Ultrasonic trapping in capillaries for trace-amount biomedical analysis

A longitudinal hemispherical standing-wave ultrasonic trap for size-selective separation of microspheres in small-diameter capillaries is described. The trap utilizes the competition between acoustic radiation forces and viscous drag forces on spheres suspended in a liquid inside 20–75-μm-diam fused silica capillaries. Experiments performed on 3.0- and 4.7-μm-diam latex spheres demonstrate the principles of trapping and verify the theoretically calculated size-dependent forces on the spheres. The spheres are detected by the use of laser-induced fluorescence. The goal is to use the trap for separation and ultrahigh-sensitivity detection of trace amounts of proteins and other macromolecules containing two antigenic sites, by binding the target molecule with high specificity to antibody-coated latex spheres.