Movable Surface Acoustic Wave Tweezers: A Versatile Toolbox for Micromanipulation

Surface acoustic wave (SAW) tweezers utilize interdigital transducers (IDTs) to generate acoustic waves to manipulate fluids and particles in a contact-free and biocompatible manner. To adapt to different requirements in various scenarios, SAW tweezers require a flexible acoustic field to achieve dynamic control of microtargets for different manipulation functions. It would be very beneficial for practicalization and popularization if a contactless micromanipulation method could exert controllable yet calibratable force to move, hold, and guide micro/nanoobjects and assemble, pattern, reconfigure, and translate micro/nanoobject swarms. However, it is difficult for traditional SAW tweezers to achieve precise and complex translation and rotation manipulation, with its operating range and degrees of freedom limited by its own device structure.
To overcome this limitation, we proposed the movable surface acoustic wave tweezers, which aimed to realize multiple functions with one device, complex and precise manipulation in a continuous manner, high applicability for different targets, and controllable and selective operation on samples with similar acoustic characteristics. The relative motion between the piezoelectric substrate and the channel allows for translation and rotation of the acoustic field, enabling the movable SAW tweezers to have a very large operation range, precise manipulation performance, and various functions.
 
Direct translation with high accuracy and high applicability
The movable SAW tweezers device has a multilayer structure. A fluid coupling layer is used to connect the channel bottom and transducers, which allows the relative motion and transmission of acoustic waves. The IDT generates acoustic waves based on input signal, which leaks into the coupling layer fluid at the Rayleigh angle, passes through the coupling layer and the channel bottom, and forms a standing field inside the channel. This method allows for direct control of particle motion. In the Figure 1, the dot draft of Mona Lisa is concatenated from frames of the trajectory of one 31.1 μm particle.
In addition to particle, a series of targets with different size, structure and acoustic characteristics are tested with this system, ranging from 10 μm (one twentieth of the SAW wavelength) to more than 1000 μm (fish eggs, > 5 times of the SAW wavelength). The motion accuracy is mainly determined by the positioning accuracy of the displacement table, so the target can have a high motion and positioning accuracy (in the level of micrometers) in a long travel distance (in the level of millimeters), without limited by the acoustic aperture.
 
Figure 1. Experimental setup and working principle of the movable SAW tweezer system. a. Movable SAW tweezer platform setup. b. Multilayer structure of the device. c. Schematic diagram of the acoustic field movement method. d. Control of the movement trajectory of a 31.1 μm particle by the movable acoustic field. The particle positions are captured frame by frame. e. Mona Lisa painting and a Mona Lisa pattern depicted by the particle trajectories. f. Schematic diagram of the cell collection method. g. Utilizing bubbles to collect cancer cells and form circular cell clusters around bubbles.

Multifunctional micromanipulation with high degrees of freedom
Based on the precise translation operation, the movable SAW tweezer can further perform more functions including cell assemble and sessile droplet fusion. But in order to fundamentally improve the degree of freedom in manipulation, we implemented the rotation function. The movable SAW tweezers can guide targets to perform in-plane rotation around any center in the XY plane and following any angle, and precise out-of-plane rotation to gradually show targets’ inner structure form different angles by exerting a local shear stress with a relatively small actuation power. All of these functions are carried out by one device, which means that the movable SAW tweezers can perform continuous translation and rotation operations on different targets without the need for transducers replacement of or signal modulation.
 
Spatial selective micromanipulation with microbubble scattered field
For most SAW micromanipulation methods, their acoustic fields affect all cells within the acoustic field region, applying a radiation force in the same way, making it difficult to independently manipulate individual cells. Using cell assemble function as an example, the acoustic field can only form cell clusters with nearby cells in a random and uncontrollable manner, which lacks selectivity and accuracy. To further optimize the performance and functions of movable acoustic tweezers, we expect them to overcome these limitations and manipulate microscale targets in a manner similar to that of a microrobot. That is, instead of just holding or moving a target, the movable acoustic tweezers should hold and use a tool.
The microbubble is introduced as a local radiation amplifier to selectively capture and move the target particles. The high acoustic characteristic impedance difference at the gas-liquid interface results in high ratio sound energy reflection, while the shorter acoustic attenuation distance of scattered waves compared to SAWs on piezoelectric substrates restricts the enhanced acoustic field in a local region. Under the combined action of coupled acoustic field and acoustic interaction force, the targeted particle or cell can be independently manipulated, with minimum influence on other unintended ones. With microbubble as a tool for SAW tweezers to perform indirect manipulation, we can selectively collect cells and form cluster around the bubble, and capture wanted cells in a laminar flow. This movable bubble significantly improves the feasibility of manipulation for particles with the size of several microns.
 
In this study, the system we propose shows the feasibility and versatility of a movable SAW field and, more importantly, breaks the chains of its own structure and gives full play to its advantages in a novel way. With further study in the future, this system is expected to be able to complete more functions and be utilized in more applications.