Айзек Азимов

Four men and a woman are reduced to a microscopic fraction of their original size, sent in a miniaturized atomic sub through a dying man's carotid artery to destroy a blood clot in his brain. If they fail, the entire world will be doomed.

Fantastic Voyage II: Destination Brain is a 1987 science fiction novel by Isaac Asimov about a group of scientists that shrink to microscopic size in order to enter a human brain so that they can retrieve memories from a comatose colleague.