Coatings for biodegradable magnesium-based supports for therapy of vascular disease: A general view

A CO-releasing coating based on carboxymethyl chitosan-functionalized graphene oxide for improving the anticorrosion and biocompatibility of magnesium alloy stent materials

Due to its biofunctions similar to NO, the CO gas signaling molecule has gradually shown great potential in cardiovascular biomaterials for regulating the in vivo performances after the implantation and has received increasing attention. To construct a bioactive surface with CO-releasing properties on the surface of magnesium-based alloy to augment the anticorrosion and biocompatibility, graphene oxide (GO) was firstly modified using carboxymethyl chitosan (CS), and then CO-releasing molecules (CORM401) were introduced to synthesize a novel biocompatible nanomaterial (GOCS-CO) that can release CO in the physiological environments. The GOCS-CO was further immobilized on the magnesium alloy surface modified by polydopamine coating with Zn2+ (PDA/Zn) to create a bioactive surface capable of releasing CO in the physiological environment. The outcomes showed that the CO-releasing coating can not only significantly enhance the anticorrosion and abate the corrosion degradation rate of the magnesium alloy in a simulated physiological environment, but also endow it with good hydrophilicity and a certain ability to adsorb albumin selectively. Owing to the significant enhancement of anticorrosion and hydrophilicity, coupled with the bioactivity of GOCS, the modified sample not only showed excellent ability to prevent platelet adhesion and activation and reduce hemolysis rate but also can promote endothelial cell (EC) adhesion, proliferation as well as the expression of nitric oxide (NO) and vascular endothelial growth factor (VEGF). In the case of CO release, the hemocompatibility and EC growth behaviors were further significantly improved, suggesting that CO molecules released from the surface can significantly improve the hemocompatibility and EC growth. Consequently, the present study provides a novel surface modification method that can simultaneously augment the anticorrosion and biocompatibility of magnesium-based alloys, which will strongly promote the research and application of CO-releasing bioactive coatings for surface functionalization of cardiovascular biomaterials and devices.

Biological response of degradation products of PEO-modified magnesium on vascular tissue cells, hemocompatibility and its influence on the inflammatory response

Cardiovascular stenting is the most widely used therapy to treat coronary artery disease caused by partial or total obstruction of the artery due to atherosclerotic plaque formation, with potentially fatal effects. There are different types of stents: bare metal stents, drug-eluting stents, bioabsorbable stents and dual therapy stents. However, they can lead to long-term complications, such as in-stent restenosis and late thrombosis. To reduce these adverse effects, research has focused on biodegradable metallic stents, since they retain the mechanical properties necessary to contain the injured artery while it is being repaired and, once their function has been fulfilled, the stent degrades without altering the system or compromising the patient's health. In this work we have evaluated the biological response of the degradation products of a bare Mg based biomaterial surface-modified by the plasma electrolytic oxidation (PEO) method on vascular tissue cells, hemocompatibility and inflammatory response. The results obtained are compatible with a biosafe material for future use as a cardiovascular implant, but it is necessary to continue with in vivo and mechanical properties tests to ensure and guarantee its use. SIGNIFICANCE STATEMENT: The development of fully bioresorbable stents is a promising alternative for the management of coronary artery disease without causing long-term problems at the implantation site. In this work, the hematological and immunological biocompatibility of bare Mg modified superficially by plasma electrolytic oxidation (PEO-Mg) was evaluated by in vitro and ex vivo assays. PEO-Mg was found to be compatible with blood and immune components surrounding the implantation site with no signs of toxicity to endothelial cells, macrophages, and arterial tissue. In addition, degradation products of PEO-Mg are eliminated by phagocytosis. However, an in-depth study of the physical and mechanical properties and in vivo biocompatibility must be carried out for its future use as a biomedical implant.

Adaptation of Vascular Smooth Muscle Cell to Degradable Metal Stent Implantation

Iron-, magnesium-, or zinc-based metal vessel stents support vessel expansion at the period early after implantation and degrade away after vascular reconstruction, eliminating the side effects due to the long stay of stent implants in the body and the risks of restenosis and neoatherosclerosis. However, emerging evidence has indicated that their degradation alters the vascular microenvironment and induces adaptive responses of surrounding vessel cells, especially vascular smooth muscle cells (VSMCs). VSMCs are highly flexible cells that actively alter their phenotype in response to the stenting, similarly to what they do during all stages of atherosclerosis pathology, which significantly influences stent performance. This Review discusses how biodegradable metal stents modify vascular conditions and how VSMCs respond to various chemical, biological, and physical signals attributable to stent implantation. The focus is placed on the phenotypic adaptation of VSMCs and the clinical complications, which highlight the importance of VSMC transformation in future stent design.

Progress in bioactive surface coatings on biodegradable Mg alloys: A critical review towards clinical translation

Mg and its alloys evince strong candidature for biodegradable bone implants, cardiovascular stents, and wound closing devices. However, their rapid degradation rate causes premature implant failure, constraining clinical applications. Bio-functional surface coatings have emerged as the most competent strategy to fulfill the diverse clinical requirements, besides yielding effective corrosion resistance. This article reviews the progress of biodegradable and advanced surface coatings on Mg alloys investigated in recent years, aiming to build up a comprehensive knowledge framework of coating techniques, processing parameters, performance measures in terms of corrosion resistance, adhesion strength, and biocompatibility. Recently developed conversion and deposition type surface coatings are thoroughly discussed by reporting their essential therapeutic responses like osteogenesis, angiogenesis, cytocompatibility, hemocompatibility, anti-bacterial, and controlled drug release towards in-vitro and in-vivo study models. The challenges associated with metallic, ceramic and polymeric coatings along with merits and demerits of various coatings have been illustrated. The use of multilayered hybrid coating comprising a unique combination of organic and inorganic components has been emphasized with future perspectives to obtain diverse bio-functionalities in a facile single coating system for orthopedic implant applications.

Assessment of the Mechanical and Corrosion Properties of Mg-1Zn-0.6Ca/Diamond Nanocomposites for Biomedical Applications

In this work, the microstructure, mechanical properties, and corrosion behavior of the Mg-1Zn-0.6Ca matrix alloy (ZX10), reinforced by adding various amounts of nanodiamond particles (0.5, 1, and 2 wt.%), prepared by the ultrasound-assisted stir-casting method, were investigated as they are deemed as potential implant materials in biomedical applications. Microstructure, nanoindentation, mechanical tensile, immersion, and potentiodynamic polarization tests were performed for evaluating the influence of the addition of nanodiamond particles on the alloy's mechanical and biocorrosion properties. The results revealed that the addition of nanodiamond particles causes a reduction in the alloy's grain size. The alloy's nanohardness and elastic modulus values increased when the amount of added nanodiamond particles were increased. The nanocomposite with an addition of 0.5% ND showed the best composition with regard to an acceptable corrosion rate as the corrosion rates are too high with higher additions of 1 or 2% NDs. At the same time, the yield strength, tensile strength, and elongation improved slightly compared to the matrix alloy.

Advancements in the Additive Manufacturing of Magnesium and Aluminum Alloys through Laser-Based Approach

Complex structures can now be manufactured easily utilizing AM technologies to meet the pre-requisite objectives such as reduced part numbers, greater functionality, and lightweight, among others. Polymers, metals, and ceramics are the few materials that can be used in AM technology, but metallic materials (Magnesium and Aluminum) are attracting more attention from the research and industrial point of view. Understanding the role processing parameters of laser-based additive manufacturing is critical to maximize the usage of material in forming the product geometry. LPBF (Laser powder-based fusion) method is regarded as a potent and effective additive manufacturing technique for creating intricate 3D forms/parts with high levels of precision and reproducibility together with acceptable metallurgical characteristics. While dealing with LBPF, some degree of porosity is acceptable because it is unavoidable; hot ripping and cracking must be avoided, though. The necessary manufacturing of pre-alloyed powder and ductility remains to be the primary concern while dealing with a laser-based additive manufacturing approach. The presence of the Al-Si eutectic phase in AlSi10Mg and AlSi12 alloy attributing to excellent castability and low shrinkage, attaining the most attention in the laser-based approach. Related studies with these alloys along with precipitation hardening and heat treatment processing were discussed. The Pure Mg, Mg-Al alloy, Mg-RE alloy, and Mg-Zn alloy along with the mechanical characteristics, electrochemical durability, and biocompatibility of Mg-based material have been elaborated in the work-study. The review article also summarizes the processing parameters of the additive manufacturing powder-based approach relating to different Mg-based alloys. For future aspects, the optimization of processing parameters, composition of the alloy, and quality of powder material used will significantly improve the ductility of additively manufactured Mg alloy by the LPBF approach. Other than that, the recycling of Mg-alloy powder hasn't been investigated yet. Meanwhile, the post-processing approach, including a homogeneous coating on the porous scaffolds, will mark the suitability in terms of future advancements in Mg and Al-based alloys.

Development of Innovative Biomaterials and Devices for the Treatment of Cardiovascular Diseases

Cardiovascular diseases have become the leading cause of death worldwide. The increasing burden of cardiovascular diseases has become a major public health problem and how to carry out efficient and reliable treatment of cardiovascular diseases has become an urgent global problem to be solved. Recently, implantable biomaterials and devices, especially minimally invasive interventional ones, such as vascular stents, artificial heart valves, bioprosthetic cardiac occluders, artificial graft cardiac patches, atrial shunts, and injectable hydrogels against heart failure, have become the most effective means in the treatment of cardiovascular diseases. Herein, an overview of the challenges and research frontier of innovative biomaterials and devices for the treatment of cardiovascular diseases is provided, and their future development directions are discussed.

Gallium containing bioactive materials: A review of anticancer, antibacterial, and osteogenic properties

The incorporation of gallium into bioactive materials has been reported to enhance osteogenesis, to influence blood clotting, and to induce anti-cancer and anti-bacterial activity. Gallium-doped biomaterials prepared by various techniques include melt-derived and sol-gel-derived bioactive glasses, calcium phosphate bioceramics, metals and coatings. In this review, we summarize the recently reported developments in antibacterial, anticancer, osteogenesis, and hemostasis properties of Ga-doped biomaterials and briefly outline the mechanisms leading to Ga biological effects. The key finding is that gallium addition to biomaterials has great potential for treating bone-related diseases since it can be efficiently transferred to the desired region at a controllable rate. Besides, it can be used as a potential substitute for antibiotics for the inhibition of infections during the initial and advanced phases of the wound healing process. Ga is also used as an anticancer agent due to the increased concentration of gallium around excessive cell proliferation (tumor) sites. Moreover, we highlight the possibility to design different therapeutic approaches aimed at increasing the efficiency of the use of gallium containing bioactive materials for multifunctional applications.

pH Stimuli-Responsive, Rapidly Self-healable Coatings Enhanced the Corrosion Resistance and Osteogenic Differentiation of Mg-1Ca Osteoimplant

Mg-Ca alloys have emerged as a promising research direction for biomedical implants in the orthopedic field. However, their clinical use is deterred by their fast corrosion rate. In this work, a pH stimuli-responsive silk-halloysite (HNT)/phytic acid (PA) self-healing coating (Silk-HNT/PA) is constructed to slow down the corrosion rate of Mg-1Ca alloy and its cell viability and osteogenic differentiation ability are enhanced. The Silk-HNT/PA coating exhibits appealing active corrosion protection, by eliciting pH-triggerable self-healing effects, while simultaneously affording superior biocompatibility and osteogenic differentiation ability. Moreover, in vivo studies by histological analysis also demonstrate better osseointegration for the Silk-HNT/PA coated Mg-1Ca alloy. In summary, the Silk-HNT/PA coating in the present study has great potential in enhancing the biomedical utility of Mg alloys.

Magnesium for Implants: A Review on the Effect of Alloying Elements on Biocompatibility and Properties

An attempt is made to cover the whole of the topic of biodegradable magnesium (Mg) alloys with a focus on the biocompatibility of the individual alloying elements, as well as shed light on the degradation characteristics, microstructure, and mechanical properties of most binary alloys. Some of the various work processes carried out by researchers to achieve the alloys and their surface modifications have been highlighted. Additionally, a brief look into the literature on magnesium composites as also been included towards the end, to provide a more complete picture of the topic. In most cases, the chronological order of events has not been particularly followed, and instead, this work is concentrated on compiling and presenting an update of the work carried out on the topic of biodegradable magnesium alloys from the recent literature available to us.

A review on magnesium alloys for biomedical applications

Magnesium (Mg) and Mg alloys are considered as potential candidates for biomedical applications because of their high specific strength, low density, and elastic modulus, degradability, good biocompatibility and biomechanical compatibility. However, the rapid corrosion rate of Mg alloys results in premature loss of mechanical integrity, limiting their clinical application in load-bearing parts. Besides, the low strength of Mg alloys restricts their further application. Thus, it is essential to understand the characteristics and influencing factors of mechanical and corrosion behavior, as well as the methods to improve the mechanical performances and corrosion resistance of Mg alloys. This paper reviews the recent progress in elucidating the corrosion mechanism, optimizing the composition, and microstructure, enhancing the mechanical performances, and controlling the degradation rate of Mg alloys. In particular, the research progress of surface modification technology of Mg alloys is emphasized. Finally, the development direction of biomedical Mg alloys in the future is prospected.

Effect of Pre-Anodized Film on Micro-Arc Oxidation Process of 6063 Aluminum Alloy

In the current investigation, micro-arc oxidation (MAO) ceramic coatings on aluminum are galvanostatically synthesized at various processing stages in an alkaline silicate system. The resultant coatings are systematically investigated in terms of the following respects: The working voltage and surface sparking evolution over the studied course of MAO are recorded by the signal acquisition system and the real-time imaging, respectively; the phase composition, the surface morphology, and the polished cross-section of the coatings are characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) assisted with an energy-dispersive X-ray spectrometer (EDS), respectively. In particular, with the help of a low-rate increase in working voltage, the evolution of the sparks, the energy consumption, and the microstructure development of aluminum in alkaline silicate electrolyte by pre-anodizing are systematically investigated. The results show that the pre-anodized film can accelerate the evolution process of MAO spark and shorten the reaction process in the early stage of MAO reaction, reducing energy consumption and improving the corrosion resistance of the MAO coating. The γ-Al2O3 phase content after pre-anodized is significantly increased in MAO coatings. In particular, the thicker the pre-anodized film (beyond 8 μm) was broken down and fragmentation thinning in the early stage of the MAO process with the presence of micro discharges. This is due to the fact that the electron transition will be released by the emission of radiative recombination and reveals obvious galvanoluminescence (GL) behavior on the surface of the pre-anodized film. Further, based on the present MAO coating microstructure, a model of coating growth after pre-anodized that evolves over time is proposed.

Topography Control of Micro-Nanosized Anatase Coating on Magnesium Alloy

Constructing surface topographies in the micro- or nanometer range is an effective way to improve the biocompatibility of biomaterials. For the present work, anatase coatings with controllable micro/nanoscale characteristics were successfully prepared on an MgZn alloy surface via solvothermal route, and their formation mechanisms are discussed. The features of the as-prepared coatings were characterized using a scanning electron microscope (SEM), a transmission electron microscope (TEM), an atomic force microscope (AFM), X-ray diffraction (XRD), and a contact angle goniometer. The corrosion behavior of the coatings was also evaluated by testing the open circuit potential (OCP) in SBF (Simulated Body Fluid). The results show that a gradual variation of the anatase coating morphologies was obtained through adjusting the solvothermal reaction conditions. With the increase of NH4F concentration in the solution, the cross-combined anatase nanosheets became more dispersed. The micro/nanostructured anatase coatings provide the MgZn alloy with good corrosion resistance, which increased with the density of anatase nanosheets in the coatings. In addition, the coatings exhibit the inhibition of platelet aggregation, and the micro/nano structures can also adsorb endothelial cells.

Influence of Femtosecond Laser Modification on Biomechanical and Biofunctional Behavior of Porous Titanium Substrates

Bone resorption and inadequate osseointegration are considered the main problems of titanium implants. In this investigation, the texture and surface roughness of porous titanium samples obtained by the space holder technique were modified with a femtosecond Yb-doped fiber laser. Different percentages of porosity (30, 40, 50, and 60 vol.%) and particle range size (100–200 and 355–500 μm) were compared with fully-dense samples obtained by conventional powder metallurgy. After femtosecond laser treatment the formation of a rough surface with micro-columns and micro-holes occurred for all the studied substrates. The surface was covered by ripples over the micro-metric structures. This work evaluates both the influence of the macro-pores inherent to the spacer particles, as well as the micro-columns and the texture generated with the laser, on the wettability of the surface, the cell behavior (adhesion and proliferation of osteoblasts), micro-hardness (instrumented micro-indentation test, P–h curves) and scratch resistance. The titanium sample with 30 vol.% and a pore range size of 100–200 μm was the best candidate for the replacement of small damaged cortical bone tissues, based on its better biomechanical (stiffness and yield strength) and biofunctional balance (bone in-growth and in vitro osseointegration).

Zinc-Doping Induces Evolution of Biocompatible Strontium-Calcium-Phosphate Conversion Coating on Titanium to Improve Antibacterial Property

Implant-associated infections (IAI) remains a common and devastating complication in orthopedic surgery. To reduce the incidence of IAI, implants with intrinsic antibacterial activity have been proposed. The surface functionalization and structure optimization of metallic implants can be achieved by surface modification using the phosphate chemical conversion (PCC) technique. Zinc (Zn) has strong antibacterial behavior toward a broad-spectrum of bacteria. Herein, Zn was incorporated into strontium-calcium-phosphate (SrCaP) coatings on titanium (Ti) via PCC method, and the influence of its doping amount on the phase, microstructure, antibacterial activity, and biocompatibility of the composite coating was researched. The results indicated that traces of Zn doping produced grain refinement of SrCaP coating with no significant effect on its phase and surface properties, while a higher Zn content induced its phase and microstructure transformed into zinc-strontium-phosphate (SrZn₂(PO₄)₂). SrCaP-Zn1 and SrCaP-Zn4 represented trace and high content Zn-doped coatings, respectively, which exhibited a similar bacterial attachment for a short time but showed inhibition of biofilm formation after continuous incubation up to 24 h. The killing rates of SrCaP-Zn1 coating for Staphylococcus aureus (S. aureus) and Escherichia coli (E. coli) reached 61.25% and 55.38%, respectively. While that data increased to 83.01% and 71.28% on SrCaP-Zn4 coating due to the more-releasing Zn²⁺. Furthermore, in vitro culture of MC3T3-E1 cells proved that the Zn-doped coatings also possessed excellent biocompatibility. This study provides a new perception for the phase and microstructural optimization of phosphate coatings on implant surfaces, as well as fabricating promising coatings with excellent biocompatibility and antimicrobial properties against IAI.

In vivo comparison of the degradation and osteointegration properties of micro-arc oxidation-coated Mg-Sr and Mg-Ca alloy scaffolds

BACKGROUND

Magnesium (Mg) alloy have biodegradation and mechanical properties that are similar to those of human bone, making it a promising candidate material for inclusion in implantable medical devices.

OBJECTIVE

The osteointegration effect of Mg alloy scaffolds with different corrosion rates were studied and evaluated in large bone defect models.

METHOD

Mg-Sr and Mg-Ca alloy scaffolds with a 20-μm Micro-arc oxidation (MAO) coating were used to repair critical bone defects for subsequent assessment of each alloy's degradation and osteointegration by X-ray, Micro-CT, fluorescence and histological examination.

RESULTS

At 12 weeks post-implantation, each defect was found to be effectively reconstructed by either of the Mg alloys based on X-ray and Micro-CT images. The corrosion rate (CR) of each Mg alloy - as calculated based on micro-computed tomography information - demonstrated that the MAO coating could provide effective protection for only 4 weeks post-surgery. From weeks 8 to 12, the CR of the Mg-Ca alloy scaffold increased from 1.34 ± 0.23 mm/y to 1.57 ± 0.16 mm/y. In contrast, the CR of the Mg-Sr alloy scaffold decreased from 0.58 ± 0.14 mm/y to 0.54 ± 0.16 mm/y. However, fluorescence and histological examination revealed more mature, closely and regularly arranged newborn osteocytes at the Mg-Ca scaffold-fracture interface e from weeks 8 to 12 after surgery.

RESULTS

The Mg-Sr scaffold was more corrosion resistant and the Mg-Ca scaffold yielded a better overall repair, which indicates that the CR of magnesium alloys matches the rate of new bone formation and is the key to repair bone defects as a bone substitute.

Biodegradability of Micro/Nanomotors: Challenges and Opportunities

Micro/nanomotors (MNMs) are miniature machines that can convert chemical or external energy into their own mechanical motions. In previous decades, significant efforts have been made to improve the performance of MNMs. For practical applications, the biodegradability of MNMs is an important aspect that must be considered, particularly in the biomedical field. In this review, recent progress in the biodegradability of MNMs and their potential applications are summarized. Different biodegradable materials, including metals and polymers, or other strategies for the fabrication of MNMs, are presented. Current challenges and future perspectives are also discussed.

Corrosion and biocompatibility behaviours of microarc oxidation/phytic acid coated magnesium alloy clips for use in cholecystectomy in a rabbit model

With the popularisation of laparoscopic cholecystectomy, ligation clips have been commonly used for ligating the cystic duct and cystic artery. However, non-degradable clips remain in the body long-term, which significantly increases the risk of the clip becoming detached. Thus, magnesium alloys have attracted tremendous attention owing to their biodegradability and good biocompatibility. However, the poor corrosion resistance hinders the clinical application of magnesium alloys with microarc oxidation/phytic acid (MAO/PA) composite coatings as protective coatings. Here, these alloys were used to hinder the rapid material degradation in aqueous solution. Electrochemical tests were conducted to evaluate the in vivo degradation behaviour in simulated body fluid (SBF) for Mg-Zn-Y-Nd alloys, and scanning electron microscopy (SEM) was used to observe the micromorphology of in vivo clip degradation. Cell toxicity, cell adhesion, and flow cytometry were performed in vitro to detect cytocompatibility. Biochemical detection of serum magnesium, serum creatinine (CREA), blood urea nitrogen (BUN), alanine transaminase (ALT), and alanine aminotransferase (AST), and haematoxylin-eosin (HE) staining of the heart, liver, and kidney tissues in vivo was conducted to determine the biocompatibility properties after surgery. Electrochemical measurements and SEM images revealed that the MAO/PA-coated magnesium alloy delayed corrosion in SBF. The apoptosis rate increased slightly with increased extract concentration. Nevertheless, MAO/PA-coated magnesium alloys still exhibited good cytocompatibility. No obvious abnormality was observed in the blood biochemical test or HE staining. Thus, MAO/PA-coated magnesium alloys exhibit better corrosion than bare magnesium. In addition, Mg-Zn-Y-Nd and MAO/PA-coated magnesium alloys exhibited no cytotoxicity, good adhesion, and biosafety.

Biodegradable magnesium-based biomaterials: An overview of challenges and opportunities

As promising biodegradable materials with nontoxic degradation products, magnesium (Mg) and its alloys have received more and more attention in the biomedical field very recently. Having excellent biocompatibility and unique mechanical properties, magnesium-based alloys currently cover a broad range of applications in the biomedical field. The use of Mg-based biomedical devices eliminates the need for biomaterial removal surgery after the healing process and reduces adverse effects induced by the implantation of permanent biomaterials. However, the high corrosion rate of Mg-based implants leads to unexpected degradation, structural failure, hydrogen evolution, alkalization, and cytotoxicity. To overcome these limitations, alloying Mg with suitable alloying elements and surface treatment come highly recommended. In this area, open questions remain on the behavior of Mg-based biomaterials in the human body and the effects of different factors that have resulted in these challenges. In addition to that, many techniques are yet to be verified to turn these challenges into opportunities. Accordingly, this article aims to review major challenges and opportunities for Mg-based biomaterials to minimize the challenges for the development of novel biomaterials made of Mg and its alloys.

Recent advances and directions in the development of bioresorbable metallic cardiovascular stents: Insights from recent human and in vivo studies

Over the past two decades, significant advancements have been made regarding the material formulation, iterative design, and clinical translation of metallic bioresorbable stents. Currently, magnesium-based (Mg) stent devices have remained at the forefront of bioresorbable stent material development and use. Despite substantial advances, the process of developing novel absorbable stents and their clinical translation is time-consuming, expensive, and challenging. These challenges, coupled with the continuous refinement of alternative bioresorbable metallic bulk materials such as iron (Fe) and zinc (Zn), have intensified the search for an ideal absorbable metallic stent material. Here, we discuss the most recent pre-clinical and clinical evidence for the efficacy of bioresorbable metallic stents and material candidates. From this perspective, strategies to improve the clinical performance of bioresorbable metallic stents are considered and critically discussed, spanning material alloy development, surface manipulations, material processing techniques, and preclinical/biological testing considerations. STATEMENT OF SIGNIFICANCE: Recent efforts in using Mg, Fe, and Zn based materials for bioresorbable stents include elemental profile changes as well as surface modifications to improve each of the three classes of materials.  Although a variety of alloys for absorbable metallic stents have been developed, the ideal absorbable stent material has not yet been discovered. This review focuses on the state of the art for bioresorbable metallic stent development. It covers the three bulk materials used for degradable stents (Mg, Fe, and Zn), and discusses their advances from a translational perspective. Strategies to improve the clinical performance of bioresorbable metallic stents are considered and critically discussed, spanning material alloy development, surface manipulations, material processing techniques, and preclinical/biological testing considerations.

Degradation behaviors and in-vivo biocompatibility of a rare earth- and aluminum-free magnesium-based stent

Biodegradable stents can provide scaffolding and anti-restenosis benefits in the short term and then gradually disappear over time to free the vessel, among which the Mg-based biodegradable metal stents have been prosperously developed. In the present study, a Mg-8.5Li (wt.%) alloy (RE- and Al-free) with high ductility (> 40%) was processed into mini-tubes, and further fabricated into finished stent through laser cutting and electropolishing. In-vitro degradation test was performed to evaluate the durability of this stent before and after balloon dilation. The influence of plastic deformation and residual stress (derived from the dilation process) on the degradation was checked with the assistance of finite element analysis. In addition, in-vivo degradation behaviors and biocompatibility of the stent were evaluated by performing implantation in iliac artery of minipigs. The balloon dilation process did not lead to deteriorated degradation, and this stent exhibited a decent degradation rate (0.15 mm/y) in vitro, but divergent result (> 0.6 mm/y) was found in vivo. The stent was almost completely degraded in 3 months, revealing an insufficient scaffolding time. Meanwhile, it did not induce possible thrombus, and it was tolerable by surrounding tissues in pigs. Besides, endothelial coverage in 1 month was achieved even under the severe degradation condition. In the end, the feasibility of this stent for treatment of benign vascular stenosis was generally discussed, and perspectives on future improvement of Mg-Li-based stents were proposed.

The effect of composite PHB coating on the biological properties of a magnesium based alloy

Magnesium alloys have been widely investigated as biodegradable cardiovascular temporal implants due to their better mechanical properties and biocompatibility, but the rapid degradation limited its application. In this study, the anodic oxidation-Cu structure was used to improve the adhesive strength and stability between poly-β-hydroxybutyrate (PHB) and magnesium alloys, and the effects of anodic oxidation magnesium alloys with copper film and PHB film (MACP) on human umbilical vein endothelial cells (HUVECs), blood compatibility and antibacterial properties were investigated in this research. As the result, the MACP structure had a stable structure and better corrosion resistance, and significant antibacterial properties. The coating would not affect the original excellent biocompatibility of the magnesium alloy. It was indicated that MACP was a potential surface modification strategy for vascular stents candidate material.

Comparison of in Vascular Bioreactors and In Vivo Models of Degradation and Cellular Response of Mg-Zn-Mn Stents

Traditional in vitro evaluation criteria of magnesium (Mg)-based stents cannot reflect the degradation process in vivo, due to the interdependence and interference between biodegradable properties and bioenvironment. The current direct and indirect evaluation approaches of in vitro biocompatibility do not have a hydrodynamic environment and vascular biological structure existing in vivo. Herein, we designed a vascular bioreactor to provide an ex vivo culture environment for vessels, which reveals the degradation behavior of Mg-Zn-Mn stent and the effect of its degradation on cells. We reported that rabbit carotid arteries could maintain native morphology and viability in the bioreactor under the best condition within a flow rate of 5.4 mL min^-1 and a culture time of one week. With this culture condition, Mg-Zn-Mn stents were implanted into the arteries in the bioreactors and compared with in vivo rabbit models. The arteries maintained cell survival in the bioreactor, but the cell attachment was absent on the stent struts, associated with a fast degradation. Conversely, the stents achieved a rapid and complete endothelialization in vivo for two weeks. This study could provide a correlation and difference of the degradation behavior and cellular response to the degradation of Mg-based stent between ex vivo and in vivo approaches.

Osteogenesis, angiogenesis and immune response of Mg-Al layered double hydroxide coating on pure Mg

Layered double hydroxides (LDHs) are widely studied to enhance corrosion resistance and biocompatibility of Mg alloys, which are promising bone implants. However, the influence of LDH coating on the osteointegration of Mg implants lacks of a systematic study. In this work, Mg-Al LDH coating was prepared on pure Mg via hydrothermal treatment. The as-prepared Mg-Al LDH coated Mg exhibited better in vitro and in vivo corrosion resistance than bare Mg and Mg(OH)2 coated Mg. In vitro culture of mouse osteoblast cell line (MC3T3-E1) suggested that Mg-Al LDH coated Mg was more favorable for its osteogenic differentiation. In vitro culture of HUVECs revealed that cells cultured in the extract of Mg-Al LDH coated Mg showed superior angiogenic behaviors. More importantly, the immune response of Mg-Al LDH coated Mg was studied by in vitro culturing murine-derived macrophage cell line (RAW264.7). The results verified that Mg-Al LDH coated Mg could induce macrophage polarize to M2 phenotype (anti-inflammatory). Furthermore, the secreted factor in the macrophage-conditioned culture medium of Mg-Al LDH group was more suitable for the bone differentiation of rat bone marrow stem cells (rBMSCs) and the angiogenic behavior of human umbilical vein endothelial cells (HUVECs). Finally, the result of femoral implantation suggested that Mg-Al LDH coated Mg exhibited better osteointegration than bare Mg and Mg(OH)2 coated Mg. With favorable in vitro and in vivo performances, Mg-Al LDH is promising as protective coating on Mg for orthopedic applications.

Protein repellent anti-coagulative mixed-charged cellulose derivative coatings

This study describes the formation of cellulose based polyelectrolyte charge complexes on the surface of biodegradable polycaprolactone (PCL) thin films. Anionic sulphated cellulose (CS) and protonated cationic amino cellulose (AC) were used to form these complexes with a layer-by-layer coating technique. Both polyelectrolytes were analyzed by charge titration methods to elucidate their pH-value dependent protonation behavior. A quartz crystal microbalance with dissipation (QCM-D) in combination with X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) were used to follow the growth, stability and water content of up to three AC/CS bi-layers in aqueous environment. This was combined with coagulation studies on one, two and three bilayers of AC/CS, measuring the thrombin formation rate and the total coagulation time of citrated blood plasma with QCM-D. Stable mixed charged bilayers could be prepared on PCL and significantly higher masses of AC than of CS were present in these complexes. Strong hydration due to the presence of ammonium and sulphate substituents on the backbone of cellulose led to a significant BSA repellent character of three bilayers of AC/CS coatings. The total plasma coagulation time was increased in comparison to neat PCL, indicating an anticoagulative nature of the coatings. Surprisingly, a coating solely composed of an AC layer significantly prolonged the total coagulation time on the surfaces although it did not prevent fibrinogen deposition. It is suggested that these cellulose derivative-based coatings can therefore be used to prevent unwanted BSA deposition and fibrin clot formation on PCL to foster its biomedical application.

Recent Advances in the Control of the Degradation Rate of PEO Treated Magnesium and Its Alloys for Biomedical Applications

Mg and Mg alloys have been studied for almost two centuries; nevertheless, commercial biomedical devices are still not available. The main issue that limits their use in the biomedical field is the rapid degradation rate combined with suitable surface properties. Novel approaches need to be designed for the development of biodegradable Mg-based devices, which could include the use of multifunctional coatings and/or new alloys designed “ad hoc”. The present article reviews on various properties, parameters and improvement methods concerning plasma electrolytic oxidation (PEO) coatings on Mg alloys substrates for biomedical applications. In this regard, (i) optimizing the PEO parameters, (ii) using additives and nanoparticles, (iii) creating combined layers of hard and/or soft particles, (iv) coating the PEO layer with a biodegradable polymer, could be the way to control their degradation rate. The review of recent scientific articles highlights that none of the techniques proposed may be preferred over the others and the need to deepen the studies for allowing the use of Mg-based devices in the biomedical field.

Interaction of different cell types with magnesium modified by plasma electrolytic oxidation

Magnesium (Mg) is a material widely used in industrial applications due to its low weight, ductility, and excellent mechanical properties. For non-permanent implants, Mg is particularly well-suited because of its biodegradability, while its degradation products are not harmful. However, Mg is chemically reactive, and cytotoxic hydrogen gas is released as part of the degradation. This adverse degradation can be tuned using plasma electrolytic oxidation (PEO). With PEO, a surface layer of MgO/Mg(OH)₂ is deposited on the surface of Mg in a controlled way. The electrolytes used during PEO influence the surface's chemistry and topography and thus expectedly the biological response of adhered cells. In this study, thin samples of commercial pure of Mg (c.p Mg) were modified by PEO guided by different electrolytes, and the biological activity was assessed on vascular cells, immune cells, and repair cells (adipose tissue-derived stromal cells, ASCs). Vascular cells were more vulnerable than ASCs for compounds released by surface-coated Mg. All surface coatings supported the proliferation of adhered ASC. Released compounds from surface-coated Mg delayed but did not block in vitro wound closure of fibroblasts monolayers. Preformed endothelial tubes were vulnerable for released compounds, while their supporting ASC was not. We conclude that PEO-based surface-coating of Mg supports adhesion and future delivery of therapeutic vascular repair cells such as ASC, but that the observed vulnerability of vascular cells for coated Mg components warrants investigations in vivo.

Traditional and bionic dynamic hip screw fixation for the treatment of intertrochanteric fracture: a finite element analysis

PURPOSE

The dynamic hip screw (DHS) is widely used for fixing intertrochanteric femur fractures. A porous bionic DHS was developed recently to avoid the stress concentration and risk of post-operative complications associated with titanium alloy DHSs. The purpose of this study was to compare the effects of traditional titanium alloy, bionic titanium alloy, and bionic magnesium alloy DHS fixation for treatment of intertrochanteric fractures using finite element analysis.

METHODS

A three-dimensional model of the proximal femur was established by human computed tomography images. An intertrochanteric fracture was created on the model, which was fixed using traditional and porous bionic DHS, respectively. The von Mises stress, maximum principal stress, and minimum principal stress were calculated to evaluate the effect of bone ingrowth on stress distribution of the proximal femur after fixation.

RESULTS

Stress concentration of the bionic DHS model was lower compared with traditional DHS fixation models. The von Mises stress, maximum principal stress, and minimum principal stress distributions of bionic magnesium alloy DHS models improved, along with simulation of the bone healing process and magnesium alloy degeneration, assumed to biodegrade completely 12 months post-operatively. The distribution of maximum principal stress in the secondary tension zone of the bionic DHS model was close to the intact bone. In the minimum principal stress, the region of minimum stress value less than - 10 MPa was significantly improved compared with traditional DHS models.

CONCLUSION

The bionic magnesium alloy DHS implant can improve the stress distribution of fractured bone close to that of intact bone while reducing the risk of post-operative complications associated with traditional internal fixations.

Research Progress on Surface Treatments of Biodegradable Mg Alloys: A Review

Mg alloys have attracted extensive attention in the biomedical fields owing to their great biocompatibility, suitable mechanical properties, and biodegradability, etc. However, the fast degradation rate restricts the application of Mg alloys. Thus, the surface treatment of Mg alloys is considered as one of the most effective ways to enhance the corrosion resistance of Mg alloys. Nevertheless, simple processing to improve the corrosion resistance can no longer meet the growing biofunctional clinical requirements. Therefore, functionalized processing has become one of the key development directions for surface treatment in the future, such as functionalized Mg alloys with antibacterial property and hydrophobicity. There are few papers that review the functionalized processing of surface treatment. This review summarized and compared the major advances of the surface treatment (anticorrosion processing and functionalized processing) of Mg alloys. Then, some potential research suggestions are proposed, which may provide a reference for the development of Mg alloys.

Study of the Morphology and Properties of Biocompatible Ca-P Coatings on Mg Alloy

Magnesium alloys are considered as potential biomaterials for use in orthopedic implantology. The main barrier to the use of Mg alloys in medicine is their overly fast and irregular degradation in body fluids. The use of protective calcium phosphate coatings to increase the corrosion resistance of Mg alloy (AM50 alloy: 4 wt.% Al, 0.3 wt.% Mn, 0.2 wt.% Zn, rest Mg) was examined in this study. The scientific goal of the study was the assessment of the influence of calcium phosphate layer morphology on the corrosion process in Ringer's solution. Modification of the coating morphology was obtained by changing the chemical composition of the phosphatizing bath using NaOH (NaAM50 sample) or ZnSO₄ (ZnAM50 sample). In practice, a more dense and uniform coating could be obtained by the immersion of AM50 alloy in a solution containing ZnSO₄ (ZnAM50 sample). In this study, an adhesion test performed on the ZnAM50 sample indicated that the critical load was 1.35 N. XRD phase analysis confirmed that the obtained coatings included dicalcium phosphate dihydrate (CaHPO₄*2H₂O). The coatings prepared on the NaAM50 and ZnAM50 samples are effective barriers against the progress of corrosion deeper into the substrate. After 120 h immersion in Ringer's solution, the volume of the evolved hydrogen was 5.6 mL/cm² for the NaAM50 and 3.4 mL/cm² for the ZnAM50 sample.

Polylactide-Based Stent Coatings: Biodegradable Polymeric Coatings Capable of Maintaining Sustained Release of the Thrombolytic Enzyme Prourokinase

The novelty of the study is the development, creation, and investigation of biodegradable polymeric membranes based on polylactide, that are capable of directed release of large molecular weight biomolecules, particularly, prourokinase protein (MW = 46 kDa). Prourokinase is a medication with significant thrombolytic activity. The created membranes possess the required mechanical properties (relative extension value from 2% to 10%, tensile strength from 40 to 85 MPa). The membranes are biodegradable, but in the absence of living cells in a water solution they decompose by less than 10% in half a year. The created membranes are capable of controlled prourokinase release into intercellular space, and the total enzymatic activity of prourokinase does not decrease by more than 12%. The daily release of prourokinase from one square centimeter of the membrane ranges from 1 to 40 μg per day depending on the technique of membrane preparation. The membranes have no acute toxic effect on cells accreting these surfaces de novo. The number of viable cells is at least 96%-97% of the overall cell count. The mitotic index of the cells growing on the surface of the polymeric films comprised around 1.5%. Histological examination did not reveal any disorders in tissues of the animals after the implantation of polymer membranes based on polylactide, both alone and as components of stent cover. Implantation of stents covered with prourokinase-containing polymers led to the formation of a mature connective tissue capsule that is thicker than in the case of uncovered stents. Thus, various polylactide-based biodegradable polymeric membranes possessing the required mechanical properties and capable of prolonged and directed release of prourokinase macromolecules are developed and investigated in the study.