Composite coatings on implants usually involve various chemical components, so manipulating the chemistry involved is the main concept behind the synthesis of the coatings for different materials. A chemical deposition is one fabrication technique, allowing specific reactions to take place on the surface of the material to be coated so that the coating sticks to the substrate. Chemical deposition can vary in different ways, and examples of chemical deposition include the surface chemical conversion, chemical vapour deposition, sol-gel deposition and dip coating. Examples of the complex version of chemical deposition are electrochemical deposition and biomimetic deposition, which will be described later [66
]. The sol-gel technique is a method whereby a chemical solution is used to produce a network of particles after the solvent from the solution has been evaporated, which is used very often in the fabrication of composite coatings because of its ability to produce multicomponent coatings of various size, shape and format [67
]. This technique is effective while being cost-effective and simple [69
]. Issues with the sol-gel method are that it is the most time consuming, and the adhesive strength between the composite coating and the material is not that strong [72
]. Chemical vapour deposition (CVD) is a widely used chemical deposition method whereby the substrate to be coated is exposed to the precursor of the material to be coated, which reacts on the substrate forming the required coating. It has many advantages as being a low cost and low maintenance procedure, which can produce a uniform coating with structural control at the nanometer level [74
]. CVD has been selected as it is low-cost, low maintenance and effective process for depositing uniform films, exhibiting good adhesion to the growing substrate; moreover, the easiness in controlling the growth rate allows a high reproducibility of the samples. However, CVD requires high vacuum and specific precursor material, which can be evaporated [78
]. Chemical conversion is a process whereby the required coating is produced on the substrate from a source in the solution. It a method widely used for the synthesis of graphene oxide [81
] and silver nanoparticles. As part of bone implants, AgNp/TiO2
composite coating on titanium has been used successfully, as mentioned in Section 2.1
. There are issues with toxicity, which have been overcome by ongoing research by the researchers mentioned in the latter section, but the anti-bacterial properties of the latter coating triumphed over the toxicity issue (Section 2.1
). There are several methods of synthesis, among which chemical reduction is one of the simplest and most commonly used [82
]. The commonly used reducing agents for this reaction are sodium borohydride, sodium dodecyl sulphate, citrate, ascorbate and elemental hydrogen [82
]. The reason behind its vast use is the fact that nanoparticles of different morphology and dimensions can be fabricated using this method [82
]. In previous work, silver nanoparticles have been successfully synthesised on the surface of TiO2
nanotubes. In this section, the chemical reduction of silver nanoparticles on the nanotubes has been discussed, while Section 3.3
has analysed the electrochemical method of synthesising the underlying nanotubes.
In the study by Gunputh et al. [39
], delta-gluconolactone was used as a reducing agent for silver ammonia with the aim of forming silver nanoparticles of diameter less than 100 nm which were formed in clusters of varied dimensions. In the latter study, the concentration of the δ-gluconolactone used was maintained at 0.002 M throughout. Initially, in method 1, the TiO2
-coated Ti-6Al-4V alloy was exposed to the mixture of the silver source, silver ammonia and gluconolactone for 10 min. The concentration of the silver ammonia (S) was varied from 0.005 M to 0.015 M, and the resulting clusters formed on the surface of the nanotubes (TNT) are illustrated in Figure 6
. TNT-S was used as a labelling aid, whereby TNT represented the nanotubes, and S represented silver ammonia used followed by the respective concentration in number. As such TNT-S 0.005 represented a silver nanoparticles-coated titanium dioxide nanotubes, whereby a concentration of 0.005 M silver ammonia was used. Panels A–C show a low magnification of the TNT-S coating, and D-F show a higher magnification of the respective figures TNT-S0.005, TNT-S0.01 and TNT-S0.015, whereby micro-clustering was observed with an increase in the size of the clusters with an increase in the concentration of silver source. The low magnification showed the coverage of the coating, while the higher magnification zoomed in to have a closer look at the morphology of the clusters.
Using the same chemical reduction method, in method 2, TNT was exposed to 0.015 M silver ammonia for 1-10 min followed by exposure to 0.002 M gluconolactone for 5 min. Panels A–C of Figure 7
show the low magnification image of the coated surfaces of S1G5, S5G5 and S10 G5, respectively, with the number being the duration for which the samples were left in the latter solution. Panels D–F show the same coatings at a higher magnification, whereby the size of the nano-clusters was seen reducing with increasing duration of exposure to silver ammonia. In both Figure 6
and Figure 7
, G represents the EDS analysis, which confirms the nanoparticles to be silver.
The clustering was assumed to happen because of the large size of the gluconolactone molecule reducing silver ammonia. The latter molecule had several –OH, and, as such, for each molecule of reducing agent, four silver components were reduced, and they would attach to each other. After each coating was synthesised, the coated material was ultrasonicated in deionised water for 10 min to remove the excessive silver attached to the surface. Then, they were exposed to simulated body fluid in triplicates (n = 3) with the aim of measuring the amount of silver released from the coating after 24 h. The micro-clustering was seen to release a significantly larger amount of silver as measured by ICP-MS as compared to the nano-clustering (Figure 8
). After analysing the distribution and release of silver from the coating, coating from method 2, TNT-S10G5 was found to be the best to be used as a coating on implants as it had a uniform distribution of nanoclusters of silver nanoparticles fully covering TNT while releasing the least amount of silver from the coating after 24 h exposure to SBF. In the human body, too much silver can be toxic, as mentioned in Section 6.2.3; as such, an ideal implant needs to release enough silver to be bactericidal while being biocompatible.
To summarise, in general, chemical deposition is a cheap and simple method of synthesising composite coating, whereby huge effort is not needed, and the resulting nano-structure and distribution of the coating can be manipulated by modifying the involved parameters, such as the concentration of chemicals, temperature or duration of exposure to the substrate.