Visual Quantitative Detection of Circulating Tumor Cells with Single‐Cell Sensitivity Using a Portable Microfluidic Device

Immunocytological technologies, molecular technologies, and functional assays are widely used for detecting circulating tumor cells (CTCs) after enrichment from patients' blood sample. Unfortunately, accessibility to these technologies is limited due to the need for sophisticated instrumentation and skilled operators. Portable microfluidic devices have become attractive tools for expanding the access and efficiency of detection beyond hospitals to sites near the patient. Herein, a volumetric bar chart chip (V‐Chip) is developed as a portable platform for CTC detection. The target CTCs are labeled with aptamer‐conjugated nanoparticles (ACNPs) and analyzed by V‐Chip through quantifying the byproduct (oxygen) of the catalytic reaction between ACNPs and hydrogen peroxide, which results in the movement of an ink bar to a concentration‐dependent distance for visual quantitative readout. Thus, the CTC number is decoded into visually quantifiable information and a linear correlation can be found between the distance moved by the ink and number of cells in the sample. This method is sensitive enough that a single cell can be detected. Furthermore, the clinical capabilities of this system are demonstrated for quantitative CTC detection in the presence of a high leukocyte background. This portable detection method shows great potential for quantification of rare cells with single‐cell sensitivity for various applications.     

Dynamic Environmental Control in Microfluidic Single‐Cell Cultivations: From Concepts to Applications

Microfluidic single‐cell cultivation (MSCC) is an emerging field within fundamental as well as applied biology. During the last years, most MSCCs were performed at constant environmental conditions. Recently, MSCC at oscillating and dynamic environmental conditions has started to gain significant interest in the research community for the investigation of cellular behavior. Herein, an overview of this topic is given and microfluidic concepts that enable oscillating and dynamic control of environmental conditions with a focus on medium conditions are discussed, and their application in single‐cell research for the cultivation of both mammalian and microbial cell systems is demonstrated. Furthermore, perspectives for performing MSCC at complex dynamic environmental profiles of single parameters and multiparameters (e.g., pH and O₂) in amplitude and time are discussed. The technical progress in this field provides completely new experimental approaches and lays the foundation for systematic analysis of cellular metabolism at fluctuating environments.     

Lipoplex‐Mediated Single‐Cell Transfection via Droplet Microfluidics

While lipoplex (cationic lipid‐nucleic acid complex)‐mediated intracellular delivery is widely adopted in mammalian cell transfection, its transfection efficiency for suspension cells, e.g., lymphatic and hematopoietic cells, is reported at only ≈5% or even lower. Here, efficient and consistent lipoplex‐mediated transfection is demonstrated for hard‐to‐transfect suspension cells via a single‐cell, droplet‐microfluidics approach. In these microdroplets, monodisperse lipoplexes for effective gene delivery are generated via chaotic mixing induced by the serpentine microchannel and co‐confined with single cells. Moreover, the cell membrane permeability increases due to the shear stress exerted on the single cells when they pass through the droplet pinch‐off junction. The transfection efficiency, examined by the delivery of the pcDNA3‐EGFP plasmid, improves from ≈5% to ≈50% for all three tested suspension cell lines, i.e., K562, THP‐1, Jurkat, and with significantly reduced cell‐to‐cell variation, compared to the bulk method. Efficient targeted knockout of the TP53BP1 gene for K562 cells via the CRISPR (clustered regularly interspaced short palindromic repeats)–CAS9 (CRISPR‐associated nuclease 9) mechanism is also achieved using this platform. Lipoplex‐mediated single‐cell transfection via droplet microfluidics is expected to have broad applications in gene therapy and regenerative medicine by providing high transfection efficiency and low cell‐to‐cell variation for hard‐to‐transfect suspension cells.     

Screening Therapeutic Agents Specific to Breast Cancer Stem Cells Using a Microfluidic Single‐Cell Clone‐Forming Inhibition Assay

Screens of cancer stem cells (CSCs)‐specific agents present significant challenges to conventional cell assays due to the difficulty in preparing CSCs ready for drug testing. To overcome this limitation, developed is a microfluidic single‐cell assay for screening breast cancer stem cell–specific agents. This assay takes advantage of the single‐cell clone‐forming capability of CSCs, which can be specifically inhibited by CSC‐targeting agents. The single‐cell assay is performed on a microfluidic chip with an array of 3840 cell‐capturing units; the single‐cell arrays are easily formed by flowing a cell suspension into the microchip. Achieved is a single cell‐capture rate of ≈60% thus allowing more than 2000 single cells to be analyzed in a single test. Over long‐term suspension culture, only a minority of cells survive and form tumorspheres. The clone‐formation rate of MCF‐7, MDA‐MB‐231, and T47D cells is 1.67%, 5.78%, and 5.24%, respectively. The clone‐forming inhibition assay is conducted by exposing the single‐cell arrays to a set of anticancer agents. The CSC‐targeting agents show complete inhibition of single‐cell clone formation while the nontargeting ones show incomplete inhibition effects. The resulting microfluidic single‐cell assay with the potential to screen CSC‐specific agents with high efficiency provides new tools for individualized tumor therapy.     

Label‐Free Optofluidic Nanobiosensor Enables Real‐Time Analysis of Single‐Cell Cytokine Secretion

Single‐cell analysis of cytokine secretion is essential to understand the heterogeneity of cellular functionalities and develop novel therapies for multiple diseases. Unraveling the dynamic secretion process at single‐cell resolution reveals the real‐time functional status of individual cells. Fluorescent and colorimetric‐based methodologies require tedious molecular labeling that brings inevitable interferences with cell integrity and compromises the temporal resolution. An innovative label‐free optofluidic nanoplasmonic biosensor is introduced for single‐cell analysis in real time. The nanobiosensor incorporates a novel design of a multifunctional microfluidic system with small volume microchamber and regulation channels for reliable monitoring of cytokine secretion from individual cells for hours. Different interleukin‐2 secretion profiles are detected and distinguished from single lymphoma cells. The sensor configuration combined with optical spectroscopic imaging further allows us to determine the spatial single‐cell secretion fingerprints in real time. This new biosensor system is anticipated to be a powerful tool to characterize single‐cell signaling for basic and clinical research.     

A Single‐Cell Assay for Time Lapse Studies of Exosome Secretion and Cell Behaviors

To understand the inhomogeneity of cells in biological systems, there is a growing demand on the capability of characterizing the properties of individual single cells. Since single‐cell studies require continuous monitoring of the cell behaviors, an effective single‐cell assay that can support time lapsed studies in a high throughput manner is desired. Most currently available single‐cell technologies cannot provide proper environments to sustain cell growth and, proliferation of single cells and convenient, noninvasive tests of single‐cell behaviors from molecular markers. Here, a highly versatile single‐cell assay is presented that can accommodate different cellular types, enable easy and efficient single‐cell loading and culturing, and be suitable for the study of effects of in vitro environmental factors in combination with drug screening. One salient feature of the assay is the noninvasive collection and surveying of single‐cell secretions at different time points, producing unprecedented insight of single‐cell behaviors based on the biomarker signals from individual cells under given perturbations. Above all, the acquired information is quantitative, for example, measured by the number of exosomes each single‐cell secretes for a given time period. Therefore, our single‐cell assay provides a convenient, low‐cost, and enabling tool for quantitative, time lapsed studies of single‐cell properties.     

Single‐Cell Detection of mRNA Expression Using Nanofountain‐Probe Electroporated Molecular Beacons

New techniques for single‐cell analysis enable new discoveries in gene expression and systems biology. Time‐dependent measurements on individual cells are necessary, yet the common single‐cell analysis techniques used today require lysing the cell, suspending the cell, or long incubation times for transfection, thereby interfering with the ability to track an individual cell over time. Here a method for detecting mRNA expression in live single cells using molecular beacons that are transfected into single cells by means of nanofountain probe electroporation (NFP‐E) is presented. Molecular beacons are oligonucleotides that emit fluorescence upon binding to an mRNA target, rendering them useful for spatial and temporal studies of live cells. The NFP‐E is used to transfect a DNA‐based beacon that detects glyceraldehyde 3‐phosphate dehydrogenase and an RNA‐based beacon that detects a sequence cloned in the green fluorescence protein mRNA. It is shown that imaging analysis of transfection and mRNA detection can be performed within seconds after electroporation and without disturbing adhered cells. In addition, it is shown that time‐dependent detection of mRNA expression is feasible by transfecting the same single cell at different time points. This technique will be particularly useful for studies of cell differentiation, where several measurements of mRNA expression are required over time.     

Photonic Microcapsules Containing Single‐Crystal Colloidal Arrays with Optical Anisotropy

Colloidal particles with a repulsive interparticle potential spontaneously form crystalline lattices, which are used as a motif for photonic materials. It is difficult to predict the crystal arrangement in spherical volume as lattices are incompatible with a spherical surface. Here, the optimum arrangement of charged colloids is experimentally investigated by encapsulating them in double‐emulsion drops. Under conditions of strong interparticle repulsion, the colloidal crystal rapidly grows from the surface toward the center of the microcapsule, forming an onion‐like arrangement. By contrast, for weak repulsion, crystallites slowly grow and fuse through rearrangement to form a single‐crystal phase. Single‐crystal structure is energetically favorable even for strong repulsion. Nevertheless, a high energy barrier to colloidal rearrangement kinetically arrests the onion‐like structure formed by heterogeneous nucleation. Unlike the isotropic onion‐shaped product, the anisotropic single‐crystal‐containing microcapsules selectively display—at certain orientations but not others—one of the distinct colors from the various crystal planes.