Just like real blood cells, the imitations could squeeze through spaces much smaller than their own diameter and absorb and release substances to order, including oxygen.

They could be used to disperse drugs, or the contrast agents used in medical imaging, throughout the body with fewer side effects than direct injection.

The fake cells could also be given to people who have lost blood instead of a blood transfusion.

Real red blood cells owe their astonishing agility to their "biconcave" or tyre-like shape, and to get the same kind of synthetic particles, Samir Mitragotri and his team got their inspiration from the way real red blood cells acquire their final shape in the body.

They start out as spherical cells, which then collapse into mature red blood cells following exposure to various substances.

Similarly, researchers found that if they added small balls made of a polymer called PLGA to a particular solvent, the spheres would collapse into a biconcave shape.

The researchers coated these 7-micrometre across, tyre-shaped particles, in a layer of protein.

When they dissolved away the polymer core, a soft biodegradable protein shell was left behind with the same mechanical properties as red blood cells.

"The soft protein shell makes them squishy and elastic. They can squeeze through capillaries smaller than their own diameter, just like real blood cells," New Scientist quoted Mitragotri as saying.

The fake cells also seem to share red blood cells' ability to transport substances.

One of the proteins Mitragotri added to the surface of the imitation blood cells was haemoglobin, the molecule that binds to oxygen in the lungs, later releasing it elsewhere in the body.

In test tube experiments, the researchers found that their haemoglobin-coated particles picked up oxygen when there was a lot around and released it later when the concentration was lower.

If the squishy particles do the same thing when injected in animals, they could be given to people instead of a blood transfusion.

Mitragotri has claimed that the particles could provide a way to get drugs into the body at a more constant concentration, or substances such as iron oxide nanoparticles, which increase contrast in magnetic resonance imaging.