Microfluidic Pneumatic Cages: A Novel Approach for In-chip Crystal Trapping, Manipulation and Controlled Chemical Treatment

Herein, we describe the fabrication and operation of a double-layer microfluidic system made of polydimethylsiloxane (PDMS). We demonstrate the potential of this device for trapping, directing the coordination pathway of a crystalline molecular material and controlling chemical reactions onto on-chip trapped structures.

The overall goal of this approach is to demonstrate the potential of this device for trapping, directing the coordination pathway of a crystalline molecular material and controlling chemical reactions onto on-chip trapped structures. This method can help answer key questions in the materials science field such as the effect of controlled chemical treatments on the properties of self-assembled structures. And it's important to underline that the number of technologies enabling controlled chemical treatment under dynamic conditions are currently very limited, hence making this approach very attractive in materials-related field.

To begin, prepare a silanized master mold using SU8 photolithography. The fouling particle is particularly sensitive to both time and temperature. Any failure to follow to the time frame and temperature described may lead to the fabrication of a non-bonded, and therefore non-functional device.

Prepare the PDMS mixture by combining 50 grams of the elastomer and 10 grams of the curing agent in a disposable weighing dish. Mix the components completely using a plastic spatula. Next, place the well-mixed PDMS into a desiccator under vacuum for 15 minutes to degas the mixture and remove the trapped bubbles.

While the first batch of PDMS is being degassed, mix a second batch using 10 grams of elastomer and 0.5 grams of the curing agent. Then, fix the master mold containing the control layer into a round 11 millimeter PTFE frame. Once the five to one mixture of PDMS has been degassed, remove it from the vacuum chamber.

Now, pour the five to one mixture of PDMS onto the control layer master mold until the mixture reaches the level of the straight vertical wall of the PTFE frame. And then place it into the desiccator. At the same time, also place the 20 to one mixture of PDMS into the desiccator and again pull a vacuum.

Degas both the coated master mold and the 20 to one ratio of PDMS for an additional 30 minutes. Then, take both of them out of the desiccator and place the control layer master mold in an oven that has been pre-heated to 80 degrees Celsius. While the control layer bakes, place the master mold for the fluidic layer onto a spin coater.

Pour around 4 milliliters of the 20 to one mixture of PDMS onto the master mold for the fluidic layer and spin coat the wafer for 40 seconds at 1200 rpms to achieve a layer that is 60 micrometers thick. After one hour of total elapsed time, open the oven, place the spin-coated wafer next to the control layer and bake them together for an additional 15 minutes at 80 degrees Celsius. Then, after 75 minutes of total elapsed time, remove both of the wafers from the oven.

First, peel off the five to one mixture of PDMS for the control layer. Cut out the chips using a razor blade. And then punch the holes for the inlets using a one millimeter biopsy punch.

Next, use adhesive tape to remove any debris from diced control layer chips. Once the chips are clean, use a stereo microscope to align the control layer chip on top of the fluidic layer master mold. Then, pour and draw the residual PDMS around the assembled chips.

And place the entire setup into an oven at 80 degrees Celsius. Bake the assembled devices overnight. The following day, take the cured assembly out of the oven and allow it to cool down to room temperature.

Then peel off the PDMS assembly from the fluidic layer master mold. Once free from the master mold, dice the fabricated double layer devices with a blade and use a 1.5 millimeter biopsy punch to form the fluidic inlets and outlets. Next, treat glass coverslips and the fluidic layer of the assembled device with a corona discharge for one minute or use oxygen plasma and then immediately bond the two surfaces together to complete the microfluidic device.

Bake the bonded double layered chips in an oven at 70 to 80 degrees Celsius for at least four hours. In order to manipulate the flow regiment using a syringe pump and a pneumatic controller, first connect the syringes previously loaded and placed in a syringe pump to the micro fluidic device's fluidic inlets and pneumatic controller system to the microfluidic device's control inlets. To visualize the flow, load one of the syringes with an aqueous dye and flow it into the chamber at a flow rate of 20 microliters per minute.

Then use the pneumatic controller system to close the valve by actuating it at three bar. It's important to note that the fluid can still flow around the valve once it is closed, and this feature is important for achieving controlled chemical treatment of trapped structure, such as coordination polymers. To open the valve, simply use the controller system to release the pressure.

While the die solution flows though the first channel, inject another aqueous fluid into the second inlet channel at the same flow rate to form an interface between the two aqueous flows. Then close the valve by actuating it at three bar. The actuation of the valve during dual flow changes the interface of the two aqueous flows.

Next change the fluid flow rates of the two syringes to 30 microliters per minute and 10 microliters per minute respectively in order to shift the interface between the two fluids. In order to visualize the ability of the valve to trap microparticles, first prepare an aqueous solution containing 10%polystyrene fluorescent microparticles by weight. Introduce the particle-laden fluid into the two inlet channels at a total flow rate of 20 microliters per minute.

Wait for two minutes until a stable flow is established. Then excite the fluorescent beads using a source with a wavelength of 488 nanometers to best view the beads. When ready, actuate the valve at three bar to close it.

Image the area of the valve to see several particles trapped underneath the valve and localized on the surface while the flow is maintained. The injection of gas through channels in the control layer squeezes the fluid layer towards the surface. This can be used to deflect fluids around the region controlled by the actuator here indicated by the absence or a rhodamine dye.

These pneumatic actuators can also be used to trap particles or cells such as these fluorescent microparticles which were trapped on the microchannel surface. Another feature of this device is its ability to trap NC2 generated coordination polymers through the actuation of the pneumatic cage. For this setup two reagent flows are used and a controlled chemical reaction takes place at the interface of the two liquids in the laminar flow.

Once trapped the coordination polymers can be chemically treated in a controlled manner by employing the pneumatic valves. If you're watching this video you should have a good understanding of how to effectively fabricate a double layer microfluidic device which can be used to conduct controlled chemical reactions onto various on tube structures. While attempting this procedure, it's important to be constrained to the time frame and temperature reported in the present protocol.

Otherwise, your effort may lead to fabrication of non-bonded or defective and therefore, non-functional devices. After its development, this technique pave the way for researchers in the field of materials science to explore various types of in-tube controlled chemical treatments with high precision using a double layer microfluidic platform.