Design of Hydroxyapatite/Magnetite (Hap/Fe 3 O 4 ) Based Composites Reinforced with ZnO and MgO for Biomedical Applications

Hydroxyapatite (HAP-- Ca 10 (PO 4 ) 6 (OH) 2 ) is a biocompatible and bioactive material that is widely used for biomedical applications, especially in bone replacements. It has good load carrying capacity; however, it lacks antibacterial property. New bio-composites based on bovine hydroxyapatite doped with, magnetite iron oxide (HAP/ Fe 3 O 4 ) matrix reinforced with ZnO and MgO nanoparticles are proposed for biomedical applications that provide improved antibacterial activity with potential to be used in magnetic therapy. Microwave sintering was used to manufacture the composites. The microstructure evolution in these composites were studied by Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS). Density, microhardness, compressive strength of the composites was measured and compared along with their magnetic properties. Finite element analysis simulations were performed for the compression tests.

Introduction that they become superparamagnetic at diameters of < 20 nm [31]. Although the role of iron in bone accrual has received little attention, a few studies have previously shown that iron restriction can have an inhibitory effect on the mineralization of osteoblasts in vitro and experimental evidence also suggests that there may be some positive association between iron metabolism and the in vitro proliferation of bone or non-bone cell lines [32][33][34][35][36][37]. Additionally, the implant associated infection is widely considered as a major concern in the field of biomedical applications and this has been the driving force for developing HAP-based biomaterials with antibacterial additives for possible use in prosthetic devices.
In our present work, we sintered Hydroxyapatite (HAP) with different concentrations of zinc oxide micro rods (ZnO) at 1250°C to produce HAP-ZnO bio composites. In vitro antimicrobial studies were carried out to understand how ZnO addition (up to 30 wt %) to HAP leads to the improvement in bacteria static/bactericidal property and thereby reduce bacterial infection on implant surface.
Other researches have also shown that the additions of reinforcing elements like ZnO and MgO in HAP/magnetic iron oxide composites could reduce the bacterial infections on the surface of the composites and increase their hardness which is a positive feature for different medical implants [2,10,11].
For this reason, the current work aims to present the design of new bio-composites based on bovine hydroxyapatite (BHA)/ nano-magnetite iron oxide (Fe 3 O 4 ) reinforced with ZnO and MgO nanoparticles. Here, in the frame of the "Bio ceramic" research project, a net shape microwave sintering procedure was used by using a few percent of paraffin to create a natural micro porous structure as an alternative to the other materials to create a porous structure. A special attention was given for the microstructural evolution with recently developed compositions to give practical significance for the application as biomaterials. Mechanical and other physical-chemical characteristics were studied in detail. The structure evolutions of these composites were observed in details by means of Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS).

HAP-Materials Processing and Microwave -Sintering
Natural HAP was obtained from calcinated fresh -young bovine bones (femurs) by following the method developed at SUPMECA/ LISMMA-Paris [2,10]. The femurs were undergone deproteinization with NaOH treatment. After repeated washing, they were heat treated at 850°C. The treated HAP powder (particle sizes of 1-2μm) was the mixed with other constituents as described in Sec 2.2.
In this work, microwave sintering process has been carried out for the manufacturing of the bio-composite materials details of which were given in earlier papers [2,10,11]. The application of microwave energy to the processing of various materials such as ceramics, metals and composites offers several advantages over conventional heating methods. Microwave heating results in lower energy costs and decreased processing times for many industrial processes. These advantages Include unique microstructure and properties, improved product yield, energy savings, reduction in manufacturing cost and synthesis of new materials. In order to compare the microstructural evolution of microwave sintered composites to the ones manufactured by conventional sintering, some specimens were sintered in an electrical -conventional furnace (High Temperature Furnace. In this project, the primary aim, however, is to use a house type microwave oven under laboratory conditions. For this reason, a house type (2.45 GHz) microwave oven was modified to be used in the manufacturing process. A special thermocouple, insulated for the microwaves, was installed to monitor the temperature during production. The accuracy of temperature measurement with this device was determined to be within -10°C of the temperature measured. The thermocouples were placed inside the alumina ceramic crucible, 2-3 mm away from the specimens. Compared to the conventional sintering, there was a slight increase in density of the specimens manufactured with microwave sintering. Since the microwave sintering took much shorter time, densification rate in the microwave sintering process may be higher than in the conventional sintering. Effective sintering time for these samples was chosen as 30 minutes.

Specimen Design and other Experimental Design
The compact geometry was prepared based on the matrix natural HAP + 20wt% magnetic iron oxide (Fe 3 O 4 ) reinforced with different percentages of nano MgO and ZnO (received from VWR-France). At the beginning of the process, a pre-treatment of doping of iron oxide was made with the reinforcements for surface activation and for increasing of homogeneous distribution of the reinforcements in the matrix. For this treatment, a very simple process has been carried out: pre-mixing and pre-heating of the magnetic iron oxide (Fe 3 O 4 ) with the reinforcement at 100-150°C followed by surface activation with hydrogen peroxide during the mixture at this temperature.
Then, the blended powders were homogenized by ball milling for two hours, then compacted by uniaxial cold isostatic pressing at a pressure of 300 MPa, intending to produce an initial green density ranging 85-90%. Cylindrical test specimens were prepared

Microstructural Evaluation
General compositions of the HAP based Biocomposites are given in the Table 1. In the frame of the common research project, only three composites were presented here. Basically, HAP was doped with pure magnetic Fe 3 O 4 after those secondary reinforcements were added as explained in the former section. HAP has a hexagonal structure with lattice parameters a = 0.942 nm and c = 0.687 nm.
The ideal formula of HAP is given as Ca 10 (PO4) 6 (OH) 2 . The atomic structure of HAP and its projection along the "c" axis are shown in Figure 1. Also, Figure 1c shows the average grain size measured of this mixture which varies from 1 to 5µm. This structure verifies that the microwave sintering is a viable manufacturing method with lower energy costs and shorter processing times for these materials.

Evaluation of Magnetic Properties for HAP-1, HAP-2 and HAP-3
Magnetic measurements have been carried out by the physicalchemistry research laboratory in Paris. Two test specimens were used for each composite and evaluated for finding the magnetic saturation values ( Figure 5) and compared certain parameters and summarized in the Table 2    In the same way, the representative cells are also meshed with quadratic tetrahedral. FE simulations are achieved using ABAQUS (2008-Supmeca-Paris) and the whole volume is meshed using 4-node C3D4 tetrahedral in ABAQUS), allowing us to improved detention the strain gradients in the matrix. After that, the prediction was efficiently compared to those obtained with finer meshes. It means that comparison of active response is made to the regular response of the reinforcement. Figure 6 indicates typical meshes formation generated for a composite with various percentages of the reinforcements and variable geometry of specimens that were used in this work.
Evidently, the macroscopic stress predicted by the FE analysis is