Bioglass has been used for bone-filling material in bone tissue engineering, but its lean mechanical strength limits its applications in load-bearing positions. Carbon nanotubes (CNTs), with their high aspect ratio and excellent mechanical properties, have the potential to strengthen and toughen bioactive glass material without offsetting its bioactivity. Therefore, in this research, multiwall carbon nanotube (MWCNT)/45S5 Bioglass composite scaffolds have been successfully prepared by means of freeze casting process. 45S5 Bioglass was synthesized by the sol-gel processing method. The obtained material was characterized with X-ray powder diffraction (XRD). The mechanical properties of the scaffolds, such as compression strength and elastic modulus, were measured. Finally, compared with the scaffolds prepared by 100% 45S5 Bioglass powders, the addition of 0.25 wt.% MWCNTs increases the compressive strength and elastic modulus of 45S5 Bioglass scaffolds from 2.08 to 4.56 MPa (a 119% increase) and 111.50 to 266.59 MPa (a 139% increase), respectively.
Tissue engineering is a concept that promotes the regeneration of host tissue by designing the scaffold that is populated with cells and signaling molecules. The scaffold is a three-dimensional substrate that can act as a template for tissue regeneration. The specific properties of ideal scaffolds for bone tissue engineering can be defined as good biocompatibility, optimal porous structure with pore interconnectivity, and ability to deliver cells. In addition, the scaffolds should possess appropriate mechanical strength and biodegradation rate without any undesirable by-products [
Carbon nanotubes (CNTs) are nanosized cylindrical carbon tubes with very large aspect ratios. CNTs can be categorized as (i) single-walled carbon nanotubes (SWCNTs) and (ii) multiwalled carbon nanotubes (MWCNTs) [
These materials have excellent characteristics that make them potentially useful in many applications such as biomaterials science and scaffolding for bone regeneration. Bone tissue compatibility of CNTs gives them an influential role on the bone formation. Also, applying CNTs in synthetic bone materials can improve their overall mechanical properties, and they act as appropriate scaffolds to promote and guide bone tissue regeneration [
CNTs have been extremely applied as reinforcing fibers in polymer and metals matrix composites [
Mattioli-Belmonte et al. [
Bioactive glasses are predestined materials to develop suitable materials for medical applications such as using as implants in the human body to repair and replace diseased or damaged bone due to their excellent bioactivity as well as biocompatibility. However, contrary to their outstanding bioactivity characteristics, these biomaterials exhibit low mechanical strength such as fracture toughness in comparison with natural bone [
Among all kinds of bioactive glasses, 45S5 Bioglass has been used in a number of medical devices due to its approval by the U.S. Food and Drug Administration (FDA). In 1969, Hench and his colleagues developed 45S5 Bioglass with the nominal composition of 46.14 mol% SiO2, 24.35 mol% Na2O, 26.91 mol% CaO, and 2.60 mol% P2O5. It has been successfully used in orthopedic and dental surgery [
Commercially produced bioactive glasses have been made by conventional glass powder manufacturing methods, that is, melting and quenching. Meanwhile, increasing research efforts are being invested in fabrication of bioactive glasses using the sol-gel technique, due to its advantages over melting-quenching processes [
Freeze casting, as an effective method for preparation of porous structures, was seen a great deal of efforts in recent years. This method includes freezing a liquid suspension, followed by sublimation of the solidified phase, and subsequent sintering to densify the walls, resulting in a porous structure with unidirectional pores in the case of unidirectional freezing, where pores are a replica of the solvent crystals [
The chemicals used in this research include tetraethyl orthosilicate (TEOS: C8H20O4Si; Merck Co.), calcium nitrate tetrahydrate (Ca(NO3)2·4H2O; Merck Co.), sodium nitrate (NaNO3; Scharlau Co.), triethyl phosphate (TEP: C6H15O4P; Merck Co.) and nitric acid (HNO3; Merck Co.) for the synthesis of the sol-gel 45S5 bioactive glass, and polyvinyl alcohol (PVA; Merck Co.) and carboxymethyl cellulose (CMC; Fluka biochemika Co.) as additives for slurry preparation, and multiwalled carbon nanotubes (MWCNT; supplied by Research Institute of Petroleum Industry) as a reinforcement phase were used for the composite preparation.
Multiwalled carbon nanotubes (MWCNTs) were prepared by spray pyrolysis, a type of catalytic chemical vapor deposition (CCVD) method, in which the carbon source, in the form of liquid hydrocarbons, acts as a solvent for the catalyst and is sprayed into the furnace. Ferrocene was used as a catalyst precursor, and hexane, a good solvent for ferrocene, was used as a carbon source. The sublimation temperature of ferrocene, the ferrocene concentration in hexane, pyrolysis temperature and time, and the flow rate of hexane and H2 were optimized to obtain MWCNTs with a high quality and a high yield. For the same of removing the impurities, MWCNTs were purified with acid leaching and air oxidation. The MWCNTs have a typical sausage-like structure and their lengths are more than several tens of micrometers. The inner and outer diameters of the MWCNTs were in the range of 15–45 and 25–70 nm, respectively. The purity and yield of the purified MWCNTs were more than 95% and 70% mass fraction, respectively [
The molar ratios of TEOS, TEP, NaNO3, and Ca(NO3)2
For composite preparation, the synthesized 45S5 Bioglass powders were reinforced with different weight fractions of MWCNTs. The porous inorganic scaffolds were prepared by controlled freezing of CNT/45S5 Bioglass slurries. Before, for purifying and stabilizing of MWCNTs, these powders were refluxed in a mixture of oxidizing acids which include sulfuric and nitric acid (with the volume ratio of 3 : 1 of sulfuric to nitric acid), for 4 h to oxidize and remove the metal catalysts and carbonaceous deposits from the inside and outside of the tube. Slurries were prepared by mixing distilled water with an organic binder such as PVA (equal to 1 wt.% of bioactive glass powder weight), a dispersant such as CMC (2 wt.% of bioactive glass), the MWCNT powder in different ratios (0, 0.1, 0.25, and 0.5 wt.% of bioactive glass, resp.), and the bioactive glass powder (20 vol.% of the slurry). The slurries were ball-milled for 24 h with alumina balls to break the agglomerated particles, and then deaired by stirring in a vacuum desiccator. To improve homogeneity and prevent the agglomeration, MWCNTs were ultrasounded for 2 h. Freezing of the slurries was done by pouring them into a PTFE mold, connected to a copper cold finger, which is placed in liquid nitrogen. With temperature decrease, the thermal energy is mainly transmitted in one direction due to the thermal conductivity coefficient of the slurry being higher than that of PTFE; therefore, the ice grows in one direction, resulting in the unidirectional microstructure of the ceramics [
A schematic of freeze casting technique for the fabrication of the MWCNT/45S5 Bioglass scaffolds.
The resulting 45S5 sol-gel derived Bioglass powders were analyzed by X-ray diffraction (XRD) with Philips PW 1800 diffractometer. This instrument works with voltage and current settings of 40 kV and 30 mA, respectively, and uses Cu-K
The samples were coated with a thin layer of gold (Au) by sputtering, and then, the microstructure of the scaffolds was observed on a scanning electron microscope (SERON Technologies Company, AIS2100) that operated at the acceleration voltage of 20 kV.
The compression test was carried out on a testing machine (Zwick/Roell) dynamic testing machine (DTM) model, Hct 400/25, Germany, with a crosshead speed of 1 mm/min according to ASTM F-2150 (load cell: 25 kN; resolution: 1 N). The samples were cylindrical in shape, with dimensions 20 mm in height and 12 mm in diameter. Each test has been repeated five times and the average amount and standard deviation (SD) of related parameters were determined.
The samples’ shrinkage was calculated at specific temperatures from the variation of the samples’ area, using (
An optical image of the fabricated 0.5 wt.% MWCNT/45S5 Bioglass composite scaffolds before and after heat treatment at 900°C, which shows the shrinkage of samples.
The density of the scaffolds (
All scaffolds exhibited porosity of ~63%, as determined by measurement of their mass and dimensions and applying (
All experiments were performed in fifth replicate. The results were given as means ± standard error (SE). Statistical analysis was performed by using One-way ANOVA and Tukey test with significance reported when
Figure
(a) XRD spectra of the synthesized MWCNTs, (b) SEM micrograph of the synthesized MWCNTs, and (c) XRD spectra of the sol-gel derived 45S5 Bioglass after sintering at 1000°C for 2 h.
It is always difficult to measure the accurate length of the MWCNTs from the SEM observation because of their twisting morphology, but the length can be estimated to be more than several tens of micrometers. As shown by the SEM image of the purified MWCNTs in Figure
Figure
In a recent experiment, Xu et al
Figure
(a) Compressive strength and (b) elastic modulus of MWCNTs/45S5 Bioglass composite scaffolds as a function of MWCNTs content.
In a recent study, Nezafati et al. [
Figure
Jia et al. [
Figure
(a) Low magnification and (b) high magnification SEM micrographs of the agglomerated MWCNTs in the scaffolds with 0.5 wt.% MWCNTs.
Cancellous or spongy bone has a compressive strength and Young’s modulus of 2–12 MPa and 20–500 MPa, respectively [
The lamellar microstructure of the scaffolds consists of plates with flat interconnected macropores between them, aligned along the ice growth direction (Figure
SEM micrograph of microstructure of MWCNTs/45S5 Bioglass composite scaffold with 63% porosity. (a) Cross sections parallel to the ice front, (b) with more details.
SEM micrograph of an open pore in MWCNTs/45S5 Bioglass composite scaffold.
Figure
(a) Low magnification and (b) high magnification SEM micrographs of homogeneous distribution of MWCNTs in the 45S5 Bioglass matrix.
As clearly shown in Figure
(a) Low magnification and (b) high magnification SEM micrographs of the bridges of CNTs between composite plates of the MWCNTs/45S5 Bioglass composite scaffold porosities (placing and crystallization of Bioglass particles on the CNT bridges).
Fine powders of Na2O-containing glass ceramics have been successfully synthesized using the sol-gel technique in aqueous solution under ambient conditions. The sol-gel, derived and sintered, 45S5 glass ceramic materials possess the essential features of Na2O-containing bioactive materials, namely, the formation of crystalline phase Na2Ca2Si3O9 during sintering, which couples well mechanical strength with appropriate biodegradability. MWCNT/45S5 Bioglass composite scaffolds have been successfully produced by means of the freeze-casting technique. The optimal content of MWCNTs in the composites was 0.25 wt.%. Compared to the scaffolds prepared by 100% 45S5 Bioglass powders, the addition of 0.25 wt.% MWCNTs increases the compressive strength and elastic modulus of 45S5 Bioglass scaffold from 2.08 to 4.56 MPa (a 119% increase) and 111.50 to 266.59 MPa (a 139% increase), respectively. The results have demonstrated that properties of prepared nanocomposite scaffolds were comparative to the natural spongy bone. Finally, it is important to point out that the mechanism of the bridge formation by CNTs between composite plates of scaffold porosities plays a critical role in the improvement of the mechanical properties of these composite scaffolds. The improved structural and physical properties of the MWCNT/45S5 Bioglass composites suggest their potential to be used as artificial scaffold matrix materials in bone tissue engineering.
The authors do not have any financial relation with the commercial identities mentioned in this paper that might lead to a conflict of interests for any of them.