Over the years, nanotechnology researcher Robert Freitas has built up an impressive and detailed body of work on how to build real, working nanorobots for medical and human enhancement purposes. In his own words:
My professional goal for the last two decades has been, and continues to be, to help make life-extending medical nanorobotics technologies happen as fast as humanly possible. ... I've been trying to figure out how to build diamondoid nanorobots, starting from current manufacturing technologies.
As noted over at the Foresight Institute, the lastest installment in this large body of work is available for those interested in the path to molecular manufacturing and programmable nanodevices that can repair our cells - or even replace our cells to perform with greater efficiency than evolved biology is capable of achieving.
Robert A. Freitas Jr., author of the Nanomedicine series of books, has just published a major new theory paper on aspects of medical nanorobot control, providing an early glimpse of future discussions of this topic that are planned to appear in Chapter 12 (Nanorobot Control) of Nanomedicine, Vol. IIB: Systems and Operations, the third volume of the series (still in preparation).
The chapter is about 5.2 MB in size and a draft preprint version may be downloaded from Freitas' nanomedicine website.
From the paper itself:
Medical nanorobotics is the most powerful form of future nanomedicine technology. Nanorobots may be constructed of diamondoid nanometer-scale parts and mechanical subsystems including onboard sensors, motors, manipulators, power plants, and molecular computers. The presence of onboard nanocomputers would allow in vivo medical nanorobots to perform numerous complex behaviors which must be conditionally executed on at least a semiautonomous basis, guided by receipt of local sensor data and constrained by preprogrammed settings, activity scripts, and event clocking, and further limited by a variety of simultaneously executing real-time control protocols.
If you're new to all this, I strongly suggest you start with a general overview like Chapter 10 of Unbounding the Future: the Nanotechnology Revolution. It may be nearly 20 years old, but the end goals it describes remain the same:
Our bodies are filled with intricate, active molecular structures. When those structures are damaged, health suffers. Modern medicine can affect the workings of the body in many ways, but from a molecular viewpoint it remains crude indeed. Molecular manufacturing can construct a range of medical instruments and devices with far greater abilities. The body is an enormously complex world of molecules. With nanotechnology to help, we can learn to repair it.
Medical nanotechnology (and the medical application of nanorobotics) should be thought of as one future rung on the ladder of advancing biotechnology and computational power. It is a forseeable better way to gain greater control over our cells, and eventually replace those cells with better hardware - a great deal of thought, planning, and development effort is presently going into the precursor technologies that will lead to medical nanorobotics. Look at Zyvex, for example, as one of the present initiatives aimed at building the tools of dry nanotechnology that will lead to molecular manufacturing that will lead to wet nanotechnology and then medical nanorobotics.
Again, this is all about the tools needed to exert greater and more precise control over all the cells in our bodies. We want to be able to manipulate cell state to order, clean out the accumulating junk that damages cells, perform in situ repairs on damage that causes cellular machinery to run awry, and shut down cells before they turn cancerous or senescent, to pick out a few examples. We want to be able to do all these things efficiently and effectively for all of our cells at once. If we can develop technology to perform these tasks well enough, then we can halt aging itself by repairing our biology faster than it falls apart.