Acoustic Micro-Manipulation and Its Biomedical Applications

Acoustic waves-and ultrasound waves in particular-are bio-compatible,with excellent transmission through biological tissues.Furthermore,the wavelength and intensity of acoustic waves can be tuned over several orders of magnitude.Most notably,the commonly used 10-300 MHz frequency range is attractive for biomedical applications,as its wavelength in water(5-150 μm)corresponds to the cellular-length scale.Thus,acoustic micro-manipulation has emerged as a promising tool for biomedical studies and clinical diagnostics.Its basis is the movement of tiny particles and objects actuated by acoustic waves,which is termed acoustophoresis.The origin of this field can be traced back to the 17th century,when Hooke described the nodal patterns formed by sand grains or flour as they spread on a vibrating drum surface[1].The method was later extended to visualize vibrations and acoustic fields,and the patterns have since been known as Chladni figures[1].Two centuries later,Rayleigh provided a theoretical explanation of the underlying principles-the acoustic streaming effect[2]and the acoustic radiation force[3].Since the 1920s and the development of piezoelectric materials and electronic devices,acoustophoresis has been exploited for the manipulation of microscopic objects.For example,in 1928,Harvey and Loomis[4]reported an intracellular protoplasmic rotation actuated by ultrasonic waves.