The technique is intended to replace existing technology, which uses high-frequency propagative surface acoustic waves (leaky SAW) to move microscopic samples on complex substrates.
CEA-Leti’s approach generates an evanescent Bessel beam in the low-frequency ultrasound (kHz range).
Thanks to the enhanced radiation force arising from the evanescent field, this beam makes it possible to pattern living organisms as small as bacteria along concentric circles on a simple substrate.
“The novelty of our approach is the ability to do exactly the same thing that people did in the propagative SAW domain using high frequency,” said CEA-Leti scientist Cédric Poulain. “In that sense, we followed the same route as in optics, where conventional (propagative) optical tweezers were replaced by the improved evanescent optical versions (using plasmon nano-optical tweezers). Here, in acoustics, the use of a very thin substrate in the low-frequency range favors the emission of evanescent sound waves.’’
Evanescent waves are localised waves in the vicinity of the emitting substrate with a very small wavelength gradient – and therefore a high level of force. To achieve such forces, conventional techniques (mainly SAW) require ultrasound waves in the MHz range, which are difficult to generate.
Furthermore, in existing propagative wave technology, waves radiate into the fluid and thus decay very quickly as they propagate along the substrate. Because the radiation of these waves wastes energy, researchers commonly refer to them as “leaky waves”. Evanescent waves do not radiate, and consequently do not decay as they propagate along the substrate.
The new technology offers several advantages over existing systems for contact-less manipulation of fluids and living cells:
- An easy-to-produce and low-cost solution that does not require cleanrooms.
- The low-frequency emission warrants both lower power consumption and an easier implementation than usual RF frequencies required for conventional SAW.
- The confinement of the energy in the vicinity of the emitter allows lower volume of the required sample, which is a key feature for biological applications.
“Our device is simply made of a commercially available but very thin glass plate that is attached to a ring-shaped piezo ceramic facing a cover that confines the liquid,” Poulain said. “We also showed that we can confine the beam within the evanescent length, which in turn increases the radiation force for some well-defined and resonant gaps.”
