Piezoelectric single crystals, which have excellent piezoelectric properties, have extensively been employed for various sensors and actuators applications. In this paper, the state–of–art in piezoelectric single crystals for ultrasonic transducer applications is reviewed. Firstly, the basic principles and design considerations of piezoelectric ultrasonic transducers will be addressed. Then, the popular piezoelectric single crystals used for ultrasonic transducer applications, including LiNbO3 (LN), PMN–PT and PIN–PMN–PT, will be introduced. After describing the preparation and performance of the single crystals, the recent development of both the single–element and array transducers fabricated using the single crystals will be presented. Finally, various biomedical applications including eye imaging, intravascular imaging, blood flow measurement, photoacoustic imaging, and microbeam applications of the single crystal transducers will be discussed.

Lead-free piezoelectric ceramics ͑1−x͒K 0.5 Na 0.5 NbO 3 -xLiSbO 3 have been fabricated by a conventional ceramic sintering technique. The results of x-ray diffraction suggest that Li + and Sb 5+ diffuse into the K 0.5 Na 0.5 NbO 3 lattices to form a solid solution with a perovskite structure. The ceramics can be well sintered at 1070-1110°C. The introduction of LiSbO 3 into the Na 0.5 K 0.5 NbO 3 solid solution decreases slightly the paraelectric cubic-ferroelectric tetragonal phase transition temperature ͑T c ͒, but greatly shifts the ferroelectric tetragonal-ferroelectric orthorhombic phase transition ͑T O-F ͒ to room temperature. Coexistence of the orthorhombic and tetragonal phases is formed at 0.05Ͻ x Ͻ 0.07 at room temperature, leading to a significant enhancement of the piezoelectric properties. For the ceramic with x = 0.06, the piezoelectric properties become optimum: piezoelectric constant d 33 = 212 pC/ N, planar and thickness electromechanical coupling factors k P = 46% and k t = 47%, respectively, remanent polarization P r = 15.0 C/cm 2 , coercive field E c = 1.74 kV/ mm, and Curie temperature T C = 358°C.

A lead-free multiferroic ceramic of BiFe0.96Sc0.04O3–BaTiO3 is a type of ABO3 perovskite structure, belonging to the R3c space group, but exhibiting poor insulation and weak multiferroicity.

Similar to optical tweezers, a tightly focused ultrasound microbeam is needed to manipulate microparticles in acoustic tweezers. The development of highly sensitive ultrahigh frequency ultrasonic transducers is crucial for trapping particles or cells with a size of a few microns. As an extra lens would cause excessive attenuation at ultrahigh frequencies, two types of 200-MHz lensless transducer design were developed as an ultrasound microbeam device for acoustic tweezers application. Lithium niobate single crystal press-focused (PF) transducer and zinc oxide self-focused transducer were designed, fabricated and characterized. Tightly focused acoustic beams produced by these transducers were shown to be capable of manipulating single microspheres as small as 5 μm two-dimensionally within a range of hundreds of micrometers in distilled water. The size of the trapped microspheres is the smallest ever reported in the literature of acoustic PF devices. These results suggest that these lensless ultrahigh frequency ultrasonic transducers are capable of manipulating particles at the cellular level and that acoustic tweezers may be a useful tool to manipulate a single cell or molecule for a wide range of biomedical applications.