Peter Thiel, a very wealthy man who often invests in biotech, is into parabiosis. Thatís the process by which the blood of the young is infused into the less-young; thereís lots of animal data showing it can combat the effects of aging.

But despite its success as a Radiohead song, donít count out on that idea being the next big thing in biotech just yet.

Thielís interest, according to Inc, is in a company called Ambrosia, which is testing whether such blood transfusions can improve age-related deterioration. Never mind, for a minute, the fun of imagining a bleak, hypercapitalist technodystopia in which billionaires like Thiel can live forever by buying the blood of the poor on an open market. Is this a real business?

Ambrosiaís founder admits that thereís no clear path to market for his method, which relies on donor blood. What would make more sense would be crafting a drug to replicate the effects of transfusion. But that has so far been hampered by a pretty serious stumbling block: No one knows why transfusions work.

In 2014, researchers singled out a protein, GDF11, that was particularly abundant in young blood versus old, making it seem like an ideal drug target. But last year, a separate team of scientists tested whether bombarding mice with GDF11 would reverse aging ó only to find that it did the opposite.

Now other companies are working on other proteins and plasmas in hopes of hitting on what makes transfusions effective. But until thereís substantive clinical data, immortality remains unattainable. Even for billionaires.