Surface acoustic waves (SAW) offer high precision, contactless control of small volumes of liquid in microfluidic devices. Through the implementation of SAW in microfluidic devices (Acoustofluidics), researchers can achieve label free sorting of cells and nanoparticles, or fluid characterization without reagents.
Acoustofluidic devices employ piezoelectric interdigital transducers (IDTs) to produce acoustic waves. IDTs are typically composed of quartz or lithium niobate (LiNbO3). IDTs allow for electrical signals to be transduced into precisely controlled acoustic waves. When these acoustic waves are generated within a medium along an interface, they will travel across the surface of the medium producing surface acoustic waves (SAWs). SAWs are generally, high frequency, variable amplitude waves causing the energy of the waves to travel across the surface and efficiently dissipate into small volumes of liquid. This dissipation allows for the actuation and precise manipulation of these small volumes of liquid (Delsing).
Microfluidic devices that utilize SAW are often composed of polydimethylsiloxane (PDMS) channels and an IDT commonly composed of lithium niobate. Plasma treatment provides irreversible, water-tight bonding of PDMS microchannel layers with PDMS layers containing the IDT.
Cell & Nanoparticle Sorting
Once fabricated, SAW devices can be utilized for cell or particle sorting and manipulation. Utilizing SAW devices allowed for novel implementation of fluorescence activated droplet sorting (FADS), and separation based on cell density and compressibility, while also being used to fabricate tumor spheroids with controllable size for biomedical research (Li, Xie, Chen). SAWs have also been used for nanoparticle sorting through the excitation of a microparticle bead bed that captures nanoparticles when a SAW is applied and releases the nanoparticles when excitation stops (Habibi). These applications of SAWs, along with others in the life sciences and sorting, present a biocompatible, label-free, contact-less and high-throughput means of manipulating cells and particles within microfluidic devices.
Fluid Characterization
Fluid characterization is another field in which SAWs is utilized. With similar benefits as those shown for life science research, the contact-less and tunable nature of SAWs allows for controlled deformation of small volumes of flowing fluid. By coupling this with image processing, the distortions created by the SAWs have be analyzed to provide information such as the viscosity, thermal conductivity and thermoelastic properties of a fluid with no additional reagents or chemical manipulations required (Khalid).
SAW devices present themselves as an easily incorporated feature for microfluidic devices which couples well with life sciences research. By harnessing the high-throughput nature of microfluidics with the contact-free, label-free benefits of acoustic waves, SAWs provide a mechanism to manipulate droplets precisely giving the user greater control and confidence in results.
Relevant Articles from Harrick Plasma Users
- Chen B, Wu Y, Ao Z, Cai H, Nunez A, Liu Y, Foley J, Nephew K, Lu X and Guo F. “High-throughput acoustofluidic fabrication of tumor spheroids”. Lab on a Chip, pubs.rsc.org, 2019, 19, 1755-1763. 10.1039/C9LC00135B
- Habibi R and Neild A. “Sound Wave Activated Nano-Sieve (SWANS) for High Performance Enrichment of Nanoparticles”. Lab on a Chip, 2019, 19, 3032-3044. 10.1039/C9LC00369J
- Khalid M, Ray A, Cohen S and Tassieri M. “Computational Image Analysis of Guided Acoustic Waves Enables Rheological Assessment of Sub-nanoliter Volumes”. ACS Nano, ACS Publications, 2019, 13, 11062-11069. 10.1021/acsnano.9b03219
- Li P, Ma Z, Zhou Y, Collins D, Wang Z and Ai Y. “Detachable Acoustophoretic System for Fluorescence-Activated Sorting at the Single-Droplet Level”. Analytical Chemistry, ACS Publications, 2019, 91, 9970-9977. 10.1021/acs.analchem.9b01708
- Wu Z, Cai H, Ao Z, Nunez A, Liu H, Bondesson M, Guo S and Guo F. “A Digital Acoustofluidic Pump Powered by Localized Fluid-Substrate Interactions”. Analytical Chemistry, ACS Publications, 2019, 91, 7097-7103 10.1021/acs.analchem.9b00069