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Acoustic Tweezers for the Life Sciences

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Acoustic Tweezers for the Life Sciences REVIEW ARTICLE https://doi.org/10.1038/s41592-018-0222-9 Acoustic tweezers for the life sciences Adem Ozcelik 1, Joseph Rufo1, Feng Guo2, Yuyang Gu1, Peng Li2, James Lata2 and Tony Jun Huang 1,2* Acoustic tweezers are a versatile set of tools that use sound waves to manipulate bioparticles ranging from nanometer-sized extracellular vesicles to millimeter-sized multicellular organisms. Over the past several decades, the capabilities of acoustic tweezers have expanded from simplistic particle trapping to precise rotation and translation of cells and organisms in three dimensions. Recent advances have led to reconfigured acoustic tweezers that are capable of separating, enriching, and pat- terning bioparticles in complex solutions. Here, we review the history and fundamentals of acoustic-tweezer technology and summarize recent breakthroughs. ew discoveries are often preceded by technological progress. However, they are dependent on particle or cell polarizability and The development of cell theory, for example, is inextricably generally require low-conductivity media, which may disrupt cell Nlinked to advances in microscopy1. Just as early advances physiology. Optoelectronic tweezers are the dynamic counterpart in the ability to visualize cells resulted in the development of cell to electrode-based electrokinetic tweezers. Instead of electrodes, a theory, recent advances in the ability to manipulate single cells and light source and photoconductive substrate induce dielectrophore- biomolecules have contributed to breakthroughs in microbiology2, sis, thus enabling dynamic manipulation at relatively low optical- molecular biology3, biophysics4, and bioanalytical chemistry5. power intensities13. However, they are constrained by the same Acoustic tweezers are an emerging platform for the precise requirement for low-conductivity media, thus restricting their use manipulation of bioparticles across a broad size range. Acoustic in many biological applications. Hydrodynamic tweezers are per- tweezers spatially and temporally manipulate matter by using the haps the simplest approach for achieving particle manipulation, by interaction of sound waves with solids, liquids, and gases. The term using fluid flows to position particles within a microchannel17. They ‘acoustical tweezers’ was first coined to describe the linear transla- are capable of a variety of applications, including trapping, focusing, tion of latex spheres and frog eggs that were trapped in an acoustic and sorting, but their controllability is rather poor, and their ability field6. Since then, a substantial number of acoustic-tweezer configu- to manipulate nanoparticles is limited. rations have been developed for applications in science and engi- Acoustic tweezers are a versatile tool that can address many of neering. Many of these acoustic-tweezer devices are modeled after the limitations of other particle-manipulation techniques. Because their predecessor, optical tweezers. Optical tweezers, invented in acoustic waves with frequencies in the kilohertz-to-megahertz 1986 (ref. 7), were quickly adopted as an invaluable tool in biology, range can be easily generated19–21, acoustic tweezers can directly chemistry, and physics, and have been used to trap viruses, bacte- manipulate particles across a length scale spanning more than five ria, and cells8,9. Despite being a powerful tool for force spectroscopy orders of magnitude (10−7 to 10−2 m). In addition, the applied acous- and biomolecular manipulation, traditional optical tweezers require tic power (10−2–10 W/cm2) and frequencies (1 kHz to 500 MHz) complex optics, including high-powered lasers and high-numerical- are similar to those used in ultrasonic imaging (2–18 MHz, less aperture objectives, and they are potentially damaging to biological than 1 W/cm2)22, which has been safely used in diagnostic applica- samples10,11. To improve the accessibility and versatility of contact- tions21,23. Studies on the biocompatibility of acoustic tweezers have free particle-manipulation technology, alternatives to optical twee- shown that their operating parameters can be optimized to avoid zers have since been developed. damage in cells24,25 and small-animal models26. For example, red Additional platforms for contactless particle manipulation rely blood cells placed in an acoustic-tweezer device for up to 30 min on different mechanisms, including magnetic12, optoelectronic13, show no changes in cell viability25, and zebrafish embryos placed plasmonic14, electrokinetic15,16, and hydrodynamic forces17 (over- in an acoustic-tweezer device for the same duration do not exhibit view of the operating parameters and system requirements for these developmental impairments or changes in mortality rates26. The techniques in Table 1). Magnetic and optical tweezers provide the versatility and biocompatibility of acoustic tweezers should allow highest degree of spatial resolution; however, manipulating particles current challenges in biology and biomedicine to be addressed, such smaller than 100 nm is challenging with either technique. Plasmonic as the isolation and detection of circulating biomarkers for cancer tweezers are a variation of optical tweezers that make use of locally diagnostics27. These biomarkers range in size from nanometer-sized enhanced electromagnetic fields on nanostructured substrates. extracellular vesicles28 to micrometer-sized circulating tumor cells Plasmonic tweezers require lower laser power and are capable of (CTCs)29. Moreover, acoustic tweezers are capable of isolating both trapping nanometer-sized particles, but the large localized inten- extracellular vesicles30 and CTCs31, capabilities valuable for oncol- sities that help to trap particles can also lead to substantial heat- ogy laboratories. For cell-to-cell and cell-to-environment interac- ing of the surrounding fluid18. As a result, thermal management of tion studies, precise control over the physical position of cells, while these devices is necessary to prevent sample heating and convective preserving normal physiology, is necessary. Acoustic tweezers can flows. Electrokinetic tweezers, which use both electrophoretic and form flexible 2D32 and 3D33 cell arrays and have been used in inter- dielectrophoretic forces, apply an electric field to trap and manipu- cellular communication studies34. Furthermore, noninvasive tools late particles across the nanometer–to-millimeter size range15,16. for manipulating organisms are required to investigate internal 1Department of Mechanical Engineering and Materials Science, Duke University, Durham, NC, USA. 2Department of Engineering Science and Mechanics, Pennsylvania State University, University Park, PA, USA. *e-mail: [email protected] NatURE METHODS | VOL 15 | DECEMBER 2018 | 1021–1028 | www.nature.com/naturemethods 1021 REVIEW ARTICLE NATURE METHODS Table 1 | Summary of different particle-manipulation platforms Technique Size range Input powera Spatial resolution Labeling required Additional system requirements (W/cm2) Acoustic tweezers 100 nm–10 10−2–10 1–10 µ m No Acoustic source mm Optical tweezers7–9 100 nm–1 106–107 0.1–1 nm Required for smaller High-powered laser, high-numerical- mm particles aperture lens Magnetic tweezers12 1 µ m–10 µ m 1–10 tesla 1–10 nm Yes Permanent magnet, superparamagnetic beads Optoelectronic tweezers13 100 nm–10 10−2–10 1–10 µ m No Photoconductive substrate, µ m low-conductivity media Plasmonic tweezers14 10 nm–1 µ m 102–104 10–100 nm No Plasmonic substrate, heat sink Electrokinetic tweezers15,16 1 nm–1 mm 104–107 V/m 0.1–1 µ m Yes Prepatterned electrodes, Low-conductivity media Hydrodynamic tweezers17 100 nm–1 N/A 1–10 µ m No Multiple pressure regulators, mm flow-control algorithm a The minimum field strength is reported for magnetic and electrokinetic tweezers. processes, such as the neuronal activity in Caenorhabditis elegans35. surface39. 1D and 2D interference patterns can be achieved by using Acoustic tweezers have been used to manipulate and rotate C. ele- sets of two and four IDTs, respectively39,40 (Fig. 1b). Suspended par- gans36 as well as larger organisms, such as zebrafish embryos26, with ticles in a standing SAW field move to pressure nodes or antinodes no adverse effects. according to their physical properties41. In addition to 2D in-plane Although acoustic tweezers have been used in various biological manipulation, standing SAWs are used to achieve 3D manipulation studies, the versatility of acoustic tweezers has proven to be a double- by exploiting the modulation of acoustic parameters (for example, edged sword. Currently, many different acoustic-tweezer platforms phase shifts and amplitude modulation), thus enabling the trapping are available, each with advantages and shortcomings; however, for position to be changed in real time33. Owing to their compact size, researchers who are not technical experts in the field, identifying the SAW-based tweezers can be conveniently integrated with microflu- acoustic-tweezer technology best suited for a particular application idic systems enabling versatile lab-on-a-chip tools40. is difficult. For example, for manipulating nanometer-sized objects, Standing-wave tweezers are mainly used for separating and pat- should an acoustic-tweezer device based on surface acoustic waves terning different types of particles and cells. Whereas BAW-based (SAWs) or bulk acoustic waves (BAWs) be used? Which acoustic- standing-wave tweezers have the advantage of handling higher vol- tweezer platform is best for handling large volumes of biofluids?
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