Concept and Proof of Principle of an Acoustofluidic Single-Particle Sorting Device Using a Spatially Confined Acoustic Active Region ^†

Abstract

We present the concept and a proof-of-principle experiment for an acoustofluidic single- particle sorter. In a microfluidic channel, a flow profile with the following three parallel fluid domains is generated: buffer fluid in the center and buffer plus particles in the lateral domains. Due to the laminar flow regime present in microfluidics, the particles essentially follow the stream line(s) along the channel. In the spatially confined sorting and detection region, by switching on the standing acoustic wave, particles of interest (POIs) are pushed into the center fluid domain, thus leaving the chip at the center outlet. For particles of non-interest (PONIs), the acoustic region remains silent, so PONIs are not centered and follow their path to the side outlet.

1. Introduction

For the acoustofluidic sorting of single particles, usually, so-called surface acoustic wave (SAW) devices [1] are used. For easier-to-fabricate and easier-to-handle bulk acoustic (BAW) devices, only one publication was found in which a detection zone and a spatially confined sorting zone were used [2]. Our concept (Figure 1) is also a bulk acoustic device and is verified in a proof-of-principle experiment (Figure 2).

2. Materials and Methods

We use a microfabricated microfluidic chip made of silicon and glass that is externally actuated by a piezoelectric plate transducer. The acoustic active region is defined geometrically by narrowing in the channel width, so that the resonance condition for a standing sound half-wave is fulfilled only there. Aside from the sorting area, the width is chosen to not equal an integer multiple of the sound half-wavelength; thus, no standing wave can develop here. Detection takes place using a suitable method, e.g., fluorescence detection using a CCD camera.

3. Discussion

In contrast to the BAW sorting device in [2], our BAW concept contains a symmetric flow profile, which is expected to exhibit the easier and more stable adjustment of flow ratios between the inlets. Adjusting the dimensions of the acoustic active region facilitates the sorting of single particles and also smaller particles. For single-particle sorting, the fluid must be sparsely populated by particles, so our high-performance particle-focusing device from previous work [3] could be used for subsequent particle enrichment.