Photoacoustic imaging is a new type of noninvasive, nonradiation imaging modality that combines the deep penetration of ultrasonic imaging and high specificity of optical imaging. Photoacoustic imaging systems employing conventional ultrasonic sensors impose certain constraints such as obstructions in the optical path, bulky sensor size, complex system configurations, difficult optical and acoustic alignment, and degradation of signal-to-noise ratio. To overcome these drawbacks, an ultrasonic sensor in the optically transparent form has been introduced, as it enables direct delivery of excitation light through the sensors. In recent years, various types of optically transparent ultrasonic sensors have been developed for photoacoustic imaging applications, including optics-based ultrasonic sensors, piezoelectric-based ultrasonic sensors, and microelectromechanical system-based capacitive micromachined ultrasonic transducers. In this paper, the authors review representative transparent sensors for photoacoustic imaging applications. In addition, the potential challenges and future directions of the development of transparent sensors are discussed.

3D printing, as a new technology, brings great freedom in the design and manufacturing of batteries. It offers unique advantages by the production of more advanced batteries with specific properties, such as good internal structural controllability, high shape conformability, and enhanced power capability and energy density. In this review, we provide an overview of 3D printing technologies in the field of customizable batteries. First, we introduce fundamental concepts of customizable batteries and the distinct advantages of 3D printing. Then, five representative 3D printing techniques are discussed along with their operating mechanisms, requirements for printing materials, manufacturing accuracy, and technical features. In addition, from the perspective of the three basic levels (pore structure, component architecture and interface, as well as battery appearance and mechanical deformability) of customizable batteries, a detailed overview is provided to gain an in‐depth understanding. The state‐of‐the‐art developments and current major challenges are summarized and discussed. Finally, potential research area is proposed to utilize 3D printed customizable batteries for practical applications.

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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.