Full Length ArticleAntibacterial effect of zinc oxide/hydroxyapatite coatings prepared by chemical solution deposition
Introduction
Combining the prevention of infectious diseases with osteogenesis induction is a key strategy to improve the outcome of surgical procedures based on titanium (Ti) implants. Incorporating antibacterial agents in hydroxyapatite (HAp, Ca(PO4)6(OH)2) coatings is one of the most effective approaches to accomplish this task. Silver (Ag) is often considered as an adequate antibacterial agent for these applications, owing to its strong antibacterial efficacy and broad-spectrum antibiotic activity [[1], [2], [3], [4], [5], [6], [7], [8]]. However, Ag is not an essential element in biological systems, and its release from an implant surface does not provide specific benefits to the human body; furthermore, released amounts of Ag ions exceeding a certain limit are known to be cytotoxic [[9], [10]]. Accordingly, in terms of the long-term safety of patients, essential elements possessing antibacterial properties are preferable as additive agents.
Zinc is an essential mineral for biological processes such as DNA synthesis, enzyme activity, and cellular metabolism [11]. Furthermore, the incorporation of Zn in an implant material promotes the proliferation and differentiation of osteoblast cells, leading to enhanced osteogenesis [[12], [13]]. Based on these properties, the addition of Zn to HAp coatings is expected to be beneficial for enhancing bone formation on a medical implant. In addition to the functions described above, Zn ions also exhibit antibacterial efficacy, although their activity is rather low compared with Ag ions. Several studies have thus investigated the antibacterial efficacy and bioactivity of HAp materials containing Zn [[14], [15], [16], [17], [18], [19], [20]]. For instance, Stanić et al. synthesized Zn-doped HAp nanocrystals and confirmed their antibacterial efficacy against three bacterial strains in a buffer solution [15]. Thian et al. also prepared Zn-substituted HAp powders containing 1.6 wt.% Zn, and confirmed their antibacterial efficacy [17]. Although these previous studies confirmed the excellent antibacterial efficacy of Zn-containing HAp powder, a reduced efficacy was observed when Zn-containing HAp was used as a coating material. For instance, although Samani et al. reported the reduction of methicillin-resistant Staphylococcus aureus (MRSA) on HAp coatings containing 2.5 wt.% Zn, they also observed MRSA reduction on Zn-free HAp coatings [20], which suggested that the antibacterial effect of Zn incorporation was negligible. The reduced antibacterial efficacy observed for the coatings was probably due to the lower amounts of Zn released from the surface, even though a higher release of Zn is also associated with an increased risk of cytotoxicity. New approaches are thus required to prepare Zn-containing HAp coatings to combine antibacterial efficacy and biocompatibility.
In the present study, we investigate a newly designed antibacterial HAp coating exploiting the so-called “contact killing” effect of ZnO, which is derived from the reactive oxygen species generated by the reaction of ZnO with water without light illumination [[21], [22]]. HAp coatings incorporated with ZnO precipitates in the topmost surface layer were prepared using chemical solution deposition and subsequent heating. The antibacterial efficacy against gram-positive and gram-negative bacterial strains and the Zn release behavior of the ZnO/HAp coatings were subsequently investigated to assess their potential as antibacterial biomaterials.
Section snippets
Preparation and characterization of coatings
The precursor solution for the HAp coatings was prepared by mixing a 3.34 M ethanolic solution of Ca(NO3)2 with an equivalent amount of 2 M aqueous solution of H3PO4. Subsequently, a 0.02, 0.2, or 1.2 M ZnO aqueous solution was added to an equivalent amount of HAp solution in order to prepare the Zn-containing HAp solution. The Zn to Ca atomic ratio ([Zn/Ca]) changed from 0.012 to 0.12 and thereafter to 1.2 with the increase in Zn concentration, whereas the Ca to P ratio ([Ca]/[P]) was set to
Properties of the ZnO/HAp coating
Table 1 lists the labels used to denote the prepared ZnO/HAp coatings, together with the corresponding [Zn]/[Ca] and [Ca]/[P] ratios determined using XRF. The [Zn]/[Ca] ratios of the coatings almost exactly match the corresponding nominal ratios used in the coating solution, whereas the [Ca]/[P] ratios are close to the stoichiometric ratio of HAp ([Ca]/[P] = 1.67), even though the ratio of the ZnHAp3 coating was slightly higher than that of the others. We attribute this difference to the error
Conclusions
HAp coatings containing precipitated ZnO were prepared on a Ti substrate via chemical solution deposition. The coatings, prepared by spin coating a ZnO/HAp solution and subsequent heating at 650 °C, consisted of crystallized HAp with ZnO precipitates loaded on the top surface region. The amount of ZnO precipitates could be controlled by changing the ZnO concentration in the deposition solution, which also affected the rate of Zn release from the surface. The ZnO/HAp coating showed excellent
Funding
A part of this work was supported by the Grants-in-aid for Scientific Research (C) from the Ministry of Education, Science, Sports, and Culture (MEXT) of Japan (grant number 15K06452).
Acknowledgment
The authors gratefully acknowledge Mr. Yamane of the Kitami Institute of Technology for his assistance with the XPS analysis.
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