Researchers have developed a line immortal stem cells that allow them to generate an unlimited supply of artificial red blood cells on demand.

If these artificial blood cells pass clinical trials, they'll be far more efficient for medical use than current red blood cell products, which have to be generated from donor blood - and would be a huge deal for patients with rare blood types, who often struggle to find matching blood donors.

The idea isn't for these immortal stem cells to replace blood donation altogether - when it comes to regular blood transfusions, donated blood still does the trick.

But it's a constant struggle to propagate red blood cells from donor blood. In the UK alone, 1.5 million units of blood need to be collected each year to meet the needs of patients, particularly those with rare blood types of conditions such as sickle-cell disease.

"Globally, there is a need for an alternative red cell product," said lead researcher Jan Frayne from the University of Bristol in the UK.

"Cultured red blood cells have advantages over donor blood, such as reduced risk of infectious disease transmission."

In the past, researchers had attempted to turn donated stem cells straight into mature red blood cells - a technique that works, but is an incredibly inefficient process.

Each stem cell only makes around 50,000 red blood cells before it dies off, at which point the researchers need a new blood donation.

And while 50,000 might sound like a lot, put that into perspective, a typical bag of blood used in hospitals contains around 1 trillion red blood cells.

To overcome this, the University of Bristol team took a different approach - they turned adult stem cells into the world's first line of immortalised 'erythroid' stem cells - erythroid refers to the process that produces red blood cells.

They've called the cell line Bristol Erythroid Line Adult or BEL-A cells.

To create these 'immortal' cells, they effectively trapped the adult stem cells in an early stage of development, which means they can divide and create red blood cells forever without dying, which avoids the need for repeat donations.

"Previous approaches to producing red blood cells have relied on various sources of stem cells which can only presently produce very limited quantities," said Frayne.

"We have demonstrated a feasible way to sustainably manufacture red cells for clinical use," she told the BBC. "We've grown litres of it."

If immortal stem cells sound familiar to you, that's because there's another famous line of immortal stem cells used in labs around the world, known as HeLa, which was taken from the tissue of a woman called Henrietta Lacks without her knowledge.

Lacks was an African American woman who had a cancerous tumour biopsied in 1951, and never knew those cells were turned into the immortal HeLa cell line, which has played a crucial role in key milestones such as the development of the polio vaccine, as well as major cancer studies, and continues to be used today.

These BEL-A immortal stem cells, on the other hand, were specifically selected from voluntarily donated blood products with the sole aim of generating adult human blood cells.

If their red blood cell products pass clinical trials, they could prove just as revolutionary and useful as Lacks' cells did.

"Scientists have been working for years on how to manufacture red blood cells to offer an alternative to donated blood to treat patients," Dave Anstee, director of the UK's National Institute of Health Research Blood and Transplant Research Unit in Red Cell products, which collaborated on the research, announced in a press statement.

"The patients who stand to potentially benefit most are those with complex and life-limiting conditions like sickle cell disease and thalassemia, which can require multiple transfusions of well-matched blood."

"The intention is not to replace blood donation but provide specialist treatment for specific patient groups," he added.

"The first therapeutic use of a cultured red cell product is likely to be for patients with rare blood groups because suitable conventional red blood cell donations can be difficult to source."

The artificial red blood cells still need to undergo clinical trials in humans before we can say for sure that they're safe and effective.

But early safety trials based on previous manufacturing methods will begin by the end of this year. If that goes to plan, the researchers will trial the BEL-A cell products in patients shortly after that.

We'll be watching the progress closely.

The research has been published in Nature Communications.