Carlo Paternoster

Influence of cross-rolling on the micro-texture and biodegradation of pure iron as biodegradable material for medical implants.

Iron-based biodegradable metals have been shown to present high potential in cardiac, vascular, orthopaedic and dental in adults, as well as paediatric, applications. These require suitable mechanical properties, adequate biocompatibility while guaranteeing a low toxicity of degradation products. For example, in cardiac applications, stents need to be made by homogeneous and isotropic materials in order to prevent sudden failures which would impair the deployment site. Besides, the presence of precipitates and pores, chemical inhomogeneity or other anisotropic microstructural defects may trigger stress concentration phenomena responsible for the early collapse of the device. Metal manufacturing processes play a fundamental role towards the final microstructure and mechanical properties of the materials. The present work assesses the effect of mode of rolling on the micro-texture evolution, mechanical properties and biodegradation behaviour of polycrystalline pure iron. Results indicated that cross-rolled samples recrystallized with lower rates than the straight-rolled ones due to a reduction in dislocation density content and an increase in intensity of {100} crystallographic plane which stores less energy of deformation responsible for primary recrystallization. The degradation resulted to be more uniform for cross-rolled samples, while the corrosion rates of cross-rolled and straight-rolled samples did not show relevant differences in simulated body solution. Finally, this work shows that an adequate compromise between biodegradation rate, strength and ductility could be achieved by modulating the deformation mode during cold rolling.

Effect of grain sizes on mechanical properties and biodegradation behavior of pure iron for cardiovascular stent application

ABSTRACT Pure iron has been demonstrated as a potential candidate for biodegradable metal stents due to its appropriate biocompatibility, suitable mechanical properties and uniform biodegradation behavior. The competing parameters that control the safety and the performance of BMS include proper strength-ductility combination, biocompatibility along with matching rate of corrosion with healing rate of arteries. Being a micrometre-scale biomedical device, the mentioned variables have been found to be governed by the average grain size of the bulk material. Thermo-mechanical processing techniques of the cold rolling and annealing were used to grain-refine the pure iron. Pure Fe samples were unidirectionally cold rolled and then isochronally annealed at different temperatures with the intention of inducing different ranges of grain size. The effect of thermo-mechanical treatment on mechanical properties and corrosion rates of the samples were investigated, correspondingly. Mechanical properties of pure Fe samples improved significantly with decrease in grain size while the corrosion rate decreased marginally with decrease in the average grain sizes. These findings could lead to the optimization of the properties to attain an adequate biodegradation-strength-ductility balance.

Effect of electrolyte composition and deposition current for Fe/Fe-P electroformed bilayers for biodegradable metallic medical applications

With its proven biocompatibility and excellent mechanical properties, iron is an excellent source material for clinical cardiac and vascular applications. However, its relatively low degradation rate limits its use for the healing and remodeling of diseased blood vessels. To address these issues, a multi-purpose fabrication process to develop a bilayer alloy composed of electroformed iron (E-Fe) and iron-phosphorus (Fe-P) was employed. Bilayers of Fe/Fe-P were produced in an electrolytic bath. The effects of electrolyte chemical composition and deposition current density (idep) on layer structure and chemical composition were assessed by scanning electron microscopy, electron probe microanalysis, X-ray diffraction and X-ray photoelectron spectroscopy. The corrosion rate was determined by potentiodynamic polarization tests. The bilayers showed an increasing amount of P with increasing NaH2PO4·H2O in the electrolyte. Fe-P structure became finer for higher P amounts. Potentiodynamic polarization tests revealed that the corrosion rate was strongly influenced by deposition conditions. For a P amount of ~2 wt.%, the corrosion rate was 1.46mm/year, which confirms the potential of this material to demonstrate high mechanical properties and a suitable corrosion rate for biomedical applications.

Synthesis, mechanical properties and corrosion behavior of powder metallurgy processed Fe/Mg2Si composites for biodegradable implant applications

Recently, Fe and Fe-based alloys have shown their potential as degradable materials for biomedical applications. Nevertheless, the slow corrosion rate limits their performance in certain situations. The shift to iron matrix composites represents a possible approach, not only to improve the mechanical properties, but also to accelerate and tune the corrosion rate in a physiological environment. In this work, Fe-based composites reinforced by Mg2Si particles were proposed. The initial powders were prepared by different combinations of mixing and milling processes, and finally consolidated by hot rolling. The influence of the microstructure on mechanical properties and corrosion behavior of Fe/Mg2Si was investigated. Scanning electron microscopy and X-ray diffraction were used for the assessment of the composite structure. Tensile and hardness tests were performed to characterize the mechanical properties. Potentiodynamic and static corrosion tests were carried out to investigate the corrosion behavior in a pseudo-physiological environment. Samples with smaller Mg2Si particles showed a more homogenous distribution of the reinforcement. Yield and ultimate tensile strength increased when compared to those of pure Fe (from 400MPa and 416MPa to 523MPa and 630MPa, respectively). Electrochemical measurements and immersion tests indicated that the addition of Mg2Si could increase the corrosion rate of Fe even twice (from 0.14 to 0.28mm·year-1). It was found that the preparation method of the initial composite powders played a major role in the corrosion process as well as in the corrosion mechanism of the final composite.

Electroforming as a New Method for Fabricating Degradable Pure Iron Stent

The first example of a documented electroforming process dates back to 1837 when a layer of electrodeposited copper was found on the surface of a printing plate. Since then, it became a basic manufacturing process to produce delicate metallic components such as nickel thin foils for solar panels, perforated screen-printing cylinders used for fabric and carpet printings, digital recording devices, etc. Recently, electroforming is used for the fabrication of iron-based materials designed for cardiovascular stents. Electroformed iron shows a higher corrosion rate in simulated biological environment; this behaviour is supposed to be influenced by its microstructure which is finer than that of iron produced with traditional techniques. A high corrosion rate can be beneficial for cardiovascular stent applications: a complete stent dissolution in 12–18 months can effectively prevent both late thrombosis and further treatment of paediatric patients, usually requiring a continuous vessel remodelling. Faster corrosion rate of iron-based material is advantageous for cardiovascular stent application in order to avoid late stent thrombosis and arterial growth mismatch in young patients leading to a secondary revascularization procedure. Electroformed iron has mechanical properties comparable to those of stainless steel (stent reference metal) with the advantage of the total dissolution of the material after the accomplishment of its function: for this reason, this metal can be considered as a valid alternative to magnesium-based materials. Nevertheless, electroforming is influenced by parameters such as electrolyte bath composition, current density, pH, temperature, additives, cathode, etc. that have a significant effect on the structure of the produced materials.

Laser surface texturing of SS316L for enhanced adhesion of HUVECs

ABSTRACT This study investigates the effects of the structured surface on the adhesion, proliferation and alignment of endothelial cells (HUVECs). The chemical state of laser-structured surface was also investigated in comparison with the non-treated surface. A novel design for biomedical applications consisting of parallel chain-like structures was realised on stainless steel surface by laser micromachining. The structures were designed to employ surface topography in the presence of micron and sub-micron features and to avoid intensive surface modification that could compromise the mechanical properties of thin devices like cardiovascular stents. The results showed that the structured surfaces favored the adhesion, proliferation and alignment of HUVECs. The proliferation and the alignment of HUVECs were pronounced when the periodic distance between two consecutive chain-like structures was 25 µm. Moreover, there was no significant difference in chemical composition on the structured surface suggesting that the cell proliferation and alignment were mainly influenced by the surface topography.

Iron-Based Degradable Implants

Iron-based degradable implants present a significant advantage over magnesium and zinc-based devices, since they possess a better combination of strength and ductility. However, their degradation inside the body proceeds too slowly. In this article, the different applications and classes of iron-based implants are examined. Moreover, testing methods for assessing their biological performances are discussed. This will allow to understand how new materials can be designed, in order that they can hit the market while possessing superior properties with respect to currently commercialized implants.