Biomedical Applications of Carbon Nanotubes

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Biomedical Applications of Carbon Nanotubes: A Critical Review Priyanka Sharma, Neelesh Kumar Mehra, Keerti Jain and N.K. Jain* Pharmaceutical Nanotechnology Research Laboratory, I.S.F. College of Pharmacy, Moga, 142001, Punjab, India Abstract: The convergence of nano and biotechnology is enabling scientific and technical knowledge for improving human well being. Carbon nanotubes have become most fascinating material to be studied and unveil new avenues in the field of nanobiotechnology. The nanometer size and high aspect ratio of the CNTs are the two distinct features, which have contributed to diverse biomedical applications. They have captured the attention as nanoscale materials due to their nanometric structure and remarkable list of superlative and extravagant properties that encouraged their exploitation for promisN.K. Jain ing applications. Significant progress has been made in order to overcome some of the major hurdles towards biomedical application of nanomaterials, especially on issues regarding the aqueous solubility/dispersion and safety of CNTs. Functionalized CNTs have been used in drug targeting, imaging, and in the efficient delivery of gene and nucleic acids. CNTs have also demonstrated great potential in diverse biomedical uses like drug targeting, imaging, cancer treatment, tissue regeneration, diagnostics, biosensing, genetic engineering and so forth. The present review highlights the possible potential of CNTs in diagnostics, imaging and targeted delivery of bioactives and also outlines the future opportunities for biomedical applications.

Keywords: Biomedical imaging, Biosensors, Carbon nanotubes, Drug delivery, Functionalization, Photothermal therapy. 1. INTRODUCTION In past decades, there has been rapid advancement in the field of nanotechnology and nanomaterials that brought many fascinating thoughts to researchers for diagnosis and treatment of various diseases. The word “nano” has been taken from the Greek word, which means dwarf (small). Norio Taniguchi coined the word “Nanotechnology” in 1974 and suggested that nanotechnology is a process of separation, consolidation, alteration and deformation of biomaterials at atomic or molecular level [1]. According to Jain et al., nanotechnology is an immense field that fetches a great deal of interest and brings evolutionary changes in scientific world by exploring numerous facts regarding the structure and properties of various materials. It is a novel scientific approach of altering physical as well as chemical properties of a substance at atomic levels [2]. Certainly, progress in nanomedicines has lately allowed us to design multifunctional nanosystems consisting of targeting, therapeutic and diagnostic functions, all in one. Like, if nanoimaging technology and targeted drug delivery approaches are combined together, the impact on cancer therapy and treatment of other diseases is considerably elevated. Currently available nano drug delivery systems include liposomes, dendrimers, nanoparticles, carbon nanotubes, *Address correspondence to this author at the Pharmaceutical Nanotechnology Research Laboratory, I.S.F. College of Pharmacy, Ghal Kalan, Ferozpur G.T. Road, Moga, 142001, Punjab, India; Tel: +91-8054114006, +919425170897; E-mail: [email protected] 1567-2018/15 $58.00+.00

nanocrystals, nanoshells, quantum dots, etc. The major purpose behind the development and use of these nanocarriers is to maximize the therapeutic response of potent drugs whilst reducing their side effects and toxicity. Among these nanomaterials, CNTs are receiving much consideration and signify new alternative for safe and effective drug delivery system on account of their unique properties [2, 3]. 1.1. Historical Perspectives The true identity of discoverer of carbon nanotubes (CNTs) is a matter of controversy. Earlier, scientists assumed that the CNTs were originally discovered by Sumio Iijima, a Japanese microscopist, in his TEM observation in 1991 [4-7], while other scientists believed that CNTs were discovered much earlier by Bacon in 1960 [8-10]. However, Single-walled carbon nanotubes (SWCNTs) were synthesized and reported in 1993 by Iijima and Ichihashi [11]. The development of SWCNTs was first proposed in 1993 in 17th June issue of Nature wherein two independent papers were published [11-13]. Even so, before the described discoveries, investigations had been performed previously for many years. Watson and Kaufmann [14] published a paper in 1946 regarding the synthesis and observation of tubular carbon structures having 100 nm diameter [15]. In 1952, two Russian scientists gave the world its first clear look of CNTs. Radushkevich and Lukyanovich published a clear TEM image of tubes made of carbon having 50-100nm diameter produced by thermal decomposition method at 600⁰C temperature using iron as catalyst [16, 17] in the Soviet Journal of Physical Chemistry [18], unfortunately their findings re© 2015 Bentham Science Publishers

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mained unnoticed worldwide [9]. In 1976, Oberlin, Endo, and Koyama [19] published a paper in which hollow carbon fibers with nanometer-scale diameters were reported to have been synthesized by vapor-growth method [18]. At Penn State University, the 14th Biennial Conference of Carbon was held in 1979 where John Abrahamson presented proof of CNTs produced on carbon anodes by arc discharge method [20, 21]. 1.2. Definition and Types of CNTs CNTs are three dimensional hexagonal arrangement of sp² hybridized, carbon based nanomaterials made up of graphite or fullerene sheets having C-C distance of ~1.4 Å [22-24]. CNTs are the third allotropes of carbon and belong to fullerene family, which was discovered by Kroto et al. [25] in 1985. These are thin graphite sheets of condensed benzene rings [2, 26] that are rolled up into seamless tubular, hollow cylinders having nano-needle shape and size [9, 27]. A comparison between SWCNTs and MWCNTs has been given in Table 1.

graphene cylindrical tubes [16]. These DWCNTs resembles SWCNTs in their similar morphology and properties. c)

Triple walled carbon nanotubes (TWCNTs):- They are characterized by the presence of three layers of graphene [24, 31].

d) Multiple walled carbon nanotubes (MWCNTs):They consist of more than 3 concentric cylindrical tubes of graphene sheets having diameter and length in the range of 1.4 to 100 nm and 1 to 50 µm, respectively [24, 29], the approximate inter-layer distance of these tubes being 0.34 nm [32]. 1.3. General Physicochemical Properties a)

High cell membrane permeability [33, 34].

b) Excellent electrical, optical, thermal and mechanical properties [16, 34]. c)

Nano size diameter, ultra light weight, and ultrahigh surface area [34].

According to their diameter, lengths and presence of the number of graphite sheets, CNTs are categorized mainly into four types, as shown in (Fig. 1):-

d) High aspect ratio (length/diameter) in the range of 1:1000 (>3) [35].

a)

Single walled carbon nanotubes (SWCNTs):- They comprise of single sheet of graphite wrapped seamlessly [28] into hollow cylindrical tube having diameter between 0.4 to 2.5 nm and length 20 to 1000 nm [9, 29, 30].

e)

Photoluminescence property [9].

f)

Rich surface chemistry [36].

b) Double walled carbon nanotubes (DWCNTs):- They are coaxial nanostructures that consist of two concentric

i)

Table 1.

g) Non-immunogenicity and biocompatibility [34, 37]. h) Photoacoustic effect [38]. pH dependent unloading of therapeutics materials [33].

Differences between SWCNTs and MWCNTs. Parameters

SWCNTs

MWCNTs

Layers of graphene

Single

Multiple

Synthesis

Metal catalyst and graphite are required for synthesis.

Can be produced without catalyst. Pure graphite is required.

Twisting

Can be easily twisted.

Cannot be easily twisted.

Bulk synthesis

Difficult (as it requires proper control over growth and atmospheric conditions)

Easy

Chance of defect

More during functionalization

Less, but once occurred it is difficult to improve

Accumulation in body

Less

More

Electrical conductivity

Good

Bad

Purity (%)

90

95

Length

20-1000 nm

1-50 µm

Diameter

0.4-2.5 nm

1.4-100 nm

Ash (catalyst residue)

< 3%

< 0.2%

Nature (Amorphous)

5-15%

2%

Thermal conductivity

~4000W/m.K

~2000W/m.K

Specific surface area

300~600 m²/g

40-300 m²/g

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Fig. (1). Types of Carbon nanotubes.

j)

Excretion by biliary pathway (94% by urine and 6% by faeces) [9].

fects, high purity, controlled size and chirality with approximately 70% yield can be synthesized by this method [42].

k) Extremely high drug cargo ability [33, 36]. l)

Neutral electrostatic potential [39].

Various properties of carbon nanotubes have been summarized in Table 2: 1.4. Advantages and Disadvantages Although CNTs have many advantages like high aspect ratio, biocompatibility, non-immunogenicity, etc. yet they also have some limitations. The advantages and disadvantages of CNTs are graphically presented in (Fig. 2). 1.5. Methods of Preparation of CNTs Carbon nanotubes (CNTs) can be synthesized by various methods as shown in (Fig. 3) [40]. Among them, few methods are discussed belowa) Electric Arc Discharge Method (EAD):- In this method demonstrated by Ebbesen and Ajayan in 1992 [41, 42], CNTs are formed by arc vaporization of highly pure graphite electrodes (anode and cathode), having 1 mm distance, in a reaction chamber comprising of inert gas, e.g. Helium (He), and argon (Ar). Carbon nanotubes (CNTs) are developed by applying high voltage current of 50 to 100 amps and pressure between 50-700 mbar that generates high temperature discharge between two electrodes and carbon rods get evaporated and deposited on the cathode in the form of rod shaped tubes. The metal catalysts such as Fe, Ni, Rh, Pt or Co are used to synthesize SWCNTs [43]. b) Laser Ablation/ Laser Vaporization Method:- This method was first reported by Smalley’s group [44, 45] (Nobel Laureate with Curl and Kroto in 1996 for discovery of fullerene) at Rice University in 1995 [5, 46, 47]. In this method, a solid graphite target doped with metal (1% of Co and Ni) is irradiated by high-power laser vaporization of pure graphite target inside a high temperature furnace at 1200⁰C temperature. A throbbing or constant laser has been used to vaporize the graphite in an oven having Argon or Helium gas. The oven is filled with these gases so as to keep the pressure at 500 torr [48, 49]. The CNTs having very less structural de-

c)

Catalytic Chemical Vapor Deposition Method (CCVD):- In CCVD method, CNTs synthesis is carried out in a quartz tube placed in a furnace. Hydrocarbons (methane, ethane, benzene, toluene, acetylene, ethylene, and propylene etc.) and electron beam or resistive heating respectively have been used as raw materials and source of energy. The molecules are broken down into the reactive carbon species at the 500-1000⁰C [24]. After deposition of catalyst on substrate, nucleation of catalyst via chemical etching (using ammonia) or thermal annealing is carried out. Carbon source is then kept in a reaction chamber in gas phase, and transformed into atomic level that will diffuse towards supported metal catalyst on which growth of CNTs takes place [16, 42].

d) Electrolysis:- Introduced in 1995 by Hsu et al this method has been used less commonly for the synthesis of CNTs was [50, 51]. The MWCNTs are synthesized by passing current through two electrodes that are immersed in molten ionic salts (LiCl) at 600ºC temperature followed by extraction of carbonaceous matter by dissolving the ionic salt in distilled water. The MWCNTs so produced have 10-15 walls with 10-20 nm diameter and length greater than 500 nm [52]. 1.6. Advances in the Synthesis of CNTs Recently, a nebulized spray pyrolysis technique has come into existence and has been utilized for the synthesis of MWCNTs. A nebulized spray is the key factor in this method, which is generated by a special ultrasonic atomizer. MWCNTs with quite consistent diameters in aligned bundles have been produced by this method. At a fixed temperature of 800°C, ferrocene (catalyst) and ethanol (solvent and carbon source) are sprayed using ultrasonic nebulizer into a tubular furnace with an argon flow rate of 1 Lt/min. Ethanol is used as a solvent as well as a carbon source because of its salient features like non-polluting, cheap, harmless byproducts (e.g., CO), and ease of handling. High growth of MWCNTs can be produced on surface. The benefit of utilizing a nebulized spray is the effortless scaling in large scale production, since the reactants are fed into the furnace constantly [16, 53].


Table 2.

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Some important properties of CNTs. Properties

Details

Family

Fullerene, third allotrope of carbon

Types

SWCNTs, DWCNTs, TWCNTs, MWCNTs

Shape

Tubular and needle

Aspect ratio

Greater than 3 (1:1000)

Solubility

Insoluble in organic and inorganic solvents

Synthesis methods

(i) Electric arc discharge, (ii) Laser vaporization/ablation, (iii) Catalytic chemical vapor deposition, (iv) Electrolysis, (v) High pressure co-conversion, (vi) Cobalt-Molybdenum catalyst method

Purification

(i) Oxidation, (ii) Chromatography, (iii) Centrifugation, (iv) Selective sedimentation, (v) Sonication

Route of administration

All

Biocompatibility

Biocompatible

Immunogenicity

Non-immunogenic

Photoluminescence

Photoluminescence

Biodegradability

Non-biodegradable

Cell uptake

Good

Blood circulation half-life (hrs.)

3- 3.5

Excretion

By biliary pathway

Modification

By: (i) Covalent method, and (ii) Non-covalent method

Fig. (2). Advantages and Disadvantages of Carbon nanotubes.



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Fig. (3). Methods of synthesis of CNTs.

2. FUNCTIONALIZATION OF CNTS Due to the intrinsic hydrophobic nature, ability to agglomerate or bundling and low aqueous solubility, the pristine CNTs are unsuitable for drug delivery and targeting. These major hurdles can be easily overcome through surface engineering/functionalization. 2.1. Need of Functionalization i.

Improves the aqueous dispersibility of CNTs

ii.

Reduces the bundling/aggregation behavior of nanotubes

iii. Enhances the internalization of CNTs within the cells iv. Increased possibility of attachment of different functional groups and chemical moieties onto the surface of CNTs 2.2. Types of Functionalization Functionalization of CNTs not only enhances their solubility/dispersibility but also provides active functional sites for attachment of drugs, targeting ligands and other agents. For example, conjugation of several hydrophilic polymers like PEG makes the CNTs long blood circulatory, i.e. stealth [29] and helps to impede in vivo opsonization and avoid reticulo endothelial system (RES) uptake. PEGylation is the most effective method to enhance the in vivo properties of CNTs [54]. Various in-vivo and in vitro studies have shown

Fig. (4). Methods of functionalization of CNTs.

that surface functionalization of pristine CNTs lessens their toxicity [55, 56]. The chemical modification of pristine CNTs can be accomplished by the methods shown in (Fig. 4 and Fig. 5). a). Covalent Functionalization Covalent functionalization is an alternative and effective approach for dispersion of CNTs via surface modification wherein various organic reagents have been used for modification on sidewall or tip of the CNTs. Before covalent functionalization, carboxylic acid (-COOH) group on CNTs surface must be activated using reagents like thionyl chloride (SOCl₂), oxalyl chloride (C₂O₂Cl₂) or Nhydroxysuccinimide (NHS) so as to obtain highly reactive intermediate groups for stable covalent bonding between nanotubes and biomolecules [57]. i.

Ends and Defects:- Several oxygenated groups such as carboxylic, ketone, phenolic, hydroxyl, and alcohol etc. are introduced at ‘end or defect’ sites by oxidative treatment. These oxidized groups can be generated by treating pristine CNTs with mixture of strong oxidative agents (H₂SO₄:HNO₃ :: 3:1 or NH₄OH:H₂O₂ :: 50:50). Images of CNTs before and after oxidation with combination of H₂SO₄ and HNO₃ have been shown in (Fig. 6) [58]. This method not only forms the oxidative groups on CNTs surface but also shortens the length of CNTs [9]. By varying the type of acid, concentration of acid or reaction conditions like sonication, duration of time, and


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Fig. (5). Common strategies for CNTs surface functionalization.

temperature; various functional groups could be incorporated for enhancing the solubility/dispersion of CNTs [29]. ii.

Side Walls:- Different functional groups are attached onto CNTs surface by gas phase reactions. Covalent side wall functionalization produces sp³ hybridized carbon sites. Various approaches for side wall functionalization are 1,3-dipole cycloaddition reaction, Diels-Alder cycloaddition reaction, esterification-amidation reaction, mechanochemical functionalization [59] and grafting of polymers [24].

(peptides and nucleic acids) [29]. Non-covalent functionalization includes:i. Van der Waals Interaction (lipids and biopolymers), ii. π-π stacking (aromatic organic compounds and nucleic acid), and iii. Hydrophobic Interaction (fluorophores and proteins) 3. PURIFICATION OF CNTs The major hurdle in nanotubes application, next to largescale production, is the impurities and low solubility/dispersion of pristine CNTs. Solubility can be enhanced by surface engineering of CNTs. Usually, pristine CNTs contain several impurities like metal particles, amorphous carbon, etc. Various methods for the purification of pristine CNTs [60] are shown in (Fig. 7). 4. PHARMACOKINETICS AND METABOLISM OF CNTs IN BODY 4.1. Pharmacokinetics and Distribution of CNTs

Fig. (6). Oxidation of carbon nanotubes using a combination of nitric acid and sulfuric acid for chemical modifications by forming carboxylate groups on the CNTs surface [58]. [Reproduced with copyright permission].

b). Non-Covalent Functionalization Non-covalent functionalization consists of Van der Waals interactions, π-π interactions and hydrophobic interactions. The major benefit of this sort of interaction is the minimum harm caused to the CNTs surface. The noncovalent attachment is claimed to maintain the aromatic structure and electronic properties of CNTs [58]. Generally, three types of molecules are employed for functionalization of CNTs: (a) surfactants, (b) polymers, and (c) biopolymers

To elucidate the in vivo performance and toxicity of nanomaterials and for their safe and efficacious applications in medical era, it is vital to have in-depth knowledge of their pharmacokinetic (PK) characteristics. Before using CNTs for clinical applications, the PK data, long-term fate, and potential toxicity should be thoroughly evaluated [61, 62]. Over the last decade, many research groups have studied to evaluate the PK, biodistribution, and toxicity of CNTs in animals [56, 63-66]. Different imaging techniques have been utilized to assess the in vivo behavior of CNTs, which include radionuclide-based techniques such as positron emission tomography (PET) and single-photon emission computer tomography (SPECT), optical imaging (e.g. with fluorescence and Raman detection), photoacoustic imaging (PAI), magnetic resonance imaging (MRI), etc. Like other nanomaterials, surface coating plays a vital role in effecting the in vivo PK and biodistribution of CNTs. The blood circulation half-



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Fig. (7). Purification of CNTs.

lives, biocompatibility profile, and aggregation of individual CNT through Van der Waals forces can significantly affected by various surface coatings. Since, it is believed that long blood circulation half-life is advantageous for enhanced tumor targeting, numerous strategies have been investigated to make SWCNTs having ultra-long blood circulation halflives. In a research, it was reported that after covalent PEGylation of SWCNTs [67], the blood circulation half-life was enhanced upto ~22.5 hr., whereas non-covalent PEGylation of SWCNTs with PEG grafted poly (maleic anhydride-alt-1octadecene) (denoted as PEG-PMHC) remarkably exhibit long blood circulation half-life (~ 22 hr) when administered intravenously into mice [68]. Earlier, instability of Pluronic coating in physiological milieu was reported, which could be quickly substituted by plasma proteins after intravenous injection [65]. This is the possible reason for a short blood circulation half-life (~1 h) of CNTs in this report. Length of CNTs greatly affects the in vivo PK of CNTs due to their similar one-dimensional shape. It was reported that macrophages can confine 220 nm-long MWCNTs more readily as compared to 825 nm-long MWCNTs, followed by higher degree of inflammation [69]. Even though, the exact length of CNTs for macrophage capture is not known, CNTs having length from 100 to 300 nm have a tendency to accumulate in the liver and spleen [66, 70, 71], CNTs having length from 300 nm to 2 µm undergone rapid renal excretion [72, 73], and high uptake in the lungs and considerable retention in the liver and spleen had been noticed with CNTs having length > 2 µm [74]. In addition to the length, the shape of CNTs also plays an important role. For example, IL-1 (Interleukin-1) was activated leading to the production of reactive oxygen species in macrophages when long, needle-shaped CNTs were used [75], which can drastically influence the PK of CNTs. The PK and toxicity of CNTs are also strongly related to the administration route. Although most of the current PK studies were focused on intravenous injection, other administration routes such as intraperitoneal injection [76], intratracheal administration [77], and oral gavage have also been investigated. It was suggested in a report that SWCNTs can

aggregate inside the body to form fiber-like structures after intraperitoneal administration [76], and induce granuloma formation when the structure length exceeded 10 µm. Although shorter aggregates ( MWNT10 > C60. SWNTs significantly impaired phagocytosis of AM at the low dose of 0.38 µg/cm2, whereas MWNT10 and C60 induced injury only at the high dose of 3.06 µg/cm2. The macrophages exposed to SWNTs or MWNT10 at a dose of 3.06 µg/cm2 showed distinctive traits of necrosis and degeneration. Also an indication of apoptotic cell death probably existed. Carbon nanomaterials with diverse geometric structures exhibit quite different cytotoxicity and bioactivity in vitro, even though they may not be precisely reflected in the comparative toxicity in vivo [109]. Different research groups use different techniques, such as the MTT assay, the use of Alamar Blue and Trypan blue to evaluate the in vitro toxicity studies of the CNTs. CNTs may interact with the dyes used in some assays, altering the colour emitted and hence leads to a false result [110]. For example, whilst most toxicology studies have indicate that COOH functionalization of SWCNTs reduces toxicity, Jos et al. establish that COOH–SWCNT causes toxicity in the Caco-2 cell line [101]. An additional limitation of in vitro assays is the inability to generate any toxicokinetic data [102]. Even if, the in vivo tests are more costly and complex, it is usually believed that they give more precise and significant data that cannot be acquired through in vitro studies. In a study, it was found that MWCNTs may cause teratogenicity [111]. In vivo studies also have the benefit that they can offer data on a wider range of parameters, such as distribution, metabolism, and elimination. Considered together, these conflicting results emphasize the complexity in assessing the CNTs toxicity. The reactivity and the general behavior of CNTs in biological media are not well understood. The safety evaluation of these nanomaterials should also take into account a careful assortment of appropriate experimental method. Thus, further studies are warranted to resolve the safety issues of CNTs so that, if found safe, CNTs may be widely and safely explored in diverse medical and non-medical applications. 8. BIOMEDICAL APPLICATIONS OF CNTs With the extravagant properties like high aspect ratio, high electrical and thermal conductivity, non-immunogenicity etc., CNTs open a new vista in the field of nanobiotechnology. CNTs offer many benefits in various applications (Fig. 9) like targeted drug delivery, imaging and diagnosis, photothermal therapy, etc. 8.1. Medical Applications a)

Nanotubes are used as carrier to deliver quantum dots (QDs) and proteins into cancer cells because QDs have photoluminescent property beneficial in imaging [47]. This may be a path breaking finding.

b) The accomplishment of bone grafting relies on the capability of scaffold that assists the natural healing process. However, the scaffold may be associated with few disadvantages like low strength and body rejection [112]. Healing process can be improved by providing a CNTs scaffold for new bone material to grow on. A study



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Fig. (9). Applications of CNTs.

revealed that CNTs could mimic the role of collagen as scaffold for growth of hydroxyapatite (HA) into bones [113]. c)

CNTs have been used for Gene therapy and stem cell therapy [48].

d) CNTs have sufficient contractility, which makes them candidates to replace damaged muscle tissue [47]. e)

The functionalization of nanotubes with PEG makes them stealth that stops White Blood Cells (WBCs) from recognizing the nanotubes as foreign materials, thus allowing them to circulate in the blood streams for longer duration of time [47].

f)

CNTs are being widely employed for effective drug delivery and imaging/diagnosis for disease treatment and health monitoring [47].

g) Surface engineered CNTs have emerged as nanocarriers for the therapeutics delivery. CNTs have shown enormous potential in specific cells targeting without affecting the normal/healthy cells and at a dosage lower than that of conventional drug delivery system [47]. 8.2. Carbon Nanotubes as Nano-composite Material Because of their extravagant mechanical properties, CNTs can be used as nano-composite materials for the development of medical devices. The elasticity, tensile strength, conductivity, toughness and durability of composite materials can be enhanced by addition of CNTs. Various composite materials produced using CNTs, include MWCNTs/Silane [114], MWCNTs/PmPV [poly-(mphenylenevinylene)] [115, 116] and SWCNTs/PmPV [117, 118]. Pristine CNTs when combined with nylon-12, form a nano-composite material that can be utilized to form microcatheters for arterial cannulation with greater biocompatibility as compared to nylononyl micro-catheter [54, 119]. 8.3. Carbon Nanotubes in Tissue Engineering CNTs can be used for tissue engineering through tracking and labeling of cells and by enhancing cellular performance. With the advancement in tissue engineering, new techniques are warranted for superior examination of engineered tissues.

CNTs may improve tracking of cells, biosensing, delivery of transfecting agents, and scaffolding. CNTs can also be incorporated into scaffolds thus facilitating structural reinforcement as well as imparting new properties such as electrical conductivity into the scaffolds to add in cell growth [54]. 8.4. Photo Thermal Therapy (PTT) Using CNTs Photo-thermal therapy (PTT) is a non-invasive therapeutic approach for cancer treatment in which photon energy is converted into heat that is sufficient to demolish cancer cells [120]. Heating sources for PTT includes near infrared (NIR) or visible light, radiofrequency waves, microwaves, and ultrasound waves that are utilized to elevate the temperature in a particular target area to destroy the cancer cells [121]. Exposure to NIR rays causes cell death by coagulative necrosis and irreversible denaturation of protein or damage of plasma membrane of the targeted cells on reaching temperature above 55oC [58, 122]. PTT is mainly of three types, i.e. traditional photothermal therapy, plasmonic photothermal therapy (PPTT) and photodynamic therapy (PDT). In traditional PTT, radiations are used along with dyes that are capable to absorb radiation at the tumor site. Plasmonic photothermal therapy is based on the use of infrared or NIR light and electron clouds. Photodynamic therapy (PDT) utilizes free radicals and radiation with photo sensitizers that make the skin more permeable to X-rays [123]. CNTs have the capability to absorb light in the NIR region, resulting in heating of the nanotubes. This distinct characteristic of nanotubes may be useful in killing cancerous cells via thermal effects. CNTs have strong optical absorption in the NIR regions (NIR-I: 700-900 nm, NIR-II: 1-1.4 µm) [124] where biological systems are known to be highly transparent and promising photoacoustic/ photothermal agents for in vivo tumor destruction [7, 125]. The property of CNTs to absorb light in NIR region leads to provoke the thermal destruction of cancer cells that are having sufficient CNTs concentrations. Targeted CNTs can be used to spare healthy cells from damage [126]. An overview of photothermal therapy is presented in (Fig. 10).


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Fig. (10). Photothermal therapy using CNTs [58]. [Reproduced with copyright permission].

De La Zerda et al. described the potential use of cyclic RGD peptides tethered SWCNTs as a contrast agent in photoacoustic imaging [127]. Welsher et al. investigated the utilization of antibody conjugated SWCNTs for probing cell surface receptors as NIR fluorescent labels [128]. Same authors further demonstrated the application of SWCNTs by high-frame-rate video imaging wherein they showed that SWCNTs could be useful in the low albedo NIR II region (1.1-1.4µm window) as fluorophores [129]. Hong et al. studied the in vivo real-time epifluorescence imaging of mouse hind limb vasculatures in the second nearinfrared region (NIR-II) using SWCNTs as fluorophores. Both high spatial (~30 µm) and temporal (