Elastomeric negative acoustic contrast particles for affinity capture assays

Acoustic Trapping and Manipulation of Hollow Microparticles under Fluid Flow Using a Single-Lens Focused Ultrasound Transducer

Microparticle manipulation and trapping play pivotal roles in biotechnology. To achieve effective manipulation within fluidic flow conditions and confined spaces, it is necessary to consider the physical properties of microparticles and the types of trapping forces applied. While acoustic waves have shown potential for manipulating microparticles, the existing setups involve complex actuation mechanisms and unstable microbubbles. Consequently, the need persists for an easily deployable acoustic actuation setup with stable microparticles. Here, we propose the use of hollow borosilicate microparticles possessing a rigid thin shell, which can be efficiently trapped and manipulated using a single-lens focused ultrasound (FUS) transducer under physiologically relevant flow conditions. These hollow microparticles offer stability and advantageous acoustic properties. They can be scaled up and mass-produced, making them suitable for systemic delivery. Our research demonstrates the successful trapping dynamics of FUS within circular tubings of varying diameters, validating the effectiveness of the method under realistic flow rates and ultrasound amplitudes. We also showcase the ability to remove hollow microparticles by steering the FUS transducer against the flow. Furthermore, we present potential biomedical applications, such as active cell tagging and navigation in bifurcated channels as well as ultrasound imaging in mouse cadaver liver tissue.

Biomolecular actuators for genetically selective acoustic manipulation of cells

The ability to physically manipulate specific cells is critical for the fields of biomedicine, synthetic biology, and living materials. Ultrasound has the ability to manipulate cells with high spatiotemporal precision via acoustic radiation force (ARF). However, because most cells have similar acoustic properties, this capability is disconnected from cellular genetic programs. Here, we show that gas vesicles (GVs)-a unique class of gas-filled protein nanostructures-can serve as genetically encodable actuators for selective acoustic manipulation. Because of their lower density and higher compressibility relative to water, GVs experience strong ARF with opposite polarity to most other materials. When expressed inside cells, GVs invert the cells' acoustic contrast and amplify the magnitude of their ARF, allowing the cells to be selectively manipulated with sound waves based on their genotype. GVs provide a direct link between gene expression and acoustomechanical actuation, opening a paradigm for selective cellular control in a broad range of contexts.

Two-Step Acoustophoresis Separation of Live Tumor Cells from Whole Blood

There is an unmet clinical need to extract living circulating tumor cells (CTCs) for functional studies and in vitro expansion to enable drug testing and predict responses to therapy in metastatic cancer. Here, we present a novel two-step acoustophoresis (A²) method for isolation of unfixed, viable cancer cells from red blood cell (RBC) lysed whole blood. The A² method uses an initial acoustofluidic preseparation step to separate cells based on their acoustic mobility. This acoustofluidic step enriches viable cancer cells in a central outlet, but a significant number of white blood cells (WBCs) remain in the central outlet fraction due to overlapping acoustophysical properties of these viable cells. A subsequent purging step was employed to remove contaminating WBCs through negative selection acoustophoresis with anti-CD45-functionalized negative acoustic contrast particles. We processed 1 mL samples of 1:1 diluted RBC lysed whole blood mixed with 10 000 DU145 cells through the A² method. Additional experiments were performed using 1000 DU145 cells spiked into 1.5 × 10⁶ WBCs in 1 mL of buffer to further elucidate the dynamic range of the method. Using samples with 10 000 DU145 cells, we obtained 459 ± 188-fold depletion of WBC and 42% recovery of viable cancer cells. Based on spiked samples with 1000 DU145 cells, our cancer cell recovery was 28% with 247 ± 156-fold WBC depletion corresponding to a depletion efficacy of ≥99.5%. The novel A² method provides extensive elimination of WBCs combined with the gentle recovery of viable cancer cells suitable for downstream functional analyses and in vitro culture.

Label-Free Isolation of Exosomes Using Microfluidic Technologies

Exosomes are cell-derived structures packaged with lipids, proteins, and nucleic acids. They exist in diverse bodily fluids and are involved in physiological and pathological processes. Although their potential for clinical application as diagnostic and therapeutic tools has been revealed, a huge bottleneck impeding the development of applications in the rapidly burgeoning field of exosome research is an inability to efficiently isolate pure exosomes from other unwanted components present in bodily fluids. To date, several approaches have been proposed and investigated for exosome separation, with the leading candidate being microfluidic technology due to its relative simplicity, cost-effectiveness, precise and fast processing at the microscale, and amenability to automation. Notably, avoiding the need for exosome labeling represents a significant advance in terms of process simplicity, time, and cost as well as protecting the biological activities of exosomes. Despite the exciting progress in microfluidic strategies for exosome isolation and the countless benefits of label-free approaches for clinical applications, current microfluidic platforms for isolation of exosomes are still facing a series of problems and challenges that prevent their use for clinical sample processing. This review focuses on the recent microfluidic platforms developed for label-free isolation of exosomes including those based on sieving, deterministic lateral displacement, field flow, and pinched flow fractionation as well as viscoelastic, acoustic, inertial, electrical, and centrifugal forces. Further, we discuss advantages and disadvantages of these strategies with highlights of current challenges and outlook of label-free microfluidics toward the clinical utility of exosomes.

Directed Assembly of Particles for Additive Manufacturing of Particle-Polymer Composites

Particle-polymer dispersions are ubiquitous in additive manufacturing (AM), where they are used as inks to create composite materials with applications to wearable sensors, energy storage materials, and actuation elements. It has been observed that directional alignment of the particle phase in the polymer dispersion can imbue the resulting composite material with enhanced mechanical, electrical, thermal or optical properties. Thus, external field-driven particle alignment during the AM process is one approach to tailoring the properties of composites for end-use applications. This review article provides an overview of externally directed field mechanisms (e.g., electric, magnetic, and acoustic) that are used for particle alignment. Illustrative examples from the AM literature show how these mechanisms are used to create structured composites with unique properties that can only be achieved through alignment. This article closes with a discussion of how particle distribution (i.e., microstructure) affects mechanical properties. A fundamental description of particle phase transport in polymers could lead to the development of AM process control for particle-polymer composite fabrication. This would ultimately create opportunities to explore the fundamental impact that alignment has on particle-polymer composite properties, which opens up the possibility of tailoring these materials for specific applications.

Rapid capture of biomolecules from blood via stimuli-responsive elastomeric particles for acoustofluidic separation

The detection of biomarkers in blood often requires extensive and time-consuming sample preparation to remove blood cells and concentrate the biomarker(s) of interest. We demonstrate proof-of-concept for a chip-based, acoustofluidic method that enables the rapid capture and isolation of a model protein biomarker (i.e., streptavidin) from blood for off-chip quantification. Our approach makes use of two key components - namely, soluble, thermally responsive polypeptides fused to ligands for the homogeneous capture of biomarkers from whole blood and silicone microparticles functionalized with similar, tethered, thermally responsive polypeptides. When the two components are mixed together and subjected to a mild thermal trigger, the thermally responsive moieties undergo a phase transition, causing the untethered (soluble) polypeptides to co-aggregate with the particle-bound polypeptides. The mixture is then diluted with warm buffer and injected into a microfluidic channel supporting a bulk acoustic standing wave. The biomarker-bearing particles migrate to the pressure antinodes, whereas blood cells migrate to the pressure node, leading to rapid separation with efficiencies exceeding 90% in a single pass. The biomarker-bearing particles can then be analyzed via flow cytometry, with a limit of detection of 0.75 nM for streptavidin spiked in blood plasma. Finally, by cooling the solution below the solubility temperature of the polypeptides, greater than 75% of the streptavidin is released from the microparticles, offering a unique approach for downstream analysis (e.g., sequencing or structural analysis). Overall, this methodology has promise for the detection, enrichment and analysis of some biomarkers from blood and other complex biological samples.

Antibody Conjugate Assembly on Ultrasound-Confined Microcarrier Particles

Bioconjugates are important next-generation drugs and imaging agents. Assembly of these increasingly complex constructs requires precise control over processing conditions, which is a challenge for conventional manual synthesis. This inadequacy has motivated the pursuit of new approaches for efficient, controlled modification of high-molecular-weight biologics such as proteins, carbohydrates, and nucleic acids. We report a novel, hands-free, semiautomated platform for synthetic manipulation of biomolecules using acoustically responsive microparticles as three-dimensional reaction substrates. The microfluidic reactor incorporates a longitudinal acoustic trap that controls the chemical reactions within a localized acoustic field. Forces generated by this field immobilize the microscale substrates against the continuous flow of participating chemical reagents. Thus, the motion of substrates and reactants is decoupled, enabling exquisite control over multistep reaction conditions and providing high-yield, high-purity products with minimal user input. We demonstrate these capabilities by conjugating clinically relevant antibodies with a small molecule. The on-bead synthesis comprises capture of the antibody, coupling of a fluorescent tag, product purification, and product release. Successful capture and modification of a fluorescently labeled antibody are confirmed via fold increases of 49 and 11 in the green (antibody)- and red (small-molecule dye)-channel median intensities determined using flow cytometry. Antibody conjugates assembled on acoustically responsive, ultrasound-confined microparticles exhibit similar quality and quantity to those prepared manually by a skilled technician.

Ultrasound Standing Wave-Based Cell-to-liquid Separation for Measuring Viscosity and Aggregation of Blood Sample

When quantifying mechanical properties of blood samples flowing in closed fluidic circuits, blood samples are collected at specific intervals. Centrifugal separation is considered as a required procedure for preparing blood samples. However, the use of centrifuge is associated with several issues, including the potential for red blood cell (RBC) lysis, clotting activation, and RBC adhesions in the tube. In this study, an ultrasonic transducer is employed to separate RBCs or diluent from blood sample. The ultrasonic radiation force is much smaller than the centrifugal force acting in centrifuge, it can avoid critical issues occurring under centrifuge. Then, the RBC aggregation and blood viscosity of the blood sample are obtained using the microfluidic technique. According to the numerical results, ultrasonic transducers exhibited a maximum quality factor at an excitation frequency of 2.1 MHz. Periodic pattern of acoustic pressure fields were visualized experimentally as a column mode. The half wavelength obtained was as 0.5 λ = 0.378 ± 0.07 mm. The experimental results agreed with the analytical estimation sufficiently. An acoustic power of 2 W was selected carefully for separating RBCs or diluent from various blood samples (i.e., Hct = 20% ~ 50%; diluent: plasma, 1x phosphate-buffered saline (PBS), and dextran solution). The present method was employed to separate fixed blood samples which tended to stack inside the tube while using the centrifuge. Fixed RBCs were collected easily with an ultrasonic transducer. After various fixed blood samples with different base solutions (i.e., glutaraldehyde solution, 1x PBS, and dextran solution) were prepared using the present method, RBC aggregation and the viscosity of the blood sample are successfully obtained. In the near future, the present method will be integrated into ex vivo or in vitro fluidic circuit for measuring multiple mechanical properties of blood samples for a certain longer period.

Detection of Rare Objects by Flow Cytometry: Imaging, Cell Sorting, and Deep Learning Approaches

Flow cytometry nowadays is among the main working instruments in modern biology paving the way for clinics to provide early, quick, and reliable diagnostics of many blood-related diseases. The major problem for clinical applications is the detection of rare pathogenic objects in patient blood. These objects can be circulating tumor cells, very rare during the early stages of cancer development, various microorganisms and parasites in the blood during acute blood infections. All of these rare diagnostic objects can be detected and identified very rapidly to save a patient's life. This review outlines the main techniques of visualization of rare objects in the blood flow, methods for extraction of such objects from the blood flow for further investigations and new approaches to identify the objects automatically with the modern deep learning methods.

Rapid and effective enrichment of mononuclear cells from blood using acoustophoresis

Effective separation methods for fractionating blood components are needed for numerous diagnostic and research applications. This paper presents the use of acoustophoresis, an ultrasound based microfluidic separation technology, for label-free, gentle and continuous separation of mononuclear cells (MNCs) from diluted whole blood. Red blood cells (RBCs) and MNCs behave similar in an acoustic standing wave field, compromising acoustic separation of MNC from RBC in standard buffer systems. However, by optimizing the buffer conditions and thereby changing the acoustophoretic mobility of the cells, we were able to enrich MNCs relative to RBCs by a factor of 2,800 with MNC recoveries up to 88%. The acoustophoretic microchip can perform cell separation at a processing rate of more than 1 × 10⁵ cells/s, corresponding to 5 µl/min undiluted whole blood equivalent. Thus, acoustophoresis can be easily integrated with further down-stream applications such as flow cytometry, making it a superior alternative to existing MNC isolation techniques.

Reducing WBC background in cancer cell separation products by negative acoustic contrast particle immuno-acoustophoresis

Cancer cells display acoustic properties enabling acoustophoretic separation from white blood cells (WBCs) with 2-3 log suppression of the WBC background. However, a subset of WBCs has overlapping acoustic properties with cancer cells, which is why label-free acoustophoretic cancer cell isolation needs additional purification prior to analysis. This paper reports for the first time a proof of concept for continuous flow acoustophoretic negative selection of WBCs from cancer cells using negative acoustic contrast elastomeric particles (EPs) activated with CD45-antibodies that specifically bind to WBCs. The EP/WBC complexes align at the acoustic pressure anti-nodes along the channel walls while unbound cancer cells focus to the pressure node in the channel center, enabling continuous flow based depletion of WBC background in a cancer cell product. The method does not provide a single process solution for the CTC separation challenge, but provides an elegant part to a multi-step process by further reducing the WBC background in cancer cell separation products derived from an initial step of label-free acoustophoresis. We report the recorded performance of the negative selection immuno-acoustophoretic WBC depletion and cancer cell recovery. To eliminate the negative impact of the separation due to the known problems of aggregation of negative acoustic contrast particles along the sidewalls of the acoustophoresis channel and to enable continuous separation of EP/WBC complexes from cancer cells, a new acoustic actuation method has been implemented where the ultrasound frequency is scanned (1.991MHz ± 100 kHz, scan rate 200 kHz ms^-1). Using this frequency scanning strategy EP/WBC complexes were acoustophoretically separated from mixtures of WBCs spiked with breast and prostate cancer cells (DU145 and MCF-7). An 86-fold (MCF-7) and 52-fold (DU145) reduction of WBCs in the cancer cell fractions were recorded with separation efficiencies of 98.6% (MCF-7) and 99.7% (DU145) and cancer cell recoveries of 89.8% (MCF-7) and 85.0% (DU145).

MicroBubble activated acoustic cell sorting

Acoustophoresis, the ability to acoustically manipulate particles and cells inside a microfluidic channel, is a critical enabling technology for cell-sorting applications. However, one of the major impediments for routine use of acoustophoresis at clinical laboratory has been the reliance on the inherent physical properties of cells for separation. Here, we present a microfluidic-based microBubble-Activated Acoustic Cell Sorting (BAACS) method that rely on the specific binding of target cells to microbubbles conjugated with specific antibodies on their surface for continuous cell separation using ultrasonic standing wave. In acoustophoresis, cells being positive acoustic contrast particles migrate to pressure nodes. On the contrary, air-filled polymer-shelled microbubbles being strong negative acoustic contrast particles migrate to pressure antinodes and can be used to selectively migrate target cells. As a proof of principle, we demonstrate the separation of cancer cell line in a suspension with better than 75% efficiency. Moreover, 100% of the microbubble-cell conjugates migrated to the anti-node. Hence a better upstream affinity-capture has the potential to provide higher sorting efficiency. The BAACS technique expands the acoustic cell manipulation possibilities and offers cell-sorting solutions suited for applications at point of care.

Investigation of polymer-shelled microbubble motions in acoustophoresis

The objective of this paper is to explore the trajectory motion of microsize (typically smaller than a red blood cell) encapsulated polymer-shelled gas bubbles propelled by radiation force in an acoustic standing-wave field and to compare the corresponding movements of solid polymer microbeads. The experimental setup consists of a microfluidic chip coupled to a piezoelectric crystal (PZT) with a resonance frequency of about 2.8MHz. The microfluidic channel consists of a rectangular chamber with a width, w, corresponding to one wavelength of the ultrasound standing wave. It creates one full wave ultrasound of a standing-wave pattern with two pressure nodes at w/4 and 3w/4 and three antinodes at 0, w/2, and w. The peak-to-peak amplitude of the electrical potential over the PZT was varied between 1 and 10V. The study is limited to no-flow condition. From Gor'kov's potential equation, the acoustic contrast factor, Φ, for the polymer-shelled microbubbles was calculated to about -60.7. Experimental results demonstrate that the polymer-shelled microbubbles are translated and accumulated at the pressure antinode planes. This trajectory motion of polymer-shelled microbubbles toward the pressure antinode plane is similar to what has been described for other acoustic contrast particles with a negative Φ. First, primary radiation forces dragged the polymer-shelled microbubbles into proximity with each other at the pressure antinode planes. Then, primary and secondary radiation forces caused them to quickly aggregate at different spots along the channel. The relocation time for polymer-shelled microbubbles was 40 times shorter than that for polymer microbeads, and in contrast to polymer microbeads, the polymer-shelled microbubbles were actuated even at driving voltages (proportional to radiation forces) as low as 1V. In short, the polymer-shelled microbubbles demonstrate the behavior attributed to the negative acoustic contrast factor particles and thus can be trapped at the antinode plane and thereby separated from particles having a positive acoustic contrast factor, such as for example solid particles and cells. This phenomenon could be utilized in exploring future applications, such as bioassay, bioaffinity, and cell interaction studies in vitro in a well-controlled environment.

Fabrication and Operation of Acoustofluidic Devices Supporting Bulk Acoustic Standing Waves for Sheathless Focusing of Particles

Acoustophoresis refers to the displacement of suspended objects in response to directional forces from sound energy. Given that the suspended objects must be smaller than the incident wavelength of sound and the width of the fluidic channels are typically tens to hundreds of micrometers across, acoustofluidic devices typically use ultrasonic waves generated from a piezoelectric transducer pulsating at high frequencies (in the megahertz range). At characteristic frequencies that depend on the geometry of the device, it is possible to induce the formation of standing waves that can focus particles along desired fluidic streamlines within a bulk flow. Here, we describe a method for the fabrication of acoustophoretic devices from common materials and clean room equipment. We show representative results for the focusing of particles with positive or negative acoustic contrast factors, which move towards the pressure nodes or antinodes of the standing waves, respectively. These devices offer enormous practical utility for precisely positioning large numbers of microscopic entities (e.g., cells) in stationary or flowing fluids for applications ranging from cytometry to assembly.

Highly parallel acoustic assembly of microparticles into well-ordered colloidal crystallites

The precise arrangement of microscopic objects is critical to the development of functional materials and ornately patterned surfaces. Here, we present an acoustics-based method for the rapid arrangement of microscopic particles into organized and programmable architectures, which are periodically spaced within a square assembly chamber. This macroscale device employs two-dimensional bulk acoustic standing waves to propel particles along the base of the chamber toward pressure nodes or antinodes, depending on the acoustic contrast factor of the particle, and is capable of simultaneously creating thousands of size-limited, isotropic and anisotropic assemblies within minutes. We pair experiments with Brownian dynamics simulations to model the migration kinetics and assembly patterns of spherical microparticles. We use these insights to predict and subsequently validate the onset of buckling of the assemblies into three-dimensional clusters by experiments upon increasing the acoustic pressure amplitude and the particle concentration. The simulations are also used to inform our experiments for the assembly of non-spherical particles, which are then recovered via fluid evaporation and directly inspected by electron microscopy. This method for assembly of particles offers several notable advantages over other approaches (e.g., magnetics, electrokinetics and optical tweezing) including simplicity, speed and scalability and can also be used in concert with other such approaches for enhancing the types of assemblies achievable.

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

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 (e.g., 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 (i.e., fluorescent label-based sorting, bead-based sorting, and label-free sorting) as well as by the physical principles underlying each sorting mechanism.

Two-dimensional spatial manipulation of microparticles in continuous flows in acoustofluidic systems

We report a modeling and experimental study of techniques to acoustically focus particles flowing through a microfluidic channel. Our theoretical model differs from prior works in that we solve an approximate 2-D wave transmission model that accounts for wave propagation in both the solid and fluid phases. Our simulations indicate that particles can be effectively focused at driving frequencies as high as 10% off of the resonant condition. This conclusion is supported by experiments on the acoustic focusing of particles in nearly square microchannels, which are studied for different flow rates, driving frequencies and placements of the lead zirconate titanate transducer, either underneath the microchannel or underneath a parallel trough. The relative acoustic potential energy and the resultant velocity fields for particles with positive acoustic contrast coefficients are estimated in the 2-D limit. Confocal microscopy was used to observe the spatial distribution of the flowing microparticles in three dimensions. Through these studies, we show that a single driving frequency from a single piezoelectric actuator can induce the 2-D concentration of particles in a microchannel with a nearly square cross section, and we correlate these behaviors with theoretical predictions. We also show that it is possible to control the extent of focusing of the microparticles, and that it is possible to decouple the focusing of microparticles in the vertical direction from the lateral direction in rectangular channels with anisotropic cross sections. This study provides guidelines to design and operate microchip-based acoustofluidic devices for precise control over the spatial arrangement of microparticles for applications such as flow cytometry and cellular sorting.

Efficient purification of CD4+ lymphocytes from peripheral blood progenitor cell products using affinity bead acoustophoresis

Processing of peripheral blood progenitor cells (PBPC) for clinical transplantation or research applications aims to effectively isolate or deplete specific cell populations, utilizing primarily magnetic or fluorescence activated sorting methods. Here, we investigated the performance of microfluidic acoustophoresis for the separation of lymphocyte subsets from PBPC, and present a novel method for affinity-bead-mediated acoustic separation of cells which can otherwise not be acoustically discriminated. As the acoustic force on a particle depends on particle size, density and compressibility, targeting of cells by affinity specific beads will generate cell-bead complexes that exhibit distinct acoustic properties relative to nontargeted cells and are, thus, possible to isolate. To demonstrate this, PBPC samples (n = 22) were obtained from patients and healthy donors. Following density gradient centrifugation, cells were labeled with anti-CD4-coated magnetic beads (Dynal) and isolated by acoustophoresis and, for comparison, standard magnetic cell sorting technique in parallel. Targeted CD4+ lymphocytes were acoustically isolated with a mean (±SD) purity of 87 ± 12%, compared with 96 ± 3% for control magnetic sorting. Viability of sorted cells was 95 ± 4% (acoustic) and 97 ± 3% (magnetic), respectively. The mean acoustic separation efficiency of CD4+ lymphocytes to the target fraction was 65 ± 22%, compared with a mean CD4+ lymphocyte recovery of 56 ± 15% for magnetic sorting. Functional testing of targeted CD4+ lymphocytes demonstrated unimpaired mitogen-mediated proliferation capacity and cytokine production. Hematopoietic progenitor cell assays revealed a preserved colony forming ability of nontarget cells post sorting. We conclude that the acoustophoresis platform can be utilized to efficiently isolate bead-labeled CD4+ lymphocytes from PBPC samples in a continuous flow format, with preserved functional capacity of both target and nontarget cells. These results open up for simultaneous affinity-bead-mediated separation of multiple cell populations, something which is not possible with current standard magnetic cell separation technology. © 2014 International Society for Advancement of Cytometry.

Nucleation and growth synthesis of siloxane gels to form functional, monodisperse, and acoustically programmable particles

Nucleation and growth methods offer scalable means of synthesizing colloidal particles with precisely specified size for applications in chemical research, industry, and medicine. These methods have been used to prepare a class of silicone gel particles that display a range of programmable properties and narrow size distributions. The acoustic contrast factor of these particles in water is estimated and can be tuned such that the particles undergo acoustophoresis to either the pressure nodes or antinodes of acoustic standing waves. These particles can be synthesized to display surface functional groups that can be covalently modified for a range of bioanalytical and acoustophoretic sorting applications.

Elastomeric negative acoustic contrast particles for capture, acoustophoretic transport, and confinement of cells in microfluidic systems

We present a particle-based method for the immunospecific capture and confinement of cells using acoustic radiation forces. Ultrasonic standing waves in microfluidic systems have previously been used for the continuous focusing of cells in rapid screening and sorting applications. In aqueous fluids, cells typically exhibit positive acoustic contrast and are thus forced toward the pressure nodes of a standing wave. Conversely, elastomeric particles exhibit negative acoustic contrast and travel toward the pressure antinodes. We have developed a class of elastomeric particles that are synthesized in bulk using a simple nucleation and growth process, providing precise control over their size and functional properties. We demonstrate that the biofunctionalization of these particles can allow the capture and transport of cells to the pressure antinodes solely via acoustic radiation forces, which may enable new acoustics-based cell handling techniques such as the washing, labeling, and sorting of cells with minimal preparatory steps.

The intersection of flow cytometry with microfluidics and microfabrication

A modern flow cytometer can analyze and sort particles on a one by one basis at rates of 50,000 particles per second. Flow cytometers can also measure as many as 17 channels of fluorescence, several angles of scattered light, and other non-optical parameters such as particle impedance. More specialized flow cytometers can provide even greater analysis power, such as single molecule detection, imaging, and full spectral collection, at reduced rates. These capabilities have made flow cytometers an invaluable tool for numerous applications including cellular immunophenotyping, CD4+ T-cell counting, multiplex microsphere analysis, high-throughput screening, and rare cell analysis and sorting. Many bio-analytical techniques have been influenced by the advent of microfluidics as a component in analytical tools and flow cytometry is no exception. Here we detail the functions and uses of a modern flow cytometer, review the recent and historical contributions of microfluidics and microfabricated devices to field of flow cytometry, examine current application areas, and suggest opportunities for the synergistic application of microfabrication approaches to modern flow cytometry.

Separation of Escherichia coli bacteria from peripheral blood mononuclear cells using standing surface acoustic waves

A microfluidic device was developed to separate heterogeneous particle or cell mixtures in a continuous flow using acoustophoresis. In this device, two identical surface acoustic waves (SAWs) generated by interdigital transducers (IDTs) propagated toward a microchannel, which accordingly built up a standing surface acoustic wave (SSAW) field across the channel. A numerical model, coupling a piezoelectric effect in the solid substrate and acoustic pressure in the fluid, was developed to provide a better understanding of SSAW-based particle manipulation. It was found that the pressure nodes across the channel were individual planes perpendicular to the solid substrate. In the separation experiments, two side sheath flows hydrodynamically focused the injected particle or cell mixtures into a very narrow stream along the centerline. Particles flowing through the SSAW field experienced an acoustic radiation force that highly depends on the particle properties. As a result, dissimilar particles or cells were laterally attracted toward the pressure nodes at different magnitudes, and were eventually switched to different outlets. Two types of fluorescent microspheres with different sizes were successfully separated using the developed device. In addition, Escherichia coli bacteria premixed in peripheral blood mononuclear cells (PBMCs) were also efficiently isolated using the SSAW-base separation technique. Flow cytometric analysis on the collected samples found that the purity of separated E. coli bacteria was 95.65%.

Fluidics

The use of fluidics is implicit in a technology named "flow cytometry," which flows a cell or particle through a sensing volume to obtain serial analysis of particles on a one by one basis. This flow of particles enables flow cytometry to collect information on multiple particle populations, giving it a distinct advantage over bulk analysis approaches. Moreover, flow cytometers can analyze thousands of particles per second in a single flowing stream. Additionally, use of volumetric sample delivery makes it possible for flow cytometers to accurately count cells and particles. Furthermore, the analysis results can be coupled with a fluidic diversion mechanism to sort and collect particles based on desired properties. Finally, when high-throughput sampling technologies are employed to rapidly change the input of the sample stream, a flow cytometer can become an integral tool for high-throughput screening. The above properties have made flow cytometry useful in a wide range of biomedical applications. In this unit we will present an overview of fluidic systems that make flow cytometry possible. This will introduce historical approaches, explanations of the commonly implemented current fluidics, and brief discussions of potential future fluidics where appropriate.

Elastomeric microparticles for acoustic mediated bioseparations

BACKGROUND

Acoustophoresis has been utilized successfully in applications including cell trapping, focusing, and purification. One current limitation of acoustophoresis for cell sorting is the reliance on the inherent physical properties of cells (e.g., compressibility, density) instead of selecting cells based upon biologically relevant surface-presenting antigens. Introducing an acoustophoretic cell sorting approach that allows biochemical specificity may overcome this limitation, thus advancing the value of acoustophoresis approaches for both the basic research and clinical fields.

RESULTS

The results presented herein demonstrate the ability for negative acoustic contrast particles (NACPs) to specifically capture and transport positive acoustic contrast particles (PACPs) to the antinode of an ultrasound standing wave. Emulsification and post curing of pre-polymers, either polydimethylsiloxane (PDMS) or polyvinylmethylsiloxane (PVMS), within aqueous surfactant solution results in the formation of stable NACPs that focus onto pressure antinodes. We used either photochemical reactions with biotin-tetrafluorophenyl azide (biotin-TFPA) or end-functionalization of Pluronic F108 surfactant to biofunctionalize NACPs. These biotinylated NACPs bind specifically to streptavidin polystyrene microparticles (as cell surrogates) and transport them to the pressure antinode within an acoustofluidic chip.

CONCLUSION

To the best of our knowledge, this is the first demonstration of using NACPs as carriers for transport of PACPs in an ultrasound standing wave. By using different silicones (i.e., PDMS, PVMS) and curing chemistries, we demonstrate versatility of silicone materials for NACPs and advance the understanding of useful approaches for preparing NACPs. This bioseparation scheme holds potential for applications requiring rapid, continuous separations such as sorting and analysis of cells and biomolecules.