BIOMEDICAL APPLICATIONS OF CARBON NANOTUBE MATERIALS
Biomedical engineering is one of the most exciting areas in which experimental work is being done with carbon nanotubes (CNTs). The unique combination of properties that carbon nanotube molecules possess and the fact that they can be used to create multifunctional materials with novel properties may mean that CNTs will become a part of entirely new medical treatments and procedures in the coming years.
Biomedical uses for CNTs can be divided into three major categories. First, there are a number of techniques that make use of individual CNT molecules. Some of these techniques take advantage of the fact that long, thin CNT molecules interact strongly with infrared light, like mini antennas, generating a strong response when even a relatively small load of CNTs are exposed to it. For example, there are techniques that use nanotubes for improved imaging within the body or to seek out and destroy cancer cells by heating up in infrared light (see examples of this research here, here, and here). Some individual CNTs are also thin enough to easily penetrate cell walls, so they can also be used for delivery of drugs of gene therapies directly into cells. Second, there are techniques that include CNTs as a part of another material. For example, CNTs can be added to hydrogel scaffolds that are used to promote cell growth; the CNTs add a stiffer mechanical structure to the soft hydrogel and can make it electrically conductive (which can be helpful for some types of cells). Finally, there are devices and experimental treatments that make use of solid, macroscopic materials made entirely of CNTs. Here at DexMat we are particularly excited about this last category, since our manufacturing methods could be used to make such materials in the future. In the remainder of this post, we want to highlight a few examples of experimental medical technology based on solid CNT materials; this isn’t meant to be an exhaustive review, just a look at some of the exciting work being done on the topic!
Solid CNT materials tend to have very high surface area (since they are made from many thin overlapping cylinders with micro- or nano-scale pores between them) and therefore have low electrochemical impedance; that is, they can form a good electrical connection with electrolytic fluids, such as saline or other fluids in the body. The low electrochemical impedance of CNT materials, coupled with the fact that they can be soft, flexible, and durable, makes them useful as electrodes which can be implanted in the body. In many cases a metal electrode might be stiff enough to cause damage to surrounding tissue, or it might suffer from a low fatigue life and risk breaking apart in the body after a certain amount of time; flexible CNT electrodes can potentially avoid these drawbacks.
Thin CNT fibers have been studied for use as electrodes to be implanted in the brain for neural monitoring or for deep brain stimulation therapy. Several studies have been performed on this application using rodents; see this work from Rice University and another from researchers at Peking University, Georgia Tech, and the Chinese Academy of Sciences. In both studies, the researchers saw a great benefit from the small size of the CNT fibers, which allowed for precise recording of signals from specific areas of the brain or even from individual neurons, and from the softness and flexibility of the fibers, which allowed them to cause less damage and inflammation to surrounding brain tissue than stiff metal electrodes might have. The researchers in Peking University also note that the CNT fibers are MRI-compatible, creating less interference with MRI brain scans than Platinum-Iridium wires of similar size.
The flexibility and durability of CNT fibers also makes them attractive for use as electrodes to be implanted in or around heart tissue. Pacemakers and implantable cardioverter defibrillators require electrodes that will remain in firm contact with the heart tissue at all times, which means they have to withstand the constant movement of the beating heart without wearing out over time; CNT materials might be a useful replacement for metals in this application. In addition to being used as electrodes to connect the heart to an electrical device, CNT materials could potentially be used on their own to form a kind of electrical bridge over damaged heart tissue that is not properly conducting electric signals, as shown by this work from Rice University and the Texas Heart Institute.
CNT electrodes do not have to be implanted in the body to be useful; they also work well on the surface of the skin. Solid CNT materials can be used as electrocardiogram electrodes to detect the body’s heartbeat, or as electromyogram electrodes to detect muscular activity. There may be a good synergy between this use of CNT materials and their incorporation into wearable technology; they may be useful in the development of smart clothing that can monitor and record heart rate and body movement for health & safety monitoring or even for athletic training. This work, in which CNTs are mixed with another polymer to form an adhesive electrode, provides one example. The effectiveness of CNT material as a skin-contact electrode was also displayed in another of our recent posts, in which we included a video of a CNT yarn touch sensor.
CNT electrodes that can be implanted into the body may be a powerful tool for future medical technology, but the use of CNT materials as skin-contact electrodes raises fewer medical concerns and requires less intensive research; for this reason, the use of CNT skin-contact electrodes is likely to be a bit closer on the horizon.
Cell and Tissue Growth
There is another way in which the high surface area of CNTs can be exploited for biomedical engineering: that surface area can provide a good structure on which cells can grow. Some research has been carried out on the use of 3D porous CNT structures as tissue culture scaffolds (see here). There are also variations on this idea in which the 3D structure is made of a different biological polymer with a network of CNTs embedded into it to provide additional structure and electrical conductivity, which beneficial for the proper growth of cardiac cells and nerve cells.
Antiviral Air Filtration
The final example we want to mention involves an application that is particularly important at the time of this writing, during the global outbreak of the COVID-19 coronavirus. Researchers at Cambridge University in the UK and Ma’alot-Tarshiha in Israel, working together with a company called Q-Flo, are studying the use of CNT mats as filters that can not only capture but also potentially destroy airborne viruses as they pass through the material. This filter is described briefly here. If this active filtration method proves effective, it would be an exciting and much-needed addition to the box of tools that health care workers and biomedical engineers use to keep us healthy and safe.
Speaking of Health and Safety...
All proposed medical applications of CNTs raise an important set of questions: are CNTs safe to use on the body or inside of the body? Do they pose an inhalation hazard? Are they at all toxic, or carcinogenic? Of course, it is difficult to give one quick and simple answer to these questions, since the health hazards of CNTs (or any substance) depend on the quantities of material used, the form of that material (Is it a powder? A solution? A solid wire?), and the method by which it is introduced to the body or the environment. We intend to devote a future blog post to this important topic in the near future and will update this post with a link to that new one when it is available.
Biomedical engineering is a perfect example of an area in which multifunctional materials, such as CNT fibers, films, and 3D scaffolds, can lead to new applications that might not have been possible with conventional materials. This is a truly exciting subset of applications for CNT materials, and we are eager to see how research in this area develops over the next few years!
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