Ultrasonic transducers are essential for a variety of ultrasound biomedical applications. Traditional ultrasonic equipment is a piezoelectric transducer, which converts electrical energy into vibration, thus producing ultrasound. With the development of piezoelectric materials, the performance of piezoelectric transducer has been enhanced. In addition, unique technologies, such as 3D printing and stretchable electrodes, provide new insights into the fabrication of ultrasonic transducers. As an important supplement to piezoelectric transducer, photoacoustic transducer has attracted much attention in recent years because of its anti-electromagnetic interference and simple manufacturing process. The device converts pulsed laser light into ultrasonic waves, based on the photoacoustic effect discovered by Bell in 1880. Laser-induced ultrasound has also been studied for biomedical imaging, nerve stimulation, cell manipulation and other biomedical fields.

Piezoelectric transducer

Piezoelectric materials used in the manufacture of ultrasonic transducers include lead-containing and lead-free materials. The characteristics of piezoelectric materials, including piezoelectric coefficient (D33) , dielectric properties, electromechanical coupling coefficient (kt) and acoustic impedance, determine the performance of the transducer. In addition, the use of piezoelectric composites has received widespread attention because of their ability to enhance electromechanical coupling, contributing to a significant increase in signal-to-noise ratio (Snr) by broadening bandwidth and increasing energy transfer.

Photoacoustic sensor

Thermal expansion and light absorbing materials are the main components of photoacoustic transducers. In general, the ideal thermal expansion material for photoacoustic transducers is polydimethylsiloxane (PDMS) because of its high Coefficient of thermal expansion, low specific heat and high transparency. The absorbing materials include metal film, carbon nano-material and perovskite. This section introduces the research of metal-PDMS composite, carbon-PDMS composite, perovskite-PDMS composite photoacoustic transducer. Table 2 shows the aggregate performance of the different photoacoustic transducers.

Because the material selection and structure design of ultrasonic transducer have great influence on its acoustic performance, seeking new materials with excellent performance and optimizing device structure are two eternal topics in the development of ultrasonic transducer. The latest research of piezoelectric materials has made a great breakthrough, Sm-PMN-PT single crystal with huge piezoelectric property and transparent PMN-PT single crystal with ultra-high voltage property have been invented, which provides a new strategy for the manufacture of ultrasonic transducer. From the point of view of environmental protection, the new non-toxic material with this super-piezoelectric property is the demand of the next generation piezoelectric transducer. Unlike piezoelectric devices, photoacoustic transducers have a more complex energy conversion process. In order to accurately control the performance of photoacoustic transducer, it is necessary to further explore device physics. At present, carbon nanomaterials PDMS composites play a leading role in photoacoustic materials, but their energy efficiency are still low. For the manufacture of next generation photoacoustic transducer, new materials with higher photoacoustic energy conversion coefficient are needed. In order to facilitate biomedical engineering applications, the development trend of ultrasonic transducer is packaging miniaturization, array design and multi-function integration. In addition, the influx of new technologies such as 3D printing, flexible electronics and artificial intelligence promises to bring innovative ideas to sensor design.