Research on particle separation microfluidic chip based on standing surface acoustic wave

Microparticle and cell separation is a key process in the study of cell properties, clinical diagnosis and disease treatment. In this paper, a particle separation method based on the combination of surface acoustic wave and microfluidic technology is proposed, which uses standing surface acoustic wave to achieve the separating of polystyrene microparticles of different sizes twice in a continuous flow. Through numerical simulation of the device model and particle trajectory simulation, the optimum particle separation parameters were determined. The particle separation microfluidic chip has successfully realized the particle separation. The separation yield of 5 μm particles is about 92.1%. The separation rate of 1 μm particles is about 83.3%.


Introduction
The efficient sorting of particles and cells is of great significance in the fields of materials [1],chemistry [2], biology [3], medicine [4]and so on.In this paper, a particle separation technique based on surface acoustic wave is proposed, which provides an unmarked, undamaged, high-precision and high-efficiency separation method.Two different standing surface acoustic wave fields are applied in the same microchannel.When the mixed particles at the entrance flow through the sound field region under the action of sheath flow, the particles of different sizes will produce different lateral displacements according to their different amounts of acoustic radiation force.After flowing out of the sound field region, they will flow out from different outlets of the microchannel and be collected.In this way, the separating of polystyrene microparticles of different sizes twice can be realized in continuous flow mode.

Working principle
As shown in Figure 1,two sets of unidirectional interdigital transducers are installed on both sides of the microchannel as inputs to form a standing wave sound field which will deflect the suspended particles in the flow channel.In addition, by tilting the interdigital electrodes on both sides of the flow channel at a certain Angle, the pressure nodes periodically distributed in the flow channel are distributed in the direction of the width of the flow channel at the oblique angle of the interdigital electrodes.When the particles move along the pressure nodes, a larger transverse separation distance will be generated to improve the efficiency of particle separation [5].

Model simulation
We use finite element simulation software to model and simulate the interfinger transducer, analyze the effect of the logarithm of the interdigital electrode and the length of the acoustic aperture on the insertion loss of the device, We determined that the logarithm of the interdigital electrodes is 30 pairs and the acoustic aperture length is 4mm.As shown in Figure 2. particles in the sound field, we studied the effects of different saw input power, main channel flow rate and ssaw field tilt Angle on the particle trajectory, and determined that the input power of the first ssaw field was 24dbm, the flow rate was 13ul/min, and the tilt Angle was 5°.
The second ssaw field has an input power of 39dbm, a flow rate of 12ul/min, and a tilt Angle of 7°.As shown in Figure 3 and Figure 4.The particle separation microfluidic chip designed in this paper is mainly composed of a microchannel for liquid transport and a surface acoustic wave device for generating a standing wave sound field, as shown in Figure 5.The surface acoustic wave device is composed of a piezoelectric substrate and a metal interfinger transducer made on its surface.After the inlet and outlet of the microchannel are drilled respectively and connected with a thin tube, the microchannel is installed in the middle of two sets of interfinger transducers on the piezoelectric substrate at a certain Angle to complete the assembly of the chip.

Results and discussion
In the particle separation experiment, under the conditions of SAW input power PI=27 dBm and PI=39 dBm and sample flow rate of 1 μL/min, for the optimal sorting rate ,5 um particles and 1um particles was measured at 92.1% and 83.3%;and for optimal particle purity,5 um particles and 1um particles was measured at 95.6% and 92.4% The microfluidic chip designed in this paper provides an optional technology for the direct separation of exosomes from human whole blood, which has important research significance and application value.In the future work, the use of human blood can be considered for experiments to get closer to the real results of the experiment.

Figure 1 .
Figure 1.Pressure node distribution and particle force analysis in microchannel

Figure 2 .
Figure 2. (a)Insertion loss of delay line models with different FEUDT electrode logarithms (b)Insertion loss of delay line models with different FEUDT electrode apertures Through modeling and simulation of the force analysis and velocity decomposition of

Figure 3 .
Figure 3. Particle trajectory in different SSAW fields with different parameters

Figure 4 .
Figure 4. Relationship between maximum separation distance and tilt Angle ɵ under different input power of SAW (a)5μm particles；(b)1μm particles 3 Device designThe particle separation microfluidic chip designed in this paper is mainly composed of a microchannel for liquid transport and a surface acoustic wave device for generating a standing wave sound field, as shown in Figure5.The surface acoustic wave device is composed of a piezoelectric substrate and a metal interfinger transducer made on its surface.After the inlet and outlet of the microchannel are drilled respectively and connected with a thin tube, the microchannel is installed in the middle of two sets of interfinger transducers on the piezoelectric substrate at a certain Angle to complete the assembly of the chip.

Figure 5 .
Figure 5. Structure diagram of particle separation microfluidic chip

Figure 6 .
Figure 6.(a) Trajectories of 5μm particles;(b) Trajectories of 1μm particles ; (c)Particle separation yield and purity 5 Conclusion The particle separation technology based on microfluidic has the advantages of high precision, high efficiency and low sample demand, and has great significance and wide application in modern chemical industry, biomedicine, material science, clinical therapy and other fields.The microfluidic chip designed in this paper provides an optional technology for the direct separation of exosomes from human whole blood, which has important research significance and application value.In the future work, the use of human blood can be considered for experiments to get closer to the real results of the experiment.Acknowledgement This work was supported by Science and Technology on Electromechanical Dynamic