Asked by: James Forbes, by email
The metabolic needs of vertebrate brains are actually fairly simple – mainly oxygen and glucose. These can be supplied by connecting the blood vessels that supply the brain with an artificial blood substitute or by immersing the blood in an artificial cerebro-spinal fluid and oxygenating that directly. Guinea pig, dog and monkey brains have all been kept alive for hours or even days after being removed.
The problem is that, without an attached body, the health of the brain can only be assessed in a fairly basic way. Generally the uptake of oxygen and presence of electrical activity are taken as evidence that the brain is alive. Since there is currently no way to reattach the severed spinal cord, it is very difficult to judge whether the brain is still conscious and fully functioning.
Subscribe to BBC Focus magazine for fascinating new Q&As every month and follow @sciencefocusQA on Twitter for your daily dose of fun science facts.