Super Artificial Cells

Researchers have used fairly simple polymers to build cell-sized sacks that are as flexible as real cell membranes, but much stronger. The sacks could be used as a new way to deliver drugs, say the researchers, whose study appears in the current issue of Science.

For years materials scientists have tried reconstructing cell membranes following nature's recipe, which calls for fat-based molecules called phospholipids as the main ingredient. But as chemical engineer Dan Hammer of the University of Pennsylvania in Philadelphia says, "The main problem with phospholipids is that they aren't very strong." Thus they would be torn apart in the bloodstream, he says. Other labs had already used fragments of polymers to make tiny sacks hundreds of nanometers across. Hammer and colleagues Dennis Discher and Frank Bates attempted to scale up this process to make vesicles more than 10 micrometers in diameter--the size of human cells.

They coated platinum electrodes with a polymer made of ethyl ethylene and ethylene oxide, immersed them in a sugar solution, then rapidly alternated a current between the electrodes, causing the slightly charged polymer molecules to form a double layer that grew away from the electrodes and eventually pinched off to form a vesicle. Some of these vesicles were as large as 50 micrometers in diameter. The team found that the sacks were nearly as bendable and expandable as real cell membranes, but up to 20 times as strong. Hammer says the vesicles could be made even stronger by using chemicals to forge bonds between the polymer molecules. Such tough vesicles might eventually be used to create artificial cells for packaging drugs, Hammer says. A drug or its gene could be wrapped in the cell, possibly through a special pore, and the cells could be equipped with extra molecules that would make them flock at a site where they're needed, such as a tumor.

The technique holds potential as a drug delivery system, but it might be too hard to make vesicles quickly enough for commercial use, says Guojun Liu, a chemist at the University of Calgary in Alberta, Canada. With this technique, Liu notes, the researchers took more than an hour to create only a handful of vesicles.