Functionally graded hydroxyapatite coatings doped with antibacterial components
Introduction
Infection, which results in morbidity and implantation failure, is a serious problem associated with prosthetic implantation, and usually leads to subsequent implant removal and debridement [1], [2], [3]. The infection mechanism has been postulated as bacterial colonization of the prosthesis at the time of implantation by direct inoculation during manipulation of tissue and implantation, or as a result of contaminated sterile implants [3]. In 1033 cases of total hip and total knee prosthetic arthroplasty infections, the major ones have been found to be caused by aerobic cocci, most commonly Staphylococcus aureus (23%), and coagulase-negative staphylococci (25%) [3]. The overall rate of prosthetic joint infection is highest in the first 6 months postoperatively, and declines continuously thereafter [3]. It has also been observed that the combined incidence of rates of total hip and knee arthroplasty infections during the first and second postoperative years, and after 2 years, were approximately 6.5, 3.2 and 1.4 per 1000 joint-years, respectively [3].
Hydroxyapatite (HA)-coated implants incorporated with antimicrobial agents are able to prevent or cure infections by releasing directly the antimicrobial agents to local regions. HA incorporated with Ag/Ag+ [2], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], Cu2+/Zn2+ [18], [21], ampicillin [23] and doxycycline [12] have been studied to evaluate their antibacterial effect. Among those, Ag incorporation has gained more attention due to its broad-spectrum antibacterial properties [4], [8]. The most common technique to incorporate Ag into HA coatings is via an ion exchange method, in which the Ca ions in HA are replaced by Ag ions while dipping the HA coatings into AgNO3 for a period of time [2], [4], [24]. The limitation of the ion exchange method is that Ag will reside mostly on the outer surface of the coating and will be quickly depleted in vivo/in vitro without long-term antibacterial effect. In order to achieve the continuous release of Ag, HA coatings doped with Ag through the entire thickness has been developed using sol–gel [10], [18], co-sputtering [5], [9], and thermal or cold spraying [25], [26]. Although Ag in small percentages can have an antibacterial effect, larger amounts can be toxic [9], and therefore optimization of the Ag concentration in the coating is critical to guarantee an optimum antibacterial effect without cytotoxicity. It has been reported that the presence of 2 wt.% Ag in HA coating processed by co-sputtering can have significant antibacterial effect to Staphylococcus aureus and Staphylococcus epidermidis without osteoblast-precursor cell cytotoxicity [9].
In this study, we investigated processing and characterization of functionally graded hydroxyapatite (FGHA) coatings incorporated with Ag as an antibacterial component. The processing technique was ion beam-assisted deposition (IBAD) with in situ heat treatment. While the amorphous top layer of the coating allows a higher release rate of silver due to its higher dissolution immediately after implantation, the crystalline layer will maintain the silver as a reservoir, resulting in long term infection protection. It is anticipated that this technique will be more efficient in infection control than conventional oral/intravenous antibiotic intake with fewer side effects. In this study, the processing of FGHA coatings with three different silver contents is reported along with their mechanical and microstructural properties. The biological responses of these coatings will be reported in a future article.
Section snippets
Material processing
Titanium disc substrates of 7.62 mm thickness were machined from a 12.7 mm diameter, 99.5% commercially pure titanium rod (Alfa Asear). Prior to deposition, the Ti substrate surfaces were prepared by wet grinding with 240-, 400- and 600-grit silicon carbide paper (Buehler) and subsequent polishing with 9, 3 and 1 μm polycrystalline diamond suspension (Buehler). In between each of the grinding and polishing steps the discs were washed and ultrasonically cleaned to prevent cross contamination of
Results
Fig. 1 shows TEM images of the cross-sections of samples FGHA-Ag1, FGHA-Ag2 and FGHA-Ag3 and Fig. 2 shows the high-resolution TEM image of FGHA-Ag2 and its fast Fourier transform. All the coatings consist of three distinct FGHA layers, as labeled in images (a-1), (b-1) and (c-1) shown in Fig. 1: an equiaxed polycrystalline interface layer adjacent to the Ti, a crystalline bottom layer and a highly-amorphous top layer. Higher magnification images of the coating interface and bottom layers are
Discussion
The coatings’ microstructures, as well as their mechanical properties, resulted from manipulating deposition parameters and substrate temperature during deposition. The well-bonded interfaces shown in Fig. 1 as well as the compositional transition interface shown in Fig. 8 illustrate that higher values for secondary beam parameters of 200 V/50 mA at the beginning of deposition facilitated the formation of a dense and compositionally graded structure. The high bond strength (greater than 83 MPa)
Conclusions
A series of functionally graded hydroxyapatite coatings incorporated with different concentrations of Ag were deposited using IBAD. TEM/STEM observations on FIB-prepared coating cross-sections show three distinct layers within the coatings: a “mixed” crystalline interface layer adjacent to the Ti substrate, a crystalline bottom layer and a mostly amorphous layer on the top surface. TEM/STEM-EDS confirmed the presence of 10–50 nm silver particles distributed throughout the coating thickness, with
Acknowledgements
The authors would like to acknowledge the financial support from the National Science Foundation (Grant No. 0600596) and access to the research facilities at the Center for Nanophase Materials Sciences (CNMS) and the Shared Research Equipment (SHaRE) User Facilities at Oak Ridge National Laboratory, both of which are supported by the Scientific User Facilities Division, Office of Basic Energy Sciences, the U.S. Department of Energy (Award No. CNMS2005-070). The authors would also like to
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