In most cases, nanoscale systems will alter in physical size upon interaction with an aqueous system. For example, it is very common for many nanostructures to adopt a different chemical form simply through relatively minor interactions; consequently, size is not a constant factor in biological interactions, noted Colvin. Furthermore, the surface area can make up a sizeable fraction of these materials, and they can be derived to make many different biomedical systems. By changing surface coatings the nanomaterial toxicity can almost be completely altered. For example, changing the surface features of the materials can change a hydro-phobic particle into a hydrophilic one. Hypothetically, surface coats could, for instance, make it possible to eat nanoscale mercury if it has the right surface coating, while it may be dangerous to eat nanoscale table salt if the surface coating was not correct. For this reason, the scientists’ typical view of toxicology, which is driven by the composition of an inorganic particle, may have to be modified for nanoscale materials, because the surface is going to affect different dimensions of environmental and health effects, according to Colvin.
Chemists and engineers interested in creating biocompatible nanostructures need to understand their interactions with biological systems. Colvin suggested that the challenge that nanomaterials pose to environmental health is that they are not one material. It is difficult to generalize about them because, similar to polymers, they represent a very broad class of systems. Many engineered nanomaterials have precisely controlled internal structures, which are structures of perfect solids. Over a third of the atoms in a nanoparticle are at the surface, and these are extremely reactive systems, which in some cases can generate oxygen radicals (see oxidative stress later in the chapter); however, nanoparticles can be tied up very tightly in covalent bonds and wrapped with a polymer. Because of the size of nanostructures, it is possible to manipulate the surface interface to allow for interactions with biological systems. Colvin noted that with the correct coating particles below 50 nm can translocate into cells relatively easily and are able to interact with channels, enzymes, and other cellular proteins. Those particles above 100 nm, based primarily on size of the particles, have more difficulty. Through the interactions with cellular machinery, there is potential for medical uses, such as drug delivery and cellular imaging.
For applications such as drug delivery devices and therapeutics, there are well-established testing regimes utilizing whole animal and in vitro testing. How-