat P, Ilham S J, Yang J I, Kashani Z, Kiani M and
Trolier-Mckinstry S 2022 32 element piezoelectric
micromachined ultrasound transducer (PMUT) phased
array for neuromodulation IEEE Open J. Ultrason.
Ferroelectr. Freq. Control 2184–93
[127] Jiang L M, Chen H, Zeng Y S, Tan Z, Wu J G, Xing J and
Zhu J G 2022 Potassium sodium niobate-based lead-free
high-frequency ultrasonic transducers for multifunctional
acoustic tweezers ACS Appl. Mater. Interfaces
14 30979–90
[128] Yoon S, Williams J, Kang B J, Yoon C, Cabrera-Munoz N,
Jeong J S, Lee S G, Shung K K and Kim H H 2015
Angled-focused 45 MHz PMN-PT single element
transducer for intravascular ultrasound imaging Sens.
Actuators A228 16–22
[129] Fei C L, Yang Y H, Guo F F, Lin P F, Chen Q, Zhou Q F and
Sun L 2018 PMN-PT single crystal ultrasonic transducer
with half-concave geometric design for IVUS imaging
IEEE Trans. Biomed. Eng. 65 2087–92
[130] Lee J, Moon J Y and Chang J H 2018 A 35 MHz/105 MHz
dual-element focused transducer for intravascular
ultrasound tissue imaging using the third harmonic
Sensors 18 2290
[131] Fleischman A, Modi R, Nair A, Talman J, Lockwood G and
Roy S 2003 Miniature high frequency focused ultrasonic
transducers for minimally invasive imaging procedures
Sens. Actuators A103 76–82
[132] Zhou Q F, Sharp C, Cannata J M, Shung K K, Feng G H and
Kim E S 2007 Self-focused high frequency ultrasonic
transducers based on ZnO piezoelectric lms Appl. Phys.
Lett. 90 113502
[133] Nguyen T P, Nguyen V T, Mondal S, Pham V H, Vu D D,
Kim B G and Oh J 2020 Improved depth-of-eld
photoacoustic microscopy with a multifocal point
transducer for biomedical imaging Sensors 20 2020
[134] Fang C, Hu H and Zou J 2020 A focused optically
transparent PVDF transducer for photoacoustic
microscopy IEEE Sens. J. 20 2313–9
[135] Park J et al 2021 Quadruple ultrasound, photoacoustic,
optical coherence, and uorescence fusion imaging with a
transparent ultrasound transducer Proc. Natl Acad. Sci.
USA 118 e1920879118
[136] Ter Haar G and Coussios C 2007 High intensity focused
ultrasound: physical principles and devices Int. J. Hyperth.
23 89–104
[137] Lynn J G, Zwemer R L, Chick A J and Miller A E 1942 A
new method for the generation and use of focused
ultrasound in experimental biology J. Gen. Physiol.
26 179–93
[138] Wu F et al 2004 Extracorporeal high intensity focused
ultrasound ablation in the treatment of 1038 patients with
solid carcinomas in China: an overview Ultrason.
Sonochem. 11 149–54
[139] Zippel D B and Papa M Z 2005 The use of MR imaging
guided focused ultrasound in breast cancer patients; a
preliminary phase one study and review Breast Cancer
12 32–38
[140] Clement G T, Sun J, Giesecke T and Hynynen K 2000 A
hemisphere array for non-invasive ultrasound brain
therapy and surgery Phys. Med. Biol. 45 3707–19
[141] Theis M et al 2023 Deep learning enables automated
MRI-based estimation of uterine volume also in patients
with uterine broids undergoing high-intensity focused
ultrasound therapy Insights Imaging 14 1
[142] Scipione R, Anzidei M, Bazzocchi A, Gagliardo C,
Catalano C and Napoli A 2018 HIFU for bone metastases
and other musculoskeletal applications Semin. Intervent
Radiol. 35 261–7
[143] Chen G S, Lin C Y, Jeong J S, Cannata J M, Lin W L,
Chang H and Shung K K 2012 Design and
characterization of dual-curvature 1.5-dimensional
high-intensity focused ultrasound phased-array transducer
IEEE Trans. Ultrason. Ferroelectr. Freq. Control 59 150–5
[144] Woo J and Roh Y 2019 Design and fabrication of an annular
array high intensity focused ultrasound transducer with an
optimal electrode pattern Sens. Actuators A290 156–61
[145] Karzova M M, Yuldashev P V, Khokhlova V A, Nartov F A,
Morrison K P and Khokhlova T D 2021 Dual-use
transducer for ultrasound imaging and pulsed focused
ultrasound therapy IEEE Trans. Ultrason. Ferroelectr.
Freq. Control 68 2930–41
[146] Kiani L 2023 Ultrasound ablation treatment for PD Nat. Rev.
Neurol. 19 197
[147] Makin I R S, Mast T D, Faidi W, Runk M M, Barthe P G and
Slayton M H 2005 Miniaturized ultrasound arrays for
interstitial ablation and imaging Ultrasound Med. Biol.
31 1539–50
[148] Zhang M Z, Narumi R, Azuma T, Okita K and Takagi S 2021
Numerical study on sector-vortex phased irradiation
method using annular array transducer in high-intensity
focused ultrasound treatment Ultrasonics 115 106464
[149] Jeong J S, Cannata J M and Shung K K 2010 Dual-focus
therapeutic ultrasound transducer for production of broad
tissue lesions Ultrasound Med. Biol. 36 1836–48
[150] Ma J G, Guo S J, Wu D, Geng X C and Jiang X N 2013
Design, fabrication, and characterization of a
single-aperture 1.5-MHz/3-MHz dual-frequency HIFU
transducer IEEE Trans. Ultrason. Ferroelectr. Freq.
Control 60 1519–29
[151] Park C Y, Kwon D S, Sung J H and Jeong J S 2017
Dual-frequency ultrasound transducer using inversion
layer technique for therapeutic ultrasound surgery IEEE
Sens. J. 17 6859–66
[152] Harvey E N 1929 The effect of high frequency sound waves
on heart muscle and other irritable tissues Am. J. Physiol.
91 284–90
[153] Fry W J, Wulff V J, Tucker D and Fry F J 2005 Physical
factors involved in ultrasonically induced changes in
living systems: I. Identication of non-temperature effects
J. Acoust. Soc. Am. 22 867–76
[154] Fry F J, Ades H W and Fry W J 1958 Production of
reversible changes in the central nervous system by
ultrasound Science 127 83–84
[155] Defeux T, Wattiez N, Tanter M, Pouget P, Aubry J F and
Younan Y 2015 Low intensity focused ultrasound
modulates monkey visuomotor behavior J. Ther.
Ultrasound 3O25
[156] Legon W, Sato T F, Opitz A, Mueller J, Barbour A,
Williams A and Tyler W J 2014 Transcranial focused
ultrasound modulates the activity of primary
somatosensory cortex in humans Nat. Neurosci. 17 322–9
[157] Mehi´
c E, Xu J M, Caler C J, Coulson N K, Moritz C T and
Mourad P D 2014 Increased anatomical specicity of
neuromodulation via modulated focused ultrasound PLoS
One 9e86939
[158] Qian X J, Lu G X, Thomas B B, Li R Z, Chen X Y,
Shung K K, Humayun M and Zhou Q F 2022 Noninvasive
ultrasound retinal stimulation for vision restoration at high
spatiotemporal resolution BME Front 2022 9829316
[159] Lee J et al 2019 A MEMS ultrasound stimulation system for
modulation of neural circuits with high spatial resolution
in vitro Microsyst. Nanoeng. 528
23
Int. J. Extrem. Manuf. 6(2024) 062001 Topical Review
[160] Yu Y Y, Zhang Z Q, Cai F Y, Su M, Jiang Q J, Zhou Q F,
Humayun M S, Qiu W B and Zheng H R 2019 A novel
racing array transducer for noninvasive ultrasonic retinal
stimulation: a simulation study Sensors 19 1825
[161] Zhou H, Niu L L, Xia X X, Lin Z R, Liu X F, Su M,
Guo R B, Meng L and Zheng H R 2019 Wearable
ultrasound improves motor function in an MPTP mouse
model of Parkinson’s disease IEEE Trans. Biomed. Eng.
66 3006–13
[162] Li G F et al 2019 Noninvasive ultrasonic neuromodulation
in freely moving mice IEEE Trans. Biomed. Eng.
66 217–24
[163] Liu W, Zhu C L and Wu D W 2021 Flexible and stretchable
ultrasonic transducer array conformed to complex surfaces
IEEE Electron Device Lett. 42 240–3
[164] Wang S Y, Meng W L, Ren Z Y, Li B X, Zhu T J, Chen H,
Wang Z, He B, Zhao D and Jiang H 2020 Ultrasonic
neuromodulation and sonogenetics: a new era for neural
modulation Front. Physiol. 11 787
[165] Liang L L, Sun C H, Zhang R T, Han S W, Wang J A, Ren N
and Liu H 2021 Piezotronic effect determined neuron-like
differentiation of adult stem cells driven by ultrasound
Nano Energy 90 106634
[166] Costa T, Shi C, Tien K, Elloian J, Cardoso F A and
Shepard K L 2021 An integrated 2D ultrasound phased
array transmitter in CMOS with pixel pitch-matched
beamforming IEEE Trans. Biomed. Circuits Syst.
15 731–42
[167] Hu Z T, Yang Y H, Xu L, Hao Y and Chen H 2022 Binary
acoustic metasurfaces for dynamic focusing of transcranial
ultrasound Front. Neurosci. 16 984953
[168] Maimbourg G, Houdouin A, Defeux T, Tanter M and
Aubry J F 2018 3D-printed adaptive acoustic lens as a
disruptive technology for transcranial ultrasound therapy
using single-element transducers Phys. Med. Biol.
63 025026
[169] Maimbourg G, Houdouin A, Defeux T, Tanter M and
Aubry J F 2020 Steering capabilities of an acoustic lens
for transcranial therapy: numerical and experimental
studies IEEE Trans. Biomed. Eng. 67 27–37
[170] Jiménez-Gambín S, Jiménez N, Benlloch J M and
Camarena F 2019 Holograms to focus arbitrary ultrasonic
elds through the skull Phys. Rev. Appl. 12 014016
[171] Jiménez-Gambín S, Jiménez N and Camarena F 2020
Transcranial focusing of ultrasonic vortices by acoustic
holograms Phys. Rev. Appl. 14 054070
[172] Naor O, Hertzberg Y, Zemel E, Kimmel E and Shoham S
2012 Towards multifocal ultrasonic neural stimulation II:
design considerations for an acoustic retinal prosthesis J.
Neural Eng. 9026006
[173] Menz M, Oralkan Ö, Khuri-Yakub P T and Baccus S A 2013
Precise neural stimulation in the retina using focused
ultrasound J. Neurosci. 33 4550–60
[174] Menz M D, Ye P, Firouzi K, Nikoozadeh A, Pauly K B,
Khuri-Yakub P and Baccus S A 2019 Radiation force as a
physical mechanism for ultrasonic neurostimulation of the
ex vivo retina J. Neurosci. 39 6251–64
[175] Gao M D et al 2017 Simulation study of an ultrasound retinal
prosthesis with a novel contact-lens array for noninvasive
retinal stimulation IEEE Trans. Neural Syst. Rehabil. Eng.
25 1605–11
[176] Xu C L, Lu G X, Kang H C, Humayun M S and Zhou Q F
2022 Design and simulation of a ring transducer array
for ultrasound retinal stimulation Micromachines
13 1536
[177] Yoo S S, Bystritsky A, Lee J H, Zhang Y Z, Fischer K,
Min B K, McDannold N J, Pascual-Leone A and
Jolesz F A 2011 Focused ultrasound modulates
region-specic brain activity Neuroimage 56 1267–75
[178] Kim C C, Kim Y, Jeong S H, Oh K H, Nam K T and Sun J J
2020 An implantable ionic wireless power transfer system
facilitating electrosynthesis ACS Nano 14 11743–52
[179] Agarwal K, Jegadeesan R, Guo Y X and Thakor N V 2017
Wireless power transfer strategies for implantable
bioelectronics IEEE Rev. Biomed. Eng. 10 136–61
[180] Mou X L and Sun H J 2015 Wireless power transfer: survey
and roadmap Proc. IEEE 81st Vehicular Technology Conf.
(VTC Spring) (IEEE) pp 1–5
[181] Maleki T, Cao N, Song S H, Kao C, Ko S C and Ziaie B 2011
An ultrasonically powered implantable micro-oxygen
generator (IMOG) IEEE Trans. Biomed. Eng. 58 3104–11
[182] Islam S and Kim A 2018 Ultrasonic energy harvesting
scheme for implantable active stent Proc.2018 IEEE Int.
Microwave Biomedical Conf. (IMBioC) (IEEE) pp 70–72
[183] Shi Q F, Wang T and Lee C 2016 MEMS based broadband
piezoelectric ultrasonic energy harvester (PUEH) for
enabling self-powered implantable biomedical devices Sci.
Rep. 624946
[184] Yang Z T, Zeng D P, Wang H, Zhao C L and Tan J W 2015
Harvesting ultrasonic energy using 1–3 piezoelectric
composites Smart Mater. Struct. 24 075029
[185] Jin P et al 2021 A exible, stretchable system for
simultaneous acoustic energy transfer and communication
Sci. Adv. 7eabg2507
[186] Sonmezoglu S, Fineman J R, Maltepe E and Maharbiz M M
2021 Monitoring deep-tissue oxygenation with a
millimeter-scale ultrasonic implant Nat. Biotechnol.
39 855–64
[187] Jiang L M, Lu G X, Yang Y, Zeng Y S, Sun Y Z, Li R Z,
Humayun M S, Chen Y and Zhou Q F 2021 Photoacoustic
and piezo-ultrasound hybrid-induced energy transfer for
3D twining wireless multifunctional implants Energy
Environ. Sci. 14 1490–505
[188] Hong Y et al 2021 A wood-templated unidirectional
piezoceramic composite for transmuscular ultrasonic
wireless power transfer Energy Environ. Sci. 14 6574–85
[189] Hinchet R, Yoon H J, Ryu H, Kim M K, Choi E K, Kim D S
and Kim S W 2019 Transcutaneous ultrasound energy
harvesting using capacitive triboelectric technology
Science 365 491–4
[190] Islam S, Oh E, Jun C, Kim J, Chang W S, Song S and Kim A
2023 Omni-directional ultrasonic powering via platonic
solid receiver for mm-scale implantable devices ACS
Mater. Lett. 51876–85
[191] Jiang L M, Lu G X, Yang Y, Xu Y, Qi F J, Li J P, Zhu B P and
Chen Y 2021 Multichannel piezo-ultrasound implant with
hybrid waterborne acoustic metastructure for selective
wireless energy transfer at megahertz frequencies Adv.
Mater. 33 2104251
[192] Yi X Y, Zheng W C, Cao H, Wang S G, Feng X L and
Yang Z T 2021 Wireless power transmission for
implantable medical devices using focused ultrasound and
a miniaturized 1–3 piezoelectric composite receiving
transducer IEEE Trans. Ultrason. Ferroelectr. Freq.
Control 68 3592–8
[193] Jiang L M, Wu B, Wei X W, Lv X, Xue H Y, Lu G X,
Zeng Y S, Xing J, Wu W J and Wu J G 2022 Flexible
lead-free piezoelectric arrays for high-efciency wireless
ultrasonic energy transfer and communication Mater.
Horiz. 92180–90
[194] Liu X Z et al 2022 An ultrasound-driven implantable
wireless energy harvesting system using a triboelectric
transducer Matter 54315–31
[195] Zhang T et al 2022 Piezoelectric ultrasound
energy–harvesting device for deep brain stimulation and
analgesia applications Sci. Adv. 8eabk0159
[196] Piech D K, Johnson B C, Shen K, Ghanbari M M, Li K Y,
Neely R M, Kay J E, Carmena J M, Maharbiz M M and
24
Int. J. Extrem. Manuf. 6(2024) 062001 Topical Review
Muller R 2020 A wireless millimetre-scale implantable
neural stimulator with ultrasonically powered bidirectional
communication Nat. Biomed. Eng. 4207–22
[197] Jiang L M, Lu G X, Zeng Y S, Sun Y Z, Kang H C,
Burford J, Gong C, Humayun M S, Chen Y and Zhou Q F
2022 Flexible ultrasound-induced retinal stimulating
piezo-arrays for biomimetic visual prostheses Nat.
Commun. 13 3853
[198] Jiang L M et al 2019 Ultrasound-induced wireless
energy harvesting for potential retinal electrical
stimulation application Adv. Funct. Mater.
29 1902522
[199] Ding X Y, Lin S C S, Kiraly B, Yue H J, Li S X, Chiang I K,
Shi J J, Benkovic S J and Huang T J 2012 On-chip
manipulation of single microparticles, cells, and
organisms using surface acoustic waves Proc. Natl Acad.
Sci. USA 109 11105–9
[200] Zeng Y S et al 2022 Manipulation and mechanical
deformation of leukemia cells by high-frequency
ultrasound single beam IEEE Trans. Ultrason. Ferroelectr.
Freq. Control 69 1889–97
[201] Hwang J Y, Kim J, Park J M, Lee C, Jung H, Lee J and
Shung K K 2016 Cell deformation by single-beam
acoustic trapping: a promising tool for measurements of
cell mechanics Sci. Rep. 627238
[202] Marzo A, Seah S A, Drinkwater B W, Sahoo D R, Long B
and Subramanian S 2015 Holographic acoustic elements
for manipulation of levitated objects Nat. Commun.
68661
[203] Thomas L, Petersson F and Andreas N 2007 Chip integrated
strategies for acoustic separation and manipulation of cells
and particles Chem. Soc. Rev. 36 492–506
[204] Chen X Y, Lam K H, Chen R M, Chen Z Y, Yu P, Chen Z P,
Shung K K and Zhou Q F 2017 An adjustable multi-scale
single beam acoustic tweezers based on ultrahigh
frequency ultrasonic transducer Biotechnol. Bioeng.
114 2637–47
[205] Fei C L, Hsu H S, Vafanejad A, Li Y, Lin P F, Li D,
Yang Y T, Kim E, Shung K K and Zhou Q F 2017
Ultrahigh frequency ZnO silicon lens ultrasonic transducer
for cell-size microparticle manipulation J. Alloys Compd.
729 556–62
[206] Zhen L Y, Liu Z D, Liu Z T, Wang Q, Liu J Q, Yao Z R and
Yang B 2023 High-density exible piezoelectric sensor
array with double working modes IEEE Sens. J.
55270–7
25
Available via license: CC BY 4.0
Content may be subject to copyright.
