Parabiosis: the idea is connecting the circulatory systems of two animals: mostly this has been done in mice. Interestingly, if you connect an old mouse to a young mouse, the young blood seems to rejuvenate the old mouse, improving its muscles and central nervous system.
Fairly recently, some people are looking fairly seriously at this in anti-aging research. But a lifetime ago, people were already thinking about it, enough so that parabiosis showed up every now and then in the world of ideas. Something like it exists in Brave New World. An improved, better-thought-out version plays in a role in Robert Heinlein’s novel Methuselah’s Children. There we find a group (the Howard families) that have long lives due to a program of selection [which is certainly possible, although slow]. Normals envy them and demand the secret – since there is none, the Howard Families are forced to flee. In their absence, the normals find the secret anyhow – they find artificial means of replicating the various biochemical mechanisms that resulted in extended lifespans in the Howard Families. One such method involved mass production of artificial ‘young” blood via tissue culture – which method is under active development today.
Peter Thiel is apparently interested in this and is even said to be taking plasma injections. Although I suspect he just thinks the cape is cool. Shaving must be hard.
Reminds me a bit of phage therapy: developed a lifetime ago, largely pushed to the side by antibiotics, but making a comeback. And of course it too was part of the mental furniture at one time – it’s central to the plot of Arrowsmith, by Sinclair Lewis.