Microparticle Manipulation by Standing Surface Acoustic Waves with Dual-frequency Excitations

This method can help address key issues in the biomedical field, such as, manipulating of microparticles and assist in the sorting in a microfluidic channel for labs-on-a-chip. The main advantage of this technique is it can enhance the tunability of the commission of standing surface acoustic wave. Demonstrating the procedure will be Yannapol, a graduate student from my group.

Begin with a mold for the microfluidic channel. This one is a negative tone photoresist pattern on a silicon wafer. More detail is provided by the photolithography pattern used to create it.

Mix 10 parts PDMS with one part of elastomer base volume ratio. Degas the mixture in a vacuum oven for about 15 minutes. After degassing, take the mixture to the mold.

There, pour it over the photoresist on the silicon wafer. Now, degas the covered silicon wafer for another 15 minutes. After that, heat the wafer in an incubator to solidify the PDSM.

When the PDMS is cooled, remove it from the wafer. The microfluidic channel is ready for the next steps in the protocol. Prepare a substrate for fabricating the transducers.

In this case, use a lithium niobate wafer four inches in diameter. At a spin coater, spin coat the wafer with positive photoresist. Then, use photolithography to pattern the photoresist with 20 150 nanometer strips in an aperture of two centimeters.

After photolithography, this is the schematic depiction of the change in cross-section of the substrate. Once the photoresist is developed, sputter 20 nanometers of chromium onto the substrate followed by 400 nanometers of aluminum. Use acetone to remove the chromium and aluminum layer on the non-exposed areas.

Take the substrate for surface treatment in oxygen plasma. Use a nitrogen to oxygen ratio of two to one with 30 watts of power for 60 seconds. When done, work with both the substrate and the microchannel.

Align the PDMS microchannel and bond it with the lithium niobate substrate. Press the two together for a few seconds to bond them. Place the integrated device in the heating chamber at 60 degrees for three hours.

After heating the chamber, the device is ready for studies using dual-frequency excitations of standing surface acoustical waves. For observations, use an inverted microscope with the device on its stage. Have the wafer with the interdigital transducers in contact with the stage and have the microchannel on top.

Connect the transducers to the amplified signal of a function generator. Arrange for fluids from a syringe pump to pass over a magnetic stirrer. Prepare the solution for the experiments.

Mix four micrometer polystyrene beads in deionized water. Spin the mixture in a vortex for two to three minutes. Follow this by placing the mixture in an ultrasound sonicator for 10 minutes.

Transfer the mixture to a three milliliter syringe. Also add a stirrer bar to the syringe. Next, place the syringe on the syringe pump.

Ensure the syringe is over the magnetic stirrer and is connected to the device inlet. Set the syringe pump flow rate to a few microliters per minute. Now, drive the device with the fundamental and third harmonic of the interdigital transducers.

Observe the stabilized microparticles under the microscope and record images with the digital camera. Begin with no excitation and vary the phase difference between the two frequencies. Use the recorded images to determine the microparticle concentration at each pressure node.

This is a plot of the pressure wave forms of a standing surface acoustic wave at the excitation frequencies of 6.2 and 18.6 megahertz. The vertical axis is the position along a 300 micrometer-wide microchannel. The curves represent different power ratios, the percentage of power in the fundamental mode versus the total power of 146 milliwatts.

Here is the acoustic radiation force applied to four micrometer microspheres in the same channel under the same power conditions. At a power ratio above 90%the force is always in phase and produces a single pressure node at the 150 micrometer position in the channel. Additional nodes appear for a power ratio below 90%When four micrometer polystyrene microspheres are initially placed at a wall of the channel, their motion is determined by the power ratio.

Note:For power ratios above 90%the particles go to the central node. For power ratios of 90%and below, they go to the side nodes. Comparison of the experimental data, plotted using symbols, with simulations plotted with dashed lines, for particle position as a function of the power ratio yields good agreement.

Note this plot shows both the upper and lower side nodes. Similar agreement is evident in a plot of the particle concentration as a function of the power ratio. While attempting this procedure, it is important to remember to tune the standing acoustic wave in the microfluidic channel by adjusting the power ratio of the dual-frequency excitation.

After its development, this technique paved the way for researchers in the field of biomedical engineering to explore rapid, effective and flexible manipulation of microparticles assist in labs-on-a-chip. Don't forget that working with electric instruments can be extremely hazardous and that precaution to prevent electric shock should always be taken while performing this procedure.