Sickling of red blood cells through rapid oxygen exchange in microfluidic drops

Computing Sickle Erythrocyte Health Index on quantitative phase imaging and machine learning

Sickle cell disease (SCD) is a genetic disorder characterized by abnormal hemoglobin and deformation of red blood cells (RBCs), leading to complications and reduced life expectancy. This study developed an in vitro assessment, the Sickle Erythrocyte Health Index, using quantitative phase imaging (QPI) and machine learning to model the health of RBCs in people with SCD. The health index combines assessment of cell deformation, sickle-shaped classification, and membrane flexibility to evaluate erythrocyte health. Using QPI and image processing, the percentage of sickle-shaped cells and membrane flexibility were quantified. Statistically significant differences were observed between individuals with and without SCD, indicating the impact of underlying pathophysiology on erythrocyte health. Additionally, sodium metabisulfite led to an increase in sickle-shaped cells and a decrease in flexibility in the sickle cell blood samples. Based on these findings, two approaches were used to calculate the Sickle Erythrocyte Health Index: one using hand-crafted features and one using learned features from deep learning models. Both indices showed significant differences between non-SCD and SCD groups and sensitivity to changes induced by sodium metabisulfite. The Sickle Erythrocyte Health Index has important clinical implications for SCD management and could be used by providers when making treatment decisions. Further research is warranted to evaluate the clinical utility and applicability of the Sickle Erythrocyte Health Index in diverse patient populations.

Rapid electrical impedance detection of sickle cell vaso-occlusion in microfluidic device

Sickle cell disease is characterized by painful vaso-occlusive crises, in which poorly deformable sickle cells play an important role in the complex vascular obstruction process. Existing techniques are mainly based on optical microscopy and video processing of sickle blood flow under normoxic condition, for measuring vaso-occlusion by a small fraction of dense sickle cells of intrinsic rigidity but not the vaso-occlusion by the rigid, sickled cells due to deoxygenation. Thus, these techniques are not suitable for rapid, point-of-care testing. Here, we integrate electrical impedance sensing and Polydimethylsiloxane-microvascular mimics with controlled oxygen level into a single microfluidic chip, for quantification of vaso-occlusion by rigid, sickled cells within 1 min. Electrical impedance measurements provided a label-free, real-time detection of different sickle cell flow behaviors, including steady flow, vaso-occlusion, and flow recovery in response to the deoxygenation-reoxygenation process that are validated by microscopic videos. Sensitivity of the real part and imaginary part of the impedance signals to the blood flow conditions in both natural sickle cell blood and simulants at four electrical frequencies (10, 50, 100, and 500 kHz) are compared. The results show that the sensitivity of the sensor in detection of vaso-occlusion decreases as electrical frequency increases, while the higher frequencies are preferable in measurement of steady flow behavior. Additional testing using sickle cell simulants, chemically crosslinked normal red blood cells, shows same high sensitivity in detection of vaso-occlusion as sickle cell vaso-occlusion under deoxygenation. This work enables point-of-care testing potentials in rapid, accurate detection of steady flow and sickle cell vaso-occlusion from microliter volume blood specimens. Quantification of sickle cell rheology in response to hypoxia, may provide useful indications for not only the kinetics of cell sickling, but also the altered hemodynamics as obseved at the microcirculatory level.

Microfluidic dual picoinjection based encapsulation of hemoglobin in alginate microcapsules reinforced by a poly(L-lysine)-g-poly(ethylene glycol)

Hemoglobin (Hb) encapsulation inside polysaccharide hydrogels has been considered a possible red blood cell (RBC) surrogate in transfusiology. Here we report on the microfluidic dual picoinjection assisted synthesis of Hb encapsulated alginate-poly(L-lysine)-g-poly(ethylene glycol) beads. This process is realized by the on-chip injections of blended Hb alginate solutions in emulsified aqueous calcium chloride (CaCl₂) droplets followed by a subsequent injection of an aqueous PLL-g-PEG into each emulsified aqueous droplet. The proposed fabrication approach was realized using a flow-focusing and two picoinjection sites in a single PDMS device. Aqueous CaCl₂ solution was emulsified and infused with Hb-alginate solution as the squeezed droplet passed through the first picoinjection site. The injection of PLL-g-PEG to reinforce the microgel and minimize the protein leaching was realized in the second picoinjection site located downstream from the first in the same microfluidic channel. In this process, monodisperse Hb-alginate-PLL-g-PEG particles with a diameter around the size of RBCs (9 μm) were obtained with around 80% of the 7.5 mg ml^-1 Hb included in the injected aqueous alginate retaining in the obtained microparticles. Microparticles with Hb loading (32.8 pg per bead) and retention (28.8 pg per bead) over a week of storage at 4 °C are in accordance with the average amount of Hb per RBC. The Hb-alginate-PLL-g-PEG microbeads fabricated in the size range of RBCs are significant for further exploration.

Microfluidic methods to advance mechanistic understanding and translational research in sickle cell disease

Sickle cell disease (SCD) is caused by a single point mutation in the β-globin gene of hemoglobin, which produces an altered sickle hemoglobin (HbS). The ability of HbS to polymerize under deoxygenated conditions gives rise to chronic hemolysis, oxidative stress, inflammation, and vaso-occlusion. Herein, we review recent findings using microfluidic technologies that have elucidated mechanisms of oxygen-dependent and -independent induction of HbS polymerization and how these mechanisms elicit the biophysical and inflammatory consequences in SCD pathophysiology. We also discuss how validation and use of microfluidics in SCD provides the opportunity to advance development of numerous therapeutic strategies, including curative gene therapies.

Multi-scale model of clogging in microfluidic devices with grid-like geometries

We propose a coarse-grained theoretical model to capture the ageing of microfluidic devices under different conditions including constant applied flow rate and constant applied pressure gradient. Microfluidic devices that sort cells by their deformability hold significant promise for medical applications. However, clogging in these microfluidic systems causes their properties to change over time and potentially limits their reliability. We compare the results of the coarse-grained model with those of stochastic simulations and with existing theoretical studies. Lastly, we apply the model to experimental data on the clogging of sickle red blood cells and discuss its wider applicability.

Cell Culture in Microfluidic Droplets

Cell manipulation in droplets has emerged as one of the great successes of microfluidic technologies, with the development of single-cell screening. However, the droplet format has also served to go beyond single-cell studies, namely by considering the interactions between different cells or between cells and their physical or chemical environment. These studies pose specific challenges linked to the need for long-term culture of adherent cells or the diverse types of measurements associated with complex biological phenomena. Here we review the emergence of droplet microfluidic methods for culturing cells and studying their interactions. We begin by characterizing the quantitative aspects that determine the ability to encapsulate cells, transport molecules, and provide sufficient nutrients within the droplets. This is followed by an evaluation of the biological constraints such as the control of the biochemical environment and promoting the anchorage of adherent cells. This first part ends with a description of measurement methods that have been developed. The second part of the manuscript focuses on applications of these technologies for cancer studies, immunology, and stem cells while paying special attention to the biological relevance of the cellular assays and providing guidelines on improving this relevance.

Droplet formation of biological non-Newtonian fluid in T-junction generators. I. Experimental investigation

The extension of microfluidics to many bioassay applications requires the ability to work with non-Newtonian fluids. One case in point is the use of microfluidics with blood having different hematocrit levels. This work is the first part of a two-part study and presents the formation dynamics of blood droplets in a T-junction generator under the squeezing regime. In this regime, droplet formation with Newtonian fluids depends on T-junction geometry; however, we found that in the presence of the non-Newtonian fluid such as red blood cells, the formation depends on not only to the channel geometry, but also the flow rate ratio of fluids, and the viscosity of the phases. In addition, we analyzed the impact of the red blood cell concentration on the formation cycle. In this study, we presented the experimental data of the blood droplet evolution through the analysis of videos that are captured by a high-speed camera. During this analysis, we tracked several parameters such as droplet volume, spacing between droplets, droplet generation frequency, flow conditions, and geometrical designs of the T junction. Our analysis revealed that, unlike other non-Newtonian fluids, where the fourth stage exists (stretching stage), the formation cycle consists of only three stages: lag, filling, and necking stages. Because of the detailed analysis of each stage, a mathematical model can be generated to predict the final volume of the blood droplet and can be utilized as a guide in the operation of the microfluidic device for biochemical assay applications; this is the focus of the second part of this study [Phys. Rev. E 105, 025106 (2022)10.1103/PhysRevE.105.025106].

In vitro assay for single-cell characterization of impaired deformability in red blood cells under recurrent episodes of hypoxia

Red blood cells (RBCs) are subjected to recurrent changes in shear stress and oxygen tension during blood circulation. The cyclic shear stress has been identified as an important factor that alone can weaken cell mechanical deformability. The effects of cyclic hypoxia on cellular biomechanics have yet to be fully investigated. As the oxygen affinity of hemoglobin plays a key role in the biological function and mechanical performance of RBCs, the repeated transitions of hemoglobin between its R (high oxygen tension) and T (low oxygen tension) states may impact their mechanical behavior. The present study focuses on developing a novel microfluidic-based assay for characterization of the effects of cyclic hypoxia on cell biomechanics. The capability of this assay is demonstrated by a longitudinal study of individual RBCs in health and sickle cell disease subjected to cyclic hypoxia conditions of various durations and levels of low oxygen tension. The viscoelastic properties of cell membranes are extracted from tensile stretching and relaxation processes of RBCs induced by the electrodeformation technique. Results demonstrate that cyclic hypoxia alone can significantly reduce cell deformability, similar to the fatigue damage accumulated through cyclic mechanical loading. RBCs affected by sickle cell disease are less deformable (significantly higher membrane shear modulus and viscosity) than normal RBCs. The fatigue resistance of sickle RBCs to the cyclic hypoxia challenge is significantly inferior to that of normal RBCs, and this trend is more significant in mature erythrocytes of sickle cells. When the oxygen affinity of sickle hemoglobin is enhanced by anti-sickling drug treatment of 5-hydroxymethyl-2-furfural (5-HMF), sickle RBCs show ameliorated resistance to fatigue damage induced by cyclic hypoxia. These results indicate an important biophysical mechanism underlying RBC senescence in which the cyclic hypoxia challenge alone can lead to mechanical degradation of the RBC membrane. We envision that the application of this assay can be further extended to RBCs in other blood diseases and other cell types.

Assessment of transient changes in oxygen diffusion of single red blood cells using a microfluidic analytical platform

Red blood cells (RBCs) capability to deliver oxygen (O₂) has been routinely measured by P50. Although this defines the ability of RBCs to carry O₂ under equilibrium states, it cannot determine the efficacy of O₂ delivery in dynamic blood flow. Here, we developed a microfluidic analytical platform (MAP) that isolates single RBCs for assessing transient changes in their O₂ release rate. We found that in vivo (biological) and in vitro (blood storage) aging of RBC could lead to an increase in the O₂ release rate, despite a decrease in P50. Rejuvenation of stored RBCs (Day 42), though increased the P50, failed to restore the O₂ release rate to basal level (Day 0). The temporal dimension provided at the single-cell level by MAP could shed new insights into the dynamics of O₂ delivery in both physiological and pathological conditions.

High-Throughput Measurements of Intra-Cellular and Secreted Cytokine from Single Spheroids Using Anchored Microfluidic Droplets

While many single-cell approaches have been developed to measure secretions from anchorage-independent cells, these protocols cannot be applied to adherent cells, especially when these cells require to be cultured in 3D formats. Here, a platform to measure secretions from individual spheroids of human mesenchymal stem cells, cultured within microfluidic droplets is introduced. The platform allows to quantify the secretions from hundreds of individual spheroids in each device, by using a secondary droplet to bring functionalized micro-beads in proximity to each spheroid. Vascular endothelial growth factor (VEGF-A) is measured on and a broad distribution of secretion levels within the population of spheroids is observed. The intra-cellular level of VEGF-A on each spheroid, measured through immuno-staining, correlates well with the extra-cellular measurement, indicating that the heterogeneities observed at the spheroid level result from variations at the intra-cellular level. Further, the molecular accumulation within the droplets is modeled and it is found that physical confinement is crucial for measurements of protein secretions. The model predicts that the time to achieve a measurement scales with droplet volume. These first measurements of secretions from individual spheroids provide several new biological and technological insights.

Individual red blood cell fetal hemoglobin quantification allows to determine protective thresholds in sickle cell disease

Polymerization of the sickle hemoglobin (HbS) is a key determinant of sickle cell disease (SCD), an inherited blood disorder. Fetal hemoglobin (HbF) is a major modulator of the disease severity by both decreasing HbS intracellular concentration and inhibiting its polymerization. However, heterocellular distribution of HbF is common in SCD. For HbS polymerization inhibition, the hypothesis of an "HbF per red blood cell (HbF/RBC) threshold" requires accurate measurement of HbF in individual RBC. To date, HbF detection methods are limited to a qualitative measurement of RBC populations containing HbF - the F cells, which are variable. We developed an accurate method for HbF quantification in individual RBC. A linear association between mean HbF content and mean RBC fluorescence by flow cytometry, using an anti-Human-HbF antibody, was obtained from non-SCD subjects presenting homogeneous HbF distribution. This correlation was then used to measure HbF/RBC. Hydroxyurea (HU) improves SCD clinical manifestations, mainly through its ability to induce HbF synthesis. The HbF distribution was analyzed in 14 SCD patients before and during HU treatment. A significant decrease in RBC population containing less than 2 pg of HbF/RBC was observed. Therefore, we tested associations for %RBC above different HbF/RBC thresholds and showed a decrease in the pathognomonic vaso-occlusive crisis incidence from the threshold of 4 pg. This quantity was also correlated with the level of sickle RBC after in vitro deoxygenation. This new method allows the comparison of HbF/RBC distributions and could be a useful tool to characterize baseline patients HbF distribution and therapeutic response to HbF inducers.

Size-Based Sorting of Emulsion Droplets in Microfluidic Channels Patterned with Laser-Ablated Guiding Tracks

We demonstrate an autonomous, high-throughput mechanism for sorting of emulsion droplets with different sizes concurrently flowing in a microfluidic Hele-Shaw channel. The aqueous droplets of varying radii suspended in olive oil are separated into different streamlines across the channel upon interaction with a shallow (depth ∼ 700 nm) inclined guiding track ablated into the polydimethylsiloxane-coated surface of the channel with focused femtosecond laser pulses. Specifically, the observed differences in the droplet trajectories along the guiding track arise due to the different scaling of the confinement force attracting the droplets into the track, fluid drag, and wall friction, with the droplet radius. In addition, the distance traveled by the droplets along the track also depends on the track width, with wider tracks providing more stable droplet guiding for any given droplet size. We systematically study the influence of the droplet size and velocity on the trajectory of the droplets in the channel and analyze the sensitivity of size-based droplet sorting for varying flow conditions. The droplet guiding and sorting experiments are complemented by modeling of the droplet motion in the channel flow using computational fluid dynamics simulations and a previously developed model of droplet guiding. Finally, we demonstrate a complete separation of droplets produced by fusion of two independent droplet streams at the inlet of the Hele-Shaw channel from unfused daughter droplets. The presented droplet sorting technique can find applications in the development of analytical and preparative microfluidic protocols.

Electrical impedance microflow cytometry with oxygen control for detection of sickle cells

Polymerization of intracellular sickle hemoglobin induced by low oxygen tension has been recognized as a primary determinant of the pathophysiologic manifestations in sickle cell disease. Existing flow cytometry techniques for detection of sickle cells are typically based on fluorescence markers or cellular morphological analysis. Using microfluidics and electrical impedance spectroscopy, we develop a new, label-free flow cytometry for non-invasive measurement of single cells under controlled oxygen level. We demonstrate the capability of this new technique by determining the electrical impedance differential of normal red blood cells obtained from a healthy donor and sickle cells obtained from three sickle cell patients, under normoxic and hypoxic conditions and at three different electrical frequencies, 156 kHz, 500 kHz and 3 MHz. Under normoxia, normal cells and sickle cells can be separated completely using electrical impedance at 156 kHz and 500 kHz but not at 3 MHz. Sickle cells, intra-patient and inter-patient show significantly different electrical impedance between normoxia and hypoxia at all three frequencies. This study shows a proof of concept that electrical impedance signal can be used as an indicator of the disease state of a red blood cell as well as the cell sickling events in sickle cell disease. Electrical impedance-based microflow cytometry with oxygen control is a new method that can be potentially used for sickle cell disease diagnosis and monitoring.

Tracking the Evolution of Transiently Transfected Individual Cells in a Microfluidic Platform

Transient gene expression (TGE) technology enables the rapid production of large amount of recombinant proteins, without the need of fastidious screening of the producing cells required for stable transfection (ST). However, several barriers must be overcome before reaching the production yields using ST. For optimizing the production yields from suspended cells using TGE, a better understanding of the transfection conditions at the single cell level are required. In this study, a universal droplet microfluidic platform was used to assess the heterogeneities of CHO-S population transiently transfected with cationic liposomes (CL) (lipoplexes) complexed with GFP-coding plasmid DNA (pDNA). A single cell analysis of GFP production kinetics revealed the presence of a subpopulation producing higher levels of GFP compared with the main population. The size of high producing (HP) cells, their relative abundance, and their specific productivity were dependent on the charge and the pDNA content of the different lipoplexes: HPs showed increased cell size in comparison to the average population, lipoplexes with positive charge produced more HPs, and lipoplexes carrying a larger amount of pDNA yielded a higher specific productivity of HPs. This study demonstrates the potential for time-resolved single-cell measurements to explain population dynamics from a microscopic point of view.

Biocompatibility of fluids for multiphase drops-in-drops microfluidics

This paper addresses the biocompatibility of fluids and surfactants in the context of microfluidics and more specifically in a drops-in-drops system for mammalian cell based drug screening. In the drops-in-drops approach, three immiscible fluids are used to manipulate the flow of aqueous microliter-sized drops; it enables merging of drops containing cells with drops containing drugs within a Teflon tube. Preliminary tests showed that a commonly-used fluid and surfactant combination resulted in significant variability in gene expression levels in Jurkat cells after exposure to a drug for four hours. This result led to further investigations of potential fluid and surfactant combinations that can be used in microfluidic systems for medium to long-term drug screening. Results herein identify a fluid combination, HFE-7500 and 5-cSt silicone oil + 0.25% Abil EM180, which enabled the drops-in-drops approach; this combination also allowed gene expression at normal levels comparable with the conventional drug screening in both magnitude and variability.

Integration and application of optical chemical sensors in microbioreactors

The quantification of key variables such as oxygen, pH, carbon dioxide, glucose, and temperature provides essential information for biological and biotechnological applications and their development. Microfluidic devices offer an opportunity to accelerate research and development in these areas due to their small scale, and the fine control over the microenvironment, provided that these key variables can be measured. Optical sensors are well-suited for this task. They offer non-invasive and non-destructive monitoring of the mentioned variables, and the establishment of time-course profiles without the need for sampling from the microfluidic devices. They can also be implemented in larger systems, facilitating cross-scale comparison of analytical data. This tutorial review presents an overview of the optical sensors and their technology, with a view to support current and potential new users in microfluidics and biotechnology in the implementation of such sensors. It introduces the benefits and challenges of sensor integration, including, their application for microbioreactors. Sensor formats, integration methods, device bonding options, and monitoring options are explained. Luminescent sensors for oxygen, pH, carbon dioxide, glucose and temperature are showcased. Areas where further development is needed are highlighted with the intent to guide future development efforts towards analytes for which reliable, stable, or easily integrated detection methods are not yet available.

A self-filling microfluidic device for noninvasive and time-resolved single red blood cell experiments

Existing approaches to red blood cell (RBC) experiments on the single-cell level usually rely on chemical or physical manipulations that often cause difficulties with preserving the RBC's integrity in a controlled microenvironment. Here, we introduce a straightforward, self-filling microfluidic device that autonomously separates and isolates single RBCs directly from unprocessed human blood samples and confines them in diffusion-controlled microchambers by solely exploiting their unique intrinsic properties. We were able to study the photo-induced oxygenation cycle of single functional RBCs by Raman microscopy without the limitations typically observed in optical tweezers based methods. Using bright-field microscopy, our noninvasive approach further enabled the time-resolved analysis of RBC flickering during the reversible shape evolution from the discocyte to the echinocyte morphology. Due to its specialized geometry, our device is particularly suited for studying the temporal behavior of single RBCs under precise control of their environment that will provide important insights into the RBC's biomedical and biophysical properties.

Droplet microfluidics for microbiology: techniques, applications and challenges

Droplet microfluidics has rapidly emerged as one of the key technologies opening up new experimental possibilities in microbiology. The ability to generate, manipulate and monitor droplets carrying single cells or small populations of bacteria in a highly parallel and high throughput manner creates new approaches for solving problems in diagnostics and for research on bacterial evolution. This review presents applications of droplet microfluidics in various fields of microbiology: i) detection and identification of pathogens, ii) antibiotic susceptibility testing, iii) studies of microbial physiology and iv) biotechnological selection and improvement of strains. We also list the challenges in the dynamically developing field and new potential uses of droplets in microbiology.

Monodisperse polyethylene glycol diacrylate hydrogel microsphere formation by oxygen-controlled photopolymerization in a microfluidic device

PEG-based hydrogels have become widely used as drug delivery and tissue scaffolding materials. Common among PEG hydrogel-forming polymers are photopolymerizable acrylates such as polyethylene glycol diacrylate (PEGDA). Microfluidics and microfabrication technologies have recently enabled the miniaturization of PEGDA structures, thus enabling many possible applications for nano- and micro- structured hydrogels. The presence of oxygen, however, dramatically inhibits the photopolymerization of PEGDA, which in turn frustrates hydrogel formation in environments of persistently high oxygen concentration. Using PEGDA that has been emulsified in fluorocarbon oil via microfluidic flow focusing within polydimethylsiloxane (PDMS) devices, we show that polymerization is completely inhibited below critical droplet diameters. By developing an integrated model incorporating reaction kinetics and oxygen diffusion, we demonstrate that the critical droplet diameter is largely determined by the oxygen transport rate, which is dictated by the oxygen saturation concentration of the continuous oil phase. To overcome this fundamental limitation, we present a nitrogen micro-jacketed microfluidic device to reduce oxygen within the droplet, enabling the continuous on-chip photopolymerization of microscale PEGDA particles.

Single-cell measurement of red blood cell oxygen affinity

Oxygen is transported throughout the body by hemoglobin (Hb) in red blood cells (RBCs). Although the oxygen affinity of blood is well-understood and routinely assessed in patients by pulse oximetry, variability at the single-cell level has not been previously measured. In contrast, single-cell measurements of RBC volume and Hb concentration are taken millions of times per day by clinical hematology analyzers, and they are important factors in determining the health of the hematologic system. To better understand the variability and determinants of oxygen affinity on a cellular level, we have developed a system that quantifies the oxygen saturation, cell volume, and Hb concentration for individual RBCs in high throughput. We find that the variability in single-cell saturation peaks at an oxygen partial pressure of 2.9%, which corresponds to the maximum slope of the oxygen-Hb dissociation curve. In addition, single-cell oxygen affinity is positively correlated with Hb concentration but independent of osmolarity, which suggests variation in the Hb to 2,3-diphosphoglycerate (2-3 DPG) ratio on a cellular level. By quantifying the functional behavior of a cellular population, our system adds a dimension to blood cell analysis and other measurements of single-cell variability.

Metabolite profiling of microfluidic cell culture conditions for droplet based screening

We investigate the impact of droplet culture conditions on cell metabolic state by determining key metabolite concentrations in S. cerevisiae cultures in different microfluidic droplet culture formats. Control of culture conditions is critical for single cell/clone screening in droplets, such as directed evolution of yeast, as cell metabolic state directly affects production yields from cell factories. Here, we analyze glucose, pyruvate, ethanol, and glycerol, central metabolites in yeast glucose dissimilation to establish culture formats for screening of respiring as well as fermenting yeast. Metabolite profiling provides a more nuanced estimate of cell state compared to proliferation studies alone. We show that the choice of droplet incubation format impacts cell proliferation and metabolite production. The standard syringe incubation of droplets exhibited metabolite profiles similar to oxygen limited cultures, whereas the metabolite profiles of cells cultured in the alternative wide tube droplet incubation format resemble those from aerobic culture. Furthermore, we demonstrate retained droplet stability and size in the new better oxygenated droplet incubation format.

Kinetics of sickle cell biorheology and implications for painful vasoocclusive crisis

We developed a microfluidics-based model to quantify cell-level processes modulating the pathophysiology of sickle cell disease (SCD). This in vitro model enabled quantitative investigations of the kinetics of cell sickling, unsickling, and cell rheology. We created short-term and long-term hypoxic conditions to simulate normal and retarded transit scenarios in microvasculature. Using blood samples from 25 SCD patients with sickle hemoglobin (HbS) levels varying from 64 to 90.1%, we investigated how cell biophysical alterations during blood flow correlated with hematological parameters, HbS level, and hydroxyurea (HU) therapy. From these measurements, we identified two severe cases of SCD that were also independently validated as severe from a genotype-based disease severity classification. These results point to the potential of this method as a diagnostic indicator of disease severity. In addition, we investigated the role of cell density in the kinetics of cell sickling. We observed an effect of HU therapy mainly in relatively dense cell populations, and that the sickled fraction increased with cell density. These results lend support to the possibility that the microfluidic platform developed here offers a unique and quantitative approach to assess the kinetic, rheological, and hematological factors involved in vasoocclusive events associated with SCD and to develop alternative diagnostic tools for disease severity to supplement other methods. Such insights may also lead to a better understanding of the pathogenic basis and mechanism of drug response in SCD.

Microfluidic long-term differential oxygenation for bacterial growth characteristics analyses

Dissolved oxygen is a critical micro-environmental factor to determine the growth characteristics of bacteria, such as cell viability, migration, aggregation and metabolic processes.

Micro-optical lens array for fluorescence detection in droplet-based microfluidics

We demonstrate the design and integration of droplet-based microfluidic devices with microoptical element arrays for enhanced detection of fluorescent signals. We show that the integration of microlenses and mirror surfaces in these devices results in an 8-fold increase in the fluorescence signal and in improved spatial resolution. Using an array of microlenses, massively parallel detection of droplets containing fluorescent dyes was achieved, leading to detection throughputs of about 2000 droplets per second and per lens, parallelized over 625 measurement points.

Microfluidic encapsulation of cells in polymer microgels

In this Concept article, recent advances in microfluidic platforms for the generation of cell-laden hydrogel particles (microgels) are reported. Advances in the continuous microfluidic encapsulation of cells in droplets and microgels are critically reviewed, and currently used methods for the encapsulation of cells in polymer microgels are discussed. An outlook on current applications and future directions in this field of research are also presented. This article will be of interest to chemists, materials scientists, cell biologists, bioengineers, and pharmacologists.

Surfactants in droplet-based microfluidics

Surfactants are an essential part of the droplet-based microfluidic technology. They are involved in the stabilization of droplet interfaces, in the biocompatibility of the system and in the process of molecular exchange between droplets. The recent progress in the applications of droplet-based microfluidics has been made possible by the development of new molecules and their characterizations. In this review, the role of the surfactant in droplet-based microfluidics is discussed with an emphasis on the new molecules developed specifically to overcome the limitations of 'standard' surfactants. Emulsion properties and interfacial rheology of surfactant-laden layers strongly determine the overall capabilities of the technology. Dynamic properties of droplets, interfaces and emulsions are therefore very important to be characterized, understood and controlled. In this respect, microfluidic systems themselves appear to be very powerful tools for the study of surfactant dynamics at the time- and length-scale relevant to the corresponding microfluidic applications. More generally, microfluidic systems are becoming a new type of experimental platform for the study of the dynamics of interfaces in complex systems.

Droplet microfluidics for single-cell analysis

This book chapter aims at providing an overview of all the aspects and procedures needed to develop a droplet-based workflow for single-cell analysis (see Fig. 10.1). The surfactant system used to stabilize droplets is a critical component of droplet microfluidics; its properties define the type of droplet-based assays and workflows that can be developed. The scope of this book chapter is limited to fluorinated surfactant systems that have proved to generate extremely stable droplets and allow to easily retrieve the encapsulated material. The formulation section discusses how the experimental parameters influence the choice of the surfactant system to use. The circuit design section presents recipes to design and integrate different droplet modules into a whole assay. The fabrication section describes the manufacturing of microfluidic chip including the surface treatment which is pivotal in droplet microfluidics. Finally, the last section reviews the experimental setup for fluorescence detection with an emphasis on cell injection and incubation.

Reversible alteration of calcium dynamics in cardiomyocytes during acute hypoxia transient in a microfluidic platform

Heart disease is the leading cause of mortality in western countries. Apart from congenital and anatomical alterations, ischemia is the most common agent causing myocardial damage. During ischemia, a sudden decrease in oxygen concentration alters cardiomyocyte function and compromises cell survival. The calcium handling machinery, which regulates the main functional features of a cardiomyocyte, is heavily compromised during acute hypoxic events. Alterations in calcium dynamics have been linked to both short- and long-term consequences of ischemia, ranging from arrhythmias to heart failure. In this perspective, we aimed at investigating the calcium dynamics in functional cardiomyocytes during the early phase of a hypoxic event. For this purpose, we developed a microfluidic system specifically designed for controlling fast oxygen concentration dynamics through a gas micro-exchanger allowing in line analysis of intracellular calcium concentration by confocal microscopy. Experimental results show that exposure of Fluo-4 loaded neonatal rat cardiomyocytes to hypoxic conditions induced changes in intracellular Ca(2+) transients. Such behavior was reversible and was detected for hypoxic levels below 5% of oxygen partial pressure. The observed changes in Ca(2+) dynamics were mimicked using specific L-type Ca(2+) channel antagonists, suggesting that alterations in calcium channel function occur at low oxygen levels. Reversible alteration in ion channel function, that takes place in response to changes in cellular oxygen, might represent an adaptive mechanism of cardiopreservation during ischemia.

Rails and anchors: guiding and trapping droplet microreactors in two dimensions

This paper presents a method to control the motion of nanolitre drops in a wide and thin microchannel, by etching fine patterns into the channel's top surface. Such control is possible for drops that are squeezed by the channel roof, by allowing them to reduce their surface energy as they enter into a local depression. The resulting gain in surface energy pulls a drop into the groove such that localized holes can be used as anchors for holding drops, while linear patterns can be used as rails to guide them along complex trajectories. An anchored drop can remain stationary indefinitely, as long as the driving flow rate is below a critical value which depends on the hole and drop sizes. By micro-fabricating holes into a grid pattern, drops can be arrayed and held in the observation field of a microscope against the mean carrier flow. Their contents can then be modulated by gas exchange with the flowing carrier oil. We demonstrate in particular how the pH or the oxygen levels within the drops can be controlled spatially and temporally, either by exposing rows of drops to two streams of oil at different gas concentrations or by periodically switching oil inputs to vary the gas concentration of drops as a function of time. Oxygen control is used to selectively deoxygenate droplets that encapsulate red blood cells from patients suffering from sickle cell disease, in order to study the polymerization of intracellular hemoglobin. Cycles of oxygenation and deoxygenation of anchored droplets induce depolymerization and polymerization of the hemoglobin, thus providing a method to simulate the cycling that takes place in physiological flows.