Deposition of Hydroxyapatite on Ti-30Nb-1Fe-1Hf alloy by Anodic Oxidation and Hydrothermal Treatment

本研究藉陽極氧化與水熱處理對低楊氏係數之Ti-30Nb-1Fe-1Hf (62GPa)合金進行表面改質以增進其表面生物活性。首先，於β-甘油磷酸鈉及醋酸鈣電解液中，以定電流方式極化Ti-Nb合金，以形成一含鈣及磷之陽極氧化膜，簡稱為陽極膜。再將試片進行250oC水熱處理以析出氫氧基磷酸鈣(HA)結晶於陽極膜表面。本文將評估陽極膜表面特性以及探討陽極氧化機制。而後，探討水熱處理機制以及影響HA析出之因素，並比較Ti-Nb合金與商用純鈦(c. p. Ti)之陽極氧化行為以及氫氧基磷酸鈣形成能力。最後，以體外細胞測試評估表面處理對Ti-Nb合金生物相容性之影響。 陽極氧化過程中，陽極膜於介電崩潰前後會依不同之機制成長。介電崩潰前，陽極膜是藉由電解液中陰離子與金屬陽離子之交互擴散成長，此行為造成陽極膜成分分布不均，其外層由高價氧化物(TiO2及Nb2O5)組成，而內層由低價氧化物(Ti2O3，TiO，NbO2及NbO)組成，此氧化層為非晶結構，具均勻厚度且局部區域有氣泡存在。另外，電解液中磷酸根離子之併入有利於穩定陽極膜之非晶結構。 隨電壓上升至一臨界電壓，陽極膜產生介電崩潰，初期介電崩潰對陽極膜成分之影響不大，僅減少氫氧根吸附。此外，c. p. Ti在介電崩潰前之陽極氧化行為與Ti-Nb合金有很大差異，因為c. p. Ti陽極膜中有許多氣泡存在，故容易造成陽極膜破裂，而此現象相對較少發生於Ti-Nb合金中。另外，Nb的存在也提高Ti-Nb合金的介電崩潰電壓。介電崩潰後，由於高能量火花放電，造成多孔的陽極膜表面。隨電壓增加，陽極膜之厚度，含鈣磷量及鈣磷比明顯增加，但結晶性以及附著強度下降。 水熱處理過程中，陽極膜與氣氛接觸之表面優先結晶化，同時鈣及磷往外表面擴散而析出高結晶性HA。另外，HA晶體之數量及形貌隨陽極氧化電壓以及水熱處理溶液pH值變化。Ti-Nb合金經250oC持溫6小時水熱處理後，陽極膜表面幾乎完全覆蓋高結晶性之HA晶體，然而，c. p. Ti表面僅析出少量粗大之HA，因此，Nb造成之非晶性陽極膜有助於水熱處理過程HA之成核。 雖然陽極氧化之高電壓(300V)係為後續析出HA之必要條件，然而，高電壓會造成陽極膜附著強度下降之風險。故，本研究藉由添加HA粉末於電解液中，可於較低電壓下(230V)製備一高鈣磷比之陽極膜，其較佳之附著強度歸因於陽極膜存在於較少的缺陷。由體外測試結果顯示，表面處理對吸光值及磷酸酶活性影響不大，但對細胞貼附行為有顯著的影響，陽極處理+水熱處理所製備之HA表面有利於促進細胞早期之貼附特性。This study employed anodic oxidation and hydrothermal treatment in a low Young’s modulus (62GPa) Ti-30Nb-1Fe-1Hf alloy (Ti-Nb) to evaluate the bioactivity of alloy surface. The purposes of this study are threefold: firstly, the Ti-Nb was anodized in the calcium acetate monohydrate (CA) and β-glycerophosphate disodium pentahydrate (β-GP) to form a anodic oxide film (AOF) that contains Ca and P. The mechanism of the anodic oxidation of Ti-Nb was studied by evaluating the microstructures and compositions of the AOF. Secondly, after hydrothermal treatment, the phenomena of the precipitation of hydroxyapatite (HA) on the AOF were studied. In addition, the anodic behaviors and HA formation ability of AOF on Ti-Nb were compared with those of commercial pure Ti (c. p. Ti). Finally, the biocompatibility of Ti-Nb alloy treated with anodic oxidation and hydrothermal treatment was tsted. The AOF formed at two different stages, before and after dielectric breakdown. Before dielectric breakdown, a thin oxide film comprising a nearly amorphous structure with localized bubbles formed. The AOF is consisted of high-valent oxides (TiO2 and Nb2O5) in the outer layer and low-valent oxides (Ti2O3, TiO, NbO2 and NbO) in the inner layer. The non-uniform distribution of composition is due to inter-diffusion of anions and cations from electrolyte and metal. The species from the electrolyte (Ca and P) could only be incorporated in the outer part of the AOF and their contents decrease with the depth of the AOF. These electrolyte-derived species can suppress crystallization. Although sparking does not really influence the composition, it reduces the number of the hydroxyl groups on the surface. Compared to c. p. Ti, AOF on Ti-Nb has less rupture of AOF and higher dielectric breakdown voltage. It can be attributed to that adding Nb can inhibit bubbles forming inside the AOF. After dielectric breakdown, AOF grows by plasma electrolytic reaction, causing the film thickness to increase rapidly, which is accompanied with the formation of numerous craters in the AOF. Increasing anodizing potential increases the contents of Ca and P and also the Ca/P ratio, but reduces the adhesion strength between the AOF and the substrate. When anodizing to 300V, the AOF has a glassy amorphous structure. After 6 h of hydrothermal treatment at 250oC, a great number of crystalline HA precipitated on the surface of AOF anodized to 300V. The morphology and population density of HA crystals can be changed by controlling the anodizing potential and the solution pH in the hydrothermal treatment. Increasing the pH of the solution in hydrothermal treatment enhanced the precipitation of HA crystals. Numerous thin columnar HA crystals that almost covered the surface of AOF were obtained when hydrothermally treated in the pH13 solution. In contrast, the same hydrothermal treatment resulted in fewer and coarser columnar HA crystals on c. p. Ti. Accordingly, Nb in Ti-Nb alloy promotes the formation of the amorphous phases in AOF which, in turn, enhances the nucleation of HA crystals during hydrothermal treatment. Although high voltage (300V) can promote HA precipitation, it also deteriorates the adhesion property between the AOF and the substrate. A new electrolyte comprising CA, β-GP and HA powders has been developed, which allows the Ti-Nb to be anodized in this HA-containing electrolyte to at a relatively low voltage (230V). Experimental Results show that the AOF anodized in the HA-containing electrolyte exhibits an improved HA forming ability during hydrothermal treatment, it is attributing to the presence of HA powders in the electrolyte that enhances substantially the Ca content and Ca/P ratio in the AOF. On the other hand, the adhesive strength is little affected due to the decrease in the size of craters residing in the AOF. Regarding the biological responses, not much difference in biocompatibility of the treated and untreated Ti-Nb surfaces was obtained. However, the anodized and hydrothermally treated surface has been found to promote the of cell attachment