Nanotubes

As one of the most interesting studies in nanomaterials, nanotube materials have a structure similar to small straws with large inner volumes, which makes it possible to fill with a variety of chemicals and biomolecules, ranging from small molecules to proteins [88, 89]. Nanotubes as a young class of material are usually composed of carbon nanotubes (CNTs) and non-carbonic nanotubes. Similar to nanofibers, nanotubes are also a type of one-dimensional macromolecules with a larger surface area, and normally exhibit similar strengthening mechanism as nano-fiber reinforcements.

Since the CNTs was first reported by Iijima [90] and Bacon [91] in 1991, they have become a biomaterial with great application prospects, especially for prosthe- ses positioned in contact with bone, like joint replacement prostheses and plates or screws for fracture fixation, as well as drug delivery systems [92]. Because of outstanding electrical, mechanical, and surface properties of CNTs, a great deal of research has been initiated in this field [93-95]. After the CNTs had been found, tungsten disulfide nanotubes (WSNTs), one of the noncarbonic nanotubes, were discovered by Rothschild et al. in 2000[10d]. Up to now, there are various noncarbonic nanotubes, such as ZnO nanotube [96], ZnS nanotube [97], NbS2 nanotube [98], boron nitride nanotubes (BNNTs) [99], NbSe2 nanotube [100], Bi2S3 nanotube [101], GaN nanotube [102], InP nanotube [103], SiO2 nanotube [104], AIN nanotube [105], and MS2 nanotube[11d]. As an important member of quasi-onedimensional family, noncarbonic nanotubes have a high volume percentage of surface area, which shows a high chemical activity and unique physical properties. In recent years, nanotubes have been synthesized from graphite and boron nitride compounds with layered structure by using various methods, such as arc discharge, chemical vapor deposition, laser ablation, carbothermal reduction, pyrolysis method, ball milling method, and hydrothermal synthesis method [92]. A new mechanism for the preparation of nanotubes from the non-layer material has developed. For example, Rothschild et al. prepared nano-diameter WS2 nanotubes by vaporsolid growth method [106]. There are two main types of nanotubes existing: the singlewalled nanotubes (SWNTs) consisting of coiled monolayer graphene sheets and the multi-walled nanotubes (MWNTs) with several graphite concentric layers. In the high-temperature methods, multi-walled carbon nanotubes (MWCNTs) can be produced by pure carbon evaporation, but the synthesis of single-walled carbon nanotubes (SWCNTs) requires the presence of metallic catalysts.

CNTs [107] has their unique structural (cylindrical nanostructure) and physical properties, such as high aspect ratio [108], strength (Young’s modulus ~1 TPa and tensile strength ~150 GPa) [109, 110], high conductivity (metallic nanotubes can carry an electric current density of 4 x 109 A/cm2) [111], and chemical stability. In recent years, some reports indicate that functionalized CNTs can improve cell compatibility of the matrix material, promote tissue regeneration and inhibit the formation of glial scar and fibrous tissue [112]. For instance, Shi et al. studied three different highly porous materials of scaffolds: poly (propylene fumarate (PPF) polymer, an ultrashort single-walled CNT (US tube) nanocomposite and a dodecylated US-tube (F-US tube) nanocomposite. The functionalized ultra-short SWCNTs nanocomposite with tunable porosity and mechanical properties possessed good performance among the composite materials, which could be a promising candidate as the ideal scaffolds materials for the bone tissue engineering applications. CNTs constitute a new type of materials and have been recognized as the most commonly used tubes reinforced materials, like fibers, in tissue engineering [113]. Because of its extraordinary properties, CNTs have been introduced into many host materials (polymer, ceramics, metal) to improve the overall properties of the reinforced composite system [114]. Remarkably, the alignment of CNTs can provide a crucial advantage in terms of directional conductivity, which is often required for the damage tissues or organs with the transmission of electrical signals [115].

Researchers have study the properties of CNT reinforced scaffold in the field of tissue repair and regeneration. Zawadzak et al. [116] deposited CNTs on the surfaces of PU foams using electrophoretic deposition (EPD) to create a new family of functional bone scaffolds. The CNTs coating was thought to promote the scaffold’s osteoconductivity and mineralization and provided a nanostructured surface and a conductivity function. Mikael et al. [117] found that the compressive strength and modulus of CNT reinforced PLGA scaffold were significantly increased (35 MPa, 510.99 MPa) compared to pure PLGA scaffolds (19 MPa and 166.38 MPa) by adding only 3% MWNTs. In addition to the random dispersion of the CNT, high electrical conductivity and mechanical strength can be achieved at low CNTs concentrations by aligning CNTs to form a connective pathway and obtain an effective charge transfer. This scaffold can be beneficial for the regeneration of nerve tissue to promote neurite outgrowth, orientation and branching [118]. Shin et al. [115] prepared the reinforced CNT-gelatin methacrylate (GelMA) hybrid, and a dose-dependent increase in mechanical properties was observed with the increase of the amount of CNTs. The tensile modulus increased about three times and the compressive modulus increased about two times by the adding 0.5 mg/ml CNTs in GelMA hybrids. Zhang et al. [119] found that the mechanical and biological properties of MWNT- reinforced PLGA improved significantly, making them potentially for tissue engineering applications, particularly for bone tissue regeneration. Compared with pure PLGA scaffolds, the tensile stress of the PLGA scaffold with 0.25% MWNTs was increased by 54% (from 5.88 to 9.08 MPa), the Young’s modulus was increased by 8% (from 163.53 to 176.83 MPa), and the elongation at break was increased by 49% (from 27.14% to 40.39%). Vozzi et al. [120] microfabricated 3D PLLA/ MWCNTs nanocomposite scaffolds for bone tissue engineering. The results suggested that the presence of CNTs increased the elastic modulus, and the cells on the PLLA/MWCNTs scaffold were more viable. Chen et al. [121] synthesized chitosan- multiwalled CNTs/HA nanocomposites (CHIMWCNTs/HA) for bone tissue engineering. These elastic modulus and compressive strength of the composites were increased sharply from 509.9 to 1089.1 MPa and from 33.2 to 105.5 MPa respectively, when multiwalled carbon/chitosan weight ratios were increased from 0 to 5%. And the cells on the surface of the composites with different weight ratios were spread out and had some filamentous pseudopods. Abarrategiet et al. [122] also proved that MWCNTs/CHI scaffolds with a well-defined micro channel porous structure had biocompatible and biodegradable supports for culture growth. Yildirim et al. [123] studied the preparation, characterization and biocompatibility of SWCNT-reinforced alginate composite scaffolds by free formation technique. In addition, Hu et al. [124] used CNTs as a substrate for culturing the hippocampal neuron growth. Mattson et al. [125] succeeded to cultivate embryonic rat-brain neurons on nanotubes, and the neurons survived and grew for 8 days in culture. Shin et al. [115] used CNTs as reinforcing agents to produce a porous hydrogel with adjustable mechanical properties.

As for noncarbonic nanotubes, their manufacturing method and properties have attracted extensive research and discussion, while their applications as reinforced materials in tissue engineering have few reports. Only a few of materials has been investigated as reinforcements in composites. Boron nitride nanotubes (BNNTs) are synthesized from boron nitrid with a stable hexagonal structure analogous to that of graphite [126]. BNNTs as an important material of noncarbonic nanotubes have excellent mechanical properties and thermal conductivity similar to CNTs, which makes it a potential candidate as reinforcements. Moreover, BNNTs have its own characteristics. BNNTs have chemical inertness, stable structure and high temperature oxidation resistance [127], so there is little effect on the ductility of the scaffolds when they reinforce polymer composites. As a kind of wide band gap semiconductors, the electrical property of BNNTs is independent from geometry. BNNTs are noncytotoxic to osteoblasts, macrophages, human embryonic kidney cells and human neuroblastoma cell line.

As a relatively new type of reinforcements, few researchers have studied the performance of BNNTs in the composite for tissue engineering. But BNNTs reinforced composites have been proved to have good thermal, mechanical, and optical properties, which makes them a promising candidate material for obtaining ideal composite material for tissue engineering scaffolds [128-130]. Lahiri et al. [131] reported that BNNTs reinforced PLC copolymer composite increased the tensile strength from 2.67 MPa in PLC to 5.59 MPa. BNNTs exhibited better expression levels of the Runx2 gene, which was the main regulator of osteoblast differentiation, and the composite had favorable biocompatibility to osteoblast and macrophages in the cytotoxicity assay. Meanwhile, Lahiri et al. [132] also proposed BNNTs as reinforcing agents to enhance HA for the stronger orthopedic implant applications. The results showed that the HA-4wt% BNNTs composite possessed excellent mechanical properties compared with HA. And the elastic modulus, hardness, wear resistance and fracture toughness were improved by 120%, 129%, 75% and 86%, respectively. BNNTs reinforcements also showed no adverse effect on osteoblast proliferation and cell viability. Zhi et al. [133] used BNNTs as reinforced agents to prepare poly(methyl methacrylate) (PMMA), polystyrene (PS), poly(vinyl butyral) (PVB), and poly(ethylene vinyl alcohol) (PEVA) composite materials to evaluate the thermal, electrical, and mechanical properties of reinforced composites. The BNNTs reinforced polymers maintained good electrical insulation and achieved higher thermal conductivity, an increase of more than 20 times. The thermal expansion coefficient of the BNNTs-loaded polymers was significantly reduced, and all the polymers remained insulated and possessed a high breakdown electric field in any case. The Vickers micro hardness tests results indicated that there was no obvious significant effect on the mechanical properties of the composites.

The inorganic WSNT is a closed polyhedron and a cylindrical crystal of a tungsten disulphide semiconductor compound having a high mechanical properties (Young’s modulus «150 GPa, bending modulus «217 GPa) [134, 135] and functional groups, which make it possible to disperse in organic solvents, polymers, epoxy, and resins. Like BNNTs, there is little research on the mechanical properties of WSNTs used as reinforcing materials for scaffolds. Elad et al. [136] evaluated the effect of embedding inorganic nanotubes of tungsten disulfide in an epoxy matrix, and the fracture toughness, shear strength, and peel strength of the resulting nanocomposites were increased by about 49%, 39% and 85%. Reddy et al. [137] found that the mechanical properties of polymeric nanocomposites could be significantly improved at very low load concentrations of WSNTs. The elastic modulus of electrospun PMMA fiber composites at 2 wt.% loading of WSNTs improved 22-fold, and their tensile strength increased 30-35% compared with pristine PMMA fibers. Lalwani et al. [138] reported that PPF nanocomposites reinforced by WSNTs at low loading concentrations (0.01-0.2 wt.%) possessed a significant enhancement (up to 28-190%) in the mechanical properties, including compressive modulus, compressive yield strength, flexural modulus, and flexural yield strength. And the efficacy of WSNTs as reinforcing agents was better than (up to 127%) or equivalent to that of carbon nanotubes (SWCNTs and MWCNTs). In general, WSNTs have the potential to be a new reinforcing material for the manufacture of scaffolds in tissue engineering, and there is still great space for further research in the future.

Titania (TiO2) is known as a class of biomaterials with excellent biocompatibility for orthopedic applications, and has been widely used as a coating for bone implants. TiO2 nanotube is highlighted as an implantation material due to its unique properties such as high specific surface area and the ability to exhibit positive cellular response. TiO2 nanotube might also be a good candidate as nanotube reinforcements in scaffolds for tissue engineering. Recently, TiO2 nanotube surfaces have been proposed as alternative architectures to enhance the interaction between the implant and tissues for orthopedic implant considerations, as well as the drug delivery systems. To date, a great number of studies have demonstrated the TiO2 nanotube surfaces exhibit a positive effect on cell behavior such as accelerated apatite formation, enhanced osteoblast adhesion, proliferation and differentiation [139]. The diameter of TiO2 nanotube shows significant influence on cell response. For instance, a recent study by Minagar et al. [140] indicated that TiO2-ZrO2-ZrTiO4 nanotubes with inner diameter (Di) ~40 nm exhibited the highest level of SaOS2 cell adhesion (41.0%), compared to 25.9%, 33.1% and 33.5% cell adhesion for nanotubes with Di 59 nm, 64 nm and 82 nm. The studies in vivo demonstrated that the TiO2 nanotube surfaces significantly improved the bone bonding strength, about nine fold compared with TiO2 gritblasted surfaces [141]. However, there is little literature on the use of TiO2 nanotubes to enhance tissue engineering scaffolds.

This nanotube structure can provide pographical cues and signals to guide the cell functions of different cell lines. It is therefore foreseen that nanotubes, both CNTs and noncarbonic nanotubes, are potential candidates as reinforcements for future biomedical applications. The future growth of interest in the research of reinforcing materials should be expected in the field of nanotubes. More future work should focus on making directional nanotubes by advanced manufacturing techniques to further improve the performance of composite materials.

 
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