Acoustic Forces in Cytometry and Biomedical Applications: Multidimensional Acoustophoresis

Abstract: Over the last decades the ongoing work in the fields of Lab-on-a-Chip and Micro-Total-Analysis-Systems has led to the discovery of new or improved ways to handle and analyse small volumes of biofluids and complex biosuspensions. The benefits of working on the microscale include: miniaturization of the analysis systems with less need for large sample volumes; temporal and spatial control of suspended particle/cell positions; low volume sheath flow lamination or mixing; novel separation techniques by using forces inherent to the microscale domain; precise regulation of sample temperatures and rapid analysis with less volumes needed to be processed. Researchers now seek to implement these techniques in integrated systems to benefit the biomedical research field as well as clinics.

Acoustophoresis, a method that utilizes acoustic forces to move particles and cells in microfluidic channels has been gaining increased attention over the last decade. The acoustophoretic method has been shown to handle a number of biosuspensions e.g., blood, cell cultures and raw milk as well as other biofluids, and comes with a variety of available unit operations e.g., free flow separation, binary density separation, particle positioning, contactless trapping, buffer changes, washing, and surface chemistry based sorting that allows integration into a wide range of application. The theoretical and experimental understanding of the acoustic radiation force which is the principal force used to manipulate particles in these systems (often generated with standing waves) has also evolved during this time. Chip-based acoustic systems have been presented in e.g., silicon, glass and PDMS, further illustrating the versatility of the method.

This dissertation presents some of the recent developments in the acoustophoretic field to illustrate how acoustic forces can be used in cytometry and biomedical applications, specifically by utilizing multiple acoustic wavelength geometries or two-dimensional particle manipulation. Paper I presents a novel way to pretreat raw milk in order to facilitate rapid quality control. Paper II extends this method by presenting a technique for label free cytometry in raw milk. Paper III showcases the ability to sort particles with fluorescence activated acoustic forces. Paper IV presents a low complexity high precision proof-of-concept sheathless impedance cytometer that can be integrated in other chip based systems. Paper V presents an improved method for concurrent blood component fractionation that requires less manual handling compared to established methods by implementing free flow separation into multiple outlets.

The theory section explains the underlying physical laws that govern the microscale fluid systems presented here. Acoustic force theory is explained in detail for better understanding of the acoustic radiation forces that act on the suspended particles and also cause media streaming. The particle manipulation section compares the different methods that are available to researchers in the biomedical microfluidic field. The microfabrication section deals with the design aspects of using various materials. Unit operations and applications specific for acoustophoresis are presented. Biofluids and cell types including blood and raw milk are discussed to underline the challenges that researchers are faced with during system design, handling and analysis. The aim of this dissertation is to provide a foundation for future development of acoustic force applications in cytometry and biomedicine.