Acoustofluidics 8: Applications of acoustophoresis in continuous flow microsystems

Abstract

This acoustofluidics tutorial focuses on continuous flow-based half wavelength resonator systems operated in the transversal mode, where the direction of the primary acoustic force acts in plane with the microchip. The transversal actuation mode facilitates integration with up- and downstream microchannel networks as well as visual control of the acoustic focusing experiment. Applications of particle enrichment in an acoustic half wavelength resonator are discussed as well as clarification of the carrier fluid from undesired particles. Binary separation of particle/vesicle/cell mixtures into two subpopulations is outlined based on the different polarities of the acoustic contrast factor. Furthermore, continuous flow separation of different particle/cell types is described where both Free Flow Acoustophoresis (FFA) and binary acoustophoresis are utilized. By capitalizing on the laminar flow regime, acoustophoresis has proven especially successful in performing bead/cell translations between different buffer systems. Likewise, the ability to controllably translate particulate matter across streamlines has opened a route to valving of cells/particles without any moving parts, where event triggered cell sorting is becoming an increasing area of activity. Recent developments now also enable measurements of fundamental cell properties such as density and compressibility by means of acoustophoresis. General aspects on working with live cells in acoustophoresis systems are discussed as well as available means to quantify the outcome of cell and particle separation experiments performed by acoustophoresis.

A thin-reflector microfluidic resonator for continuous-flow concentration of microorganisms: a new approach to water quality analysis using acoustofluidics

“…42,43 Continuous-flow concentration of particles in acoustofluidic devices is usually performed via generation of an ultrasonic standing wave (USW) within a fluid chamber, causing suspended particles to focus in a confined liquid volume under the action of acoustic radiation forces (ARFs). 44 In order to obtain a significant increase in particle concentration, the fluid flow must be split into the particle-rich and the particle-depleted fractions. 44,45 Based on this functioning principle, concentration of a range of particles including bacterial spores, biological cells and polystyrene spherical micro-beads has been demonstrated.…”

“…44 In order to obtain a significant increase in particle concentration, the fluid flow must be split into the particle-rich and the particle-depleted fractions. 44,45 Based on this functioning principle, concentration of a range of particles including bacterial spores, biological cells and polystyrene spherical micro-beads has been demonstrated. 15,38,[46][47][48][49][50][51] Nordin and Laurell have recently demonstrated a very significant increase in concentration of red blood cells and prostate cancer cells (up to a maximum of ~200-fold), using a two-stages half-wavelength resonator.…”

Acoustofluidics 23: acoustic manipulation combined with other force fields

“…This series of tutorial articles has explored the theory underlying acoustic radiation forces 1 and acoustically-induced streaming 2 , has described experimental techniques for evaluating these effects 3,4 and has discussed a variety of applications [5][6][7][8] based on both bulk acoustic waves 9 and surface acoustic waves 10 . It has become apparent that the acoustic phenomena employed have a potentially large scale of action 11 , can trap and manipulate relatively large particles and agglomerates 8 , are appropriate for biological use 12 , and are suitable for integration with many microfluidic fabrication techniques 13 .…”

Ultrasound assisted particle and cell manipulation on-chip

“…Two recent review papers distinguish between applications that involve the trapping of particles and cells [13] and those that use radiation forces within continuous flow microsystems [2]. Trapping of particles can be used to enhance bioassays [80], by increasing bead interactions in agglutination assays [81], or by trapping particles on functionalised surfaces [14].…”

“…In these, the role of the ultrasound is usually to transfer energy from a convenient location for its generation, through transduction of electrical to mechanical energy by piezoelectric material, to a location where it is applied. For example, ultrasound may be applied to the surface of the human body for non-invasive hyperthermic surgery [1], or it may be applied at a remote location in a lab-on-chip system to control or sort particles within a microfluidic channel [2].…”