改善鎂－5 wt.%錫合金機械與抗腐蝕性質及其應用於植體材料之研究

Abstract

本研究旨在改善鎂－5 wt.% 錫合金(以下縮寫為Mg-5Sn)的機械性質與抗腐蝕性質，使其成為更適合應用在植體的生醫材料。為得到具有更加機械性質的鎂合金，本研究透過等通道擠型固化純鎂粉末，得到具有奈米晶粒的鎂材。使用純鎂粉末是為了更清楚了解不同的等通道擠型參數對粉末固化的影響。被銅殼包覆的鎂粉末沿著BC路徑進行等通道擠型固化，並於不同溫度（200、250及300°C）下分別經過不同道次（1、2、4道次）擠型。透過掃描式電子顯微鏡分析，可以了解不同擠型過程對固化後顯微結構的影響。固化後的密度則利用阿基米德法得知。不同擠型過程對固化後機械性質的影響則以微硬度以及抗壓強度來比較。經研究後發現，鎂粉於300 °C、4道次的擠型後，可以得到最好的固化效果，且可以維持奈米級的晶粒。其固化後的密度可以達到98.4%的鎂理想密度，49Hv的硬度，以及100MPa的抗壓降伏強度。然而擠型過程中無可避免的仍會於粉末間生成氧化物，因此固化後鎂材的機械性質無法有效的提升，鑄造鎂材仍為鎂合金應用於植體材料較佳的選擇，後續實驗將以本研究團隊自行鑄造之Mg-5Sn為研究對象。 將Mg-5Sn應用於植體材料，必須大幅的提升Mg-5Sn的抗腐蝕性質。本研究使用磷酸進行表面前處理，披覆具生物相容性的聚醯亞胺薄膜，來提升Mg-5Sn的抗腐蝕性質。磷酸鹽表面前處理可粗化鎂合金表面，使鎂合金表面可與聚醯亞胺披覆層產生機械連鎖，藉此增加披覆層的附著力。並且藉由X光繞射儀以及電子能譜儀發現，磷酸鹽表面前處理在Mg-5Sn表面所沉積的磷酸鹽，與披覆層形成化學鍵結。此化學鍵結可產生化學錨定效應，更進一步的增加Mg-5Sn與聚醯亞胺披覆層的附著力；此外，亦大幅的提升了聚醯亞胺披覆層於Mg-5Sn上的抗腐蝕能力（Ecorr = −0.86 V/SCE, Icorr = 4.9×〖10〗^(-2) μA cm-2 並且腐蝕速率小於 0.1 ml cm-2 day-1），證明此化學鍵結相當穩定，可以抵擋來自腐蝕液中離子的侵蝕，並且避免其與Mg-5Sn表面發生電化學反應。這個在抗腐蝕性質上大幅度的提升，使得Mg-5Sn更適合作為植體材料。 為了更進一步的提升Mg-5Sn作為植體材料的應用性，本研究建立了一個建基於骨頭重組與生長的數值化植體設計模型，以有效率的設計Mg-5Sn植體形狀。為了驗證此數值化設計模型的正確性，此模型先套用於設計商用Ti6Al4V植體的骨頭復原腔室。爾後，此設計模型再套用於設計相同的植體，但植體材料為Mg-5Sn，並將其設計結果與Ti6Al4V植體相比，以凸顯使用Mg-5Sn為植體材料的優異性。結果顯示，以Mg-5Sn作為植體材料，植體的骨頭復原腔室比Ti6Al4V植體來得大，因此可以有更多的骨長入。於Mg-5Sn植體周圍健康的骨頭，亦比Ti6Al4V植體周圍健康的骨頭來得多。亦即使用Mg-5Sn植體，可大幅的減少應力遮蔽效應。Mg-5Sn植體的表面積，也比Ti6Al4V植體來得大，將可達到更好的骨整合效果。其受力後的位移，亦小於達成骨整合所需之最小位移（40μm）。

This study focused on improving the mechanical properties and corrosion resistance of Mg-5 wt.% Sn alloy (abbreviated to Mg-5Sn in the following article) to be a biomaterial for implants. Firstly, to improve the mechanical properties of Mg-5Sn, Mg powder consolidated by equal channel angular extrusion (ECAE) was investigated. The use of Mg powder was to simplify the experiment to understand the effect of different ECAE parameters on powder consolidation. Cu cans filled with Mg powder were ECAE processed for one, two and four passes via the BC route at 200, 250 and 300 °C. The microstructure of ECAE-processed samples was observed by optical microscope and scanning electron microscope. The density of each sample was determined using Archimedes’ principle. Microhardness and compression tests were conducted to investigate the mechanical properties of each ECAE-processed sample. The best consolidation was achieved after four passes of ECAE at 300 °C which can still maintain the nano-grain structure. Density at 98.4% of the ideal density of bulk Mg was achieved, microhardness was 49 Hv, and compressive yield stress was 100 MPa. However, the oxides (MgO) which form between Mg particles are inevitable resulting in no significant enhancement of mechanical properties of consolidated Mg. Therefore, in the following research, the investigated material will be the as-cast Mg-5Sn made by our researching group. The method to improve the corrosion resistance which is a crucial issue for Mg-5Sn to be applied to implant material was also studied. Phosphate surface pretreatment (PSPT) and poly[pyromellitic dianhydride-co-4,4’-oxydianiline] polyimide (PMDA-ODA PI) coating were used to provide corrosion protection for Mg-5Sn. PSPT enhanced the adhesion of PI coating by increasing the surface roughness for better mechanical interlocking. The phosphates formed on the alloy surface act as anchor groups. Energy dispersive X-ray spectrometer and X-ray photoelectron spectroscopy results indicated that bonds formed between anchor groups and PI coating. The bonds provided chemical anchoring and enhanced the adhesion of PI coating. The increase of corrosion resistance of PI coating on Mg-5Sn (Ecorr = −0.86 V/SCE, Icorr = 4.9×10−2 μA cm-2 and corrosion rate far less than 0.1 ml cm-2 day-1) showed the bonds were stable enough to stand the attack of electrolyte and inhibit the electrochemical reaction at the interface. The improvement in corrosion resistance makes Mg-5Sn a more suitable implant material. For further application of Mg-5Sn as an implant material, a numerical method based on the remodeling of bone was established to design the Mg-5Sn implant. The correctness of the method was firstly verified by applying to design the healing chamber in commercial Ti6Al4V dental implant. After that, the method was applied to design the healing chamber in Mg-5Sn dental implant. The designed Mg-5Sn implant was then compared with the designed Ti6Al4V implant, showing the advantages of Mg-5Sn for the implant application. The Mg-5Sn implant exhibited larger healing chamber and surface area which can allow larger amount of bone ingrowth and better osseointegration than the Ti6Al4V implant. The amount of healthy surrounding bone is also larger for the use of Mg-5Sn implant than the use of Ti6Al4V implant. These demonstrate that the Mg-5Sn implants can allow more bone ingrowth, less the stress shielding effect, and thus improve the implant stability. The micromotion of loaded Mg-5Sn implant is also smaller than the limit value for osseointegration.