Two-Aperture Microfluidic Probes as Flow Dipole: Theory and Applications

Open-Top Patterned Hydrogel-Laden 3D Glioma Cell Cultures for Creation of Dynamic Chemotactic Gradients to Direct Cell Migration

The laminar flow profiles in microfluidic systems coupled to rapid diffusion at flow streamlines have been widely utilized to create well-controlled chemical gradients in cell cultures for spatially directing cell migration. However, within hydrogel-based closed microfluidic systems of limited depth (≤0.1 mm), the biomechanical cues for the cell culture are dominated by cell interactions with channel surfaces rather than with the hydrogel microenvironment. Also, leaching of poly(dimethylsiloxane) (PDMS) constituents in closed systems and the adsorption of small molecules to PDMS alter chemotactic profiles. To address these limitations, we present the patterning and integration of a PDMS-free open fluidic system, wherein the cell-laden hydrogel directly adjoins longitudinal channels that are designed to create chemotactic gradients across the 3D culture width, while maintaining uniformity across its ∼1 mm depth to enhance cell-biomaterial interactions. This hydrogel-based open fluidic system is assessed for its ability to direct migration of U87 glioma cells using a hybrid hydrogel that includes hyaluronic acid (HA) to mimic the brain tumor microenvironment and gelatin methacrylate (GelMA) to offer the adhesion motifs for promoting cell migration. Chemotactic gradients to induce cell migration across the hydrogel width are assessed using the chemokine CXCL12, and its inhibition by AMD3100 is validated. This open-top hydrogel-based fluidic system to deliver chemoattractant cues over square-centimeter-scale areas and millimeter-scale depths can potentially serve as a robust screening platform to assess emerging glioma models and chemotherapeutic agents to eradicate them.

Emerging open microfluidics for cell manipulation

Cell manipulation is the foundation of biochemical studies, which demands user-friendly, multifunctional and precise tools. Based on flow confinement principles, open microfluidics can control the movement of microscale liquid in open space. Every position of the circuit is accessible to external instruments, making it possible to perform precise treatment and analysis of cells at arbitrary target positions especially at the single-cell/sub-cell level. Benefiting from its unique superiority, various manipulations including patterned cell culture, 3D tissue modelling, localized chemical stimulation, online cellular factor analysis, single cell sampling, partial cell treatment, and subcellular free radical attack can be easily realized. In this tutorial review, we summarize two basic ideas to design open microfluidics: open microfluidic networks and probes. The principles of mainstream open microfluidic methods are explained, and their recent important applications are introduced. Challenges and developing trends of open microfluidics are also discussed.

A Microfluidic Probe Integrated Device for Spatiotemporal 3D Chemical Stimulation in Cells

Numerous in vitro studies have been conducted in conventional static cell culture systems. However, most of the results represent an average response from a population of cells regardless of their local microenvironment. A microfluidic probe is a non-contact technology that has been widely used to perform local chemical stimulation within a restricted space, providing elaborated modulation and analysis of cellular responses within the microenvironment. Although microfluidic probes developed earlier have various potential applications, the two-dimensional structure can compromise their functionality and flexibility for practical use. In this study, we developed a three-dimensional microfluidic probe integrated device equipped with vertically oriented microchannels to overcome crucial challenges and tested the potential utility of the device in biological research. We demonstrated that the device tightly regulated spatial diffusion of a fluorescent molecule, and the flow profile predicted by simulation replicated the experimental results. Additionally, the device modulated the physiological Ca²⁺ response of cells within the restricted area by altering the local and temporal concentrations of biomolecules such as ATP. The novel device developed in this study may provide various applications for biological studies and contribute to further understanding of molecular mechanisms underlying cellular physiology.

Microelectrofluidic probe for sequential cell separation and patterning

Cell separation and patterning are of interest to several biological and medical applications including rare cell isolation and co-culture models. Numerous microfluidic devices have been used for cell separation and patterning, however, the typical closed channel configuration comes with challenges and limitations. Here, we report a dielectrophoresis (DEP) enabled microelectrofluidic probe (MeFP) for sequentially separating and patterning of mammalian cells in an open microfluidic system. The MeFP is a microfluidic probe with injection and aspiration apertures, integrated with an array of micro-hump electrodes on its tip. Aligning the MeFP parallel, and in close proximity, to a conductive substrate forms a vertical pin-plate electrode configuration that allows for an integration of DEP forces within the hydrodynamic flow confinement. Upon confining a heterogeneous cell suspension in the gap between the MeFP and the substrate, target cells are selectively captured on the micro-hump electrodes using positive DEP forces, and then deposited on the substrate in defined patterns. Characterization of the MeFP showed an increase in cell-capture efficiency when the MeFP is of a higher microfluidic multipole configuration. Separation of cancer cells from T lymphocytes was demonstrated with capture purity as high as 89.6%. Deposited patterns of isolated cells match the numerically calculated particle trajectories of the evaluated microfluidic multipoles configurations. By adjusting the flow configuration of the MeFP, we show that the patterned co-culture of two different cell types can be dynamically controlled for homotypic and heterotypic cell interaction studies. This work presents a multifunctional microfluidic tool that bio-fabricates selective multicellular patterns directly on an open substrate without the need for confined conduits.

Open Microfluidic Capillary Systems

Open microfluidic capillary systems are a rapidly evolving branch of microfluidics where fluids are manipulated by capillary forces in channels lacking physical walls on all sides. Typical channel geometries include grooves, rails, or beams and complex systems with multiple air-liquid interfaces. Removing channel walls allows access for retrieval (fluid sampling) and addition (pipetting reagents or adding objects like tissue scaffolds) at any point in the channel; the entire channel becomes a "device-to-world" interface, whereas such interfaces are limited to device inlets and outlets in traditional closed-channel microfluidics. Open microfluidic capillary systems are simple to fabricate and reliable to operate. Prototyping methods (e.g., 3D printing) and manufacturing methods (e.g., injection molding) can be used seamlessly, accelerating development. This Perspective highlights fundamentals of open microfluidic capillary systems including unique advantages, design considerations, fabrication methods, and analytical considerations for flow; device features that can be combined to create a "toolbox" for fluid manipulation; and applications in biology, diagnostics, chemistry, sensing, and biphasic applications.

Microfluidic multipoles theory and applications

Microfluidic multipoles (MFMs) have been realized experimentally and hold promise for "open-space" biological and chemical surface processing. Whereas convective flow can readily be predicted using hydraulic-electrical analogies, the design of advanced microfluidic multipole is constrained by the lack of simple, accurate models to predict mass transport within them. In this work, we introduce the complete solutions to mass transport in multipolar microfluidics based on the iterative conformal mapping of 2D advection-diffusion around a simple edge into dipoles and multipolar geometries, revealing a rich landscape of transport modes. The models are validated experimentally with a library of 3D printed devices and found in excellent agreement. Following a theory-guided design approach, we further ideate and fabricate two classes of spatiotemporally reconfigurable multipolar devices that are used for processing surfaces with time-varying reagent streams, and to realize a multistep automated immunoassay. Overall, the results set the foundations for exploring, developing, and applying open-space microfluidic multipoles.

3D Printed Microfluidic Probes

In this work, we fabricate microfluidic probes (MFPs) in a single step by stereolithographic 3D printing and benchmark their performance with standard MFPs fabricated via glass or silicon micromachining. Two research teams join forces to introduce two independent designs and fabrication protocols, using different equipment. Both strategies adopted are inexpensive and simple (they only require a stereolithography printer) and are highly customizable. Flow characterization is performed by reproducing previously published microfluidic dipolar and microfluidic quadrupolar reagent delivery profiles which are compared to the expected results from numerical simulations and scaling laws. Results show that, for most MFP applications, printer resolution artifacts have negligible impact on probe operation, reagent pattern formation, and cell staining results. Thus, any research group with a moderate resolution (≤100 µm) stereolithography printer will be able to fabricate the MFPs and use them for processing cells, or generating microfluidic concentration gradients. MFP fabrication involved glass and/or silicon micromachining, or polymer micromolding, in every previously published article on the topic. We therefore believe that 3D printed MFPs is poised to democratize this technology. We contribute to initiate this trend by making our CAD files available for the readers to test our "print & probe" approach using their own stereolithographic 3D printers.

Spatially resolved microfluidic stimulation of lymphoid tissue ex vivo

The lymph node is a structurally complex organ of the immune system, whose dynamic cellular arrangements are thought to control much of human health. Currently, no methods exist to precisely stimulate substructures within the lymph node or analyze local stimulus-response behaviors, making it difficult to rationally design therapies for inflammatory disease. Here we describe a novel integration of live lymph node slices with a microfluidic system for local stimulation. Slices maintained the cellular organization of the lymph node while making its core experimentally accessible. The 3-layer polydimethylsiloxane device consisted of a perfusion chamber stacked atop stimulation ports fed by underlying microfluidic channels. Fluorescent dextrans similar in size to common proteins, 40 and 70 kDa, were delivered to live lymph node slices with 284 ± 9 μm and 202 ± 15 μm spatial resolution, respectively, after 5 s, which is sufficient to target functional zones of the lymph node. The spread and quantity of stimulation were controlled by varying the flow rates of delivery; these were predictable using a computational model of isotropic diffusion and convection through the tissue. Delivery to two separate regions simultaneously was demonstrated, to mimic complex intercellular signaling. Delivery of a model therapeutic, glucose-conjugated albumin, to specific regions of the lymph node indicated that retention of the drug was greater in the B-cell zone than in the T-cell zone. Together, this work provides a novel platform, the lymph node slice-on-a-chip, to target and study local events in the lymph node and to inform the development of new immunotherapeutics.

Stagnation point flows in analytical chemistry and life sciences

Isolated microfluidic stagnation points – formed within microfluidic interfaces – have come a long way as a tool for characterizing materials, manipulating micro particles, and generating confined flows and localized chemistries.