On-chip open microfluidic devices for chemotaxis studies

3D cellular invasion platforms: how do paper-based cultures stack up?

Cellular invasion is the gateway to metastasis, which is the leading cause of cancer-related deaths. Invasion is driven by a number of chemical and mechanical stresses that arise in the tumor microenvironment. In vitro assays are needed for the systematic study of cancer progress. To be truly predictive, these assays must generate tissue-like environments that can be experimentally controlled and manipulated. While two-dimensional (2D) monolayer cultures are easily assembled and evaluated, they lack the extracellular components needed to assess invasion. Three-dimensional (3D) cultures are better suited for invasion studies because they generate cellular phenotypes that are more representative of those found in vivo. This feature article provides an overview of four invasion platforms. We focus on paper-based cultures, an emerging 3D culture platform capable of generating tissue-like structures and quantifying cellular invasion. Paper-based cultures are as easily assembled and analyzed as monolayers, but provide an experimentally powerful platform capable of supporting: co-cultures and representative extracellular environments; experimentally controlled gradients; readouts capable of quantifying, discerning, and separating cells based on their invasiveness. With a series of examples we highlight the potential of paper-based cultures, and discuss how they stack up against other invasion platforms.

A Novel Microdevice for Rapid Neutrophil Purification and Phenotyping in Type 2 Diabetes Mellitus

Neutrophil dysfunction is strongly linked to type 2 diabetes mellitus (T2DM) pathophysiology, but the prognostic potential of neutrophil biomarkers remains largely unexplored due to arduous leukocyte isolation methods. Herein, a novel integrated microdevice is reported for single-step neutrophil sorting and phenotyping (chemotaxis and formation of neutrophil extracellular traps (NETosis)) using small blood volumes (fingerprick). Untouched neutrophils are purified on-chip from whole blood directly using biomimetic cell margination and affinity-based capture, and are exposed to preloaded chemoattractant or NETosis stimulant to initiate chemotaxis or NETosis, respectively. Device performance is first characterized using healthy and in vitro inflamed blood samples (tumor necrosis factor alpha, high glucose), followed by clinical risk stratification in a cohort of subjects with T2DM. Interestingly, "high-risk" T2DM patients characterized by severe chemotaxis impairment reveal significantly higher C-reactive protein levels and poor lipid metabolism characteristics as compared to "low-risk" subjects, and their neutrophil chemotaxis responses can be mitigated after in vitro metformin treatment. Overall, this unique and user-friendly microfluidics immune health profiling strategy can significantly aid the quantification of chemotaxis and NETosis in clinical settings, and be further translated into a tool for risk stratification and precision medicine methods in subjects with metabolic diseases such as T2DM.

Orthogonal Screening of Anticancer Drugs Using an Open-Access Microfluidic Tissue Array System

Screening for potential drug combinations presents significant challenges to the current microfluidic cell culture systems, due to the requirement of flexibility in liquid handling. To overcome this limitation, we present here an open-access microfluidic tissue array system specifically designed for drug combination screening. The microfluidic chip features a key structure in which a nanoporous membrane is sandwiched by a cell culture chamber array layer and a corresponding media reservoir array layer. The microfluidic approach takes advantage of the characteristics of the nanoporous membrane: on one side, this membrane permits the flow of air but not liquid, thus acting as a flow-stop valve to enable automatic cell distribution; on the other side, it allows diffusion-based media exchange and thus mimics the endothelial layer. In synergy with a liquid-transferring platform, the open-access microfluidic system enables complex multistep operations involving long-term cell culture, medium exchange, multistep drug treatment, and cell-viability testing. By using the microfluidic protocol, a 10 × 10 tissue array was constructed in 90 s, followed by schedule-dependent drug testing. Morphological and immunohistochemical assays indicated that the resultant tumor tissue was faithful to that in vivo. Drug-testing assays showed that the incorporation of the nanoporous membrane further decreased killing efficacy of the anticancer agents, indicating its function as an endothelial layer. Robustness of the microfluidic system was demonstrated by implementing a three-factor, three-level orthogonal screening of anticancer drug combinations, with which 67% of the testing (9 vs. 27) was saved. Experimental results demonstrated that the microfluidic tissue system presented herein is flexible and easy-to-use, thus providing an ideal tool for performing complex multistep cell assays with high efficiencies.

Studying Electrotaxis in Microfluidic Devices

Collective cell migration is important in various physiological processes such as morphogenesis, cancer metastasis and cell regeneration. Such migration can be induced and guided by different chemical and physical cues. Electrotaxis, referring to the directional migration of adherent cells under stimulus of electric fields, is believed to be highly involved in the wound-healing process. Electrotactic experiments are conventionally conducted in Petri dishes or cover glasses wherein cells are cultured and electric fields are applied. However, these devices suffer from evaporation of the culture medium, non-uniformity of electric fields and low throughput. To overcome these drawbacks, micro-fabricated devices composed of micro-channels and fluidic components have lately been applied to electrotactic studies. Microfluidic devices are capable of providing cells with a precise micro-environment including pH, nutrition, temperature and various stimuli. Therefore, with the advantages of reduced cell/reagent consumption, reduced Joule heating and uniform and precise electric fields, microfluidic chips are perfect platforms for observing cell migration under applied electric fields. In this paper, I review recent developments in designing and fabricating microfluidic devices for studying electrotaxis, aiming to provide critical updates in this rapidly-growing, interdisciplinary field.

In Vitro Studies on a Microfluidic Sensor with Embedded Obstacles Using New Antibacterial Synthetic Compounds (1-TDPPO) Mixed Prop-2-en-1-one with Difluoro Phenyl

This paper describes the use of an analytical microfluidic sensor for accelerating chemo-repellent response and strong anti-bacterial 1-(Thien-2-yl)-3-(2, 6-difluoro phenyl) prop-2-en-1-one (1-TDPPO). The chemically-synthesized antimicrobial agent, which included prop-2-en-1-one and difluoro phenyl groups, was moving through an optically transparent polydimethylsiloxane (PDMS) microfluidic sensor with circular obstacles arranged evenly. The response, growth and distribution of fluorescent labeling Pseudomonas aeruginosa PAO1 against the antimicrobial agent were monitored by confocal laser scanning microscope (CLSM). The microfluidic sensor along with 1-TDPPOin this study exhibits the following advantages: (i) Real-time chemo-repellent responses of cell dynamics; (ii) Rapid eradication of biofilm by embedded obstacles and powerful antibacterial agents, which significantly reduce the response time compared to classical methods; (iii) Minimal consumption of cells and antimicrobial agents; and (iv) Simplifying the process of the normalization of the fluorescence intensity and monitoring of biofilm by captured images and datasets.

A Customizable Chamber for Measuring Cell Migration

Cell migration is a vital part of immune responses, growth, and wound healing. Cell migration is a complex process that involves interactions between cells, the extracellular matrix, and soluble and non-soluble chemical factors (e.g., chemoattractants). Standard methods for measuring the migration of cells, such as the Boyden chamber assay, work by counting cells on either side of a divider. These techniques are easy to use; however, they offer little geometric modification for different applications. In contrast, microfluidic devices can be used to observe cell migration with customizable concentration gradients of soluble factors^1,2. However, methods for making microfluidics based assays can be difficult to learn. Here, we describe an easy method for creating cell culture chambers to measure cell migration in response to chemical concentration gradients. Our cell migration chamber method can create different linear concentration gradients in order to study cell migration for a variety of applications. This method is relatively easy to use and is typically performed by undergraduate students. The microchannel chamber was created by placing an acrylic insert in the shape of the final microchannel chamber well into a Petri dish. After this, poly(dimethylsiloxane) (PDMS) was poured on top of the insert. The PDMS was allowed to harden and then the insert was removed. This allowed for the creation of wells in any desired shape or size. Cells may be subsequently added to the microchannel chamber, and soluble agents can be added to one of the wells by soaking an agarose block in the desired agent. The agarose block is added to one of the wells, and time-lapse images can be taken of the microchannel chamber in order to quantify cell migration. Variations to this method can be made for a given application, making this method highly customizable.

Seamless Combination of Fluorescence-Activated Cell Sorting and Hanging-Drop Networks for Individual Handling and Culturing of Stem Cells and Microtissue Spheroids

Open microfluidic cell culturing devices offer new possibilities to simplify loading, culturing, and harvesting of individual cells or microtissues due to the fact that liquids and cells/microtissues are directly accessible. We present a complete workflow for microfluidic handling and culturing of individual cells and microtissue spheroids, which is based on the hanging-drop network concept: The open microfluidic devices are seamlessly combined with fluorescence-activated cell sorting (FACS), so that individual cells, including stem cells, can be directly sorted into specified culturing compartments in a fully automated way and at high accuracy. Moreover, already assembled microtissue spheroids can be loaded into the microfluidic structures by using a conventional pipet. Cell and microtissue culturing is then performed in hanging drops under controlled perfusion. On-chip drop size control measures were applied to stabilize the system. Cells and microtissue spheroids can be retrieved from the chip by using a parallelized transfer method. The presented methodology holds great promise for combinatorial screening of stem-cell and multicellular-spheroid cultures.

Lymphocytes can self-steer passively with wind vane uropods

A wide variety of cells migrate directionally in response to chemical or mechanical cues, however the mechanisms involved in cue detection and translation into directed movement are debatable. Here we investigate a model of lymphocyte migration on the inner surface of blood vessels. Cells orient their migration against fluid flow, suggesting the existence of an adaptive mechano-tranduction mechanism. We find that flow detection may not require molecular mechano-sensors of shear stress, and detection of flow direction can be achieved by the orientation in the flow of the non-adherent cell rear, the uropod. Uropods act as microscopic wind vanes that can transmit detection of flow direction into cell steering via the on-going machinery of polarity maintenance, without the need for novel internal guidance signalling triggered by flow. Contrary to chemotaxis, which implies active regulation of cue-dependent signalling, upstream flow mechanotaxis of lymphocytes may only rely on a passive self-steering mechanism.

Engineered three-dimensional microfluidic device for interrogating cell-cell interactions in the tumor microenvironment

Stromal cells in the tumor microenvironment play a key role in the metastatic properties of a tumor. It is recognized that cancer-associated fibroblasts (CAFs) and endothelial cells secrete factors capable of influencing tumor cell migration into the blood or lymphatic vessels. We developed a microfluidic device that can be used to image the interactions between stromal cells and tumor cell spheroids in a three dimensional (3D) microenvironment while enabling external control of interstitial flow at an interface, which supports endothelial cells. The apparatus couples a 200-μm channel with a semicircular well to mimic the interface of a blood vessel with the stroma, and the design allows for visualization of the interactions of interstitial flow, endothelial cells, leukocytes, and fibroblasts with the tumor cells. We observed that normal tissue-associated fibroblasts (NAFs) contribute to the "single file" pattern of migration of tumor cells from the spheroid in the 3D microenvironment. In contrast, CAFs induce a rapid dispersion of tumor cells out of the spheroid with migration into the 3D matrix. Moreover, treatment of tumor spheroid cultures with the chemokine CXCL12 mimics the effect of the CAFs, resulting in similar patterns of dispersal of the tumor cells from the spheroid. Conversely, addition of CXCL12 to co-cultures of NAFs with tumor spheroids did not mimic the effects observed with CAF co-cultures, suggesting that NAFs produce factors that stabilize the tumor spheroids to reduce their migration in response to CXCL12.

A microfluidic-enabled mechanical microcompressor for the immobilization of live single- and multi-cellular specimens

A microcompressor is a precision mechanical device that flattens and immobilizes living cells and small organisms for optical microscopy, allowing enhanced visualization of sub-cellular structures and organelles. We have developed an easily fabricated device, which can be equipped with microfluidics, permitting the addition of media or chemicals during observation. This device can be used on both upright and inverted microscopes. The apparatus permits micrometer precision flattening for nondestructive immobilization of specimens as small as a bacterium, while also accommodating larger specimens, such as Caenorhabditis elegans, for long-term observations. The compressor mount is removable and allows easy specimen addition and recovery for later observation. Several customized specimen beds can be incorporated into the base. To demonstrate the capabilities of the device, we have imaged numerous cellular events in several protozoan species, in yeast cells, and in Drosophila melanogaster embryos. We have been able to document previously unreported events, and also perform photobleaching experiments, in conjugating Tetrahymena thermophila.

Recent developments in microfluidics-based chemotaxis studies

Microfluidic devices can better control cellular microenvironments compared to conventional cell migration assays. Over the past few years, microfluidics-based chemotaxis studies showed a rapid growth. New strategies were developed to explore cell migration in manipulated chemical gradients. In addition to expanding the use of microfluidic devices for a broader range of cell types, microfluidic devices were used to study cell migration and chemotaxis in complex environments. Furthermore, high-throughput microfluidic chemotaxis devices and integrated microfluidic chemotaxis systems were developed for medical and commercial applications. In this article, we review recent developments in microfluidics-based chemotaxis studies and discuss the new trends in this field observed over the past few years.