Abstract
Mechanical phenotyping of single cells is an emerging tool for cell classification, enabling assessment of effective parameters relating to cells' interior molecular content and structure. Here, we present iso-acoustic focusing, an equilibrium method to analyze the effective acoustic impedance of single cells in continuous flow. While flowing through a microchannel, cells migrate sideways, influenced by an acoustic field, into streams of increasing acoustic impedance, until reaching their cell-type specific point of zero acoustic contrast. We establish an experimental procedure and provide theoretical justifications and models for iso-acoustic focusing. We describe a method for providing a suitable acoustic contrast gradient in a cell-friendly medium, and use acoustic forces to maintain that gradient in the presence of destabilizing forces. Applying this method we demonstrate iso-acoustic focusing of cell lines and leukocytes, showing that acoustic properties provide phenotypic information independent of size.
Highlights
Mechanical phenotyping of single cells is an emerging tool for cell classification, enabling assessment of effective parameters relating to cells’ interior molecular content and structure
Because this scattering is governed by differences in mass density r and adiabatic compressibility k between the cell and the surrounding medium, there exists a medium condition for which the acoustic contrast F and force Frad are zero, and the acoustically induced sideways velocity urad vanishes
To a good approximation the iso-acoustic point (IAP) is the location at which Zmed equals the effective acoustic impedance Zcell of the cell (Supplementary Note 1)
Summary
Mechanical phenotyping of single cells is an emerging tool for cell classification, enabling assessment of effective parameters relating to cells’ interior molecular content and structure. We introduce a size-insensitive method, iso-acoustic focusing (IAF), that can analyze cells based on the previously uncharted parameter of effective acoustic impedance This equilibrium method can be viewed as a microfluidic analog to density gradient centrifugation or iso-electric focusing. 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. To develop this method, we have first identified a suitable molecule (iodixanol) to alter the acoustic properties of the cell-culture medium such that cells can have positive, zero or negative acoustic contrast depending on the molecular concentration. We utilize here a recent finding that acoustic impedance gradients are self-stabilized in resonant acoustic fields, which counteracts any gravitational relocation of the laminated liquids due to density differences[34]
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