Biocompatibility

Any medical nanobot will have to interact closely with the chemicals of the body. Whatever the robot is built of, its surface must not provoke an allergic response. Most medical applications will require the detection and/or release of chemicals. The outside of a nanobot will be immersed in fluid, but the inside will probably be dry, at least with some types of mechanism. The interface between a nanomachine and the chemical environment of the body will form a large part of the design.

Carbon is an extremely versatile molecule; it can form linear or zig-zag chains, rings (benzene and other aromatic compounds), buckyballs (spherical molecules), sheets (graphite and buckytubes), or blocks (diamond). Chemists have been able to bond organic molecules to each of these forms of carbon, so we will be able to design the surface separately from the workings of the nanobot. We have been implanting gizmos into the body for decades, so we already know some materials we can use to make biocompatible surfaces. We can design surfaces that will remain separate from the body's tissues, or that will attract tissues such as bones or blood vessels to attach to them. Future research will give us more flexibility, but what we have today is good enough for most applications. Recently, researchers have even been able to make neurons grow through holes in a silicon chip, for the purpose of sensing the signals.

Each chemical compound has a certain characteristic shape, and also a pattern of electric charge on its surface. A pocket or pit of the same shape and lined with the opposite charge pattern will attract the desired chemical. This can be used to sense the presence of the chemical. If the pit is movable, it can be rotated inside the machine to take in chemicals for processing--a close-fitting pit would exclude most or all of the water and undesired chemicals, and deliver the desired chemical precisely packaged for the interior mechanism to work on. Likewise, a substance synthesized inside the machine can be moved outside; deforming the pit or changing the pattern of charge will make the chemical float away. Antibodies are nature's version of such pits; they attach themselves to chemicals with amazing specificity. Artificial pits or "binding sites" that attract specific molecules have been constructed.

Biotech researchers are already extracting molecular motors of several types from cells, and building systems to test the capabilities of the motors. Other researchers are building intricate shapes out of DNA molecules--an application nature never planned for, but potentially useful nevertheless.