Fantastic Voyage II: Destination Brain is a 1987 science fiction novel by Isaac Asimov about a group of scientists that shrink down to microscopic size in order to enter a human brain so that they can retrieve memories from a comatose colleague.
It is a sequel to Fantastic Voyage, the official novelization Asimov wrote (with minor changes to fix plot holes) for the 1966 film; but because Asimov did not have rights to the film's characters, it shares only the central concept of scientists shrinking to enter a human body for medical and political reasons.
The voyage takes place in a fictional mid- to late-21st century Soviet Union, inside the comotose body of an eccentric Soviet scientist. An American scientist was kidnapped by the Soviets to take part in the voyage. At the time of the novel's publication, the Soviet Union was still in existence.
This novel is implied to be included in the Robot/Foundation Series' fictional universe. On page 327 of the novel, an American scientist describes fictional components of a robotic positronic brain.