Name: one-step approach to fabricating polydimethylsiloxane microfluidic channels of different geometric sections by sequential wet etching processes
Uploaded: 2018-09-13T14:00:00
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The authors gratefully acknowledge the support provided by the National Health Research Institutes (NHRI) in Taiwan under the Innovative Research Grant (IRG) (EX106-10523EI), the Taiwan Ministry of Science and Technology (MOST 104-2218-E-032-004, 104-2221-E-001-015-MY3, 105-2221-E-001-002-MY2, 105-2221-E-032-006, 106-2221-E-032-018-MY2), and the Academia Sinica Career Development Award. The authors would like to thank Heng-Hua Hsu for proofreading the manuscript.

1. Fabrication of Microfluidic Devices with Single-Layer Channel Layouts
NOTE: In this paper, the soft lithography method3 is adopted for fabricating microfluidic devices made of PDMS materials, to demonstrate how to manufacture channels with various sections.
<ol>
	<li>Creation of master molds for a PDMS layer with designed topology features
	<ol>
		<li>Design channel layouts on a PDMS layer for a single etching process or etching in sequence.</li>
	...

<ol>
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Over the past decades, microfluidics has offered promising means by which experimental platforms for chemical and biomedical research can be constructed systematically1,2,3,4,5. The platforms have also presented their capabilities of investigating several cellular functions in vivo under physiological microenvironment conditions via in vitro ...

Microfluidic techniques have drawn attention over the past decades because of their intrinsic advantages for a variety of biomedical and chemical research and applications. Several material usage options for constructing microfluidic chips are available nowadays, such as polymers, ceramics, and silicon materials. To the best of our knowledge, among the microfluidic materials, PDMS is the most common one due to its appropriate material properties for various microfluidics research and applications, including its optical and biological compatibilities with particles, fluids, and extremely small living organisms1,2,3,4,5. Furthermore, the surface chemical and structure mechanical properties of PDMS materials can be adjusted to facilitate microelectromechanical and mechanobiological studies by applying such polymer-based microfluidic devices10,11,12. Concerning the manufacturing of microfluidic devices with designed channel patterns, soft lithography replica molding methods are usually applied to create the microfluidic channels by utilizing their corresponding master molds which are composed of photolithography-patterned photoresist layers and silicon wafer substrates12. Owing to the nature of molding approaches using silicon wafers with patterned photoresist layers, the microfluidic channels commonly have regular cross sections of rectangular shapes with identical heights.
Recently, researchers have made significant progress in biomedical studies which deal with, for instance, sorting particles and cells using hydrophoresis, separating blood plasma, and enriching white blood cells by applying microfluidic chips with channels of different heights or geometric sections6,7,8,9. Such sorting and separating functions of microfluidics for biomedical applications are realized by customizing channels with different geometric sections. Several studies have been devoted to the manufacture of microfluidic channels with cross sections of different geometry features by fabricating master molds with specific surface patterns of various heights or non-rectangular cross sections. These studies on mold fabrication include such techniques as multi-step photolithography, photoresist reflow, and grey-scale lithography13,14,15. Inevitably, the existing techniques involve finely crafted photomasks or a precise alignment in multi-step manufacturing processes, which may substantially enhance the complexity levels of the corresponding fabrication of microfluidic channels. So far, several attempts have been made on single-step manufacturing processes for microfluidic channels of various sections, but the respective techniques are highly restricted to specific cross-sectional shapes of channels16.
Over the past two decades, in addition to the molding approaches for fabricating PDMS microfluidic channels with various sections, etching techniques for patterning PDMS channels with geometric features have become the fabrication of choice in a variety of microfluidic applications. For instance, PDMS wet etching is exploited along with multi-layer PDMS bonding for constructing a pneumatic actuated cell culture device of microfluidics with reconstituted organ-level lung functions17. The PDMS wet etching technique is employed together with PDMS casting on cylindrical microwells machined by computer-aided control systems for fabricating 3D PDMS microneedle arrays18. PDMS dry etching is used to make PDMS microstructures as parts of micro-electromechanical actuators19,20. Porous PDMS membranes with designed pore layouts are also fabricated through dry etching processes21. Both the wet and the dry etching techniques can be integrated into patterning PDMS films with designated geometric shapes22.
However, the etching techniques for forming PDMS channel structures with complex section shapes have not been commonly applied because of their intrinsic limitations on microfluidic fabrication. First, while the techniques of PDMS wet etching utilizing laminar flows of chemicals for creating microfluidic channels of various sections have been established, the subsequent channel section formation is still restricted because of the basic characteristics of isotropic chemical etching processes23. Furthermore, even though there seems to be reasonable space for controlling the channel section geometries in a microfluidics fabrication using the PDMS dry etching techniques20, the required etching time is usually too long (in terms of hours) to be practical for manufacturing microfluidic chips. In addition, the etching selectivity between PDMS materials and the corresponding masking photoresist layers might be low in general, and the resulted etched depths for the channels are, thus, not acceptable20.
In this paper, we develop a one-step approach to fabricate microfluidic channels of different geometric cross sections by PDMS sequential wet etching processes (hereafter referred to as SWEP). The SWEP begin with a PDMS microfluidic device with single-layer channels. With assorted layout designs of the channels, fabricating microfluidic channels with different geometric sections of various kinds can be achieved through sequential etching processes. The sequential etching only needs an etchant to be introduced into specific channels of the planned single-layer layouts embedded in PDMS materials. Compared to conventional PDMS fabrication processes, the SWEP just require one further step to fabricate microfluidic channels of non-rectangular sections or various heights. The proposed SWEP provide a straightforward and simple way of fabricating microfluidic channels with various sections along the flow direction, which can significantly simplify the processes in the aforementioned methods.

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			<td style="width:217px;">Sigma-Aldrich Co., St Louis, MO</td>
			<td style="width:135px;">F6300</td>
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			<td style="width:135px;">Ver. 1.51</td>
			<td>Imaging Processing Program&#160;</td>
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			<td style="width:217px;">Leica Microsystems, Wetzlar, Germany</td>
			<td style="width:135px;">DMI 6000 B</td>
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			<td style="width:217px;">Nordson MARCH, Concord CA</td>
			<td style="width:135px;">PX-250</td>
			<td>Oxygen plasma surface treatment</td>
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			<td style="width:217px;">Dow Corning, Midland, MI</td>
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			<td style="width:135px;">427446</td>
			<td>PE 205, 10&#39;</td>
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			<td style="width:217px;">ELS Technology, Hsinchu, Taiwan</td>
			<td style="width:135px;">ELS 306MA</td>
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			<td style="width:135px;">SU-8 2050</td>
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			<td style="width:135px;">Y020100</td>
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		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:211px;">Surgical Blade</td>
			<td style="width:217px;">Feather, Osaka, Japan</td>
			<td style="width:135px;">5005093</td>
			<td>PDMS cutting</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:211px;">Syringe Pump</td>
			<td style="width:217px;">Chemyx, Houston, TX</td>
			<td style="width:135px;">Fusion 400</td>
			<td></td>
		</tr>
		<tr height="35">
			<td height="35" style="height:35px;width:211px;">Tetra-n-butylammonium Fluoride (TBAF)</td>
			<td style="width:217px;">Alfa Aesar, Ward Hill, MA</td>
			<td style="width:135px;">A10588</td>
			<td></td>
		</tr>
	</tbody>
</table>

one-step approach to fabricating polydimethylsiloxane microfluidic channels of different geometric sections by sequential wet etching processes

Polydimethylsiloxane (PDMS) materials are substantially exploited to fabricate microfluidic devices by using soft lithography replica molding techniques. Customized channel layout designs are necessary for specific functions and integrated performance of microfluidic devices in numerous biomedical and chemical applications (e.g., cell culture, biosensing, chemical synthesis, and liquid handling). Owing to the nature of molding approaches using silicon wafers with photoresist layers patterned by photolithography as master molds, the microfluidic channels commonly have regular cross sections of rectangular shapes with identical heights. Typically, channels with multiple heights or different geometric sections are designed to possess particular functions and to perform in various microfluidic applications (e.g., hydrophoresis is used for sorting particles and in continuous flows for separating blood cells6,7,8,9). Therefore, a great deal of effort has been made in constructing channels with various sections through multiple-step approaches like photolithography using several photoresist layers and assembly of different PDMS thin sheets. Nevertheless, such multiple-step approaches usually involve tedious procedures and extensive instrumentation. Furthermore, the fabricated devices may not perform consistently and the resulted experimental data may be unpredictable. Here, a one-step approach is developed for the straightforward fabrication of microfluidic channels with different geometric cross sections through PDMS sequential wet etching processes, that introduces etchant into channels of planned single-layer layouts embedded in PDMS materials. Compared to the existing methods for manufacturing PDMS microfluidic channels with different geometries, the developed one-step approach can significantly simplify the process to fabricate channels with non-rectangular sections or various heights. Consequently, the technique is a way of constructing complex microfluidic channels, which provides a fabrication solution for the advancement of innovative microfluidic systems.

Recently, a large number of studies have been made on the fabrication of microfluidic devices with channels of different sections by lithography replica molding13,14,15 and PDMS etching techniques17,18,19,20,21,<sup class="x...

Watch this Scientific Journal Video about One-Step Approach to Fabricating Polydimethylsiloxane Microfluidic Channels of Different Geometric Sections by Sequential Wet Etching Processes at JoVE.com

One-Step Approach to Fabricating Polydimethylsiloxane Microfluidic Channels of Different Geometric Sections by Sequential Wet Etching Processes

Several methods are available for the fabrication of channels of non-rectangular sections embedded in polydimethylsiloxane microfluidic devices. Most of them involve multistep manufacturing and extensive alignment. In this paper, a one-step approach is reported for fabricating microfluidic channels of different geometric cross sections by polydimethylsiloxane sequential wet etching.

Polydimethylsiloxane (PDMS) materials are substantially exploited to fabricate microfluidic devices by using soft lithography replica molding techniques. ...

This method can help provide key fabrication techniques in the microfluidics field such as for the manufacturing of channels with different geometric cross sections in a polydimethylsiloxane polycarbonate microfluidic device. The main advantage of this technique is that the microfluidic channels can be fabricated through a one-step approach by sequential wet etching processes without tedious procedures and extensive alignments. The implications of this technique extend toward development of complex microfluidic systems because fabrication processes for channels with non-rectangular sections or various highs can be significantly simplified using this method.Though this method can provide insight into microfluidic device fabrication, it can also be applied to other desired microenvironment conditions such as flux distribution or mixture of substance in channels. Begin the procedure with a prepared master mold. In this case, the negative mold created with photoresist is on a four-inch silicon wafer.Here is a computer-aided design image of the inverted topology for the channel layouts. Proceed by combining PDMS and its catalyst in a clean plastic cup and mixing it in a power stirrer. When the mixture is homogenous, put the cup in a desiccator connected to a vacuum pump for 60 minutes.After retrieving the PDMS mixture, take it to the master mold. Pour 20 grams of the mixture on top of the mold. Try not to introduce any bubbles.Next, obtain a Petri dish and pour 30 grams of the PDMS mixture into it. Place the covered mold and the Petri dish in a desiccator to eliminate bubbles. After 60 minutes, transfer the covered mold and the Petri dish to a 60-degree Celsius oven to cure for four hours.Following the curing step, allow the PDMS to cool to room temperature. Then, use a scalpel and tweezers to detach the cured PDMS from the mold. Continue to taylor the PDMS layer with the scalpel so that it covers the channel layout.Use a biopsy punch to open the channel access ports. Now, work with the PDMS in the Petri dish. Use a scalpel and tweezers to remove it from the dish.Then, cut the PDMS layer to the same dimensions as the molded layer. Take the PDMS layers to a surface treatment machine. Orient the layers to expose the designed channels of one PDMS layer and the featureless surface of the other.Expose the layers to oxygen plasma for 40 seconds. When done, the two surfaces have to be bonded. Bond the two layers by placing their treated surfaces in contact.Then, place the bonded PDMS layers in a 60 degrees Celsius oven for at least 30 minutes. Prepare a microfluidic device to characterize PDMS wet etching. Use a simple design with a single layer and a straight rectangular channel.In this device, there is one waste outlet and one inlet. Note that the top and bottom PDMS layers are depicted as separated in this image. Set the device up so that it can be observed and photographed with a microscope.For the etchant solution, use TBAF and NMP in a volume to volume ratio of one to 10. Draw the etchant into a 10 milliliter syringe with a stainless steel blunt needle. Put the syringe on a syringe pump near the microscope.Then, connect it to the inlet of the device. Guide the output from the outlet to a waste container. Run the syringe pump with the etchant at a 150 microliter per minute flow rate.Use bright-field microscope views to make sure the etched channel along the flow direction has uniform width and to help confirm the flow rate and etchant composition. Capture time series images of the channel cross section with four-times magnification during the etching process for analysis. Set up a microscope to view and photograph a microfluidic device.In this case, design the device so that the etchant and buffer inlets flow into different regions. Here is a schematic of the inlets and channels of the device from above and in cross section. Over time, the different action of the etchant versus the buffer will create a new cross section, demonstrated in this schematic of the final cross section and top view photograph.Place the device on the microscope stage. Prepare an etchant solution of TBAF and NMP and an NMP buffer solution. Fill two 10-milliliter syringes with buffer solution and one with etchant.Put the syringes in position on a syringe pump near the device. Make the appropriate connections for the device, including those for waste removal. Run the syringe pump with the etchant at a 50 microliter per minute flow rate.In this video, the initial channel geometry is revealed with red dye. Over time, the etchant causes the channel wall thickness to vary. Time the wet etching process to ensure the process yields the proper channel geometry.Prepare the microscope with the syringe pump nearby. This is the design used to create PDMS channels for a microfluidic mixer with different sections. It has alternating mirrored cross sectional geometry that would be difficult to achieve with standard processes.Set up the device on the microscope and connect it with the syringe pump. Introduce the TBAF NFP etchant solution at the port marked outlet. Observe the microfluidic channel as the chamber evolves over time, as in these top view photos recorded over two hours.Time the wet etching process to ensure the proper channel geometries. When done, characterize the device. Prepare a syringe with distilled water and another with fluorescein sodium salt solution to place in the syringe pump.Place the microfluidic device on the microscope and connect to each syringe to an inlet with a 20-microliter per minute flow rate. Draw waste from the outlet. Take fluorescence microscope images of the channel from above at predetermined positions.For comparison, record images under the same circumstances with an unetched version of the microfluidic device. It is possible to fabricate microfluidic channels with various geometric cross sections with sequential wet etching. This panel summarizes the arrangement of etchant inlets in single-layer PDMS channels to produce the cross-shaped cross section demonstrated in this video.Note the etchant goes into the central channel and the buffer into the side channels. The same arrangement of channels, along with switching the channels that carry etchant and buffer, produces a dumbbell-shaped cross section. A different channel arrangement similar to the microfluidic mixer, produces a bell-shaped cross section using only etchant.While attempting this procedure, it is important to remember to with the inlets of devices to avoid etchant leakage and further induce PDMS leakage at the inlets. After watching this video, you should have a good understanding of how to perform a single-step approach to successfully fabricate PDMS microfluidic devices with different geometric cross section of channels with sequential wet etching processes.

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