Steve Connor: The promise of an unlimited supply for transfusions

More than two million units of blood – some 250,000 gallons – are handled by the transfusion service each year. Yet shortages can and do happen, especially around holiday periods and special events, such as the recent World Cup, when many donors stayed at home.

But one of the greatest problems facing the transfusion service is the growing menace of new infectious agents which can pass undetected into the blood supply. Hepatitis C, HIV and more recently the infectious "prion" behind Creutzfeldt-Jacob disease have all emerged in the population before there was a reliable blood test to screen donated blood.

The issue of emerging infections is likely to get worse with greater international travel and the continued encroachment of humans on the wild places where many of these animal-borne diseases lurk in their natural reservoirs. West Nile virus, for example, is passed on by mosquitoes, but in 2002 the first cases in the US occurred as a result of transmission from infected blood donors.

Developing safe, synthetic alternatives to blood is not, therefore, of simple academic interest. The central function of red blood cells is to carry oxygen around the body and for many decades scientists have laboured over potential alternatives to living red blood cells, from oxygen-carrying chemicals known as perfluorocarbons to the use of the natural oxygen-carrying pigment, haemoglobin.

None of these approaches has really proved a success. However, the manufacture of "synthetic" red blood cells derived from embryonic stem cells makes perfect logical sense. The red blood cell is the vehicle the body uses to oxygenate the tissues and everything we know of embryonic stem cells suggests they can be made to produce red cells. Stem cells from four-day-old spare IVF embryos are also likely to be free of infectious diseases.

Scientists have shown already that mature red blood cells can be made from the adult blood stem cells found in the bone marrow, although these stem cells have limited ability to replicate. Stem cells derived from embryos, however, can multiply ad infinitum, meaning that in theory just one "universal donor" of blood type "O-negative" could supply the country's entire needs.