原子層沉積技術披覆二氧化鈦薄膜於鎳鈦形狀記憶合金生醫植入材之研究

Abstract

鎳鈦形狀記憶合金(Nickel-titanium shape memory alloys)具有優異的形狀記憶效應、超彈性效應、耐疲勞的特性與生物相容性等特性，而被廣泛使用於生醫用途中，如：心血管支架、微創手術中引導絲、結石移除器、人工心臟瓣膜與齒列矯正線等。然而鎳鈦合金在生理環境中，易受到腐蝕而有鎳離子溶出之問題，其鎳離子之溶出易對人體造成過敏、毒性與致癌性，而有健康上之疑慮。現今許多研究著重於鎳鈦合金上之表面改質，以阻擋鎳離子溶出，進而提升生醫植入材之生物相容性，增加植入材之穩定性，但傳統薄膜製程之階梯包覆性差與膜層厚度過厚，而侷限了鎳鈦合金之形狀記憶效應與超彈性效應之應用。故本研究利用原子層沉積技術(Atomic Layer Deposition, ALD)於鎳鈦形狀記憶合金上施鍍奈米尺度之氧化鈦(Titanium dioxide, TiO2)膜層，以探討所能承受之應變量與生物相容性。 本研究中使用電漿增強型原子層沉積技術(Plasma-enhanced atomic layer deposition, PE-ALD)以沉積氧化鈦膜層於鎳鈦形狀記憶合金上，並且比較不同循環次數下之膜層性質，膜層評估方法包括：顯微結構觀察、親疏水性、彎曲測試、抗腐蝕性測試、鎳離子溶出檢測與骨細胞相容性，以探討原子層沉積技術沉積氧化鈦膜層於鎳鈦形狀記憶合金上之可行性。 研究結果顯示，利用原子層沉積技術施鍍於鎳鈦合金上之氧化鈦膜層，經橫截面觀察可發現到，氧化鈦膜層為均勻性佳且與基材間貼覆性良好；施鍍氧化鈦之膜層相較基材疏水，其接觸角並不會隨著膜層厚度而有些微增加的趨勢；隨著沉積膜層厚度越厚，膜層之抗腐蝕能力與鎳阻擋效率皆有提升；體外試驗中，氧化鈦膜層相較基材不具細胞毒性並且可提供細胞相容性，因此利用原子層沉積技術施鍍氧化鈦膜層可增加鎳鈦植入材的生物相容性。

NiTi shape memory alloys (SMAs) have been widely applied in biomedical fields due to their excellent shape memory effect, pseudoelasticity, fatigue performance and biomedical properties, such as the cardiovascular stent, guide wire, stone extractors use in the cystic duct, artificial heart valve and orthodontic wire. However, NiTi alloys would release out the Ni ion in the physiological environment during corrosion and might induce the serious problems of allergic reaction, poisoning and causing cancer. Therefore, surface modification is employed to decrease the Ni ion release of NiTi alloy and make it more biocompatible, thereby improving its long-term stability. The general thin film deposition techniques are poor in step coverage and are too thick in film thickness. The thick coating couldn’t bear the large strain, so the shape memory effect and/or the pseudoelasticity of NiTi alloys would be limited due to the thick coating. In this research, the atomic layer deposition (ALD) process was applied to deposit the titanium dioxide (TiO2) with different nano-scale thickness on the NiTi substrates, and then the strain-bearing ability and the biocompatibility of ALD-TiO2 were analyzed. In this study, the plasma enhanced-atomic layer deposition process was applied to deposit titanium dioxide on the NiTi SMAs surfaces and the film properties with deposition of 50, 100, 200 cycles were compared. Several methods have been used to evaluate the properties of ALD films, including the microstructure observation, water contact angle, bending test, corrosion resistance, the nickel release and the biocompatibility in vitro. The experimental results show the ALD-TiO2 films are uniform and the interfaces between the ALD-TiO2 films and NiTi substrates fit very well. The coating films are more hydrophobic than the NiTi substrates. The water contact angles of ALD-TiO2 films only change slightly as the deposition cycles increases. The ALD-TiO2 films can effectively inhibit the Ni release from NiTi matrix. Meanwhile, NiTi SMAs with ALD-TiO2 films can exhibit better corrosion resistance in simulate body fluid than the bare NiTi SMAs. In Vitro, the ALD-TiO2 films do not exhibit any toxicity. These results indicate that the biocompatibility of the NiTi SMAs can be effectively improved by TiO2 films deposited by ALD technique.