Dynamic degradation of porous magnesium under simulated environment of human cancellous bone

Abstract

Biodegradable metals have been suggested for bone scaffold applications due to their mechanical properties that are better for load bearing applications. Among biodegradable metals, magnesium and its alloy are the most investigated materials due to their mechanical properties which are closer to the cancellous bone and could prevent complications such as an aseptic loosening of stress shielding effects, and potentially to be used as bone scaffolds. Bone adapts the mechanical loading from the physiological activities that induced the movement of bone marrow passing through the porous structure of cancellous bone due to the pressure differences. The aim of this research is to analyse the degradation behaviour of porous magnesium under dynamic degradation test for bone scaffold applications. Interconnected holes of porous magnesium have been developed with various percentages of porosity (30%, 41% and 55%) and are fabricated using computer numerical control (CNC) machine. Dynamic immersion test rigs are specifically designed to simulate environment of human cancellous bone. There are two types of tests that have been conducted in this study: (1) fluid flow with different flowrates (0.025, 0.4 and 0.8 ml/min) and (2) fluid flow integrated cyclic loading (different cyclic loading (1000, 2000 and 3500 με) under constant flowrate of 0.025 ml/min). A dynamic immersion test has been conducted for 24, 48 and 72 hours. The results showed that the specimen with a higher percentage of porosity as well as the exposed surface area degrades faster compared to the others. The degradation product formation and clogging pores phenomenon are influenced by the level of flow rates. The effects of different flow rates towards the mechanical integrity of porous magnesium have shown a huge drop of 95% from their original mechanical properties within 3 days, which have deteriorated in both functions; porosity and degradation time. The variation in flowrates used showed that degradation of the material is seven times higher compared to the static immersion test environment. Furthermore, the influenced of integrating fluid flow and cyclic loading have increased the relative weight loss and degradation rate as high as 61.56% and 93.67%, respectively. Additionally, the mechanical properties have improved and increased from 53% to 87% as compared to dynamic immersion test using the mechanical stimulus of fluid flow only. Therefore, the dynamic immersion test with integrated cyclic loading was more reliable and provides realistic environment for degradation assessment compared to static immersion test for bone scaffold application as this study using the boundary of human cancellous bone environment.