Soft Lithographic Procedure for Producing Plastic Microfluidic Devices with View-ports Transparent to Visible and Infrared Light

The over goal of this microfabrication protocol is to produce plastic microfluidic devices compatible with Fourier transform infrared microspectroscopy in a simple and cost effective way. These methods can help with vast understanding of cellular biochemistry. It provides us simplified access to infrared microspectroscopy, a label free, no damaging imaging technique which can retrieve biochemical maps of live cells.

The main advantage of the this technique is that it reduces the need for accessing micro-fabrication facility and it uses plastic as the main component for the final devices. Demonstrating the procedure will be Mona Suryana, a research assistant from our laboratory. To begin this procedure, prepare the silicon primary mold as outlined in the text protocol.

Next, mix the PDMS elastomer and the curing agent at 10 to one ratio. The total amount mixed is such that the resulting PDMS is approximately one to 1.5 millimeters in thickness. After a thorough mixing, transfer the mixture to a vacuum jar.

Lower the pressure until it is between one and 10 millibar. Wait for 15 minutes or until there are no visible bubbles to degas the mixture. Pour the degassed PDMS onto the prepared silicon mold.

Transfer the mold to the vacuum jar and lower the pressure for 15 minutes to degas the mixture once again. Then transfer the mold to a hot plate or oven. Heat the mold at 70 degrees Celsius for two hours to cure the mixture.

Remove the cured PDMS from the heat and let it cool to room temperature. Use a razor blade to cut the PDMS along the edges of the silicon mold. Using a pair of tweezers, pinch one corner, and carefully peel the PDMS replica off the silicon mold.

Next, transfer the PDMS mold to a plasma cleaner. Set the chamber pressure between one and 10 millibar and treat the PDMS mold with oxygen plasma at 60 watts for 30 seconds with 20 SCCM of oxygen flow. After this, place the mold in a vacuum jar and add about 50 microliters of silane.

Leave the jar in the vacuum state for two hours. To begin, design or acquire acrylic templates, as outlined in the text protocol and prepare PDMS elastomer and curing agent as shown at the beginning of this procedure. Pour the degassed PDMS mixture on the acrylic templates until the topmost surface is submerged, about one millimeter below the liquid surface.

Then transfer the submerged templates to a vacuum jar. Degas the PDMS using the same process as before. Then transfer the submerged templates to a hot plate or an oven.

Heat at 60 degrees Celsius for two hours to cure the mixture. Remove the cured PDMS from the heat source and let it cool to room temperature. Using a razor blade, cut the PDMS along the edges of the acrylic templates.

Next use a pair of tweezers to pinch one corner and carefully peel the PDMS off. After this, prepare the second PDMS replica as outlined in the text protocol. To begin fabricating the patterned half of the device, treat a calcium fluoride window with oxygen plasma, at 60 watts for 30 seconds, with 20 SCCM of oxygen flow.

Carefully place the first PDMS template, the one with the small pillars, onto a flat surface. Then, place the treated calcium fluoride window in the center of the template. Press gently to ensure the window is fully in contact with the PDMS.

Next, place a UV transparent plate on the backside of the PDMS mold, aligned with the location of the central chamber. Press gently to ensure it is fully in contact with the PDMS. Place the mold onto the PDMS template with the fluidic pattern face down and with the fluidic chamber aligned to the center of the window.

Then gradually dispense drops of NOA at the inlet of the PDMS template. Allow the NOA to slowly fill the cavity. After the cavity is completely filled, cure the NOA by exposing the mold to UV light.

Carefully remove the UV transparent plate from the mold. Gently peel the PDMS mold from the top of the NOA layer. After this remove the NOA layer.

To begin fabricating the flat half of the device, treat the calcium fluoride window with oxygen plasma at 60 watts for 30 seconds with 20 SCCM of oxygen flow. Carefully place the second PDMS template, the one without the small pillars, onto a flat surface. Place the treated calcium fluoride window in the center of template.

Press gently to ensure the window is fully in contact with the PDMS. Next acquire a one millimeter thick sheet of PDMS that is five centimeters by 3.5 centimeters. Place this sheet on top of the calcium fluoride window, aligned with the center of the template.

Press gently to ensure the sheet is fully in contact with the window. Gradually dispense drops of NOA at the inlet of the PDMS template. Allow the NOA to slowly fill the cavity.

After the cavity is completely filled, cure the NOA by exposing the mold to UV light. Peel off the PDMS layer. Then carefully remove the cured NOA layer from the PDMS template.

Place one half of the device on top of the other, with the calcium fluoride windows aligned. Gently press at the corners of the NOA layers, fixing the position of the two halves. Next, acquire PDMS discs and rectangles as outlined in the text protocol.

Place the PDMS discs in the corresponding openings of the device. Next, place the PDMS rectangles with precut openings onto each side. Transfer the entire assembly to the vacuum press sandwiching it between two plates.

Then, seal the plastic bag. Turn on the vacuum pump to evacuate the assembly and allow it to run for at least 10 minutes. Using a broadband mercury vapor lamp at 270 watts, expose the evacuated assembly to UV light for 15 minutes.

After this, turn off the vacuum pump and let the assembly slowly vent to atmospheric pressure before removing the final device from the assembly. In this procedure, a plastic microfluidic device with view-ports transparent to visible and infrared light is fabricated. Transmittance spectra are then acquired to compare a brand new calcium fluoride window, one half of the fabricated device, and the complete device.

As can be seen, all three exhibit a transmittance over 80%up to mid-infrared, indicating a high degree of transparency in this range. While the spectrum for the full device exhibits an interference pattern caused by the air gap between the two windows, these spectra demonstrate that the fabrication process does not alter the transparency of calcium fluoride windows up through the mid-infrared range. Once mastered, this technique can be done in one hour if it is performed properly and the required templates and molds are ready.

After watching this video, you should have good understanding of how to produce plastic microfluidic devices compatible with Fourier transform infrared spectroscopy using replica molding with PDMS and the capillarity filling process with UV curable resins. Don’t forget that working with with silane and UV light can be extremely hazardous and always conduct procedure-based risk assessments and wear appropriate person protective equipment while performing this procedure.