Acoustofluidic separation of cells and particles

Abstract

Acoustofluidics, the integration of acoustics and microfluidics, is a rapidly growing research field that is addressing challenges in biology, medicine, chemistry, engineering, and physics. In particular, acoustofluidic separation of biological targets from complex fluids has proven to be a powerful tool due to the label-free, biocompatible, and contact-free nature of the technology. By carefully designing and tuning the applied acoustic field, cells and other bioparticles can be isolated with high yield, purity, and biocompatibility. Recent advances in acoustofluidics, such as the development of automated, point-of-care devices for isolating sub-micron bioparticles, address many of the limitations of conventional separation tools. More importantly, advances in the research lab are quickly being adopted to solve clinical problems. In this review article, we discuss working principles of acoustofluidic separation, compare different approaches of acoustofluidic separation, and provide a synopsis of how it is being applied in both traditional applications, such as blood component separation, cell washing, and fluorescence activated cell sorting, as well as emerging applications, including circulating tumor cell and exosome isolation.

Keywords: Chemistry; Engineering; Nanoscience and technology.

Fig. 1. Generating acoustic waves via piezoelectric materials.

a When a voltage is applied to the electrodes, the piezoelectric material expands and contracts normal to the surface. This mode of vibration is called thickness mode. b For some material orientations, when a voltage is applied, the piezoelectric material will deform in the horizontal direction. This mode of vibration is called shear mode. c By exciting interdigitated transducers (IDTs) patterned on a piezoelectric crystal, vibrations can be generated on the surface of the material in the form of surface acoustic waves (SAWs). The wavelength of the SAW (λ) is dependent on the width and spacing between IDT fingers

Fig. 2. Schematic representation of general acoustofluidic separation techniques.

a Standing acoustic waves are excited in the half-wavelength resonator formed by a silicon-based microfluidic channel. b Microfluidic channels are formed by stainless steel or other high acoustic impedance materials by stacking several layers together. A transducer is attached to the channel to excite vibrations. Standing waves are generated by the reflection of waves at steel/fluid interface. c PDMS channels are bonded in between two IDTs. A standing SAW is formed on the piezoelectric substrate by the interference of oppositely propagating SAWs. Upon contact, SAWs leak into the fluid domain in the form of leaky waves. d One pair of IDTs generate traveling SAWs that leak into the fluid inside the PDMS microfluidic channel. e Particles are directed towards lower (higher) acoustic pressure regions through the effect of acoustic radiation force (Fr) if the acoustic contrast factor (Φ) is larger (smaller) than zero

Fig. 3. Acoustofluidic separation of blood components.

a Separating blood cells from plasma by acoustic waves for the applications of blood wash or plasmapheresis. Reprinted with permission from the American Chemical Society. b Separating platelets from blood cells by a SAW device. Reprinted with permission from the Royal Society of Chemistry. c High-throughput separation of platelets and blood cells using a BAW technique. Reprinted with permission from the Royal Society of Chemistry. d Separation of mononuclear cells (lymphocytes and monocytes) from blood using a two-stage acoustofluidic separation device. Reprinted with permission from the Nature Publishing Group

Fig. 4. Acoustofluidic separation of cancer cells.

a Separation of cancer cells using BAW techniques. Reprinted with permission from the American Chemical Society. b Circulating tumor cells (CTCs) are separated from WBCs by using a tilted-angle SAW-based device. Reprinted with permission from the National Academy of Sciences. c High-throughput isolation of CTCs by using standing SAW and PDMS/glass hybrid channel. Reprinted with permission from the John Wiley & Sons, Inc

Fig. 5. Acoustofluidic separation of bacteria.

a Separation of bacteria from blood cells using a standing SAW technique. Reprinted with permission from the American Chemical Society. b Blood cells are deflected by tilted-angle SAW field and thus separated from bacteria. Reprinted with permission from the IOP Publishing. c Separation and enrichment of bacteria by acoustofluidics. Reprinted with permission from the American Chemical Society

Fig. 6. Acoustofluidic separation and manipulation of nanoparticles.

a Purification of exosomes from microvesicles by standing SAW. Reprinted with permission from the American Chemical Society. b Isolation of exosomes from whole blood by an integrated acoustic separation platform. Reprinted with permission from the National Academy of Sciences. c Focusing nanoparticles using micro-vortex induced by high-frequency focused SAW. Reprinted with permission from the Royal Society of Chemistry. d Separation of nanoparticles by integrating acoustic radiation force and dielectrophoresis. Reprinted with permission from the Royal Society of Chemistry

Fig. 7. Acoustofluidic droplet separation.

a Deflection of droplets by traveling SAW-induced acoustic streaming. Reprinted with permission from the Royal Society of Chemistry. b Separation of droplets by changing the frequencies of slanted IDT. Reprinted with permission from the American Society of Chemistry. c Separation of cell-encapsulated alginate beads based on density. Reprinted with permission from the AIP Publishing

Fig. 8. Acoustofluidic-based single particle/cell sorting.

a A schematic of an acoustic sorting device using focused IDTs (not to scale). A sample solution with cells/particles to be sorted is infused from the middle inlet and a buffer solution is infused from both side inlets. b When a confined acoustic field is generated through focused IDTs, a polystyrene particle, labeled as 2, is pushed to the collection outlet from its initial path. Scale bar: 50 μm. c A schematic of an acoustic sorting device based on traveling SAWs using focused IDTs (not to scale). d A time-averaged image showing that 3 μm single particles are pushed to the lower outlet using 300 μs, 30 mW pulsed focused traveling SAWs with a beam width of 25 μm. Scale bar: 50 μm. Figures in a, b are adapted from ref. , and figures in c, d are adapted from ref. with permission from the Royal Society of Chemistry