镁合金腐蚀研究进展(21)—Mg-4Li-1Ca表面微弧氧化膜/壳聚糖自降解机理

    调控医用镁合金腐蚀速率和微环境pH值面临巨大挑战。这是因为镁过快的降解速率与骨生长速率不相匹配。另外，腐蚀降解导致微环境的高碱性也不利于细胞生长。前人的工作和我们的工作表明，镁合金表面聚乳酸膜（PLLA）和壳聚糖（CS）能一定程度地提高镁合金的耐蚀性能和生物相容性。微弧阳极氧化膜（MAO）具有较高的硬度、与基体冶金结合和较好的耐蚀性。其多孔性也存在不足，长期浸泡后期可能反而加快基体腐蚀。但其多孔性也为高分子涂层提供更好的机械结合位点，有利于高分子涂层的结合。

    我们的前期研究表明，Mg-Li-Ca表面经冷冻干燥得到的多孔MAO/PLLA膜在Hank's溶液中浸泡140h后，pH值范围为7.3-7.8.这说明PLLA的水解和酸化可中和调节镁腐蚀降解的pH值到适宜细胞生长的微环境。但PLLA的膨胀腐蚀和剥落也带来了临床应用的风险。相比PLLA，CS降解速率慢，另具有抗菌功能。因此，CS改性开始受到关注。我们课题组利用层层组装（layer-by-layer assembly）CS/聚谷氨酸(poly-L-glutamic acid)，实现了提高镁合金耐蚀性和抗菌性能的目的。郑玉峰教授课题组及其他研究者发现CS的分子量和层数对镁基体耐蚀有影响，证实CS可降低溶液pH值。但未见有镁合金表面CS膜极性和降解机理的报道。

    本项工作发现，相对于镁合金基体，CS的自腐蚀电位更负，为阴极性。为此，我们提出了Mg-4Li-1Ca表面微弧氧化膜/壳聚糖自降解模型，并阐明其机理。

    此项研究发表在Surface and Coating Technology (2018, 34: 1-11 )。题目为：Self-degradation of micro-arc oxidation/chitosan composite coating on Mg-4Li-1Ca alloy （可免费下载）。

  图1 （a）MAO（b）MAO/CS-1,（C）MAO/CS-2,（d）MAO/CS-3

图2 极化曲线

图3 电位-时间曲线

图4 电化学阻抗

Abstract

Regulating degradation rate and moderate pH micro-environment for biodegradable magnesium alloys face huge challenge. The chemical and morphological characteristics of micro-arc oxidation (MAO) and chitosan (CS) composite coatings, fabricated on Mg-4Li-1Ca alloy, are analyzed through field-emission scanning electronic microcopy, energy dispersive X-ray spectroscopy, X-ray diffraction and Fourier transform infrared spectroscopy. Corrosion resistance of the samples is evaluated via hydrogen evolution, potentiodynamic polarization and electrochemical impedance spectroscopy in Hank's solution. Results indicated that the MAO and CS coating enhances the corrosion resistance and antibacterial growth activity. With increasing immersion time, the degradation of the MAO/CS coatings gives rise to a decrease in pH value and leads to a rapid increase in hydrogen evolution rate after an immersion in Hank's solution after 100 h. The MAO/CS coatings retain the solution pH at a moderate level (≤8.25). A novel self-degradation mechanism of the MAO/CS coating on Mg-Li-Ca alloy is proposed due to the fact that MAO/CS coating is cathodic relative to the substrate.

链接：曾荣昌发表论文目录（Rong-Chang Zeng's Publications）(2000-)