Researchers have revealed how high-frequency sound waves can be used to build new materials,
make smart nanoparticles and even deliver drugs to the lungs for painless, needle-free
vaccinations.
While sound waves have been part of science and medicine for decades – ultrasound was
first used for clinical imaging in 1942 and for driving chemical reactions in the 1980s
– the technologies have always relied on low frequencies.
Now researchers at RMIT University have shown how high frequency sound waves could revolutionise
the field of ultrasound-driven chemistry.
A new review published in the journal Advanced Science reveals the bizarre effects of these
sound waves on materials and cells, such as molecules that seem to spontaneously order
themselves after being hit with the sonic equivalent of a semi-trailer.
The researchers also detail various exciting applications of their pioneering work, including,
Drug delivery to the lungs, Drug-protecting nanoparticles, Breakthrough smart materials,
and Nano-manufacturing 2D materials.
The researchers have spent over a decade researching the interaction of sound waves at frequencies
above 10 MHz with different materials.
They generate high-frequency sound waves on a microchip to precisely manipulate fluids
or materials.
Ultrasound has long been used at low frequencies – 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 localised pressure cooker.
But it turns out that if you up the frequency, these reactions change completely.
When high frequency sound waves were transmitted into various materials and cells, the researchers
saw behaviour that had never been observed with low-frequency ultrasound.
They have seen self-ordering molecules that 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,
so it’s incredibly puzzling how something so tiny can be precisely manipulated with
something so big.
It’s like driving a truck through a random scattering of Lego bricks, then finding those
pieces stack nicely on top of each other.
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 – the basis for the various drug delivery technologies
patented by the RMIT research team.
One of these patented devices is a cheap, lightweight and portable advanced nebuliser
that can precisely deliver large molecules such as DNA and antibodies, unlike existing
nebulisers.
This opens the potential for painless, needle-free vaccinations and treatments.
The nebuliser 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 nebuliser 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.
In addition, the researchers have shown irradiating cells with the high-frequency sound waves
allows therapeutic molecules to be inserted into the cells without damage, a technique
that can be used in emerging cell-based therapies.
The team has used the sound waves to drive crystallisation for the sustainable production
of metal-organic frameworks, or MOFs.
Predicted to be the defining material of the 21st century, MOFs 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 RMIT team has developed a clean,
sound wave-driven technique that can produce a customised MOF in minutes and can be easily
scaled up for efficient mass production.
Sound waves can also be used for nano-manufacturing 2D materials, which are used in many applications
from flexible electric circuits to solar cells.
The next steps for the RMIT team are focused on scaling up the technology.