الفهرس 
ABSTRACTOnly 14 pages are availabe for public view
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Abstract
Hydroxyapatite has decreased the hydrophobicity of the surface which is demonstrated by the decreasing contact angle with increasing the percentage of hydroxyapatite due to its hydrophilic nature. The near infrared laser increases the hydroxylation of the carbon nanotubes and oxidation of APTES alkyl groups regaining an increase in the amount of hydroxyl surface groups, lowering the contact angles, and improving the surface energy [35].
Silanization is a simple, less time consuming and less hazardous technique than electrophoretic deposition which offers both mechanical and chemical adhesion as well as even coat thickness. It is also a low temperature technique that does not alter the properties of the composite and does not form cracks like plasma spraying [36]. The results obtained in our study are higher than those of shear bond strength values obtained with electrophoretic deposition coating when the hydroxyapatite was used as a major phase of the nanocomposite [37]. With reference to Figure 8, the types of bonds formed between carbon nanotubes and APTES as well as those between CNT-HA nanocomposites and APTES are shown. Carbon nanotubes are attached to the APTES either by the amine or the oxygen of the silane group. The addition of hydroxyapatite, in case of the nanocomposites, has offered more bonding sites for the APTES as shown in the diagram. This has led to the increase in shear bond strength with the CNT - 1% HA group more than the CNT - 0.5% HA and the CNT - 0% HA coated groups. As mentioned previously, near infrared radiation induces carbon nanotubes hydroxylation and APTES oxidation leading to increased binding sites in the form of hydroxyl groups thus increasing the bond strengths of the near infrared radiated groups [35]. As the surface is subjected to near infrared laser, chelation between the silane and the calcium phosphate occurs similar to the effect mentioned by Ramachandruni et al. between silane containing adhesive and calcium phosphate of dentin which has also improved the bond strength [38]. The adhesive failure indicates that the coating is even much stronger than the values obtained
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which agrees with the high bond strengths of the CNT - 1% HA laser treated group as almost all of its failure modes were adhesive. The epoxy adhesive was not able to delaminate the coat leaving it almost intact on the titanium surface while the adhesive was left on the counterpart. Cohesive failure in the epoxy adhesive is mainly due to error of chance as it only happened in two of the thirty-six specimens. The mixed failure contained some detachments from the coat and the epoxy adhesive as well which also gives a good indication of bond strength as the detached coat represented a lesser area compared to the detached epoxy adhesive (Figure 3 and Table 3) [39, 40].
For pure hydroxyapatite coatings, the simulated body fluid solutions cause degradation and cracking to their surfaces, yet since the major phase used in our composites was carbon nanotubes and the hydroxyapatite particles were chemically bonded to the carbon nanotubes, the coats did not suffer from such degradation [41]. This also explains the homogenous non-cracked appearance of the coats after soaking in simulated body fluid (Figure 5, S1-3).
One of the reasons that could explain the difference in calcium phosphate deposition between the laser and non-laser treated groups is the hydroprocessing theory. The solubility of calcium phosphate ions in solutions decreases when the temperature is increased [42]. Light waves in the near infrared range have been transformed into heat energy all over the implant coat due to the presence of the carbon nanotubes which possess the inherent property of photothermal effect leading to an increase in the temperature of the coat. This rise in temperature, according to the theory, led to an increased deposition of the calcium and phosphate on the coat due to their lowered solubility in the simulated body fluid solution. Calcium immobilization occurs first where the calcium binds to the carbon nanotubes since functionalized carbon nanotubes have high affinity to calcium. This is indicated by the formation of calcium carbide after 7 days of impregnation in simulated body fluid. Afterwards, nucleation occurs using the calcium as a nidus or nucleating
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center for phosphate attraction and calcium phosphate compounds formation and crystallization [43]. According to the temperature of the surface, the shape of the calcium phosphate particles is determined. Calcium phosphate spheres form at temperatures below 40°C, plates and cubes form at temperatures about 60°C and needles form at 140°C.
Another reason is due to the presence of the of hydroxyl groups on the surface of carbon nanotubes forming a negative lamella above the surface of the coat attracting positively charged calcium from the solution followed by the phosphate. Also, the free amine groups of the APTES which are positively charged have also attracted phosphate groups in the solution followed by calcium. The groups that were coated with the nanocomposite had an additional advantage of having hydroxyapatite crystals along the length of the nanotubes creating nucleating sites for calcium phosphate crystal formation. They also had another advantage, according to Figure 8, as the APTES bonded by its silane part with the hydroxyapatite of the nanocomposite thus leaving free amine groups which added another site of calcium phosphate deposition around the nanocomposite [44]. When it comes to carbon nanotubes, the APTES often binds by its amine group creating C-N bonds and the silane bonds to the titanium leaving no free amine groups. Carbon nanotubes absorb light energy in the range of 700-900 nm and emit radiation in the range of 950-1400 nm which results in promotion of mass transport of ions from solution to the nucleating centers on the carbon nanotubes and hydroxyapatite surfaces. This produces larger calcium phosphate crystals over time (Figure 7E) as indicated by Scherrer’s formula in the laser treated groups compared to the non-laser treated ones and increases the degree of crystallinity of the whole coat (Figure 7D) [45, 46]. Octacalcium phosphate is considered a precursor for hydroxyapatite which explains the decrease in its amount over time (Figure 7B) where the octacalcium phosphate is being transformed to hydroxyapatite or attracting more calcium phosphate particles to be deposited as
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hydroxyapatite (Figure 7A). The calcium polyphosphate is an amorphous phase that was induced by the laser treatment of the discs and was absent from the non-laser treated group (Figure 7C). This phase in itself is an enhancer of mesenchymal stem cell osteogenic differentiation which adds to the advantages of laser treatment of the coated discs especially for an in-vivo use [47].
This study explored the efficacy of applying nanocomposite coats with carbon nanotubes as the major phase and nano hydroxyapatite as the minor phase onto titanium dental implants. The coats have shown increased wettability, bond strength and calcium phosphate deposition which all serve the purpose of having better osseointegration and bone formation. The laser treatment of the coat has also offered an additional advantage of the photo-thermal effect enhancing even further the hydrophilicity, bond strength of the coat to titanium substrate and calcium phosphate deposition rate. Such an effect coupled with this implant coat would also enhance cellular activity, cellular adhesion and decrease the time needed for bone formation and osseointegration if this coat is to be applied in an in-vivo setting.