Open access microfluidic device for the study of cell migration during chemotaxis

A Bi-Directional Acoustic Micropump Driven by Oscillating Sharp-Edge Structures

This paper proposes a bi-directional acoustic micropump driven by two groups of oscillating sharp-edge structures: one group of sharp-edge structures with inclined angles of 60° and a width of 40 μm, and another group with inclined angles of 45° and a width of 25 μm. One of the groups of sharp-edge structures will vibrate under the excitation of the acoustic wave generated with a piezoelectric transducer at its corresponding resonant frequency. When one group of sharp-edge structures vibrates, the microfluid flows from left to right. When the other group of sharp-edge structures vibrates, the microfluid flows in the opposite direction. Some gaps are designed between the sharp-edge structures and the upper surface and the bottom surface of the microchannels, which can reduce the damping between the sharp-edge structures and the microchannels. Actuated with an acoustic wave of a different frequency, the microfluid in the microchannel can be driven bidirectionally by the inclined sharp-edge structures. The experiments show that the acoustic micropump, driven by oscillating sharp-edge structures, can produce a stable flow rate of up to 125 μm/s from left to right, when the transducer was activated at 20.0 kHz. When the transducer was activated at 12.8 kHz, the acoustic micropump can produce a stable flow rate of up to 85 μm/s from right to left. This bi-directional acoustic micropump, driven by oscillating sharp-edge structures, is easy to operate and shows great potential in various applications.

Switching from Chemical to Electrical Micromotor Propulsion across a Gradient of Gastric Fluid via Magnetic Rolling

To address and extend the finite lifetime of Mg-based micromotors due to the depletion of the engine (Mg-core), we examine electric fields, along with previously studied magnetic fields, to create a triple-engine hybrid micromotor for driving these micromotors. Electric fields are a facile energy source that is not limited in its operation time and can dynamically tune the micromotor mobility by simply changing the frequency and amplitude of the field. Moreover, the same electrical fields can be used for cell trapping and transport as well as drug delivery. However, the limitations of these propulsion mechanisms are the low pH (and high conductivity) environment required for Mg dissolution, while the electrical propulsion is quenched at these conditions as it requires low conductivity mediums. In order to translate the micromotor between these two extreme medium conditions, we use magnetic rolling as means of self-propulsion along with magnetic steering. Interestingly, electrical propulsion also necessitates at least the partial consumption of the Mg, resulting in a sufficient geometrical asymmetry of the micromotor. We have successfully demonstrated the rapid propulsion switching capability of the micromotor, from chemical to electrical motions, via magnetic rolling within a microfluidic device with the concentration gradient of the simulated gastric fluid. Such triple-engine micromotor propulsion holds considerable promise for in vitro studies mimicking gastric conditions and performing various bioassay tasks.

Macropinocytosis and Cell Migration: Don't Drink and Drive…

Macropinocytosis is a nonspecific mechanism by which cells compulsively "drink" the surrounding extracellular fluids in order to feed themselves or sample the molecules therein, hence gaining information about their environment. This process is cell-intrinsically incompatible with the migration of many cells, implying that the two functions are antagonistic. The migrating cell uses a molecular switch to stop and explore its surrounding fluid by macropinocytosis, after which it employs the same molecular machinery to start migrating again to examine another location. This cycle of migration/macropinocytosis allows cells to explore tissues, and it is key to a range of physiological processes. Evidence of this evolutionarily conserved antagonism between the two processes can be found in several cell types-immune cells, for example, being particularly adept-and ancient organisms (e.g., the social amoeba Dictyostelium discoideum). How macropinocytosis and migration are negatively coupled is the subject of this chapter.

A Concentration Gradients Tunable Generator with Adjustable Position of the Acoustically Oscillating Bubbles

It is essential to control concentration gradients at specific locations for many biochemical experiments. This paper proposes a tunable concentration gradient generator actuated by acoustically oscillating bubbles trapped in the bubble channels using a controllable position based on the gas permeability of polydimethylsiloxane (PDMS). The gradient generator consists of a glass substrate, a PDMS chip, and a piezoelectric transducer. When the trapped bubbles are activated by acoustic waves, the solution near the gas–liquid interface is mixed. The volume of the bubbles and the position of the gas–liquid interface are regulated through the permeability of the PDMS wall. The tunable concentration gradient can be realized by changing the numbers and positions of the bubbles that enable the mixing of fluids in the main channel, and the amplitude of the applied voltage. This new device is easy to fabricate, responsive, and biocompatible, and therefore has great application prospects. In particular, it is suitable for biological research with high requirements for temporal controllability.

Natural killer cell migration control in microchannels by perturbations and topography

Natural killer (NK) cells are lymphocytes which play an important role in the immune system by recognizing and killing potentially malignant cells without antigen sensitization, and could be utilized in cancer therapy. NK cell migration is an essential process to find and kill target cells, which is well known to be driven by the chemotaxis effect. NK cells also experience a topographical effect induced by the extracellular matrix (ECM) during their migration. However, topographical effects on NK cell locomotion in three dimensional (3D) environments are not well studied yet. In this work, polydimethylsiloxane based platforms containing microchannels with different types of perturbations and decorated with various surface patterns were fabricated to systematically study the topographical effect on NK cell migration with and without the chemotaxis effect. The results showed that perturbation sites in channels induced pauses and reversals in chemotaxis driven NK cell migration. Surface topography such as gratings in confined environments could introduce directional preference to NK cell movement even without chemoattractants. These findings showed that NK cell migration could be controlled by contact guidance, which provides future possibility to manipulate NK cell migration in controlled in vitro bioengineering systems. Results in this study showed that the complex topography of 3D microenvironments in the ECM could have significant effects on NK cell migration in different tissues and organs, and provided insight for explaining the dynamics of NK cell activities in clinical experiments.

Migration of Dictyostelium discoideum to the Chemoattractant Folic Acid

Dictyostelium discoideum can be grown axenically in a cultured media or in the presence of a natural food source, such as the bacterium Klebsiella aerogenes (KA). Here we describe the advantages and methods for growing D. discoideum on a bacterial lawn for several processes studied using this model system. When grown on a bacterial lawn, D. discoideum show positive chemotaxis towards folic acid (FA). While these vegetative cells are highly unpolarized, it has been shown that the signaling and cytoskeletal molecules regulating the directed migration of these cells are homologous to those seen in the motility of polarized cells in response to the chemoattractant cyclic adenosine monophosphate (cAMP). Growing D. discoideum on KA stimulates chemotactic responsiveness to FA. A major advantage of performing FA-mediated chemotaxis is that it does not require expression of the cAMP developmental program and therefore has the potential to identify mutants that are purely unresponsive to chemoattractant gradients. The cAMP-mediated chemotaxis can appear to fail when cells are developmentally delayed or do not up-regulate genes needed for cAMP-mediated migration. In addition to providing robust chemotaxis to FA, cells grown on bacterial lawns are highly resistant to light damage during fluorescence microscopy. This resistance to light damage could be exploited to better understand other biological processes such as phagocytosis or cytokinesis. The cell cycle is also shortened when cells are grown in the presence of KA, so the chances of seeing a mitotic event increases.

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.

Cell Blebbing in Confined Microfluidic Environments

Migrating cells can extend their leading edge by forming myosin-driven blebs and F-actin-driven pseudopods. When coerced to migrate in resistive environments, Dictyostelium cells switch from using predominately pseudopods to blebs. Bleb formation has been shown to be chemotactic and can be influenced by the direction of the chemotactic gradient. In this study, we determine the blebbing responses of developed cells of Dictyostelium discoideum to cAMP gradients of varying steepness produced in microfluidic channels with different confining heights, ranging between 1.7 μm and 3.8 μm. We show that microfluidic confinement height, gradient steepness, buffer osmolarity and Myosin II activity are important factors in determining whether cells migrate with blebs or with pseudopods. Dictyostelium cells were observed migrating within the confines of microfluidic gradient channels. When the cAMP gradient steepness is increased from 0.7 nM/μm to 20 nM/μm, cells switch from moving with a mixture of blebs and pseudopods to moving only using blebs when chemotaxing in channels with confinement heights less than 2.4 μm. Furthermore, the size of the blebs increases with gradient steepness and correlates with increases in myosin-II localization at the cell cortex. Reduction of intracellular pressure by high osmolarity buffer or inhibition of myosin-II by blebbistatin leads to a decrease in bleb formation and bleb size. Together, our data reveal that the protrusion type formed by migrating cells can be influenced by the channel height and the steepness of the cAMP gradient, and suggests that a combination of confinement-induced myosin-II localization and cAMP-regulated cortical contraction leads to increased intracellular fluid pressure and bleb formation.

A real-time assay for neutrophil chemotaxis

Neutrophils are the predominant cells during acute phases of inflammation, and it is now recognized that these leukocytes play an important role in modulation of the immune response. Directed migration of these cells to the sites of injury, known as chemotaxis, is driven by chemoattractants present at the endothelial cell surface or in the extracellular matrix (ECM). Since uncontrolled or excessive neutrophil chemotaxis is involved in pathological conditions such as atherosclerosis and severe asthma, studying the chemical cues triggering neutrophil migration is essential for understanding the biology of these cells and developing new anti-inflammatory therapies. Although several methods have been developed to evaluate neutrophil chemotaxis, these techniques are generally labor-intensive or alter the native form of these cells and their physiological response. Here we report a rapid, non-invasive, impedance-based, and label-free assay for real-time assessment of neutrophil chemotaxis.

An open-chamber flow-focusing device for focal stimulation of micropatterned cells

Microfluidic devices can deliver soluble factors to cell and tissue culture microenvironments with precise spatiotemporal control. However, enclosed microfluidic environments often have drawbacks such as the need for continuous culture medium perfusion which limits the duration of experiments, incongruity between microculture and macroculture, difficulty in introducing cells and tissues, and high shear stress on cells. Here, we present an open-chamber microfluidic device that delivers hydrodynamically focused streams of soluble reagents to cells over long time periods (i.e., several hours). We demonstrate the advantage of the open chamber by using conventional cell culture techniques to induce the differentiation of myoblasts into myotubes, a process that occurs in 7-10 days and is difficult to achieve in closed chamber microfluidic devices. By controlling the flow rates and altering the device geometry, we produced sharp focal streams with widths ranging from 36 μm to 187 μm. The focal streams were reproducible (∼12% variation between units) and stable (∼20% increase in stream width over 10 h of operation). Furthermore, we integrated trenches for micropatterning myoblasts and microtraps for confining single primary myofibers into the device. We demonstrate with finite element method (FEM) simulations that shear stresses within the cell trench are well below values known to be deleterious to cells, while local concentrations are maintained at ∼22% of the input concentration. Finally, we demonstrated focused delivery of cytoplasmic and nuclear dyes to micropatterned myoblasts and myofibers. The open-chamber microfluidic flow-focusing concept combined with micropatterning may be generalized to other microfluidic applications that require stringent long-term cell culture conditions.

Design of a microfluidic device to quantify dynamic intra-nuclear deformation during cell migration through confining environments

The ability of cells to migrate through tissues and interstitial spaces is an essential factor during development and tissue homeostasis, immune cell mobility, and in various human diseases. Deformation of the nucleus and its associated lamina during 3-D migration is gathering increasing interest in the context of cancer metastasis, with the underlying hypothesis that a softer nucleus, resulting from reduced levels of lamin A/C, may aid tumour spreading. However, current methods to study the migration of cells in confining three dimensional (3-D) environments are limited by their imprecise control over the confinement, physiological relevance, and/or compatibility with high resolution imaging techniques. We describe the design of a polydimethylsiloxane (PDMS) microfluidic device composed of channels with precisely-defined constrictions mimicking physiological environments that enable high resolution imaging of live and fixed cells. The device promotes easy cell loading and rapid, yet long-lasting (>24 hours) chemotactic gradient formation without the need for continuous perfusion. Using this device, we obtained detailed, quantitative measurements of dynamic nuclear deformation as cells migrate through tight spaces, revealing distinct phases of nuclear translocation through the constriction, buckling of the nuclear lamina, and severe intranuclear strain. Furthermore, we found that lamin A/C-deficient cells exhibited increased and more plastic nuclear deformations compared to wild-type cells but only minimal changes in nuclear volume, implying that low lamin A/C levels facilitate migration through constrictions by increasing nuclear deformability rather than compressibility. The integration of our migration devices with high resolution time-lapse imaging provides a powerful new approach to study intracellular mechanics and dynamics in a variety of physiologically-relevant applications, ranging from cancer cell invasion to immune cell recruitment.

A spatiotemporally controllable chemical gradient generator via acoustically oscillating sharp-edge structures

The ability to generate stable, spatiotemporally controllable concentration gradients is critical for resolving the dynamics of cellular response to a chemical microenvironment. Here we demonstrate an acoustofluidic gradient generator based on acoustically oscillating sharp-edge structures, which facilitates in a step-wise fashion the rapid mixing of fluids to generate tunable, dynamic chemical gradients. By controlling the driving voltage of a piezoelectric transducer, we demonstrated that the chemical gradient profiles can be conveniently altered (spatially controllable). By adjusting the actuation time of the piezoelectric transducer, moreover, we generated pulsatile chemical gradients (temporally controllable). With these two characteristics combined, we have developed a spatiotemporally controllable gradient generator. The applicability and biocompatibility of our acoustofluidic gradient generator are validated by demonstrating the migration of human dermal microvascular endothelial cells (HMVEC-d) in response to a generated vascular endothelial growth factor (VEGF) gradient, and by preserving the viability of HMVEC-d cells after long-term exposure to an acoustic field. Our device features advantages such as simple fabrication and operation, compact and biocompatible device, and generation of spatiotemporally tunable gradients.

Quantitative analysis of Caenorhabditis elegans chemotaxis using a microfluidic device

Caenorhabditis elegans, one of the widely studied model organisms, sense external chemical cues and perform relative chemotaxis behaviors through its simple chemosensory neuronal system. To study the mechanism underlying chemosensory behavior, a rapid and reliable method for quantitatively analyzing the worms' behaviors is essential. In this work, we demonstrated a microfluidic approach for investigating chemotaxis responses of worms to chemical gradients. The flow-based microfluidic chip was consisted of circular tree-like microchannels, which was able to generate eight flow streams containing stepwise chemical concentrations without the difference in flow velocity. Worms' upstream swimming into microchannels with various concentrations was monitored for quantitative analysis of the chemotaxis behavior. By using this microfluidic chip, the attractive and repellent responses of C. elegans to NaCl were successfully quantified within several minutes. The results demonstrated the wild type-like repellent responses and severely impaired attractive responses in grk-2 mutant animals with defects in calcium influx. In addition, the chemotaxis analysis of the third stage larvae revealed that its gustatory response was different from that in the adult stage. Thus, our microfluidic method provided a useful platform for studying the chemosensory behaviors of C. elegans and screening of chemosensation-related chemical drugs.

Small molecule-folic acid modification on nanopatterned PDMS and investigation on its surface property

Folic acid (or folate, FA) has attracted considerable attention for cancer therapy. As one small molecule, its receptor (folate receptor, FR) is significantly overexpressed on the surface of many human tumor cells compared with normal cells. In this work, physical and chemical coupled modification method, that is the combination of nanoimprinting technique and graft polymerization, was adopted to modify FA on nanopatterned polydimethylsiloxane (PDMS) surface for possible application in micro-nanofluidic cytology. The surface property of differently treated PDMS was characterized by FTIR, AFM and contact angle measurement. AO/PI double staining, cell counting and MTT method were performed to examine the potential influence of FA modified nanopatterned PDMS on human cervical carcinoma (HeLa) cell behavior. Both FA modification and nanostructure have positive effect on the growth and viability of HeLa cells. It is the first time that the small molecule-folic acid was used to immobilize on the surface of PDMS in order to improve its surface property.

Geometry-Driven Polarity in Motile Amoeboid Cells

Motile eukaryotic cells, such as leukocytes, cancer cells, and amoeba, typically move inside the narrow interstitial spacings of tissue or soil. While most of our knowledge of actin-driven eukaryotic motility was obtained from cells that move on planar open surfaces, recent work has demonstrated that confinement can lead to strongly altered motile behavior. Here, we report experimental evidence that motile amoeboid cells undergo a spontaneous symmetry breaking in confined interstitial spaces. Inside narrow channels, the cells switch to a highly persistent, unidirectional mode of motion, moving at a constant speed along the channel. They remain in contact with the two opposing channel side walls and alternate protrusions of their leading edge near each wall. Their actin cytoskeleton exhibits a characteristic arrangement that is dominated by dense, stationary actin foci at the side walls, in conjunction with less dense dynamic regions at the leading edge. Our experimental findings can be explained based on an excitable network model that accounts for the confinement-induced symmetry breaking and correctly recovers the spatio-temporal pattern of protrusions at the leading edge. Since motile cells typically live in the narrow interstitial spacings of tissue or soil, we expect that the geometry-driven polarity we report here plays an important role for movement of cells in their natural environment.

A microfluidic technique to probe cell deformability

Here we detail the design, fabrication, and use of a microfluidic device to evaluate the deformability of a large number of individual cells in an efficient manner. Typically, data for ~10(2) cells can be acquired within a 1 hr experiment. An automated image analysis program enables efficient post-experiment analysis of image data, enabling processing to be complete within a few hours. Our device geometry is unique in that cells must deform through a series of micron-scale constrictions, thereby enabling the initial deformation and time-dependent relaxation of individual cells to be assayed. The applicability of this method to human promyelocytic leukemia (HL-60) cells is demonstrated. Driving cells to deform through micron-scale constrictions using pressure-driven flow, we observe that human promyelocytic (HL-60) cells momentarily occlude the first constriction for a median time of 9.3 msec before passaging more quickly through the subsequent constrictions with a median transit time of 4.0 msec per constriction. By contrast, all-trans retinoic acid-treated (neutrophil-type) HL-60 cells occlude the first constriction for only 4.3 msec before passaging through the subsequent constrictions with a median transit time of 3.3 msec. This method can provide insight into the viscoelastic nature of cells, and ultimately reveal the molecular origins of this behavior.

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.

Delineating the core regulatory elements crucial for directed cell migration by examining folic-acid-mediated responses

Dictyostelium discoideum shows chemotaxis towards folic acid (FA) throughout vegetative growth, and towards cAMP during development. We determined the spatiotemporal localization of cytoskeletal and signaling molecules and investigated the FA-mediated responses in a number of signaling mutants to further our understanding of the core regulatory elements that are crucial for cell migration. Proteins enriched in the pseudopods during chemotaxis also relocalize transiently to the plasma membrane during uniform FA stimulation. In contrast, proteins that are absent from the pseudopods during migration redistribute transiently from the PM to the cytosol when cells are globally stimulated with FA. These chemotactic responses to FA were also examined in cells lacking the GTPases Ras C and G. Although Ras and phosphoinositide 3-kinase activity were significantly decreased in Ras G and Ras C/G nulls, these mutants still migrated towards FA, indicating that other pathways must support FA-mediated chemotaxis. We also examined the spatial movements of PTEN in response to uniform FA and cAMP stimulation in phospholipase C (PLC) null cells. The lack of PLC strongly influences the localization of PTEN in response to FA, but not cAMP. In addition, we compared the gradient-sensing behavior of polarized cells migrating towards cAMP to that of unpolarized cells migrating towards FA. The majority of polarized cells make U-turns when the cAMP gradient is switched from the front of the cell to the rear. Conversely, unpolarized cells immediately extend pseudopods towards the new FA source. We also observed that plasma membrane phosphatidylinositol 3,4,5-trisphosphate [PtdIns(3,4,5)P3] levels oscillate in unpolarized cells treated with Latrunculin-A, whereas polarized cells had stable plasma membrane PtdIns(3,4,5)P3 responses toward the chemoattractant gradient source. Results were similar for cells that were starved for 4 hours, with a mixture of polarized and unpolarized cells responding to cAMP. Taken together, these findings suggest that similar components control gradient sensing during FA- and cAMP-mediated motility, but the response of polarized cells is more stable, which ultimately helps maintain their directionality.

On-chip open microfluidic devices for chemotaxis studies

Microfluidic devices can provide unique control over both the chemoattractant gradient and the migration environment of the cells. Our work incorporates laser-machined micro and nanofluidic channels into bulk fused silica and cover slip-sized silica wafers. We have designed “open” chemotaxis devices that produce passive chemoattractant gradients without an external micropipette system. Since the migration area is unobstructed, cells can be easily loaded and strategically placed into the devices with a standard micropipette. The reusable monolithic glass devices have integral ports that can generate multiple gradients in a single experiment. We also used cover slip microfluidics for chemotaxis assays. Passive gradients elicited from these cover slips could be readily adapted for high throughput chemotaxis assays.We have also demonstrated for the first time that cells can be recruited into cover slip ports eliciting passive chemoattractant gradients. This proves, in principle, that intravital cover slip configurations could deliver controlled amounts of drugs, chemicals, or pathogens as well as recruit cells for proteomic or histological analysis in living animals while under microscopic observation. Intravital cover slip fluidics will create a new paradigm for in vivo observation of biological processes.

How do control-based approaches enter into biology?

Control is intrinsic to biological organisms, whose cells are in a constant state of sensing and response to numerous external and self-generated stimuli. Diverse means are used to study the complexity through control-based approaches in these cellular systems, including through chemical and genetic manipulations, input-output methodologies, feedback approaches, and feed-forward approaches. We first discuss what happens in control-based approaches when we are not actively examining or manipulating cells. We then present potential methods to determine what the cell is doing during these times and to reverse-engineer the cellular system. Finally, we discuss how we can control the cell's extracellular and intracellular environments, both to probe the response of the cells using defined experimental engineering-based technologies and to anticipate what might be achieved by applying control-based approaches to affect cellular processes. Much work remains to apply simplified control models and develop new technologies to aid researchers in studying and utilizing cellular and molecular processes.

Femtosecond laser machined microfluidic devices for imaging of cells during chemotaxis

Microfluidic devices designed for chemotaxis assays were fabricated on fused silica substrates using femtosecond laser micromachining. These devices have built-in chemical concentration gradient forming structures and are ideally suited for establishing passive diffusion gradients over extended periods of time. Multiple gradient forming structures, with identical or distinct gradient forming characteristics, can be integrated into a single device, and migrating cells can be directly observed using an inverted microscope. In this paper, the design, fabrication, and operation of these devices are discussed. Devices with minimal structure sizes ranging from 3 to 7 lm are presented. The use of these devices to investigate the migration of Dictyostelium discoideum cells toward the chemoattractant folic acid is presented as an example of the devices' utility.

Integrated and diffusion-based micro-injectors for open access cell assays

Currently, most microfluidic devices are fabricated with embedded micro-channels and other elements in a close form with outward connections. Although much functionality has been demonstrated and a large number of applications have been developed, they are not easy for routine operation in biology laboratories where most in vitro cell processing still relies on the use of culture dishes, glass slides, multi-well plates, tubes, pipettes, etc. We report here an open access device which consists of an array of isolated micro-channels plated on a large culture surface, each of them having tiny nozzles for localized drug delivery. In a diffusion dominant regime, steady gradients of molecule concentration could be obtained and varied by changing the flow rate inside the micro-channels. As assay examples, cell staining and drug-induced cell apoptosis were demonstrated, showing fast cell responses in close proximity of the nozzles.

A modular cell culture device for generating arrays of gradients using stacked microfluidic flows

Microfluidics has become increasingly important for the study of biochemical cues because it enables exquisite spatiotemporal control of the microenvironment. Well-characterized, stable, and reproducible generation of biochemical gradients is critical for understanding the complex behaviors involved in many biological phenomena. Although many microfluidic devices have been developed which achieve these criteria, the ongoing challenge for these platforms is to provide a suitably benign and physiologically relevant environment for cell culture in a user-friendly format. To achieve this paradigm, microfluidic designs must consider the full scope of cell culture from substrate preparation, cell seeding, and long-term maintenance to properly observe gradient sensing behavior. In addition, designs must address the challenges associated with altered culture conditions and shear forces in flow-based devices. With this consideration, we have designed and characterized a microfluidic device based on the principle of stacked flows to achieve highly stable gradients of diffusible molecules over large areas with extremely low shear forces. The device utilizes a benign vacuum sealing strategy for reversible application to pre-established cell cultures. We apply this device to an existing culture of breast cancer cells to demonstrate the negligible effect of its shear flow on migratory behavior. Lastly, we extend the stacked-flow design to demonstrate its scalable architecture with a prototype device for generating an array of combinatorial gradients.