PROVIDENCE — Iron particles so small that a few thousand of them, lined end to end, would be as wide as a human hair may help doctors pierce the slimy shield that protects bacteria infecting artificial hips, knees and other implants, Brown University researchers said.

Such infections plague as many as 1 in 40 recipients of hip and knee implants. They are difficult to stop, without actually removing the implant, because the bacteria use the shield to keep antibiotics at bay.

Brown graduate student Erik Taylor and Thomas Webster, an associate professor of engineering and orthopedics, created the iron-oxide nanoparticles and used magnets to first pull them against the shield, then force them to penetrate both it and the underlying bacteria cells, like cannonballs puncturing the hull of a ship.

“When nanoparticles get into the bacteria, they create these pores like a sponge, and the insides of the bacteria ooze out of those pores and they die,” said Webster.

If that weren’t enough, the excess iron seems toxic to the bacteria as well.

“Then all the little remnants of biofilm start disintegrating,” he said.

“The next neat part is that those same magnetic particles seem to increase bone growth, which is just as exciting. You’re growing bone where bone should be,” to integrate the implant into the body, according to the researcher, whose findings appear in the International Journal of Nanomedicine.

The drug-free treatment is still experimental. The researchers have yet to try it in animals, but they are planning experiments with rats.

Several questions remain, not the least of which is whether it will work in humans.

It is also not clear whether every bacteria cell needs to be pummeled with nanoparticles for the treatment to be effective.

“We do see 100-percent killing in our in vitro [test tube] studies,” said Webster. “We do have an immune system [that wants to attack bacteria]. You may just have to help it out a bit with the iron particles.”

The Brown team also has a range of different-sized cannonballs to choose from. Webster said their nanoparticles can be as small as 2 nanometers and as large as 80 nanometers. A typical human hair is about 80,000 nanometers wide.

gemery@projo.com