| Magnesium alloys have attracted great attention and become the hotspot in respectof biodegradable bone implant in load bearing applications due to theirbiodegradability and excellent mechanical properties that close to human bones.However, plenty research results indicated that the main problems which may hinderthe application of magnesium alloy in human body are its rapid corrosion rate inbody fluid and the bacterial infection to the surgical site. The rapid corrosion ratecould lead to a large number of bubbles and metal ions around the surgical sitewhich are adverse to the human body, and eventually cause the mechanical loss andfailure of the implants. Therefore, the corrosion resistance of magnesium alloy needsto be improved and so does the biocompatibility according to clinical demands forbone implants. Surface modification is an effective way to perfect the surfaceperformance while reserve the matrix properties. It is feasible to endow themagnesium alloy excellent corrosion and mold resistance through different surfacemodification technologies.Based on the biological requirements of bone tissues, the composite coatingscontaining corrosion resistance and antimicrobial films are fabricated by MAO-HAprocess (adding HA particles in electrolyte during micro-arc oxidation process) anddip-coating process in Ag-containing solutions because of the broad-spectrumantibacterial property of Ag. During the dip-coating process, we firstly investigatethe feasibility of poly-dopamine film as the drug carriers, then we fabricate theAg-PDA coating as the antimicrobial film.This study firstly demonstrated the effectiveness of applying a bio-compositecoating as the corrosion resistance coating on magnesium alloy. The morphologiesand structure were characterized by SEM, EDS, XPS and XRD, The resultssuggested that HA particles took part in MAO process, getting decomposed intoCa3(PO4)2under a high temperature caused by the intense micro-arc dischargegenerated at high applied voltage. In the meantime, synthesis of Mg3(PO4)2waspromoted under the high temperature. The sintering-caused phases Ca3(PO4)2and Mg3(PO4)2combined with HA composed the bioactive coating. Testified by scratchtest, the composite coating showed high bonding strength reaching to30MPa, whichmet the requirement for bonding strength of HA coating in the surgical implantapplication. Through electrochemical test and short-term immersion test, the Ecorrofthe sample was improved to-1.55V and Icorrdeclined to1.1×10-5A/cm2. The H2evolution was postponed and the corrosion rate calculated through H2volume wasdeclined to5.83mm·y-1. The compressive strength of MAO-HA group dropped to226MPa after immersed15days in SBF, but it was still higher than relatively highcompressive strength of human bones. So the MAO-HA coating protected Mg alloyfrom rapid corrosion and maintained mechanical strength higher at the initialimmersion period, which benefited from its nonporous structure and high bondingstrength to the Mg substrate. The increasing content of Ca and P elements inimmersed coatings suggested that the coating could induce the deposition of apatitethrough partial dissolution of its components, showing biodegradable feature whichwas an important requirement to induce the precipitation of bone-like apatite on theimplant surface.SEM and AFM morphologies showed the PDA film made by dip-coating indopamine solution filled up the micro pores and narrowed down its pore size to lessthan200nm. XPS results found functional groups in the PDA films, and thefunctional groups not only combined the MAO-HA coating with chemical bonds, butalso reacted with positive ions in the solution. The electrochemical testing provedthat the corrosion resistance was further improved through PDA film. Comparedwith the MAO-HA coating, the contact angle of PDA film was much lower,suggesting that the hydrophily was improved by dip-coating process. In themeantime, the cytotoxicity and cell proliferation of the MAO-HA coating and PDAfilm were measured by MTS, PDA film was nontoxic and had bettercytocompatibility that could promote the proliferation of cells. The results displayedthat PDA film was qualified to be the drug carrier of antibacterial agents.Based on the analysis of PDA coating, we tried to fabricate antibacterial coatingby dip-coating process in the AgNO3and dopamine solution. The SEM resultsindicated that Ag particles could cover the micropores and separate uniformly on the MAO-HA coating surface when the concentration of AgNO3was3g/L. XPSanalyses manifested that Ag+interacted with the functional groups of PDA, in whichway reinforced the bonding strength between them and availed to control the drugrelease, as well as guarantee the durable antibacterial effect.Therefore, this study indicated that the composite coating obtained by MAO-HAprocess and dip-coating process may be suitable for synthesizing promising boneimplant materials in clinical application. |
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