Anja C. Hänzi

Magnesium alloys for temporary implants in osteosynthesis: In vivo studies of their degradation and interaction with bone

This study investigates the bone and tissue response to degrading magnesium pin implants in the growing rat skeleton by continuous in vivo microfocus computed tomography (μCT) monitoring over the entire pin degradation period, with special focus on bone remodeling after implant dissolution. The influence of gas release on tissue performance upon degradation of the magnesium implant is also addressed. Two different magnesium alloys - one fast degrading (ZX50) and one slowly degrading (WZ21) - were used for evaluating the bone response in 32 male Sprague-Dawley rats. After femoral pin implantation μCTs were performed every 4 weeks over the 24 weeks of the study period. ZX50 pins exhibited early degradation and released large hydrogen gas volumes. While considerable callus formation occurred, the bone function was not permanently harmed and the bone recovered unexpectedly quickly after complete pin degradation. WZ21 pins kept their integrity for more than 4 weeks and showed good osteoconductive properties by enhancing bone accumulation at the pin surface. Despite excessive gas formation, the magnesium pins did not harm bone regeneration. At smaller degradation rates, gas evolution remained unproblematic and the magnesium implants showed good biocompatibility. Online μCT monitoring is shown to be suitable for evaluating materials degradation and bone response in vivo, providing continuous information on the implant and tissue performance in the same living animal.

Design strategy for biodegradable Fe-based alloys for medical applications☆

The aim of this article is to describe a design strategy for the development of new biodegradable Fe-based alloys offering a performance considered appropriate for temporary implant applications, in terms of both an enhanced degradation rate compared to pure iron, and suitable strength and ductility. The design strategy is based on electrochemical, microstructural and toxicological considerations. The influence of alloying elements on the electrochemical modification of the Fe matrix and the controlled formation of noble intermetallic phases is deployed. Such intermetallic phases are responsible for both an increased degradation rate and enhanced strength. Manganese and palladium have been shown to be suitable alloying additions for this design strategy: Mn lowers the standard electrode potential, while Pd forms noble (Fe,Mn)Pd intermetallics that act as cathodic sites. We discuss the efficiency and the potential of the design approach, and evaluate the resulting characteristics of the new alloys using metal-physical experiments including electrochemical measurements, phase identification analysis and electron microscopy studies. The newly developed Fe-Mn-Pd alloys reveal a degradation resistance that is one order of magnitude lower than observed for pure iron. Additionally, the mechanical performance is shown to be adjustable not only by the choice of alloying elements but also by heat treatment procedures; high strength values >1400MPa at ductility levels >10% can be achieved. Thus, the new alloys offer an attractive combination of electrochemical and mechanical characteristics considered suitable for biodegradable medical applications.

On the biodegradation performance of an Mg–Y–RE alloy with various surface conditions in simulated body fluid

This study documents the influence of different surface conditions produced by various heat treatments on the in vitro degradation performance of an Mg-Y-RE alloy (WE43) investigated by immersion in simulated body fluid. WE43 samples were, respectively (i) annealed at 525 degrees C (plus artificial aging at 250 degrees C in one case) and afterwards polished; and (ii) polished, annealed at 500 degrees C in air and subsequently investigated in the oxidized state. Thermogravimetric analysis (TGA) indicates a mass gain during oxidation in air, following a square-root law over time. X-ray diffraction spectra imply a growing Y(2)O(3) layer upon oxidation, and Auger electron spectroscopy depth profiles show an increased oxide layer thickness which develops according to the behavior observed by TGA. Macroscopically, the degradation performance of the differently heat-treated samples can be divided into two groups. Annealed and polished samples show a fast and homogeneous degradation which slows with time. Their degradation behavior is approximated by a parabolic law. Oxidized samples exhibit a slow initial degradation rate which increases when the protection of the oxide layer is reduced. Overall, they reveal a sigmoidal degradation behavior. Here the differing degradation performances of the annealed-polished and the oxidized samples are related to the different surface conditions and explained on the basis of a depletion hypothesis.

In vivo degradation performance of micro-arc-oxidized magnesium implants: A micro-CT study in rats

Biodegradable Mg alloys are of great interest for osteosynthetic applications because they do not require surgical removal after they have served their purpose. In this study, fast-degrading ZX50 Mg-based implants were surface-treated by micro-arc oxidation (MAO), to alter the initial degradation, and implanted along with untreated ZX50 controls in the femoral legs of 20 male Sprague-Dawley rats. Their degradation was monitored by microfocus computed tomography (μCT) over a total observation period of 24weeks, and histological analysis was performed after 4, 12 and 24weeks. While the MAO-treated samples showed almost no corrosion in the first week, they revealed an accelerated degradation rate after the third week, even faster than that of the untreated ZX50 implants. This increase in degradation rate can be explained by an increase in the surface-area-to-volume ratio of MAO-treated implants, which degrade inhomogeneously via localized corrosion attacks. The histological analyses show that the initially improved corrosion resistance of the MAO implants has a positive effect on bone and tissue response: The reduced hydrogen evolution (due to reduced corrosion) makes possible increased osteoblast apposition from the very beginning, thus generating a stable bone-implant interface. As such, MAO treatment appears to be very interesting for osteosynthetic implant applications, as it delays implant degradation immediately after implantation, enhances fracture stabilization, minimizes the burden on the postoperatively irritated surrounding tissue and generates good bone-implant connections, followed by accelerated degradation in the later stage of bone healing.

Design strategy for microalloyed ultra-ductile magnesium alloys

This article describes a design strategy deployed in developing ultra-ductile Mg alloys based on a microalloying concept, which aims to restrict grain growth considerably during alloy casting and forming. We discuss the efficiency of the design approach, and evaluate the resulting microstructural and mechanical properties. After processing, the so-designed alloys ZQCa3 (Mg–3Zn–0.5Ag–0.25Ca–0.15Mn, in wt.%) and ZKQCa3 (Mg–3Zn–0.5Zr–0.5Ag–0.25Ca–0.15Mn, in wt.%) reveal very fine grains (<10 µm), high ductility (elongation to fracture of up to 30%) at moderate strength or high strength (ultimate tensile strength of up to 350 MPa) at reasonable ductility. These properties are explained based on thermodynamic modelling, microstructure analysis including transmission electron microscopy studies, and microstructural and mechanical testing after annealing, and are compared to a related commercial alloy (ZK31).

Design strategy for new biodegradable Mg-Y-Zn alloys for medical applications

Abstract The aim of this article is to describe the design strategy deployed in developing new biodegradable Mg–Y–Zn alloys. The development approach is based on a microalloying concept which aims to restrict grain growth considerably during alloy casting and forming. We discuss the efficiency of the design approach, and evaluate the characteristics of the new alloys using metal-physical experiments, thermodynamic calculations and transmission electron microscopy analysis. Our results show that after extrusion the alloys have very fine grains (< 10 m), exhibit high ductility (uniform elongation: 17 – 20 %) at considerable strength (ultimate tensile strength: 250 – 270 MPa) and reveal the presence of finely distributed intermetallic particles which are stable upon annealing. Due to an attractive combination of mechanical, electrochemical and biological properties, the new alloys are very promising not only for applications in medicine but also in other fields.

Towards biodegradable wireless implants

A new generation of partially or even fully biodegradable implants is emerging. The idea of using temporary devices is to avoid a second surgery to remove the implant after its period of use, thereby improving considerably the patients comfort and safety. This paper provides a state-of-the-art overview and an experimental section that describes the key technological challenges for making biodegradable devices. The general considerations for the design and synthesis of biodegradable components are illustrated with radiofrequency-driven resistor–inductor–capacitor (RLC) resonators made of biodegradable metals (Mg, Mg alloy, Fe, Fe alloys) and biodegradable conductive polymer composites (polycaprolactone–polypyrrole, polylactide–polypyrrole). Two concepts for partially/fully biodegradable wireless implants are discussed, the ultimate goal being to obtain a fully biodegradable sensor for in vivo sensing.

Design considerations for achieving simultaneously high-strength and highly ductile magnesium alloys

A comprehensive scheme of phase configuration optimization in the Mg–Zn–Ca(–Zr) system by thermodynamic simulations and microstructural analyses is presented. A composition window of 0.2–0.4 wt% Ca and 5–6 wt% Zn is defined as optimal for establishing a complex heterogeneous microstructure allowing for enhanced ductility and simultaneously high strength of the material. Literature data analysis and our own results confirm the enhanced performance of alloys from this composition window.

Design Strategy for Microalloyed Ultra-Ductile Magnesium Alloys for Medical Applications

The aim of this article is to describe the design strategy deployed in developing new bioabsorbable Mg–Y–Zn alloys. The development approach is based on a microalloying concept, which aims to restrict grain growth considerably during alloy casting and forming. We discuss the efficiency of the design approach, and evaluate the characteristics of the new alloys using metal-physical experiments, thermodynamic calculations, and TEM analysis. Our results show that after extrusion the alloys have very fine grains (<10m), exhibit high ductility (uniform elongation: 17–20%) at considerable strength (ultimate tensile strength: 250–270 MPa), and reveal the presence of finely distributed intermetallic particles, which are stable upon annealing. Due to an attractive combination of mechanical, electrochemical and biological properties, the new alloys are very promising not only for applications in medicine but also in other fields.

The influence of heat treatment and plastic deformation on the bio-degradation of a Mg-Y-RE alloy.

In this study the bio-degradation behavior of a Mg-Y-RE alloy in different heat treatment states with respect to the alloys potential application as biodegradable implant material was investigated by electrochemical impedance spectroscopy in two body-similar fluids. The heat treatments increase the degradation resistance of the alloy and lead to the formation of a thermal oxide layer on the sample surface and to a change in microstructure such as the distribution of yttrium. The varying Y distribution in the alloy does not significantly influence the degradation behavior, and all samples show a similar low polarization resistance. However, samples with a thermal oxide layer, which consists mainly of Y(2)O(3), degrade much more slowly and feature remarkably high polarization resistance. Nevertheless, in some cases localized corrosion attack occurs and drastically impairs performance. Cracks in the oxide layer, intentionally induced by straining of the samples and which in practice could originate from the implantation process, reduce the corrosion resistance. However, these samples perform still better than polished specimens and show a macroscopically homogeneous degradation behavior without localized corrosion. Microscopically, corrosion attacks start at the cracks and undermining of the oxide layer occurs with time. For all the material conditions a remarkable dependence of the degradation rate on the electrolyte is noted.