<title>Micro-organism manipulation and microparticle arrangement by the use of ultrasonic standing waves</title>

Abstract

Ultrasonic waves exert acoustic pressure on microparticles in liquid. Consequently microparticles are trapped at the nodes of a standing wave that is excited between a pair of ultrasonic transducers. If two orthogonal standing waves are excited by using two pairs of transducers, microparticles are trapped at the intersecting points of the nodes. A larger trapping force is attainable by ultrasonic trapping than by laser trapping that utilizes a weak optical pressure. Therefore ultrasonic trapping is suitable for the manipulation of biological tissues which easily suffer thermal damage by the exposure to a focused laser beam. Microorganisms such as euglena and paramecia were trapped by the ultrasonic waves of approximately 3 MHz. Trapped microorganisms could be transferred to desired positions by changing the ultrasonic frequency. The aggregation of microorganisms was achievable by the cyclic frequency change. This ultrasonic trapping technique was also used to fabricate composite materials with lattice structure; i.e., polymer, glass, or metal particles were ultrasonically arranged in a polymer matrix during the solidification process of the host polymer.