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Localized Corrosion Studies for Magnesium Implant Devices

Periodic Reporting for period 1 - MAGPLANT (Localized Corrosion Studies for Magnesium Implant Devices)

Reporting period: 2016-09-01 to 2018-08-31

Magnesium exhibits a promising applicability biodegradable implant material.Biodegradable implants allow musculoskeletal healing while safely degrading in the physiological environment to finally give place to the completely restored tissue. This technology spares injured patients to multiple surgical interventions and harmful side effects than can arise from conventional implant materials such as Ti-based alloys and stainless steel. Mainstream biodegradable materials, like PGA and PLA, can display unsatisfactory mechanical strength and acidic degradation products. Statistics tell us that metallic implants are removed 19 to 54% of the time depending on the fracture type. Implant removal has been considered as the biggest drawback by 91% of the patients in a recent study conducted in the UK, with 80% of the enquired patients showing willingness to participate in clinical trials for comparison between bio-resorbable implants and permanent ones. Economically, biodegradable implants can be more cost-effective up to seven hundred dollars less per implant. Hence, the socioeconomic demand for bio-resorbable prosthetic devices is undeniable and asks for improved solutions that can satisfy the necessities of all involved parts, from patients to health authorities. Magnesium is the lightest engineering metal with high strength-to-weight ratio, it abundantly present in bone and an essential element in body metabolism, making it biocompatible. Its mechanical properties like the elastic modulus and the compressive yield strength are close to those of bone. Although the initial use of Mg-based implants can be dated back to 1878, its use for bioapplications was abandoned in the mid-20th century, due to the fast corrosion rate in chloride containing media, which is the case of the human body. Magnesium suffers mainly from localized corrosion with side propagating pitting, with homogeneous corrosion being hardly relevant. Because Mg is anodic to any other engineering metal, macro and micro galvanic coupling between the metal and other metal impurities and secondary phases are key aspects in controlling the degradation rate. An additional complication is hydrogen evolution deriving from the cathodic partial reactions, which can cause toxic effects for the body tissue and induce local embrittlement. Pitting and hydrogen embrittlement are two of the most propelling forms of causing premature implant failure due to stress corrosion cracking. It is also an essential element in body metabolism. Today, the biodegradability of Mg and Mg-alloys is a hot topic motivating the development of corrosion controlling strategies in order to validate the applicability of these materials for artificial orthopedic load bearing structures such as pins, plates, screws, etc.
During the MAGPLANT project, localized electrochemical techniques like micro-ISE (Ion-Selective Electrodes), SVET (Scanning Vibrating Electrode Technique) and microamperometry have been used to investigate Mg corrosion processes in various corrosion environments including the magnesium/cellular interface.
The project was structured in five work packages, with a progressive character. The objective of the first work package was to secure the existence of measuring sensors in anticipation of the possible limitations imposed by the corrosion of magnesium, electrolyte media and interaction with cells and cellular metabolic products. Boron doped diamond (BDD) microelectrodes were developed, based on previously existing knowledge, as possible alternatives to liquid membrane microcapillary electrodes for pH measurements. An alternative version of such BDD microelectrodes can work as replacement for platinum microelectrodes as dissolved oxygen sensors. This work package was developed mainly at the partner institution, the University of Aveiro.
The remaining tasks of each work package were developed at the host institution, the Helmholtz-Zentrum Geesthacht.
The second work package was set to clarify aspects of localized corrosion of pure magnesium. Two main topics of interest were developed in this work package: the influence of dissolved oxygen on the corrosion of magnesium and characterization of microgalvanic corrosion caused by transition metal impurities. Two scientific publications resulted from this work package, one which is already published under open access. The second one is undergoing final internal revision and is to be submitted soon.
Additionally, pure magnesium was characterized in terms of pH mapping under immersion in electrolytes with increasing complexity towards real physiological conditions: NaC < SBF < α-MEM < DMEM. This task from WP2 was used as a starting point for characterization of four selected magnesium alloys in WP3: Mg0.51Ca Mg10Gd, Mg0.2Ca0.5Zn Mg0.2Ca4Zn. The alloys were characterized in similar media as pure magnesium in WP2. The corrosion products of all materials were analysed by scanning electron microscopy and energy dispersive X-ray spectroscopy in order to establish a possible correlation between immersion conditions and influence of alloying elements.
WP4 was designed to gather information about tribocorrosion on magnesium. The measurements would consist on assessing localized corrosion on the friction zone of Mg-alloys, using a pin-on-disc setup with a microelectrode coupled to the rider, at different distances from the contact zone, in NaCl, PBS and SBF. Despite the success of the remaining work packages, WP4 was not executed due to technical reasons, as there were difficulties on setting an exact distance between the microelectrodes and the pin-on-disc rotating setup.
The last work package concerned the characterization of magnesium corrosion under cell culture conditions. As a model system brain tumour cells were cultured in FBS supplemented DMEM with pure magnesium as a substrate. Local pH measurements were conducted in DMEM. The choice for tumour cells derived from the expected establishment of an acidic extra cellular environment. The high pH created by corroding magnesium allowed a more straightforward and conclusive interpretation of the collected data. Besides pH mapping of the near-surface of cell covered magnesium substrates, linear pH gradients were characterized along the Z axis, from the bulk electrolyte, across the cell layer until the magnesium surface. The latter set of experiments proved very important in the assessment of the interface between magnesium and the cell layer. The characterization of this region is vital for understanding the evolution of corrosion rate over time, as well as determining cell viability conditions.
The project benefited by innovative approaches, challenging some ideas that have been rooted in the magnesium community for many years. An example of such was the demonstration of oxygen reduction during magnesium corrosion. Another example is the reinforcement of a theory developed by colleagues from the HZG, which is growing in acceptance, regarding the redeposition of heavy metal impurities as a corrosion enhancer for magnesium. During MAGPLANT, this theory was explored by studying the detrimental effect and mechanisms of nickel in magnesium. WP5 was also innovative in demonstrating how SIET (Scanning Ion-selective Electrode Technique) can be used to study the interface between magnesium and cells, which is most critical for understanding the degradation mechanism and for future alloy design.