A continuous flow microparticle filter that combines megahertz frequency ultrasonic standing waves and laminar flow is described. The filter has a 0.25 mm, single half wavelength, acoustic pathlength at right angles to the flow. Standing wave radiation pressure on suspended particles drives them towards the centre of the acoustic pathlength. Clarified suspending phase from the region closest to the filter wall is drawn away through a downstream outlet. Experimental tests achieved >1000-fold clearance of 5 μm yeast cells, at a sample flow rate of 6 ml min −1 , from which the clarified aliquot is 1 ml min −1 . At this flow rate the average residence time in the sound field was 1 h was less than 1 K. The design criteria considered in the fabrication of this high performance device are discussed. A theoretical model of the filter’s efficiency, which considers the action of primary radiation force and the particle distribution across a laminar flow profile is presented here. The model predicts that totally clarified filtrate (i.e. zero suspended particles) may be drawn from the downstream outlet. The system described offers a generic approach to automated filtration in some applications. It is continuous flow thereby solving many of the problems of automation presented by batch filter methods and centrifuges. It could be developed for both larger scale and microfluidic applications.

Force field particle filter, combining ultrasound standing waves and laminar flow

The forces on suspended particles in acoustic fields are reviewed briefly and the theoretical modelling of ultrasonic separators based on piezoelectrically excited layered resonators is described. Two flow-through resonator chamber concepts for ultrasonic particle (bio-cell) separation are investigated: (a) the coagulation or sedimentation approach, (b) the so-called h-shaped ultrasonic separator. The h-shaped ultrasonic separator is analysed by combining for the first time the mathematical modelling of the laminar flow with the acoustic force based velocity field of the particles relative to the suspension medium. This allows a complete modelling of the resonator's particle separation performance. Examples for separation chamber designs optimized by use of the mathematical model are presented and the calculated particle traces in the h-resonator are shown and compared with experimental results. For direct comparison of different ultrasonic flow through separator concepts a separation performance figure is introduced and its value is given for the two investigated separator concepts for the sample suspensions of polystyrene spheres, yeast and spirulina cells in (salt) water. The presented results are of importance for the state of the art design of acoustic cell filters for perfusion type bioreactors, as recently launched at the biotechnology market, as well as for the ultrasonic separation of plant (algae) cells under low gravity conditions, where the sedimentation concept fails.

Ultrasonic separation of suspended particles

An apparatus for acoustic concentration of particles in a fluid flow includes a substantially acoustically transparent membrane and a vibration generator that define a fluid flow path therebetween. The fluid flow path is in fluid communication with a fluid source and a fluid outlet and the vibration generator is disposed adjacent the fluid flow path and is capable of producing an acoustic field in the fluid flow path. The acoustic field produces at least one pressure minima in the fluid flow path at a predetermined location within the fluid flow path and forces predetermined particles in the fluid flow path to the at least one pressure minima.

Acoustic concentration of particles in fluid flow

The present invention includes an apparatus and corresponding method for concentrating analytes within a fluid flowing through a tube using acoustic radiation pressure. The apparatus includes a function generator that outputs a radio frequency electrical signal to a transducer that transforms the radio frequency electric signal to an acoustic signal and couples the acoustic signal to the tube. The acoustic signal is converted within the tube to acoustic pressure that concentrates the analytes within the fluid.

Ultrasonic analyte concentration and application in flow cytometry

The present invention is a method and apparatus for acoustically manipulating one or more particles.

W. Bolek

Papers

Enhanced synchronized ultrasonic and flow-field fractionation of suspensions