Three-dimensional heating and patterning dynamics of particles in microscale acoustic tweezers

Robust global arrangement by coherent enhancement in Huygens-Fresnel traveling surface acoustic wave interference field

The application of standing surface acoustic wave (SSAW) tweezers based on backpropagation superposition to achieve precise behavior manipulation of microscale cells and even nanoscale bacteria has been widely studied and industrialized. However, the structure requires multiple transducer components or full channel resonance. It is very challenging to design a simple structure for nano-control by complex acoustic field. In this study, a reflector-interdigital transducer (R-IDT) acoustofluidic device based on unilateral coherence enhancement is proposed to achieve SSAW definition features of periodic particle capture positions. The SAW device based on a unilateral transducer can not only generate leaky-SAW in water-filled microchannel, but also have a contribution of spherical waves in the vibration area of the substrate-liquid interface due to the Huygens-Fresnel diffractive principle. Both of them form a robust time-averaged spatial periodicity in the pressure potential gradient, accurately predicting the lateral spacing of these positions through acoustic patterning methods. Furthermore, a reflector based on Bragg-reflection is used to suppress backward transmitted SAW and enhance forward conducted SAW beams. By using a finite element model, R-IDT structure's amplitude enhances 60.78% compared to single IDT structure. The particle manipulation range of the diffractive acoustic field greatly improves, verified by experimental polystyrene microspheres. Besides, biocompatibility is conformed through red blood cells and Bacillus subtilis. We investigate the overall shift of periodic pressure field that can still occur when the phase changes. This work provides a simpler and low-cost solution for the application of acoustic tweezer in biological cell culture and filtering.

From rectangular to diamond shape: on the three-dimensional and size-dependent transformation of patterns formed by single particles trapped in microfluidic acoustic tweezers

Generally, the pattern formed by individual particles trapped inside a microfluidic chamber by a two-dimensional standing acoustic wave field has been considered only the result of the acoustic radiation force. Previous studies showed that particles can be trapped at the local minima and maxima of the first-order pressure and velocity fields. Thus, either a rectangular or a diamond pattern can be formed solely depending on the particle size, when the acoustic field is unchanged, and the material properties of the particles and the fluid are fixed. In this paper, we report about the co-existence of different patterns with particles of the same size. The actual shape of the patterns depends mainly on the ratio between particle diameter and wavelength. In addition, particles were found to be trapped at locations that coincide with the position of antinodes, even though the particles have a positive acoustic contrast factor. These phenomena imply that the trapping of individual particles cannot be described by the acoustic radiation force solely. Hence, further research is required, taking the viscous drag force caused by the fluid flow induced by the acoustic streaming effect into account.

Bisymmetric coherent acoustic tweezers based on modulation of surface acoustic waves for dynamic and reconfigurable cluster manipulation of particles and cells

Acoustic tweezers based on surface acoustic waves (SAWs) have raised great interest in the fields of tissue engineering, targeted therapy, and drug delivery. Generally, the complex structure and array layout design of interdigital electrodes would restrict the applications of acoustic tweezers. Here, we present a novel approach by using bisymmetric coherent acoustic tweezers to modulate the shape of acoustic pressure fields with high flexibility and accuracy. Experimental tests were conducted to perform the precise, contactless, and biocompatible cluster manipulation of polystyrene microparticles and yeast cells. Stripe, dot, quadratic lattice, hexagonal lattice, interleaved stripe, oblique stripe, and many other complex arrays were achieved by real-time modulation of amplitudes and phase relations of coherent SAWs to demonstrate the capability of the device for the cluster manipulation of particles and cells. Furthermore, rapid switching among various arrays, shape regulation, geometric parameter modulation of array units, and directional translation of microparticles and cells were implemented. This study demonstrated a favorable technique for flexible and versatile manipulation and patterning of cells and biomolecules, and it has the advantages of high manipulation accuracy and adjustability, thus it is expected to be utilized in the fields of targeted cellular assembly, biological 3D printing, and targeted release of drugs.