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.
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.
