Plug-and-play acoustic tweezer enables droplet centrifugation on silicon superstrate with surface multi-layered microstructures

Previous works have demonstrated the use of acoustic waves to manipulate microparticles and biological samples on bare glass and silicon superstrates. Traditionally, the surface acoustic wave (SAW) is converted into a Lamb wave propagating within a bare silicon superstrate via a thin coupling agent. The potential impact of acoustically driven microfluidics could be further augmented if applied to superstrates integrated with sensing structures that would allow sequential analysis after sample treatment. In this work, we demonstrate the applicability of acoustically-driven micro-centrifugation on a superstrate with multiple thin film layers to mimic sensing structures on a chip, and characterize the underlying performance. We propose an integrated plug-and-play platform comprised of a reusable SAW device interfaced to a disposable surface-micromachined silicon (SMS) superstrate processed with five layers of thin films. To address the shortcomings of existing coupling agents, we examine and compare the transmission efficiency and long-term stability of four kinds of coupling agents with the aim to enable disposable acoustofluidics applications. To investigate the effect of different superstrates on the performance of droplet centrifugation, we characterize and compare centrifugation on different superstrates. High-performance concentration was realized on the SMS superstrate under different conditions, such as input power, temperature, droplet volume, and particle size and density. In terms of more advanced fluidic handling functionality on a multi-layered SMS superstrate, we demonstrate ultrasonic isopycnic separation between microbeads differentiated by density on the SMS superstrate. The results herein lay the groundwork for realizing particle concentration on complex superstrates processed with multilayer films that represent a range of microfabricated sensors towards a broader goal of integrating acoustically driven concentration capabilities and sensing for applications in diagnostics and fundamental analysis.