Nanophobic or Nanophilic?

by | Jun 6, 2011

Carbon nanotubes have the potential to unlock effective treatments for that most delicate of systems – the mammalian central nervous system. But conflicting studies and reports on potential toxicity underline the need for careful, standardized, and thorough research protocols.

One of the notable motifs of the work of the late US fiction author Michael Crichton was the cautionary tale of scientific inquisitiveness outstripping scientific caution. Be it dinosaurs, extraterrestrial bacteria, or outright extraterrestrials, curiosity most definitely ends up killing a number of scientific cats and usually wreaks untold havoc on an unwary public. Interestingly, one of Crichton’s later novels, Prey, published in 2002, tapped into the little-known (at that time) field of nanotechnology. While avoiding the now-infamous ‘gray goo’ scenario, the escape of autonomous swarms of nanorobots from research laboratories nonetheless makes for a chilling read.

Nanotechnology is, of course, nowhere so advanced as to be commercializing self-aware automatons, but good progress is being made, with developments and improvements to existing technologies and new applications, and ‘nano-something’ becoming increasingly common as that special ingredient that will block the sun more effectively, keep your tie stain-free, or enhance your killer smash in tennis. Leaving aside for the moment that this is, in many cases, simply a rebranding exercise on science that has existed for decades, nano-something is rapidly becoming the defining technobabble of early 21st century gadgetry.

One of the fields of research in which carbon nanotubes in particular are envisaged to play a critical role is in treatment for conditions affecting the central nervous system (CNS). The CNS is a delicate and complex system that lacks the ability to self-repair in adult humans, meaning many diseases that affect it are difficult if not impossible to treat, and physical damage is equally challenging to repair. As CNTs are able to penetrate cell membranes, they can be internalized by neurons, and their hollow structure would enable the transport and effective targeting of drugs to treat many debilitating CNS conditions. However, the question of the inherent toxicity of CNTs, as well as of the processes and chemicals used to make them physiologically compatible (CNTs are naturally hydrophobic and insoluble, and must be coated or functionalized in order to impart solubility) remains an open and serious issue. With no governing or regulatory body acting for nanotechnology as a whole (analogous to the FDA for drug development), oversight and standardization of testing are lacking.

A recent essay on this topic by a noted team of nanoscience researchers in Trieste and Strasbourg highlights the complexity of these issues, reporting on research results that show widely varying responses of neural tissue response to CNTs, some of which conflict with one another. CNTs are shown to be both toxic and non-toxic, sometimes even when using the same functionalization procedures and solvents. The problem arises from the myriad methods in which CNTS can be manufactured, purified, functionalized and coated. One batch shows toxicity, another does not; at the least, one has to identify and eliminate the potential effects of synthesis method, CNT type (how many walls), CNT size (length and width) aggregation, contamination (perhaps metal left over from synthesis), and purity. And then we come to the processes for making the CNTs soluble: which solvents, covalent or noncovalent, which coating, what impact does the coating alone have, what concentration of solvents or surfactants? And then finally, the product itself – toxic or non-toxic? Does concentration matter? What does the dose-response curve look like?

Such uncertainty calls for scientific caution, argue the authors (and I agree entirely). Many factors are involved in the production of biocompatible CNTs, and all of them may influence the final behavior of the nanotubes. Establishing rigorous methods for the assessment of nanostructure toxicity including detailed physical and chemical characterization of the as-manufactured nanomaterials, control testing on all added ingredients, and standard procedures for measurement will be invaluable in ensuring that evaluations of the risks and benefits of nanotubes and other nanostructures for medical applications are accurate and consistent. As nanotechnology starts to filter its way into our waste disposal systems with its obsolete first generation of technology, the issue of hazardous waste will be another in which a thoughtful, standardized approach will be needed. Nanotechnology has the potential to save lives, but to do so, it first requires for itself a clean bill of health.