Microwell Array Method for Rapid Generation of Uniform Agarose Droplets and Beads for Single Molecule Analysis

Microwell Confined Electro-Coalescence for Rapid Formation of High-Throughput Droplet Array

Droplet array is widely applied in single cell analysis, drug screening, protein crystallization, etc. This work proposes and validates a method for rapid formation of uniform droplet array based on microwell confined droplets electro-coalescence of screen-printed emulsion droplets, namely electro-coalescence droplet array (ECDA). The electro-coalescence of droplets is according to the polarization induced electrostatic and dielectrophoretic forces, and the dielectrowetting effect. The photolithographically fabricated microwells are highly regular and reproducible, ensuring identical volume and physical confinement to achieve uniform droplet array, and meanwhile the microwell isolation protects the paired water droplets from further fusion and broadens its feasibility to different fluidic systems. Under optimized conditions, a droplet array with an average diameter of 85 µm and a throughput of 106 in a 10 cm × 10 cm chip can be achieved within 5 s at 120 Vpp and 50 kHz. This ECDA chip is validated for various microwell geometries and functional materials. The optimized ECDA are successfully applied for digital viable bacteria counting, showing comparable results to the plate culture counting. Such an ECDA chip, as a digitizable and high-throughput platform, presents excellent potential for high-throughput screening, analysis, absolute quantification, etc.

Web hybrid chain reaction enhanced fluorescent magnetic bead array for digital nucleic acid detection

The detection of biomarkers at low concentrations is important in clinical diagnostic analyses and has attracted continuous research. In this work, absolute quantification of hepatitis B virus (HBV) DNA was achieved using magnetic beads with isothermal, enzyme-free DNA nanostructure for fluorescence amplification. Firstly, the DNA-functionalized bead captured the target nucleic acid in the form of sandwich hybridization, and the individual target lighted up the entire bead by isothermal web hybridization chain reaction (wHCR). After the microarray scanning, the target nucleic acids can be digitally quantified based on the Poisson statistics. Therefore, the fluorescent bead assay enabled precise detection of HBV DNA down to 5 fM level without external calibration curves. Moreover, this method not only specifically distinguished single-base mismatched sequences, but also obtained the quantitative detection of HBV DNA in serum samples. Unlike routine digital detection usually combined with complex compartment partitioning operations, the amplification structure immobilized on beads can be conducted in microcentrifuge tubes with a volume of microliter scale. This work expands the application of magnetic beads in the digital quantitative detection via enzyme-free and isothermal method.

Multiplex Digital Polymerase Chain Reaction on a Droplet Array SlipChip for Analysis of KRAS Mutations in Pancreatic Cancer

Pancreatic cancer is a terminal disease with high mortality and very poor prognosis. A sensitive and quantitative analysis of KRAS mutations in pancreatic cancer provides a tool not only to understand the biological mechanisms of pancreatic cancer but also for diagnosis and treatment monitoring. Digital polymerase chain reaction (PCR) is a promising tool for KRAS mutation analysis, but current methods generally require a complex microfluidic handling system, which can be challenging to implement in routine research and point-of-care clinical diagnostics. Here, we present a droplet-array SlipChip (da-SlipChip) for the multiplex quantification of KRAS G12D, V, R, and C mutant genes with the wild-type (WT) gene background by dual color (FAM/ROX) fluorescence detection. This da-SlipChip is a high-density microwell array of 21,696 wells of 200 pL in 4 by 5424 microwell format with simple loading and slipping operation. It does not require the same precise alignment of microfeatures on the different plates that are acquired by the traditional digital PCR SlipChip. This device can provide accurate quantification of both mutant genes and the WT KRAS gene. We collected tumor tissue, paired normal pancreatic tissue, and other normal tissues from 18 pancreatic cancer patients and analyzed the mutation profiles of KRAS G12D, V, R, and C in these samples; the results from the multiplex digital PCR on da-SlipChip agree well with those of next-generation sequencing (NGS). This da-SlipChip moves digital PCR closer to the practical point-of-care applications not only for detecting KRAS mutations in pancreatic cancer but also for other applications that require precise nucleic acid quantification with high sensitivity.

Sensitive and Low-Bias Transcriptome Sequencing Using Agarose PCR

Transcriptome sequencing has emerged as an important research tool for exploring the mysteries of life at the single-cell level. However, its wide application is limited by the bias associated with the amplification reactions which is essential for library building of trace RNA. In this study, low-melting-point agarose was added to the amplification reactions to take advantage of its molecular crowding effect and polymer cross-linked structure to improve the sensitivity of the reactions and reduce bias. To further evaluate the performance of the method, it was applied to transcriptome sequencing of microregion samples from brain tissue sections of mice with Parkinson's disease at the single cell level. The results showed that agarose PCR had better performance than in-tube PCR. Further application of agarose PCR to transcriptome library sequencing could obtain data closer to that of unamplified. With the addition of low melting point agarose, the sensitivity of the amplification reaction was significantly increased, while homogeneity was increased by approximately 2-fold. Not only that, but this work also provides 11% sensitivity improvement for spatial transcriptomic study on Parkinson's disease-associated gene detection. The agarose PCR provides a new tool for efficient and homogeneous amplification of trace samples and can be widely used for spatial transcriptome library sequencing and studies.

Accessible, fast and easy fabrication of hydrophilic-in-hydrophobic microdroplet arrays

Microdroplet arrays (MDAs) are powerful tools for digital immunoassays, high-throughput screening and single cell analysis. However, MDAs are usually produced with cleanroom processes, which are associated with high costs and low availability. Furthermore, in order to obtain robust and stable MDAs based on hydrophilic spots surrounded by a hydrophobic background, the chemistry must be strictly controlled, which is challenging using shared equipment. Here, we developed a new method to fabricate MDA substrates independently from the cleanroom. A small and low-cost in-house built system to collimate the light source was assembled for photopatterning a negative resist, and spots with diameters down to 4 μm were obtained, with only 3% to 5% spot-to-spot variation across the same sample and high batch-to-batch reproducibility. The use of a negative photoresist enabled the formation of a hydrophobic coating in solution which yielded high-quality MDAs. The feasibility for carrying out digital assays was demonstrated by measuring anti-Tau antibody in sample buffers containing bovine serum albumin, with no noticeable surface fouling. The reported, robust, cost-effective, and fast process could hence lower the threshold to fabricate and use MDAs for digital immunoassays and other microcompartmentalization-based applications.

Scalable Fabrication and Use of 3D Structured Microparticles Spatially Functionalized with Biomolecules

Microparticles with defined shapes and spatial chemical modification can interface with cells and tissues at the cellular scale. However, conventional methods to fabricate shaped microparticles have trade-offs between the throughput of manufacture and the precision of particle shape and chemical functionalization. Here, we achieved scalable production of hydrogel microparticles at rates of greater than 40 million/hour with localized surface chemistry using a parallelized step emulsification device and temperature-induced phase-separation. The approach harnesses a polymerizable polyethylene glycol (PEG) and gelatin aqueous two-phase system (ATPS) which conditionally phase separates within microfluidically generated droplets. Following droplet formation, phase separation is induced and phase separated droplets are subsequently cross-linked to form uniform crescent and hollow shell particles with gelatin functionalization on the boundary of the cavity. The gelatin localization enabled deterministic cell loading in subnanoliter-sized crescent-shaped particles, which we refer to as nanovials, with cavity dimensions tuned to the size of cells. Loading on nanovials also imparted improved cell viability during analysis and sorting using standard fluorescence activated cell sorters, presumably by protecting cells from shear stress. This localization effect was further exploited to selectively functionalize capture antibodies to nanovial cavities enabling single-cell secretion assays with reduced cross-talk in a simplified format.

A Portable Digital Loop-Mediated Isothermal Amplification Platform Based on Microgel Array and Hand-Held Reader

Digital polymerase chain reaction (dPCR) has found widespread applications in molecular diagnosis of various diseases owing to its sensitive single-molecule detection capability. However, the existing dPCR platforms rely on the auxiliary procedure to disperse DNA samples, which needs complicated operation, expensive apparatus, and consumables. Besides, the complex and costly dPCR readers also impede the applications of dPCR for point-of-care testing (POCT). Herein, we developed a portable digital loop-mediated isothermal amplification (dLAMP) platform, integrating a microscale hydrogel (microgel) array chip for sample partition, a miniaturized heater for DNA amplification, and a hand-held reader for digital readout. In the platform, the chip with thousands of isolated microgels holds the capability of self-absorption and partition of DNA samples, thus avoiding auxiliary equipment and professional personnel operations. Using the integrated dLAMP platform, λDNA templates have been quantified with a good linear detection range of 2-1000 copies/μL and a detection limit of 1 copy/μL. As a demonstration, the epidermal growth factor receptor L858R gene mutation, a crucial factor for the susceptibility of the tyrosine kinase inhibitor in non-small-cell lung cancer treatment, has been accurately identified by the dLAMP platform with a spiked plasma sample. This work shows that the developed dLAMP platform provides a low-cost, facile, and user-friendly solution for the absolute quantification of DNA, showing great potential for the POCT of nucleic acids.

Photolithographic Patterning of FluorAcryl for Biphilic Microwell-Based Digital Bioassays and Selection of Bacteria

FluorAcryl 3298 (FA) is a UV-curable fluoroacrylate polymer commonly employed as a chemically resistant, hydrophobic, and oleophobic coating. Here, FA was used in a cleanroom-based microstructuring process to fabricate hydrophilic-in-hydrophobic (HiH) micropatterned surfaces containing femtoliter-sized well arrays. A short protocol involving direct UV photopatterning, an etching step, and final recovery of the hydrophobic properties of the polymer produced patterned substrates with micrometer resolution. Specifically, HiH microwell arrays were obtained with a well diameter of 10 μm and various well depths ranging from 300 nm to 1 μm with high reproducibility. The 300 nm deep microdroplet array (MDA) substrates were used for digital immunoassays, which presented a limit of detection in the attomolar range. This demonstrated the chemical functionality of the hydrophilic and hydrophobic surfaces. Furthermore, the 1 μm deep wells could efficiently capture particles such as bacteria, whereas the 300 nm deep substrates or other types of flat HiH molecular monolayers could not. Capturing a mixture of bacteria expressing red- and green-fluorescent proteins, respectively, served as a model for screening and selection of specific phenotypes using FA-MDAs. Here, green-fluorescent bacteria were specifically selected by overlaying a solution of gelatin methacryloyl (GelMA) mixed with a photoinitiator and using a high-magnification objective, together with custom pinholes, in a common fluorescence microscope to cross-link the hydrogel around the bacteria of interest. In conclusion, due to the straightforward processing, versatility, and low-price, FA is an advantageous alternative to more commonly used fluorinated materials, such as CYTOP or Teflon-AF, for the fabrication of HiH microwell arrays and other biphilic microstructures.

Slip formation of a high-density droplet array for nucleic acid quantification by digital LAMP with a random-access system

Digital nucleic acid analysis (digital NAA) is an important tool for the precise quantification of nucleic acids. Various microfluidic-based approaches for digital NAA have been developed, but most methods require complex auxiliary control instruments, cumbersome device fabrication, or inconvenient preparation processes. A SlipChip is a microfluidic device that can generate and manipulate liquid partitions through simple movements of two microfluidic plates in close contact. However, the traditional SlipChip requires accurate alignment of microfeatures on different plates; therefore, the dimensions of the microwells and density of partitions can be constrained. Here, we developed a droplet array SlipChip (da-SlipChip) that can form droplets of various sizes at high density in a single slipping step. This process does not require precise overlapping microfeatures on different plates; therefore, the design flexibility and partition density can be significantly increased. We quantified SARS-CoV-2 nucleic acids extracted from the COVID-19 pseudovirus by digital loop-mediated isothermal amplification (LAMP) on a da-SlipChip with 21 696 of 0.25 nL droplets, and the results were in good agreement with those of the commercial digital PCR method of Stilla. Furthermore, we demonstrated a random-access system with a single-throughput fluorescence imager and a stackable thermal control instrument with nine independent heating modules. This random-access system with the da-SlipChip can greatly improve the total throughput and flexibility for digital isothermal nucleic acid quantification and significantly reduce the total waiting time.

Digital Loop-Mediated Isothermal Amplification-Based Absolute Methylation Quantification Revealed Hypermethylated DAPK1 in Cervical Cancer Patients

The aberrant methylation of many genes has been reported to be associated with various carcinomas. Accurate detection of the methylation level could provide critical insights into the diagnostic analysis of diseases. Here, a sensitive HpaII-edited absolute droplet loop-mediated isothermal amplification (HEADLAMP) method based on methylation-sensitive restriction enzyme (MSRE) HpaII was developed for the digital quantification of DNA methylation. Methylation levels of the death-associated protein kinase 1 (DAPK1) gene that is associated with many cancers were studied using β-actin as an internal reference. DAPK1 (2.5 pM) with 0.01% methylation (250 aM) can be detected with the conventional HpaII-edited LAMP assay. Using HEADLAMP, as low as 1% methylation level can be distinguished with an estimated limit of detection of 5 aM (ca. 3 copies/μL). Moreover, HEADLAMP can detect low levels of methylated DAPK1 in normal L-02 cells, while the conventional assay cannot. Finally, HEADLAMP was applied to the detection of DAPK1 methylation in 20 clinical tissue samples, which revealed hypermethylated DAPK1 in cervical cancer patients. We envisage potential applications of this robust, specific, and sensitive HEADLAMP assay in epigenetic studies and early clinical diagnosis.

Nucleic Acids Analysis

Nucleic acids are natural biopolymers of nucleotides that store, encode, transmit and express genetic information, which play central roles in diverse cellular events and diseases in living things. The analysis of nucleic acids and nucleic acids-based analysis have been widely applied in biological studies, clinical diagnosis, environmental analysis, food safety and forensic analysis. During the past decades, the field of nucleic acids analysis has been rapidly advancing with many technological breakthroughs. In this review, we focus on the methods developed for analyzing nucleic acids, nucleic acids-based analysis, device for nucleic acids analysis, and applications of nucleic acids analysis. The representative strategies for the development of new nucleic acids analysis in this field are summarized, and key advantages and possible limitations are discussed. Finally, a brief perspective on existing challenges and further research development is provided.

A fully automated centrifugal microfluidic system for sample-to-answer viral nucleic acid testing

The outbreak of virus-induced infectious diseases poses a global public-health challenge. Nucleic acid amplification testing (NAAT) enables early detection of pandemic viruses and plays a vital role in preventing onward transmission. However, the requirement of skilled operators, expensive instrumentation, and biosafety laboratories has hindered the use of NAAT for screening and diagnosis of suspected patients. Here we report development of a fully automated centrifugal microfluidic system with sample-in-answer-out capability for sensitive, specific, and rapid viral nucleic acid testing. The release of nucleic acids and the subsequent reverse transcription loop-mediated isothermal amplification (RT-LAMP) were integrated into the reaction units of a microfluidic disc. The whole processing steps such as injection of reagents, fluid actuation by rotation, heating and temperature control, and detection of fluorescence signals were carried out automatically by a customized instrument. We validate the centrifugal microfluidic system using oropharyngeal swab samples spiked with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) armored RNA particles. The estimated limit of detection for armored RNA particles is 2 copies per reaction, the throughput is 21 reactions per disc, and the assay sample-to-answer time is approximately 70 min. This enclosed and automated microfluidic system efficiently avoids viral contamination of aerosol, and can be readily adapted for virus detection outside the diagnostic laboratory.

A fully automated centrifugal microfluidic system for sample-to-answer viral nucleic acid testing

The outbreak of virus-induced infectious diseases poses a global public-health challenge. Nucleic acid amplification testing (NAAT) enables early detection of pandemic viruses and plays a vital role in preventing onward transmission. However, the requirement of skilled operators, expensive instrumentation, and biosafety laboratories has hindered the use of NAAT for screening and diagnosis of suspected patients. Here we report development of a fully automated centrifugal microfluidic system with sample-in-answer-out capability for sensitive, specific, and rapid viral nucleic acid testing. The release of nucleic acids and the subsequent reverse transcription loop-mediated isothermal amplification (RT-LAMP) were integrated into the reaction units of a microfluidic disc. The whole processing steps such as injection of reagents, fluid actuation by rotation, heating and temperature control, and detection of fluorescence signals were carried out automatically by a customized instrument. We validate the centrifugal microfluidic system using oropharyngeal swab samples spiked with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) armored RNA particles. The estimated limit of detection for armored RNA particles is 2 copies per reaction, the throughput is 21 reactions per disc, and the assay sample-to-answer time is approximately 70 min. This enclosed and automated microfluidic system efficiently avoids viral contamination of aerosol, and can be readily adapted for virus detection outside the diagnostic laboratory.

Electronic Supplementary Material

Supplementary material is available for this article at 10.1007/s11426-020-9800-6 and is accessible for authorized users.

Facile Method for Fabricating Microfluidic Chip Integrated with Microwell Arrays for Cell Trapping

With the development of biomedical technology, personalized diagnosis and treatment at the single-cell level are becoming more important in the medical field. As one of the most powerful tools, microfluidic chips have shown significant potential for various applications related to cell separation, cell proliferation, and cell behavior analysis. However, fabricating microfluidic devices requires complicated procedures and high-cost equipment. In this study, an optofluidic maskless lithography method was proposed for rapid fabrication of microfluidic devices integrated with microwells. Through the use of this approach, microwells can be on-line designed and the exposure patterns can be modulated. Single or multi polystyrene microspheres were successfully trapped by using the designed microwells. The capture of MCF-7 cells and cell arrays indicated that the microfluidic devices fabricated in the present study can be applied for cell research.

Locus-patterned sequence oriented enrichment for multi-dimensional gene analysis

Multi-dimensional gene analysis provides in-depth insights into gene sequence, locus variations and molecular abundance, whereas it is vulnerable to the perturbation of complex reaction networks and always compromises on the discrimination of analogous sequences. Here, we present a sequence oriented enrichment method patterned by the prescribed locus without crosstalk between concurrent reactions. Energetically favourable structures of nucleic acid probes are theoretically derived and oriented to a specific gene locus. We designed a pair of universal probes for multiple conserved loci to avoid side reactions from undesired interactions among increased probe sets. Furthermore, competitive probes were customized to sink analogues for differentiating the reaction equilibrium and kinetics of sequence enrichment from the target, so variant loci can be synchronously identified with nucleotide-level resolution. Thus, the gene locus guides sequence enrichment and combinatorial signals to create unique codes, which provides access to multidimensional and precise information for gene decoding.

Slip-driven microfluidic devices for nucleic acid analysis

Slip-driven microfluidic devices can manipulate fluid by the relative movement of microfluidic plates that are in close contact. Since the demonstration of the first SlipChip device, many slip-driven microfluidic devices with different form factors have been developed, including SlipPAD, SlipDisc, sliding stripe, and volumetric bar chart chip. Slip-driven microfluidic devices can be fabricated from glass, quartz, polydimethylsiloxane, paper, and plastic with various fabrication methods: etching, casting, wax printing, laser cutting, micromilling, injection molding, etc. The slipping operation of the devices can be performed manually, by a micrometer with a base station, or autonomously, by a clockwork mechanism. A variety of readout methods other than fluorescence microscopy have been demonstrated, including both fluorescence detection and colorimetric detection by mobile phones, direct visual detection, and real-time fluorescence imaging. This review will focus on slip-driven microfluidic devices for nucleic acid analysis, including multiplex nucleic acid detection, digital nucleic acid quantification, real-time nucleic acid amplification, and sample-in-answer-out nucleic acid analysis. Slip-driven microfluidic devices present promising approaches for both life science research and clinical molecular diagnostics.

Recent advances in single cell manipulation and biochemical analysis on microfluidics

Single cell analysis has become of great interest with unprecedented capabilities for the systematic investigation of cell-to-cell variation in large populations. Rapid and multi-parametric analysis of intercellular biomolecules at the single-cell level is imperative for the improvement of early disease diagnosis and personalized medicine. However, the small size of cells and the low concentration levels of target biomolecules are critical challenges for single cell analysis. In recent years, microfluidic platforms capable of handling small-volume fluid have been demonstrated to be powerful tools for single cell analysis. In addition, microfluidic techniques allow for precise control of the localized microenvironment, which yield more accurate outcomes. Many different microfluidic techniques have been greatly improved for highly efficient single-cell manipulation and highly sensitive detection over the past few decades. To date, microfluidics-based single cell analysis has become the hot research topic in this field. In this review, we particularly highlight the advances in this field during the past three years in the following three aspects: (1) microfluidic single cell manipulation based on microwells, micropatterns, droplets, traps and flow cytometric methods; (2) detection methods based on fluorescence, mass spectrometry, electrochemical, and polymerase chain reaction-based analysis; (3) applications in the fields of small molecule detection, protein analysis, multidrug resistance analysis, and single cell sequencing with droplet microfluidics. We also discuss future research opportunities by focusing on key performances of throughput, multiparametric target detection and data processing.

Target trapping and in situ single-cell genetic marker detection with a focused optical beam

Optical trapping of single particles or cells with the capability of in situ bio-sensing or genetic profiling opens the possibility of rapid screening of biological specimens. However, common optical tweezers suffer from the lack of long-range forces. Consequently, their application areas are predominantly limited to target manipulation instead of biological diagnostics. To solve this problem, we herein report an all-in-one approach by combining optical forces and convective drag forces generated through localized optothermal effect for long-range target manipulation. The device consists of a 2D array of gold coated polydimethylsiloxane (PDMS) micro-wells, which are immersed by colloidal particles or cell solution. Upon excitation of a 785-nm laser, the hydrodynamic convective force and optical forces will drag the targets of interest into their designated micro-wells. Moreover, the plasmonic thermal dissipation provides a constant temperature environment for following cell analysis procedures of cell isolation, lysis and isothermal nucleic acid amplification for the detection of genetic markers. With the merits of fabrication simplicity, short sample-to-answer cycle time and the compatibility with optical microscopes, the reported technique offers an attractive and highly versatile approach for on-site single cell analysis systems.

Digital Loop-Mediated Isothermal Amplification on a Commercial Membrane

In this work, we report digital loop-mediated isothermal amplification (LAMP) or reverse-transcription LAMP (RT-LAMP) on a commercial membrane, without the need for complex chip fabrication or use of specialized equipment. Due to the pore size distribution, the theoretical error for digital LAMP on these membranes was analyzed, using a combination of Random Distribution Model and Multivolume Theory. A facile peel-off process was developed for effective droplet formation on the commercial track-etched polycarbonate (PCTE) membrane. Each pore functions as an individual nanoreactor for single DNA amplification. Absolute quantification of bacteria genomic DNA was realized with a dynamic range from 11 to 1.1 × 10⁵ copies/μL. One-step digital RT-LAMP was also successfully performed on the membrane for the quantification of MS2 virus in wastewater. With the introduction of new probes, the positive pores can be easily distinguished from negative ones with 100 times difference in fluorescence intensities. Finally, the cost of a disposable membrane is less than $0.10/piece, which, to the best of our knowledge, is the most inexpensive way to perform digital LAMP. The membrane system offers opportunities for point-of-care users or common laboratories to perform digital quantification, single cell analysis, or other bioassays in an inexpensive, flexible, and simplified way.

Generating Microdroplet Array on Photonic Pseudo-paper for Absolute Quantification of Nucleic Acids

Here, we propose a new platform for digital nucleic acid assay based on a microdroplet array. The hydrophilic-superhydrophobic property of the pseudopaper photonic nitrocellulose substrate is employed to divide an aqueous sample containing the target nucleic acid into hundreds of microdroplets for loop-mediated isothermal amplification (LAMP). The LAMP reaction leads to the production of fluorescent calcein. By counting the number of microdroplets that are fluorescent, the target nucleic acid (i.e., Staphylococcus aureus DNA) with the copy number ranging from 1 to 1000 can be absolutely quantified without using sophisticated microfluidic devices. The effect of fluorescence enhancement attributed to the photonic nitrocellulose can effectively amplify the detection signal and reduce the amplification time. Thus, we believe that the proposed platform for digital nucleic acid assay based on the microdroplet array is promising for rapid and cost-effective bioanalytical applications.

Microfluidic and Paper-Based Devices for Disease Detection and Diagnostic Research

Recent developments in microfluidic devices, nanoparticle chemistry, fluorescent microscopy, and biochemical techniques such as genetic identification and antibody capture have provided easier and more sensitive platforms for detecting and diagnosing diseases as well as providing new fundamental insight into disease progression. These advancements have led to the development of new technology and assays capable of easy and early detection of pathogenicity as well as the enhancement of the drug discovery and development pipeline. While some studies have focused on treatment, many of these technologies have found initial success in laboratories as a precursor for clinical applications. This review highlights the current and future progress of microfluidic techniques geared toward the timely and inexpensive diagnosis of disease including technologies aimed at high-throughput single cell analysis for drug development. It also summarizes novel microfluidic approaches to characterize fundamental cellular behavior and heterogeneity.