As a well-known medical imaging methodology, intravascular ultrasound (IVUS) imaging plays a critical role in diagnosis, treatment guidance and post-treatment assessment of coronary artery diseases. By cannulating a miniature ultrasound transducer mounted catheter into an artery, the vessel lumen opening, vessel wall morphology and other associated blood and vessel properties can be precisely assessed in IVUS imaging. Ultrasound transducer, as the key component of an IVUS system, is critical in determining the IVUS imaging performance. In recent years, a wide range of achievements in ultrasound transducers have been reported for IVUS imaging applications. Herein, a comprehensive review is given on recent advances in ultrasound transducers for IVUS imaging. Firstly, a fundamental understanding of IVUS imaging principle, evaluation parameters and IVUS catheter are summarized. Secondly, three different types of ultrasound transducers (piezoelectric ultrasound transducer, piezoelectric micromachined ultrasound transducer and capacitive micromachined ultrasound transducer) for IVUS imaging are presented. Particularly, the recent advances in piezoelectric ultrasound transducer for IVUS imaging are extensively examined according to their different working mechanisms, configurations and materials adopted. Thirdly, IVUS-based multimodality intravascular imaging of atherosclerotic plaque is discussed. Finally, summary and perspectives on the future studies are highlighted for IVUS imaging applications.

The objective of this article is to review the recent advancement in piezoelectric micromachined ultrasound transducer (PMUT) technology and the associated piezoelectric materials, device fabrication and characterization, as well as applications. PMUT has been an active research topic since the late 1990s because of the ultrasound application needs of low cost large 2D arrays, and the promising progresses on piezoelectric thin films, semiconductors, and micro/nano-electromechanical system technology. However, the industrial and medical applications of PMUTs have not been very significant until the recent success of PMUT based fingerprint sensing, which inspired growing interests in PMUT research and development. In this paper, recent advances of piezoelectric materials for PMUTs are reviewed first by analyzing the material properties and their suitability for PMUTs. PMUT structures and the associated micromachining processes are next reviewed with a focus on the complementary metal oxide semiconductor compatibility. PMUT prototypes and their applications over the last decade are then summarized to show the development trend of PMUTs. Finally, the prospective future of PMUTs is discussed as well as the challenges on piezoelectric materials, micro/nanofabrication and device integration.

Continuous blood pressure monitoring in everyday life is important and necessary to detect and control high blood pressure in advance. While the existing blood pressure monitoring techniques are well suited for applications in current clinical settings, they are inadequate for next-generation wearable long-term monitoring of blood pressure on a daily basis. In this study, a flexible piezo-composite ultrasonic sensor was reported, for the first time, for continuous blood pressure measurement through ultrasonic motion tracking of blood vessel wall. A flexible piezo-composite ultrasonic sensor was designed and fabricated with a layer of PZT-5A/polydimethylsiloxane (PDMS) anisotropic 1-3 composite and silver nanowire based stretchable electrodes. The material properties and dimensions of the sensor were determined according to the volume fraction of PZT-5A and the material properties of PZT-5A and PDMS. The experimental results illustrated that the flexible sensor possessed adequate bandwidth and sensitivity for blood pressure monitoring. Continuous blood pressure measurement was successfully conducted with the ulnar artery on a volunteer’s right arm. Compared with the measurement results using a clinical ultrasound probe and a commercial upper arm blood monitor, the results obtained in this study demonstrated the capability of the proposed flexible sensor to continuously monitor blood pressure waveforms during cardiac cycles. The flexible sensor provides a promising solution for noninvasive, nonocclusive and calibration-free blood pressure monitoring. It has great potential to be integrated into a wearable ultrasonic healthcare sensing system for blood pressure and flow monitoring.

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.

While fingerprint technology has been widely used in mobile devices for user identification, the existing fingerprint sensors can only capture 2D images of the fingerprint and are thus vulnerable to spoofing attacks using 2D replicas of fingerprint. Ultrasonic fingerprint recognition via imaging the structures beneath the human skin can be a promising approach for preventing spoofing attacks on fingerprint-based identification devices. In this study, under-display ultrasonic fingerprint recognition (UDUFR) was investigated for the first time via imaging a finger vessel underneath the fingerprint. A 40 MHz ultrasonic fingerprint sensor composed of PZT-5H piezoelectric active material was firstly developed, which demonstrated broad bandwidth (73.9%) and high loop sensitivity (-27.5 dB) for UDUFR applications. UDUFR experiments were performed to demonstrate the effectiveness of the proposed technique using a two-layer polydimethylsiloxane (PDMS) phantom which consists of a dummy fingerprint in the surface layer and a finger vessel mimicker in the inner layer. Electronic images of the fingerprint and finger vessel with a resolution of 500 × 500 DPI were successfully obtained. The results reported in this study open up new avenues for the next generation of robust and secure UDUFR technology.