While most of us science-literate folks are watching the biotech revolution with tentative optimism, hoping for innovations like medicines that have no side effects because they’re tuned to a patient’s genes, or livers and kidneys grown to order for people with organ failure, some intrepid souls are taking much larger leaps. Based on the fledgling promise of stem cells and brain-machine interfaces, they wonder: Why tolerate chronic pain, or suffer irrevocable injury in accidents? Why become forgetful, get sick, or grow old?