The blood-brain barrier, a dense layer of tightly packed cells that line brain capillaries like a regiment of infantrymen, has always been the bane of neuromedicine. True, this line of defense protects the brain from all manner of potentially harmful chemicals. But it keeps most medications out, too. Scientists have spent decades searching for ways to breach the barrier just long enough for Alzheimer’s, Parkinson’s or antitumor drugs to slip through. Now, researchers say, they are finally onto something.

The new method employs micro­bubbles—­small, preformed bubbles made up of a simple gas surrounded by a rigid lipid cell. Scientists at Harvard, the Massachusetts Institute of Technology, Columbia and other institutions are developing ways of injecting the bubbles into the bloodstream and guiding them by ultrasound to the blood-brain barrier. The bubbles then pry open the barrier at specific points targeted by the ultrasound beam. Once the barrier is breached, scientists inject drug-coated, magnetically charged nanoparticles into the patient and utilize MRI beams to guide them to the precise spot where they are most needed. So far rodent studies have shown as much as a 20 percent increase in the amount of antitumor or Alzheimer’s medication that reaches the brain when ultrasound and microbubbles are used.

Microbubbles are only the latest and most promising in a string of recent projects aimed at solving the blood-brain barrier problem. These include running a catheter into the brain capillaries and designing a whole suite of drugs that would trick the brain into letting them cross. “Microbubbles are less invasive and more cost-effective than other things we’ve come up with,” says Nathan McDannold, who is a radiologist and researcher at Brig­ham and Women’s Hospital in Boston.

Scientists need to iron out some kinks, however. The main problem is how to increase the intensity of ultrasound to a level that will work in humans without causing tissue damage. McDannold believes that researchers are making rapid progress on all fronts. “It’s not quite ready for humans yet,” he says. “But we are getting there, quickly.”