While sound waves have been part of science and medicine for decades, the technologies have always relied on low frequencies. Researchers now have shown how high-frequency sound waves could revolutionize the field of ultrasound-driven chemistry. One of the effects of these sound waves on materials and cells is that molecules seem to spontaneously order themselves after being hit with the sonic equivalent of a semi-trailer.
The process can enable sustainable production of super-porous nanomaterials that can be used to store, separate, release, and protect almost anything. It also provides precise, cost-effective, and fast exfoliation of atomically thin quantum dots and nanosheets.
Medical applications include nebulization technology that could deliver lifesaving drugs and vaccines by inhalation rather than through injections and encapsulating drugs in special nano-coatings to protect them from deterioration, control their release over time, and ensure they precisely target the right places in the body like tumors or infections.
The research team generated high-frequency sound waves on a microchip to precisely manipulate fluids or materials. Ultrasound has long been used at low frequencies of around 10 kHz to 3 MHz to drive chemical reactions — a field known as sonochemistry. At these low frequencies, sonochemical reactions are driven by the violent implosion of air bubbles. This process, known as cavitation, results in huge pressures and ultra-high temperatures — like a tiny and extremely localized pressure cooker.
When high-frequency sound waves were transmitted into various materials and cells, the researchers saw behavior that had never been observed with low-frequency ultrasound.
Self-ordering molecules seem to orient themselves in the crystal along the direction of the sound waves. The sound wavelengths involved can be over 100,000 times larger than an individual molecule.
While low-frequency cavitation can often destroy molecules and cells, they remain mostly intact under the high-frequency sound waves. This makes them gentle enough to use in biomedical devices to manipulate biomolecules and cells without affecting their integrity.
One application is an inexpensive, lightweight, and portable advanced nebulizer that can precisely deliver large molecules such as DNA and antibodies, unlike existing nebulizers. The Respite nebulizer uses high-frequency sound waves to excite the surface of the fluid or drug, generating a fine mist that can deliver larger biological molecules directly to the lungs. The nebulizer technology can also be used to encapsulate a drug in protective polymer nanoparticles in a one-step process, bringing together nano-manufacturing and drug delivery.
The team also has used the sound waves to drive crystallization for the sustainable production of metal-organic frameworks (MOFs), which are ideal for sensing and trapping substances at minute concentrations to purify water or air, and can also hold large amounts of energy for making better batteries and energy storage devices.
While the conventional process for making a MOF can take hours or days and requires the use of harsh solvents or intensive energy processes, the researchers developed a clean, sound wave-driven technique that can produce a customized MOF in minutes and can be easily scaled up for efficient mass production. The team successfully tested the approach on copper- and iron-based MOFs.
Sound waves can also be used for nano-manufacturing of 2D materials, which are used in myriad applications from flexible electric circuits to solar cells. Sound wave-generating microchips can be produced using the standard processes for mass fabrication of silicon chips for computers, opening the possibility of producing industrial quantities of materials with sound waves through massive parallelization using thousands of chips simultaneously.