Hydrodynamic focusing--a versatile tool

A 3D printed sheath flow interface for surface enhanced Raman spectroscopy (SERS) detection in flow

Surface enhanced Raman spectroscopy (SERS) is an effective technique for detecting molecules in aqueous solutions due to its insensitivity to water, which makes it especially useful for biological samples. Utilizing SERS in flow can aid in a variety of applications such as metabolomics, pharmaceuticals, and diagnostics. The ability to 3D print complex objects enables rapid dissemination of prototypes. A 3D printed flow cell for sheath flow SERS detection has been developed that can incorporate a variety of planar substrates. The 3D printed flow cell incorporates hydrodynamic focusing, a sheath flow, that confines the analyte near the SERS substrate. Since the SERS signal obtained relies on the interaction between analyte molecules and nanostructures, sheath flow increases the detection efficiency and eliminates many issues associated with SERS detection in solution. This device was optimized by analyzing both molecules and particles with and without using sheath flow for SERS detection. Our results show that the flow rates can be optimized to increase the SERS signal obtained from a variety of analytes, and that the signal was increased when using sheath flow. This 3D printed flow cell offers a straightforward method to disseminate this technology and to facilitate online SERS detection.

Light-field flow cytometry for high-resolution, volumetric and multiparametric 3D single-cell analysis

Imaging flow cytometry (IFC) combines flow cytometry and fluorescence microscopy to enable high-throughput, multiparametric single-cell analysis with rich spatial details. However, current IFC techniques remain limited in their ability to reveal subcellular information with a high 3D resolution, throughput, sensitivity, and instrumental simplicity. In this study, we introduce a light-field flow cytometer (LFC), an IFC system capable of high-content, single-shot, and multi-color acquisition of up to 5,750 cells per second with a near-diffraction-limited resolution of 400-600 nm in all three dimensions. The LFC system integrates optical, microfluidic, and computational strategies to facilitate the volumetric visualization of various 3D subcellular characteristics through convenient access to commonly used epi-fluorescence platforms. We demonstrate the effectiveness of LFC in assaying, analyzing, and enumerating intricate subcellular morphology, function, and heterogeneity using various phantoms and biological specimens. The advancement offered by the LFC system presents a promising methodological pathway for broad cell biological and translational discoveries, with the potential for widespread adoption in biomedical research.

Hydrodynamic Assembly of Astrocyte Cells in Conductive Hollow Microfibers

The manufacturing of 3D cell scaffoldings provides advantages for modeling diseases and injuries as it enables the creation of physiologically relevant platforms. A triple-flow microfluidic device is developed to rapidly fabricate alginate/graphene hollow microfibers based on the gelation of alginate induced with CaCl2 . This five-channel microdevice actualizes continuous mild fabrication of hollow fibers under an optimized flow rate ratio of 300:200:100 µL min-1 . The polymer solution is 2.5% alginate in 0.1% graphene and a 30% polyethylene glycol solution is used as the sheath and core solutions. The biocompatibility of these conductive microfibers by encapsulating mouse astrocyte cells (C8D1A) within the scaffolds is investigated. The cells can successfully survive both the manufacturing process and prolonged encapsulation for up to 8 days, where there is between 18-53% of live cells on both the alginate microfibers and alginate/graphene microfibers. These unique 3D hollow scaffolds can significantly enhance the available surface area for nutrient transport to the cells. In addition, these conductive hollow scaffolds illustrate unique advantages such as 0.728 cm3  gr-1 porosity and two times more electrical conductivity in comparison to alginate scaffolds. The results confirm the potential of these scaffolds as a microenvironment that supports cell growth.

Squeezed state in the hydrodynamic focusing regime for Escherichia coli bacteria detection

Flow cytometry is an essential technique in single particle analysis and cell sorting for further downstream diagnosis, exhibiting high-throughput and multiplexing capabilities for many biological and biomedical applications. Although many hydrodynamic focusing-based microfluidic cytometers have been demonstrated with reduced size and cost to adapt to point-of-care settings, the operating conditions are not characterized systematically. This study presents the flow transition process in the hydrodynamic focusing mechanism when the flow rate or the Reynolds number increases. The characteristics of flow fields and mass transport were studied under various operating conditions, including flow rates and microchannel heights. A transition from the squeezed focusing state to the over-squeezed anti-focusing state in the hydrodynamic focusing regime was observed when the Reynolds number increased above 30. Parametric studies illustrated that the focusing width increased with the Reynolds number but decreased with the microchannel height in the over-squeezed state. The microfluidic cytometric analyses using microbeads and E. coli show that the recovery rate was maintained by limiting the Reynolds number to 30. The detailed analysis of the flow transition will provide new insight into microfluidic cytometric analyses with a broad range of applications in food safety, water monitoring and healthcare sectors.

Rapid Production of Nanoscale Liposomes Using a 3D-Printed Reactor-In-A-Centrifuge: Formulation, Characterisation, and Super-Resolution Imaging

Nanoscale liposomes have been extensively researched and employed clinically for the delivery of biologically active compounds, including chemotherapy drugs and vaccines, offering improved pharmacokinetic behaviour and therapeutic outcomes. Traditional laboratory-scale production methods often suffer from limited control over liposome properties (e.g., size and lamellarity) and rely on laborious multistep procedures, which may limit pre-clinical research developments and innovation in this area. The widespread adoption of alternative, more controllable microfluidic-based methods is often hindered by complexities and costs associated with device manufacturing and operation, as well as the short device lifetime and the relatively low liposome production rates in some cases. In this study, we demonstrated the production of liposomes comprising therapeutically relevant lipid formulations, using a cost-effective 3D-printed reactor-in-a-centrifuge (RIAC) device. By adjusting formulation- and production-related parameters, including the concentration of polyethylene glycol (PEG), temperature, centrifugation time and speed, and lipid concentration, the mean size of the produced liposomes could be tuned in the range of 140 to 200 nm. By combining selected experimental parameters, the method was capable of producing liposomes with a therapeutically relevant mean size of ~174 nm with narrow size distribution (polydispersity index, PDI ~0.1) at a production rate of >8 mg/min. The flow-through method proposed in this study has potential to become an effective and versatile laboratory-scale approach to simplify the synthesis of therapeutic liposomal formulations.

A microfluidic platform for the controlled synthesis of architecturally complex liquid crystalline nanoparticles

Soft-matter nanoparticles are of great interest for their applications in biotechnology, therapeutic delivery, and in vivo imaging. Underpinning this is their biocompatibility, potential for selective targeting, attractive pharmacokinetic properties, and amenability to downstream functionalisation. Morphological diversity inherent to soft-matter particles can give rise to enhanced functionality. However, this diversity remains untapped in clinical and industrial settings, and only the simplest of particle architectures [spherical lipid vesicles and lipid/polymer nanoparticles (LNPs)] have been routinely exploited. This is partially due to a lack of appropriate methods for their synthesis. To address this, we have designed a scalable microfluidic hydrodynamic focusing (MHF) technology for the controllable, rapid, and continuous production of lyotropic liquid crystalline (LLC) nanoparticles (both cubosomes and hexosomes), colloidal dispersions of higher-order lipid assemblies with intricate internal structures of 3-D and 2-D symmetry. These particles have been proposed as the next generation of soft-matter nano-carriers, with unique fusogenic and physical properties. Crucially, unlike alternative approaches, our microfluidic method gives control over LLC size, a feature we go on to exploit in a fusogenic study with model cell membranes, where a dependency of fusion on particle diameter is evident. We believe our platform has the potential to serve as a tool for future studies involving non-lamellar soft nanoparticles, and anticipate it allowing for the rapid prototyping of LLC particles of diverse functionality, paving the way toward their eventual wide uptake at an industrial level.

Spatially selective cell treatment and collection for integrative drug testing using hydrodynamic flow focusing and shifting

Hydrodynamic focusing capable of readily producing and controlling laminar flow facilitates drug treatment of cells in existing microfluidic culture devices. However, to expand applications of such devices to multiparameter drug testing, critical limitations in current hydrodynamic focusing microfluidics must be addressed. Here we describe hydrodynamic focusing and shifting as an advanced microfluidics tool for spatially selective drug delivery and integrative cell-based drug testing. We designed and fabricated a co-flow focusing, three-channel microfluidic device with a wide cell culture chamber. By controlling inlet flow rates of sample and two side solutions, we could generate hydrodynamic focusing and shifting that mediated precise regulation of the path and width of reagent and drug stream in the microfluidic device. We successfully validated a hydrodynamic focusing and shifting approach for spatially selective delivery of DiI, a lipophilic fluorophore, and doxorubicin, a chemotherapeutic agent, to tumor cells in our device. Moreover, subsequent flowing of a trypsin EDTA solution over the cells that were exposed to doxorubicin flow allowed us to selectively collect the treated cells. Our approach enabled downstream high-resolution microscopy of the cell suspension to confirm the nuclear delivery of doxorubicin into the tumor cells. In the device, we could also evaluate in situ the cytotoxic effect of doxorubicin to the tumor cells that were selectively treated by hydrodynamic flow focusing and shifting. These results show that hydrodynamic focusing and shifting enable a fast and robust approach to spatially treat and then collect cells in an optimized microfluidic device, offering an integrative assay tool for efficient drug screening and discovery.

Microfluidic Tissue Engineering and Bio-Actuation

Bio-hybrid technologies aim to replicate the unique capabilities of biological systems that could surpass advanced artificial technologies. Soft bio-hybrid robots consist of synthetic and living materials and have the potential to self-assemble, regenerate, work autonomously, and interact safely with other species and the environment. Cells require a sufficient exchange of nutrients and gases, which is guaranteed by convection and diffusive transport through liquid media. The functional development and long-term survival of biological tissues in vitro can be improved by dynamic flow culture, but only microfluidic flow control can develop tissue with fine structuring and regulation at the microscale. Full control of tissue growth at the microscale will eventually lead to functional macroscale constructs, which are needed as the biological component of soft bio-hybrid technologies. This review summarizes recent progress in microfluidic techniques to engineer biological tissues, focusing on the use of muscle cells for robotic bio-actuation. Moreover, the instances in which bio-actuation technologies greatly benefit from fusion with microfluidics are highlighted, which include: the microfabrication of matrices, biomimicry of cell microenvironments, tissue maturation, perfusion, and vascularization.

A low-cost flow cell for flow cytometry

Flow cytometry is an essential analytical technique used in biomedical diagnostics to measure properties of cells, micro-organisms, and particles. Laser light is scattered from particles focused in a flow cell and collected by light sensors, where the intensity of the scattered light is a function of the scattering angle, the refractive index of the particle and surrounding medium, the wavelength of light, and the size and the shape of the particle. One of the critical parts of the cytometer is the flow cell where the particle stream is constrained into a tight region within 10-30 μm using hydrodynamic focusing. The conventional flow cells use thick quartz flow cells, which are expensive and therefore not suitable for instruments targeted for resource-constrained settings. We demonstrate a compact, economical, bio-compatible flow cell assembly design that incorporates inexpensive and easily available capillaries attached to sturdy polymer fixtures in a simple manner that performs the focusing of a sample stream of particles. The flow cell has been tested by studying the relation between sample core diameter, and sample and sheath flow rates. Small-angle scattering (forward scatter) and wide-angle scattering (side scatter) have been captured for the enumeration and characterization of particles. We show excellent agreement between the size distribution obtained via direct imaging and that obtained from light scattering. The flow cell was also used to successfully size white blood cells in human blood samples.

Current Development in Interdigital Transducer (IDT) Surface Acoustic Wave Devices for Live Cell In Vitro Studies: A Review

Acoustics have a wide range of uses, from noise-cancelling to ultrasonic imaging. There has been a surge in interest in developing acoustic-based approaches for biological and biomedical applications in the last decade. This review focused on the application of surface acoustic waves (SAW) based on interdigital transducers (IDT) for live-cell investigations, such as cell manipulation, cell separation, cell seeding, cell migration, cell characteristics, and cell behaviours. The approach is also known as acoustofluidic, because the SAW device is coupled with a microfluidic system that contains live cells. This article provides an overview of several forms of IDT of SAW devices on recently used cells. Conclusively, a brief viewpoint and overview of the future application of SAW techniques in live-cell investigations were presented.

Microfluidic technologies for the synthesis and manipulation of biomimetic membranous nano-assemblies

Microfluidics has been proposed as an attractive alternative to conventional bulk methods used in the generation of self-assembled biomimetic structures, particularly where there is a desire for more scalable production. The approach also allows for greater control over the self-assembly process, and parameters such as particle architecture, size, and composition can be finely tuned. Microfluidic techniques used in the generation of microscale assemblies (giant vesicles and higher-order multi-compartment assemblies) are fairly well established. These tend to rely on microdroplet templation, and the resulting structures have found use as comparmentalised motifs in artificial cells. Challenges in generating sub-micron droplets have meant that reconfiguring this approach to form nano-scale structures is not straightforward. This is beginning to change however, and recent technological advances have instigated the manufacture and manipulation of an increasingly diverse repertoire of biomimetic nano-assemblies, including liposomes, polymersomes, hybrid particles, multi-lamellar structures, cubosomes, hexosomes, nanodiscs, and virus-like particles. The following review will discuss these higher-order self-assembled nanostructures, including their biochemical and industrial applications, and techniques used in their production and analysis. We suggest ways in which existing technologies could be repurposed for the enhanced design, manufacture, and exploitation of these structures and discuss potential challenges and future research directions. By compiling recent advances in this area, it is hoped we will inspire future efforts toward establishing scalable microfluidic platforms for the generation of biomimetic nanoparticles of enhanced architectural and functional complexity.

Nano-Lipid-Carriers for the Treatment of Vitiligo: A Recent Update, Pathophysiology and Mechanism of Drug Delivery

BACKGROUND

Vitiligo is an auto-immune disease with white-coloured disfiguring patches or spots on the skin surface. It is highly prevalent in several corners of the world. This disease spreads in different age groups.

AIM

The main aim of this review is to provide an overview of pathophysiology and lipid-based nano-carriers for the treatment of vitiligo.

METHODS

The conventional delivery systems available have limited efficacy due to less retention of the drug in the epidermal layer where melanocytes reside. In order to overcome these issues, an advanced delivery system with lipid-based nano-carriers for the treatment of vitiligo was proven to be better as per the literature explored.

RESULTS

This review summarizes the pathophysiology along with novel lipid-based nano-carriers for the treatment of vitiligo.

CONCLUSION

Though various treatment regimens are present for the disease, nano-lipid carrier systems are significantly gaining importance nowadays, due to their high effectivity in topically acting on the target site. Nano-lipid carrier systems such as liposomes, ethosomes, transethosomes and transferosomes can be said to be at the top of the list in acting effectively against vitiligo or several other tropical diseases.

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.

Disease diagnostics using hydrodynamic flow focusing in microfluidic devices: Beyond flow cytometry

The multi-disciplinary field of microfluidics has the potential to provide solutions to a diverse set of problems. It offers the advantages of high-throughput, continuous, rapid and expeditious analysis requiring minute quantities of sample. However, even as this field has yielded many mass-manufacturable and cost-efficient point-of-care devices, its direct and practical applications into the field of disease diagnostics still remain limited and largely overlooked by the industry. This review focuses on the phenomenon of hydrodynamic focusing and its potential to materialize solutions for appropriate diagnosis and prognosis. The study aims to look beyond its intended cytometric applications and focus on unambiguous disease detection, monitoring, drug delivery, studies conducted on DNA and highlight the instances in the scientific literature that have proposed such approach.

Transferosomes as nanocarriers for drugs across the skin: Quality by design from lab to industrial scale

Transferosomes, also known as transfersomes, are ultradeformable vesicles for transdermal applications consisting of a lipid bilayer with phospholipids and an edge activator and an ethanol/aqueous core. Depending on the lipophilicity of the active substance, it can be encapsulated within the core or amongst the lipid bilayer. Compared to liposomes, transferosomes are able to reach intact deeper regions of the skin after topical administration delivering higher concentrations of active substances making them a successful drug delivery carrier for transdermal applications. Most transferosomes contain phosphatidylcholine (C18) as it is the most abundant lipid component of the cell membrane, and hence, it is highly tolerated for the skin, decreasing the risk of undesirable effects, such as hypersensitive reactions. The most common edge activators are surfactants such as sodium deoxycholate, Tween® 80 and Span® 80. Their chain length is optimal for intercalation within the C18 phospholipid bilayer. A wide variety of drugs has been successfully encapsulated within transferosomes such as phytocompounds like sinomenine or apigenin for rheumatoid arthritis and leukaemia respectively, small hydrophobic drugs but also macromolecules like insulin. The main factors to develop optimal transferosomal formulations (with high drug loading and nanometric size) are the optimal ratio between the main components as well as the critical process parameters for their manufacture. Application of quality by design (QbD), specifically design of experiments (DoE), is crucial to understand the interplay among all these factors not only during the preparation at lab scale but also in the scale-up process. Clinical trials of a licensed topical ketoprofen transferosomal gel have shown promising results in the alleviation of symptons in orthreothritis with non-severe skin and subcutaneous tissue disorders. However, the product was withdrawn from the market which probably was related to the higher cost of the medicine linked to the expensive manufacturing process required in the production of transferosomes compared to other conventional gel formulations. This example brings out the need for a careful formulation design to exploit the best properties of this drug delivery system as well as the development of manufacturing processes easily scalable at industrial level.

Single-cell technologies in reproductive immunology

The maternal-fetal interface represents a unique immune privileged site that maintains the ability to defend against pathogens while orchestrating the necessary tissue remodeling required for placentation. The recent discovery of novel cellular families (innate lymphoid cells, tissue-resident NK cells) suggests that our understanding of the decidual immunome is incomplete. To understand this complex milieu, new technological developments allow reproductive immunologists to collect increasingly complex data at a cellular resolution. Polychromatic flow cytometry allows for greater resolution in the identification of novel cell types by surface and intracellular protein. Single-cell RNA-seq coupled with microfluidics allows for efficient cellular transcriptomics. The extreme dimensionality and size of data sets generated, however, requires the application of novel computational approaches for unbiased analysis. There are now multiple dimensionality reduction (tSNE, SPADE) and visualization tools (SPICE) that allow researchers to efficiently analyze flow cytometry data. Development of computational tools has also been extended to RNA-seq data (including scRNA-seq), which requires specific analytical tools. Here, we provide an overview and a brief primer for the reproductive immunology community on data acquisition and computational tools for the analysis of complex flow cytometry and RNA-seq data.

In Situ Photophysical Characterization of π-Conjugated Oligopeptides Assembled via Continuous Flow Processing

Bioinspired materials have been developed with the aim of harnessing natural self-assembly for precisely engineered functionality. Microfluidics is poised to play a key role in the directed assembly of advanced materials with ordered nano and mesoscale features. More importantly, there is a strong need for understanding the kinetics of continuous assembly processes. In this work, we describe a continuous microfluidic system for the assembly and alignment of synthetic oligopeptides with π-conjugated cores using a three-dimensional (3D) flow focusing of inlet reactant streams. This system facilitates in situ confocal fluorescence microscopy and in situ fluorescence lifetime imaging microscopy (FLIM), which can be used in unprecedented capacity to characterize the integrity of peptides during the assembly process. To achieve continuous assembly, we integrate chevron patterns in the ceiling and floor of the microdevice to generate a 3D-focused sheath flow of the reactant peptide. Consequently, the peptide stream is directed toward an acidic triggering stream in a cross-slot geometry which mediates assembly into higher-order fiber-like structures. Using this approach, the focused peptide stream is assembled using a planar extensional flow, which ensures high degrees of microstructural alignment within the assembled material. We demonstrate the efficacy of this approach using three different synthetic oligopeptides, and in all cases, we observe the efficient and continuous assembly of oligopeptides. In addition, finite element simulations are used to guide device design and to validate 3D focusing. Overall, this approach presents an efficient and effective method for the continuous assembly and alignment of ordered materials using microfluidics.

Cell Cytometry: Review and Perspective on Biotechnological Advances

Cell identification and enumeration are essential procedures within clinical and research laboratories. For over 150 years, quantitative investigation of body fluids such as counts of various blood cells has been an important tool for diagnostic analysis. With the current evolution of point-of-care diagnostics and precision medicine, cheap and precise cell counting technologies are in demand. This article reviews the timeline and recent notable advancements in cell counting that have occurred as a result of improvements in sensing including optical and electrical technology, enhancements in image processing capabilities, and contributions of micro and nanotechnologies. Cell enumeration methods have evolved from the use of manual counting using a hemocytometer to automated cell counters capable of providing reliable counts with high precision and throughput. These developments have been enabled by the use of precision engineering, micro and nanotechnology approaches, automation and multivariate data analysis. Commercially available automated cell counters can be broadly classified into three categories based on the principle of detection namely, electrical impedance, optical analysis and image analysis. These technologies have many common scientific uses, such as hematological analysis, urine analysis and bacterial enumeration. In addition to commercially available technologies, future technological trends using lab-on-a-chip devices have been discussed in detail. Lab-on-a-chip platforms utilize the existing three detection technologies with innovative design changes utilizing advanced nano/microfabrication to produce customized devices suited to specific applications.

Grow with the Flow: When Morphogenesis Meets Microfluidics

Developmental biology has advanced the understanding of the intricate and dynamic processes involved in the formation of an organism from a single cell. However, many gaps remain in the knowledge of embryonic development, especially regarding tissue morphogenesis. A possible approach to mimic such phenomena uses pluripotent stem cells in in vitro morphogenetic models. Herein, these systems are summarized with emphasis on the ability to better manipulate and control cellular interfaces with either liquid or solid materials using microengineered tools, which is critical for attaining deeper insights into pattern formation and stem cell differentiation during organogenesis. The role of conventional and customized cell-culture systems in supporting important advances in the field of morphogenesis is discussed, and the fascinating role that material sciences and microengineering currently play and are expected to play in the future is highlighted. In conclusion, it is proffered that continued microfluidics innovations when applied to morphogenesis promise to provide important insights to advance many multidisciplinary fields, including regenerative medicine.

Label-Free Monitoring of Diffusion in Microfluidics

Label-free, real-time detection of concentration gradients is demonstrated in a microfluidic H-filter, using an integrated photonic crystal slab sensor to monitor sample refractive index with spatial resolution. The recorded diffusion profiles reveal root-mean-square diffusion lengths for non-fluorescing and non-absorbing molecules, both small (glucose, 180 Da) and large (bovine serum albumin, 67 kDa).

Optofluidic restricted imaging, spectroscopy and counting of nanoparticles by evanescent wave using immiscible liquids

Conventional flow cytometry (FC) suffers from the diffraction limit for the detection of nanoparticles smaller than 100 nm, whereas traditional total internal reflection (TIR) microscopy can only detect few samples near the solid-liquid interface mostly in static states. Here we demonstrate a novel on-chip optofluidic technique using evanescent wave sensing for single nanoparticle real time detection by combining hydrodynamic focusing and TIR using immiscible flows. The immiscibility of the high-index sheath flow and the low-index core flow naturally generate a smooth, flat and step-index interface that is ideal for the TIR effect, whose evanescent field can penetrate the full width of the core flow. Hydrodynamic focusing can focus on all the nanoparticles in the extreme centre of the core flow with a width smaller than 1 μm. This technique enables us to illuminate every single sample in the running core flow by the evanescent field, leaving none unaffected. Moreover, it works well for samples much smaller than the diffraction limit. We have successfully demonstrated the scattering imaging and counting of 50 nm and 100 nm Au nanoparticles and also the fluorescence imaging and counting of 200 nm beads. The effective counting speeds are estimated as 1500, 2300 and 2000 particles per second for the three types of nanoparticles, respectively. The optical scattering spectra were also measured to determine the size of individual Au nanoparticles. This provides a new technique to detect nanoparticles and we foresee its application in the detection of molecules for biomedical analyses.

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.

An open-chamber flow-focusing device for focal stimulation of micropatterned cells

Microfluidic devices can deliver soluble factors to cell and tissue culture microenvironments with precise spatiotemporal control. However, enclosed microfluidic environments often have drawbacks such as the need for continuous culture medium perfusion which limits the duration of experiments, incongruity between microculture and macroculture, difficulty in introducing cells and tissues, and high shear stress on cells. Here, we present an open-chamber microfluidic device that delivers hydrodynamically focused streams of soluble reagents to cells over long time periods (i.e., several hours). We demonstrate the advantage of the open chamber by using conventional cell culture techniques to induce the differentiation of myoblasts into myotubes, a process that occurs in 7-10 days and is difficult to achieve in closed chamber microfluidic devices. By controlling the flow rates and altering the device geometry, we produced sharp focal streams with widths ranging from 36 μm to 187 μm. The focal streams were reproducible (∼12% variation between units) and stable (∼20% increase in stream width over 10 h of operation). Furthermore, we integrated trenches for micropatterning myoblasts and microtraps for confining single primary myofibers into the device. We demonstrate with finite element method (FEM) simulations that shear stresses within the cell trench are well below values known to be deleterious to cells, while local concentrations are maintained at ∼22% of the input concentration. Finally, we demonstrated focused delivery of cytoplasmic and nuclear dyes to micropatterned myoblasts and myofibers. The open-chamber microfluidic flow-focusing concept combined with micropatterning may be generalized to other microfluidic applications that require stringent long-term cell culture conditions.

Microfluidic chips with multi-junctions: an advanced tool in recovering proteins from inclusion bodies

Active recombinant proteins are used for studying the biological functions of genes and for the development of therapeutic drugs. Overexpression of recombinant proteins in bacteria often results in the formation of inclusion bodies, which are protein aggregates with non-native conformations. Protein refolding is an important process for obtaining active recombinant proteins from inclusion bodies. However, the conventional refolding method of dialysis or dilution is time-consuming and recovered active protein yields are often low, and a cumbersome trial-and-error process is required to achieve success. To circumvent these difficulties, we used controllable diffusion through laminar flow in microchannels to regulate the denaturant concentration. This method largely aims at reducing protein aggregation during the refolding procedure. This Commentary introduces the principles of the protein refolding method using microfluidic chips and the advantage of our results as a tool for rapid and efficient recovery of active recombinant proteins from inclusion bodies.

Quantifying the volume of single cells continuously using a microfluidic pressure-driven trap with media exchange

We demonstrate a microfluidic device capable of tracking the volume of individual cells by integrating an on-chip volume sensor with pressure-activated cell trapping capabilities. The device creates a dynamic trap by operating in feedback; a cell is periodically redirected back and forth through a microfluidic volume sensor (Coulter principle). Sieve valves are positioned on both ends of the sensing channel, creating a physical barrier which enables media to be quickly exchanged while keeping a cell firmly in place. The volume of individual Saccharomyces cerevisiae cells was tracked over entire growth cycles, and the ability to quickly exchange media was demonstrated.

Ultrasensitive surface-enhanced Raman scattering flow detector using hydrodynamic focusing

Label-free, chemical specific detection in flow is important for high throughput characterization of analytes in applications such as flow injection analysis, electrophoresis, and chromatography. We have developed a surface-enhanced Raman scattering (SERS) flow detector capable of ultrasensitive optical detection on the millisecond time scale. The device employs hydrodynamic focusing to improve SERS detection in a flow channel where a sheath flow confines analyte molecules eluted from a fused silica capillary over a planar SERS-active substrate. Increased analyte interactions with the SERS substrate significantly improve detection sensitivity. The performance of this flow detector was investigated using a combination of finite element simulations, fluorescence imaging, and Raman experiments. Computational fluid dynamics based on finite element analysis was used to optimize the flow conditions. The modeling indicates that a number of factors, such as the capillary dimensions and the ratio of the sheath flow to analyte flow rates, are critical for obtaining optimal results. Sample confinement resulting from the flow dynamics was confirmed using wide-field fluorescence imaging of rhodamine 6G (R6G). Raman experiments at different sheath flow rates showed increased sensitivity compared with the modeling predictions, suggesting increased adsorption. Using a 50 ms acquisition, a sheath flow rate of 180 μL/min, and a sample flow rate of 5 μL/min, a linear dynamic range from nanomolar to micromolar concentrations of R6G with a limit of detection (LOD) of 1 nM is observed. At low analyte concentrations, rapid analyte desorption is observed, enabling repeated and high-throughput SERS detection. The flow detector offers substantial advantages over conventional SERS-based assays such as minimal sample volumes and high detection efficiency.

A hydrodynamic focusing microchannel based on micro-weir shear lift force

A novel microflow cytometer is proposed in which the particles are focused in the horizontal and vertical directions by means of the Saffman shear lift force generated within a micro-weir microchannel. The proposed device is fabricated on stress-relieved glass substrates and is characterized both numerically and experimentally using fluorescent particles with diameters of 5 μm and 10 μm, respectively. The numerical results show that the micro-weir structures confine the particle stream to the center of the microchannel without the need for a shear flow. Moreover, the experimental results show that the particles emerging from the micro-weir microchannel pass through the detection region in a one-by-one fashion. The focusing effect of the micro-weir microchannel is quantified by computing the normalized variance of the optical detection signal intensity. It is shown that the focusing performance of the micro-weir structure is equal to 99.76% and 99.57% for the 5-μm and 10-μm beads, respectively. Overall, the results presented in this study confirm that the proposed microcytometer enables the reliable sorting and counting of particles with different diameters.