Carbon nanotubes: Properties,biomedical applications,advantages and risks in patients and occupationally-exposed workers

Abstract

Since the beginning of the 21st century, carbon-based nanomaterials (CNTs) have been introduced in pharmacy and medicine for drug delivery system in therapeutics. CNTs have proved able to transport a wide range of molecules across membranes and into living cells; therefore, they have attracted great interest in biomedical applications such as advanced imaging, tissue regeneration, and drug or gene delivery. Although there are many data on the advantages in terms of higher efficacy and less adverse effects, several recent findings have reported unexpected toxicities induced by CNTs. The dose, shape, surface chemistry, exposure route, and purity play important roles in these differential toxicities. Mapping these risks as well as understanding their molecular mechanisms is a crucial step in the development of any CNT-containing nanopharmaceuticals. This paper seeks to provide a comprehensive review of all articles published on cellular response to CNTs, underlining their therapeutic applications and possible toxicity in patients and occupationally exposed workers.

Keywords

carbon nanomaterials exposed workers nanodrugs nanotoxicology

Nanotechnology has opened a window for the development of diverse organic and inorganic drug carriers, known as nanoparticles. The greatest advantage is its potential to create novel structures with enhanced abilities to translocate through cell membranes, and increased solubilization, stability, and bioavailability of biomolecules, thereby enhancing their delivery efficiency.^1–3

Nanoparticles are generally <1000 nm in at least one dimension, and consist of different biodegradable materials such as natural or synthetic polymers, lipids, or metals as phospholipids, lactic acid, chitosan, dextran, polyethylene glycol (PEG), cholesterol, carbon, silica.⁴

Carbon nanotubes (CNTs), in particular, have been introduced in pharmacy and medicine for drug delivery system in therapeutics since the beginning of the 21st century.⁵ They have an ultra-small size, large surface area to mass ratio, and high reactivity, which are different from bulk materials (in microscale) of the same composition; in addition, they are able to adsorb or conjugate with a wide variety of therapeutic molecules (drugs, proteins, antibodies, DNA, enzymes, etc.) and they have been proven to be an excellent vehicle for drug delivery by penetrating into the cells directly and keeping the drug intact without metabolism during transport in the body.⁵ Although these characteristics are associated with highly desirable properties (e.g. mechanical, electrical, chemical) for medical uses, they are also the main factors that make them potentially dangerous to human health. Importantly, this risk is not only for patients, but also for workers including researchers, manufacturers and people involved in the preparation of the therapeutic infusions.^1,6

This paper seeks to provide a comprehensive review of all articles published on CNTs underlining their potential toxicity in both patients and occupationally exposed workers.

Structure

Carbonic structures can form diverse shapes and configurations both within compounds and as elementary substances. The numerous carbon forms (allotropes) identified to date include naturally occurring minerals (such as graphite, diamond, and coal) and fullerenes (such as buckyballs, graphene, and carbon nanotubes), which can be artificially synthesized and have more recently been found in nature. Graphene is a flat monolayer of carbon atoms arranged in a two-dimensional hexagonal lattice.¹¹ Moreover, single or multiple graphene sheets can be folded into cylindrical structures, to give single-walled (SWCNTs) and multi-walled (MWCNTs) CNTs, and into carbon nanofibers (CNF).⁷

SWCNTs are mono-cylindrical carbon layers with a diameter in the range of 0.4–2.0 nm organized in harmony with chiral, armchair, and zigzag arrangements (Figure 1). Double-walled CNTs (DWCNTs) consist of only two tubes having similar sizes to SWCNTs. However, MWCNTs consist of a number of cylindrical carbon sheets with an average diameter of 1–3 nm for the central cylindrical tubes and 2–100 nm for the external cylindrical tubes (Figure 1). MWCNTs structures can be split into two categories based on their arrangements of graphite layers. The Ru Rotating sheets of graphite are arranged in concentric cylinders. In the Parchment model, a single sheet of graphite is rolled in around itself, resembling a scroll of parchment or a rolled newspaper.⁷

Figure 1.

Single-walled carbon nanotube is a sheet of graphite rolled up into a tube. Depending on the direction of hexagons, nanotubes can be classified as either zigzag, armchair or chiral. Multi-walled carbon nanotubes consist of multiple rolled layers (concentric tubes) of graphene.

Carbon nanotubes are the strongest and stiffest materials discovered to date in terms of tensile strength and elasticity, respectively, they are one-dimensional electrical conductors, although intrinsic superconductivity has been reported,⁸ and they are very good thermal conductors.⁸ MWCNTs exhibit a striking telescoping property.⁹ These CNT properties give them a wide range of applications in electronics, the chemical industry, and medicine.

Production and workplace exoposure to CNTS

Since CNTs were discovered by Iijima in 1991 studies of CNTs have progressed rapidly in many fields.¹⁰ CNTs were firstly used as additives to various structural materials for electronics, optics, plastics, and other materials in nanotechnology fields. Different techniques have been developed to produce nanotubes in appropriate amounts, including arc discharge, laser ablation, high-pressure carbon monoxide disproportionation, and chemical vapor deposition (CVD).⁵ Most of these processes take place in a vacuum or with gases. In arc discharge the carbon contained in the negative electrode sublimates because of the high-discharge temperatures. The yield for this method is up to 30% by weight and it produces both SWCNTs and MWCNTs with lengths of up to 50 µm with few structural defects.¹¹ In laser ablation a pulsed laser vaporizes a graphite target in a high-temperature reactor while an inert gas is bled into the chamber. Nanotubes develop on the cooler surfaces of the reactor as the vaporized carbon condenses. A water-cooled surface may be included in the system to collect the nanotubes. The laser ablation method yields around 70% and produces primarily SWCNTs with a controlled size determined by the reaction temperature. However, it is more expensive than both arc discharge and CVD.¹² In plasma torch method, a gas mixture composed of argon, ethylene, and ferrocene is introduced into a microwave plasma torch, where it is atomized by the atmospheric pressure plasma, which has the form of an intense ‘flame’. The fumes created by the flame are found to contain SWNT, metallic and carbon nanoparticles, and amorphous carbon. The induction thermal plasma method can produce up to 2 g of nanotube material per minute, which is higher than the arc discharge or the laser ablation methods.¹³ The catalytic vapor phase deposition of carbon was reported in 1952 but it was not until 1993 that CNTs were processed in this manner. During CVD, a substrate is prepared with a layer of nickel, cobalt, and iron particles. The substrate is heated to approximately 700°C. To initiate the growth of nanotubes, two gases are bled into the reactor: a process gas (such as ammonia, nitrogen, or hydrogen) and a carbon-containing gas (such as acetylene, ethylene, ethanol, or methane). Nano-tubes grow at the sites of the metal catalyst.¹³ Of the different methods used for nanotube synthesis, CVD is the most promising for industrial scale-up, because of its price/unit ratio.¹⁴

Workers are involved with CNTs throughout the nanomaterial lifecycle, from research in laboratories, through start-up efforts, manufacture, incorporation of CNTs into products, manipulating CNT containing products, and ﬁnally through disposal and recycling of CNTs.^1,15 Proposed occupational air exposure limits (OELs) for speciﬁc types of CNTs have ranged in literature from approximately 2 to 50 mg/m³ based on the estimated risk of non-cancerous adverse lung effects (inflammation and fibrosis).¹⁵

The main work practice recommendations described are: containing tasks in processes that could result in the release of airborne CNTs in the workplace; handling of CNT as slurries rather than dry powders where possible; and dispensing, weighing, or sonication processes should be performed on well-designed ventilated benches or in effective ventilated exposures, performing initial and periodic area and personal exposure monitoring.¹⁵ Medical surveillance of workers involved with CNTs must include initial and periodic evaluations of the respiratory system and exposure registries should be developed for all workers with potential exposure to CNTs.¹⁵

Biomedical application

Since the beginning of the 21st century, CNTs have been introduced in pharmacy and medicine for drug delivery system in therapeutics. A number of biomedical applications of CNTs are proposed including drug vectors, biomolecule, gene delivery to cells or organs, tissue regeneration, and biosensor diagnostics and analysis.¹¹ In fact, due to their high surface area, excellent chemical stability, and rich electronic polyaromatic structure, CNTs are able to adsorb or conjugate with a wide variety of therapeutic molecules (drugs, proteins, antibodies, DNA, enzymes, etc.) and to carry them near the targeted cell.^1,14 The drug can either be loaded into the CNT structure or be attached to the CNT surface. Two different systems of drug delivery conjugates with CNT have been described either without internalization of the CNT carrier or both the drug and the CNT carrier can enter the cells either via the endocytosis pathway or via the insertion and diffusion pathway. The internalization method is more effective than surface attachment. In fact, in the former method, after entering the cells, the intracellular environment degrades the conjugated drug releasing pharmacological agents inside the cells, while in the surface attachment method, the drug can be degraded in the physiological fluids prior to be internalized by the cells.¹¹ These novel approaches in drug delivery were first used to bind antineoplastic and antibiotic drugs to CNTs for cancer and infection treatment, respectively.¹

Uses in anti-cancer treatment

Many anti-cancer drugs, such as epirubicin, doxorubicin, cisplatin, methotrexate, quercetin, and paclitaxel, have been conjugated with functionalized CNTs and successfully assessed both in vitro and in vivo.¹ The chemotherapy agents can be bound to a complex formed by CNT and antibody against antigen overexpressed on the cancerous cell surface. In this way it is possible to carry the drug mostly at the level of the tumor cells. Drugs can be linked with a magnetic CNT complex, obtained by fixing a layer of magnetite (Fe₃O₄) nanoparticle on the surface of the nanotubes. In this case, the system CNT drugs can be guided by an externally placed magnet to target a desired organ interested by the cancer cell localization sparing normal counterparts.¹⁶). Moreover, due to their tiny size and accessible external modifications, CNTs are able to cross the blood-brain barrier (BBB) by various mechanisms targeting for acting as effective delivery carriers to treat brain tumors.¹⁷

CNTs showed some peculiarity in targeting the drug in different tumor types. Taghdisi et al., for example, reported that a tertiary complex of Sgc8c aptamer, daunorubicin, and SWCNT can be internalized effectively into human T cell leukemia cells (MOLT-4 cells).¹⁸

A water soluble SWCNT-Paclitaxel (PTX) conjugate has been found to be highly efficient in suppressing tumor growth when compared with free taxol in a murine 4T1 breast cancer cell model, likely for both the extended blood circulation and enhanced permeability and retention (EPR) effect by SWCNT.¹⁹

Ji et al. developed a highly effective drug delivery system (DDS) based on chitosan and folic acid modified SWCNTs for controllable loading/release of the anticancer agent doxorubicin. The obtained DDS not only effectively killed the hepatocellular carcinoma SMMC-7721 cell lines and depressed the growth of liver cancer but also displayed lesser in vivo toxicity than free doxorubicin.²⁰

Some studies have demonstrated that CNTs used as carriers can be effectively applied in anti-tumor immunotherapy.²¹ These approaches are based on stimulation of the patient’s immune system in order to hit malignant tumor cells. This stimulation can be achieved by the administration of a cancer vaccine or a therapeutic antibody used as drug. In vitro, the conjugation of CNTs with tumor immunogens can activate immune effector T cells; this action is due to the high avidity of antigen on the surface and to the negative charge of CNTs.²¹ In this way, it is possible to administer inert materials (CNTs conjugated to tumor antigens) to the patients avoiding the administration of biological material increasing both risks and costs linked to the methods of preparation of the immunogens.²¹

Antimicrobial application

Functionalized CNTs can be used in vaccination procedures. CNTs were shown to activate cells deriving from the innate immune system, such as monocytes, macrophages, and dendritic cells.²¹ Microarray profiling of a monocytic cell line, THP-1, showed that CNTs, both functionalized and non-functionalized, activate several genes involved in monocyte response to infection or vaccination, such as nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB), interleukin-1β (IL-1β), IL-6, tumor necrosis factor-α (TNF-α), among others.²²

CNTs activate the class II Major Immuno-compatibility Complex (MHC) in antigen pre-senting cells which can, in turn, promote an antibody-based response. The linkage of either a bacterial or viral antigen to CNTs allows the maintenance of antigen conformation integrity, thereby, inducing antibody response increasing both the specificity and the sensitivity.⁵

It has also been suggested that SWNT conjugated with unmethylated CpG DNA motifs can be used as an adjuvant in vaccines. Functionalized CNTs have been demonstrated to be able to act as carriers for antimicrobial agents, such as the antifungal amphotericin B. CNTs covalently attached to amphotericin B reduced the undesired toxicity by about 40% as compared to the free drug.²³

Moreover, CNTs themselves might have antimicrobial activity through oxidation of the intracellular antioxidant glutathione, resulting in increased oxidative stress on the bacterial cells and eventual pathogen death.⁵

Other applications

The linkages of other biomolecules such as genes, proteins, DNA, and biosensors to CNTs have been also assessed for gene therapy and tissue regeneration. CNTs can effectively transport the genes inside mammalian cells, maintaining their integrity. In fact, when bound to SWCNTs, DNA probes are protected from enzymatic cleavage and interference from nucleic acid binding proteins; consequently, DNA-SWCNT complex exhibits superior biostability and increased self-delivery capability of DNA in comparison to DNA used as free moieties. CNTs can interact directly with DNA through Van der Waals and hydrophobic forces.²⁴

Siu et al. found both significant uptake and gene silencing in the tumor tissue of Cy3-labeled siRNA raised against the B-raf oncogene (siBraf) linked to SWCNTs.²⁵ Al-Jamal et al. had reported that amino-functionalized CNT complexed with siRNA were able to lead to functional rehabilitation in an induced stroke model after stereotactic administration.²⁶

SWCNTs have strong optical absorption from ultraviolet (UV) to near-infrared (NIR) regions, which can be used for photothermal therapy and photoacoustic imaging from the heat they generate from NIR light absorption. SWCNTs appear to be an excellent platform for biomedical molecular imaging.⁵

Polymethyl methacrylate denture base material modified with multiwalled carbon nanotubes showed better results in terms of fatigue resistance, flexural strength, and resilience compared to conventional materials used in dentistry.²⁷ Besides these main applications of CNTs, they have been shown as a powerful tool for enantiomer separation of chiral drugs and chemicals in the pharmaceutical industry.⁵

Factors determining CNT toxicity

The toxicity of CNTs can be influenced by a wide range of factors and characteristics. Morphology, physical and chemical properties, including size, shape, charge and/or agglomeration state, and purity can be involved in the induction of CNT toxicity.²⁸

Due to their shape, CNTs can penetrate cell membrane via a spiraling or winding motion. The subsequent interaction with different cellular target sites, induces a cytotoxic response in the cells (i.e. primary macrophages). In fact, it has been suggested that the large contact area of long nanoparticles with cell surface receptors, puts a strain on the cytoskeleton of the phagocyte during phagocytosis, impeding this process. Various studies have shown that CNTs with longer length and larger diameter have greater toxicity than smaller ones, although some researchers have found the opposite.^1,28

Metal impurities are additional factors that can determine CNT toxicity, resulting in cell death through various mechanisms including both mitochondrial destruction and oxidative stress.²⁹ It has been established by various research groups that functionalization can significantly improve the dispersibility and biocompatibility, while reducing the toxicity of CNTs.⁶

Organ toxicity of medical CNTs

Respiratory system

One of the most important doors and target organs for CNT is the respiratory system and one of the most widespread routes of human exposure to airborne CNT is inhalation in the workplace and the environment (Figure 2).

Figure 2.

Illustration of the pathways of exposure to carbon nanotubes (CNTs) and associated adverse effects. CNTs can be internalized in cells by inhalation, ingestion, or dermal exposure. Inhaled nanodrugs can pass through epithelia of the respiratory tract into the interstitium and access the bloodstream directly or via lymphatic pathways. Successively, the bloodstream transport nanodrugs to the central nervous system, liver, kidneys, and other organs. Moreover, they can be directly ingested or alternatively, inhaled CNTs can also arrive in gastrointestinal tract. Once CNTs are internalized in cells, they can induce organ-specific toxicity.

Respiratory exposure to CNT can cause important adverse respiratory effects, such as multifocal granulomas, peribronchial inflammation, progressive interstitial fibrosis, chronic inflammatory responses, collagen deposition, pleural lesions and gene mutations, at least in experimental animal studies. The deposition of CNT in the respiratory tract is largely determined by the particle aerodynamic or thermodynamic diameter that gives the probability to the NP to reach deep alveoli that have a size of approximately 20 nm.³⁰ Therefore, although composed of the same carbon-carbon bond arrangement, Mercer et al. found large differences in the distribution, toxicity and clearance rates of the CNTs from the lungs, depending on the size of their structures.³¹

Clearance of larger MWCNT structures occurs by reduction in the lung burden of MWCNTs within alveolar macrophages. On the other hand, smaller MWCNT structures in general and SWCNT are retained mainly within the alveolar septa and sub-pleural tissues producing a more evident progressive fibrotic reaction in the lungs.³¹

Although most of the studies for this type of NPs were performed on animal models using intratracheal instillation technique, which is not the usual way of exposure, most of the literature agrees in describing that chronic inflammation and oxidative stress observed during and after exposure to CNT can induce adverse health effects such as, genotoxicity and cancer. In fact, Wang at al. demonstrated that chronic exposure to CNT can produce malignant transformation of human lung small airway epithelial cells.³¹ Sargent et al. showed that inhaled CNTs are strong promoters of pulmonary adenomas and adenocarcinomas in B6C3F1 mice.³² Results from these studies suggest that caution should be taken during use, production, and processing of CNTs to limit human inhalation exposures.

Intraperitoneal instillation of MWCNTs in mice has been associated with abdominal mesothelioma. MWCNTs deposited in the distal alveoli can migrate to the intrapleural space, and MWCNTs injected in the intrapleural space can cause lesions at the parietal pleura.³³ However, additional studies are required to determine whether pulmonary exposure to MWCNTs can induce pleural lesions or mesothelioma.

Due to their characteristics, like nano-sized dimensions and high relative surface area, CNTs have been associated with development and aggravation of respiratory allergies. Both SWCNTs and MWCNTs increased serum levels of allergen-specific IgE, the number of eosinophils in bronchial alveolar fluids, and the secretion of Th2-associated cytokines in the MLN, all being important mediators in allergic airway responses.³⁴

Blood and vascular system

Nanodrugs can access the blood stream not only after intravenous injection but they can cross epithelia of the respiratory tract and pass into the interstitium and in the vascular system either directly or via lymphatic pathways also after inhalation (Figure 2). Nemmar et al. found that inhaled (99 m)Tc-labeled carbon particles (100 nm) pass into the blood circulation 1 min after exposure. Once arrived in the blood stream, CNTs could induce the same adverse biological effects of the intravenously injected carbon nanodrugs.³⁵ Sachar et al. showed that treatment with SWCNTs results in acute membrane damage and eventual erythrocytes lysis. On the other hand, several studies have demonstrated that CNTs have immunostimulatory properties in absence of cytotoxicity in white blood cells.³⁶

CNTs injected into the blood have also been reported to induce platelet aggregation in the hepatic microvasculature of healthy mice in association with prothrombotic changes on the endothelial surface of the hepatic microvessels. In addition, they accelerate the rate of vascular thrombosis in rat carotid artery.³⁷ Moreover, transforming growth factor-β1(TGF-β1), which is involved in platelet activation, has been found to be significantly increased.³⁷ Salvador-Morales et al. reported that CNTs can directly bind some plasma protein as fibrinogen and apolipoproteins and trigger activation of complement cascade.³⁸

CNTs were proven to have a proinflammatory action on endothelial cells, inhibition of cell growth, and reduction of endothelial nitric oxide synthase.³⁹ Castranova et al. showed that pulmonary exposure to MWCNTs depresses the ability of coronary arterioles to respond to dilators. These cardiovascular effects may result from neurogenic signals derived by sensory irritant receptors in the lung.⁴⁰

Central nervous system

Drug transport from the bloodstream to the CNS is hindered by the presence of an endothelium characterized by a low permeability, namely the BBB, whose cells are linked by tight junctions not allowing the passage of most part of known drugs. NPs may produce potential toxicity on human neural cells because of their ability to pass through biological membranes.¹ Effects from the presence (or even accumulation) of NPs in the brain and through the BBB have not yet been fully studied (Figure 2).

Wang et al. exposed PC12 cells, a commonly used in vitro model for neurotoxicity study, to two kinds of commercially available SWCNTs. They found that CNTs induced oxidative stress in neuronal cells, with the possibility of occurrence of diseases related to cellular injuries such as neurodegenerative disorders.⁴¹ Bardi et al. demonstrated that oxidation of the nanotube surface can contribute to a sustained inflammatory reaction in the healthy brain.⁴² In a separate study, up to 30 µg mL⁻¹ SWCNTs significantly decreased the overall DNA content in chicken embryonic spinal cord or dorsal root ganglia.⁴³

Gastrointestinal tract

Another door for NP penetration into the body is the gastrointestinal tract (GIT). In addition to direct ingestion, a proportion of inhaled particulate materials are eventually removed via the GIT, after being mobilized up the trachea via the mucociliary escalator (Figure 2).¹ GIT is a selective mucosal barrier that represents a considerable surface area, estimated at 300 m² in the adult human, for potential interaction with NPs.¹

CNTs have greater absorption in GIT for their smaller size but they had little adverse effects on GIT. SWCNT-COOH appears to inhibit efflux pump activity in Caco-2 cells or co-cultures through interaction with the P-glycoprotein (P-gp) efflux system, with increased cellular accumulation of the pump substrate, rhodamine-123.⁴⁴ SWCNT-COOH also modulated the tight junctions through perturbation of zonulin-1 distribution, a tight junction marker protein. These findings were considered as evidence in favor to the fact that CNTs could enhance paracellular permeability via disruption of tight junctions in GIT.⁴⁵

Beliaeva et al. showed a significant change in the fine structure of the villi of the small intestine in male mice, which consumed water containing CNTs. In particular, they found an increasing number of unstructured villi and proliferation of epithelial cells, most pronounced in animals exposed for up to 2 months.⁴⁶ Purified carboxylated functionalized MWCNT can induce hepatotoxicity in Swiss-Webster mice through oxidative stress activation.⁴⁷ Moreover, Ji et al. found that gene expressions for GPCRs (G protein-coupled receptors), cholesterol biosynthesis, the metabolism by cytochrome P450, the natural-killer-cell-mediated cytotoxicity, TNF-alpha, and NF-kappaB signaling pathway changed in mouse liver exposed to MWCNT.⁴⁸

Skin

The effects of carbon nanodrugs are more limited (Figure 1). It has been found in vitro that CNTs induce pro-inflammatory responses in human keratinocyte cells in skin (Figure 2). Murray et al. showed that topical exposure of SKH-1 mice (5 days, with daily doses of 40 µg/mouse, 80 µg/mouse, or 160 µg/mouse) to unpurified SWCNT caused oxidative stress, depletion of glutathione, oxidation of protein thiols and carbonyls, elevated myeloperoxidase activity, an increase of dermal cell numbers, and skin thickening resulting from the accumulation of polymorphonuclear leukocytes (PMNs) and mast cells. Taken together, these data indicate that topical exposure to unpurified SWCNT induced free radical generation, oxidative stress, and inflammation, thus causing dermal toxicity.⁴⁹

Shvedova et al. indicate that dermal exposure to unrefined SWCNT may lead to dermal toxicity due to accelerated oxidative stress in the skin of exposed workers in a cellular model. In fact, after 18 h of exposure of HaCaT to SWCNT, they found an increase of intracellular free radicals, accumulation of peroxidative products, antioxidant depletion, and loss of cell viability, with ultrastructural and morphological changes in cultured skin cells.⁵⁰

However, more research is needed to investigate the toxic cellular effects of NPs on skin both in vitro and in vivo.

Conclusions

The use of nanotechnology in medicine is expected to spread rapidly. In recent years, the pharmaceutical industry has used nanoparticles to reduce toxicity and side effects of drugs.^1,2 Until now, it was not realized that NPs as CNTs systems themselves could impose risks to both patients and exposed workers.¹ In fact, as clearly described by Kostarelos and colleagues, ‘The use of CNTs in medicine is now at the crossroads between a proof-of-principle concept and an established preclinical candidate for a variety of therapeutic and diagnostic applications. Progress towards clinical trials will depend on the outcomes of efficacy and toxicology studies, which will provide the necessary risk-to-benefit assessments for carbon nanotube based materials’.⁵¹

CNTs are not completely inert materials and can be endowed with intrinsic cytotoxicity that can cause potential deleterious effects in normal tissues. Experimental studies have given evidence that factors determining CNT toxicity include size, length, diameter, purity, production method, and functionalization. However, these studies have been conducted prevalently in vitro and in animals, and it is difficult to translate results from animal studies to humans, because exposure dose in experiments are frequently higher than that of the occupational and living environments, and also the exposure systems are not the real ones.

For these reasons more toxicological investigations, particularly in vivo, of different forms functionalized CNTs are highly recommended before they can be effectively used in clinical study and then marketed worldwide.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

References

Lamberti

Zappavigna

Sannolo

. (2014) Advantages and risks of nanotechnologies in cancer patients and occupationally exposed workers. Expert Opinion on Drug Delivery 11: 1087–1101.

Caraglia

Marra

Misso

. (2012) Tumour-specific uptake of anti-cancer drugs: the future is here. Current Drug Metabolism 13: 4–21.

Wang

Chen

. (2009) Advances of cancer therapy by nanotechnology. Cancer Research and Treatment 41: 1–11.

Sharma

Madhunapantula

Robertson

(2012) Toxicological considerations when creating nanoparticle-based drugs and drug delivery systems. Expert Opinion on Drug Metabolism & Toxicology 8: 47–69.

Pham-Huy

Dramou

. (2013) Carbon nanotubes: applications in pharmacy and medicine. BioMed Research International 2013: 578290.

Madani

Mandel

Seifalian

(2013) A concise review of carbon nanotube’s toxicology. Nano Reviews 3: 4.

Wang

Sun

Bao

. (2011) Cytotoxicity of single-walled carbon nanotubes on PC12 cells. Toxicology in Vitro 25: 242–250.

Jakubinek

Johnson

White

. (2010) Novel method to produce single-walled carbon nanotube films and their thermal and electrical properties. Journal of Nanoscience and Nanotechnology 10: 8151–8157.

Popov

Lozovik

Kulish

. (2010) High frequency electromechanical memory cells based on telescoping carbon nanotubes. Journal of Nanoscience and Nanotechnology 10: 4322–4328.

10.

Iijima

(1991) Helical microtubules of graphitic carbon. Nature 354: 56–58.

11.

Hirlekar

Yamagar

Garse

. (2009) Carbon nanotubes and its applications: a review. Asian Journal of Pharmaceutical and Clinical Research 2: 17–27.

12.

Chico

Crespi

Benedict

. (1996) Pure carbon nanoscale devices: nanotube heterojunctions. Physical Review Letters 76: 971–974.

13.

Ganesh

(2013) Single walled and multi walled carbon nanotube structure. Synthesis and Applications 2: 311–318.

14.

Patole

Alegaonkar

Lee

. (2008) Optimization of water assisted chemical vapor deposition parameters for super growth of carbon nanotubes. Carbon 46: 1987–1993.

15.

Schulte

Kuempel

Zumwalde

. (2012) Focused actions to protect carbon nanotube workers. American Journal of Industrial Medicine 55: 395–411.

16.

Masotti

Caporali

(2013) Preparation of magnetic carbon nanotubes (Mag-CNTs) for biomedical and biotechnological applications. International Journal of Molecular Science 14: 24619–24642.

17.

Caraglia

De Rosa

Salzano

. (2012) Nanotech revolution for the anti-cancer drug delivery through blood-brain barrier. Current Cancer Drug Targets 12: 186–196.

18.

Taghdisi

Lavaee

Ramezani

. (2011) Reversible targeting and controlled release delivery of daunorubicin to cancer cells by aptamer-wrapped carbon nanotubes. European Journal of Pharmaceutics and Biopharmaceutics 77: 200–206.

19.

Liu

Chen

Davis

. (2008) Drug delivery with carbon nanotubes for in vivo cancer treatment. Cancer Research 68: 6652–6660.

20.

Lin

. (2012) Targeted therapy of SMMC-7721 liver cancer in vitro and in vivo with carbon nanotubes based drug delivery system. Journal of Colloid Interface Science 365: 143–149.

21.

Gottardi

Douradinha

(2013) Carbon nanotubes as a novel tool for vaccination against infectious diseases and cancer. Journal of Nanobiotechnology 11: 30.

22.

Pondman

Sobik

Nayak

. (2014) Complement activation by carbon nanotubes and its influence on the phagocytosis and cytokine response by macrophages. Nanomedicine 10: 1287–1299.

23.

Rosen

Mattix

Rao

. (2009) Carbon nanotubes and infectious diseases. In: Hunter

(ed.) Nanomedicine in Health and Disease. London: Science Publishers, pp. 249–267.

24.

Hollanda

Lobo

Lancellotti

. (2014) Graphene and carbon nanotube nanocomposite for gene transfection. Materials Science and Engineering: C 39: 288–298.

25.

Siu

Chen

Zheng

. (2014) Non-covalently functionalized single-walled carbon nanotube for topical siRNA delivery into melanoma. Biomaterials 35: 3435–3442.

26.

Al-Jamal

Gherardini

Bardi

. (2011) Functional motor recovery from brain ischemic insult by carbon nanotube-mediated siRNA silencing. Proceedings of the National Academy of Sciences of the United States of America 108: 10952–10957.

27.

Wang

Tao

. (2014) Characterization of multiwalled carbon nanotube-polymethyl methacrylate composite resins as denture base materials. Journal of Prosthetic Dentistry 111: 318–326.

28.

Luo

Song

. The toxicity and pharmacokinetics of carbon nanotubes as an effective drug carrier. Current Drug Metabolism 14: 879–890.

29.

Meng

Jiang

Chen

. (2013) Inhibitory effects of multiwall carbon nanotubes with high iron impurity on viability and neuronal differentiation in cultured PC12 cells. Toxicology 313: 49–58.

30.

Ferreira

Cemlyn-Jones

Robalo Cordeiro

(2013) Nanoparticles, nanotechnology and pulmonary nanotoxicology. Revista Portuguesa de Pneumologia 19: 28–37.

31.

Wang

Stueckle

Mishra

. (2014) Neoplastic-like transformation effect of single-walled and multi-walled carbon nanotubes compared to asbestos on human lung small airway epithelial cells. Nanotoxicology 8: 485–507.

32.

Sargent

Porter

Staska

. (2014) Promotion of lung adenocarcinoma following inhalation exposure to multi-walled carbon nanotubes. Particle and Fibre Toxicology 11: 3.

33.

Nagai

Okazaki

Chew

. (2011) Diameter and rigidity of multiwalled carbon nanotubes are critical factors in mesothelial injury and carcinogenesis. Proceedings of the National Academy of Sciences of the United States of America 108: E1330–1338.

34.

Nygaard

Hansen

Samuelsen

. Single-walled and multi-walled carbon nanotubes promote allergic immune responses in mice. Toxicology Science 109: 113–123.

35.

Nemmar

Hoet

Vanquickenborne

. (2002) Passage of inhaled particles into the blood circulation in humans. Circulation 105: 411–414.

36.

Sachar

Saxena

(2011) Cytotoxic effect of poly-dispersed single walled carbon nanotubes on erythrocytes in vitro and in vivo. PLoS One 6: e22032.

37.

Guidetti

Consonni

Cipolla

. (2012) Nanoparticles induce platelet activation in vitro through stimulation of canonical signalling pathways. Nanomedicine 8: 1329–1336.

38.

Semberova

De Paoli Lacerda

Simakova

. (2009) Carbon nanotubes activate blood platelets by inducing extracellular Ca2+ influx sensitive to calcium entry inhibitors. Nano Letters 9: 3312–3317.

39.

Khandoga

Stampfl

Takenaka

. (2004) Ultrafine particles exert prothrombotic but not inflammatory effects on the hepatic microcirculation in healthy mice in vivo. Circulation 109: 1320–1325.

40.

Castranova

Schulte

Zumwalde

(2013) Occupational nanosafety considerations for carbon nanotubes and carbon nanofibers. Accounts of Chemical Research 46: 642–649.

41.

Wang

Sun

Bao

. (2012) Vitamin E renders protection to PC12 cells against oxidative damage and apoptosis induced by single-walled carbon nanotubes. Toxicology In Vitro 26: 32–41.

42.

Bardi

Nunes

Gherardini

. (2013) Functionalized carbon nanotubes in the brain: cellular internalization and neuroinflammatory responses. PLoS One 8: e80964.

43.

Belyanskaya

Weigel

Hirsch

. Effects of carbon nanotubes on primary neurons and glial cells. Neurotoxicology 30: 702–711.

44.

Jos

Pichardo

Puerto

. Cytotoxicity of carboxylic acid functionalized single wall carbon nanotubes on the human intestinal cell line Caco-2. Toxicology In Vitro 23: 1491–1496.

45.

Lai

Blazer-Yost

Clack

. (2013) Protein expression profiles of intestinal epithelial co-cultures: effect of functionalised carbon nanotube exposure. International Journal of Biomedical Nanoscience and Nanotechnology 3: 1–2.

46.

Beliaeva

Mikhaĭlova

Sycheva

. (2012) Effect of multi-walled carbon nanotubes on the morphofunctional state of the small intestine cells of mice. Gigiena i Sanitariia 6: 58–61.

47.

Patlolla

Berry

Tchounwou

(2011) Study of hepatotoxicity and oxidative stress in male Swiss-Webster mice exposed to functionalized multi-walled carbon nanotubes. Molecular and Cellular Biochemistry 358: 189–199.

48.

Zhang

. (2009) The hepatotoxicity of multi-walled carbon nanotubes in mice. Nanotechnology 20: 445101.

49.

Murray

Kisin

Leonard

. (2009) Oxidative stress and inflammatory response in dermal toxicity of single-walled carbon nanotubes. Toxicology 257: 161–171.

50.

Shvedova

Castranova

Kisin

. (2003) Exposure to carbon nanotube material: assessment of nanotube cytotoxicity using human keratinocyte cells. Journal of Toxicology and Environmental Health 66: 1909–1926.

51.

Kostarelos

Bianco

Prato

(2009) Promises, facts and challenges for carbon nanotubes in imaging and therapeutics. Nature Nanotechnology 4: 627–633.