Electro-chemical deposition of nano hydroxyapatite-zinc coating on titanium metal substrate
© The Author(s). 2017
Received: 20 March 2017
Accepted: 28 July 2017
Published: 13 August 2017
Titanium is an inert metal that does not induce osteogenesis and has no antibacterial properties; it is proposed that hydroxyapatite coating can enhance its bioactivity, while zinc can contribute to antibacterial properties and improve osseointegration.
A nano-sized hydroxyapatite-zinc coating was deposited on commercially pure titanium using an electro-chemical process, in order to increase its surface roughness and enhance adhesion properties.
The hydroxyapatite-zinc coating was attained using an electro-chemical deposition in a solution composed of a naturally derived calcium carbonate, di-ammonium hydrogen phosphate, with a pure zinc metal as the anode and titanium as the cathode. The applied voltage was −2.5 for 2 h at a temperature of 85 °C. The resultant coating was characterized for its surface morphology and chemical composition using a scanning electron microscope (SEM), energy dispersive x-ray spectroscope (EDS), and Fourier transform infrared (FT-IR) spectrometer. The coated specimens were also evaluated for their surface roughness and adhesion quality.
Hydroxyapatite-zinc coating had shown rosette-shaped, homogenous structure with nano-size distribution, as confirmed by SEM analysis. FT-IR and EDS proved that coatings are composed of hydroxyapatite (HA) and zinc. The surface roughness assessment revealed that the coating procedure had significantly increased average roughness (Ra) than the control, while the adhesive tape test demonstrated a high-quality adhesive coat with no laceration on tape removal.
The developed in vitro electro-chemical method can be employed for the deposition of an even thickness of nano HA-Zn adhered coatings on titanium substrate and increases its surface roughness significantly.
Titanium metal is one of the most widely used biomedical orthopedic materials because of its decent mechanical properties . However, as an inert material, it cannot induce osteogenesis and has no antibacterial properties . In order to improve surface bioactivity of titanium substrates, numerous methods have been proposed to cover it with bio-ceramic coatings . Various clinical studies demonstrated that the hydroxyapatite coating of prosthesis can promote earlier osseous response which could increase the prosthesis fixation and the bonding strength [3–5].
Titanium implants are usually placed in contact with bones and gingival tissues so they are partially exposed to the oral cavity during and after implantation. This increases the hazard of bacterial infection, which is known as peri-implantitis [6, 7].
For centuries, Zinc (Zn) as one of the essential elements of tissues in the human body has a stimulating role in the metabolism of bones and has been used as bacteriostatic and bactericidal agents [8, 9]. Zinc can enhance the retention strength and osseointegration of implants [10, 11], by stimulating alkaline-phosphatase activity and collagen production, thus can increase bone deposition and reduce bone resorption . Zn deficiency results in skeletal changes, including retardation of skeletal growth  and prolonged bone recovery . Moreover, Zn species are also known to possess excellent antibacterial qualities. Zinc showed inhibitory effects against several bacteria, including Streptococcal mutans [14–16].
The metals’ antibacterial activity has been contingent on their contact surface; thus, a greater nanoparticles’ surface area permits larger interfaces and increases their interactions with other particles .
Although HA coatings revealed an enhanced bone attachment and thus better implants integration, long-term coating stability is quite a provoking concern . Numerous coating techniques like plasma spraying, sol-gel, electrophoretic deposition, electro deposition have been employed to deposit hydroxyapatite on titanium implants. Plasma spraying is the most widely used technique for coating, but it leads to decomposition of HA due to the high temperature used, and it cannot be employed for complex structures. In electrophoretic deposition, high voltage was applied to the metal surface in order to attract the dispersed particles which leads to anodic polarization of metal substrate. This might increase the corrosion risk of metal and suppress the adhesion of HA particles [19–21]. Electro-chemical deposition (ED) is a frequently used approach with increasing popularity, due to variability of coating composition, process simplicity, and its applicability for multidimensional implant surfaces .
The aim of the present work was to develop well-adhered and uniform hydroxyapatite-zinc coatings on titanium metal substrate, through an in vitro electro-chemical deposition method. The coating was characterized for functional chemical group, surface morphology, surface chemical analysis, surface roughness, and coat adhesive bonding by Fourier transform infrared spectrometer (FT-IR), scanning electron microscope (SEM), energy dispersive spectroscope (EDS), profilometer, and tape adhesive test respectively.
Commercially pure Ti (CpTi) grade II specimens were cut down into plates with dimensions 10 × 10 × 2 mm and used as substrates (cathode material) for depositing HA and Zn. CpTi specimens were polished with successive grades of silicon carbide papers, ultra-sonicated in acetone (99.5%, EM Science), rinsed in distilled water, and then air dried at room temperature, before they were used for the electro-chemical process.
Electro-chemical deposition of HA and Zn
Characterization of the deposited HA-Zn coating
The coating was scrapped from Ti specimen's surface and investigated for its chemical structure using FT-IR spectroscopy. The powder was investigated by double-beam dispersive IR spectrometer (Nicolet iS10, Thermo Electron Corporation, UK) which utilized the selected range of 400 to 4000 wave numbers (cm−1) at 4 cm−1 resolution and averaging of 100 scans. Two milligrams of scrapped powder was mixed with 300 mg of KBr and pressed into a disc before the measurement.
Surface characterization of coatings
Scanning electron microscope (SEM) (JSM 6300, JEOL, Japan) and energy dispersive spectroscope (EDS) were used to examine the morphological qualities and the elemental composition of the HA-Zn deposits. The working distance was 15 mm at 20 V. Three specimens were examined for each group of the study.
Surface-mechanical testing of coatings
Roughness of coatings
Specimens of control and HA-Zn coated groups were evaluated by a surface roughness profilometer tester (Surftest SJ-210, Mitutoyo Corporation, Tokyo, Japan,) according to ISO 4287-1997  with a diamond tip radius of 5 μm, a scanning speed 0.5 mm/s, a resolution of 0.01 μm, a Gaussian filter, and a cut-off length of 8 mm. Seven specimens from each group were scanned and evaluated for the average roughness parameter, each specimen was scanned five times, and the mean was calculated in µm. The roughness parameter (Ra) values were compared for statistical significance using the Student t test in SPSS software version 20 (SPSS Inc. Chicago, IL, USA).
Coating adhesion test
The adhesion of coating is qualitatively assessed by the tape test. A standard test method (Tape test-ASTM D 3359-97) was used for assessing the adhesion of the HA-Zn coating on the titanium substrate. In this method, a part of a pressure-sensitive adhesive tape (masking tape, M&G pen AJD97355) is pressed against the coating by the use of a pencil eraser for 90 s. The tape is then rapidly removed (without jerk movements) at 180° angle, and the degree of film removal is detected when the tape is pulled off. Because an integral coating with substantial adhesion is often not detached at all, the sternness of the test is typically improved by making a figure X cutting into the coat using a sharp scalpel with enough pressure to reach the metal substrate, then applying the tape and remove it. The denuded area is inspected for removal of coating from the substrate, and then the adhesion is ranked by relating the detached part of the coat versus a recognized rating scale. The test is repeated for three other locations in the same specimen. Coverage of coated substrate was computed using Matlab (version7.1) .
SEM and EDS results
The Student t test of the control and coated specimen roughness Ra (μm)
Number of specimens
Mean ± (SD)
Standard error mean
0.34 ± (0.06)
1.09 ± (0.16)
Adhesive test results
Following the examination of X cut areas after the adhesive tape removal; the adhesion was rated to be 5A, as no peeling or coat removal occurred along the incisions' length or at their intersection.
Metallic orthopedic prosthesis is most commonly used due to its good mechanical properties, but its failure mostly occurs due to the lack of proper bone bonding and/or the occurrence of post-operative infections. Hydroxyapatite is commonly used as a bone filler biomaterial or as a coat for titanium prosthesis due to its decent biocompatibility, osseoconductivity, and bioactivity . However, as a ceramic material, HA still has lower mechanical properties . The biological apatite differs from synthetic apatite because the former contains numerous cationic substitutions, such as Zn2+, Na+, Mg2+, and has smaller size than synthetic apatite [28, 29]. It was proposed that the addition of zinc to hydroxyapatite had led to a reduction in inflammatory reaction and an improvement of bioactivity [28, 30].
Plasma spraying, sol-gel, and electrophoretic deposition has been all utilized to deposit HA on titanium implants, with some difficulties and worries of suppressing the HA particles’ adhesion, anodic polarization of metal substrate, and increasing metals’ corrosion risk [19–21]. Electrochemical deposition (ED) is the selected approach in this study due to its simplicity, easiness of parameters control, uniform coating thickness produced, and its applicability for multidimensional implant surfaces .
In the current study, an electrochemical deposition was applied to prepare nano-HA-Zn coating on titanium metal aiming to improve bioactivity, osseointegration, and preventing peri-implantitis. At this early point of research, the coatings’ procedure was accustomed to produce a uniform thickness of HA-Zn coating, characterize its chemical structure, observe its surface morphology, and evaluate the surface roughness and coat adhesive properties.
Recycling of natural-derived resources is a challenging task that may have both environmental and economical profits. Cuttlebone fishery is a naturally derived biomaterial that was used as a source of calcium during the electrochemical deposition process in this study. It was confirmed in the IR spectra (Fig. 2) that Ca(NO3)2·4H2O resulted from the reaction of CaCO3 of cuttlebone and nitric acid . The selected time for electrochemical deposition of HA-Zn coating was 2 h; as by then, the formation of a white detectable coating had occurred and could be scrapped for IR spectral analysis. After preparation of HA-Zn coating, the analyzed powder appeared to still have the HA characterization. Li et al. prepared Zn-HA coatings through a hydrothermal method and found that the FT-IR spectra of Zn-HA has no significant changes than the as-prepared HA ; this Zn-HA spectrum paralleled with this study.
Yang et al. prepared a Zn-HA coating on Ti plates by an electrochemical process, and the SEM examination showed irregularly shaped rod-like crystals with hexagonal cross-section; this corresponded well with the current study results. They also concluded that a Zn-HA coating improves proliferation and differentiation of osteoblasts and would enhance implant osseointegration .
Ceramic coatings must have good adhesion to the implant to act as a barrier and assure good protection to the substrate. The adhesion test was performed in this study to verify the adequacy of the coating thickness.
An improvement of coating adhesion occurs as their thickness decrease, although very thin coatings may not attain the protection requirements . Contrariwise, it is recognized that thick ceramic coatings may develop cracks after the deposition procedure . The adhesive tape test read the highest score (5A); this might be attributed to the fine homogenous, closely packed, coating particles that appear crack free and highly sintered, as proved by the SEM results in Figs. 4, 5, and 6.
Dental implants do exist with various geometries, different lengths and diameters, and features, such as, pits, pores, vents, and slots. Essentially, a highly rough surface produces better initial stability and anchorage. Moreover, a rough surface with a larger surface area facilitates particles exchange between the implant and surrounding tissues. It could be concluded that such coatings with an increased surface area could have better clinical performance . This developed electro-deposition process, can be applied to deposit a nano-HA-Zr coating to complex implant surfaces and thus increases their surface area, surface roughness, initial stability and clinical performance.
Supplementary, biocompatibility, anti-bacterial activity, and in vivo investigations are required to correlate between the HA-Zn coating properties and their effect on bone formation and osseo-integration.
The electro-chemical method can be employed for HA-Zn coating deposition on titanium metal, where Ca source was a recycled cuttlebone fish to precipitate HA phases. Using a Zn anode on a low-sustained voltage was able to induce an even coat thickness of HA-Zn precipitation and increase the surface roughness significantly.
The authors would like to express their gratitude for Dr. Sherif Kishk, Professor of Communication and Electrical Engineering, Faculty of Engineering, Mansoura University, for his help in photographing and analyzing the coating for adhesion test.
All authors carried out the experimental study conception and design. FR helped in the experimental part of the study. NA did the data acquisition and interpretation. NE performed the statistical analysis, drafted the manuscript, and revised it critically for important intellectual content. FR had given final approval of the version to be published. All authors read and approved the final manuscript.
El-Wassefy N, Aref N, and Reicha F declare that they have no competing interests.
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