Carbon nanotubes (CNTs) are cylindrical nano-sized structures made entirely out of carbon atoms. Each carbon atom is covalently bonded to three other carbon atoms, forming strong hexagonal structures from which the tube is made of. This type of structure is also known as a fullerene.
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Carbon nanotubes can be found either in the form of single-walled carbon nanotubes (SWCTs), or have more than one layer called multi-walled carbon nanotubes (MWCTs).
The use of carbon nanostructures dates back to 1985 when the first carbon nanotube was discovered. Since this time, the field has undergone rapid development and will play an important role in the future of material and medical technology.
How are drugs loaded onto carbon nanotubes (CNTs)?
Most drugs can be covalently bonded onto a carbon nanotube, non-covalently adsorbed to the nanotube, or encapsulated inside. The choice of method for loading the drug onto the carbon nanotube depends on the structure and size of the drug.
These options all tend to reliably transport the drug to its target, but improvements to the strength of the bonding between the carbon nanotube and the drug are being made to improve the efficiency of this drug delivery method.
In order to target an antimicrobial drug to certain types of cells while using a carbon nanotube as a delivery system, a specific targeting ligand must also be attached as well as the drug. These targeting ligands, such as small peptides or antibodies, only bind to specific molecules and can, therefore, be used to narrow down the cells that the drug is delivered to.
The use of these targeting ligands does not produce perfect specificity to an exact cell or receptor target, but it greatly increases the effectiveness of the bound drug by ensuring more of the drug reaches the target area.
Modifying carbon nanotubes for drug delivery
Further modification of standard carbon nanotubes is needed in order to produce effective functional carbon nanotubes for drug delivery. These modifications normally involve adding different functional groups to the carbon nanotube with an aim to increase their solubility and decrease any toxic effects.
Fluorescent or radiolabeled compounds are also often attached to carbon nanotubes (initially used for tumor targeting) in order to provide a method of tracking of where the drug has been distributed to.
After the carbon nanotubes have been modified effectively, they can then be administered to the body where they can target specific types of cell and cross the cell membranes. The drug contents are released into these cells and the cell death follows.
Advantages of carbon nanotubes over other antimicrobial drug delivery systems
One important factor for antimicrobial drug delivery is cell penetration. This is the ability for a drug to be taken up by the target cell in order to exert its desired effect. Without cell penetration, the drug can often never have a strong effect - even if it is very potent per se.
Carbon nanotubes show a very good level of cell penetration compared to other drug delivery systems like liposomes and dendrimers, meaning that they can administer more drug to cells and have an increased antimicrobial effect.
They are also able to hold more drug content for their size than most other antimicrobial drug delivery systems, making them a very efficient choice for drug delivery.
Carbon nanotubes also have a low level of toxicity in the body and do not produce an immune response. This means they can be at relatively high concentrations in the body without much negative effect.
In general, carbon nanotubes have the highest strength to weight ratio of any known material, making them very stable when containing drugs. This allows them to proliferate within the target tissue for longer without being broken down.
All of these factors make carbon nanotubes very advantageous as a method of delivering targeted antimicrobial drugs. They can contain high quantities of the drug, and have a very low chance of reacting with its cargo before delivery to the target site, as CNTs are relatively inert.
Sources
- Osipov, V., et al. Carbon Nanostructures. BLTP. theor.jinr.ru/disorder/carbon.html.
- What are Carbon Nanotubes? Understanding Nano. understandingnano.com/what-are-carbon-nanotubes.html.
- Bianco, A., et al. (2005). Applications of carbon nanotubes in drug delivery. Current Opinion in Chemical Biology. doi.org/10.1016/j.cbpa.2005.10.005.
- Liu, Z., et al. (2010). Carbon nanotubes in biology and medicine: In vitro and in vivo detection, imaging, and drug delivery. Nano Research. doi.org/10.1007/s12274-009-9009-8.
- Sengel-Turk, C. T. & Alpturk, Onur. (2018). Carbon Nanotubes for Drug Delivery. researchgate.net/publication/324654187_Carbon_Nanotubes_for_Drug_Delivery.