Carbon Nanotube Cancer Treatment

Abstract

In the world today, there are countless types of ailments. Cancer, one the most devastating ailments in the world, currently has no known definite cures but researchers are trying to change this disturbing fact. Recently, there have been new studies on carbon nanotubes which have given cancer patients and their loved ones new hope to beat this terrible disease. This type of research could be the future of fighting cancer or other diseases and it is going to be up to chemical engineers and people in related fields to continue this research to better the world. For our paper we intend to elaborate on the discussion of this existing technology and to tell how it is being developed into a better cancer fighter. We are going to research what new innovations in carbon nanotechnology there are and how they are being used to help in the fight against cancer. Our research is intended to show that these innovations have the potential to save countless lives and hopefully will one day completely replace chemotherapy as a cancer treatment.

Key Words : cancer, carbon, cells, nanotechnology, nanotubes

INTRODUCTION

A recent and innovative technological advancement in the world of chemistry has been found and it could be one the best ways to fight cancer that people know of. This innovation is carbon nanotechnology and it is already being used to kill certain cancer cells that are being studied in laboratories. Doctors, researchers, and chemical engineers alike all could play a part in helping this new technology take off as a new cancer treatment. If this treatment is completely verified by the scientific community, it could replace painful chemotherapy and other current treatments that are currently implemented to kill cancer.

Dr. John Kanzius’s Idea

Dr. John Kanzius was a retired electrical engineer with some incredible ideas. He was only 58 years old when he was diagnosed with a form of leukemia that made doctors suspect that he only had nine months to live. Instead of sitting around and waiting for his final day to come, he decided to be proactive and try to find a cure for his ailment. He knew that heating up cancerous (malignant) cells would kill them so he just needed a mechanism for which he could get into the body and heat the malignant cells effectively without killing other healthy cells. He had the idea to try to infuse metal particles into malignant cells and heat the metal up to kill these cells. He knew that metal heats up when exposed to radiofrequency energy so now all he needed to do was find a metal that effectively and harmlessly could be infused with malignant cells. He experimented with several types of metals like nickel but wasn’t nearly as successful as when he used carbon nanotubes. In his experiments at the University of Texas M.D. Anderson Cancer Center, two lines of liver cancer cells and one line of pancreatic cancer cells were successfully destroyed. It seemed as though Dr. Kanzius was on to something big. [1]

Properties of Carbon

Carbon is one of the most versatile and useful elements on earth. Structures of molecules that contain carbon actually make up an entire subset of chemistry called organic chemistry, named so because all compounds that contain carbon are considered organic. The most basic structures of carbon are carbon chains that can contain any number of carbons from two to as many as can be imagined. Carbon is an element that wants four electrons in its outer shell. In chemistry terms, this means that it wants to bond to four other elements or form double bonds which would make it only bond to three other elements. In other cases carbon can form triple bonds where it would only bond to two other elements but this is not as common as single and double bonds. Carbon chains have the special ability to form carbon rings by two of the carbons at the end of the chain linking together. Like carbon chains, carbon rings can have countless carbons in them but, unlike chains, rings can have a minimum of three carbons in them because a ring cannot be made out of only two carbons. Smaller rings are very unstable and the bigger the rings get, the more stable they get. There is an exception to this though; the most stable of these rings is the cyclohexa-1,3,5-triene ring, more commonly known as a benzene ring. The “cyclohexa” part of the name comes from the molecule having six carbons in a ring formation. The 1,3, and 5 numbers in the name come from where the double bonds are located and the “triene” part comes from the molecule having three double bonds.

FIGURE 1 Above is a picture of a benzene ring molecule. notice that each carbon molecule is bonded to three other atoms which makes each a strong sp2 bond [2]. The dotted lines in the picture represent alternating double bonds. These alternating double bonds also give each carbon in the ring a sp2 formation meaning each carbon has three atoms around it. This is more stable than a sp3 bond because the double bond in the sp2 formation is favorable. Sp2 formations are more favorable because molecules like more of an “s” character in their formations than more “p” character. The main reason why this ring is more stable though is that there is no steric strain on it. Steric strain is the strain that elements in a compound put on other elements in a compound. In all other carbon rings, the hydrogen molecules put strain on the other hydrogen molecules because they are too close together. That is why larger carbon rings are more stable than smaller ones with the exception of benzene because of how perfectly the hydrogen atoms are placed away from each other. These benzene rings are the makeup of carbon nanotechnology [3]. They are fused together like building blocks to make long tubes. This is done by replacing every hydrogen atom with a corresponding carbon atom as seen here:

FIGURE 2 This is what the carbon nanotubes look like when all the benzene rings are put together like a puzzle in their signature tube shape [4]. This technology is what made Dr. John Kanzius’s idea possible. Without these nanotubes, there would be no efficient metal to be the mechanism of his idea. An Overview of Carbon Nanotubes Since their discovery by Iijima in 1991, carbon nanotubes are seen by many as the breakthrough nanotechnology of the future. They are simple in design and yet have several intriguing and advantageous properties associated with them. There are several different types of carbon nanotubes such as single-walled, multi-walled, and nanotorus nanotubes. This paper, however, will be discussing single-walled carbon nanotubes due to their relevance to tumor ablation. Single-walled carbon nanotubes (SWNTs) are macromolecules derived from a single sheet of graphene rolled into a seamless cylinder (as shown in the previous picture) ending in a hemispherical fullerene cap on either side. Graphene is just all the benzene rings put together like a puzzle as shown here:

FIGURE 3 This is how to compute the chiral vectors in a graphene sheet [5]. Their unique lattice configuration of hexagonal carbon rings which comprise the graphene causes the SWNTs to be very strong and rigid. Linkage by sp2 bonds serves to strengthen the structural stability further. In fact SWNTs are so strong that some of their other applications involve body armor. They have a circumference of around 20 to 40 atoms, diameter of one nanometer, and are only microns long. This makes them an effective tool in the fields of medicine and engineering. SWNTs are 1-D systems defined by either their length, diameter, and chirality or by their indices (n,m) which describe the chiral vector. Diameter is determined by the formula d=Ch/pi, where Ch is the length of the chiral vector, while indices can be found using the vector equation R=na1+ma2 [6]. These determinants define the SWNT as metallic or semi-conducting as well as give them their optical, electrical, thermal, and elastic properties. SWNTs are currently of such interest to the medical community due to their unique thermal properties. Carbon nanotubes are able to retain massive amounts of thermal energy compared to other types of materials due to their structure. SWNTs are very effective thermal conductors, stemming from an ability known as ballistic conduction. During ballistic conduction there is an uninterrupted flow of charge or other energy carrying particles over the length of the nanotube. This makes SWNTs efficient heat carriers. This is a promising aspect for thermal ablation because the efficiency of this process leads to a decrease in heat loss and therefore less thermal energy is being released into the surrounding cells, resulting in a reduction of structural damage to the body. The diameter of a SWNT depends on the temperature at which it was synthesized and increases as the temperature increases, making them stable up to 4000 degrees Kelvin in a vacuum. Thermal conductivity is also affected by temperature [6]. At temperatures below 30 degrees Kelvin the relationship between temperature and conductivity is temperature dependent. This is believed to be due to phonon activity. Thermal conductivity can be calculated using the equation K=(the sum of) Cv(z)^2*T. When graphed the relationship is linear at low temperatures of 7K to 25K. [7].

FIGURE 3 Temperature dependance of SWNTs [8]. Phonons are another thermal property of SWNTs. A phonon is a quantum of vibrational energy within a solid generated when outside forces cause the lattice structure to vibrate [9]. Evidence of these particles is present in the fact that SWNTs have a high conductivity level as well as a very low specific heat. The equation used to determine phonon thermal conductivity is: Kzz=(the sum of) Cv(z)^2*T [10]. Analysis of measurements from an experiment performed at Harvard showed that SWNTs have a room temperature thermal conductivity greater than 200W/m*K. Along its axis an SWNT can have a conductivity of as much as 3500W/m*K. Phonons are capable of transporting energy throughout the nanotube. The energy content of a phonon can be determined by the equation (n+1/2)hf where n is a positive integer [7].

Functionalization of Carbon Nanotubes

One problematic limitation of SWNTs is their inability to dissolve in a solvent. This is due to the uncommonly strong interactions between Van der Waals forces in an aggregate which cause SWNTs to have an extremely high binding energy. These forces can attain a value of up to 500eV per millimeter; therefore, separating them for practical applications requires an enormous amount of energy. Insolubility was once a major limiting factor for carbon nanotube research as it prevented any further study. It also suggested that SWNTs would not be naturally compatible with biological systems. However, the problem of incompatibility has recently been overcome. Through extensive experiments and investigative studies, scientists concluded that chemically functionalizing SWNTs allows them to become soluble without critically compromising their inherent properties. Further manipulations and usages of carbon nanotubes are now possible. After functionalization takes place the properties of the SWNT tend to associate themselves with properties of other materials. This effect is especially useful in the field of nanomedicine [11].

A chemical functional group is attached to a SWNT either covalently or non-covalently. When being used for medical purposes SWNTs are normally covalently functionalized because the use of weak non-covalent bonds can result in the separation of the functional group from the nanotube, rendering it useless. This also presents a threat to the patient’s health as non-functionalized carbon nanotubes are known to be cytotoxic. Functional groups can be covalently attached to the benzene rings lining the SWNT’s sidewall (sidewall functionalization), the fullerene caps, and even structural defects (defect functionalization). Not only does functionalization make SWNTs water soluble, it also greatly reduces or even eliminates cytotoxicity. This makes them safe for medical use [11].

Functionalization of SWNTs occurs by way of 1,3 dipolar cycloadditions in an organic pericyclic chemical reaction. In this case a 1,3 dipole has a separation of charge across three different atoms and reacts with a dipolarophile. The Prato reaction, or the 1,3 dipolar cycloaddition of azomethine ylides to SWNTs, is a common practice [11].

Using Kanzius’s Idea to Fight Cancer

SWNTs functionalized for biological systems have an interesting relationship with cells. Somatic cells naturally internalize these specialized carbon nanotubes (at 12% to 15%) through CNT-cell interactions. They move into cells through the process of endocytosis or penetrate the cell membrane by piercing it like a needle. They are then able to enter the cytoplasm and nucleus. While this is a useful ability, in order to fight cancer functionalized SWNTs must be targeted specifically to malignant tumor cells, ensuring that healthy cells are not adversely affected by the treatment. Cancer cells are mutations derived from normally functioning cells. The cancer cells can therefore be distinguished from healthy cells by locating alterations on them that are not on healthy cells. Coating functionalized SWNTs with peptides and other cell-binding ligands such as monoclonal antibodies (mAbs) allows them to target specific cancerous cells [12]. MAbs are identical monospecific antibodies cloned from a parent immune cell and all have an affinity for the same antigen. These mAbs can be engineered to specifically bind to any substrate. Therefore, once chemically bound to a functionalized SWNT, they can be made to detect and attach only to cancerous cells. Specific cell targeting can be attained through the use of the integrin avB3 mAb. AvB3 is a targeting molecule with an extremely high specificity for the avB3 positive antigens on cancer cells. Neutravidin conjugated mAbs are also useful for extra binding capabilities but have the unfortunate concern of having nonspecific properties. Neutravidin protein can be chemically modified without affecting its activity and it is water soluble. Another innovative binding technique used by Professor Hongjie Dai (from Stanford University) is coating SWNTs with folic acid (vitamin B). Cancerous cells have a high affinity for these SWNTs because they have a high number of folic acid receptors lining the cell membrane [13].

Treatment using functionalized SWNTs can begin after they make their way inside the tumor. Floating in the cytoplasm and nucleus they are harmless. No effects will be seen until the patient is placed inside a radiofrequency or NIR field. These two types of radiation were chosen for their ability to pass through the body without damaging body tissue [14]. Radiofrequency waves have the tendency to penetrate further into the body. Once inside the field SWNTs can effectively convert radiofrequency energy or NIR into heat. They absorb the arriving waves of radiation, giving them energy and in turn causing them to vibrate [15]. The vibrational movement causes heat to be produced and thermal properties to activate. Vibration of the lattice structure releases phonons which transfer the heat energy throughout the length of the nanotube. Heat is then dispersed inside the tumor from the entire surface area of the SWNTs causing overheating, protein denaturation, and eventually malignant cell death [16]. Using SWNTs also boosts the effectiveness of chemotherapy treatment. Chemotherapy is normally delivered in high enough doses to be toxic to cancer cells, causing other health issues. But used in collaboration with SWNTs chemotherapy drugs may be given to patients in smaller, less harmful doses. Tests indicate that when the SWNTs are heated they force the area in which they are located into a hyperthermic state. This condition, known as hyperthermia, can give chemotherapy drugs more access to a larger area of the tumor through the blood vessels. When the body overheats blood vessels naturally expand in an attempt to dissipate any excess heat. Widening of the blood vessels in the tumor allows chemotherapy drugs to be delivered further in and over a larger surface area of the tumor at a faster rate. Putting them in contact with more cells increases apoptosis and furthers the likelihood for success.

Preclinical Trials with Kanzius’s Idea

SWNTs have shown a lot of promise in the medical field. Currently, many researchers are taking on studies related to and involving the medical applications of carbon nanotubes. Supported by the American Association of Cancer Research, the Alliance for NanoHealth, and NASA [13], these preclinical trials have shown confirmed positive results along with a great deal of success. Several experiments have already been completed including a test run by Dr. Stephen Curley. Dr. Curley works as a professor in M.D. Anderson’s Department of Surgical Oncology in Houston, Texas. He conducted the experiment with the help of John Kanzius and Rice University nanotechnology specialists. The experiment was designed to test the effectiveness of SWNT cancer treatments in malignant liver cells. Malignant human HepG2 and Hep3B liver cells were used as well as pancreatic adenocarcinoma cells. In the lab some cancerous liver cells were cultured independently and laid out in 96 well culture plates. Functionalized SWNTs were then added to the cell cultures and exposed to a 13.56-megahertz field of radiofrequency for 2 minutes, causing the SWNTs to vibrate and release thermal energy. This effectively destroyed the cancer cell lines with in vitro SWNTs. "These are promising, even exciting, preclinical results in this liver cancer model." says senior author Steven Curley [18].

Another study conducted at the same lab showed the effects SWNTs on cancer in vitro inside the body. A group of rabbits, all implanted with the same form of hepatic VX2 liver cancer, were used as test subjects. An experimental group had Kentera functionalized SWNTs injected directly into their tumors and then immediately placed inside a radiofrequency field and positioned so the tumor would get maximum field energy. Rabbits were left in the 600W field for 2 minutes each in order to heat the SWNTs to the desired temperature. There were two other control groups being tested as well. One group was treated with just radiofrequency waves while the other was given only a direct injection of SWNTs. After treatment was complete, the livers were removed and evaluated. The experiment resulted in the malignant tumor cells being completely destroyed in the experimental group with no noted adverse side effects to the test subjects. However there was evidence of some damage to healthy liver cells 2-5 millimeters from the tumor. It was concluded that the damage was caused by free floating SWNTs, further emphasizing the importance of specificity and bond type. Neither control group had any tumor shrinkage or other beneficial effects from their respective treatments [17, 18].

Carbon Nanotubes Fighting Cancer Without Implementing Kanzius’s Idea

Carbon nanotubes are slowly being proved to fight cancer through the ideas previously discussed but; are there other ways carbon nanotubes can help fight cancer? According to engineers and researchers at Yale University, the answer to this question is yes. They said that defects in the carbon nanotubes can actually cause the body’s immune system to respond better. This is because the defects cause T cell antigens to circulate through the blood which in turn, causes the immune system to be stimulated. This immune system stimulation could greatly advance the current system of adoptive immunotherapy which is implemented in fighting cancer. Adoptive immunotherapy is a process by which blood is extracted from a patient’s body. It is separated from the body because T cells are produced more efficiently outside of cancer patient’s bodies because tumors can block T cells from being produced in large numbers in an infected body. Scientists can make T cells be produced outside the body in this extracted blood by using different substances that promote T cell production. The more T cells that are produced in this blood, the more the immune system will be stimulated when the blood is returned to the patient’s body. Carbon nanotubes work better than other T cell stimulants, like polystyrene, because the T cell antigens cluster around the microscopic defects of the carbon nanotubes. The current methods of adoptive immunotherapy take about three weeks to produce an efficient amount of T cells while carbon nanotubes actually only take about one third of this time to produce the same number of T cells. The researchers at Yale say that the carbon nanotubes are completely harmless to the body because they are being used in the blood outside the body. The only aspect of this new adoptive immunotherapy that the Yale researchers need to perfect is how to get the nanotubes out of the blood after they are done with helping produce the T cells. Once this is perfected, carbon nanotubes will have yet another use towards the fight for cancer [19].

Another method of fighting cancer with the nanotubes is currently being tested by an engineering professor at the University of Delaware, Balaji Panchapakesan. His experiments are closely related to Kanzius’s idea with heating up the SWNTs but it is different with the heat that is used and the effects that the heat has on the nanotubes. First the carbon nanotubes are put in water to make them absorb it. Then, an 800 nanometer laser is shot at the SWNTs which causes them to actually explode because of the water that is absorbed by them. These water molecules heat up to about 100 degrees Celsius when they are hit with the laser that has an intensity of approximately 50 to 100 mW/cm2. At this temperature, the water evaporates and develops extreme pressures which in turn cause the SWNTs to explode [20]. SWNTs explosive properties have been experimented with since 2002 as military weapons and even rocket propellants, so Panchapakesan decided to try to make them explode on a much smaller scale and kill cancerous cells. These “micro explosions” kill all of the malignant cells that surround the explosion [21].

Why Use Carbon Nanotubes Instead of Other Cancer Treatments?

Cancer is a complex and destructive disease with an infamous reputation for being difficult to cure completely. Currently the most effective clinically recommended cancer fighting treatments are chemotherapy, radiation, and thermal ablation. Chemotherapy works by targeting and destroying all rapidly dividing cells by sending toxic chemo drugs through the bloodstream. Radiation has the same effect but instead uses targeted beams of radiation to bombard the tumor. Thermal ablation is less commonly used. It works by directly inserting needle-like electrodes into tumors [22]. Radiofrequency waves or near infrared radiation (NIR) are used to heat the electrodes. While all of these treatments have been proven effective, they have a number of terrible and painful side effects. Along with causing apoptosis in the malignant cells, they also ravage healthy cells, damaging healthy tissue beyond repair and sometimes even worsening their condition. Any quickly dividing cell in the body is attacked and destroyed by chemo and radiation. 5% to 40% of the time, thermal ablation treatment is ineffective, causing incomplete tumor destruction and damaging healthy tissue far outside of the target. It is also a limited treatment, only able to be performed in the liver, breast, kidney, and lungs. This results in side effects such as nausea, vomiting, fatigue, hair loss, skin irritation, acid reflux, and lack of salivary function. Other dangerous side effects have the potential to occur in parts of the body including the intestines, skin, and bone marrow due to the nature of the treatments. Unlike these treatments, SWNTs do not cause any of these side effects in the patient’s body when they are implemented in fighting cancer [23].

Production of carbon Nanotubes

The discovery of carbon nanotubes and their applications has caused them to be in high demand. Today labs around the country have developed several methods for nanotube production in high quantities. Some of these methods include arc discharge, laser ablation, gas-phase production, and vapor liquid solid growth [24]. These methods allow carbon nanotubes to be widely available for research and other technical applications.

The arc discharge method was the original method and therefore most widely used for SWNT production. During this process electrical currents passing from an anode to a cathode at low pressure ignite helium gas which in turn sublimates an amount of carbon. However to get a SWNT a metal catalyst such as iron, nickel, or cobalt must be added to the system [24]. The anode is coated with the metal and then the process begins. Temperatures at this point can range from 3000 degrees Celsius to 4000 degrees Celsius. SWNTs are then deposited on the cathode in the soot material. SWNTs produced by arc discharge are very straight, have few structural defects, and usually have a production yield of about 30% [25].

Another principle technique used to produce SWNTs is called laser ablation. Laser ablation produces nanotubes in abundance, having a regular yield of up to 10 grams. In this method, carbon mixed with cobalt and nickel is placed inside a low pressure furnace. The carbon is then hit repeatedly with a high intensity dual-pulsed laser. The newly formed SWNTs are then carried to a cooling area by an inert gas that was flooded into the furnace. These SWNTs are produced in long ropes or hexagonal crystals [24]. Gas-phase production involves SWNTs being synthesized from carbon monoxide. The carbon monoxide molecules are pushed through a gas-phase chemical-vapor-deposition processor (HiPco) with high pressure (30 to 50 atm) and with a high temperature (900 to 1100 degrees Celsius). They flow on catalytic clusters of iron which serve as catalytic particles for which the SWNTs can form and grow on. The iron is inserted into the HiPco in the form of Fe(CO)5 because this was found to be the most efficient way to have iron enter the process. The process happens through carbon disproportionation which is shown by the following equation: CO + CO ↔ CO2 + C This process generally yields SWNTs of 97 mole percent purity and are generated at high rates of speed. They have been generated at speeds of up to 450 mg/h which is fast compared to other methods of production [26].

The vapor liquid solid method is a less common method than the others but it is still useful. To produce nanotubes this way, a supersaturated liquid is placed in a container with sublimated carbon vapor. The vaporized carbon is then absorbed into the liquid. After it is absorbed, precipitation occurs and the precipitate drops out of the liquid to collect in hollow cylindrical structures at the bottom. Once the precipitation is complete a high yield of vertical SWNTs are left on the bottom of the container [24].

Other Nanotube Properties That Can Help Fight Cancer

The latest technology with carbon nanotubes is how they are being manipulated in the body with magnets. Researchers can use the magnetic properties of the nanotubes to prevent cancerous cells from migrating to other parts of the body and can even use magnetic fields to move cancerous cells to less dangerous places in the body. Also, these magnetic properties can be used in combination with the carbon nanotube’s ability to interact with biomolecules to move certain cancer fighting drugs through the body [27]. By attaching a peptide that is specifically attracted to tumor cells to the SWNTs, they can navigate themselves to the tumor cells. Once at the tumor, the carbon nanotube is swallowed up by the cells through there permeable membranes and the drugs are released into the cells which kills them. This swallowing of the SWNTs by the cell happens because the nanotubes are designed to be amphilic which means that they can penetrate the cell’s membrane. An additional property of SWNTs that is helpful with this method of cancer fighting is their size. They have surface areas of up to 2600 m2/g which is quite large for these kind of molecules. This means that large quantities of cancer fighting drugs, like doxorubicin, can be stored in the SWNTs and released to kill as many cancerous cells as possible in only one session [28].

The fact that SWNTs interact with biomolecules so well is also conducive to lowering toxicity in cells that are damaged by cancerous cells. The same property is used in this except instead of the carbon nanotubes being inserted into malignant cells, they are inserted into useful cells in the body that have just been damaged by the cancerous ones. Instead of attaching cancer fighting drugs to the carbon nanotubes, this time RNAs, proteins, or plasmids are attached so that they can enter cells that the patient’s body wants to keep and help these cells lower their toxicity. This alternative could be a possible solution for improving current gene therapy [28].

Conclusion

Carbon nanotechnology has proven itself useful in many areas of science, especially in chemical engineering. Innovations in carbon nanotube technology have already revolutionized the world as we know it and could revolutionize cancer treatments as well if enough time is put into researching the power that they have in killing malignant cells. Researchers and engineers alike are obligated to push to find better cancer fighters and this could be the path that takes them there. If carbon nanotubes are pushed to their full potential, countless lives could be saved from this disease.

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Acknowledgements

We would like to thank the librarians for their continued support in helping us with this paper. We would also like to thank Jacob George, our cochair, for his help with revising our paper. And, of course, we would like to thank Dr. Budny for teaching the course.