A Newly Discovered Form of Sound Wave Could Change Drug Delivery

Surface reflected bulk waves offer a way of delivering fragile medicines to difficult-to-access lung tissues.

Acoustics experts from Australia's RMIT University have created a new variety of hybridized sound wave, the first in over 50 years of acoustics research. Their work, which is described in the current issue of Advanced Materials, has "revolutionary" implications in an unexpected field: stem cell therapy.

The RMIT group calls its new hybrid waves "surface reflected bulk waves," as they exist as a combination of two existing varieties of acoustic waves known as bulk waves and surface waves. While powerful, the waves are also gentle enough to be employed in biomedical applications such as manipulating stem cells in non-disruptive and non-destructive ways, thus opening up new possibilities for the cells' usage.

A surface wave can be imagined as an ocean wave, where the wave propagates along the surface of the body of water without having much of an effect on everything beneath it. That is, a swimmer might simply duck underwater to escape most of the force of an approaching breaker. Sound waves can propagate like this too, where a "surface" might represent the interface between substances of different densities.

A particularly interesting form of surface wave is the surface acoustic wave (SAW). These are, naturally, acoustic waves—waves of alternating compression and relative decompression in some material—propagating along the surface of an elastic material. It's a more specific phenomenon and one employed widely in signal processing applications; your phone probably has a few SAW devices built into it.

Where the elastic material is actually a solid, SAWs are known as Rayleigh waves. An example would be a wave moving along the surface of a crystal or piezoelectric material (a material designed to convert applied surface stress to electricity).

Surface acoustic wave in crystal substrate. Image: Wiki

A more recent application of SAWs and Rayleigh waves is in microfluidics. It's possible to very efficiently transfer SAWs to a surrounding liquid, so electronically-driven SAWs can be used to precisely control such actions as pumping and mixing at very small scales.

This is the angle from which the RMIT researchers are coming at things: using SAWs to nebulize liquids for pulmonary drug delivery, e.g. drugs delivered as mists nasally directly to the lungs. Simply, sound waves are made to smash into liquids, with the result being those liquids shattered (atomized) into many tiny droplets.

Unfortunately, SAWs have so far been unable to atomize liquids at sufficient rates to be very useful, as the paper explains, thus "frustrating efforts to translate an otherwise attractive and potentially powerful technology for pulmonary drug administration into clinical practice given its many advantages, such as the ability to generate aerosol sizes that are optimal for deep lung deposition and the ability to deliver next generation therapeutic molecules without degradation."

The problem is that as you increase the energy of the waves, resulting in higher and higher amplitudes, the device itself becomes vulnerable. At a certain point, the thing just breaks.

What the current paper describes is a way of increasing this maximum power by combining surface waves with another sort of wave, known as a bulk wave. A bulk wave is the opposite idea. Rather than just the surface of a material, a bulk wave exerts itself on the whole mess. This is more what we'd expect of a sound wave as it travels through the air—the whole material, from top to bottom, vibrating as a single entity.

The method the group describes is based on recapturing wave energy "leaked" by surface acoustic waves into their underlying substrate. Usually, the goal is to suppress this leakage, but instead, the new method harnesses it, capturing waves reflected around the edges of the SAW material and using them to activate very tiny droplets of liquid. The result are very powerful waves (they have high-frequencies) still gentle enough (low amplitudes) to manipulate fragile stem cells; many low-energy droplets can be created very quickly, potentially cutting the time it takes to deliver a nebulized vaccine to a patient from 30 minutes to 30 seconds.

"The reason is running these device at very high frequencies [of] 10 MHz and above compared to typical transducers working in the 10-100 KHz range," Amgad Rezk, the current paper's lead author, told me. "The field reversal—waves going up and down—is too fast to be felt by these sensitive biosamples; for example, it is a lot faster than the relaxation time of DNA strand."

One upshot of this is that it should become possible to deliver fragile stem cells—and other sensitive materials, such as vaccines—in more effective ways to difficult-to-access lung tissues.