It’s difficult to sidestep hyperbole when speaking of carbon nanotubes. Vastly stronger than steel, infinitely lighter than silk, and significantly more flexible than either material — in terms of thermal and electrical conductivity — carbon nanotubes offer exciting possibilities for any number of industries.
There’s an obvious temptation to speak of uncharted territory in the realm of targeted drug delivery, for instance. Already, carbon nanotubes (CNTs) have been investigated for their ability to deliver chemotherapy drugs and antibiotics to specific cellular targets. At just a couple of nanometers in width, these amazing synthetic structures can pass through virtually any cellular membrane unimpeded, without triggering any toxic response. And that’s just the beginning.
What Are Carbon Nanotubes?
Although they may have been discovered considerably earlier, most people credit the discovery of carbon nanotubes to Japanese scientist, Sumio Iijima, who first described the curious structures in 1991.
Carbon nanotubes are unimaginably thin-cylindrical structures made from sheets of carbon just one atom thick. The process begins with graphene. Comprised of an almost impossibly gossamer-thin layer of carbon atoms, arranged in a single-atom-thick honeycomb lattice, graphene is essentially a two-dimensional sheet. It is one of many allotropes, or alternative forms, carbon may assume.
Diamond is another familiar example. In diamond, carbon atoms are arranged in a three-dimensional lattice. Despite the delicate appearance of graphene, the chemical bonds among individual atoms are significantly stronger than in diamond’s three-dimensional lattice. Graphene is about 100 times stronger than steel, for example.
When a sheet of graphene is coaxed into forming a cylinder, a carbon nanotube results. Nanotubes made from graphene sheets possess the greatest tensile strength of any known material. Multiple nanotubes self-align to form “ropes”.
However, tubes may easily tangle, and may require deagglomeration and selective length reduction to become optimally useful, especially for biological applications. High-shear processors and other technologies may be used to enhance the performance characteristics of agglomerated tubes.
Folding Determines Variable Properties
Like an Asian-origami artist summoning diverse shapes from a single sheet of paper, it’s possible to alter the properties of a given nanotube through manipulation of the angles used during folding. Folding along a 90-degree axis yields the simplest, so-called “armchair” configuration, for example. Fold slightly more along the bias, and you get a tube called “zigzag”. Another tweak yields the “chiral” conformation. Each has differing thermal, electrical and physical properties.
Carbon nanotubes are now being generated with either single or multiple walls. Single-walled carbon nanotubes (SWNTs) typically have a diameter of just a nanometer or two — about 50,000 times less than the diameter of a human hair. But the potential length limit — if one exists — has not yet been reached. Presently, half-meter-long nanotubes have been reported.
Multi-walled carbon nanotubes (MWNTs) consist of tubes within tubes, like Russian nesting dolls. With post-production tweaks, researchers report it’s possible to reach “near-ultimate” tensile strength with MWNTs.
Bionanotechnology is Here
In 2000, the cost of SWNTs was about $1,500 per gram. By 2010, prices had plummeted to just $50 per gram. Needless to say, this dramatic but welcome drop in the cost of a key raw material has sparked a modern gold rush of sorts, as scientists scramble to investigate new CNT applications for industries as varied as optics, electronics, pharmacology and medicine.
The prospects for new pharmacological and medical applications alone are exhilarating. Given their infinitesimal size, CNTs can readily pass through cell membranes. The race is on to design CNTs with convenient payloads that may include everything from drugs, to genes, to biomolecules to vaccines and so on to be delivered with precision to previously unreachable places. Other potential applications include biosensor diagnostics and tissue regeneration.
The latter involves the potential use of collagen-coated CNTs as molecular scaffolds to grow tissues, bone, and even artificial-implantable organs. Because they resist biodegradation, CNTs are expected to be superior to present candidate materials.
Pristine CNTs are highly hydrophobic, so they must be bonded to other more biocompatible molecules to become suitable for deployment in the human body. Methods to achieve this level of functionality have already been developed. They involve covalent and non-covalent functionalization processes that either covalently bond or physically encase nanotubes with hydrophilic molecules.
These “functionalized nanotubes” are then capable of carrying molecular payloads across cytoplasmic and nuclear membranes, without eliciting any toxic effects. This renders the drug-CNT conjugate significantly safer — and more effective — for drug delivery than many existing drug formulations.
Present and future biomedical applications include anti-tumor immunotherapy, infection therapy, gene therapy by DNA delivery, biosensor vehicles for diagnosis and detection, enantioseparation of chiral drugs and biochemicals, local anti-tumor hyperthermia therapy, tissue regeneration and artificial implants, neurodegenerative disease therapy (nanotubes cross the blood-brain barrier with ease) and applications involving free-radical scavenging activities (antioxidants), among other uses.
In truth, we’ve barely scratched the surface regarding what’s possible with CNT technology. One thing’s clear, though, carbon nanotubes are transforming the future of biomedical and pharmacological science. The future is bright, but the full potential of these remarkable structures remains to be elucidated.
Brady Veitch works for Microfluidics and has over 10 years of experience in the healthcare, biotech, pharma, and medical device sectors. For more information on this topic, or to contact Brady, send an email here.