Artificial Cells

"That was our insight," said Hammer. "We realized that there's nothing that prevents a polymer from forming a bilayer like a phospholipid would."

But polymersomes have one huge advantage: they can be controlled. By adding in different molecules, researchers are learning to manipulate their abilities and make them do things that biological cells just can't manage.

For example, polymersomes can be made strong. While it's true that the phospholipids in natural membranes hold together, they don't bond with each other very tightly. They move around within the cell membrane, and, without the pressure of a watery environment, they fall apart.

Credit: University of Pennsylvania Giant, 2-20 mm, polymersomes in phosphate buffered saline – visualized by phase-contrast microscopy (internal solution of 300 mM sucrose).

Polymersomes, on the other hand, can be designed so that they cling to each other tightly. Their atoms can bond not only within a single polymer, but also to the polymers next to them. This is called cross-linking, and it vastly increases the strength of artificial cells. (It's cross-linking that stiffens the curls in a beauty-shop permanent enough to keep the shape of the hair-do.) In fact, between cross-linking and the increased molecular weight of the polymers, polymersomes are a thousand-fold stronger than phospholipid cells.

"Probably the main advantage from NASA's point of view," says Hammer, "is that once the polymersomes are crosslinked, the cells become durable enough to be dehydrated into a powder." They can be stored easily, for a long time, and without taking up much space. In other words, it would be a perfect way to carry extra blood for medical emergencies on long distance voyages in outer space.

That, in fact, is the use that he and his colleagues initially envisioned, says Hammer. But they quickly realized that the polymersomes could be used for transporting other things.

Hammer explains: It's easy to encapsulate many kinds of molecules with polymersomes; such artificial cells could then be sent throughout the body. Because their outer membrane consists of molecules that don't interact with cells, polymersomes are invisible to the immune system. They can travel unhampered through the bloodstream.