Three-Dimensionally Printed Microfluidic Cross-flow System for Ultrafiltration/Nanofiltration Membrane Performance Testing

The overall goal of this procedure is to demonstrate the design and operation of a 3D printed microfluidic cross-flow cell for the purpose of testing the performance and fouling propensity of polymer membranes. This method can help researchers in the membrane field, to study membrane performance and fouling propensity using microfluidics at elevated pressures and well-defined flow conditions with minimal costs. Advantages of this small scale setup includes the ability to perform separations of small amounts of high volume materials, to perform membrane studies requiring expensive reagency materials and to test multiple new membrane formulations or modifications in parallel.

Begin by designing the microfluidic device in two separate parts using a computer aided drafting program. To make the bottom layer, use the rectangle tool to draw a 40 millimeter by 60 millimeter rectangle. At one corner of the rectangle, use the circle tool to create a 6.2 millimeter diameter circle.

Center this circle 10 millimeters from the left edge, and 10 millimeters from the bottom edge. Then, use the linear pattern tool to replicate the holes across the rectangle with 20 millimeter spacing for a total of six holes. Next, use the fillet tool to fillet the rectangles with a radius of one millimeter.

Then, extrude the part 10 millimeters using the extrude tool. In the center of the top face, use the rectangle tool to create a 30 millimeter by one millimeter rectangle. Then, use the extrude cut tool to cut a 0.2 millimeter deep channel into the surface.

Next, use the circle tool to make a one millimeter diameter circle at the end of the flow channel. Then, use the line tool in order to construct a path connecting the circle to the nearest 40 millimeter by 10 millimeter face, including a four millimeter radius made with the fillet tool. Using the swept cut tool, make a cut along this path.

Next, go to the right plane view, and use the circle tool to create a 3.9 millimeter diameter circle at the center of the flow path. Then, use the extrude cut tool to cut down eight millimeters in order to allow room for fittings. Repeat this same process on the other side of the rectangular channel in order to create a mirrored image.

To design the top layer of the microfluidic test system, first make a rectangle with six screw holes having the same dimensions as the bottom layer. Next, in the center of the top face, create a 30 millimeter by one millimeter by 0.5 millimeter permeate channel using the rectangle tool to first create a 30 millimeter by one millimeter rectangle. And then, using the extrude cut tool, to cut the rectangle 0.5 millimeters into the top surface.

Now, use the the circle tool to make a one millimeter circle centered in the permeate channel, five millimeters from one end of the channel. Then, as before, use the line and the fillet tools to construct a path which includes a four millimeter radius band to connect the circle to one of the one centimeter by six centimeter faces of the rectangle. Use the swept cut tool to cut along the path and create a channel.

Then, use the circle tool to create an additional 3.9 millimeter diameter circle with its center on the permeate path. And use the extrude cut tool to cut the circle eight millimeters into the surface. At the part's top 40 millimeter edges, with the rectangle tool, create rectangles 40 millimeters by five millimeters, adding a four millimeter radius with the fillet tool.

Use the extrude tool, to extrude three millimeters downward for handles. Finally, print the top and bottom pieces using a multi-material photopolymer 3D printer, with a hard transparent polymer. Include a 0.05 millimeter overcoating with a soft, rubbery polymer on the face of each part that contains the channel, in accordance with the manufacturer's protocol.

To begin, set up the microfluidics, including the pump, the valves, the pressure transducer, the servos, the flow meter, the regulators, and the microcontroller as described in the accompanying text protocol. Then, configure the system to measure four membranes in parallel by opening all of the valves to the flow cells. Place one pump inlet tube into the ultra pure water reservoir, and place the other inlet tube into a reservoir containing 0.08 grams per liter of BSA solution.

Using a syringe, prime the tubing to remove all of the air bubbles in the system. Next, place the ultrafiltration membranes onto the bottom part of the flow cells so that the active side of the membrane is towards the feed channels. Then, close the cells, and feed the screws through the holes in the device.

Fasten the nuts by hand initially. Then, using a wrench to tighten them by alternating around the device so that there is even pressure on the chamber. Do not overtighten any one side, as improper tightening may lead to water leakage.

Next, select the ultra pure water using the reservoir switch. Set the pump flow rate to eight milliliters per minute and then start the pump. With the pump running, adjust the pressure regulator to 0.4 bar.

Monitor the flex values of the membranes using data acquisition software, and adjust the pressure regulator until an average flux of between 180 and 220 liters per square meter per hour is achieved. Replace an individual membrane before running the experiment, if it is not able to get within 20 liters per square meter per hour of this range. When ready, set up the experimental run by first selecting the ultra pure water in reservoir one for 60 minutes with a constant flux of between 180 and 220 liters per square meter per hour.

Then, select the BSA in reservoir two, for an additional 420 minutes with manual control of the pressure regulator. Select the ultra pure water in reservoir one for the final 15 minutes of the experiment to flush the system. Use manual control of the pressure regulator during this step.

Then, set the reservoir switch to auto and start the experiment. After the experimental run has completed, shut the system down and remove the membranes from flow cells. Then, rinse out the BSA inlet tubing by drawing ultra pure water through it with a syringe.

To measure the performance in fouling of the nanofiltration membranes, a flow meter was connected to the line to measure the permeate flux as various solutions were flowed through the system. First, the system was equilibrated with water for 45 minutes, followed by 45 minutes of 10 millimolar magnesium sulfate. The system was then returned to pure water for an additional 45 minutes until a stable flux was achieved before 0.8 grams per liter of BSA was fed into the system for 45 minutes.

Finally, the system was returned to pure water for the final 45 minutes. The decrease in flux, compared to the flux of a control membrane under the conditions of 10 millimolar NaCl, indicated membrane fouling due to BSA. The graph shown here, displays the fouling of ultrafiltration membranes.

The flux across the 50 kilodalton membranes is shown in blue. And the flux across the 30 kilodalton membranes is shown in red. While the 50 kilodalton membrane had a higher normalized flux at the termination of the experiment, the difference was not significant.

We found that the multi-material 3D inkjet photopolymerization base technology was essential to producing a flow cell that was robust and included a printed rubber-like gasket layer, which was crucial to seal the flow cell and prevent leakages. After watching this video, you should have a good understanding of how to design and operate your own custom 3D printed microfluidic cross-flow cell for ultrafiltration or nanofiltration membrane applications.