Surface-Acoustic-Wave-Driven Acoustic Tweezing in a Silicon Microfluidic Chip

Surface-acoustic-wave (SAW)-driven acoustic tweezers (ATs) have been widely explored for high-resolution ultrasonic sample manipulation. Among these, hybrid ATs comprising a reusable SAW chip and a disposable glass or silicon microfluidic (MF) chip as a superstrate offer advantages such as reduced experimental costs and minimized cross-contamination between experiments. However, maximizing the acoustic pressure within the MF channel requires efficient acoustic coupling between the SAW and the MF structure. In this work, we investigate the frequency-dependent characteristics of acoustophoresis of 50-MHz hybrid ATs composed of an SAW chip and a silicon MF chip. We elucidate the role of the bulk acoustic wave (BAW) resonances in the silicon substrate in facilitating the formation of acoustic standing waves (SWs) within the MF channel. The experimental results demonstrate the generation of acoustic pressures up to $2.1~\pm ~0.5$ MPa inside the channel. The fabricated device was successfully used to probe the transient viscoelastic deformation of HEK293T cells and to trap motile cells Tetrahymena. These findings highlight the potential of the proposed hybrid ATs as a platform for acousto-mechanical testing of soft matter and biological samples.