Pump-less static microfluidic device for analysis of chemotaxis of Pseudomonas aeruginosa using wetting and capillary action

Fugetaxis of Cell-in-Catalytic-Coat Nanobiohybrids in Glucose Gradients

Manipulation and control of cell chemotaxis remain an underexplored territory despite vast potential in various fields, such as cytotherapeutics, sensors, and even cell robots. Herein is achieved the chemical control over chemotactic movement and direction of Jurkat T cells, as a representative model, by the construction of cell-in-catalytic-coat structures in single-cell nanoencapsulation. Armed with the catalytic power of glucose oxidase (GOx) in the artificial coat, the nanobiohybrid cytostructures, denoted as Jurkat[Lipo_GOx] , exhibit controllable, redirected chemotactic movement in response to d-glucose gradients, in the opposite direction to the positive-chemotaxis direction of naïve, uncoated Jurkat cells in the same gradients. The chemically endowed, reaction-based fugetaxis of Jurkat[Lipo_GOx] operates orthogonally and complementarily to the endogenous, binding/recognition-based chemotaxis that remains intact after the formation of a GOx coat. For instance, the chemotactic velocity of Jurkat[Lipo_GOx] can be adjusted by varying the combination of d-glucose and natural chemokines (CXCL12 and CCL19) in the gradient. This work offers an innovative chemical tool for bioaugmenting living cells at the single-cell level through the use of catalytic cell-in-coat structures.

Bacterial chemotaxis in static gradients quantified in a biopolymer membrane-integrated microfluidic platform

Chemotaxis is a fundamental bacterial response mechanism to changes in chemical gradients of specific molecules known as chemoattractant or chemorepellent. The advancement of biological platforms for bacterial chemotaxis research is of significant interest for a wide range of biological and environmental studies. Many microfluidic devices have been developed for its study, but challenges still remain that can obscure analysis. For example, cell migration can be compromised by flow-induced shear stress, and bacterial motility can be impaired by nonspecific cell adhesion to microchannels. Also, devices can be complicated, expensive, and hard to assemble. We address these issues with a three-channel microfluidic platform integrated with natural biopolymer membranes that are assembled in situ. This provides several unique attributes. First, a static, steady and robust chemoattractant gradient was generated and maintained. Second, because the assembly incorporates assembly pillars, the assembled membrane arrays connecting nearby pillars can be created longer than the viewing window, enabling a wide 2D area for study. Third, the in situ assembled biopolymer membranes minimize pressure and/or chemiosmotic gradients that could induce flow and obscure chemotaxis study. Finally, nonspecific cell adhesion is avoided by priming the polydimethylsiloxane (PDMS) microchannel surfaces with Pluronic F-127. We demonstrated chemotactic migration of Escherichia coli as well as Pseudomonas aeruginosa under well-controlled easy-to-assemble glucose gradients. We characterized motility using the chemotaxis partition coefficient (CPC) and chemotaxis migration coefficient (CMC) and found our results consistent with other reports. Further, random walk trajectories of individual cells in simple bright field images were conveniently tracked and presented in rose plots. Velocities were calculated, again in agreement with previous literature. We believe the biopolymer membrane-integrated platform represents a facile and convenient system for robust quantitative assessment of cellular motility in response to various chemical cues.

Recent Developments in Bacterial Chemotaxis Analysis Based on the Microfluidic System

Bacterial motility in response to chemicals, also called bacterial chemotaxis, is a critical ability to search for the optimal environment to ensure the survival of bacterial species. Recent advances in microbiology have allowed the engineering of bacterial chemotactic properties. Conventional methods for characterizing bacterial motility are not able to fully monitor chemotactic behavior. Developments in microfluidic technology have enabled the designing of new experimental protocols in which spatiotemporal control of the cellular microenvironment can be achieved, and in which bacterial motility can be precisely and quantitatively measured and compared. This review provides an overview of recent developments of and new insights into microfluidic systems for chemotaxis assay.

Scalable Printing of Bionic Multiscale Channel Networks Through Digital Light Processing-Based Three-Dimensional Printing Process

Digital light processing (DLP)-based printing process has been used to print microfeature-sized constructs and architectures for biomedical applications; the key challenge is to achieve both large printing size and high accuracy at the same time. Here we reported a scalable DLP-based three-dimensional (3D) printing system with scalable resolution and building size, which was used for printing of multiscale hydrogel fractal bionic channels. Scalable printing was achieved by moving the convex lens of the printing system, and thus, each single micromirror of the digital micromirror device chip corresponded to the single-pixel size scaling from 6 to12 μm. Using this system, we were able to use poly (ethylene glycol) diacrylate to fabricate a variety of multiscale architectures, such as regular fractal Y-shaped channels, and more irregular and intricate geometries, such as biomimetic capillary vascular networks. Blue and red food dye solutions were able to freely fill all these channels in the scaffolds, from the trunk (>1500 μm in width) to small branch (∼30 μm in width) by capillarity. Cell experiments were carried out to certify the biocompatibility of printed multiscale biomimetic channel networks. This work reveals significant progress in printing multiscale constructs with both large printing size and high precision in scalable DLP-based 3D printing.

Mkit: A cell migration assay based on microfluidic device and smartphone

Mobile sensing based on the integration of microfluidic device and smartphone, so-called MS2 technology, has enabled many applications over recent years, and continues to stimulate growing interest in both research communities and industries. In particular, it has been envisioned that MS2 technology can be developed for various cell functional assays to enable basic research and clinical applications. Toward this direction, in this paper, we describe the development of a MS2-based cell functional assay for testing cell migration (the Mkit). The system is constructed as an integrated test kit, which includes microfluidic chips, a smartphone-based imaging platform, the phone apps for image capturing and data analysis, and a set of reagent and accessories for performing the cell migration assay. We demonstrated that the Mkit can effectively measure purified neutrophil and cancer cell chemotaxis. Furthermore, neutrophil chemotaxis can be tested from a drop of whole blood using the Mkit with red blood cell (RBC) lysis. The effects of chemoattractant dose and gradient profile on neutrophil chemotaxis were also tested using the Mkit. In addition to research applications, we demonstrated the effective use of the Mkit for on-site test at the hospital and for testing clinical samples from chronic obstructive pulmonary disease patient. Thus, this developed Mkit provides an easy and integrated experimental platform for cell migration related research and potential medical diagnostic applications.

A pressure-driven gas-diffusion/permeation micropump for self-activated sample transport in an extreme micro-environment

The pressure-driven gas-diffusion/permeation micropump is highlighted for stable microdroplet/liquid delivery under extreme conditions,e.g.high temperature, and a three-dimensional, long-distance and complex-topology microchannel.

Quantifying 3D chemotaxis in microfluidic-based chips with step gradients of collagen hydrogel concentrations

Cell migration is an essential process involved in crucial stages of tissue formation, regeneration or immune function as well as in pathological processes including tumor development or metastasis. During the last few years, the effect of gradients of soluble molecules on cell migration has been widely studied, and complex systems have been used to analyze cell behavior under simultaneous mechano-chemical stimuli. Most of these chemotactic assays have, however, focused on specific substrates in 2D. The aim of the present work is to develop a novel microfluidic-based chip that allows the long-term chemoattractant effect of growth factors (GFs) on 3D cell migration to be studied, while also providing the possibility to analyze the influence of the interface generated between different adjacent hydrogels. Namely, 1.5, 2, 2.5 and 4 mg ml^-1 concentrations of collagen type I were alternatively combined with 5, 10 or 50 ng ml^-1 concentrations of PDGF and VEGF (as a negative control). To achieve this goal, we have designed a new microfluidic device including three adjacent chambers to introduce hydrogels that allow the generation of a collagen concentration step gradient. This versatile and simple platform was tested by using dermal human fibroblasts embedded in 3D collagen matrices. Images taken over a week were processed to quantify the number of cells in each zone. We found, in terms of cell distribution, that the presence of PDGF, especially in small concentrations, was a strong chemoattractant for dermal human fibroblasts across the gels regardless of their collagen concentration and step gradient direction, whereas the effects of VEGF or collagen step gradient concentrations alone were negligible.

Chemotaxis detection towards chlorophenols using video processing analysis

To our knowledge, this communication is the first report of chemotaxis towards chlorophenols by any bacteria. We used a recently published method based on the agarose in-plug assay combined with video processing analysis and we also present a new index of bacterial mean speed for these assays.

Perspectives in flow-based microfluidic gradient generators for characterizing bacterial chemotaxis

Chemotaxis is a phenomenon which enables cells to sense concentrations of certain chemical species in their microenvironment and move towards chemically favorable regions. Recent advances in microbiology have engineered the chemotactic properties of bacteria to perform novel functions, but traditional methods of characterizing chemotaxis do not fully capture the associated cell motion, making it difficult to infer mechanisms that link the motion to the microbiology which induces it. Microfluidics offers a potential solution in the form of gradient generators. Many of the gradient generators studied to date for this application are flow-based, where a chemical species diffuses across the laminar flow interface between two solutions moving through a microchannel. Despite significant research efforts, flow-based gradient generators have achieved mixed success at accurately capturing the highly subtle chemotactic responses exhibited by bacteria. Here we present an analysis encompassing previously published versions of flow-based gradient generators, the theories that govern their gradient-generating properties, and new, more practical considerations that result from experimental factors. We conclude that flow-based gradient generators present a challenge inherent to their design in that the residence time and gradient decay must be finely balanced, and that this significantly narrows the window for reliable observation and quantification of chemotactic motion. This challenge is compounded by the effects of shear on an ellipsoidal bacterium that causes it to preferentially align with the direction of flow and subsequently suppresses the cross-flow chemotactic response. These problems suggest that a static, non-flowing gradient generator may be a more suitable platform for chemotaxis studies in the long run, despite posing greater difficulties in design and fabrication.

Biofilm development of an opportunistic model bacterium analysed at high spatiotemporal resolution in the framework of a precise flow cell

Life of bacteria is governed by the physical dimensions of life in microscales, which is dominated by fast diffusion and flow at low Reynolds numbers. Microbial biofilms are structurally and functionally heterogeneous and their development is suggested to be interactively related to their microenvironments. In this study, we were guided by the challenging requirements of precise tools and engineered procedures to achieve reproducible experiments at high spatial and temporal resolutions. Here, we developed a robust precise engineering approach allowing for the quantification of real-time, high-content imaging of biofilm behaviour under well-controlled flow conditions. Through the merging of engineering and microbial ecology, we present a rigorous methodology to quantify biofilm development at resolutions of single micrometre and single minute, using a newly developed flow cell. We designed and fabricated a high-precision flow cell to create defined and reproducible flow conditions. We applied high-content confocal laser scanning microscopy and developed image quantification using a model biofilm of a defined opportunistic strain, Pseudomonas putida OUS82. We observed complex patterns in the early events of biofilm formation, which were followed by total dispersal. These patterns were closely related to the flow conditions. These biofilm behavioural phenomena were found to be highly reproducible, despite the heterogeneous nature of biofilm.

Microfluidic partition with in situ biofabricated semipermeable biopolymer membranes for static gradient generation

We report an in situ biofabrication strategy that conveniently partitions microfluidic networks into physically separated while chemically communicating microchannels with semipermeable biopolymer membranes, which enable the facile generation of static gradients for biomedical applications. The biofabrication of parallel biopolymer membranes was initiated with the dissipation of trapped air bubbles in parallel apertures in polydimethylsiloxane (PDMS) microfluidic devices, followed by tunable membrane growth with precise temporal and spatial control to the desired thickness. Static gradients were generated within minutes and well maintained over time by pure diffusion of molecules through the biofabricated semipermeable membranes. As an example application, the static gradient of alpha factor was generated to study the development of the "shmoo" morphology of yeast over time. The in situ biofabrication provides a simple approach to generate static gradients and an ideal platform for biological applications where flow-free static gradients are indispensable.

Video processing analysis for the determination and evaluation of the chemotactic response in bacterial populations

The aim of the present work was to design a methodology based on video processing to obtain indicators of bacterial population motility that allow the quantitative and qualitative analysis and comparison of the chemotactic phenomenon with different attractants in the agarose-in plug bridge method. Video image sequences were processed applying Shannon's entropy to the intensity time series of each pixel, which conducted to a final pseudo colored image resembling a map of the dynamic bacterial clusters. Processed images could discriminate perfectly between positive and negative attractant responses at different periods of time from the beginning of the assay. An index of spatial and temporal motility was proposed to quantify the bacterial response. With this index, this video processing method allowed obtaining quantitative information of the dynamic changes in space and time from a traditional qualitative assay. We conclude that this computational technique, applied to the traditional agarose-in plug assay, has demonstrated good sensitivity for identifying chemotactic regions with a broad range of motility.

Microfluidic static droplet array for analyzing microbial communication on a population gradient

Quorum sensing (QS) is a type of cell-cell communication using signal molecules that are released and detected by cells, which respond to changes in their population density. A few studies explain that QS may operate in a density-dependent manner; however, due to experimental challenges, this fundamental hypothesis has never been investigated. Here, we present a microfluidic static droplet array (SDA) that combines a droplet generator with hydrodynamic traps to independently generate a bacterial population gradient into a parallel series of droplets under complete chemical and physical isolation. The SDA independently manipulates both a chemical concentration gradient and a bacterial population density. In addition, the bacterial population gradient in the SDA can be tuned by a simple change in the number of sample plug loading. Finally, the method allows the direct analysis of complicated biological events in an addressable droplet to enable the characterization of bacterial communication in response to the ratio of two microbial populations, including two genetically engineered QS circuits, such as the signal sender for acyl-homoserine lactone (AHL) production and the signal receiver bacteria for green fluorescent protein (GFP) expression induced by AHL. For the first time, we found that the population ratio of the signal sender and receiver indicates a significant and potentially interesting partnership between microbial communities. Therefore, we envision that this simple SDA could be a useful platform in various research fields, including analytical chemistry, combinatorial chemistry, synthetic biology, microbiology, and molecular biology.

Effect of temperature on biofilm formation by Antarctic marine bacteria in a microfluidic device

Polar biofilms have become an increasingly popular biological issue because new materials and phenotypes have been discovered in microorganisms in the polar region. Various environmental factors affect the functionality and adaptation of microorganisms. Because the polar region represents an extremely cold environment, polar microorganisms have a functionality different from that of normal microorganisms. Thus, determining the effective temperature for the development of polar biofilms is crucial. Here, we present a simple, novel one-pot assay for analysis of the effect of temperature on formation of Antarctic bacterial biofilm using a microfluidic system where continuous temperature gradients are generated. We find that a specific range of temperature is required for the growth of biofilms. Thus, this microfluidic approach provides precise information regarding the effective temperature for polar biofilm development with a new high-throughput screening format.

A “Light-up” 1D supramolecular nanoprobe for silver ions based on assembly of pyrene-labeled peptide amphiphiles: cell-imaging and antimicrobial activity

The histidine-coated fibrils response to Ag⁺ with fluorescence enhancement was developed through a rational design based on the aqueous self-assembly of peptides for potential use as cell-imaging and antimicrobial agents.

A novel microfluidic co-culture system for investigation of bacterial cancer targeting

Although bacterial cancer targeting in animal models has been previously demonstrated and suggested as a possible therapeutic tool, a thorough understanding of the mechanisms responsible for cancer specificity would be required prior to clinical applications. To visualize bacterial preference for cancer cells over normal cells and to elucidate the cancer-targeting mechanism, a simple microfluidic platform has been developed for in vitro studies. This platform allows simultaneous cultures of multiple cell types in independent culture environments in isolated chambers, and creates a stable chemical gradient across a collagen-filled passage between each of these cell culture chambers and the central channel. The established chemical gradient induces chemotactic preferential migration of bacteria toward a particular cell type for quantitative analysis. As a demonstration, we tested differential bacterial behavior on a two-chamber device where we quantified bacterial preference based on the difference in fluorescence intensities of green fluorescence protein (GFP)-expressing bacteria at two exits of the collagen-filled passages. Analysis of the chemotactic behavior of Salmonella typhimurium toward normal versus cancer hepatocytes using the developed platform revealed an apparent preference for cancer hepatocytes. We also demonstrate that alpha-fetoprotein (AFP) is one of the key chemo-attractants for S. typhimurium in targeting liver cancer.