Six-Month Long In Vitro Degradation Tests of Biodegradable Twinning-Induced Plasticity Steels Alloyed with Ag for Stent Applications

Abstract

Twinning-induced plasticity (TWIP) Fe–Mn–C steels are biodegradable metals with far superior mechanical properties to any biodegradable metal, including Mg alloys, used in commercially available devices. For this reason, the use of Fe–Mn–C alloys to produce thinner and thinner implants can be exploited for overcoming the device size limitations that biodegradable stents still present. However, Fe–Mn steels are known to form a phosphate layer on their surface over long implantation times in animals, preventing device degradation in the required timeframe. The introduction of second phases in such alloys to promote galvanic coupling showed a short-term promise, and particularly the use of Ag looked especially effective. Nonetheless, the evolution of the corrosion mechanism of quaternary Fe–Mn–C–Ag alloys over time is still unknown. This study aims at understanding how corrosion changes over time for a TWIP steel alloyed with Ag using a simple static immersion setup. The presence of Ag promoted some galvanic coupling just in the first week of immersion; this effect was then suppressed by the formation of a mixed carbonate/hydroxide layer. This layer partly detached after 2 months and was replaced by a stable phosphate layer, over which a new carbonate/hydroxide formed after 4 months, effectively hindering the sample degradation. Attachment of phosphates to the surface matches 1-year outcomes from animal tests reported by other authors, but this phenomenon cannot be predicted using immersion up to 28 days. These results demonstrate that immersion tests of Fe-based degradable alloys can be related to animal tests only when they are carried out for a sufficiently long time and that galvanic coupling with Ag is not a viable strategy in the long term. Future works should focus more on surface modifications to control the interfacial behavior rather than alloying in the bulk.

FeMn with Phases of a Degradable Ag Alloy for Residue-Free and Adapted Bioresorbability

“…The hypothesis of inhibiting depositions is supported by the formation of gaps around the Ag and AgCaLa phases. No depositions exist at the beginning of degradation, and the potential difference can effectively increase the dissolution of the matrix (FeMn), resulting in gaps [ 71 ]. During the development of the gaps, the inhibiting layer develops, and further anodic dissolution does not occur.…”

“…A significant difference between the degradation in NC, RL, and HS is present. HS is the only solution containing P, and, thus, Ca-P compounds may form layers that significantly affect further degradation [ 13 , 19 , 28 , 30 , 36 , 45 , 71 ]. Therefore, the degradation in HS might be suppressed by the deposition of compounds originating from the solution.…”

“…However, inhibiting layers are developed. Thus, the development of an additional mechanism to prevent the deposition of such layers is required to enable the effectiveness of potential differences [ 71 ].…”

Ultrashort Laser Texturing for Tuning Surface Morphology and Degradation Behavior of the Biodegradable Fe–20Mn Alloy for Temporary Implants

“…[ 19 ] Pure iron materials have low degradation rate (0.16 mm year −1 [ 20 ] ) and, as a consequence, the possibility of modifying such rate in order to meet the clinical needs could be interesting. [ 21 ] Different techniques have been approached to answer this main concern, such as alloying procedures, [ 22 ] laser surface treatments, and bulk modifications. [ 3,4,10,15,23 ]…”

“…[19] Pure iron materials have low degradation rate (0.16 mm year À1 [20] ) and, as a consequence, the possibility of modifying such rate in order to meet the clinical needs could be interesting. [21] Different techniques have been approached to answer this main concern, such as alloying procedures, [22] laser surface treatments, and bulk modifications. [3,4,10,15,23] Even though the addition of Mn in Fe-based alloys can promote an increase of the degradation rate, [24,25] a further step in terms of corrosion rate can be tuned by the modification not only of the chemical composition but also of the surface morphology, ensuring the desired life time for any specific device.…”