| As bone tissue materials, magnesium and its alloys exhibit excellent properties, such as biodegradability, biocompatibility and mechanical properties. They can avoid the secondary operation and reduce the "stress shielding" effect. However, magnesium and its alloys have some shortcomings such as fast degradation and poor bioactivity in human body environment. Therefore, starting from the structure of natural bone, AZ91D based CaP/chitosan bone impant material with an appropriate degradation rate and good bioactivity was prepared in this study. The effects of process parameters on bonding force between the coating and metal substrate were optimized. The bioactivity and the degradation performance of the implant material in m-SBF were investigated. To get ceramic coating with better bonding property, bioactivity and loading medicine ability, carbon nanotubes (CNTs) or graphene (GNS) was added respectively, and their behaviour was studied.Under2:3of volume ratio of acetic acid in aqueous solution and ethanol suspension solution, the optimized parameters were:45mL/L of acetic acid,5g/L of nHA and1.25g/L of CS in aqueous solution;5g/L of nHA in ethanol suspension solution,40V of deposition voltage. Among them, the concentration of acetic acid and voltage had significant effect on bonding force. Bonding property of CaP/chitosan coating obtained in the optimized condition was favorable. The chemical compositions of the prepared CaP/chitosan coating included CaHPO4·2H2O(DCPD), Ca10(PO4)6(OH)2(HA), Ca(OH)2and chitosan. Ca(OH)2transforced into DCPD and HA after CaP/chitosan coating was soaked in PBS for5d. The electrochemical results showed that the coating thickness affected the degradation rate of AZ91D based CaP/chitosan in m-SBF and the CaP/chitosan coating had a good protective effect on AZ91D when the coating thickness was0.40~0.40mm. CDC code of equivalent circuit of Nyquist for two capacitive loops and one inductance loop was confirmed to be Ru(Cd(Rct(CSRS(RLL)))) by analyzing mathematical description of Faraday impedance. CaP/chitosan coating and MAO film effectively protected AZ91D from corroding in m-SBF, including a main protective role of CaP/chitosan coating and a secondary role of MAO. The form mechanism of CaP/chitosan coating was put forward.With the increase of immersion time in m-SBF, HA and HCA changed from flake-like to spherical-shaped pattern, and the spherical structure was a stable form in m-SBF. The corrosion reaction of AZ91D based CaP/chitosan in m-SBF was orderly controlled by electrochemical and limited layer diffusion process, electrochemical and semi-infinite diffusion process, and semi-infinite diffusion process with immersion time. The effect of Mg2+concentration in the SBF on growing behaviour of HA and HCA was: with Mg2+concentration in SBF changing from1×CMg2+to4×CMg2+, a dissolving process of nHA was accompanied by growth of HCA, however, from4×CMg2+to8×CMg2+, a growing process of nHA changed to be dominant and a growing rate of HCA became slow. While nHA and HCA presented a dissolving trend from8×CMg2+to10×CMg2+and the excess Mg2+in m-SBF inhibited the total growth process of nHA and HCA.CNTs and GNS were respectively added into the CaP/chitosan coating by EPD, and the obtained CaP/chitosan/CNTs or GNS coating could protect the corrosion of metal matrix in m-SBF. CNTs could promote the crystal growth of nHA and HCA when the CaP/chitosan/CNTs coating was soaked in m-SBF. The bioactivity and bonding capacity of the obtained CaP/chitosan/CNTs coating were satisfied when the concentration of CNTs in electrophoresis solution was increased from0.1to0.25g/L. GNS was beneficial to the growth of phosphate when AZ91D based CaP/chitosan/GNS was soaked in m-SBF. HCA was gradually mineralized while nHA first dissolved and then deposited with the increase of immersion time in m-SBF. The mineralization process of phosphate in the coating was obvious and the bonding capacity of the CaP/chitosan/GNS coating was better when the concentration of GNS in electrophoresis solution was increased from0.005to0.03g/L. Gentamicin was respectively loaded into the CaP/chitosan/CNTs or GNS coating by immersion loading. Experimental results showed the gentamicin was totally released when CaP/chitosan coating was immersed into m-SBF for5days. However, for the CaP/chitosan coating added CNTs of GNS, the releasing rate obviously decreased, and releasing time extended. Moreover, the order for the loading amount of gentamicin was: CaP/chitosan/CNTs coating> CaP/chitosan/GNS coating> CaP/chitosan coating. A EPD loading method for CNTs or GNS being loaded into CaP/chitosan coating was presented. The effective releasing time for the obtained CaP/chitosan/CNTs or GNS coating with medicine reached to more than10days, and the releasing behaviour was almost not affected by release medium of PBS or m-SBF. |
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