Eighty to 90 percent of combat deaths occur within an hour of wounding, and at least 50 percent of those die because of severe blood loss. The treatment for uncontrolled bleeding is the administration of red blood cells, plasma and platelets or their substitutes, according to the U.S. Army Combat Casualty Care Research Program.

The Defense Advanced Research Projects Agency is beginning a program this month to grow Type O negative -- universal donor -- blood outside the human body for combat casualties.

The need is great -- at least 16,000 troops have been severely wounded in Iraq and Afghanistan and many need additional blood.

The program would develop an automated progenitor cell-based culture system to produce red blood cells of a normal structure and function. Scientists proved they could recreate mouse blood from mouse bone marrow stem cells in 1999, but generating large amounts of a particular type of human blood ready for transfusion is a far greater challenge.

The military is also looking into ways of extending the shelf life of donated blood for use on the battlefield.

Blood products are temperature sensitive, making them difficult to maintain in austere environments like Iraq and Afghanistan. Red blood cells last only six weeks under constant refrigeration and shipment at 1 to 10 degrees Celsius. Plasma is supplied frozen and must be maintained at -20 degrees Celsius and must be shipped with dry ice; it has to be thawed and refrigerated before use. Platelets have just a five-day shelf life. They must be maintained at 20 to 24 degrees Celsius with constant gentle agitation.