Investment casting of porous Mg-alloy networks biomechanically tuned for bone implant applications

Abstract

Manufacturing technology has been refined and described for the fabrication of honeycomb-based bioresorbable networks for temporal bone replacement applications. Two novel techniques, digital light processing and investment casting, were utilized to produce customized, shape-optimized cellular constructs with additional orifices promoting tissue ingrowth during osteo-regeneration. For this purpose, a conventional magnesium casting alloy (AZ91) was chosen. Numerical simulations were conducted to predict the compressive behavior of the proposed biodegradable lightweight scaffolds. Spatial castings were adjusted to possess mechanical properties comparable to the ones of cortical or trabecular bones. Two kinds of protective coatings (plasma electrolytic oxidation and organic ones based on natural polyphenols from tea extract) were deposited and characterized. They can be utilized to control the degradation rate during exploitation to achieve a predictable implant lifespan. The elaborated layers aim to mitigate the rapid corrosion of magnesium substrates by prolonging their bioresorption time and thus expanding their applicability in osseointegration. To evaluate this assumption, immersion tests in phosphate-buffered saline were performed, showing better chemical resistance of PEO coating and as-cast sample (for both mass gain by below 1%), and visible increase in mass of sample coated with organic coating (increase by almost 5%). Compressive strength results from numerical approach were further validated by experimental compression tests, showing that PEO coating deteriorated compressive strength by almost 3%, and organic coating improved it by over 9%. Results achieved in numerical approach were better than expected for stiffer sample, and slightly lower for the one with bigger pores.