Introduction: Biodegradable metals emerged as potential candidates for biomedical application to replace permanent implants and avoid long-term risks[1]. For this purpose, biodegradable alloys should be biocompatible and their degradation rate should ideally match tissue-healing process. In the litterature, various tests-systems and solutions were used to evaluate biocompatibility and degradation rate, therefore it becomes difficult to compare the results, because the degradation behavior is affected not only by the chemical composition of the material, but also by the environmental conditions. In a CO2 rich medium, like blood, various carbonates with various stability could form. for example, Fe ones are not stable[2] wheras mn ones are considered more stable. this work aimed to investigate the influence of CO2 presence on the degradation behavior of new austenitic Fe-Mn-C alloy, in an effort to standardize the degradation test method for highly CO2 sensitive materials.
Materials and Methods: Fe-Mn-C samples were obtained by vacuum and melted under uncontaminated conditions at CNR-IENI (LC, Italy). The material was worked to 0.45 mm thick sheets that were thermally treated for 1 h at 800 C in Ar + 5% H2 atmosphere (TT) and tested. A modified Hanks' solution (MH) was used for static degradation test (ASTM G31). Two groups of five samples were aged up to 14 days, one under controlled atmosphere (CA, 37 C; 5% CO2 ) and another one in ambient atmosphere (AA, 37 C; 0.04% CO2). Scanning electron microscopy, electron dispersion spectroscopy, X-ray diffraction and atomic absorption spectroscopy (AAS) were used to characterize surfaces and degradation products. Degradation rate was calculated based on the weight loss.
Results and Discussion: The degradation rate ratio between AA samples and CA samples ones was approximately two. MnCO3 crystals mainly covered degraded surfaces of CA samples, while Fe oxides and hydro-oxides were found mainly in degradation products. For AA samples, Fe and Mn oxides and hydro-oxides were found both on the degraded surfaces and in the degradation products. AAS investigations showed that Fe ions concentration in exhausted solution was higher than Mn one for both Ca and AA samples. The results clearly evidenced that different CO2 concentrations led to different degradation patterns regarding this Fe-Mn-C alloy. Indeed the formation of stable carbonates (such as MnCO3) adhering to the sample surface significantly affected its degradation rate.
Conclusion: The degradation behavior resulted to be strongly affected by material chemistry, the composition of chemical solution used and the test atmosphere. An accurate understanding of these parameters is needed both to compare the results obtained by different researchers, especially in attempt to predict the correlation between in-vitro and in-vivo experiences.
Stéphane Turgeon; Vicky Dodier; Daniel Marcotte; Nathalie Moisan; Jean Frenette; Marc Choquette PhD; André Ferland; ACDI & PCBF; NSERC-Canada, CIHR-Canada, FRQ-NT-Québec, MRI-Québec/Italy
References:
[1] Hoffmann R. et al. Circulation 1996; 94(6); 1247-54
[2] T.L. Webb et al. Academic Press, New York, 1970, p 327