In vitro studies of carbon nanotubes biocompatibility
Introduction
Carbon nanotubes, due to their specific structure/texture and properties, may play a significant role in the development of carbon materials for medicine, the main body of which includes pyrocarbons, glassy carbon, carbon fibers, carbon–carbon composites, and diamond-like layers [1], [2], [3], [4]. The medical applications of these materials are determined by the following properties: biocompatibility in contact with blood, bone, cartilage and soft tissues; biofunctionality understood as the ability of taking over certain functions of tissues by a mutual adjustment of implants and tissues properties [5], [6], [7]. Fields of current applications of carbon biomaterials are given in Table 1.
The electronic structure and the surface morphology of carbon nanotubes, which probably determine their biocompatibility, are typical of graphite-like structures [11]. They can be distinguished by a tubular construction in the nanometer range and by high strength and Young modulus [12]. The combination of these properties may open new fields of application, including those in medicine. Like other fibrous materials, nanotubes can be used as substrates for the regeneration of tissue. Due to their high electrical conductivity, the latter process can be additionally assisted by electrostimulation during the cell cultures and the tissue formation [13].
Two forms of carbon nanotubes can exist: single-wall (SWNTs) and multiwalled (MWNTs) [14], [15]. They can be open-ended after specific chemical treatments [16], or may have closed tips. In the latter case they may be used as biosensors [17]. The accessible canals of open-ended nanotubes may facilitate the migration of metabolites or growth agents, and may also be used as a drug carrier [18]. Very good mechanical properties, the possibility of surface machining as well as the ability to form functional groups constitute good fundaments for the use of nanotubes in the fabrication of composite materials.
The possibility of forming composites with polymer matrices, both biostable (polysulfone, PEEK), and bioresorbable (PLA, PGLA, co-polymers) is of particular importance in the case of medical applications [19], [20], [21]. This opens opportunities for the manufacture of multifunctional implants useful in many areas of medicine. However, with resorbable polymers, the relation between the resorption time and the time of tissue healing is of significant importance. A too high resorption rate may lead to a release of nanotubes from the composite materials into the living body.
In the case of the ceramic matrix composites, an improvement of the fracture toughness can be expected, which may be particularly important in the case of manufacturing reinforced nanostructure ceramics. The nanocomposite system of nanotubes reinforced hydroxyapatite [22] may be included in this category.
The use of the advantages of nanotubes in medical applications, particularly their tubular morphology and their excellent electrical and mechanical properties relies heavily on their biocompatibility. Although both the nature of carbon and positive experiences to date with various forms of carbon would suggest also a good biocompatibility of nanotubes, basic cellular tests must be performed in order to allow projects and application works to be opened in medicine. In the present work, the viability of fibroblasts, osteoblasts and osteocalcin concentrations in osteoblasts cultures in the presence of high purity multiwalled carbon nanotubes (MWNTs) has been examined, as well as the degree of cells stimulation, based on the amount of released collagen type I, IL-6 and oxygen free radicals.
Section snippets
Preparation and characterization of the nanotubes
In our experiments, we used high purity multiwalled carbon nanotubes (MWNTs) prepared by catalytic decomposition of acetylene on a CoO/MgO solid solution catalyst, according to the process described in Refs. [23], [24]. This process allows a large scale and selective production of MWNTs, free of any disordered carbon, that makes the purification very easy by simple dissolution of the catalyst precursor in HCl. The elemental analysis on the purified material gives: C = 96 wt%, H = 0.85 wt%, Mg = 200 ppm
Results and discussion
The examination of fibroblasts and osteoblasts viability on polysulfone films covered with nanotubes indicates a small decrease of the viability of all the examined cells, as compared to the viability obtained on pure polysulfone films (Fig. 4). This decrease may be related to the nature of the substance itself, as well as to its surface state. In the case of other carbon materials, the effect of their surface roughness on the cells viability was found to be of importance [26]. The same effect
Conclusion
The cellular tests performed in this study confirm a good biocompatibility of nanotubes, which is similar to that of polysulfone currently used in medicine. The high level of viability of the examined cells in contact with the nanotubes, the unchanged level of osteocalcin released from osteoblasts, the lack of pro-inflammatory IL-6 cytokine as well as free radicals induction, point out a good cellular biocompatibility of nanotubes. The slight increase of collagen formation induced on nanotubes
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