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Iso-acoustic focusing

Analyzing cells' acoustic properties

A cancer cell, a lymphocyte and neutrophil is seen, where the cancer cell has the lowest acoustic impediance and the neutrophil has the highest.
Schematic representation of iso-acoustic focusing for size-insensitive separation of cells based on their acoustic impedance.

In iso-acoustic focusing (IAF), individual cells are analyzed based on the previously uncharted parameter of effective acoustic impedance [Augustsson et al., Nature Communications, 2016]. This equilibrium method can be viewed as a microfluidic analog to density gradient centrifugation or iso-electric focusing. Cells flowing through a microchannel migrate sideways, influenced by an acoustic field, into flow streams of ever increasing acoustic impedance. Finally, the individual cells reach their iso-acoustic point (IAP), at which the acoustic contrast between the cell and the surrounding liquid becomes zero, and the sideways displacement ceases. Cell-specific differences in effective acoustic impedance translate to a spatial dispersion of the cell population transverse to the flow, enabling continuous label-free analysis of individual cells.

The neutrophils and the monocytes have been seperated along the vertical flow direction.
Fluorescence image overlay of neutrophils (green) and monocytes (red) at their individual iso-acoustic points in an acoustic impedance gradient (blue). The acoustic resonance across the width of the channel is indicated by red dashed lines.
Before the inlet, there is Histopaque with whole blood on its sides. Before the outlet, the whole blood is divided in focused red blood cells and plasma, respectively.
Whole blood is layered on both sides of a central cell free stream of cell density centrifugation media, Histopaque. Upon onset of sound, red blood cells (RBC) and granulocytes penetrate the media while monocytes become trapped at the interfaces to the high density medium.

Separating cells based on acoustic properties

By this approach, we recently presented the first one-step acoustic separation of mononuclear cells (MNCs) from undiluted human whole blood [Alsved et al. Microtas, Basel, 2019 (PDF)].