Manufacturing and processing techniques for fabrication of iron-based BMs

A new emerging class of biomaterials: Biodegradable metals C

onventional metallic biomaterials for medical applications are made of corrosion resistant materials and are designed to remain intact in a human body for a long time. They are characterized by appropriate mechanical properties (for searched applications) and excellent corrosion resistance. Although over the last decades permanent materials have been largely used, they have certain disadvantages. Implant for bone fracture fixation should be removed after one or two years after implantation. From an economic point of view, noticeable cost for healthcare systems and implant removal constitute 30% of orthopedic procedures.

Biodegradable metals (BMs) could adapt the human body and eventually dissolve when their presence is no longer required. Such materials would help patients suffering from fractures of long bones or limb length discrepancies. Further, the expenses of multiple procedures including implantation, removal, and re-implantation of the permanent implant could be avoided. Another example is a metallic stent used to treat blockages in the coronary arteries. This small implant can cause an immune response that leads to the formation of blood clots or restenosis.

The use of bioabsorbable stents drastically reduces, if not annihilate, some potentially long-term clinical problems related to permanent stents including chronic inflammation, late stent thrombosis, in-stent restenosis and stent strut fracture which can damage the local vasculature. The concept of an absorbable stent is to keep the occluded arteries open during the remodeling period and degrade harmlessly afterward when its mechanical scaffolding effect is no longer needed. The stent material and its corrosion product must be non-toxic and compatible with the vascular environment [14–17]. BM stents, as well as bone fixation implants, have other advantages in pediatric applications showing the ability to adjust to the tissue growth. The biodegradable implant should be able to compromise its degradation and mechanical integrity after implantation and during tissue healing period. The BM should start its dissolution just after the implantation, which results from low corrosion resistance comparing to inert material. The modulable corrosion rate is needed to retain the optimal mechanical integrity of the implant until the tissue remodeling is achieved.

Development of Zn-based biodegradable materials

The first consideration for alloying element selection to develop biodegradable Zn alloys is elemental toxicity. Elements with potential toxicological problems must preferably be avoided in the initial design stage for biodegradable alloys. The degradation products of the alloys should be non-toxic and readily absorbable by the surrounding tissues. Only limited types of Zn alloy systems have been investigated as potential biodegradable materials thus far [48–58]. Various alloying elements such as Mg, Ag, Sr, Ca, Li, Mn and Cu have been added to Zn to improve its mechanical performance. In Fig. 1.5. the mechanical properties of investigated Zn-based alloys in the cast and wrought conditions are collected. As seen, cast alloys display a wide range of tensile strength, ranging between 70 MPa and 250 MPa, but very poor ductility (below 4%).

The highest tensile strength can be found in Zn-1.5Mg-Ca/Sr alloys due to the combined strengthening contributions of brittle Zn + Mg2Zn11 eutectic constituents, CaZn13 and SrZn13 intermetallic phases. However, the general mechanical performance, considered as the combination of strength and ductility, for all the reported cast Zn alloys listed in Fig. 1.5a is far below the requirements for the implant materials. Additional improvement in properties could be achieved by post thermomechanical treatments through tailoring the microstructural features such as grain size, secondary phase size/distribution and crystallographic texture. Fig. 1.5b summarizes the tensile strength and fracture elongation of several wrought Zn-based alloys explored to use in the biomedical application during the last two years [47,48,50–55,57–63]. However, by now, no Zn-based alloys have been introduced fulfilling the benchmark values required for an ideal absorbable stent/bone implant material.

For Zn-Mg based alloys, which are the most investigated alloys, a parabolic trend can be seen, so that with increasing Mg concentration UTS increases, while the elongation to failure decreases concurrently. However, it is apparent that any percentage of Mg higher than 0.5 wt% results in a drastic drop in fracture elongation. This trend is attributed to the increasing volume fraction of the brittle MgZn11 intermetallic phase. Conversely, in case of Zn-Ag alloys, with increasing Ag content from 2.5 wt.% to 7.0 wt.%, despite a meaningful improvement of tensile strength, fracture elongation remains in the range 32-36%.

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Composite strengthening / Particle dispersion hardening

Metal matrix composites (MMCs) which introduce micometer-sized reinforcing particles in a bulk matrix offer opportunities to tailor material’s mechanical performances. With the emersion of nanomaterials, nanocomposites have developed with outstanding properties, suppressing the limitations for metals and conventional composites. The possibility of combining different materials (e.g. metal – ceramic – non-metal) provides the opportunity for infinite variations.

The properties of these novel materials are arisen from the properties of their individual components. Fig. 1.7 shows the distribution of the composite materials into three distinct groups of materials. Reinforcing of Fe-based BMs would have various objectives which could be summarized as follows: (i) improvement of mechanical strength, while keeping acceptable ductility and toughness, (ii) increasing the fatigue resistance, (iii) increasing the Young’s modulus, (iv) increasing corrosion rate in physiological environment resulting from micro-galvanic coupling and (v) improving the biocompatibility by introducing bioactive reinforcements. The improved mechanical performances of MMCs are arisen from several strengthening mechanisms such as: (i) load transfer effect, (ii) Hall-Petch strengthening, (iii) Orowan strengthening, (iv) mismatch in coefficient of thermal expansion (CTE) and elastic modulus (EM).

1.1. Permanent implant materials
1.2. A new emerging class of biomaterials: Biodegradable metals
1.3. Mg-based biodegradable metals
1.3.1. Development of Mg-based biodegradable materials
1.3.2. In vitro corrosion mechanism
1.3.3. In vivo and clinical performance
1.4. Zn-based biodegradable metals
1.4.1. Development of Zn-based biodegradable materials
1.4.2. In vitro corrosion mechanism
1.4.3. In vivo performance
1.5. Fe-based biodegradable metals
1.5.1. Methods for strengthening Fe-based biodegradable materials Work hardening Solution hardening Precipitation hardening Grain refining hardening Composite strengthening / Particle dispersion hardening
1.5.2. Pure Fe, its alloys and composites for BMs applications Pure Fe Fe-Mn system Other Fe-based alloys systems Iron-based biodegradable composites Surface modifications for enhanced degradation rate of Fe-based alloys
1.5.3. Manufacturing and processing techniques for fabrication of iron-based BMs Electroforming Severe plastic deformation techniques Metal Injection Molding Cold Gas Dynamic Spraying 3D Printing
1.5.4. In vitro corrosion mechanism
1.5.5. In vivo performance
2.Strategies, objectives and structure of the thesis
2.1. Importance of research on development of biodegradable implants
2.2. Main challenge for iron-based BMs- objective of the project
2.3. Objectives
2.4. Material selection
2.4.1. Iron matrix reinforced by magnesium silicide particles
2.5. Manufacturing process selection and equipment set-up.
2.5.1. Fabrication of the specimens
3.Synthesis, mechanical properties and corrosion behavior of powder metallurgy processed Fe/Mg2Si composites for biodegradable implant applications
3.1. Résumé
3.2. Abstract
3.3. Introduction
3.4. Materials and methods
3.4.1. Characterization
3.4.2. Corrosion behavior
3.5. Experimental results
3.5.1. Microstructure
3.5.2. Mechanical properties
3.5.3. Corrosion behavior of Fe and Fe/Mg2Si composites
3.5.4. Corrosion products characterization
3.6. Discussion
3.6.1. Microstructure and mechanical properties
3.6.2. Corrosion behavior
3.7. Conclusions
3.8. Acknowledgements
4.Understanding the effect of the reinforcement addition on corrosion behavior of Fe/Mg2Si composites for biodegradable implant applications
4.1. Résumé
4.2. Abstract
4.3. Introduction
4.4. Materials and methods
4.4.1. Sample preparation
4.4.2. Metallographic examination
4.4.3. Corrosion behavior
4.4.4. Corrosion characterization
4.5. Experimental results
4.5.1. Microstructure
4.5.2. Electrochemical measurements
4.5.3. Static immersion tests
4.6. Discussion
4.7. Conclusions
5.Long-term in vitro degradation behaviour of Fe and Fe/Mg2Si composites for biodegradable implant applications
5.1. Résumé
5.2. Abstract
5.3. Introduction
5.4. Materials and methods
5.4.1. Synthesis of composites
5.4.2. Metallographic examination
5.4.3. Degradation behavior
5.4.4. Degradation characterization
5.5.1. Microstructure of as-received specimens
5.5.2. XRD and FTIR analysis of degraded samples
5.5.3. Degradation morphologies of Fe and its composites after 20, 50 and 100 days of immersion in modified Hanks’ solution
5.5.4. Degradation precipitates
5.5.5. Degradation rates and ion release
5.6. Discussion
5.6.1. Protective film structure and formation
5.7. Conclusions
5.8. Acknowledgements
6.General discussion
6.1. Development of MMCs for BM applications
6.2. Determination of corrosion rate and identification of corrosion products
6.3. Periods considered to measure the corrosion rates in vitro
7.Conclusions & Perspectives
7.1. Conclusions
7.2. Perspectives

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