Carbon nanotubes are a fascinating class of organic molecule. Imagine a hexagonal grid drawn on a sheet of paper, with all of the hexagons sharing common sides. At all of the common corners (where the lines intersect) lies a carbon atom. Now, imagine that you take that piece of paper and tape two sides together to make a rolled tube. What you’re looking at is a fairly accurate representation of a carbon nanotube. Each of the lines connecting the carbon atoms is a rigid bond, and since all of the hexagons share common sides, the entire structure is rigid. The inside of the tube is hollow and the cavity size can vary depending on how “tight” the tube is rolled. The internal diameter can vary from just a few tens of nanometers up to around 500 nanometers; since each nanometer is only 0.000000001 meters, the nanotubes just look like a shiny black powder to the human eye. A microscope is needed to see the actual hollow tubular structure.
Scientists are interested in carbon nanotubes for lots of different reasons, but a research article from the Journal of Materials Chemistry that I read today discusses nanotubes as possible drug delivery systems. The human body is pretty much incapable of breaking down and metabolizing a carbon nanotube, as structures similar to the tubes don’t occur naturally. Humans haven’t evolved any enzymes which can attack the carbon molecules. Therefore, carbon nanotubes can pass throughout the bodys systems and still retain its hollow tube shape. This particular research article, which was submitted by Australian chemists, outlined a method by which the nanotubes could be filled with a liquid and then “capped” off by a plastic coating over the top and bottom of the tube. Almost like a Pringles can, to use an analogy.
The chemists outline their synthetic method in some detail. They chose nanotubes which averaged 100 nanometers in diameter, and soaked a small sample of the microscopic tubes in a water solution for several hours. Water molecules are small enough that they can fit into the hollow center of the tubes, and the random molecular motion of the water molecules drives a process of diffusion; the water molecules gradually find their way into the center of the tubes until all of the tubes are water-logged. The researchers used water in their tests, but the liquid could just have easily been any number of liquid small-molecule pharmaceuticals. After the water had flooded the tubes, the carbon powder was filtered from the water and then the carbon nanotubes were placed in a toluene solution of a polymer called polycaprolactone. As the polymer molecules begin to diffuse into the tubes, the polymer hits the water lining the inside of the tubes and precipitates out of solution. This solidifies a plastic “bottle cap” coating on the tops and bottoms of the hollow tubes, effectively sealing in the water molecules that had drifted into the carbon nanostructures. The tubes are then filtered and dried.
The result, according to the scientists analysis, is a collection of nanotubes that still resembled a shiny black powder but which were actually acting more like “nanocapsules”. Just like a honeycomb which is filled with honey and then sealed with wax, the nanotubes serve as a convenient carrier for the liquid at their center. If someone were to ingest the nanotubes – simply by swallowing them – the plastic tube caps would be dissolved (the polymer is biodegradable and biocompatible) and the liquid would then be released. The emptied hollow tubes would then just be excreted by the body, as humans have no way to digest the actual nanotubes.
This type of technology is paving the way for future drug delivery systems. Obviously, it’s not ready for commercialization just yet. Scientists need to develop a method of directing the nanotubes to a particular part of the body; this will accumulate the dosage at the site of a tumor, or a disease, and allow the chemical payload of the tubes to be released precisely where it’s needed. However, this research is a fantastic proof-of-concept for the development of a nanocapsule, and the article itself is a very interesting read.
The source of this article can be found at: http://www.rsc.org/Publishing/Journals/JM/article.asp?doi=b714541c