In vitro and in vivo corrosion of the novel magnesium alloy Mg–La–Nd–Zr: influence of the measurement technique and in vivo implant location

Multi-modal investigation of the bone micro- and ultrastructure, and elemental distribution in the presence of Mg-xGd screws at mid-term healing stages

Magnesium (Mg) - based alloys are becoming attractive materials for medical applications as temporary bone implants for support of fracture healing, e.g. as a suture anchor. Due to their mechanical properties and biocompatibility, they may replace titanium or stainless-steel implants, commonly used in orthopedic field. Nevertheless, patient safety has to be assured by finding a long-term balance between metal degradation, osseointegration, bone ultrastructure adaptation and element distribution in organs. In order to determine the implant behavior and its influence on bone and tissues, we investigated two Mg alloys with gadolinium contents of 5 and 10 wt percent in comparison to permanent materials titanium and polyether ether ketone. The implants were present in rat tibia for 10, 20 and 32 weeks before sacrifice of the animal. Synchrotron radiation-based micro computed tomography enables the distinction of features like residual metal, degradation layer and bone structure. Additionally, X-ray diffraction and X-ray fluorescence yield information on parameters describing the bone ultrastructure and elemental composition at the bone-to-implant interface. Finally, with element specific mass spectrometry, the elements and their accumulation in the main organs and tissues are traced. The results show that Mg-xGd implants degrade in vivo under the formation of a stable degradation layer with bone remodeling similar to that of Ti after 10 weeks. No accumulation of Mg and Gd was observed in selected organs, except for the interfacial bone after 8 months of healing. Thus, we confirm that Mg-5Gd and Mg-10Gd are suitable material choices for bone implants.

Localized drug delivery graphene bioscaffolds for cotransplantation of islets and mesenchymal stem cells

In the present work, we developed, characterized, and tested an implantable graphene bioscaffold which elutes dexamethasone (Dex) that can accommodate islets and adipose tissue–derived mesenchymal stem cells (AD-MSCs). In vitro studies demonstrated that islets in graphene–0.5 w/v% Dex bioscaffolds had a substantial higher viability and function compared to islets in graphene-alone bioscaffolds or islets cultured alone (P < 0.05). In vivo studies, in which bioscaffolds were transplanted into the epididymal fat pad of diabetic mice, demonstrated that, when islet:AD-MSC units were seeded into graphene–0.5 w/v% Dex bioscaffolds, this resulted in complete restoration of glycemic control immediately after transplantation with these islets also showing a faster response to glucose challenges (P < 0.05). Hence, this combination approach of using a graphene bioscaffold that can be functionalized for local delivery of Dex into the surrounding microenvironment, together with AD-MSC therapy, can significantly improve the survival and function of transplanted islets.

Biodegradation ZK50 magnesium alloy compression screws: Mechanical properties, biodegradable characteristics and implant test

BACKGROUND

Magnesium alloy implants have lower stress load and can be absorbed gradually, but their degradation rates are too fast generally. A magnesium alloy contained 5% Zn and 0.5% Zr (ZK50) which have lower degradation rate are designed to be applied to cannulated bone screw.

METHODS

An oxidation heat treatment of 380 °C for 2 h proceeds to modify the ZK50 Mg alloy (ZK50-H). The microstructure observation, degradation tests and Biocompatibility analysis are proceeded between ZK50 and ZK50-H. Finally, a mini-pig implantation test is proceeded to provide a reference of implant application for future pre-clinical evaluation.

RESULTS

The heat treatment can improve the mechanical properties. A passive ceramic layer formed after simulated body fluid (SBF) solution immersion can restrict the degradation effectively. The cytotoxicity test shows the initial biosafety of ZK50 Mg alloy. A mini-pig implantation test of bone screw has proceeded to confirm the advanced biocompatibility. The ZK50-H screws can maintain enough support at least 8 weeks which the fracture of bone can get curing. The excellent osteoinduction of ZK50-H has a positive effect to growth of new bones and help the mini-pig regain heal faster in 12 weeks.

CONCLUSION

This study shows ZK50-H Mg alloy screw is a feasible degradation implant and can be carried out the next-step clinical experiments.

Bio-inspired biomaterial Mg-Zn-Ca: a review of the main mechanical and biological properties of Mg-based alloys

The toxicity of alloying elements in magnesium alloys used for biomedical purposes is an interesting and innovative subject, due to the great technological advances that would result from their application in medical devices (MDs) in traumatology. Recently promising results have been published regarding the rates of degradation and mechanical integrity that can support Mg alloys; this has led to an interest in understanding the toxicological features of these emerging biomaterials. The growing interest of different segments of the MD market has increased the determination of different research groups to clarify the behavior of alloying elements in vivo. This review covers the influence of the alloying elements on the body, the toxicity of the elements in Mg-Zn-Ca, as well as the mechanical properties, degradation, processes of obtaining the alloy, medical approaches and future perspectives on the use of the Mg in the manufacture of MDs for various medical applications.

A Collagen Based Cryogel Bioscaffold that Generates Oxygen for Islet Transplantation

The aim of this work was to develop, characterize and test a novel 3D bioscaffold matrix which can accommodate pancreatic islets and provide them with a continuous, controlled and steady source of oxygen to prevent hypoxia-induced damage following transplantation. Hence, we made a collagen based cryogel bioscaffold which incorporated calcium peroxide (CPO) into its matrix. The optimal concentration of CPO integrated into bioscaffolds was 0.25wt.% and this generated oxygen at 0.21±0.02mM/day (day 1), 0.19±0.01mM/day (day 6), 0.13±0.03mM/day (day 14), and 0.14±0.02mM/day (day 21). Accordingly, islets seeded into cryogel-CPO bioscaffolds had a significantly higher viability and function compared to islets seeded into cryogel alone bioscaffolds or islets cultured alone on traditional cell culture plates; these findings were supported by data from quantitative computational modelling. When syngeneic islets were transplanted into the epididymal fat pad (EFP) of diabetic mice, our cryogel-0.25wt.%CPO bioscaffold improved islet function with diabetic animals re-establishing glycemic control. Mice transplanted with cryogel-0.25wt.%CPO bioscaffolds showed faster responses to intraperitoneal glucose injections and had a higher level of insulin content in their EFP compared to those transplanted with islets alone (P<0.05). Biodegradability studies predicted that our cryogel-CPO bioscaffolds will have long-lasting biostability for approximately 5 years (biodegradation rate: 16.00±0.65%/year). Long term implantation studies (i.e. 6 months) showed that our cryogel-CPO bioscaffold is biocompatible and integrated into the surrounding fat tissue with minimal adverse tissue reaction; this was further supported by no change in blood parameters (i.e. electrolyte, metabolic, chemistry and liver panels). Our novel oxygen-generating bioscaffold (i.e. cryogel-0.25wt.%CPO) therefore provides a biostable and biocompatible 3D microenvironment for islets which can facilitate islet survival and function at extra-hepatic sites of transplantation.

Formation mechanism, degradation behavior, and cytocompatibility of a double-layered structural MAO/rGO-CaP coating on AZ31 Mg

Microarc oxidation coated magnesium attracts increasing attention recently, owing to its excellent anti-corrosion and wear-resistance properties. However, some drawbacks like micropores on the MAO surface reduce the corrosion resistance of the coatings, which requires post treatment. In the present work, a specific double layered structural MAO/rGO-CaP coating was produced to seal the micropores on the MAO coating and further enhance the corrosion resistance. The structure, cytocompatibility, electrochemical properties, and long-term corrosion behavior of the composite coatings were investigated. XRD results show that the coatings are mainly composed of CaHPO₄ (DCP) and Ca₅(PO₄)₃OH (HA). Cytocompatibility evaluation indicates that the rGO in the coating shows no cytotoxicity. Corrosion potential of the bottom MAO coating is enhanced significantly by the rGO-CaP top coatings from -1.58 V to -1.02 V. Long term soaking test reveals that a longer chemical stable coating was produced. The results suggest a potential application of the MAO/rGO-CaP coating in practice.

Biocompatibility of bioabsorbable Mg-Ca alloys with rare earth elements addition

The objectives were to investigate the mechanical strength and biocompatibility of Mg2Ca2Gd and Mg1Ca2Nd (wt%) alloys developed for biomedical application as implantable bioabsorbable devices. Samples were implanted in New-Zealand rabbits tibia for 3, 6 and 8 weeks and compatibility analysis involved whole blood test, biochemistry, histopathology, histology, and radiographs. Refinement in grains were observed in Mg₂Ca₂Gd alloy; and Mg₅Gd, Mg₄₁Nd₅, α-Mg and Mg₂Ca phases were identified. Polarization curves revealed easier oxidation of Mg2Ca2Gd alloy, smaller values of corrosion rate and a higher polarization resistance of Mg1Ca2Nd. Adequate compatibility of both alloys was identified with pre-osteoblast stem cells. Red and white cells stayed compatible with reference ranges. Enzymes from liver and kidneys stayed at regular values and samples from kidneys and liver tissues presented similar organization to control animals. Histological displays from implantation sites disclosed well-structured tissues with evidences of bone cells activities compatible with the new bone tissues observed. Radiographs from tibias did not revealed relevant gas pockets. Mg2Ca2Gd alloy demonstrated faster degradation. Adequate biocompatibility was observed in Mg-Ca alloys with RE addition, being potential candidates for development of metallic implantable bioabsorbable devices.

In Vivo Simulation of Magnesium Degradability Using a New Fluid Dynamic Bench Testing Approach

The degradation rate of magnesium (Mg) alloys is a key parameter to develop Mg-based biomaterials and ensure in vivo-mechanical stability as well as to minimize hydrogen gas production, which otherwise can lead to adverse effects in clinical applications. However, in vitro and in vivo results of the same material often differ largely. In the present study, a dynamic test bench with several single bioreactor cells was constructed to measure the volume of hydrogen gas which evolves during magnesium degradation to indicate the degradation rate in vivo. Degradation medium comparable with human blood plasma was used to simulate body fluids. The media was pumped through the different bioreactor cells under a constant flow rate and 37 °C to simulate physiological conditions. A total of three different Mg groups were successively tested: Mg WE43, and two different WE43 plasma electrolytically oxidized (PEO) variants. The results were compared with other methods to detect magnesium degradation (pH, potentiodynamic polarization (PDP), cytocompatibility, SEM (scanning electron microscopy)). The non-ceramized specimens showed the highest degradation rates and vast standard deviations. In contrast, the two PEO samples demonstrated reduced degradation rates with diminished standard deviation. The pH values showed above-average constant levels between 7.4–7.7, likely due to the constant exchange of the fluids. SEM revealed severe cracks on the surface of WE43 after degradation, whereas the ceramized surfaces showed significantly decreased signs of corrosion. PDP results confirmed the improved corrosion resistance of both PEO samples. While WE43 showed slight toxicity in vitro, satisfactory cytocompatibility was achieved for the PEO test samples. In summary, the dynamic test bench constructed in this study enables reliable and simple measurement of Mg degradation to simulate the in vivo environment. Furthermore, PEO treatment of magnesium is a promising method to adjust magnesium degradation.

Assessment of magnesium-based biomaterials: from bench to clinic

This review presents the operation procedures of commonly used standard methods for assessment of Mg-based biomaterials from bench to clinic.

Monitoring and Assessing the Degradation Rate of Magnesium-Based Artificial Bone In Vitro Using a Wireless Magnetoelastic Sensor

A magnetoelastic-based (MB) sensor was employed as a novel method to monitor and assess the degradation rate of magnesium-based artificial bone (MBAB) in vitro, which can be used as an implant to repair a bone defect, providing a quantitative method to depict the degradation rate of MBAB. MBABs were fabricated by the Pro/Engineering software and a precision machine tool using high-purity (HP) magnesium. The MB sensor was embedded in the neutral surface of MBAB by an unharmful quick adhesive, forming the MB sensor-embedded MBAB (EMBAB). The modified simulated body fluid (MSBF) media (PH = 7.4), mimicking the human internal environment, and the NaOH media (PH = 12), accelerating EMBAB’s degradation, were used to immerse the EMBAB for 15 days at 37 °C. The EMBAB was then tested daily on a self-developed experimental platform to monitor the relative output power under a 100 N external force. The results showed that the relative output power of the sensing coil gradually increased with the EMBAB’s degradation. The degradation rate of the EMBAB could be calculated on the basis of the changes of the relative output power caused by the MB sensor and of the degradation time. With the EMBAB’s degradation, an increasing strain directly worked on the MB sensor, significantly changing the value of the relative output power, which means that the EMBAB was characterized by a quick degradation rate. During the 15 days of the experiment, the degradation rates on the 7th and 15th days were 0.005 dbm/day and 0.02 dbm/day, and 0.02 dbm/day and 0.04 dbm/day in MSBF and alkaline media, respectively. Therefore, the MB sensor provides a wireless and passive method to monitor and assess the degradation rate of bone implants in vitro.

Silane coatings of metallic biomaterials for biomedical implants: A preliminary review

In response to increased attention in literature, this work provides a qualitative review surrounding the application of silane-based coatings of metallic biomaterials for biomedical implants. Included herein is both a brief summary of existing knowledge and concepts regarding silane-based thin films, along with an analysis of recent peer-reviewed publications and advances towards their practical application for biomedical coatings. Specifically, the review identifies innovative silane-based coatings according to their molecular identity and film structure and analyses their impact on the biocorrosion resistance, protein adsorption, cell viability, and antimicrobial properties of the overall coated implant. It is shown that a range of common silanes clearly exhibit promising properties for biomedical implant coatings, but further work is needed, particularly on mechanisms of physiological interaction and characteristic effects of silane functional groups, before seeing clinical use. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 106B: 2901-2918, 2018.

In vivo quantification of hydrogen gas concentration in bone marrow surrounding magnesium fracture fixation hardware using an electrochemical hydrogen gas sensor

Magnesium (Mg) medical devices are currently being marketed for orthopedic applications and have a complex degradation process which includes the evolution of hydrogen gas (H₂). The effect of H₂ exposure on relevant cell types has not been studied; and the concentration surrounding degrading Mg devices has not been quantified to enable such mechanistic studies. A simple and effective method to measure the concentration of H₂ in varying microenvironments surrounding Mg implants is the first step to understanding the biological impact of H₂ on these cells. Here, the in vivo measurement of H₂ surrounding fracture fixation devices implanted in vivo is demonstrated. An electrochemical H₂ microsensor detected increased levels of H₂ at three anatomical sites with a response time of about 30 s. The sensor showed the H₂ concentration in the bone marrow at 1 week post-implantation (1460 ± 320 µM) to be much higher than measured in the subcutaneous tissue (550 ± 210 µM) and at the skin surface (120 ± 50 µM). Additionally, the H₂ concentrations measured in the bone marrow exceeded the concentration in a H₂ saturated water solution (∼800 µM). These results suggest that H₂ emanating from Mg implants in bone during degradation pass through the bone marrow and become at least partially trapped because of slow permeation through the bone. This study is the first to identify H₂ concentrations in the bone marrow environment and will enable in vitro experiments to be executed at clinically relevant H₂ concentrations to explore possible biological effects of H₂ exposure.

STATEMENT OF SIGNIFICANCE

An electrochemical H₂ sensor was used to monitor the degradation of a Mg fracture fixation system in a lapine ulna fracture model. Interestingly, the H₂ concentration in the bone marrow is 82% higher than H₂ saturated water solution. This suggests H₂ generated in situ is trapped in the bone marrow and bone is less permeable than the surrounding tissues. The detectable H₂ at the rabbit skin also demonstrates a H₂ sensor's ability to monitor the degradation process under thin layers of tissue. H₂ sensing shows promise as a tool for monitoring the degradation of Mg alloy in vivo and creating in vitro test beds to more mechanistically evaluate the effects of varying H₂ concentrations on cell types relevant to osteogenesis.

The Effects of Static and Dynamic Loading on Biodegradable Magnesium Pins In Vitro and In Vivo

Here we systematically assess the degradation of biodegradable magnesium pins (as-drawn pure Mg, as-cast Mg-Zn-Mn, and extruded Mg-Zn-Mn) in a bioreactor applying cyclical loading and simulated body fluid (SBF) perfusion. Cyclical mechanical loading and interstitial flow accelerated the overall corrosion rate, leading to loss of mechanical strength. When compared to the in vivo degradation (degradation rate, product formation, uniform or localized pitting, and stress distribution) of the same materials in mouse subcutaneous and dog tibia implant models, we demonstrate that the in vitro model facilitates the analysis of the complex degradation behavior of Mg-based alloys in vivo. This study progresses the development of a suitable in vitro model to examine the effects of mechanical stress and interstitial flow on biodegradable implant materials.

Magnesium alloys: A stony pathway from intensive research to clinical reality. Different test methods and approval-related considerations

The first degradable implant made of a magnesium alloy, a compression screw, was launched to the clinical market in March 2013. Many different complex considerations are required for the marketing authorization of degradable implant materials. This review gives an overview of existing and proposed standards for implant testing for marketing approval. Furthermore, different common in vitro and in vivo testing methods are discussed. In some cases, animal tests are inevitable to investigate the biological safety of a novel medical material. The choice of an appropriate animal model is as important as subsequent histological examination. Furthermore, this review focuses on the results of various mechanical tests to investigate the stability of implants for temporary use. All the above aspects are examined in the context of development and testing of magnesium-based biomaterials and their progress them from bench to bedside. A brief history of the first market launch of a magnesium-based degradable implant is given. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 105A: 329-347, 2017.