Acoustic focusing with engineered node locations for high-performance microfluidic particle separationElectronic supplementary information (ESI) available: Details and results of first generation (gen1) device characterization, optimization of transducer attachment, and PCR protocol details. See DOI: 10.1039/c4an00034j

Acoustofluidic devices for manipulating microparticles in fluids are appealing for biological sample processing due to their gentle and high-speed capability of sorting cell-scale objects. Such devices are generally limited to moving particles toward locations at integer fractions of the fluid channel width (1/2, 1/4, 1/6, etc. ). In this work, we introduce a unique approach to acoustophoretic device design that overcomes this constraint, allowing us to design the particle focusing location anywhere within the microchannel. This is achieved by fabricating a second fluid channel in parallel with the sample channel, separated from it by a thin silicon wall. The fluids in both channels participate to create the ultrasound resonance, while only one channel processes the sample, thus de-coupling the fluidic and acoustic boundaries. The wall placement and the relative widths of the adjacent channels define the particle focusing location. We investigate the operating characteristics of a range of these devices to determine the configurations that enable effective particle focusing and separation. The results show that a sufficiently thin wall negligibly affects focusing efficiency and location compared to a single channel without a wall, validating the success of this design approach without compromising separation performance. Using these principles to design and fabricate an optimized device configuration, we demonstrate high-efficiency focusing of microspheres, as well as separation of cell-free viruses from mammalian cells. These "transparent wall" acoustic devices are capable of over 90% extraction efficiency with 10 μm microspheres at 450 μL min −1 , and of separating cells (98% purity), from viral particles (70% purity) at 100 μL min −1 . We present a new approach to acoustofluidic device design with a secondary channel separated from the main channel by a thin wall. This allows off-center placement of acoustic nodes, which enables high-efficiency and high-throughput separation of cell-scale objects.