Carbon Polymer Nanocomposites For Tissue Engineering Biology Essay


Abstract: Nanotechnology research provides a fundamental understanding of phenomena and materials that enable the creation and use of structures, devices and systems that have novel properties and functions because of their small size. The introduction of nanotubes in nanotechnology field began a revolutionary era as CNT's with 100 times more strength than that of steel are 1/6th the weight of the steel. These can be a very good substitution for load bearing structures of the body. The ultimate goal in tissue engineering is to make a structure that can replace the lost tissue and provide support to the surrounding matrix.

1. Overview

1.1 Introduction

Carbon nanotubes with unique electronic and mechanical properties are the latest addition to the field of nanotechnology. Single-wall carbon nanotubes (SWCNTs) can be thought of as rolled graphite sheets with fullerene resembling structures at the apex. The electronic properties of SWCNTs depend on the angle of wrapping and graphite sheet diameter, which characterizes them as either metallic or semi-conducting SWCNTs. The carbon nanotubes introduction in the field of nanotechnology might compensate for the drawbacks in micro-technology. Tissue engineering is a very challenging field and requires the tissue engineered substitute to regenerate, restore and maintain the biological structure and function lost by the host tissue. Hence, the goal is to construct something that can replace the tissue and support the extra cellular matrix.

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By combining the properties of nanotubes with a biocompatible polymer we can obtain a new generation of tissue-engineered implants that can reconstruct natural tissue subjected to high mechanical stress. The use of polymer in bio medical area is very limited as the primary criteria to be eligible for this field is bio compatibility. Also, the polymer has to satisfy necessities such as implant integrity, when used for load bearing parts of the body. The polymers with low strength cannot satisfy these needs. Hence, materials such as glass fibers and carbon are mixed to increase the polymer strength. All life forms contain carbon and nanotubes are only made from carbon having the same scale size of DNA. Hence, carbon nanotube that contain biocompatible polymer structures can have a successful application in biomedical science.

The conductive properties of the nanotubes reflects in the composite with well aligned nanotubes inside the structure that is used as implants for stimulating neuronal growth and new bone formation and bone remodeling by the use of electrical stimuli. Articular cartilages get inured quite frequently. Since the regeneration is very limited for them, such injuries lead to permanent disability and joint dysfunction. This problem can be solved by the use of organic materials such as tissue grafts, inorganic fibers such as carbon fibers, polyester, hydroxyapatite, polymer hydrogels and sponges as artificial cartilage.

Biomedical polymers are roughly characterized into two major groups namely, biostable and biodegradable polymers. Biostable polymers are sufficiently durable and can be used for long-term as artificial organs like veins, arteries, aortic valves and skeletal joints. In contrast, Biodegradable polymers are used for short-term in the body and then they gradually decompose into tiny

molecules and are metabolized or excreted. Many biocompatible polymers are used as scaffolds for tissue engineering. Carbon nanotubes have 100 times more strength than that of steel and 1/6th of weight of the steel and are highly suitable for the implants of bone and cartilage, which bear the load of the body. Also, the flexibility in bending and moving of 2-dimensional graphite

sheet reflects the gliding movement of the joint cartilages.

PLA is a alpha-hydroxy polyester that has a characteristic mass loss in uniform manner. This uniform mass loss of PLA makes it suitable as an ideal biodegradable implant for cartilage repair. The degradation period for PLA is enough for maintaining the integrity of the tissue structure during the regeneration of the cartilage. The nanofiber surface provides large amount of surface area. Another very important factor for implants is the pore size as cells are selective to certain pore sizes through which they can migrate and proliferate. High porosity of nanofiber scaffold resembles the porous structure of the cartilage. Electrospinning is used to obtain the PLA fibers. During this process, there is a continuous charge flow from the source to the target. This charge aligns the nanotubes in a arranged way along the length of the fibers to achieve good electrical conductivity in the composite.

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Of the total mass of cartilage matrix, about 70% is water and approximately 95% of the dry weight is composed of collagen and proteoglycans. Chondrocytes makes up less than 5% of the volume of cartilage. The conductivity of Carbon nanotubes containing nanofibers represents the conductivity in extra cellular matrix present when there is wound healing or remodeling of the bone. The developing bone surfaces contain un-mineralized collagen fibrils which give rise to mechanic-electrical effect that is responsible for bone remodeling. Hence, carbon nanotubes that contain conductive polymer nanofiber matrix is expected to pass electrical stimulation for growth and healing as well as maintaining the shape of bone and cartilage. The nanofibers also provide nanoscale perfection and support and guide cell growth towards the direction of the nanofibers. Also, futher advantage of the CNT's incorporation is enhancement of the integrity of the scaffold almost equal to the strength of cartilage.

1.2 Properties of an ideal scaffold

In tissue engineering, an ideal scaffold must be compatible to the tissue environment. It should not produce any antigenic or toxic reaction to the body. The tissue structure should be strong enough to be a fair representation of the original replacement. Also, it should rapidly adhere to the wound area when it is implanted in the body. It must either be biodegradable and is eventually completely excreted from the body after regeneration of the new tissue or should be biostable and must resist wear and tear. As, the wear particles from such nondegradable scaffolds may prove to be toxic to the tissues in the surrounding. Further, the product should be economical so that every class of patients can use it. Lastly, an ideal scaffold should have an indefinite shelf life and also its storage should be cheap and easy.

1.3 Objective

In the recent times despite of the few studies on polymer matrix with CNTs, the area of electrospun-nanofiber matrix as a suitable biodegradable scaffold has not been studied enough. The use of these polymers in tissue-engineering is a very attractive alternative as the inflammatory response to the implant recedes after the scaffold is resorbed. Here we will use a biodegradable polymer PLA (Poly -L -lactic acid) in the form of nanofibers to provide adequate surface area, broad range of pore size and highly porous structure to allow and support cartilage cell growth at nano scale. Further, the polymer is reinforced with the single-walled carbon nanotubes to provide good physical integrity and electrical conductivity at nano stage. So, the objective of this study is to develop a polymer nanofibernanotube scaffold for cartilage that bear the loads and thereby systematically evaluate the scaffold.

2. Carbon nano tubes

2.1 Introduction

The fabrication and properties of carbon nanofibers are useful in this project because of the fibers'expected conductivity properties. These materials viz carbon nanotubes are very good conductors. But they have some problems like they can not be easily obtained, their structure cant be controlled and results once taken cant be repeated again and again. Also, there are problems associated with the thin films. They are not thin as per the desired requirements, they cant be usd to build a circuit etc. whereas, carbon fibres are very thin, the results obtained can be reproduced and are very easy to manipulate. Polyacrylonitrile (PAN) is a widely used precursor for carbon fibers. Also, PAN can be easily transformed into a polymer solution by dissolving it in dimethylformamide (DMF), and then it can be used for electrospinning. After electrospinning, the polymer polymer solution obtained has fibers at the sub-micron level. These fibers are then heat-treated on a vacuum furnace to obtain carbon nanofibers.

Recent experimental and theoretical investigations of carbon nanotubes reflect that carbon nanotuubes have properties suitable for applications in the areas of nanoscale electronics, mechanics, and composites etc. There have been some attempts to manipulate nanotubes to actually build nanoscale electronics, but they haven't given any positive results yet. Successful control of the fabrication and manipulation of carbon nanofibers should enable us to be able to build nanoscale electronics. We have been able to successfully control some of the properties for the nanofibres fabrication. The thickness of the fibres can be controlled by varying the voltage and viscosity of the fibers. In addition, the carbon content in the fiber can be controlled by changing the heating temperature. Other factors such as distance, the collection plate shape, angle of the syringe, etc. can also alter the properties of nanofibers

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There are two main types of carbon nanotubes that have high structural perfection i.e Single-walled nanotubes and Multiwalled nanotubes. Single-walled nanotubes (SWNTs) consist of a single graphite sheet wrapped into a cylindrical tube seamlessly.

Figure 1: Types of SWNT's

Multiwalled nanotubes (MWNTs) consist of a seamless array of such nanotubes which are nested concentrically similar to rings of a tree trunk.

Figure 2: Multi walled nano tubes

Although they are structurally similar to a single sheet of graphite, which is a semiconductor with zero band gap, SWNTs can be either metallic or semiconducting, which wholly depends on the direction about which the graphite sheet is rolled for the formation of a nanotube cylinder. This graphite sheet plane direction and the diameter of the nanotube can be obtained from a pair of integers (a, b) that represent the nanotube type (1). The nanotube is either of the armchair (a= b), zigzag (a=0 or b = 0), or chiral (any other a and b) variety, this depends on the appearance of the belt of carbon bonds around the diameter of the nanotube. All types of armchair SingleWalled NanoTubess are metals and those with a - b=3k, where k is a nonzero integer are semiconductors with a tiny band gap and all the remaining others are semiconductors with a band gap that inversely depends on the diameter of the nanotube (1). The electrical properties of perfect MWNTs are very similar to those of perfect SWNTs, as the coupling between the cylinders is weak in MWNTs. Due to the almost one-dimensional electronic structure, electronic transport in metallic SWNTs and MWNTs occurs ballistically which is without scattering over long lengths of nanotubes, which enables them to carry high currents with essentially almost no heating. Phonons can also easily travel along the nanotube: The thermal conductivity measured at room temperature for an individual MWNT (>3000W/m_K) is greater than the thermal conductivity of natural diamond (2000 W/m_K).and the graphite basal plane (2000 W/m_K). The materials also exhibit Superconductivity but only at low temperatures, and have a transition temperature of ~0.55 K for 1.4-nm-diameter Single Walled Nano Tubes and ~5 K for 0.5-nm-diameter Single Walled Nano Tubes cultured in zeolites.

Small-diameter Single Walled Nano Tubes are relatively stiff and exhibit exceptional srenghth, which means that they have a high Young's modulus and a very high tensile strength. Theoretical and literature journals/reports of such mechanical parameters can prove to be confusing at times, because few authors use the total occupied cross sectional area and the rest take into account the much smaller van der Waals area for defining Young's modulus and tensile strength. With the total area per nanotube in a bundle of nanotube for normalizing the applied force to obtain the applied stress, the Young's modulus calculated for an individual (10, 10) nanotube is ~0.6 TPa, which proves to be consistent with the theoretical measurements. As, nanotube ropes of small-diameter have been elastically extended by ~6% before breaking, the Single Walled Nano Tube strength figured from the product of the aforemetioned strain and Young's modulus is ~37 GPa, which is almost equal to the maximum strength of silicon carbide nanorods (~53 GPa). This modulus of ~0.64 TPa is almost equal to the modulus of silicon carbide nanofibers (~0.66 TPa) but it is lower than the modulus of highly oriented pyrolytic graphite (~1.06 TPa). The density-normalized modulus and strength of this typical Single Walled Nano Tubes are ~19 and ~56 times respectively when compared to steel wire and ~2.4 and ~1.7 times when compared to silicon carbide nanorods. This property is very important and of utmost use for applications needing light structural materials. However, to achieve all of these properties of individual Single Walled Nano Tubes in assemblies of nanotube found in continuous sheets and fibers is still a challenge.

2.2 Nanotube Synthesis and Processing

Single Walled Nano Tubes and Multi Walled Nano Tubes are generally made by discharge of carbon-arc, carbon laser ablation, or deposition of chemical vapor(typically on catalytic particles). Diameters of nanotubes range from ~0.5 to ~3 nm for Single Walled Nano Tubes and from ~1.5 to at least 100 nm for Multi Walled Nano Tubes. Properties of nanotubes can hence be controlled by changing the diameter. Unfortunately, Single Walled Nano Tubes are very expensive and are produced only on a small scale currently. Almost all scientists depend on the production facilities started by Rick Smalley of Rice University for purified and genuine Single Walled Nano Tubes, on nanotubes produced by laser ablation, and now on the high-pressure carbon monoxide (HiPco) nanotubes of Carbon Nanotechnology, Inc.(CNI. Hence it is hoped that price will be brought down by this production.

All synthesis methods for Single Walled Nano Tubes known presently give rise to major concentrations of impurities. Metal catalysts that are carbon-coated contaminate the HiPco route nanotubes, and both metal catalysts that are carbon-coated are formed by the carbon-arc route. Acid treatment removes these impurities buit in turn introduces other many kinds of impurities, and also it might degrade length and perfection of nanotubes. Also, it adds to nanotube cost. Furthermore, the usual synthetic routes give rise to mixtures of various nanotubes that are semiconducting and metallic especially for electric devices. Metallic Single Walled Nano Tubes can be easily damaged selectively by electrical heating, so that only the semiconducting nanotubes required for nanotube field-effect transistors (NTFETs) remain. But, no method to generate enough amount of one type of nanotubes is yet recognized.

However, profitable access to Multi Walled Nano Tubes is less challenging. Hyperion Catalysis International, Inc., pioneered the manufacture of MWNTs in large quantities in the first half of 1990s. But, these nanotubes have not been widely accessible to and used by researchers, because Hyperion has usually sold nanotubes compounded as a minority element in plastics and has always required agreements restricting independent pursuit of customer patents. Also, Multi Walled Nano Tubes manufactured catalytically using gas-phase pyrolysis have very elevated defect densities when compared with the ones manufactured by the much more expensive carbon-arc process. However, the tubes produced catalytically are substantial for various applications, principally because they can be straightforwardly produced without any major pollution by carbonaceous impurities.

Nanotube be it sheets, fibers or composites must all maintain the properties of the individual nanotubes as far as achievable. A common problem here is that the impurities very easily coat the surface of nanotubes. Even a very fine nanometer-thick coating can concern spreadiblity of nanotubes, composite binding in and the electrical and mechanical properties of junctions between nanotubes. Also, Single Wall Nano Tubes generally get shaped into bundles of parallel tubes such that the entire surface area of the individual nanotubes is not typically available for the transfer of stress within the matrix.

Figure 3: TEM micrograph (18) showing the lateral packing of 1.4-nm-diameter SWNTs in a bundle.

Nanotube sheets, also known as "nanotube paper" or "bucky paper", are normally prepared by filtering Single Wall Nano Tubes isolated in a liquid, then peeling off the produced sheet from the filter and washing and drying it, and finally annealing the sheet at very high temperatures to eradicate impurities. Had Single Wall Nano Tubes been not so costly and if there were a need by the companies, one could make nanotube sheets ona similar scale using similar methods as used to manufacture ordinary paper. But, the maximum Young's modulus of sheets manufactured using the filtration process does not significantly exceed the modulus of ordinary organic polymer sheets and it increases from ~0.3 to ~6 GPa as extra precaution and care is taken in eradicating derived impurities ("bucky goo") which were introduced during purification.

Now days, advanced polymer containing Single Wall Nano Tubes are being produced by melt spinning and in aligning the nanotubes using drawing. But, the melt viscosity becomes too elevated for usual melt spinning when the nanotube content is more than 10%, and established enhancements in strength and modulus are smaller than those forecasted using the rule of mixtures. Vigolo and many others researchers have developed a coagulation-based process that enables us to spin nonstop fibers that contain almost Single Wall Nano Tubes. Presently, however, the production rate from the coagulation bath is very low, the loading of the nanotube in the spinning solution is low and the alignment is not very good of the nanotubes. The maximum modulus derived for fibers spun by a modification of Vigolo and others' coagulation-based process is ~50 GPa, more than an order of magnitude lower than the intrinsic modulus of individual Single Wall Nano Tubes. Trace poly(vinyl alcohol) synthesized from the coagulation solution helps in binding the nanotubes together in air more successfully than do van der Waals interactions, and it is also responsible for causing fiber swelling and corresponding degradation of mechanical properties in aqueous electrolytes. Its exclusion by pyrolysis decreases Young's modulus to ~15 GPa. Another major problem for these spun fibers is Creep. A newly developed fiber-spinning method for Single Wall Nano Tubes, which appears to include a lyotropic liquid crystal phase, elevates the concentrationof the nanotube in the spinning solution by more than an order of magnitude results in oriented nanotube fibers. Advancement in coupling between nanotubes emerges necessary to optimize the Young's modulus and tensile strength of these spun nanotube fibers which are currently very low. With selected area of deposition of catalyst, nanotubes have been developed as jungles of vertically aligned Multi Walled Nano Tubes, nanoprobes, and structures for field emission displays


Figure 4: Scanning electron microscope (SEM) image of an array of MWNTs grown as a nanotube forest

3. Spectroscopic study

3.1 Nanotubes and Polymers interaction

the interaction between conjugated polymer and nanotube is widely studied. In order to probe the structure and property relationship in both carbon nanotubes and conjugated polymers Raman spectroscopy is used. The polymer PmPV spectrum and carbon nanotube spectrum can be used to contrast between the spectra of PmPV-nanotube composite spectra. The wavelength used to excite is 676nm.

Figure 5: Comparative Raman spectra for SWNT, 1% SWNT composite and PmPV.

The above figure shows a dominating peak corresponding to the nanotube spectra at 1580 cm-1. This spectra of polymer were dominated by multiple modes, and have a central mode at 1600 cm-1. The PmPV spectrum depicts two well defined bands at 1330 cm-1 assigned for vinyl bond and it shows multiple modes for nanotubes at 1590cm-1, 1610 cm-1 and 1627 cm-1

3.2 On pure, unreacted SWNTs

Raman spectra of pure unadulterated SWNTs shows a characteristic breathing mode of Single walled carbon nanotubes at 186 cm-1. The broader peak at 1580 cm -1 corresponds to the sp2 hybridized carbon stretching mode.

Figure6: Raman spectra of pure, untreated SWNTs

3.3. Study of single-layered nanotubes with electron beam

Kiang proved that the geometry of nanotubes is many a times distorted from their regular straight and cylindrical form. Although nanotubes are always expected to be tough, resistant and strong, sometimes defects arising in the graphite sheets can bend and twist the nanotubes sharply without tearing.

Transmission Electron Microscopy observation shows that the rigidity and flexiblity of single walled carbon nanotube depends on their diameter. Bending was frequently seen in tubes having diameter more than 2 nanometers while the tubes with smaller diameter than 2 nanometers preserved their regular cylindrical shapes. Defects can frequently occur when nanotubes entangle with each. The rest of the nanotubes other than those defected appear to maintain their unchanged diameter along the length. This constant diameter of the tubes proposes that the twisting and folding of single layered

nanotubes happened after the formation of the nanotubes and not before their formation.

Figure 7: TEM image showing twisting of the nanotubes

Figure 8: TEM images showing folding of single walled nanotubes