Accurate and high throughput cell sorting is a critical enabling technology in molecular and cellular biology, biotechnology, and medicine While conventional methods can provide high efficiency sorting in short timescales, advances in microfluidics have enabled the realization of miniaturized devices offering similar capabilities that exploit a variety of physical principles We classify these technologies as either active or passive Active systems generally use external fields (eg, acoustic, electric, magnetic, and optical) to impose forces to displace cells for sorting, whereas passive systems use inertial forces, filters, and adhesion mechanisms to purify cell populations Cell sorting on microchips provides numerous advantages over conventional methods by reducing the size of necessary equipment, eliminating potentially biohazardous aerosols, and simplifying the complex protocols commonly associated with cell sorting Additionally, microchip devices are well suited for parallelization, enabling complete lab-on-a-chip devices for cellular isolation, analysis, and experimental processing In this review, we examine the breadth of microfluidic cell sorting technologies, while focusing on those that offer the greatest potential for translation into clinical and industrial practice and that offer multiple, useful functions We organize these sorting technologies by the type of cell preparation required (ie, fluorescent label-based sorting, bead-based sorting, and label-free sorting) as well as by the physical principles underlying each sorting mechanism

Microfluidic cell sorting: a review of the advances in the separation of cells from debulking to rare cell isolation

Techniques that can dexterously manipulate single particles, cells, and organisms are invaluable for many applications in biology, chemistry, engineering, and physics. Here, we demonstrate standing surface acoustic wave based “acoustic tweezers” that can trap and manipulate single microparticles, cells, and entire organisms (i.e., Caenorhabditis elegans) in a single-layer microfluidic chip. Our acoustic tweezers utilize the wide resonance band of chirped interdigital transducers to achieve real-time control of a standing surface acoustic wave field, which enables flexible manipulation of most known microparticles. The power density required by our acoustic device is significantly lower than its optical counterparts (10,000,000 times less than optical tweezers and 100 times less than optoelectronic tweezers), which renders the technique more biocompatible and amenable to miniaturization. Cell-viability tests were conducted to verify the tweezers’ compatibility with biological objects. With its advantages in biocompatibility, miniaturization, and versatility, the acoustic tweezers presented here will become a powerful tool for many disciplines of science and engineering.

https://www.pnas.org/content/pnas/109/28/11105.full.pdf

On-chip manipulation of single microparticles, cells, and organisms using surface acoustic waves

The recent introduction of surface acoustic wave (SAW) technology onto lab-on-a-chip platforms has opened a new frontier in microfluidics. The advantages provided by such SAW microfluidics are numerous: simple fabrication, high biocompatibility, fast fluid actuation, versatility, compact and inexpensive devices and accessories, contact-free particle manipulation, and compatibility with other microfluidic components. We believe that these advantages enable SAW microfluidics to play a significant role in a variety of applications in biology, chemistry, engineering and medicine. In this review article, we discuss the theory underpinning SAWs and their interactions with particles and the contacting fluids in which they are suspended. We then review the SAW-enabled microfluidic devices demonstrated to date, starting with devices that accomplish fluid mixing and transport through the use of travelling SAW; we follow that by reviewing the more recent innovations achieved with standing SAW that enable such actions as particle/cell focusing, sorting and patterning. Finally, we look forward and appraise where the discipline of SAW microfluidics could go next.

Surface acoustic wave microfluidics

The need for efficient cell separation, an essential preparatory step in many biological and medical assays, has led to the recent development of numerous microscale separation techniques. This review describes the current state-of-the-art in microfluidics-based cell separation techniques. Microfluidics-based sorting offers numerous advantages, including reducing sample volumes, faster sample processing, high sensitivity and spatial resolution, low device cost, and increased portability. The techniques presented are broadly classified as being active or passive depending on the operating principles. The various separation principles are explained in detail along with popular examples demonstrating their application toward cell separation. Common separation metrics, including separation markers, resolution, efficiency, and throughput, of these techniques are discussed. Developing efficient microscale separation methods that offering greater control over cell population distribution will be important in realizing true point-of-care (POC) lab-on-a-chip (LOC) systems.

Microfluidics for cell separation.

The progress in microfabrication and lab-on-a-chip technologies has been a major area of development for new approaches to bioanalytics and integrated concepts for cell biology. Fundamental advances in the development of elastomer based microfluidics have been driving factors for making microfluidic technology available to a larger scientific community in the past years. In line with this, microfluidic separation of cells and particles is currently developing rapidly where key areas of interest are found in designing lab-on-a-chip systems that offer controlled microenvironments for studies of fundamental cell biology. More recently industrial interests are seen in the development of micro chip based flow cytometry technology both for preclinical research and clinical diagnostics. This critical review outlines the most recent developments in microfluidic technology for cell and particle separation in continuous flow based systems. (130 references)

Continuous separation of cells and particles in microfluidic systems

This work introduces a method of continuous particle separation through standing surface acoustic wave (SSAW)-induced acoustophoresis in a microfluidic channel. Using this SSAW-based method, particles in a continous laminar flow can be separated based on their volume, density and compressibility. In this work, a mixture of particles of equal density but dissimilar volumes was injected into a microchannel through two side inlets, sandwiching a deonized water sheath flow injected through a central inlet. A one-dimensional SSAW generated by two parallel interdigital transducers (IDTs) was established across the channel, with the channel spanning a single SSAW pressure node located at the channel center. Application of the SSAW induced larger axial acoustic forces on the particles of larger volume, repositioning them closer to the wave pressure node at the center of the channel. Thus particles were laterally moved to different regions of the channel cross-section based on particle volume. The particle separation method presented here is simple and versatile, capable of separating virtually all kinds of particles (regardless of charge/polarization or optical properties) with high separation efficiency and low power consumption.

Continuous particle separation in a microfluidic channel via standing surface acoustic waves (SSAW).

Selective actuation of a single microswimmer from within a diverse group would be a first step toward collaborative guided action by a group of swimmers. Here we describe a new class of microswimmer that accomplishes this goal. Our swimmer design overcomes the commonly-held design paradigm that microswimmers must use non-reciprocal motion to achieve propulsion; instead, the swimmer is propelled by oscillatory motion of an air bubble trapped within the swimmer's polymer body. This oscillatory motion is driven by the application of a low-power acoustic field, which is biocompatible with biological samples and with the ambient liquid. This acoustically-powered microswimmer accomplishes controllable and rapid translational and rotational motion, even in highly viscous liquids (with viscosity 6,000 times higher than that of water). And by using a group of swimmers each with a unique bubble size (and resulting unique resonance frequencies), selective actuation of a single swimmer from among the group can be readily achieved.

/pdf/selectively-manipulable-acoustic-powered-microswimmers-2ypctgpobb.pdf

Selectively manipulable acoustic-powered microswimmers

We demonstrate a tunable in-plane optofluidic microlens with a 9× light intensity enhancement at the focal point. The microlens is formed by a combination of a tunable divergent air–liquid interface and a static polydimethylsiloxane lens, and is fabricated using standard soft lithography procedures. When liquid flows through a straight channel with a side opening (air reservoir) on the sidewall, the sealed air in the side opening bends into the liquid, forming an air–liquid interface. The curvature of this air–liquid interface can be conveniently and predictably controlled by adjusting the flow rate of the liquid stream in the straight channel. This change in the interface curvature generates a tunable divergence in the incident light beam, in turn tuning the overall focal length of the microlens. The tunability and performance of the lens are experimentally examined, and the experimental data match well with the results from a ray-tracing simulation. Our method features simple fabrication, easy operation, continuous and rapid tuning, and a large tunable range, making it an attractive option for use in lab-on-a-chip devices, particularly in microscopic imaging, cell sorting, and optical trapping/manipulating of microparticles.

Zak Stratton

Papers

Continuous particle separation in a microfluidic channel via standing surface acoustic waves (SSAW).

Selectively manipulable acoustic-powered microswimmers

Tunable optofluidic microlens through active pressure control of an air–liquid interface