Flexible Ultrasonic Transducer Research Articles

Flexible Ultrasonic Transducers for Wearable Biomedical Applications: A Review on Advanced Materials, Structural Designs, and Future Prospects.

Due to the rapid developments in materials science and fabrication techniques, wearable devices have recently received increased attention for biomedical applications, particularly in medical ultrasound imaging, sensing, and therapy. Ultrasound is ubiquitous in biomedical applications because of its non-invasive nature, nonionic radiating, high precision, and real-time capabilities. While conventional ultrasound transducers are rigid and bulky, flexible transducers can be conformed to curved body areas for continuous sensing without restricting tissue movement or transducer shifting. This article comprehensively reviews the application of flexible ultrasound transducers in the field of biomedical imaging, sensing, and therapy. First, we review the background of flexible ultrasound transducers. Following that, we discuss advanced materials and fabrication techniques for flexible ultrasound transducers and their enabling technology status. Lastly, we highlight and summarize some promising preliminary data with recent applications of flexible ultrasound transducers in biomedical imaging, sensing, and therapy. We also provide technical barriers, challenges, and future perspectives for further research and development.

Recent Advances in Flexible Ultrasonic Transducers: From Materials Optimization to Imaging Applications.

Ultrasonic (US) transducers have been widely used in the field of ultrasonic and photoacoustic imaging system in recent years, to convert acoustic and electrical signals into each other. As the core part of imaging systems, US transducers have been extensively studied and achieved remarkable progress recently. Imaging systems employing conventional rigid US transducers impose certain constraints, such as not being able to conform to complex surfaces and comfortably come into contact with skin and the sample, and meet the applications of continuous monitoring and diagnosis. To overcome these drawbacks, significant effort has been made in transforming the rigid US transducers to become flexible and wearable. Flexible US transducers ensure self-alignment to complex surfaces and maximize the transferred US energy, resulting in high quality detection performance. The advancement in flexible US transducers has further extended the application range of imaging systems. This review is intended to summarize the most recent advances in flexible US transducers, including advanced functional materials optimization, representative US transducers designs and practical applications in imaging systems. Additionally, the potential challenges and future directions of the development of flexible US transducers are also discussed.

Air-Borne Ultrasonic Transducers Based on Cross-Linked Polypropylene Ferro/Piezoelectrets

Air-borne ultrasonic transducers based on irradiated cross-linked polypropylene (IXPP) ferro/piezoelectret films, which exhibit a high longitudinal piezoelectric activity and a low acoustic impedance, were analyzed, fabricated, and measured in this work. Equivalent circuits and finite element models for IXPP film transducers were built to predict and confirm the transducer performances. Circular IXPP transducers which are 20 mm in diameter show a maximum radiated pressure of 3.2 Pa (104 dB) driven by 100 Vrms and a receiving sensitivity of 8 mV/Pa when resonance occurs at around 45 kHz, both at a distance of 60 cm, as confirmed by analytical and numerical models. Pulse-echo response of the transducers was obtained from theoretical calculation and simulation as well, indicating a −6 dB bandwidth of around 25% is achieved. Besides, the directivity patterns of IXPP film transducers at various frequencies were measured and analyzed. This work may introduce a novel material for flexible ultrasonic transducers with a demand of high sensitivity.

Preliminary Investigation of Magnetic Resonance Imaging Guided Beamforming in Flexible Type Ultrasonic Array Transducers

A flexible-shaped ultrasonic array probe that can be used in a high magnetic field environment in an MRI gantry has been developed. Given that this probe can be fixed according to the shape of the skull’s surface, it is particularly applicable for imaging in the brain. To perform ultrasonic beamforming using a bent probe, it is necessary to measure the bent shape. Therefore, in this research, the curvature of the probe was estimated using MRI. A phantom with ellipse surface close to the shape of a skull was created using a 3D printer. The probe was arranged along the phantom surface to perform MRI and ultrasonic beamforming. The ultrasonic array transducer had 192 elements made from 1-3 composite piezoelectric materials with an element spacing of 0.3 mm. Eight MR position markers were attached to the probe in parallel in two rows of four each, with the array transducer in between. The delay time of each element in dynamic focusing for reception was calculated from its position estimated by the curvature of the probe. To evaluate the feasibility of this method, a B-mode image of 0.98 mm-diameter-thread targets placed in water was generated. The beam width at half maximum of the echo peak in the lateral direction from the thread target set up near the transmission focal point was calculated based on the B-mode image. It was found that the beam width of 1.32 mm in the proposed method, which was close to the thread diameter, whereas that was 4.38 mm in the conventional method that did not consider the bending of the probe. Consequently, the proposed beamforming technique is feasible for ultrasonic imaging through an arbitrary curved surface. Practical applications with a head phantom mimicking skull and cerebral tissue are expected.

Flexible piezoelectric micro ultrasonic transducer array integrated on various flexible substrates

Flexible ultrasonic transducers have attracted much attention in industrial and biomedical applications. However, most of them are fabricated based on a single substrate by complex micromachining techniques. In this paper, a bending mode based flexible piezoelectric micro ultrasonic transducer (PMUT) array has been designed and fabricated through an improved simple and robust adhesive bonding fabrication process. The developed flexible PMUT array consists of 3 × 3 elements and its resonant frequency is about 161 kHz. Our device can be fabricated on various flexible substrates and pasted on concave, convex, and undulant surfaces, reaching an output signal of 75 mVpp under 100 Vpp excitation. The characterizations of the device were performed to verify its excellent flexibility, mechanical stability, and lifespan. Its acoustic performance was also evaluated using diverse time of flight (TOF) tests in air ambient, demonstrating its potential use in flexible acoustic devices or wearable range finder.

Shape Estimation Algorithm for Ultrasound Imaging by Flexible Array Transducer.

A flexible ultrasonic array transducer able to be attached to the body has the potential to achieve long-term continuous unconstrained ultrasound (US) imaging. However, the quality of reconstructed US images is affected by the accuracy of the estimated array shape because the array shape is primitive for time-delay calculation in delay-and-sum beamforming. In this study, we propose an algorithm for estimating the array shape only from the backscattered US signal without using any external device. The proposed algorithm is based on the assumption that beam-summed images reconstructed using an array shape estimated at higher accuracy would have smaller entropy. The array shape is estimated by searching for the shape with minimal entropy, which was used as the index of the beam-summed image quality. Simulation experiments and phantom experiments were used to evaluate the proposed algorithm. In the simulation experiments, three different array shapes with 2.0 MHz, 20 elements, and 0.8-mm pitch transducers were estimated. In the phantom experiments, the array shapes of commercially available linear, convex, and concave transducers were estimated. The results showed that the proposed algorithm can estimate the correct array shape with an average element position error of less than one-eighth of the wavelength of the transmitted signals. These results indicate that the proposed algorithm can achieve sufficiently accurate shape estimation and has the potential to enable clear US imaging with flexible array transducers.

A flexible laser ultrasound transducer for Lamb wave-based structural health monitoring

Lamb waves are widely used in structural health monitoring (SHM) for plate-like structures. In this paper, a new and flexible ultrasonic transducer with a high photo-acoustic conversion efficiency was proposed by using candle soot nanoparticles (CSNPs) and polydimethylsiloxane (PDMS). Experimental results demonstrate that the developed transducer can generate a longitudinal wave with a short duration of 0.28 μs under the illumination of a nanosecond laser pulse. The amplitude of the excited longitudinal wave is 10 times that of the signal generated by the traditional laser ultrasound technique. Further, wedge-shape transducers were developed to excite Lamb waves in a 1.5-mm thick aluminum substrate by the oblique incidence method. The specific dA0 and S0 modes of the Lamb wave with the central frequency of 647 kHz were successfully excited in the aluminum plate. Based on the synthetic aperture focusing imaging technique, a delay-and-sum signal processing method was adopted for damage location in the plate by using the A0 mode Lamb. A 3.5-mm defect was well imaged and the results demonstrate that the developed flexible photo-acoustic transducer can be a good alternative method for SHM.

Multi-instrumentation monitoring for the curing process of a composite structure

The phenomena occurring during curing process is one of the major issues to deal in order to improve the physical properties of a composite structure. In this work, a multi-instrumentation setup is conceived in order to have a better understanding of composites curing using different devices (tunnelling junction sensors, optical fibres and flexible ultrasonic transducers). The sensors are simultaneously embedded or placed on the surface of a thick carbon-epoxy plate for monitoring its autoclave cycle. By means of all signals, an evolution of temperature, internal strains and physical state changes is described. A qualitative statement from all measurements is provided to have a first approach of the initial state of the composite plate. With this methodology, the initial state after composite manufacturing can be considered in the structure design procedure to improve its mechanical performance.

Flexible Ultrasonic Transducer Array with Bulk PZT for Adjuvant Treatment of Bone Injury.

Flexible electronic devices are developing rapidly, especially in medical applications. This paper reports an arrayed flexible piezoelectric micromachined ultrasonic transducer (FPMUT) with a sandwich structure for adjuvant treatment of bone injury. To make the device conformable and stretchable for attaching to the skin surface, the flexible substrate of polydimethylsiloxane (PDMS) was combined with the flexible metal line interconnection between the bulk lead zirconate titanate (PZT) arrays. Simulations and experiments were carried out to verify the resonant frequency and tensile property of the reported FPMUT device. The device had a resonant frequency of 321.15 KHz and a maximum sound pressure level (SPL) of 180.19 dB at the distance of 5 cm in water. In addition, detailed experiments were carried out to test its acoustic performance with different pork tissues, and the results indicated good ultrasound penetration. These findings confirm that the FPMUT shows unique advantages for adjuvant treatment of bone injury.

Low-Temperature Bonding of PZT (PbZrTiO3) and Flexible Printed Circuits Using Sn52In Solder Anisotropic Conductive Films for Flexible Ultrasonic Transducers

Solder anisotropic conductive films (ACFs) consist of conductive solder particles, polymer resin, and flux activator. The flux activator removes solder oxide of solder particles by generated acid for the solder wetting during a thermocompression bonding process. In this article, eutectic Sn52In solder (Tm = 118°) particles were used as the conductive solder particle in the solder ACFs to reduce <130° ACFs’ bonding temperature. Novel Sn52In solder ACFs were fabricated using a comma-roll film coater and used to interconnect Au-coated PZT (PbZrTiO3) and flexible printed circuits (FPCs). PZT and FPCs interconnects using Sn52In solder ACFs were successfully demonstrated with the formation of good solder joints and reliable electrical resistances. According to the energy-dispersive spectrometer (EDS) line scan analysis, excellent solder wetting was confirmed by the presence of Au–In and Ni–Sn intermetallic compounds (IMCs) at the PZT and FPCs interface. Sn52In solder ACFs’ joints showed no open-circuit failure due to the excellent metallurgical bonding during a pressure cooker test (100% relative humidity and 2 atm at 121°) and a dynamic bending reliability test (the minimum bending radius of 7 mm). Therefore, Sn52In solder ACFs’ bonding can provide excellent metallurgical interconnection for flexible ultrasonic transducer packaging.

Morphological-adaptive photoacoustic tomography with flexible transducer and flexible orientation light.

Although conventional photoacoustic tomography (PAT) provides high optical contrast and high ultrasound resolution in depth, its fixed-energy-distribution laser and unchangeable-shaped detector hinder its implementations on clinical routine breast screening. In this Letter, for the first time, to the best of our knowledge, we present a fully flexible photoacoustic coupling system (FPACS) that integrates optical fiber bundles and a flexible ultrasonic transducer for morphological-adaptive breast detection. The experimental results show that the system has better resolution and higher energy utilization than traditional PAT systems, even at the edge of the imaging region. Moreover, the system can achieve morphological adaption, suggesting that the FPACS provides the potential for clinical routine breast screening.

The Use of Flexible Ultrasound Transducers for the Detection of Laser-Induced Guided Waves on Curved Surfaces at Elevated Temperatures.

In this study, a flexible ultrasonic transducer (FUT) was applied in a laser ultrasonic technique (LUT) for non-destructive characterization of metallic pipes at high temperatures of up to 176 °C. Compared with normal ultrasound transducers, a FUT is a piezoelectric film made of a PZT/PZT sol-gel composite which has advantages due to its high sensitivity, curved surface adaptability and high temperature durability. By operating a pulsed laser in B-scan mode along with the integration of FUT and LUT, a multi-mode dispersion spectrum of a stainless steel pipe at high temperature can be measured. In addition, dynamic wave propagation behaviors are experimentally visualized with two dimensional scanning. The images directly interpret the reflections from the interior defects and also can locate their positions. This hybrid technique shows great potential for non-destructive evaluation of structures with complex geometry, especially in high temperature environments.

Sputtered ZnO film on aluminium foils for flexible ultrasonic transducers

Nanocrystalline ZnO films with both C-axis vertical grown and inclined angled grown were sputter-deposited onto aluminium foils (50μm thick) and characterised for using as flexible ultrasonic transducers. As-deposited C-axis grown ZnO films were annealed at different temperatures up to 600°C to enhance film crystallinity and reduce film stress. The C-axis grown ZnO film on the Al foil were bonded onto steel plates, and the pulse-echo tests verified a good performance (with dominant longitudinal waves) of the ultrasonic transducers made from both as-deposited and post-annealed films. Inclined angled ZnO films on the Al foil glued onto steel plates generated mixed shear and longitudinal waves in the pulse-echo test.

Applications of Flexible Ultrasonic Transducer Array for Defect Detection at 150 °C

In this study, the feasibility of using a one dimensional 16-element flexible ultrasonic transducer (FUT) array for nondestructive testing at 150 °C is demonstrated. The FUT arrays were made by a sol-gel sprayed piezoelectric film technology; a PZT composite film was sprayed on a titanium foil of 75 μm thickness. Since the FUT array is flexible, it was attached to a steel pipe with an outer diameter of 89 mm and a wall thickness of 6.5 mm at 150 °C. Using the ultrasonic pulse-echo mode, pipe thickness measurements could be performed. Moreover, using the ultrasonic pulse-echo and pitch-catch modes of each element of FUT array, the defect detection was performed on an Al alloy block of 30 mm thickness with a side-drilled hole (SDH) of ϕ3 mm at 150 °C. In addition, a post-processing algorithm based on the total focusing method was used to process the full matrix of these A-scan signals of each single transmitter and multi-receivers, and then the phase-array image was obtained to indicate this defect- SDH. Both results show the capability of FUT array being operated at 150 °C for the corrosion and defect detections.

Flexible ultrasonic transducer arrays for health monitoring of an aircraft component

The damage detection capability of a flexible ultrasonic transducer (FUT) array bonded onto a planar and a curved surface are presented. The piezoelectric lead zirconate titanate composite (PZT-c) film FUT arrays are fabricated on a 75 μm titanium membrane substrate using a sol-gel spray technique. The test article was a complex aluminium component that is representative of aircraft structural complexity. Room temperature curable adhesive is used as the bonding material and ultrasonic couplant between the FUT and the test article. The glue has been successfully tested in the temperature range of -80°C to 100°C, which covers the sensor operating temperatures commonly required for aircraft structures. For a planar surface, the selected FUT arrays were able to detect the fasteners and the 2.54 mm-long electrical discharge machining (EDM) notch up to a distance of 176 mm with sufficient signal-to-noise ratio. The pulse-echo measurements obtained by the FUT array were compared with those of a commercial 10 MHz ultrasonic transducer (UT). The results showed that the performance of the FUT array was as good as the commercial UT. Another FUT array was bonded onto a curved surface of the test article using the same adhesive. No measurement using commercial UTs was taken on the curved surface because restricted access around this area did not allow such measurement. The pulse-echo measurements confirmed the detection of EDM notches of 2.54 mm and 1.27 mm in length. The experimental results demonstrated the potential of FUT arrays for aircraft structural health monitoring.

Flexible metallic ultrasonic transducers for structural health monitoring of pipes at high temperatures

Piezoelectric films have been deposited by a sol-gel spray technique onto 75-μm-thick titanium and stainless steel (SS) membranes and have been fabricated into flexible ultrasonic transducers (FUTs). FUTs using titanium membranes were glued and those using SS membranes brazed onto steel pipes, procedures that serve as on-site installation techniques for the purpose of offering continuous thickness monitoring capabilities at up to 490 °C. At 150 °C, the thickness measurement accuracy of a pipe with an outer diameter of 26.6 mm and a wall thickness of 2.5 mm was estimated to be 26 μm and the center frequency of the FUT was 10.8 MHz. It is demonstrated that the frequency bandwidth of the FUTs and SNR of signals using glue or brazing materials as high-temperature couplant for FUTs are sufficient to inspect the steel pipes even with a 2.5 mm wall thickness.

High-temperature integrated and flexible ultrasonic transducers for nondestructive testing

Integrated ultrasonic transducers (IUTs) and flexible ultrasonic transducers (FUTs) are presented for nondestructive testing at high temperatures. These transducers are made of sol–gel-sprayed piezoelectric thick (>40 μm) ceramic films. The ceramic materials are lead-zirconate-titanate, bismuth titanate and lithium niobate which are for thickness measurements up to 150, 400 and 800 °C, respectively. The IUT can also be deposited onto one end of a long ultrasonic delay line to perform nondestructive testing at the other end at even higher temperatures. FUTs made of bismuth titanate films onto thin stainless steel foils are also used for thickness measurements at 300 °C with a high-temperature couplant sandwiched between the FUT and a steel substrate. All experiments at high temperatures were performed in pulse-echo mode and ultrasonic echoes with signal-to-noise ratios above 20 dB were obtained. The center operation frequencies of both IUTs and FUTs range from 4.4 to 10.7 MHz.

In situ ice and structure thickness monitoring using integrated and flexible ultrasonictransducers

Two types of ultrasonic sensors are presented for in situ capability development of ice detectionand structure thickness measurement. These piezoelectric film based sensors have been fabricatedby a sol–gel spray technique for aircraft environments and for temperatures ranging from−80 to100 °C. In one sensor type, piezoelectric films of thickness greater than40 µm are deposited directly onto the interior of a 1.3 mm thick aluminum (Al) alloycontrol surface (stabilizer) of an aircraft wing structure as integrated ultrasonictransducers (UTs). In the other sensor type, piezoelectric films are coated onto a50 µm thick polyimide membrane as flexible UTs. These were subsequently glued onto similarlocations at the same control surfaces. In situ monitoring of stabilizer outer skin thicknesswas performed. Ice build-up ranging from a fraction of 1 mm to less than 1.5 mm was alsodetected on a 3 mm thick Al plate. Measurements using these ultrasonic sensors agreed wellwith those obtained by a micrometer. Tradeoffs of these two approaches are presented.

Flexible piezoelectric transducer for ultrasonic inspection of non-planar components

This paper presents the fabrication and characterisation of a flexible ultrasonic transducer using commercially available PZT-5A piezoelectric fibers which are lapped to form rectangular piezoelectric elements. The key feature in the device construction is the inclusion of gaps between the piezoelectric fibers to ensure good flexibility in the plane normal to the fiber direction. The spatial response of the transducer ultrasonic output was assessed using acoustographic imaging. The flexibility of the transducer and its applicability in pulse-echo mode on curved sections was demonstrated by testing on a 38 mm diameter steel rod. The transducer response was found to be broad band and highly non uniform but good pulse-echo performance was achieved at 5 MHz.

Flexible ultrasonic transducers

Flexible ultrasonic transducers (UTs) consisting of a metal foil, a piezoelectric ceramic film, and a top electrode have been developed. The flexibility is realized owing to the porosity of piezoelectric film and the thinness of metal foil. In this paper, the stainless steel (SS), lead-zirconate-titanate (PZT)/PZT composite and silver paste were chosen as metal foil, piezoelectric film, and top electrode materials, respectively. The SS foil serves as both substrate and bottom electrode. The PZT/PZT piezoelectric composite film is made by the sol-gel spray technique. PZT/PZT films of thicknesses from 40 to 70 microm were fabricated onto SS foils. The capability of these flexible sensors operated in the pulse-echo mode for nondestructive testing on flat and curved surfaces of different materials at room temperature and 160 degrees C has been demonstrated. Numerical simulations of the effects of the metal foil thickness on the ultrasonic performance of flexible UTs also were carried out, and the results are in reasonable agreement with experimental data. In addition, a PZT/PZT flexible transducer showed a signal strength comparable with that obtained by a commercial room temperature broad bandwidth transducer.