Standing surface acoustic waves, and the mechanics of acoustic tweezer manipulation of eukaryotic cells

Manipulation by focused ultrasound is an emerging technology with much promise for non-contact handling of microscale objects. A particularly promising approach for achieving this with living cells involves incorporating standing surface acoustic waves (SSAWs) into a microfluidic device. SSAWs must be tuned to provide the necessary range of acoustic radiation force (ARF), but models enabling this tuning have neglected the mechanics of the cells themselves, treating cells as rigid or homogenous spheres, and have also neglected energy transfer from the substrate to the fluid at the Rayleigh angle. We therefore applied Mie scattering theory to develop a model of the ARF arising from a SSAW impacting an idealized eukaryotic cell in an inviscid fluid. The cell was treated as a three-layered body with a nucleus, cytoplasm, and cortical layer. Results showed strong dependence on cell structures and the Rayleigh angle that can be harnessed to develop novel applications for cell manipulation and sorting. ARF can be tuned using the new model to both push away and pull back a cell towards the sound source. The proposed analytical model provides a foundation for design of microfluidic systems that manipulate and sort cells based upon their mechanical properties.