| 摘要 | Ti-24Nb-4Zr-8Sn（wt.%，简称Ti2448）是一种高强度、低模量、耐腐蚀、无生物毒性的新型β钛合金，在医学领域具有良好的应用前景。但Ti2448为生物惰性材料，需要对其进行表面改性以提高其生物活性及生物相容性。纳米管阵列是一种中空的管结构，具有很大的比表面积，而且其管径和长度可以通过实验参数来控制。若将纳米管阵列结构作为Ti2448合金的表面涂层，并在管中填充药物或其他负载物，将有利于提高其表面生物相容性。
本文通过阳极氧化法在Ti2448合金表面制备一层具有不同尺寸的纳米管氧化膜，研究氧化电压、氧化时间、后续热处理、合金元素、合金组织等各种因素对纳米管的影响，并对该氧化膜的生物活性及组织相容性进行评价。
通过控制阳极氧化工艺参数，在Ti2448表面制备出平均外径为30 nm - 315 nm，厚度为 约 14 μm的纳米管阵列氧化膜。结果表明，在一定的电解液中，10 V - 80 V的电压范围内均能制备出纳米管；在相同的氧化时间下，纳米管的直径和厚度随着氧化电压的增加而增加；在相同的氧化电压下，随着氧化时间的增加，纳米管的厚度增加，但直径基本保持不变。纳米管氧化膜表层为非晶态氧化物，组成为TiO2、Nb2O5、SnO2 和少量ZrO2；随着纳米管厚度的增加出现低价态氧化物Ti2O3、TiO、NbO2 、NbO。在不同温度对纳米管氧化膜进行热处理，450 oC时出现锐钛矿TiO2，750 oC时有部分金红石和Nb2O5析出；热处理消除了纳米管中的F元素，同时还能保持纳米管结构的完整性。研究了Ti2448表面纳米管的生长过程，结果表明，随着氧化时间的增加，初始形成的上层纳米孔结构逐渐溶解，下层自有序的纳米管稳定的向基体推进，长度的增加变得缓慢。另外，Ti2448合金中多种合金元素的协同作用导致纳米管的生长与纯Ti不同，其中Nb氧化物的抗腐蚀性阻止了Ti的过度腐蚀，从而增加Ti2448形成纳米管的电压范围；Zr氧化物的易溶性促进纳米管的初始形成和稳定阶段的持续生长，有利于得到较厚的纳米管氧化膜。由于不同相之间晶体结构和化学成分含量不同，因此两相组织的Ti2448表面不能生成规则均匀的纳米管阵列。
对经过不同处理前后的纳米管氧化膜进行模拟人体生理溶液浸泡，考察了Ti2448纳米管表面的生物活性。结果表明，非晶型纳米管、锐钛矿纳米管、经预钙化处理后的锐钛矿纳米管，均能加速诱导羟基磷灰石的生成；锐钛矿纳米管比非晶态纳米管更利于Ca-P化合物膜层的生长；经过预钙化处理后的锐钛矿纳米管活性最高，在SBF中浸泡2周后就能形成一层均匀、结晶度高的羟基磷灰石。
对具有纳米管氧化膜的Ti2448进行成骨细胞培养，评价了Ti2448纳米管表面的组织相容性。结果表明，纳米管结构不仅提高Ti2448的表面能，还为细胞伪足的延伸提供大量表面位点，利于引导成骨细胞的粘附；纳米管氧化膜改性后的Ti2448合金表面细胞增殖能力显著提高，这主要与纳米尺寸的表面形貌、粗糙度、亲疏水性、表面能、化学成分等因素有关；Ti2448纳米管氧化膜表面的ALP活性显著高于未处理表面。; Ti-24Nb-4Zr-8Sn (wt.%，abbreviated as Ti2448), a new kind of β type titanium alloy, possesses desired properties for biomedical application, e.g., high strength, low elastic modulus, good corrosion resistance and nontoxicity. Since Ti2448 is bio-inert, surface modification for good bioactivity and biocompatibility should be performed. Nanotube array, the diameter and length of which can be easily controlled by electrochemical parameters, is a kind of nanotubular architecture with high specific surface area. If the nanotube array can be fabricated on Ti2448 alloy surface, to be loaded with drugs or other useful nanoparticales in tubular structure, it will be great valuable for this alloy in biomedical application.
In this research, oxide nanotube layers with different dimensions were fabricated on Ti2448 alloy, and the influences of applied potential, anodizing time, post heat treatment, alloy elements, alloy microstructure on nanotubes were investigated. In addition, the biological properties of the formed nanotube layer were evaluated.
Highly ordered and vertically orientated nanotubes of 30 nm ~ 315 nm in average outer diameter, and up to 14 μm in length were fabricated on Ti2448 surface by electrochemical anodic oxidation in potential range from 10 V to 80 V. The diameter and length both increase with increasing potential; and anodizing time has significant influence on length while no obvious change on diameter. The outermost nanotubes consist of an amorphous mixed oxides of TiO2、Nb2O5、SnO2 and ZrO2, while Ti2O3、TiO、NbO2 and NbO are gradually developed with increasing length of nanotubes. After heat treatment at different temperatures, anatase-type TiO2 emerges at 450 oC, and additional rutile-type TiO2 and Nb2O5 crystallize at 750 oC with major anatase remains. During the process of heat treatment, F contained in nanotube can be eliminated, simultaneously integrity of tube structure can be kept. During the process of nanotube growth, the upper nanoporous layer formed in the beginning gradually dissolves with increasing time, while lower nanotube structure grows inward the substrate with decreased rate. Since three different alloy elements are contained in Ti2448, the growth of nanotubes is different from that on pure Ti, e.g., Nb-oxide with high corrosion resistance improves flexibility in the potential range; and Zr-oxide with high solubility is beneficial for the formation and steady growth of nanotubes, finally increased length can be obtained. Because of different crystalline structure and content of chemical composition in different phases, nanotube layer can not be formed on Ti2448 with α+β microstructure.
The bioactivity was evaluated by immersing Ti2448 with nanotube layers in SBF. The results show that amorphous, anatase, pre-calcified anatase nanotubes can all accelerate the formation of hydroxyapatite. However, anatase is better than amorphous nanotubes in inducing hydroxyapatite; and pre-calcified anatase nanotubes show highest bioactivity since uniform and crystallized hydroxyapatite can be formed after 2 weeks.
The histocompatibility of nanotube layers was evaluated using osteoblast cell culture. The results show that nanotubes can not only improve the surface energy but also provide lots of sites for filopodia protruding into the nanotube architecture, thus enhance cell adhesion. Additionally, cell proliferation on Ti2448 with nanotube layer can be improved as a result of synergy influence of different factors, e.g., nanoscale morphology, roughness, hydrophilicity, surface energy and chemical composition. Among different surfaces, nanotube layer shows highest ALP activity, indicating good cell differentiation. |
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