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Photoacoustic imaging (PAI) is drawing extensive attention and gaining rapid development as an emerging biomedical imaging technology because of its high spatial resolution, large imaging depth, and rich optical contrast. PAI has great potential applications in endoscopy, but the progress of endoscopic PAI was hindered by the challenges of manufacturing and assembling miniature imaging components. Over the last decade, microelectromechanical systems (MEMS) technology has greatly facilitated the development of photoacoustic endoscopes and extended the realm of applicability of the PAI. As the key component of photoacoustic endoscopes, micromachined ultrasound transducers (MUTs), including piezoelectric MUTs (pMUTs) and capacitive MUTs (cMUTs), have been developed and explored for endoscopic PAI applications. In this article, the recent progress of pMUTs (thickness extension mode and flexural vibration mode) and cMUTs are reviewed and discussed with their applications in endoscopic PAI. Current PAI endoscopes based on pMUTs and cMUTs are also introduced and compared. Finally, the remaining challenges and future directions of MEMS ultrasound transducers for endoscopic PAI applications are given.

Facilitated by microelectromechanical systems (MEMS) technology, MEMS speakers or microspeakers have been rapidly developed during the past decade to meet the requirements of the flourishing audio market. With advantages of a small footprint, low cost, and easy assembly, MEMS speakers are drawing extensive attention for potential applications in hearing instruments, portable electronics, and the Internet of Things (IoT). MEMS speakers based on different transduction mechanisms, including piezoelectric, electrodynamic, electrostatic, and thermoacoustic actuation, have been developed and significant progresses have been made in commercialization in the last few years. In this article, the principle and modeling of each MEMS speaker type is briefly introduced first. Then, the development of MEMS speakers is reviewed with key specifications of state-of-the-art MEMS speakers summarized. The advantages and challenges of all four types of MEMS speakers are compared and discussed. New approaches to improve sound pressure levels (SPLs) of MEMS speakers are also proposed. Finally, the remaining challenges and outlook of MEMS speakers are given.

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