For the new study, “we turned down the dials” on the ultrasound device, Airan said. The intensity of the ultrasound used in these experiments was about 1/10th to 1/100th of the intensity used in clinical ablation procedures. The ultrasound in these experiments was delivered in a series of short staccato pulses separated by periods of rest, giving the targeted brain tissue plenty of time to cool off between pulses. Rats exposed numerous times to the experimental protocol showed no evidence of tissue damage from it.
The nanoparticles, which Airan has been perfecting for several years, are biocompatible, biodegradable, liquid-filled spheres averaging 400 nanometers (about 15-millionths of an inch) in diameter. Their surfaces consist of a copolymer matrix in which the drug of choice is encaged. Roughly 3 million molecules of a drug typically dot the surface of one of these nanoparticles.
Each nanoparticle encloses a droplet of a substance called perfluorocarbon. Buffeted by ultrasound waves at the right frequency, these liquid cores begin shaking and expanding until the copolymer matrix coating the surface ruptures, setting the trapped drug molecules free. Propofol, like all psychoactive drugs, easily diffuses through the otherwise formidable blood-brain barrier. But having crossed this barrier, the drug is quickly soaked up by brain tissue, so that it never gets farther than about a half-millimeter from the capillary where it’s been released.
Airan and his colleagues injected these particles intravenously into experimental rats and explored focused ultrasound’s potential for targeted drug delivery.