Fake blood is easy to make if you’re a movie director.

A little corn syrup, a little food coloring, and you’ve got all the gore your heart could desire. But you don’t want to inject that into your veins and be like, ‘oh, that’s going to help’.

Real blood is made up of cells that do lots of things, like closing wounds and fighting infections. But the thing we really need blood for is oxygen transport. Blood transfusions save lives, but there just isn’t enough donated blood to go around.

Even what we can scrounge up and refrigerate has an expiration date of around 42 days, so it’s extremely perishable in remote areas without refrigeration. So there is lots of motivation to develop a blood substitute, but it is really hard to do. Even if you have artificial blood that looks good in the lab, making sure it’s safe enough to use in the world is a whole other problem.

Medical professionals have a long wish list of things that a blood substitute should be able to do. First and foremost, it should be able to carry oxygen and drop it off at cells. It should last more than a few weeks at various temperatures without getting too damaged, and be easy to manufacture in decent amounts.

It should also be safe: it shouldn’t spread bloodborne diseases or cause dangerous immune responses in people with different blood types. In recent decades, there have been three main approaches to making blood substitutes that meet at least some of these criteria. One involves a class of synthetic chemicals called perfluorocarbons, which are mostly made of carbon and fluorine atoms and aren’t chemically reactive.

Perfluorocarbons can bind with gases like oxygen and carbon dioxide to carry them around like red blood cells do. But they’re hydrophobic molecules, and don’t mix easily with our water-filled bloodstreams. That means they have to be mixed with other chemicals to form what’s called an emulsion before they’re put into our bodies.

Now, a few perfluorocarbon-based blood substitutes are out there, but there haven’t been very many clinical trials and a lot of research has stalled. There have been concerns with safety, like treatments causing flu-like symptoms and other complications, and efficacy, like patients needing to breathe in extra oxygen because the perfluorocarbons can’t transport it as well as normal blood can. A second strategy is to use the actual oxygen-carrying protein in blood: hemoglobin.

Regrettably, we can’t just throw extra hemoglobin into people and call the problem solved. Your body keeps it tied up inside red blood cells for a reason -- hemoglobin can be toxic if it’s just floating around, because it’s really chemically reactive and can bind to different parts of cells, which messes with how they work. Past attempts have tried chemically modifying hemoglobin to not cause toxicity, but a lot of them still appeared to be unsafe or just didn’t work at the clinical trial stage.

Currently, some research groups are trying to make different kinds of synthetic chemical envelopes that keep hemoglobin tucked safely away, just like your red blood cells do. But it’s a tricky process: like, if these envelopes are too small, they might escape out of blood vessels. But still, this technology is appealing because a synthetic molecular shell means there’s no blood type problem.

And since it was never inside another human being, the chances for disease transmission are low. One team even thinks their molecule-wrapped hemoglobin can be freeze-dried and reconstituted in water for use, which means it’ll have a very long shelf life. But these hemoglobin-based substitutes are still a long way away from human trials.

That is not the case for this third approach: the UK’s National Health Service is set to test a blood substitute made of real red blood cells as early as this year! They’re hard to make, but not impossible -- thanks to stem cell technology. See, your body has lots of specialized cells, like red blood cells or brain cells or skin cells… all the cells.

And stem cells can turn into more specialized types when they’re exposed to different signals, from chemical cocktails to physical contact. The stem cells in this artificial blood treatment comes from sources like adult bone marrow or umbilical cord blood. And, with the right signals, they become functionally identical to the red blood cells already inside of us.

It’s hard to make mature cells from stem cells in the first place, and it’s really hard to make a lot them. So the researchers say they don’t intend for this to be a mass-market, one-size-fits-all solution to the blood shortage. Instead, it’s like a boutique approach.

Doctors could tailor small batches of blood for patients with rare blood types or other unique needs for transfusions. There probably isn’t a single solution to the need for artificial blood right now, because our red blood cells just do so many things so very well. But there’s room for different scientific approaches to work together, and hopefully some of them will start to work even maybe just a little bit sometime soon.

For more on the science of blood, check out our video where I talk all about the history of blood transfusions–I found it fascinating. Thank you for watching, and don’t forget to go to youtube.com/scishow and subscribe!