Hydrodynamic Cell Trapping for High Throughput Single-Cell Applications

Metabolic and Oxidative Stress Effects on the Spectroelectrochemical Behavior of Single Pseudomonas aeruginosa Cells

Pseudomonas aeruginosa is an opportunistic human pathogen capable of causing a wide range of diseases in immunocompromised patients. In order to better understand P. aeruginosa behavior and virulence and to advance drug therapies to combat infection, it would be beneficial to understand how P. aeruginosa cells survive stressful conditions, especially environmental stressors. Here, we report on a strategy that measures potential-dependent fluorescence of individual P. aeruginosa cells, as a sentinel, for cellular response to starvation, hunger, and oxidative stress. This is accomplished using a micropore electrode array capable of trapping large numbers of isolated, vertically oriented cells at well-defined spatial positions in order to study large arrays of single cells in parallel. We find that conditions promoting either starvation or oxidative stress produce discernible changes in the fluorescence response, demonstrated by an increase in the prevalence of fluorescence transients, one of three canonical spectroelectrochemical behaviors exhibited by single P. aeruginosa cells. In contrast, more modest nutrient limitations have little to no effect on the spectroelectrochemical response when compared to healthy cells in the stationary phase. These findings demonstrate the capabilities of micropore electrode arrays for studying the behavior of single microbial cells under conditions where the intercellular spacing, orientation, and chemical environment of the cells are controlled. Realizing single-cell studies under such well-defined conditions makes it possible to study fundamental stress responses with unprecedented control.

On-Chip Analysis of Protein Secretion from Single Cells Using Microbead Biosensors

The profiling of the effector functions of single immune cells─including cytokine secretion─can lead to a deeper understanding of how the immune system operates and to potential diagnostics and therapeutical applications. Here, we report a microfluidic device that pairs single cells and antibody-functionalized microbeads in hydrodynamic traps to quantitate cytokine secretion. The device contains 1008 microchambers, each with a volume of ∼500 pL, divided into six different sections individually addressed to deliver an equal number of chemical stimuli. Integrating microvalves allowed us to isolate cell/bead pairs, preventing cross-contamination with factors secreted by adjacent cells. We implemented a fluorescence sandwich immunoassay on the biosensing microbeads with a limit of detection of 9 pg/mL and were able to detect interleukin-8 (IL-8) secreted by single blood-derived human monocytes in response to different concentrations of LPS. Finally, our platform allowed us to observe a significant decrease in the number of IL-8-secreting monocytes when paracrine signaling becomes disrupted. Overall, our platform could have a variety of applications for which the analysis of cellular function heterogeneity is necessary, such as cancer research, antibody discovery, or rare cell screening.

How to Perform a Microfluidic Cultivation Experiment—A Guideline to Success

As a result of the steadily ongoing development of microfluidic cultivation (MC) devices, a plethora of setups is used in biological laboratories for the cultivation and analysis of different organisms. Because of their biocompatibility and ease of fabrication, polydimethylsiloxane (PDMS)-glass-based devices are most prominent. Especially the successful and reproducible cultivation of cells in microfluidic systems, ranging from bacteria over algae and fungi to mammalians, is a fundamental step for further quantitative biological analysis. In combination with live-cell imaging, MC devices allow the cultivation of small cell clusters (or even single cells) under defined environmental conditions and with high spatio-temporal resolution. Yet, most setups in use are custom made and only few standardised setups are available, making trouble-free application and inter-laboratory transfer tricky. Therefore, we provide a guideline to overcome the most frequently occurring challenges during a MC experiment to allow untrained users to learn the application of continuous-flow-based MC devices. By giving a concise overview of the respective workflow, we give the reader a general understanding of the whole procedure and its most common pitfalls. Additionally, we complement the listing of challenges with solutions to overcome these hurdles. On selected case studies, covering successful and reproducible growth of cells in MC devices, we demonstrate detailed solutions to solve occurring challenges as a blueprint for further troubleshooting. Since developer and end-user of MC devices are often different persons, we believe that our guideline will help to enhance a broader applicability of MC in the field of life science and eventually promote the ongoing advancement of MC.

High-Throughput Cell Trapping in the Dentate Spiral Microfluidic Channel

Cell trapping is a very useful technique in a variety of cell-based assays and cellular research fields. It requires a high-throughput, high-efficiency operation to isolate cells of interest and immobilize the captured cells at specific positions. In this study, a dentate spiral microfluidic structure is proposed for cell trapping. The structure consists of a main spiral channel connecting an inlet and an out and a large number of dentate traps on the side of the channel. The density of the traps is high. When a cell comes across an empty trap, the cell suddenly makes a turn and enters the trap. Once the trap captures enough cells, the trap becomes closed and the following cells pass by the trap. The microfluidic structure is optimized based on the investigation of the influence over the flow. In the demonstration, 4T1 mouse breast cancer cells injected into the chip can be efficiently captured and isolated in the different traps. The cell trapping operates at a very high flow rate (40 μL/s) and a high trapping efficiency (>90%) can be achieved. The proposed high-throughput cell-trapping technique can be adopted in the many applications, including rapid microfluidic cell-based assays and isolation of rare circulating tumor cells from a large volume of blood sample.

Advances in Microfluidics-Based Technologies for Single Cell Culture

Single cell culture has been considered one of the fundamental tools for single cell studies. Complex biological systems evolve from single cells, and the cells within biological systems are intrinsically heterogeneous. Therefore, culturing and understanding the behaviors of single cells are of great interest for both biological research and clinical studies. In recent years, advances in microfluidics-based technologies have demonstrated unprecedented capabilities for single cell studies, and they have made high-throughput single cell cultures possible. Microfluidic systems enable precise control of the microenvironment for single cell culture and monitoring of the behavior of single cells in real time. In addition, microfluidic devices can consist of upstream cell sorting and cell isolation, and they can also be seamlessly integrated with various downstream analysis methods. Therefore, microfluidic technologies can obtain data about the performance at the single-cell level, providing information that cannot be achieved by studying the ensemble behavior of cell colonies. In this review, the recent developments in droplet-based microfluidics, microwell-based microfluidics, trap-based microfluidics and SlipChip-based microfluidics for the study of single cell culture is focused on. Perspectives on future improvement regarding single cell culture and its related research opportunities are also provided.

Microfluidic systems for hydrodynamic trapping of cells and clusters

Microfluidic devices have been widely applied to trapping and isolation of cells and clusters for controllable intercellular environments and high-throughput analysis, triggering numerous advances in disease diagnosis and single-cell analysis. Passive hydrodynamic cell trapping is one of the simple and effective methods that has been gaining attention in recent years. Our aim here is to review the existing passive microfluidic trapping approaches, including microposts, microfiltration, microwells, and trapping chambers, with emphasis on design principles and performance. We summarize the remarkable advances that hydrodynamic trapping methods offer, as well as the existing challenges and prospects for development. Finally, we hope that an improved understanding of hydrodynamic trapping approaches can lead to sophisticated and useful platforms to advance medical and biological research.

Dynamic screening and printing of single cells using a microfluidic chip with dual microvalves

Inoculation of single cells into separate culture chambers is one of the key requirements in single-cell analysis. This paper reports an innovative microfluidic chip integrating two pneumatic microvalves to screen and print single cells onto a well plate. The upper and lower size limits of cells can be dynamically controlled by regulating the deformation of two adjacent microvalves. Numerical simulations were employed to systematically study the influence of membrane dimensions and pressure on the deflection of a valve. A mathematical model was then modified to predict the size of cells captured by a microvalve at various pressures. The membrane deflection was further studied using confocal imaging. The critical pressure trapping beads of various sizes was experimentally determined. These experiments validated the accuracy of both numerical simulations and the mathematical model. Furthermore, single beads and endothelial cells with the desired size range were screened using dual valves and printed onto well plates with 100% efficiency. Viability studies suggested that the screening process had no significant impact on cells. This device enables dynamic regulation of both the lower and the upper size limits of cells for printing. It has significant application potential in inoculating cells with desired sizes for various fields such as clonal expansion, monoclonality development and single-cell genomic studies.

Microfluidic Single-Cell Manipulation and Analysis: Methods and Applications

In a forest of a hundred thousand trees, no two leaves are alike. Similarly, no two cells in a genetically identical group are the same. This heterogeneity at the single-cell level has been recognized to be vital for the correct interpretation of diagnostic and therapeutic results of diseases, but has been masked for a long time by studying average responses from a population. To comprehensively understand cell heterogeneity, diverse manipulation and comprehensive analysis of cells at the single-cell level are demanded. However, using traditional biological tools, such as petri-dishes and well-plates, is technically challengeable for manipulating and analyzing single-cells with small size and low concentration of target biomolecules. With the development of microfluidics, which is a technology of manipulating and controlling fluids in the range of micro- to pico-liters in networks of channels with dimensions from tens to hundreds of microns, single-cell study has been blooming for almost two decades. Comparing to conventional petri-dish or well-plate experiments, microfluidic single-cell analysis offers advantages of higher throughput, smaller sample volume, automatic sample processing, and lower contamination risk, etc., which made microfluidics an ideal technology for conducting statically meaningful single-cell research. In this review, we will summarize the advances of microfluidics for single-cell manipulation and analysis from the aspects of methods and applications. First, various methods, such as hydrodynamic and electrical approaches, for microfluidic single-cell manipulation will be summarized. Second, single-cell analysis ranging from cellular to genetic level by using microfluidic technology is summarized. Last, we will also discuss the advantages and disadvantages of various microfluidic methods for single-cell manipulation, and then outlook the trend of microfluidic single-cell analysis.

Time Sequential Single-Cell Patterning with High Efficiency and High Density

Single-cell capture plays an important role in single-cell manipulation and analysis. This paper presents a microfluidic device for deterministic single-cell trapping based on the hydrodynamic trapping mechanism. The device is composed of an S-shaped loop channel and thousands of aligned trap units. This arrayed structure enables each row of the device to be treated equally and independently, as it has row periodicity. A theoretical model was established and a simulation was conducted to optimize the key geometric parameters, and the performance was evaluated by conducting experiments on MCF-7 and Jurkat cells. The results showed improvements in single-cell trapping ability, including loading efficiency, capture speed, and the density of the patterned cells. The optimized device can achieve a capture efficiency of up to 100% and single-cell capture efficiency of up to 95%. This device offers 200 trap units in an area of 1 mm², which enables 100 single cells to be observed simultaneously using a microscope with a 20× objective lens. One thousand cells can be trapped sequentially within 2 min; this is faster than the values obtained with previously reported devices. Furthermore, the cells can also be recovered by reversely infusing solutions. The structure can be easily extended to a large scale, and a patterned array with 32,000 trap sites was accomplished on a single chip. This device can be a powerful tool for high-throughput single-cell analysis, cell heterogeneity investigation, and drug screening.

Pipette Petri Dish Single-Cell Trapping (PP-SCT) in Microfluidic Platforms: A Passive Hydrodynamic Technique

Microfluidics-based biochips play a vital role in single-cell research applications. Handling and positioning of single cells at the microscale level are an essential need for various applications, including genomics, proteomics, secretomics, and lysis-analysis. In this article, the pipette Petri dish single-cell trapping (PP-SCT) technique is demonstrated. PP-SCT is a simple and cost-effective technique with ease of implementation for single cell analysis applications. In this paper a wide operation at different fluid flow rates of the novel PP-SCT technique is demonstrated. The effects of the microfluidic channel shape (straight, branched, and serpent) on the efficiency of single-cell trapping are studied. This article exhibited passive microfluidic-based biochips capable of vertical cell trapping with the hexagonally-positioned array of microwells. Microwells were 35 μm in diameter, a size sufficient to allow the attachment of captured cells for short-term study. Single-cell capture (SCC) capabilities of the microfluidic-biochips were found to be improving from the straight channel, branched channel, and serpent channel, accordingly. Multiple cell capture (MCC) was on the order of decreasing from the straight channel, branch channel, and serpent channel. Among the three designs investigated, the serpent channel biochip offers high SCC percentage with reduced MCC and NC (no capture) percentage. SCC was around 52%, 42%, and 35% for the serpent, branched, and straight channel biochips, respectively, for the tilt angle, θ values were between 10–15°. Human lung cancer cells (A549) were used for characterization. Using the PP-SCT technique, flow rate variations can be precisely achieved with a flow velocity range of 0.25–4 m/s (fluid channel of 2 mm width and 100 µm height). The upper dish (UD) can be used for low flow rate applications and the lower dish (LD) for high flow rate applications. Passive single-cell analysis applications will be facilitated using this method.

Manipulation of cells with laser microbeam scissors and optical tweezers: a review

The use of laser microbeams and optical tweezers in a wide field of biological applications from genomic to immunology is discussed. Microperforation is used to introduce a well-defined amount of molecules into cells for genetic engineering and optical imaging. The microwelding of two cells induced by a laser microbeam combines their genetic outfit. Microdissection allows specific regions of genomes to be isolated from a whole set of chromosomes. Handling the cells with optical tweezers supports investigation on the attack of immune systems against diseased or cancerous cells. With the help of laser microbeams, heart infarction can be simulated, and optical tweezers support studies on the heartbeat. Finally, laser microbeams are used to induce DNA damage in living cells for studies on cancer and ageing.

Digital microfluidics for time-resolved cytotoxicity studies on single non-adherent yeast cells

Single cell analysis (SCA) has gained increased popularity for elucidating cellular heterogeneity at genomic, proteomic and cellular levels. Flow cytometry is considered as one of the most widely used techniques to characterize single cell responses; however, its inability to analyse cells with spatio-temporal resolution poses a major drawback. Here, we introduce a digital microfluidic (DMF) platform as a useful tool for conducting studies on isolated yeast cells in a high-throughput fashion. The reported system exhibits (i) a microwell array for trapping single non-adherent cells by shuttling a cell-containing droplet over the array, and allows (ii) implementation of high-throughput cytotoxicity assays with enhanced spatio-temporal resolution. The system was tested for five different concentrations of the antifungal drug Amphotericin B, and the cell responses were monitored over time by time lapse fluorescence microscopy. The DMF platform was validated by bulk experiments, which mimicked the DMF experimental design. A correlation analysis revealed that the results obtained on the DMF platform are not significantly different from those obtained in bulk; hence, the DMF platform can be used as a tool to perform SCA on non-adherent cells, with spatio-temporal resolution. In addition, no external forces, other than the physical forces generated by moving the droplet, were used to capture single cells, thereby avoiding cell damage. As such, the information on cellular behaviour during treatment could be obtained for every single cell over time making this platform noteworthy in the field of SCA.