Acoustic Techniques for Microfluidic Particle Sorting
Microfluidic systems can integrate various sorting methods based on the physical properties of particles, but can also be used with external fields, such as acoustics, optics, and magnetism. The use of sound waves to classify particles of various sizes is effective and biocompatible.
Due to the label-free, biocompatibility, and non-contact nature of the acoustic fluid separation technology, it can be used to separate cells and other biological particles with high yield, high purity, and biocompatibility. By carefully designing and adjusting the applied sound field, automated immediate care equipment for isolating submicron biological particles can be developed. Therefore, many limitations of traditional separation tools can be overcome and a variety of clinical problems can be solved. This article introduces you to acoustic technology as a method of separating and classifying particles in a microfluidic system.
Different Waves Used for Acoustofluidics in Microfluidics
Two different wave modes are commonly used to classify particles: surface acoustic wave (SAW) and bulk acoustic wave (BAW). When the wave interacts with the medium, two acoustic phenomena occur-acoustic streaming and acoustic radiation force, which act on the particles in the microchannel. Either or a combination of these two phenomena is used to classify particles. The waves are generally created from interdigitated transducers (IDT) comprising electrodes on the sides of the channel.
It propagates on the surface of the substrate and is usually produced by a half-wavelength resonator. There are two different SAWs, traveling surface acoustic waves (TSAW) and standing wave surface acoustic waves (SSAW), depending on how they are generated: using a set of IDTs or two different sources.
It is a wave propagating in a medium, usually generated by a bulk piezoelectric transducer instead of an IDT.
In order to optimize the overall performance of the experiment, careful research is needed to select the best frequency range and the most accurate geometry for the microfluidic device. The efficiency of the acoustic fluid method largely depends on the size of the particles being studied and the physical properties of the fluid used. It is also important that acoustic technology is best suited for particle size ranges from micrometers to 10-20 micrometers.
Free-flow acoustophoresis is the classification of particles in a microfluidic device according to the size of the particles, in which the particles are basically driven to the pressure node by the force of acoustic radiation when transporting with the flow. This acoustic radiation force is proportional to the cube of the particle radius, so the larger the particle, the stronger the force acting on the particle, and then the faster it moves to the pressure node.
Based on this, free-flow acoustophoresis is a good method of multiple fractionations. Compared with smaller particles, larger particles move to the pressure node faster, resulting in particles of different sizes in different positions in the microfluidic channel. Using an outlet of the same size as the particles, the particles can be sorted in a simple way. Free-flow acoustic swimming is popular in medical applications. For example, it is used to separate white blood cells, red blood cells and platelets.
Figure.1 (a) A single-stage free flow acoustophoresis device, which can separate 5 different types of particles. (b) A multi-stage free flow acoustophoresis device. (Dalili A, et al. 2019)
In addition to size, differences in density and compressibility can be used to separate and sort particles. The separation process of binary acoustophoresis is based on the idea that two particles may have different acoustic contrast factors. Particles with positive acoustic factors are driven to the pressure node by the main acoustic force. For particles with negative acoustic factors, the force acts in the opposite direction, and the particles move to the antinodes. For a half-wavelength resonator, the particles are separated between the center outlet and the side outlets.
Based on this, particles or encapsulated cells of the same size but different densities can be separated and classified. Binary separation is very important for applications in the medical field, such as removing lipid emboli from blood flowing out during cardiac surgery.
By manipulating the frequency of acoustic waves, the particles in the microfluidic device can be classified. By adjusting the magnitude of the main acoustic radiation force, two different particles can be aligned to different pressure nodes. To get the best results, excellent control of frequency and time interval as well as temperature is required. Switching frequency is also a good technique for sorting similar particles to multiple outlets. Depending on the frequency of the application, the particles are driven to different pressure nodes.
Figure.2 The ABPS consists of two independently controlled, serially connected stages, each characterized by a channel width w, and piezo actuation amplitude V and frequency f. A particle mixture of varied sizes is introduced into the sides of stage 1 alongside a central buffer flow. Due to the volume dependence of the acoustic radiation force, larger particles are focused faster than smaller particles. The larger, selected particles are reintroduced into the sides of stage 2 and subject to another round of independently controlled focusing. (Adams J. D, et al. 2010)
- Dalili A, et al. (2019). "A Review of Sorting, Separation and Isolation of Cells and Microbeads for Biomedical Applications: Microfluidic Approaches." Analyst. 144: 87-113.
- Adams J. D, et al. (2010). "Tunable Acoustophoretic Band-pass Particle Sorter." Appl Phys Lett. 97(6): 064103.
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