| Summary: | 碩士 === 國立中興大學 === 材料科學與工程學系所 === 103 === This study investigated crevice corrosion behavior in the gap of pure magnesium bone-screw fixation system effects on screw-plate pullout strength. Casting pure magnesium undergoes grains refined process to improve its mechanical properties. Then put on lathe cutting to cut into screw-plate system tensile specimen. Fix the screw-plate fixation system in simulated body fluid (SBF) and maintain in 37 ºC. Took out the immersed specimens after different immersion time and undergo tensile test. After tensile test, mounted the specimen in epoxy and ground it until the cross section of screw-plate fixation. Analyzed its fracture cross section by SEM-BEI signal, this study founds all specimen breaks at screw head, even specimens after 30 weeks immersion in SBF did not stripped. This indicates screw thread after corroded still retained enough strength for screw-plate interlocking. Analyze the corroded thread by EDS founds the corrosion product is Mg(OH)2. Calculate from density and molar mass of Mg and Mg(OH)2, showing it will expand 1.76 times when Mg turns into Mg(OH)2. Suppose the corroded thread retain enough strength is contributed by the expansion of corrosion product. To prove this supposition, another experiment was done; cut pure magnesium by lathe cutting to create a pin without thread and plate also without thread. Combine pin-plate specimen in SBF then test its tensile strength with different immersion time. This test proves corrosion product do produce holding strength by expansion. It also proves holding strength of biodegradable screw-plate fixation system made by pure magnesium will not be weaken by corrode. Switch Mg plate specimen into S316L Stainless steel to simulate human bone, what do not corroded in human body, to test will the corrosion product still produce same holding strength if only one side corroded. The result shows corrosion product of pure magnesium not only enhance the screw-plate fixation strength, but also enhance screw-bone inter locking strength. To study the effects of sells behavior on corrosion products, plane specimens were used to immerse in SBF with different time, then analyzed their composition and cells attachment, proliferation and cytotoxicity. Finding corrosion product is not only Mg(OH)2, but also some Ca, P-rich oxides and hydroxides on surface of corrosion products. For cells test, this study found corrosion products on specimens with 0~48 hours immersion time are benefit for cells to grow and adhesion. But specimens with longer immersion time show cytotoxic and decrease of attached cells number. This predicted to be longer immersion time creates thicker Mg(OH)2 film, and Mg(OH)2 will dissolve into culture medium making solution abundant of OH and become too base for cells to survive. However, during 0~48 hours immersion test show corrosion products do help pure magnesium to have better biocompatibility as being a biodegradable implants.
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