Red blood cells travel through the bloodstream delivering vital oxygen to body tissues and taking away unwanted carbon dioxide - and they have to squeeze through blood vessels as thin as 3 micrometres across to do it. But in some diseases, such as malaria and sickle cell disease, red blood cells lose this ability to deform.
Because of the small size of red blood cells and the demanding work they do, nobody has succeeded in making artificial versions to help people with such conditions.
Now though Joseph DeSimone, a chemical engineer at the University of North Carolina at Chapel Hill, US, thinks he knows how.
He has created tiny sacks of the polymer polyethylene glycol just 8 micrometres across - in the range of human red blood cells - that are capable of deforming in a way that allows them to pass through the tiniest capillaries.
Polyethylene glycol is biologically benign, but binds easily with other substances, which makes it ideal for carrying cargo through the blood, says DeSimone.
For example, a haemoglobin-type molecule carried inside the bag could deliver oxygen to the body and carry away carbon dioxide. The bags could also deliver drugs instead, or help as contrast agents for scans such as magnetic resonance imaging, PET or ultrasound.
DeSimone has injected the particles into mice with "no adverse side effects", but there is no news yet of more extensive tests.