Dear Colleagues,

In comparison with their conventional bulk PZT-based counterparts, ultrasonic transducers based on MEMS technologies boast advantages including miniaturized volume, broadened bandwidth, low power consumption, ease of two dimension (2D) array fabrication, and facile integration with ICs. These advantages make them promising candidates in 3D ultrasonic medical imaging, portable/handle ultrasonic diagnostic device, and long-term and in situ nondestructive detection. In the past decades, a great number of studies have focused on the design, modeling, and fabrication of MEMS ultrasonic transducers, including capacitive micromachined ultrasonic transducers (CMUTs) and piezoelectric micromachined ultrasonic transducers (PMUTs). Remarkable progress has been achieved in improving their performance, in areas such as electromechanical coupling efficiency and transmitting and receiving resistivity. Real-time 3D ultrasonic imaging has been demonstrated, and commercially available handheld imaging devices (such as ultrasound-on-chip probes by Butterfly Network, Inc.) have been developed for portable medical diagnostics applications. However, with the increasing requirements and extending scope of practical applications, new challenges are rising and continuous effort is needed. For instance, air-coupled MEMS ultrasonic transducers are in high demand in air-coupled sensing situations, such as biometric recognition, range finding, and 3D gesture recognition, which have widespread applications in the emerging fields of human–machine interfaces, intelligent robotics, and non-contact controlled electronics. MEMS ultrasonic transducers with high output acoustic pressure and low power consumption are required in order to overcome the exponential decay of acoustic energy in air environments and enhance the propagation distance and transducer response. Beyond sensing ultrasound, power ultrasound is ushering in a new era because of its significant applications in ultrasonic therapy. Benefiting from its low power consumption, portability, and noninvasiveness, MEMS transducer-based ultrasonic therapy has become a widely accepted and easily accessible approach for various treatments for critical illness, such as kidney stone comminution, cancer hyperthermia therapies, transcranial sonothrombolysis for brain stroke treatments, and ultrasound neuroregulation for Alzheimer's disease and Parkinson's disease. In these applications, MEMS transducers with high output power are required in order to provide enough ultrasound energy to interact with human tissues or organs. To solve these newly raised and unmet requirements in the aforementioned applications, a vast number of investigations should be carried out to innovate the modeling, design, fabrication, and packaging technologies of MEMS ultrasonic transducers.

Therefore, this Special Issue focuses on recent advance in modeling, design, fabrication, packaging, and applications of MEMS ultrasonic transducers. Both original research and review papers are welcome.

Dr. Zhikang Li
Prof. Dr. Libo Zhao
Prof. Dr. Jin Xie
Guest Editors

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• CMUTs
• PMUTs

• Principle innovation

• Structure design
• Theoretical modeling
• Fabrication
• Packaging
• Front-end circuits
• Performance enhancement
• Emerging applications