Such flexible, potentially long-lasting particles hold great potential for drug delivery. But Mitragotri has not yet looked to see whether the synthetic cells can stand up to the most difficult test: remaining in circulation. Proving that the particles remain in the bloodstream and do not prompt an immune attack is a critical step that will require testing in animals.

"Back in 1966, I made similar [particles] that can change in shape and in size," says artificial blood researcher Thomas Chang from McGill University in Quebec, Canada. Those cells, he says, could also squeeze through capillary tubes and were about the same size as red blood cells. The problem was that even synthetic cells one-eighth of the size of regular blood cells were purged from the blood within 30 seconds. (By the 1970s, researchers found that artificial blood particles work best at 200 nanometers or less--30 times smaller than red blood cells.) "The main thing is to show that they remain in circulation," Chang says.

Even the most advanced synthetic particles get cleared out of the blood incredibly rapidly. "The longest circulating nanoparticle ever lasted about 24 hours, so there's a need for developing an approach to something that can circulate in the bloodstream for a long period of time," says Jeffrey Karp, a Harvard-MIT professor of health science and technology. But the new research could be a big step in that direction, he says, if the body keeps the synthetic cells circulating for as long as two to three months, like real red blood cells. Karp says that the production methods that Mitragotri and his colleagues used could be scaled up without much difficulty.

Assuming that the cells stand up to the circulatory test of time, "I would think that anybody who's trying to use a nanoparticle-like system for delivery or for imaging would have good reason to go with these particles," says Daniel Pack, a drug-delivery researcher at the University of Illinois at Urbana-Champaign.

Mitragotri says that the next step will be animal testing. He also wants to look into other ways to mimic nature's delivery methods. "We started with red blood cells, but there are many others I can think of that might be of interest, like viruses and bacteria," he says. "You have your synthetic world on one side, and your biological world on the other, and we want to bridge the gap as best we can between these two extremes."