The IEEE Spectrum is running a feature on the development of biological, prosthetic and mixed ("biohybrid") replacement organs through applied regenerative medicine or simply better, smarter materials science. "Part human, part machine, replacement organs may one day extend your life ... Bob is worried about getting old. As an engineer, Bob knows that the body is just one big system that runs by chemical gradients and electrical impulses. So why is it so difficult to come up with replacement parts when bodies like his start to break down?" It certainly isn't easy; the body is a very complex machine. Beyond the required scientific advances, there are no reasons why we can't just replace body parts as they fail, however - except the brain, which is going to require better technologies for in situ repair.