Acoustofluidic Fluorescence Activated Cell Sorter

Acoustofluidic Actuation of Living Cells

Acoutofluidics is an increasingly developing and maturing technical discipline. With the advantages of being label-free, non-contact, bio-friendly, high-resolution, and remote-controllable, it is very suitable for the operation of living cells. After decades of fundamental laboratory research, its technical principles have become increasingly clear, and its manufacturing technology has gradually become popularized. Presently, various imaginative applications continue to emerge and are constantly being improved. Here, we introduce the development of acoustofluidic actuation technology from the perspective of related manipulation applications on living cells. Among them, we focus on the main development directions such as acoustofluidic sorting, acoustofluidic tissue engineering, acoustofluidic microscopy, and acoustofluidic biophysical therapy. This review aims to provide a concise summary of the current state of research and bridge past developments with future directions, offering researchers a comprehensive overview and sparking innovation in the field.

On Chip Sorting of Stem Cell-Derived β Cell Clusters Using Traveling Surface Acoustic Waves

There is a critical need for sorting complex materials, such as pancreatic islets of Langerhans, exocrine acinar tissues, and embryoid bodies. These materials are cell clusters, which have highly heterogeneous physical properties (such as size, shape, morphology, and deformability). Selecting such materials on the basis of specific properties can improve clinical outcomes and help advance biomedical research. In this work, we focused on sorting one such complex material, human stem cell-derived β cell clusters (SC-β cell clusters), by size. For this purpose, we developed a microfluidic device in which an image detection system was coupled to an actuation mechanism based on traveling surface acoustic waves (TSAWs). SC-β cell clusters of varying size (∼100-500 μm in diameter) were passed through the sorting device. Inside the device, the size of each cluster was estimated from their bright-field images. After size identification, larger clusters, relative to the cutoff size for separation, were selectively actuated using TSAW pulses. As a result of this selective actuation, smaller and larger clusters exited the device from different outlets. At the current sample dilutions, the experimental sorting efficiency ranged between 78% and 90% for a separation cutoff size of 250 μm, yielding sorting throughputs of up to 0.2 SC-β cell clusters/s using our proof-of-concept design. The biocompatibility of this sorting technique was also established, as no difference in SC-β cell cluster viability due to TSAW pulse usage was found. We conclude the proof-of-concept sorting work by discussing a few ways to optimize sorting of SC-β cell clusters for potentially higher sorting efficiency and throughput. This sorting technique can potentially help in achieving a better distribution of islets for clinical islet transplantation (a potential cure for type 1 diabetes). Additionally, the use of this technique for sorting islets can help in characterizing islet biophysical properties by size and selecting suitable islets for improved islet cryopreservation.

Passive Focusing of Single Cells Using Microwell Arrays for High-Accuracy Image-Activated Sorting

Sorting single cells from a population was of critical importance in areas such as cell line development and cell therapy. Image-based sorting is becoming a promising technique for the nonlabeling isolation of cells due to the capability of providing the details of cell morphology. This study reported the focusing of cells using microwell arrays and the following automatic size sorting based on the real-time recognition of cells. The simulation first demonstrated the converged streamlines to the symmetrical plane contributed to the focusing effect. Then, the influence of connecting microchannel, flowing length, particle size, and the sample flow rate on the focusing effect was experimentally analyzed. Both microspheres and cells could be aligned in a straight line at the Reynolds number (Re) of 0.027-0.187 and 0.027-0.08, respectively. The connecting channel was proved to drastically improve the focusing performance. Afterward, a tapered microwell array was utilized to focus sphere/cell spreading in a wide channel to a straight line. Finally, a custom algorithm was employed to identify and sort the size of microspheres/K562 cells with a throughput of 1 event/s and an accuracy of 97.8/97.1%. The proposed technique aligned cells to a straight line at low Reynolds numbers and greatly facilitated the image-activated sorting without the need for a high-speed camera or flow control components with high frequency. Therefore, it is of enormous application potential in the field of nonlabeled separation of single cells.

Acoustophoresis around an elastic scatterer in a standing wave field

Acoustofluidic systems often employ prefabricated acoustic scatterers that perturb the imposed acoustic field to realize the acoustophoresis of immersed microparticles. We present a numerical study to investigate the time-averaged streaming and radiation force fields around a scatterer. Based on the streaming and radiation force field, we obtain the trajectories of the immersed microparticles with varying sizes and identify a critical transition size at which the motion of immersed microparticles in the vicinity of a prefabricated scatterer shifts from being streaming dominated to radiation dominated. We consider a range of acoustic frequencies to reveal that the critical transition size decreases with increasing frequency; this result explains the choice of acoustic frequencies in previously reported experimental studies. We also examine the impact of scatterer material and fluid properties on the streaming and radiation force fields, as well as on the critical transition size. Our results demonstrate that the critical transition size decreases with an increase in acoustic contrast factor: a nondimensional quantity that depends on material properties of the scatterer and the fluid. Our results provide a pathway to realize radiation force based manipulation of small particles by increasing the acoustic contrast factor of the scatterer, lowering the kinematic viscosity of the fluid, and increasing the acoustic frequency.

Advances in single-cell metabolomics to unravel cellular heterogeneity in plant biology

Single-cell metabolomics is a powerful tool that can reveal cellular heterogeneity and can elucidate the mechanisms of biological phenomena in detail. It is a promising approach in studying plants, especially when cellular heterogeneity has an impact on different biological processes. In addition, metabolomics, which can be regarded as a detailed phenotypic analysis, is expected to answer previously unrequited questions which will lead to expansion of crop production, increased understanding of resistance to diseases, and in other applications as well. In this review, we will introduce the flow of sample acquisition and single-cell techniques to facilitate the adoption of single-cell metabolomics. Furthermore, the applications of single-cell metabolomics will be summarized and reviewed.

Convergence of flow cytometry and bacteriology. Current and future applications: a focus on food and clinical microbiology

Since its development in the 1960s, flow cytometry (FCM) was quickly revealed a powerful tool to analyse cell populations in medical studies, yet, for many years, was almost exclusively used to analyse eukaryotic cells. Instrument and methodological limitations to distinguish genuine bacterial signals from the background, among other limitations, have hampered FCM applications in bacteriology. In recent years, thanks to the continuous development of FCM instruments and methods with a higher discriminatory capacity to detect low-size particles, FCM has emerged as an appealing technique to advance the study of microbes, with important applications in research, clinical and industrial settings. The capacity to rapidly enumerate and classify individual bacterial cells based on viability facilitates the monitoring of bacterial presence in foodstuffs or clinical samples, reducing the time needed to detect contamination or infectious processes. Besides, FCM has stood out as a valuable tool to advance the study of complex microbial communities, or microbiomes, that are very relevant in the context of human health, as well as to understand the interaction of bacterial and host cells. This review highlights current developments in, and future applications of, FCM in bacteriology, with a focus on those related to food and clinical microbiology.

Continuous Flow Separation of Live and Dead Cells Using Gravity Sedimentation

The separation of target cell species is an important step for various biomedical applications ranging from single cell studies to drug testing and cell-based therapies. The purity of cell solutions is critical for therapeutic application. For example, dead cells and debris can negatively affect the efficacy of cell-based therapies. This study presents a cost-effective method for the continuous separation of live and dead cells using a 3D resin-printed microfluidic device. Saccharomyces cerevisiae yeast cells are used for cell separation experiments. Both numerical and experimental studies are presented to show the effectiveness of the presented device for the isolation of dead cells from cell solutions. The experimental results show that the 3D-printed microfluidic device successfully separates live and dead cells based on density differences. Separation efficiencies of over 95% are achieved at optimum flow rates, resulting in purer cell populations in the outlets. This study highlights the simplicity, cost-effectiveness, and potential applications of the 3D-printed microfluidic device for cell separation. The implementation of 3D printing technology in microfluidics holds promise for advancing the field and enabling the production of customized devices for biomedical applications.

Power-controlled acoustofluidic manipulation of microparticles

Recently, surface acoustic wave (SAW) based acoustofluidic separation of microparticles and cells has attracted increasing interest due to accuracy and biocompatibility. Precise control of the input power of acoustofluidic devices is essential for generating optimum acoustic radiation force to manipulate microparticles given their various parameters including size, density, compressibility, and moving velocity. In this work, an acoustophoretic system is developed by employing SAW based interdigital electrode devices. Power meters are applied to closely monitor the incident and reflected powers of the SAW device, which are associated with the separation efficiency. There exists a range of input powers to migrate the microparticles to the pressure node due to their random locations when entering the SAW field. Theoretical analysis is performed to predict a proper input power to separate mixtures of polystyrene microspheres, and the end lateral position of microspheres being acoustically separated. The separation efficiency of four sizes of microspheres, including 20 µm, 15 µm, 10 µm, and 5 µm, is calculated and compared with experimental results, which suggest the input power for separating the mixture of these microspheres. The study provides a practical guidance on operating SAW devices for bioparticle separation using the incident power as a control parameter.

Acoustofluidics - changing paradigm in tissue engineering, therapeutics development, and biosensing

For more than 70 years, acoustic waves have been used to screen, diagnose, and treat patients in hundreds of medical devices. The biocompatible nature of acoustic waves, their non-invasive and contactless operation, and their compatibility with wide visualization techniques are just a few of the many features that lead to the clinical success of sound-powered devices. The development of microelectromechanical systems and fabrication technologies in the past two decades reignited the spark of acoustics in the discovery of unique microscale bio applications. Acoustofluidics, the combination of acoustic waves and fluid mechanics in the nano and micro-realm, allowed researchers to access high-resolution and controllable manipulation and sensing tools for particle separation, isolation and enrichment, patterning of cells and bioparticles, fluid handling, and point of care biosensing strategies. This versatility and attractiveness of acoustofluidics have led to the rapid expansion of platforms and methods, making it also challenging for users to select the best acoustic technology. Depending on the setup, acoustic devices can offer a diverse level of biocompatibility, throughput, versatility, and sensitivity, where each of these considerations can become the design priority based on the application. In this paper, we aim to overview the recent advancements of acoustofluidics in the multifaceted fields of regenerative medicine, therapeutic development, and diagnosis and provide researchers with the necessary information needed to choose the best-suited acoustic technology for their application. Moreover, the effect of acoustofluidic systems on phenotypic behavior of living organisms are investigated. The review starts with a brief explanation of acoustofluidic principles, the different working mechanisms, and the advantages or challenges of commonly used platforms based on the state-of-the-art design features of acoustofluidic technologies. Finally, we present an outlook of potential trends, the areas to be explored, and the challenges that need to be overcome in developing acoustofluidic platforms that can echo the clinical success of conventional ultrasound-based devices.

Image-based cell sorting using focused travelling surface acoustic waves

Sorting cells is an essential primary step in many biological and clinical applications such as high-throughput drug screening, cancer research and cell transplantation. Cell sorting based on their mechanical properties has long been considered as a promising label-free biomarker that could revolutionize the isolation of cells from heterogeneous populations. Recent advances in microfluidic image-based cell analysis combined with subsequent label-free sorting by on-chip actuators demonstrated the possibility of sorting cells based on their physical properties. However, the high purity of sorting is achieved at the expense of a sorting rate that lags behind the analysis throughput. Furthermore, stable and reliable system operation is an important feature in enabling the sorting of small cell fractions from a concentrated heterogeneous population. Here, we present a label-free cell sorting method, based on the use of focused travelling surface acoustic wave (FTSAW) in combination with real-time deformability cytometry (RT-DC). We demonstrate the flexibility and applicability of the method by sorting distinct blood cell types, cell lines and particles based on different physical parameters. Finally, we present a new strategy to sort cells based on their mechanical properties. Our system enables the sorting of up to 400 particles per s. Sorting is therefore possible at high cell concentrations (up to 36 million per ml) while retaining high purity (>92%) for cells with diverse sizes and mechanical properties moving in a highly viscous buffer. Sorting of small cell fraction from a heterogeneous population prepared by processing of small sample volume (10 μl) is also possible and here demonstrated by the 667-fold enrichment of white blood cells (WBCs) from raw diluted whole blood in a continuous 10-hour sorting experiment. The real-time analysis of multiple parameters together with the high sensitivity and high-throughput of our method thus enables new biological and therapeutic applications in the future.

Screening of a high-yield strain of avermectin B1a by colony analysis in situ

Avermectin, an agricultural antibiotic, is widely used as an agricultural insecticide and an important lead compound of antibiotics. It is manufactured by Streptomyces avermitilis through fermentation. Manufacturers pay special attention to screening for strains with high fermentation capacity based on morphological properties of the colony and by the result of shake flask fermentation. These traditional screening methods are time-consuming and labor-intensive and require specialized equipment. Moreover, evaluation of colony appearance is highly subjective. To improve and accelerate the screening process, we developed a rapid in situ screening method. Forty-four strains isolated naturally from the spores of industrial high-yielding strains were studied. The data show that the colony fermentation titer is highly correlated with the yield from the shake flask fermentation of avermectin, and the Pearson's R is 0.990. The total titer of avermectins by shake flask fermentation is also highly correlated with the B_1a titer (Pearson's R is 0.994). This result also shows that strains can be quickly screened by analyzing the colony titer. Pigment rings of the colonies that appeared after growing and maturing on the new medium plate were analyzed. The chosen colonies were directly marked and punched and then extracted with methanol. The fermentation ability can be evaluated by measuring the absorbance at 245 nm. This methodology can be applied in both natural breeding and mutation breeding conditions. By continuously breeding from 2008 to 2020, the flask titer of avermectin B_1a increased from 4582 ± 483 to 9197 ± 1134 μg/mL.

Efficient recovery of potent tumour-infiltrating lymphocytes through quantitative immunomagnetic cell sorting

Adoptive cell therapies require the recovery and expansion of highly potent tumour-infiltrating lymphocytes (TILs). However, TILs in tumours are rare and difficult to isolate efficiently, which hinders the optimization of therapeutic potency and dose. Here we show that a configurable microfluidic device can efficiently recover potent TILs from solid tumours by leveraging specific expression levels of target cell-surface markers. The device, which is sandwiched by permanent magnets, balances magnetic forces and fluidic drag forces to sort cells labelled with magnetic nanoparticles conjugated with antibodies for the target markers. Compared with conventional cell sorting, immunomagnetic cell sorting recovered up to 30-fold higher numbers of TILs, and the higher levels and diversity of the recovered TILs accelerated TIL expansion and enhanced their therapeutic potency. Immunomagnetic cell sorting also allowed us to identify and isolate potent TIL subpopulations, in particular TILs with moderate levels of CD39 (a marker of T-cell reactivity to tumours and T-cell exhaustion), which we found are tumour-specific, self-renewable and essential for the long-term success of adoptive cell therapies.

Label-free imaging flow cytometry for analysis and sorting of enzymatically dissociated tissues

Biomedical research relies on identification and isolation of specific cell types using molecular biomarkers and sorting methods such as fluorescence or magnetic activated cell sorting. Labelling processes potentially alter the cells’ properties and should be avoided, especially when purifying cells for clinical applications. A promising alternative is the label-free identification of cells based on physical properties. Sorting real-time deformability cytometry (soRT-DC) is a microfluidic technique for label-free analysis and sorting of single cells. In soRT-FDC, bright-field images of cells are analyzed by a deep neural net (DNN) to obtain a sorting decision, but sorting was so far only demonstrated for blood cells which show clear morphological differences and are naturally in suspension. Most cells, however, grow in tissues, requiring dissociation before cell sorting which is associated with challenges including changes in morphology, or presence of aggregates. Here, we introduce methods to improve robustness of analysis and sorting of single cells from nervous tissue and provide DNNs which can distinguish visually similar cells. We employ the DNN for image-based sorting to enrich photoreceptor cells from dissociated retina for transplantation into the mouse eye.

Potential of the acoustic micromanipulation technologies for biomedical research

Acoustic micromanipulation technologies are a set of versatile tools enabling unparalleled micromanipulation capabilities. Several characteristics put the acoustic micromanipulation technologies ahead of most of the other tweezing methods. For example, acoustic tweezers can be adapted as non-invasive platforms to handle single cells gently or as probes to stimulate or damage tissues. Besides, the nature of the interactions of acoustic waves with solids and liquids eliminates labeling requirements. Considering the importance of highly functional tools in biomedical research for empowering important discoveries, acoustic micromanipulation can be valuable for researchers in biology and medicine. Herein, we discuss the potential of acoustic micromanipulation technologies from technical and application points of view in biomedical research.

Sheathless acoustic based flow cell sorter for enrichment of rare cells

Cell enrichment is a powerful tool in many kinds of cell research, especially in applications with low abundance cell types. In this work, we developed a microfluidic fluorescence activated cell sorting device that was able to perform on-demand, low loss cell detection, and sorting. The chip utilizes three-dimensional acoustic standing waves to position all cells in the same fluid velocity regime without sheath. When the cells pass through a laser interrogation region, the scattering and fluorescent signals are detected, translated and transported to software. The target cells are then identified by gating on the plots. Short bursts of standing acoustic waves are triggered by order from PC to sort target cells within predefined gating region. For very low abundance and rare labeled lymphocytes mixed with high concentration unlabeled white blood cells (WBCs), (1-100 labeled lymphocytes are diluted in 10⁶ WBCs in 1 ml volume fluid), the device is able to remove more than 98% WBCs and recover labeled lymphocytes with efficiency of 80%. We further demonstrated that this device worked with real clinical samples by successfully isolating fetal nucleated red blood cells (FNRBCs) in the blood samples from pregnant women with male fetus. The obtained cells were sequenced and the expressions of (sex determining region Y) SRY genes were tested to determine fetal cell proportion. In genetic analysis, the proportion of fetal cells in the final picked sample is up to 40.64%. With this ability, the device proposed could be valuable for biomedical applications involving fetal cells, circulating tumor cells, and stem cells.

Mattertronics for programmable manipulation and multiplex storage of pseudo-diamagnetic holes and label-free cells

Manipulating and separating single label-free cells without biomarker conjugation have attracted significant interest in the field of single-cell research, but digital circuitry control and multiplexed individual storage of single label-free cells remain a challenge. Herein, by analogy with the electrical circuitry elements and electronical holes, we develop a pseudo-diamagnetophoresis (PsD) mattertronic approach in the presence of biocompatible ferrofluids for programmable manipulation and local storage of single PsD holes and label-free cells. The PsD holes conduct along linear negative micro-magnetic patterns. Further, eclipse diode patterns similar to the electrical diode can implement directional and selective switching of different PsD holes and label-free cells based on the diode geometry. Different eclipse heights and junction gaps influence the switching efficiency of PsD holes for mattertronic circuitry manipulation and separation. Moreover, single PsD holes are stored at each potential well as in an electrical storage capacitor, preventing multiple occupancies of PsD holes in the array of individual compartments due to magnetic Coulomb-like interaction. This approach may enable the development of large programmable arrays of label-free matters with high throughput, efficiency, and reliability as multiplex cell research platforms.

Sheathless microflow cytometer utilizing two bulk standing acoustic waves

In recent years, microflow cytometry has become a popular research field because of its potential to provide low-cost and disposable chips for complex cell analyses. Herein, we demonstrate a sheathless microflow cytometer which integrates a bulk standing acoustic wave based microchip capable of three dimensional cell focusing. Flow cytometry was successfully demonstrated using this system with a coefficient of variation (CV) of 2.16% with standard calibration beads. The sensitivities calibrated by rainbow beads are 518 MEFL in fluorescein Isothiocyanate (FITC) channel and 264 MEPE in P-phycoerythrin (PE) channels, respectively. The linearities are more than 99% in both channels. The capability of the proposed microflow cytometer is further demonstrated by immunologically labeled leukocytes differentiation in blood. This acoustic-based microflow cytometer did not require any sheath flows or complex structures and can be mass produced. Because of the simple fluid channel, the chip can be easily made pipeless, disposable for applications requiring no cross contamination. Moreover, with the gentle and bio-compatible acoustic waves used, this technique is expected to maintain the viability of cells and other bioparticles.

Rapid and precise tumor cell separation using the combination of size-dependent inertial and size-independent magnetic methods

Circulating tumor cells (CTCs) play a significant role in cancer diagnosis and treatment monitoring. One of the major challenges in isolating and detecting rare CTCs from blood is that white blood cells (WBCs) have a size overlap with the target CTCs. To address this issue, we constructed a three-stage i-Mag device integrated with passive inertial microfluidics and active magnetophoresis, enabling rapid and precise separation of tumor cells from blood. The first-stage spiral inertial sorter was applied to rapidly remove small-sized red blood cells (RBCs), and then the second-stage serpentine inertial focuser and the third-stage magnetic sorter were used for removing the magnetically labeled WBCs size-independently, to significantly purify the captured tumor cells. Then, the separation performance of our i-Mag device was explored. The results indicated rapid and precise separation of breast cancer cells from diluted whole blood at a high separation efficiency of 93.84% and at a high purity of 51.47%. The purity of the collected tumor cells could be further improved to 93.60% when the blood dilution ratio was increased. We also successfully applied our i-Mag device for the isolation and detection of trace tumor cells. Our i-Mag device has numerous advantages, such as enabling high-throughput processing and high-precision separation, requiring easy manufacturing at a low cost, and providing tumor antigen-independent operation. We believe that the i-Mag device has great potential to act as a precise tool for separating various bioparticles.

Inertial microfluidics: Recent advances

Inertial microfluidics has attracted significant attentions in last decade due to its superior advantages of high throughput, label- and external field-free operation, simplicity, and low cost. A wide variety of channel geometry designs were demonstrated for focusing, concentrating, isolating, or separating of various bioparticles such as blood components, circulating tumor cells, bacteria, and microalgae. In this review, we first briefly introduce the physics of inertial migration and Dean flow for allowing the readers with diverse backgrounds to have a better understanding of the fundamental mechanisms of inertial microfluidics. Then, we present a comprehensive review of the recent advances and applications of inertial microfluidic devices according to different channel geometries ranging from straight channels, curved channels to contraction-expansion-array channels. Finally, the challenges and future perspective of inertial microfluidics are discussed. Owing to its superior benefit for particle manipulation, the inertial microfluidics will play a more important role in biology and medicine applications.

Acoustic Microfluidic Separation Techniques and Bioapplications: A Review

Microfluidic separation technology has garnered significant attention over the past decade where particles are being separated at a micro/nanoscale in a rapid, low-cost, and simple manner. Amongst a myriad of separation technologies that have emerged thus far, acoustic microfluidic separation techniques are extremely apt to applications involving biological samples attributed to various advantages, including high controllability, biocompatibility, and non-invasive, label-free features. With that being said, downsides such as low throughput and dependence on external equipment still impede successful commercialization from laboratory-based prototypes. Here, we present a comprehensive review of recent advances in acoustic microfluidic separation techniques, along with exemplary applications. Specifically, an inclusive overview of fundamental theory and background is presented, then two sets of mechanisms underlying acoustic separation, bulk acoustic wave and surface acoustic wave, are introduced and discussed. Upon these summaries, we present a variety of applications based on acoustic separation. The primary focus is given to those associated with biological samples such as blood cells, cancer cells, proteins, bacteria, viruses, and DNA/RNA. Finally, we highlight the benefits and challenges behind burgeoning developments in the field and discuss the future perspectives and an outlook towards robust, integrated, and commercialized devices based on acoustic microfluidic separation.

Acoustic fields and microfluidic patterning around embedded micro-structures subject to surface acoustic waves

Recent research has shown that interactions between acoustic waves and microfluidic channels can generate microscale interference patterns with the application of a traveling surface acoustic wave (SAW), effectively creating standing wave patterns with a traveling wave. Forces arising from this interference can be utilized for precise manipulation of micron-sized particles and biological cells. The patterns that have been produced with this method, however, have been limited to straight lines and grids from flat channel walls, and where the spacing resulting from this interference has not previously been comprehensively explored. In this work we examine the interaction between both straight and curved channel interfaces with a SAW to derive geometrically deduced analytical models. These models predict the acoustic force-field periodicity near a channel interface as a function of its orientation to an underlying SAW, and are validated with experimental and simulation results. Notably, the spacing is larger for flat walls than for curved ones and is dependent on the ratio of sound speeds in the substrate and fluid. Generating these force-field gradients with only travelling waves has wide applications in acoustofluidic systems, where channel interfaces can potentially support a range of patterning, concentration, focusing and separation activities by creating locally defined acoustic forces.

Morpho-Rheological Fingerprinting of Rod Photoreceptors Using Real-Time Deformability Cytometry

Distinct cell-types within the retina are mainly specified by morphological and molecular parameters, however, physical properties are increasingly recognized as a valuable tool to characterize and distinguish cells in diverse tissues. High-throughput analysis of morpho-rheological features has recently been introduced using real-time deformability cytometry (RT-DC) providing new insights into the properties of different cell-types. Rod photoreceptors represent the main light sensing cells in the mouse retina that during development forms apically the densely packed outer nuclear layer. Currently, enrichment and isolation of photoreceptors from retinal primary tissue or pluripotent stem cell-derived organoids for analysis, molecular profiling, or transplantation is achieved using flow cytometry or magnetic activated cell sorting approaches. However, such purification methods require genetic modification or identification of cell surface binding antibody panels. Using primary retina and embryonic stem cell-derived retinal organoids, we characterized the inherent morpho-mechanical properties of mouse rod photoreceptors during development based on RT-DC. We demonstrate that rods become smaller and more compliant throughout development and that these features are suitable to distinguish rods within heterogenous retinal tissues. Hence, physical properties should be considered as additional factors that might affect photoreceptor differentiation and retinal development besides representing potential parameters for label-free sorting of photoreceptors. © 2019 The Authors. Cytometry Part A published by Wiley Periodicals, Inc. on behalf of International Society for Advancement of Cytometry.

Adhesive cell patterning technique using ultrasound vibrations

This paper investigated an ultrasound vibration cell patterning technique. The ultrasound cell culture dish consisted of a culture dish with a glass bottom and a glass disc with a piezoelectric ring that generated a resonance flexural vibration mode on the bottom of the dish. The growth of HeLa cells on the dish was observed under ultrasound excitation for 24 h. Large ultrasound vibrations on the dish inhibited the cell growth. The acoustic field was predicted with finite element analysis and it was found that the cell growth depended strongly on both the acoustic field in the culture medium and the vibration distribution of the dish. The ultrasound vibrations did not affect the viability of the cells, and the cell growth could be controlled by the flexural vibration of the cultured dish.

Acoustofluidic separation of cells and particles

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.

Label-free enrichment of primary human skeletal progenitor cells using deterministic lateral displacement

Skeletal stem cells (SSCs) are present in bone marrow (BM) and offer great potential for bone regenerative therapies. However, in the absence of a unique marker, current sorting approaches remain challenging in the quest for simple strategies to deliver SSCs with consistent regeneration and differentiation capacities. Microfluidics offers the possibility to sort cells marker-free, based on intrinsic biophysical properties. Recent studies indicate that SSCs are stiffer than leukocytes and are contained within the larger cell fraction in BM. This paper describes the use of deterministic lateral displacement (DLD) to sort SSCs based on cell size and stiffness. DLD is a technology that uses arrays of micropillars to sort cells based on their diameter. Cell deformation within the device can change the cell size and affect sorting - here evidenced using human cell lines and by fractionation of expanded SSCs. Following sorting, SSCs remained viable and retained their capacity to form clonogenic cultures (CFU-F), indicative of stem cell potential. Additionally, larger BM cells showed enhanced capacity to form CFU-F. These findings support the theory that SSCs are more abundant within the larger BM cell fraction and that DLD, or other size-based approaches, could be used to provide enriched SSC populations with significant implications for stem cell research and translation to the clinic.

Applications of Acoustofluidics in Bioanalytical Chemistry

Acoustics has a broad spectrum of applications, ranging from noise cancelation to ultrasonic imaging. In the past decade, there has been increasing interest in developing acoustic-based methods for biological and biomedical applications. This Perspective summarizes the recent progress in applying acoustofluidic methods (i.e., the fusion of acoustics and microfluidics) to bioanalytical chemistry. We describe the concepts of acoustofluidics and how it can be tailored to different types of bioanalytical applications, including sample concentration, fluorescence-activated cell sorting, label-free cell/particle separation, and fluid manipulation. Examples of each application are given, and the benefits and limitations of these methods are discussed. Finally, our perspectives on the directions that developing solutions should take to address the bottlenecks in the acoustofluidic applications in bioanalytical chemistry are presented.

Analysis of Biomechanical Properties of Hematopoietic Stem and Progenitor Cells Using Real-Time Fluorescence and Deformability Cytometry

Stem cell mechanics, determined predominantly by the cell's cytoskeleton, plays an important role in different biological processes such as stem cell differentiation or migration. Several methods to measure mechanical properties of cells are currently available, but most of them are limited in the ability to screen large heterogeneous populations in a robust and efficient manner-a feature required for successful translational applications. With real-time fluorescence and deformability cytometry (RT-FDC), mechanical properties of cells in suspension can be screened continuously at rates of up to 1,000 cells/s-similar to conventional flow cytometers-which makes it a suitable method not only for basic research but also for a clinical setting. In parallel to mechanical characterization, RT-FDC allows to measure specific molecular markers using standard fluorescence labeling. In this chapter, we provide a detailed protocol for the characterization of hematopoietic stem and progenitor cells (HSPCs) in heterogeneous mobilized peripheral blood using RT-FDC and present a specific morpho-rheological fingerprint of HSPCs that allows to distinguish them from all other blood cell types.

Microfluidic technologies in cell isolation and analysis for biomedical applications

Efficient platforms for cell isolation and analysis play an important role in applied and fundamental biomedical studies. As cells commonly have a size of around 10 microns, conventional handling approaches at a large scale are still challenged in precise control and efficient recognition of cells for further performance of isolation and analysis. Microfluidic technologies have become more prominent in highly efficient cell isolation for circulating tumor cells (CTCs) detection, single-cell analysis and stem cell separation, since microfabricated devices allow for the spatial and temporal control of complex biochemistries and geometries by matching cell morphology and hydrodynamic traps in a fluidic network, as well as enabling specific recognition with functional biomolecules in the microchannels. In addition, the fabrication of nano-interfaces in the microchannels has been increasingly emerging as a very powerful strategy for enhancing the capability of cell capture by improving cell-interface interactions. In this review, we focus on highlighting recent advances in microfluidic technologies for cell isolation and analysis. We also describe the general biomedical applications of microfluidic cell isolation and analysis, and finally make a prospective for future studies.

Microfluidic Based Optical Microscopes on Chip

Last decade's advancements in optofluidics allowed obtaining an ever increasing integration of different functionalities in lab on chip devices to culture, analyze, and manipulate single cells and entire biological specimens. Despite the importance of optical imaging for biological sample monitoring in microfluidics, imaging is traditionally achieved by placing microfluidics channels in standard bench-top optical microscopes. Recently, the development of either integrated optical elements or lensless imaging methods allowed optical imaging techniques to be implemented in lab on chip systems, thus increasing their automation, compactness, and portability. In this review, we discuss known solutions to implement microscopes on chip that exploit different optical methods such as bright-field, phase contrast, holographic, and fluorescence microscopy.

New advances in microfluidic flow cytometry

In recent years, researchers are paying the increasing attention to the development of portable microfluidic diagnostic devices including microfluidic flow cytometry for the point-of-care testing. Microfluidic flow cytometry, where microfluidics and flow cytometry work together to realize novel functionalities on the microchip, provides a powerful tool for measuring the multiple characteristics of biological samples. The development of a portable, low-cost, and compact flow cytometer can benefit the health care in underserved areas such as Africa or Asia. In this article, we review recent advancements of microfluidics including sample pumping, focusing and sorting, novel detection approaches, and data analysis in the field of flow cytometry. The challenge of microfluidic flow cytometry is also examined briefly.

Microfluidic approaches for cell-based molecular diagnosis

The search for next-generation biomarkers has enabled cell-based diagnostics in a number of disciplines ranging from oncology to pharmacogenetics. However, cell-based diagnostics are still far from clinical reality due to the complex assays and associated protocols which typically require cell isolation, lysis, DNA extraction, amplification, and detection steps. Leveraging recent advances in microfluidics, many biochemical assays have been translated onto microfluidic platforms. We have compared and summarized recent advances in modular approaches toward the realization of fully-integrated, cell-based molecular diagnostics for clinical and point-of-care applications.

High-throughput single-cell mechanical phenotyping with real-time deformability cytometry

Mechanical properties of cells can serve as a label-free marker of cell state and function and their alterations have been implicated in processes such as cancer metastasis, leukocyte activation, or stem cell differentiation. Over recent years, new techniques for single-cell mechanical characterization at high throughput have been developed to accelerate discovery in the field of mechanical phenotyping. One such technique is real-time deformability cytometry (RT-DC), a robust technology based on microfluidics that performs continuous mechanical characterization of cells in a contactless manner at rates of up to 1000 cells per second. This tremendous throughput allows for comparison of large sample numbers and precise characterization of heterogeneous cell populations. Additionally, parameters acquired in RT-DC measurements can be used to determine the apparent Young's modulus of individual cells. In this chapter, we present practical aspects important for the implementation of the RT-DC methodology, including a description of the setup, operation principles, and experimental protocols. In the latter, we describe a variety of preparation procedures for samples originating from different sources including 2D and 3D cell cultures as well as blood and tissue-derived primary cells, and discuss obstacles that may arise during their measurements. Finally, we provide insights into standard data analysis procedures and discuss the method's performance in light of other available techniques.

Label-free sorting of soft microparticles using a bioinspired synthetic cilia array

Isolating cells of interest from a heterogeneous population has been of critical importance in biological studies and clinical applications. In this study, a novel approach is proposed for utilizing an active ciliary system in microfluidic devices to separate particles based on their physical properties. In this approach, the bottom of the microchannel is covered with an equally spaced cilia array of various patterns which is actuated by an external stimuli. 3D simulations are carried out to study cilia-particle interaction and isolation dynamic in a microfluidic channel. It is observed that these elastic hair-like filaments can influence particle's trajectories differently depending on their biophysical properties. This modeling study utilizes immersed boundary method coupled with the lattice Boltzmann method. Soft particles and cilia are implemented through the spring connected network model and point-particle scheme, respectively. It is shown that cilia array with proper stimulation is able to continuously and non-destructively separate cells into subpopulations based on their size, shape, and stiffness. At the end, a design map for fabrication of a programmable microfluidic device capable of isolating various subpopulations of cells is developed. This biocompatible, label-free design can separate cells/soft microparticles with high throughput which can greatly complement existing separation technologies.

Pumpless Microflow Cytometry Enabled by Viscosity Modulation and Immunobead Labeling

Major challenges of miniaturizing flow cytometry include obviating the need for bulky, expensive, and complex pump-based fluidic and laser-based optical systems while retaining the ability to detect target cells based on their unique surface receptors. We addressed these critical challenges by (i) using a viscous liquid additive to control flow rate passively, without external pumping equipment, and (ii) adopting an immunobead assay that can be quantified with a portable fluorescence cell counter based on a blue light-emitting diode. Such novel features enable pumpless microflow cytometry (pFC) analysis by simply dropping a sample solution onto the inlet reservoir of a disposable cell-counting chamber. With our pFC platform, we achieved reliable cell counting over a dynamic range of 9-298 cells/μL. We demonstrated the practical utility of the platform by identifying a type of cancer cell based on CD326, the epithelial cell adhesion molecule. This portable microflow cytometry platform can be applied generally to a range of cell types using immunobeads labeled with specific antibodies, thus making it valuable for cell-based and point-of-care diagnostics.

Acoustic Actuation of in situ Fabricated Artificial Cilia

We present on-chip acoustic actuation of in situ fabricated artificial cilia. Arrays of cilia structures are UV polymerized inside a microfluidic channel using a photocurable polyethylene glycol (PEG) polymer solution and photomasks. During polymerization, cilia structures are attached to a silane treated glass surface inside the microchannel. Then, the cilia structures are actuated using acoustic vibrations at 4.6 kHz generated by piezo transducers. As a demonstration of a practical application, DI water and fluorescein dye solutions are mixed inside a microfluidic channel. Using pulses of acoustic excitations, and locally fabricated cilia structures within a certain region of the microchannel, a waveform of mixing behavior is obtained. This result illustrates one potential application wherein researchers can achieve spatiotemporal control of biological microenvironments in cell stimulation studies. These acoustically actuated, in situ fabricated, cilia structures can be used in many on-chip applications in biological, chemical and engineering studies.

On-demand acoustic droplet splitting and steering in a disposable microfluidic chip

On-chip droplet splitting is one of the fundamental droplet-based microfluidic unit operations to control droplet volume after production and increase operational capability, flexibility, and throughput. Various droplet splitting methods have been proposed, and among them the acoustic droplet splitting method is promising because of its label-free operation without any physical or thermal damage to droplets. Previous acoustic droplet splitting methods faced several limitations: first, they employed a cross-type acoustofluidic device that precluded multichannel droplet splitting; second, they required irreversible bonding between a piezoelectric substrate and a microfluidic chip, such that the fluidic chip was not replaceable. Here, we present a parallel-type acoustofluidic device with a disposable microfluidic chip to address the limitations of previous acoustic droplet splitting devices. In the proposed device, an acoustic field is applied in the direction opposite to the flow direction to achieve multichannel droplet splitting and steering. A disposable polydimethylsiloxane microfluidic chip is employed in the developed device, thereby removing the need for permanent bonding and improving the flexibility of the droplet microfluidic device. We experimentally demonstrated on-demand acoustic droplet bi-splitting and steering with precise control over the droplet splitting ratio, and we investigated the underlying physical mechanisms of droplet splitting and steering based on Laplace pressure and ray acoustics analyses, respectively. We also demonstrated droplet tri-splitting to prove the feasibility of multichannel droplet splitting. The proposed on-demand acoustic droplet splitting device enables on-chip droplet volume control in various droplet-based microfluidic applications.

Fluorescence activated cell sorting via a focused traveling surface acoustic beam

Fluorescence activated cell sorting (FACS) has become an essential technique widely exploited in biological studies and clinical applications. However, current FACS systems are quite complex, expensive, bulky, and pose potential sample contamination and biosafety issues due to the generation of aerosols in an open environment. Microfluidic technology capable of precise cell manipulation has great potential to reinvent and miniaturize conventional FACS systems. In this work, we demonstrate a benchtop scale FACS system that makes use of a highly focused traveling surface acoustic wave beam to sort out micron-sized particles and biological cells upon fluorescence interrogation at ∼kHz rates. The highly focused acoustic wave beam has a width of ∼50 μm that enables highly accurate sorting of individual particles and cells. We have applied our acoustic FACS system to isolate fluorescently labeled MCF-7 breast cancer cells from diluted whole blood samples with the purity of sorted MCF-7 cells higher than 86%. The cell viability before and after acoustic sorting is higher than 95%, indicating excellent biocompatibility that should enable a variety of cell sorting applications in biomedical research.

Particle separation by phase modulated surface acoustic waves

High efficiency isolation of cells or particles from a heterogeneous mixture is a critical processing step in lab-on-a-chip devices. Acoustic techniques offer contactless and label-free manipulation, preserve viability of biological cells, and provide versatility as the applied electrical signal can be adapted to various scenarios. Conventional acoustic separation methods use time-of-flight and achieve separation up to distances of quarter wavelength with limited separation power due to slow gradients in the force. The method proposed here allows separation by half of the wavelength and can be extended by repeating the modulation pattern and can ensure maximum force acting on the particles. In this work, we propose an optimised phase modulation scheme for particle separation in a surface acoustic wave microfluidic device. An expression for the acoustic radiation force arising from the interaction between acoustic waves in the fluid was derived. We demonstrated, for the first time, that the expression of the acoustic radiation force differs in surface acoustic wave and bulk devices, due to the presence of a geometric scaling factor. Two phase modulation schemes are investigated theoretically and experimentally. Theoretical findings were experimentally validated for different mixtures of polystyrene particles confirming that the method offers high selectivity. A Monte-Carlo simulation enabled us to assess performance in real situations, including the effects of particle size variation and non-uniform acoustic field on sorting efficiency and purity, validating the ability to separate particles with high purity and high resolution.

On-chip ultrasonic manipulation of microparticles by using the flexural vibration of a glass substrate

As biotechnology develops, techniques for manipulating and separating small particles such as cells and DNA are required in the life sciences. This paper investigates on-chip manipulation of microparticles in small channels by using ultrasonic vibration. The chip consists of a rectangular glass substrate with a cross-shaped channel (cross-section: 2.0×2.0mm²) and four lead zirconate titanate transducers attached to the substrate's four corners. To efficiently generate the flexural vibration mode on the chip, we used finite element analysis to optimize the configurations of the glass substrate and transducers. Silicon carbide microparticles with an average diameter of 50μm were immersed in the channels, which were filled with ethanol. By applying an in-phase input voltage of 75V at 225kHz to the four transducers, a flexural vibration mode with a wavelength of 13mm was excited on the glass substrate, and this flexural vibration generated an acoustic standing wave in the channel. The particles could be trapped at the nodal lines of the standing wave. By controlling the driving phase difference between the two pairs of transducers, the vibrational distribution of the substrate could be moved along the channels so that the acoustic standing wave moved in the same direction. The trapped particles could be manipulated by the two-phase drive, and the transport direction could be switched at the junction of the channels orthogonally by changing the combination of the driving condition to four transducers.

Surface acoustic wave devices for chemical sensing and microfluidics: A review and perspective

Surface acoustic waves (SAWs), are electro-mechanical waves that form on the surface of piezoelectric crystals. Because they are easy to construct and operate, SAW devices have proven to be versatile and powerful platforms for either direct chemical sensing or for upstream microfluidic processing and sample preparation. This review summarizes recent advances in the development of SAW devices for chemical sensing and analysis. The use of SAW techniques for chemical detection in both gaseous and liquid media is discussed, as well as recent fabrication advances that are pointing the way for the next generation of SAW sensors. Similarly, applications and progress in using SAW devices as microfluidic platforms are covered, ranging from atomization and mixing to new approaches to lysing and cell adhesion studies. Finally, potential new directions and perspectives on the field as it moves forward are offered, with a specific focus on potential strategies for making SAW technologies for bioanalytical applications.

Selective particle and cell capture in a continuous flow using micro-vortex acoustic streaming

Acoustic streaming has emerged as a promising technique for refined microscale manipulation, where strong rotational flow can give rise to particle and cell capture. In contrast to hydrodynamically generated vortices, acoustic streaming is rapidly tunable, highly scalable and requires no external pressure source. Though streaming is typically ignored or minimized in most acoustofluidic systems that utilize other acoustofluidic effects, we maximize the effect of acoustic streaming in a continuous flow using a high-frequency (381 MHz), narrow-beam focused surface acoustic wave. This results in rapid fluid streaming, with velocities orders of magnitude greater than that of the lateral flow, to generate fluid vortices that extend the entire width of a 400 μm wide microfluidic channel. We characterize the forces relevant for vortex formation in a combined streaming/lateral flow system, and use these acoustic streaming vortices to selectively capture 2 μm from a mixed suspension with 1 μm particles and human breast adenocarcinoma cells (MDA-231) from red blood cells.