Introduction: The development of new implant materials and/or the investigation of risk potential for already applied materials systems depend on a better understanding of corrosion processes of biodegradable implant alloys. For this analytical methods with high lateral resolution and high sensitivity have to be applied since degradation products are formed on the micrometer scale and changes in local tissue concentrations or in organs have to be determined at the µg/g to µg/kg level. Challenges for accurate measurements also arise from the lack of appropriate reference materials, the composite nature of the implants or the application of analytically difficult elements, e.g. aluminum, fluorine or rare earth elements in or on the degradable alloys[1],[2].
Materials and Methods: Mass spectrometry (SIMS, ICP-MS, LA-ICP-MS), X-ray spectroscopy (µXRF, XRD, PIXE, XPS), optical emission spectroscopy (ICP-OES), Infrared and Raman spectroscopy have been applied for the determination of corrosion products at the implant (in vitro and ex vivo) as well as for the determination of accumulation factors for critical alloy components in liver, kidney, brain, blood and bone. Reference materials were synthesized according to the matrix conditions of bone or tissue samples and adapted calibration strategies were applied for the quantification. Mg alloys with a wide variety of alloying components (Ag, Al, Be, F, Li, REE, Zn, Zr, and others) served as samples for all in vitro and in vivo investigations.
Results and Discussion: Investigation of in vitro and ex vivo samples showed different degradation rates and products depending on alloy composition. Especially metal cations like aluminum and the REE were fixed in reaction products with low solubility thus leading to drastically reduced degradation rates for these alloy components. Best results for the quantification of degradation products around the implants were obtained by LA-ICP-MS and PIXE (Fig.1). An accumulation in various tissues and bone was detected for Li, Al and the REE (up to 100 fold enrichment). The interaction of proteins with components of the formed corrosion layer could be shown by SY-IR measurements. Best results for the quantification of tissues by solid state spectroscopic methods were produced when polymer reference materials and an adapted quantification procedure were used, for bone samples composites of polymers and hydroxyl apatite were used as references. Even for REE in Mg alloys an accurate quantification was possible despite of their chemical and physicochemical similarity and the interferences present for all methods. Difficulties in quantification were observed for Li, Al and F.
Conclusion: A combination of several methods and adapted references and calibration strategies are necessary to obtain reliable information about the degradation process of biodegradable alloys. Although most elements of these alloys could be measured with the methods and references developed in this work investigation is still challenging for elements like fluorine and aluminum in tissue or carbon and oxygen in low concentrations on the surface of implant alloys due to technical and methodical limitations.
References:
[1] F. Witte, et. al, Current Opinion in Solid State & Materials Science 12 (2008) 63-72
[2] Vogt et al., Magnesium, 2009, Wiley-VCH, p.1162-1174