This work was supported by a grant from the Ministry of Science and Technology (105-2628-E-400-001-MY2), and the Ph.D. Program in Tissue Engineering and Regenerative Medicine, National Chung Hsing University and National Health Research Institutes.

1. Fabrication of a wafer mold by soft lithography
NOTE: Mask pattern data is available in our previous publication14.
<ol>
	<li>Dehydrate a 4-inch silicon wafer in a 120 &#176;C oven for 15 min.</li>
	<li>Spin coat 4 g of SU-8 2 negative photoresist onto a 4-inch silicon wafer at 1,000 rpm for 30 s to create a 5 &#181;m thick layer (layer #1).</li>
	<li>Soft bake layer #1 on a 65 &#176;C hotplate for 1 min and then transfer layer #1 to a 95 &#176;C hotplate for 3 min.</li>
	<li>Cool layer #1 to room temperature, place it onto the holder of the semi-automatic mask aligner, and align with the layer #1 chrome-plated photomask (capture-site layer).</li>
	<li>Expose layer #1 with 365 nm UV light at a dose of 150 mJ/cm2.</li>
	<li>Remove layer #1 from the aligner and post-bake on a 65 &#176;C hotplate for 1 min. Transfer layer #1 to a 95 &#176;C hotplate for 1 min.</li>
	<li>Cool layer #1 to room temperature. Immerse in a propylene glycol monomethyl ether acetate solution to wash away the uncrosslinked photoresist for 2 min. Gently dry with nitrogen gas to reveal a layer #1 alignment mark.</li>
	<li>Cover the layer #1 alignment mark by an adhesive tape, spin coat 4 g of SU-8 10 negative photoresist onto the layer #1 at 1,230 rpm for 30 s to create a 25 &#181;m thick layer #2.</li>
	<li>Remove the tape, soft bake layer #2 on a 65 &#176;C hotplate for 3 min, and then transfer layer #2 to a 95 &#176;C hotplate for 7 min.</li>
	<li>Cool layer #2 to room temperature, place layer #2 onto the holder of the semi-automatic mask aligner, and align the layer #2 chrome-plated photomask (bypass channel layer) to the layer #1 alignment mark.</li>
	<li>Expose layer #2 with 365 nm UV light at a dose of 200 mJ/cm2.</li>
	<li>Remove layer #2 from the aligner and post-bake on a 65 &#176;C hotplate for 1 min and transfer layer #2 to a 95 &#176;C hotplate for 3 min.</li>
	<li>Cool layer #2 to room temperature, and cover the layer #1 alignment mark by adhesive tape. Spin coat 4 g of SU-8 2050 negative photoresist onto layer #2 at 1,630 rpm for 30 s to create a 100 &#181;m thick layer #3.</li>
	<li>Remove the tape, soft bake layer #3 on a 65 &#176;C hotplate for 5 min, and then transfer layer #3 to a 95 &#176;C hotplate for 20 min.</li>
	<li>Cool layer #3 to room temperature, place layer #3 onto the holder of the semi-automatic mask aligner, and align the layer #3 chrome-plated photomask (culture chamber layer) to the layer #1 alignment mark.</li>
	<li>Expose layer #3 with 365 nm UV light at a dose of 240 mJ/cm2.</li>
	<li>Remove layer #3 from the aligner and post-bake on a 65 &#176;C hotplate for 5 min. Transfer layer #3 to a 95 &#176;C hotplate for 10 min.</li>
	<li>Cool layer #3 to room temperature. Immerse in a propylene glycol monomethyl ether acetate solution to washed away the uncrosslinked photoresist for 10 min, and gently dry with nitrogen gas.</li>
</ol>
2. PDMS device preparation for multiple single cell capture
<ol>
	<li>Place the wafer mold and the weighing dish containing 100 &#181;L of trichlorosilane in a desiccator (only for silanization) and apply a vacuum (-85 kPa) for 15 min. 
	NOTE: Silanize the wafer surface with trichlorosilane to create hydrophobic surface properties before PDMS castingso that it can effortlessly be peeled off from the wafer PDMS mold.</li>
	<li>Stop the vacuum, and then silanize the wafer mold in the desiccator (only for silanization) at 37 &#176;C for at least 1 h.</li>
	<li>Mix PDMS base and PDMS curing agent in a ratio of 10:1. Pour a total of 20 g of mixed PDMS onto the wafer mold in a 15 cm dish.</li>
	<li>Place the 15 cm dish into a desiccator and apply vacuum (-85 kPa) for 1.5 min. Then remove the 15 cm dish from the desiccator. Keep for 20 min at room temperature. Finally, remove residual air bubbles in PDMS with nitrogen gas.</li>
	<li>Place the 15 cm dish in an oven at 65 &#176;C for 2-4 h to cure PDMS.</li>
	<li>Remove the PDMS replica from the wafer mold, and then punch a 1.5 mm inlet and a 0.5 mm outlet on the PDMS using a 1.5 mm inner diameter and a 0.5 mm inner diameter puncher (Figure 1C).</li>
	<li>Clean the PDMS replica and the slide surface with removable tape and then treat the surface with oxygen plasma (100 W for 14 s).</li>
	<li>Manually align the PDMS replicas with the slide and bring them into contact with each other.</li>
	<li>Place the PDMS slide in a 65 &#176;C oven for 1 day. 
	NOTE: Permanent bonding between the slide and the PDMS replica is achieved to form the device.</li>
	<li>Immerse the PDMS device in a container filled with phosphate buffered saline and place into a desiccator. Then apply vacuum (-85 kPa) for 15 min to remove air bubbles.</li>
	<li>Place the PDMS device in a cell culture hood and sterilize the device with UV light (light wavelength: 254 nm) for 30 min.</li>
	<li>Replace the PDMS device buffer with medium (DMEM-F12 basal medium containing 1% antibiotic and 10% fetal bovine serum) and incubate the PDMS device at 4 &#176;C for 1 day. This prevents cells from adhering to the PDMS surface.</li>
</ol>
3. Preparation of a single-cell suspension
NOTE: Cell types include human lymphatic endothelial cells (LECs), human OSCC TW2.6 cells expressing WNT5B-specific shRNA (WNT5B sh4) and vector control (pLKO-GFP) which were obtained from our previous study15. Please refer to our previous publication for detailed cultivation steps.
<ol>
	<li>Remove the culture medium when the cells achieve 70-80% confluence. Then gently wash the cells with 5 mL of sterile PBS three times.</li>
	<li>Add 1 mL of DMEM-F12 medium containing 1 &#181;M fluorescent dye into WNT5B sh4 and pLKO-GFP cells (use MV2 medium for LECs) and then incubate the cells for 30 min at room temperature. 
	NOTE: LECs were stained with green chloromethylfluorescein diacetate (CMFDA) Dye, WNT5B sh4 cells were stained with blue 7-amino-4-chloromethylcoumarin (CMAC) Dye and pLKO-GFP cells were stained with red Dil fluorescent dye.</li>
	<li>Gently wash the cells with 5 mL of sterile PBS three times.</li>
	<li>Remove the PBS and add 2 mL of 0.25% Trypsin-EDTA (0.05% Trypsin-EDTA for LECs).</li>
	<li>Incubate the cells for 4 min at room temperature and then gently tap the tissue culture dish to promote cells detachment.</li>
	<li>Add 4 mL of DMEM-F12 medium to disperse WNT5B sh4 and pLKO-GFP cells (For LECs use 3 mL of MV2 medium and 1 mL of trypsin neutralizer solution). Then transfer the cells into a 15 mL tube, and centrifuge at 300 x g for 3 min.</li>
	<li>Remove the supernatant, and resuspend the cell pellet in 1 mL of DMEM-F12 medium gently. Count the number of live cells in a hemocytometer by using the standard Trypan Blue exclusion method16. Prepare 1 mL of cell suspension at 3 x 105 cells/mL concentration in DMEM-F12 medium, and then keep cells on ice to prevent cell aggregation. 
	NOTE: In order to improve single-cell capture efficiency, careful preparation of the single cell suspension with well-dissociated is required.</li>
</ol>
4. Multiple single-cell capture and triple single-cell culture
<ol>
	<li>Connect a poly-tetrafluoroethene (PTFE) tube between the outlet of the device and syringe pump. Remove the medium and add 1 &#181;L of cell suspension at a concentration of 3 x 105 cells/mL into the inlet of the PDMS device.</li>
	<li>Load the cell suspension into the device by a syringe pump at a flow rate of 0.3 &#181;L/min (Figure 2A). Flow direction is from the inlet to the outlet. 
	NOTE: Load immediately after adding the cell suspension into the inlet to prevent cell sedimentation.</li>
	<li>Add 1 &#181;L of DMEM-F12 medium into the inlet of the PDMS device after step 4.2.Load the DMEM-F12 medium into the device by a syringe pump at a flow rate of 0.3 &#181;L/min (Figure 2B). Flow direction was flowing from the inlet to the outlet.</li>
	<li>Load 0.3 &#181;L of DMEM-F12 medium into the device by a syringe pump at a flow rate of 10 &#181;L/min (Figure 2C). Flow direction was flowing from the outlet to the inlet.</li>
	<li>Repeat steps 4.1 to 4.4 to load other cell types into the device.</li>
	<li>After completing the cell capture, use a microscope with 4x lens to image each culture chamber. 
	NOTE: The fluorescence emissions of the cells was used to identify and count the number of individual cells in each culture chamber.</li>
	<li>Remove the PTFE tube and seal the inlet and the outlet with polyolefin tape to create a closed culture system.</li>
	<li>Move the PDMS device to a 10 cm culture dish and add 10 mL of sterile PBS around the PDMS device to avoid evaporation of the medium from the device.</li>
	<li>Transfer the culture dish to an incubator (37 &#176; C, 5% CO2 and 95% humidity) for triple single-cell culture.</li>
	<li>Microscopically observe and photograph cell growth every 12 h.</li>
</ol>

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The authors declare that they have no competing financial interests.

The intercellular interactions of various cells in the tumor microenvironment play an important role in the progression of the tumor17. In order to understand the mechanism of cell-cell interactions, co-culture systems are used as a common analytical method. However, multiple cell types and the heterogeneity of the cells themselves have led to experimental complexity and analytical difficulties.
The hydrodynamic shuttling chip allows multiple single-cell loading in the ...

Efficient capture of different types of single cells and providing sufficient culture space are needed for single cell co-culture experiments of multiple types of single cells1. Limiting dilution is the most commonly used method to prepare the single cells for such experiments, due to the low cost of equipment required. However, due to the Poisson distribution limitation, the maximum single cell acquisition probability is only 37%, making the experimental operation laborious and time-consuming2. In contrast, using fluorescence activated cell sorting (FACS) can overcome the Poisson distribution limitation to high-efficiently prepare single cells3. However, FACS may not be accessible to some laboratories due to expensive instrumentation and maintenance cost. Microfabricated devices have been recently developed for single cell trapping4, single cell pairing5, and single cell culture applications. These devices are advantageous based on their ability to accurately manipulate single cells6, perform high-throughput experiments, or reduce sample and reagent consumption. However, performing single-cell co-culture experiments with multiple cell types with the current microfluidic devices is still challenging due the limitation of Poisson distribution1,7,8, or inability of the devices to capture more than two types of single cells4,5,6,9,10.
For example, Yoon et al. reported a microfluidic device for cell-cell interaction study11. This device uses the probabilistic method to pair cells in one chamber. However, it can only achieve the pairing of two different cell types due to geometric restrictions in the device structure. Another report from Lee et al. demonstrated a deterministic method to capture and pair single cells12. This device is increases pairing efficiency by the deterministic method but it is limited by the prolonged operation time required to pair cells. Specifically, the second cell capture can only be performed after the first captured cell is attached to the surface after 24 h. Zhao et al. reported a droplet-based microfluidic device to capture two types of a single cell13. We can found that the droplet-based microfluidic device is still limited to the Poisson distribution and can only be used on non-attached cells, and it is not possible to change the culture solution during the cultivation process.
Previously, we have developed a microfluidic &#34;hydrodynamic shuttling chip&#34; that utilizes deterministic hydrodynamic forces to capture multiple types of single-cell into the culture chamber and can subsequently perform cell co-culture experiment to analyze individual cell migration behavior under cell-cell interactions14. The hydrodynamic shuttling chip comprises an arrayed sets of units that each contains a serpentine by-pass channel, a capture-site, and a culture chamber. By using the difference in flow resistance between the serpentine by-pass channel and the culture chamber, and a specially designed operation procedure, different types of single cells can be repeatedly captured into the culture chamber. Notably, the ample space of the culture chamber can not only prevent the cell from being flushed during cell capture out but also provide sufficient space for the cells to spread, proliferate and migrate, allowing for observing of live single-cell interactions. In this article, we focus on the production of this device and detailed protocol steps.

<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>3M Advanced Polyolefin Diagnostic Microfluidic Medical Tape</td>
			<td>3M Company</td>
			<td>9795R</td>
			<td></td>
		</tr>
		<tr>
			<td>Antibiotics</td>
			<td>Biowest</td>
			<td>L0014-100</td>
			<td>Glutamine-Penicillin-Streptomycin</td>
		</tr>
		<tr>
			<td>AutoCAD software</td>
			<td>Autodesk</td>
			<td>AutoCAD LT 2011</td>
			<td>Part No. 057C1-74A111-1001</td>
		</tr>
		<tr>
			<td>CellTracke Blue CMAC Dye</td>
			<td>Invitrogen</td>
			<td>C2110</td>
			<td></td>
		</tr>
		<tr>
			<td>CellTracker Green CMFDA Dye</td>
			<td>Invitrogen</td>
			<td>C7025</td>
			<td></td>
		</tr>
		<tr>
			<td>Conventional oven</td>
			<td>YEONG-SHIN company</td>
			<td>ovp45</td>
			<td></td>
		</tr>
		<tr>
			<td>Desiccator</td>
			<td>Bel-Art Products</td>
			<td>F42020-0000</td>
			<td>Space saver vacuum desiccator 190 mm white base</td>
		</tr>
		<tr>
			<td>DiIC12(3) cell membrane dye</td>
			<td>BD Biosciences</td>
			<td>354218</td>
			<td>Used as a cell tracker</td>
		</tr>
		<tr>
			<td>DMEM-F12 medium</td>
			<td>Gibco</td>
			<td>11320-082</td>
			<td></td>
		</tr>
		<tr>
			<td>Endothelial Cell Growth Medium MV 2</td>
			<td>PromoCell</td>
			<td>C-22022</td>
			<td></td>
		</tr>
		<tr>
			<td>Fetal bovine serum Hyclone</td>
			<td>Thermo</td>
			<td>SH30071.03HI</td>
			<td></td>
		</tr>
		<tr>
			<td>Hamilton 700 series Glass syringe ( 0.1 ml )</td>
			<td>Hamilton</td>
			<td>80630</td>
			<td>100 &#181;L, Model 710 RN SYR, Small Removable NDL, 22s ga, 2 in, point style 2</td>
		</tr>
		<tr>
			<td>Harris Uni-Core puncher</td>
			<td>Ted Pella Inc.</td>
			<td>15075</td>
			<td>with 1.5mm inner-diameter</td>
		</tr>
		<tr>
			<td>Harris Uni-Core puncher</td>
			<td>Ted Pella Inc.</td>
			<td>15071</td>
			<td>with 0.5mm inner-diameter</td>
		</tr>
		<tr>
			<td>Hotplate</td>
			<td>YOTEC company</td>
			<td>YS-300S</td>
			<td></td>
		</tr>
		<tr>
			<td>Msak aligner</td>
			<td>Deya Optronic CO.</td>
			<td>A1K-5-MDA</td>
			<td></td>
		</tr>
		<tr>
			<td>Oxygen plasma</td>
			<td>NORDSON MARCH</td>
			<td>AP-300</td>
			<td></td>
		</tr>
		<tr>
			<td>Plasma cleaner</td>
			<td>Nordson</td>
			<td>AP-300</td>
			<td>Bench-Top Plasma Treatment System</td>
		</tr>
		<tr>
			<td>Polydimethylsiloxane (PDMS) kit</td>
			<td>Dow corning</td>
			<td>Sylgard 184</td>
			<td></td>
		</tr>
		<tr>
			<td>Poly-tetrafluoroethene (PTFE)</td>
			<td>Ever Sharp Technology, Inc.</td>
			<td>TFT-23T</td>
			<td>inner diameter, 0.51 mm; outer diameter, 0.82 mm</td>
		</tr>
		<tr>
			<td>Removable tape</td>
			<td>3M Company</td>
			<td>Scotch Removable Tape 811</td>
			<td></td>
		</tr>
		<tr>
			<td>Silicon wafer</td>
			<td>Eltech corperation</td>
			<td>SPE0039</td>
			<td></td>
		</tr>
		<tr>
			<td>Spin coater</td>
			<td>Synrex Co., Ltd.</td>
			<td>SC-HMI 2&#34; ~ 6&#34;</td>
			<td></td>
		</tr>
		<tr>
			<td>Stereomicroscope</td>
			<td>Leica Microsystems</td>
			<td>Leica E24</td>
			<td></td>
		</tr>
		<tr>
			<td>SU-8 10 negative photoresist</td>
			<td>MicroChem</td>
			<td>Y131259</td>
			<td></td>
		</tr>
		<tr>
			<td>SU-8 2 negative photoresist</td>
			<td>MicroChem</td>
			<td>Y131240</td>
			<td></td>
		</tr>
		<tr>
			<td>SU-8 2050 negative photoresist</td>
			<td>MicroChem</td>
			<td>Y111072</td>
			<td></td>
		</tr>
		<tr>
			<td>SU-8 developer</td>
			<td>Grand Chemical Companies</td>
			<td>GP5002-000000-72GC</td>
			<td>Propylene glycol monomethyl ether acetate</td>
		</tr>
		<tr>
			<td>Syringe pump</td>
			<td>Harvard Apparatus</td>
			<td>703007</td>
			<td></td>
		</tr>
		<tr>
			<td>Trichlorosilane</td>
			<td>Gelest, Inc</td>
			<td>SIT8174.0</td>
			<td>Tridecafluoro-1,1,2,2-tetrahydrooctyl. Hazardous. Corrosive to the respiratory tract, reacts violently with water.</td>
		</tr>
		<tr>
			<td>Trypsin Neutralizer Solution</td>
			<td>Gibco</td>
			<td>R-002-100</td>
			<td></td>
		</tr>
	</tbody>
</table>

establishing single-cell based co-cultures in a deterministic manner with a microfluidic chip

Cell co-culture assays have been widely used for studying cell-cell interactions between different cell types to better understand the biology of diseases including cancer. However, it is challenging to clarify the complex mechanism of intercellular interactions in highly heterogeneous cell populations using conventional co-culture systems because the heterogeneity of the cell subpopulation is obscured by the average values; the conventional co-culture systems can only be used to describe the population signal, but are incapable of tracking individual cells behavior. Furthermore, conventional single-cell experimental methods have low efficiency in cell manipulation because of the Poisson distribution. Microfabricated devices are an emerging technology for single-cell studies because they can accurately manipulate single cells at high-throughput and can reduce sample and reagent consumption. Here, we describe the concept and application of a microfluidic chip for multiple single-cell co-cultures. The chip can efficiently capture multiple types of single cells in a culture chamber (~46%) and has a sufficient culture space useful to study the cells&#39; behavior (e.g., migration, proliferation, etc.) under cell-cell interaction at the single-cell level. Lymphatic endothelial cells and oral squamous cell carcinoma were used to perform a single-cell co-culture experiment on the microfluidic platform for live multiple single-cell interaction studies.

The device has a three-layer structure as shown by the cross-section photograph of a cut PDMS device (Figure 1A). The first layer contains a capture-site (6.0 &#181;m in width and 4.6 &#181;m in height) that connects the culture chamber and the by-pass channel. The difference in flow resistance between the culture chamber and the by-pass channel causes the cells to flow into the capture position and fill the entrance of the small path. After ...

Watch this Scientific Journal Video about Establishing Single-Cell Based Co-Cultures in a Deterministic Manner with a Microfluidic Chip at JoVE.com

Establishing Single-Cell Based Co-Cultures in a Deterministic Manner with a Microfluidic Chip

This report describes a microfluidic chip-based method to set up a single cell culture experiment in which high-efficiency pairing and microscopic analysis of multiple single cells can be achieved.

Cell co-culture assays have been widely used for studying cell-cell interactions between different cell types to better understand the biology of diseases ...

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Research

JoVE Journal

Bioengineering

5.9K Views.  National Health Research Institutes. This report describes a microfluidic chip-based method to set up a single cell culture experiment in which high-efficiency pairing and microscopic analysis of multiple single cells can be achieved.

Video: Establishing Single-Cell Based Co-Cultures in a Deterministic Manner with a Microfluidic Chip

A Microfluidic-based Hydrodynamic Trap for Single Particles

The ability to confine and manipulate single particles in free solution is a key enabling technology for fundamental and applied science. Methods for particle trapping based on optical, magnetic, electrokinetic, and acoustic techniques have led to major advancements in physics and biology ranging from the molecular to cellular level. In this article, we introduce a new microfluidic-based technique for particle trapping and manipulation based solely on hydrodynamic fluid flow. Using this method, we demonstrate trapping of micro- and nano-scale particles in aqueous solutions for long time scales. The hydrodynamic trap consists of an integrated microfluidic device with a cross-slot channel geometry where two opposing laminar streams converge, thereby generating a planar extensional flow with a fluid stagnation point (zero-velocity point). In this device, particles are confined at the trap center by active control of the flow field to maintain particle position at the fluid stagnation point. In this manner, particles are effectively trapped in free solution using a feedback control algorithm implemented with a custom-built LabVIEW code. The control algorithm consists of image acquisition for a particle in the microfluidic device, followed by particle tracking, determination of particle centroid position, and active adjustment of fluid flow by regulating the pressure applied to an on-chip pneumatic valve using a pressure regulator. In this way, the on-chip dynamic metering valve functions to regulate the relative flow rates in the outlet channels, thereby enabling fine-scale control of stagnation point position and particle trapping. The microfluidic-based hydrodynamic trap exhibits several advantages as a method for particle trapping. Hydrodynamic trapping is possible for any arbitrary particle without specific requirements on the physical or chemical properties of the trapped object. In addition, hydrodynamic trapping enables confinement of a "single" target object in concentrated or crowded particle suspensions, which is difficult using alternative force field-based trapping methods. The hydrodynamic trap is user-friendly, straightforward to implement and may be added to existing microfluidic devices to facilitate trapping and long-time analysis of particles. Overall, the hydrodynamic trap is a new platform for confinement, micromanipulation, and observation of particles without surface immobilization and eliminates the need for potentially perturbative optical, magnetic, and electric fields in the free-solution trapping of small particles.

In this article, we present a microfluidic-based method for particle confinement based on hydrodynamic flow. We demonstrate stable particle trapping at a fluid stagnation point using a feedback control mechanism, thereby enabling confinement and micromanipulation of arbitrary particles in an integrated microdevice.

The ability to confine and manipulate single particles in free solution is a key enabling technology for fundamental and applied science.  Methods for ...

A Microfluidic Chip for the Versatile Chemical Analysis of Single Cells

We present a microfluidic device that enables the quantitative determination of intracellular biomolecules in multiple single cells in parallel. For this purpose, the cells are passively trapped in the middle of a microchamber. Upon activation of the control layer, the cell is isolated from the surrounding volume in a small chamber. The surrounding volume can then be exchanged without affecting the isolated cell. However, upon short opening and closing of the chamber, the solution in the chamber can be replaced within a few hundred milliseconds. Due to the reversibility of the chambers, the cells can be exposed to different solutions sequentially in a highly controllable fashion, e.g. for incubation, washing, and finally, cell lysis. The tightly sealed microchambers enable the retention of the lysate, minimize and control the dilution after cell lysis. Since lysis and analysis occur at the same location, high sensitivity is retained because no further dilution or loss of the analytes occurs during transport. The microchamber design therefore enables the reliable and reproducible analysis of very small copy numbers of intracellular molecules (attomoles, zeptomoles) released from individual cells. Furthermore, many microchambers can be arranged in an array format, allowing the analysis of many cells at once, given that suitable optical instruments are used for monitoring. We have already used the platform for proof-of-concept studies to analyze intracellular proteins, enzymes, cofactors and second messengers in either relative or absolute quantifiable manner.

In this article we present a microfluidic chip for single cell analysis. It allows the quantification of intracellular proteins, enzymes, cofactors, and second messengers by means of fluorescent assays or immunoassays.&#160;

We present a microfluidic device that enables the quantitative determination of intracellular biomolecules in multiple single cells in parallel. For this ...

Microfluidic Fabrication of Polymeric and Biohybrid Fibers with Predesigned Size and Shape

A &#8220;sheath&#8221; fluid passing through a microfluidic channel at low Reynolds number can be directed around another &#8220;core&#8221; stream and used to dictate the shape as well as the diameter of a core stream.&#160;Grooves in the top and bottom of a microfluidic channel were designed to direct the sheath fluid and shape the core fluid.&#160;By matching the viscosity and hydrophilicity of the sheath and core fluids, the interfacial effects are minimized and complex fluid shapes can be formed.&#160;Controlling the relative flow rates of the sheath and core fluids determines the cross-sectional area of the core fluid.&#160;Fibers have been produced with sizes ranging from 300 nm to ~1 mm, and fiber cross-sections can be round, flat, square, or complex as in the case with double anchor fibers. Polymerization of the core fluid downstream from the shaping region solidifies the fibers.&#160;Photoinitiated click chemistries are well suited for rapid polymerization of the core fluid by irradiation with ultraviolet light.&#160;Fibers with a wide variety of shapes have been produced from a list of polymers including liquid crystals, poly(methylmethacrylate), thiol-ene and thiol-yne resins, polyethylene glycol, and hydrogel derivatives. Minimal shear during the shaping process and mild polymerization conditions also makes the fabrication process well suited for encapsulation of cells and other biological components.

Two adjacent fluids passing through a grooved microfluidic channel can be directed to form a sheath around a prepolymer core; thereby&#160;determining both shape and cross-section. Photoinitiated polymerization, such as thiol click chemistry, is well suited for rapidly solidifying the core fluid into a microfiber with predetermined size and shape.

A &#8220;sheath&#8221; fluid passing through a microfluidic channel at low Reynolds number can be directed around another &#8220;core&#8221; stream and ...

A Microfluidic Chip for ICPMS Sample Introduction

This protocol discusses the fabrication and usage of a disposable low cost microfluidic chip as sample introduction system for inductively coupled plasma mass spectrometry (ICPMS). The chip produces monodisperse aqueous sample droplets in perfluorohexane (PFH). Size and frequency of the aqueous droplets can be varied in the range of 40 to 60 &#181;m and from 90 to 7,000&#160;Hz, respectively. The droplets are ejected from the chip with a second flow of PFH and remain intact during the ejection. A custom-built desolvation system removes the PFH and transports the droplets into the ICPMS. Here, very stable signals with a narrow intensity distribution can be measured, showing the monodispersity of the droplets. We show that the introduction system can be used to quantitatively determine iron in single bovine red blood cells. In the future, the capabilities of the introduction device can easily be extended by the integration of additional microfluidic modules.

We present a discrete droplet sample introduction system for inductively coupled plasma mass spectrometry (ICPMS). It is based on a cheap and disposable microfluidic chip that generates highly monodisperse droplets in a size range of 40&#8722;60 &#181;m at frequencies from 90 to 7,000 Hz.

This protocol discusses the fabrication and usage of a disposable low cost microfluidic chip as sample introduction system for inductively coupled plasma ...

The Multi-organ Chip - A Microfluidic Platform for Long-term Multi-tissue Coculture

The ever growing amount of new substances released onto the market and the limited predictability of current in vitro test systems has led to a high need for new solutions for substance testing. Many drugs that have been removed from the market due to drug-induced liver injury released their toxic potential only after several doses of chronic testing in humans. However, a controlled microenvironment is pivotal for long-term multiple dosing experiments,&#160;as even minor alterations in extracellular conditions may greatly influence the cell physiology. We focused within our research program on the generation of a microengineered bioreactor, which can be dynamically perfused by an on-chip pump and combines at least two culture spaces for multi-organ applications. This circulatory system mimics the in vivo conditions of primary cell cultures better and assures a steadier,&#160;more quantifiable extracellular relay of signals to the cells. 
For demonstration purposes, human liver equivalents, generated by aggregating differentiated HepaRG cells with human hepatic stellate cells in hanging drop plates, were cocultured with human skin punch biopsies for up to 28 days inside the microbioreactor. The use of cell culture inserts enables the skin to be cultured at an air-liquid interface, allowing topical substance exposure. The microbioreactor system is capable of supporting these cocultures at near physiologic fluid flow and volume-to-liquid ratios, ensuring stable and organotypic culture conditions. The possibility of long-term cultures enables the repeated exposure to substances. Furthermore, a vascularization of the microfluidic channel circuit using human dermal microvascular endothelial cells yields a physiologically more relevant vascular model.

Here, we present a protocol to coculture primary cells, tissue models and punch biopsies in a microfluidic multi-organ chip for up to 28 days. Human dermal microvascular endothelial cells, liver aggregates and skin biopsies were successfully combined in a common media circulation.

The ever growing amount of new substances released onto the market and the limited predictability of current in vitro test systems has led to a high need ...

Electrotaxis Studies of Lung Cancer Cells using a Multichannel Dual-electric-field Microfluidic Chip

The behavior of directional cell migration under a direct current electric-field (dcEF) is referred to as electrotaxis. The significant role of physiological dcEF in guiding cell movement during embryo development, cell differentiation, and wound healing has been demonstrated in many studies. By applying microfluidic chips to an electrotaxis assay, the investigation process is shortened and experimental errors are minimized. In recent years, microfluidic devices made of polymeric substances (e.g., polymethylmethacrylate, PMMA, or acrylic) or polydimethylsiloxane (PDMS) have been widely used in studying the responses of cells to electrical stimulation. However, unlike the numerous steps required to fabricate a PDMS device, the simple and rapid construction of the acrylic micro&#64258;uidic chip makes it suitable for both device prototyping and production. Yet none of the reported devices facilitate the efficient study of the simultaneous chemical and dcEF effects on cells. In this report, we describe our design and fabrication of an acrylic-based multichannel dual-electric-field (MDF) chip to investigate the concurrent effect of chemical and electrical stimulation on lung cancer cells. The MDF chip provides eight combinations of electrical/chemical stimulations in a single test. The chip not only greatly shortens the required experimental time but also increases accuracy in electrotaxis studies.

Many microfluidic devices have been developed for use in the study of electrotaxis. Yet, none of these chips allows the efficient study of the simultaneous chemical and electric-field (EF) effects on cells. We developed a polymethylmethacrylate-based device that offers better-controlled coexisting EF and chemical stimulation for use in electrotaxis research.

The behavior of directional cell migration under a direct current electric-field (dcEF) is referred to as electrotaxis. The significant role of ...

A Microfluidic Platform for High-throughput Single-cell Isolation and Culture

Studying the heterogeneity of single cells is crucial for many biological questions, but is technically difficult. Thus, there is a need for a simple, yet high-throughput, method to perform single-cell culture experiments. Here, we report a microfluidic chip-based strategy for high-efficiency single-cell isolation (~77%) and demonstrate its capability of performing long-term single-cell culture (up to 7 d) and cellular heterogeneity analysis using clonogenic assay. These applications were demonstrated with KT98 mouse neural stem cells, and A549 and MDA-MB-435 human cancer cells. High single-cell isolation efficiency and long-term culture capability are achieved by using different sizes of microwells on the top and bottom of the microfluidic channel. The small microwell array is designed for precisely isolating single-cells, and the large microwell array is used for single-cell clonal culture in the microfluidic chip. This microfluidic platform constitutes an attractive approach for single-cell culture applications, due to its flexibility of adjustable cell culture spaces for different culture strategies, without decreasing isolation efficiency.

Here, we present a protocol for isolating and culturing single cells with a microfluidic platform, which utilizes a new microwell design concept to allow for high-efficiency single cell isolation and long-term clonal culture.

Studying the heterogeneity of single cells is crucial for many biological questions, but is technically difficult. Thus, there is a need for a simple, yet ...

Co-culture of Living Microbiome with Microengineered Human Intestinal Villi in a Gut-on-a-Chip Microfluidic Device

Here, we describe a protocol to perform long-term co-culture of multi-species human gut microbiome with microengineered intestinal villi in a human gut-on-a-chip microphysiological device. We recapitulate the intestinal lumen-capillary tissue interface in a microfluidic device, where physiological mechanical deformations and fluid shear flow are constantly applied to mimic peristalsis. In the lumen microchannel, human intestinal epithelial Caco-2 cells are cultured to form a &#39;germ-free&#39; villus epithelium and regenerate small intestinal villi. Pre-cultured microbial cells are inoculated into the lumen side to establish a host-microbe ecosystem. After microbial cells adhere to the apical surface of the villi, fluid flow and mechanical deformations are resumed to produce a steady-state microenvironment in which fresh culture medium is constantly supplied and unbound bacteria (as well as bacterial wastes) are continuously removed. After extended co-culture from days to weeks, multiple microcolonies are found to be randomly located between the villi, and both microbial and epithelial cells remain viable and functional for at least one week in culture. Our co-culture protocol can be adapted to provide a versatile platform for other host-microbiome ecosystems that can be found in various human organs, which may facilitate in vitro study of the role of human microbiome in orchestrating health and disease.

We describe an in vitro protocol to co-culture gut microbiome and intestinal villi for an extended period using a human gut-on-a-chip microphysiological system.

Here, we describe a protocol to perform long-term co-culture of multi-species human gut microbiome with microengineered intestinal villi in a human ...

Image-guided, Laser-based Fabrication of Vascular-derived Microfluidic Networks

This detailed protocol outlines the implementation of image-guided, laser-based hydrogel degradation for the fabrication of vascular-derived microfluidic networks embedded in PEGDA hydrogels. Here, we describe the creation of virtual masks that allow for image-guided laser control; the photopolymerization of a micromolded PEGDA hydrogel, suitable for microfluidic network fabrication and pressure head-driven flow; the setup and use of a commercially available laser scanning confocal microscope paired with a femtosecond pulsed Ti:S laser to induce hydrogel degradation; and the imaging of fabricated microfluidic networks using fluorescent species and confocal microscopy. Much of the protocol is focused on the proper setup and implementation of the microscope software and microscope macro, as these are crucial steps in using a commercial microscope for microfluidic fabrication purposes that contain a number of intricacies. The image-guided component of this technique allows for the implementation of 3D image stacks or user-generated 3D models, thereby allowing for creative microfluidic design and for the fabrication of complex microfluidic systems of virtually any configuration. With an expected impact in tissue engineering, the methods outlined in this protocol could aid in the fabrication of advanced biomimetic microtissue constructs for organ- and human-on-a-chip devices. By mimicking the complex architecture, tortuosity, size, and density of in vivo vasculature, essential biological transport processes can be replicated in these constructs, leading to more accurate in vitro modeling of drug pharmacokinetics and disease.

This protocol outlines the implementation of image-guided, laser-based hydrogel degradation to fabricate vascular-derived, biomimetic microfluidic networks embedded in poly(ethylene glycol) diacrylate (PEGDA) hydrogels. These biomimetic microfluidic systems may be useful for tissue engineering applications, generation of in vitro disease models, and fabrication of advanced "on-a-chip" devices.

This detailed protocol outlines the implementation of image-guided, laser-based hydrogel degradation for the fabrication of vascular-derived microfluidic ...

Dry Film Photoresist-based Electrochemical Microfluidic Biosensor Platform: Device Fabrication, On-chip Assay Preparation, and System Operation

In recent years, biomarker diagnostics became an indispensable tool for the diagnosis of human disease, especially for the point-of-care diagnostics. An easy-to-use and low-cost sensor platform is highly desired to measure various types of analytes (e.g., biomarkers, hormones, and drugs) quantitatively and specifically. For this reason, dry film photoresist technology - enabling cheap, facile, and high-throughput fabrication - was used to manufacture the microfluidic biosensor presented here. Depending on the bioassay used afterwards, the versatile platform is capable of detecting various types of biomolecules. For the fabrication of the device, platinum electrodes are structured on a flexible polyimide (PI) foil in the only clean-room process step. The PI foil serves as a substrate for the electrodes, which are insulated with an epoxy-based photoresist. The microfluidic channel is subsequently generated by the development and lamination of dry film photoresist (DFR) foils onto the PI wafer. By using a hydrophobic stopping barrier in the channel, the channel is separated into two specific areas: an immobilization section for the enzyme-linked assay and an electrochemical measurement cell for the amperometric signal readout.
The on-chip bioassay immobilization is performed by the adsorption of the biomolecules to the channel surface. The glucose oxidase enzyme is used as a transducer for electrochemical signal generation. In the presence of the substrate, glucose, hydrogen peroxide is produced, which is detected at the platinum working electrode. The stop-flow technique is applied to obtain signal amplification along with rapid detection. Different biomolecules can quantitatively be measured by means of the introduced microfluidic system, giving an indication of different types of diseases, or, in regard to therapeutic drug monitoring, facilitating a personalized therapy.

A microfluidic biosensor platform was designed and fabricated using low-cost dry film photoresist technology for the rapid and sensitive quantification of various analytes. This single-use system allows for the electrochemical readout of on-chip-immobilized enzyme-linked assays by means of the stop-flow technique.

In recent years, biomarker diagnostics became an indispensable tool for the diagnosis of human disease, especially for the point-of-care diagnostics. An ...