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NASA Science News for May 29, 2003

Artificial Cells

NASA-supported researchers are learning to make designer cells for
dehydrated blood supplies and space-age medicines. 

May 29, 2003:  Red blood cells are great at carrying oxygen.
Unfortunately, that's about all they do. Let's face it: with a little
bit of help, they could be a lot more useful. 

Imagine, for example, blood cells that could carry all kinds of
things--medication as well as oxygen. Imagine blood that could be
dehydrated, and stored for months or even years at a time. It could
be carried by medics onto a battlefield--or by astronauts into outer
space. Imagine blood that could be used for transfusions with no risk
of AIDS or any other disease. 

A group of university researchers are helping NASA develop an
artificial cell that can do all this and more. 

Bioengineers Dan Hammer and Dennis Discher of the University of
Pennsylvania and Frank Bates of the University of Minnesota have
created a special kind of molecule--a polymer--that forms something
very like a cell membrane, and they've been able to coax these
membranes into artificial cells, or polymersomes, that are stronger
and more easily manageable than the real thing. 

A polymer is simply a chain of smaller molecules that have been
linked together. The cellulose in plants and the wool on sheep are
natural polymers. Man-made polymers can be found in everything from
nylon stockings to car parts to furniture stuffing. 

The polymers used in polymersomes are larger and heavier than the
natural molecules in cell membranes: They've got a molecular weight
of over 3600, compared to about 750 for phospholipids, the fatty acid
molecules used by cells. 

Manmade molecules can be crafted with an important characteristic,
which many naturally occurring molecules share; they can be
engineered to be amphiphilic, where one end seeks water, and the
other end avoids it. In a water-based solution, such molecules
spontaneously arrange themselves into a double-layer with their
hydrophobic (water fearing) tails in the middle and their
hydrophilic (water loving) heads on the outside. 

"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. 

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. 

Polymersomes can also be engineered so that some types of cells do
react to them. Hammer, Discher and colleagues can add to their
polymersomes particular molecules that latch onto the cells they're
targeting. Typically, says Hammer, the polymersomes float through the
bloodstream for about 18 hours before they reach their destination
and grab onto the target cells. 

The key word is "target." Doctors using polymersomes wouldn't have to
pepper the entire body with medications. They could be targeted--sent
only to the places they're needed. Arthritis medications, for
example, could be sent only to a patient's swollen fingers, without
the risk of causing reactions elsewhere. Polymersomes could carry
cancer-zapping pharmaceuticals directly to a tumor. They could
incorporate imaging agents like iron oxide particles, which can be
detected by magnetic resonance imaging. If these particles are
encapsulated into polymersomes designed to latch onto cancer cells,
they'd be able to locate small tumor cells that have migrated through
the body.

Polymersomes could theoretically be designed to carry both the
imaging agents that locate a problem, and the medication that treats
it. 

Using manmade materials to produce an artificial cell is "a highly
novel concept," says Hammer. "I think that NASA saw this as a
wonderful material, and they wanted to see how far it could evolve."
In some conditions, he says, polymersomes take on shapes that are
very reminiscent of the ones biological cells take on when, for
instance, they're dividing. 

And Hammer and his colleagues are still exploring the possibilities.
They're experimenting with different types of polymers, to see how
the capabilities of artificial cells can be expanded. 

The most exciting applications of polymersomes, believes Hammer, are
still to come. 

Credits & Contacts
Author: Karen Miller 
Responsible NASA official: Ron Koczor 
Production Editor: Dr. Tony Phillips 
Curator: Bryan Walls 
Media Relations: Steve Roy

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