Researchers in the U.K. have achieved something of a world first: they have manufactured blood in the lab, which they’ve since administered to humans. The clinical trial will aim to test the safety and effectiveness of the lab-made blood in at least 10 healthy people. Two volunteers have already received a dose.

The scientists — from the University of Cambridge, the National Health Service and the University of Bristol — are keen to find out whether their novel blood can last as long as normal red blood cells (which normally stay alive for about 120 days inside the human body) and whether there are any side effects.

Blood Donations

Transfusing donated blood has saved countless lives, allowing patients to get through complicated operations in good health. Blood products also help to treat chronic conditions such as sickle cell anemia. But blood donation, as a system, has many drawbacks.

It requires a complicated infrastructure to collect and deliver blood where it’s needed safely. That requires adequate refrigeration all along the route, and while that might be relatively straightforward for developed countries, it remains a challenge elsewhere in the world. Rarer blood types also suffer from dwindling supplies in the blood banks, which often means it’s harder to find a suitable blood match for certain racial and ethnic groups. It’s also costly to maintain the infrastructure; the average donation of less than half a liter of blood costs the U.K.’s National Health Service approximately £130 ($155).

Read More: What Blood Types Can Reveal About Our Health

That’s why scientists from around the world, often funded by military agencies, have been searching for more practical alternatives for decades. Still, it’s an endeavor that has thus far enjoyed limited success.

“After 9/11, the U.S. Army invested millions of dollars in producing a blood replicant to be used for casualties in the battlefield, but it came to naught,” says Lt. Col. Matthew Armstrong, who studies the fluid dynamics of blood at the United States Military Academy at West Point.

Blood Transfusions to Lab-Grown Blood

Back in the 1600s, doctors tried transfusing milk and wine into their bleeding patients — needless to say, it didn’t go well. Then they moved on to using sheep’s blood. That didn’t work, either. Physicians eventually realized that human blood from a donor (with the same blood type as the recipient) is needed. A British doctor finally performed the first successful transfusion in 1818.

By World War II, the blood donation infrastructure was sufficiently developed to transport large volumes of blood across great distances to reach those in need. In 1940, before the U.S. formally entered the fray, the “Plasma for Britain” campaign shipped 13 million units of blood from the U.S. to those in the U.K. who were already fighting the Nazis. Then, following the war, attention turned towards finding more reliable, lab-made alternatives to traditional blood.

One approach has been to synthesize artificial substances that perform the same oxygen-carrying role as blood. Another line of attack has been to harness the power of stem cells to make real blood cells. This latest study is the most recent victory for those in the stem cell camp. 

The researchers started with blood donations and used specialized magnets to isolate stem cells, which were then transferred into a lab environment. Here, the scientists ensured the ideal conditions to make these stem cells proliferate into large numbers of red blood cells. It took about three weeks to grow 15 billion red blood cells; that might sound like a lot, but healthy adults have between 3.92 and 5.65 million red cells in just a microliter (one-millionth of a liter) of their blood.

Although the clinical trial is only using minimal amounts of lab-grown blood for now, the researchers hope to one day cultivate large volumes for people with rare blood types. It will be a challenge, however, to scale up this technology in a cost-effective way.