Electrophoretic Deposition of Bioadaptive Drug Delivery Coatings on Magnesium Alloy for Bone Repair

Molybdate and Phosphate Cross-Linked Chitosan Films for Corrosion Protection of Hot-Dip Galvanized Steel

Environmentally friendly and sustainable methods to protect hot-dip galvanized (HDG) steel from corrosion are extensively studied. Films of the biopolymer polyelectrolyte chitosan were ionically cross-linked in this work with the well-known corrosion inhibitors phosphate and molybdate. Layers on this basis are presented as components in a protective system and could, e.g., be applied in pretreatments similar to a conversion coating. For the preparation of the chitosan-based films, a procedure involving sol-gel chemistry and wet-wet application was utilized. Homogeneous films of few micrometers thickness were obtained on HDG steel substrates after thermal curing. Properties of chitosan-molybdate and chitosan-phosphate films were compared with purely passive epoxysilane-cross-linked chitosan, and pure chitosan. Delamination behavior of a poly(vinyl butyral) (PVB) weak model top coating studied by scanning Kelvin probe (SKP) showed an almost linear time dependence over >10 h on all systems. Delamination rates were 0.28 mm h^-1 (chitosan-molybdate) and 0.19 mm h^-1 (chitosan-phosphate), ca. 5% of a non-cross-linked chitosan reference and slightly higher than of the epoxsyilane cross-linked chitosan. Immersion of the treated zinc samples over 40 h in 5% NaCl solution yielded a 5-fold increase of the resistance in the chitosan-molybdate system, as evidenced by electrochemical impedance spectroscopy (EIS). Ion exchange of electrolyte anions with molybdate and phosphate triggers corrosion inhibition, presumably by reaction with the HDG surface as well described in the literature for these inhibitors. Thus, such surface treatments have potential for application, e.g., in temporary corrosion protection.

Composite Nanocoatings of Biomedical Magnesium Alloy Implants: Advantages, Mechanisms, and Design Strategies

The rapid degradation of magnesium (Mg) alloy implants erodes mechanical performance and interfacial bioactivity, thereby limiting their clinical utility. Surface modification is among the solutions to improve corrosion resistance and bioefficacy of Mg alloys. Novel composite coatings that incorporate nanostructures create new opportunities for their expanded use. Particle size dominance and impermeability may increase corrosion resistance and thereby prolong implant service time. Nanoparticles with specific biological effects may be released into the peri-implant microenvironment during the degradation of coatings to promote healing. Composite nanocoatings provide nanoscale surfaces to promote cell adhesion and proliferation. Nanoparticles may activate cellular signaling pathways, while those with porous or core-shell structures may carry antibacterial or immunomodulatory drugs. Composite nanocoatings may promote vascular reendothelialization and osteogenesis, attenuate inflammation, and inhibit bacterial growth, thus increasing their applicability in complex clinical microenvironments such as those of atherosclerosis and open fractures. This review combines the physicochemical properties and biological efficiency of Mg-based alloy biomedical implants to summarize the advantages of composite nanocoatings, analyzes their mechanisms of action, and proposes design and construction strategies, with the purpose of providing a reference for promoting the clinical application of Mg alloy implants and to further the design of nanocoatings.

Advances in Multifunctional Bioactive Coatings for Metallic Bone Implants

To fix the bone in orthopedics, it is almost always necessary to use implants. Metals provide the needed physical and mechanical properties for load-bearing applications. Although widely used as biomedical materials for the replacement of hard tissue, metallic implants still confront challenges, among which the foremost is their low biocompatibility. Some of them also suffer from excessive wear, low corrosion resistance, infections and shielding stress. To address these issues, various coatings have been applied to enhance their in vitro and in vivo performance. When merged with the beneficial properties of various bio-ceramic or polymer coatings remarkable bioactive, osteogenic, antibacterial, or biodegradable composite implants can be created. In this review, bioactive and high-performance coatings for metallic bone implants are systematically reviewed and their biocompatibility is discussed. Updates in coating materials and formulations for metallic implants, as well as their production routes, have been provided. The ways of improving the bioactive coating performance by incorporating bioactive moieties such as growth factors, osteogenic factors, immunomodulatory factors, antibiotics, or other drugs that are locally released in a controlled manner have also been addressed.

Potential bioactive coating system for high-performance absorbable magnesium bone implants

Magnesium alloys are considered the most suitable absorbable metals for bone fracture fixation implants. The main challenge in absorbable magnesium alloys is their high corrosion/degradation rate that needs to be controlled. Various coatings have been applied to magnesium alloys to slow down their corrosion rates to match their corrosion rate to the regeneration rate of the bone fracture. In this review, a bioactive coating is proposed to slow down the corrosion rate of magnesium alloys and accelerate the bone fracture healing process. The main aim of the bioactive coatings is to enhance the direct attachment of living tissues and thereby facilitate osteoconduction. Hydroxyapatite, collagen type I, recombinant human bone morphogenetic proteins 2, simvastatin, zoledronate, and strontium are six bioactive agents that show high potential for developing a bioactive coating system for high-performance absorbable magnesium bone implants. In addition to coating, the substrate itself can be made bioactive by alloying magnesium with calcium, zinc, copper, and manganese that were found to promote bone regeneration.

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Bioactive-coated magnesium implant could accelerate bone fracture healing time to match with magnesium degradation.

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Hydroxyapatite, collagen type I, recombinant human bone morphogenetic proteins 2, simvastatin, zoledronate, and strontium are high potential bioactive coating materials.

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The incorporation of Ca, Zn, Cu, Sr, and Mn in Mg base-metal could further enhance bone formation.

Regulation of Magnesium Matrix Composites Materials on Bone Immune Microenvironment and Osteogenic Mechanism

Despite magnesium based metal materials are widely used in bone defect repair, there are still various deficiencies, and their properties need to be optimized. Composites synthesized with magnesium based metal as matrix are the research hotspot, and the host immune response after biomaterial implantation is very important for bone binding. By studying the immunoregulation of bone biomaterials, it can regulate the immune response in the process of osteogenesis and create a good local immune microenvironment, which is conducive to biomaterials to reduce inflammatory response and promote good bone binding. This article introduces the osteogenic mechanism of magnesium based metal materials and its regulation on bone immune microenvironment in detail.

In vitro evaluation of freeze-drying chitosan-mineralized collagen/Mg-Ca alloy composites for osteogenesis

Magnesium (Mg) alloy with good mechanical properties and biodegradability is considered as one of the ideal bone repair materials. However, the rapid corrosion of Mg-based metals can pose harm to the function of an implant in clinical applications. In this study, micro-arc oxidation coating was prepared on the surface of the Mg-Ca matrix, then the chitosan and mineralized collagen (nano-hydroxyapatite/collagen; nHAC) were immobilized on the surface of the MAO/Mg-Ca matrix to construct the CS-nHAC/Mg-Ca composites of different component proportions (the ratio of CS to nHAC is 2:1, 1:1, and 1:2, respectively). The corrosion resistance, osteogenic activity, and angiogenic ability were extensively investigated. The results indicated that the CS-nHAC reinforcement materials can improve the corrosion resistance of the Mg matrix significantly and promote the proliferation and adhesion of mouse embryo osteoblast precursor cells (MC3T3-E1) and human umbilical vein endothelial cells (HUVECs). In addition, the CS-nHAC/Mg-Ca composites can not only promote the alkaline phosphatase (ALP) activity and extracellular matrix mineralization of MC3T3-E1 cells but also enhance the migration motility and vascular endothelial growth factor (VEGF) expression of HUVECs. Meanwhile, the 2CS-1nHAC/Mg-Ca composite exhibited the optimum function characteristics compared with other samples. Therefore, considering the improvement of corrosion resistance and biocompatibility, the CS-nHAC/Mg-Ca composites are expected to be a promising orthopedic implant.

Amorphous calcium magnesium phosphate nanocomposites with superior osteogenic activity for bone regeneration

The seek of bioactive materials for promoting bone regeneration is a challenging and long-term task. Functionalization with inorganic metal ions or drug molecules is considered effective strategies to improve the bioactivity of various existing biomaterials. Herein, amorphous calcium magnesium phosphate (ACMP) nanoparticles and simvastatin (SIM)-loaded ACMP (ACMP/SIM) nanocomposites were developed via a simple co-precipitation strategy. The physiochemical property of ACMP/SIM was explored using transmission electron microscope (TEM), Fourier transform infrared spectroscopy (FTIR), powder X-ray diffraction (XRD) and high-performance liquid chromatograph (HPLC), and the role of Mg²⁺ in the formation of ACMP/SIM was revealed using X-ray absorption near-edge structure (XANES). After that, the transformation process of ACMP/SIM in simulated body fluid (SBF) was also tracked to simulate and explore the in vivo mineralization performance of materials. We find that ACMP/SIM releases ions of Ca²⁺, Mg²⁺ and PO43−, when it is immersed in SBF at 37°C, and a phase transformation occurred during which the initially amorphous ACMP turns into self-assembled hydroxyapatite (HAP). Furthermore, ACMP/SIM displays high cytocompatibility and promotes the proliferation and osteogenic differentiation of MC3T3-E1 cells. For the in vivo studies, lamellar ACMP/SIM/Collagen scaffolds with aligned pore structures were prepared and used to repair a rat defect model in calvaria. ACMP/SIM/Collagen scaffolds show a positive effect in promoting the regeneration of calvaria defect after 12 weeks. The bioactive ACMP/SIM nanocomposites are promising as bone repair materials. Considering the facile preparation process and superior in vitro/vivo bioactivity, the as-prepared ACMP/SIM would be a potential candidate for bone related biomedical applications.

Co-culture of BMSCs and HUVECs with simvastatin-loaded gelatin nanosphere/chitosan coating on Mg alloy for osteogenic differentiation and vasculogenesis

Mg alloys are increasingly being investigated as a versatile and economical alternative for developing bone repair implants because of their high mechanical strength, wide availability, adjustable structure and properties. In this study, magnesium alloy WE43 is coated on both sides with gelatin nanosphere/chitosan (GNs/CTS), a coating enhanced by incorporating simvastatin (SIM). SIM-loaded GNs/CTS coated magnesium alloy can promote the osteogenic differentiation of bone mesenchymal stem cells (BMSCs). BMSCs and human umbilical vein endothelial cells (HUVECs) are co-cultured through transwell systems. The release of SIM from the coating is found to increase the secretion of chemokine and angiogenic factors from BMSCs, which promote the migration and tube formation of HUVECs, respectively. Bone morphogenetic protein secreted by HUVECs is seen to increase by the release of SIM from the coating, promoting the osteogenic differentiation of BMSCs. The secretion of chemokines from HUVECs promote the migration of BMSCs. The coated magnesium alloy substrate loaded with SIM is found to regulate the osteogenic differentiation of BMSCs. The study of the paracrine interaction between BMSCs and HUVECs proves that the applied coating promotes both osteogenic differentiation and vascularization, thus demonstrating a new approach for the design of bone repair materials based on magnesium alloys.

Pulsed laser deposition of nanostructured bioactive glass and hydroxyapatite coatings: Microstructural and electrochemical characterization

Bioactive coatings on metallic implants promote osseointegration between bone and implant interfaces. A suitable coating enhances the life span of the implant and reduces the requirement of revision surgery. The coating process needs to be optimized such that it does not alter the bioactivity of the material. To understand this, the biocompatibility of nanostructured bioactive glass and hydroxyapatite-coated Titanium substrate by pulsed laser deposition method is evaluated. Raman and IR spectroscopic techniques based on silica and phosphate functional groups mapping have confirmed homogeneity in coatings by pulse laser deposition method. Comparative studies on nanostructured bioactive glass and hydroxyapatite on titanium surface elaborated the significance of bioactivity, hemocompatibility, and cytocompatibility of the coated surface. Notably, both hydroxyapatite and bioactive glass show good hemocompatibility in powder form. Hemocompatibility and cytocompatibility results validate the enhanced sustenance for hydroxyapatite coating. These results signify the importance of the choice of coating methodology of bioceramics towards implant applications.

Tuning the Biointerface: Low-Temperature Surface Modification Strategies for Orthopedic Implants to Enhance Osteogenic and Antimicrobial Activity

As both the average life expectancy and incidence of bone tissue reconstruction increases, development of load-bearing implantable materials that simultaneously enhance osseointegration while preventing postoperative infection is crucial. To address this need, significant research efforts have been dedicated to developing surface modification strategies for metallic load-bearing implants and scaffolds. Despite the abundance of strategies reported, many address only one factor, for example, surface chemistry or topography. Furthermore, the incorporation of surface features to increase osteocompatibility can increase the probability of infection, by encouraging the formation of bacterial biofilms. To truly advance this field, research efforts must focus on developing multifunctional coatings that concurrently address these complex and competing requirements. In addition, particular emphasis should be placed on utilizing surface modification processes that are versatile, low cost, and scalable, for ease of translation to mass manufacturing and clinical use. The aim of this short Review is to highlight recent advances in scalable and multifunctional surface modification techniques that obtain a programmed response at the bone tissue/implant interface. Low-temperature approaches based on macromolecule immobilization, electrochemical techniques, and solution processes are discussed. Although the strategies discussed in this Review have not yet been approved for clinical use, they show great promise toward developing the next generation of ultra-long-lasting biomaterials for joint and bone tissue repair.

A Comprehensive Review on Surface Modifications of Biodegradable Magnesium-Based Implant Alloy: Polymer Coatings Opportunities and Challenges

The development of biodegradable implants is certainly intriguing, and magnesium and its alloys are considered significant among the various biodegradable materials. Nevertheless, the fast degradation, the generation of a significant amount of hydrogen gas, and the escalation in the pH value of the body solution are significant barriers to their use as an implant material. The appropriate approach is able to solve this issue, resulting in a decrease the rate of Mg degradation, which can be accomplished by alloying, surface adjustment, and mechanical treatment. Surface modification is a practical option because it not only improves corrosion resistance but also prepares a treated surface to improve bone regeneration and cell attachment. Metal coatings, ceramic coatings, and permanent polymers were shown to minimize degradation rates, but inflammation and foreign body responses were also suggested. In contrast to permanent materials, the bioabsorbable polymers normally show the desired biocompatibility. In order to improve the performance of drugs, they are generally encapsulated in biodegradable polymers. This study summarized the most recent advancements in manufacturing polymeric coatings on Mg alloys. The related corrosion resistance enhancement strategies and future potentials are discussed. Ultimately, the major challenges and difficulties are presented with aim of the development of polymer-coated Mg-based implant materials.

Bio-inspired multifunctional collagen/electrospun bioactive glass membranes for bone tissue engineering applications

Treatment of bone disease and disorders is often challenging due to its complex structure. Each year millions of people needs bone substitution materials with quick recovery from diseases conditions. Synthetic bone substitutes mimicking structural, chemical and biological properties of bone matrix structure will be very obliging and of copious need. In this work, we reported on the fabrication of bioinspired, biomimetic, multifunctional bone-like three-dimensional (3D) membranes made up of inorganic bioactive glass fibers matrixed organic collagen structure. The 3D structure is arranged as a stacked-layer similar to the order of apatite and neotissue formation. Comparative studies on collagen, collagen with hollow and solid bioactive glass fibers evidenced that, collagen/hollow bioactive glass is mechanically robust, has optimal hydrophilicity, simultaneously promotes bioactivity and in situ forming drug delivery. The 3D membrane displays outstanding mechanical properties apropos to the bioactive glass fibers arrangement, with its Youngs modulus approaching the modulus of cortical bone. The in vitro cell culture studies with fibroblast cells (3T3) on the membranes display enhanced cell adhesion and proliferation with the cell alignment similar to anisotropic cell alignment found in the native bone extracellular matrix. The membranes also support 3D cell culturing and exhibits cell proliferation on the membrane surface, which extends the possibility of its bone tissue engineering application. The alkaline phosphatase assessment and alizarin red staining of osteoblast cells (MG63) depicted an enhanced osteogenic activity of the membranes. Notable Runx2, Col-Type-1 mRNA, osteocalcin, and osteonectin levels were found to be significantly increased in cells grown on the collagen/hollow bioactive glass membrane. This membrane also promotes vascularization in the chick chorioallantoic membrane model. The results altogether evidence this multifunctional 3D membrane could potentially be utilized for treatment of bone defects.

Controllable Synthesis of Nanostructured Ca-P Coating on Magnesium Alloys via Sodium Citrate Template-Assisted Hydrothermal Method and Its Corrosion Resistance

In this study, a nanostructured needle-like hydroxyapatite (HA) coating was prepared by the sodium citrate template-assisted hydrothermal method on magnesium alloy (AZ31). The influence of sodium citrate on the composition, microstructure and corrosion behavior of the coatings was studied. The results showed that with the increase in the mole ratio of Ca/sodium citrate from 1 to 13, the coating gradually changed from the needle-like morphology of HA to the flake morphology of β-tricalcium phosphate (β-TCP), which was related to the existing form of citrate in the solution and the trend of complexation reaction. When the mole ratio of Ca/sodium citrate was 1, the HA coating sample with the nano needle-like morphology had a high corrosion resistance (Rt = 235.300 ± 3.584 kΩ·cm2), which was almost 200 times that of the naked AZ31 alloy. Moreover, the corrosion rates of the Ca-P coated AZ31 alloy stabilized at about 0.55 mm/year and could provide more than 56 days of corrosion protection to the samples, which approximated the degradation rate requirement for biomaterials used as bone fixture.

MXene-Based Hydrogels Endow Polyetheretherketone with Effective Osteogenicity and Combined Treatment of Osteosarcoma and Bacterial Infection

After an osteosarcoma resection, the risks of cancer recurrence, postoperative infection, and large bone loss still threaten patients' health. Conventional treatment relies on implanting orthopedic materials to fill bone defects after surgery, but it has no ability of destroying residual tumor cells and preventing bacterial invasion. To tackle this challenge, here, we develop a novel multifunctional implant (SP@MX/GelMA) that mainly consists of MXene nanosheets, gelatin methacrylate (GelMA) hydrogels, and bioinert sulfonated polyetheretherketone (SP) with the purpose of facilitating tumor cell death, combating pathogenic bacteria, and promoting osteogenicity. Because of the synergistic photothermal effects of MXene and polydopamine (pDA), osteosarcoma cells are effectively killed on the multifunctional coatings under 808 nm near-infrared (NIR) irradiation through thermal ablation. After loading tobramycin (TOB), the SP@MX-TOB/GelMA implants display robust antibacterial properties against Gram-negative/Gram-positive bacteria. More importantly, the multifunctional implants are demonstrated to have superior cytocompatibility and osteogenesis-promoting capability in terms of cell replication, spreading, alkaline phosphatase activity, calcium matrix mineralization, and in vivo osseointegration. Accordingly, such photothermally controlled multifunctional implants not only defeat osteosarcoma cells and bacteria but also intensify osteogenicity, which hold a greatly promising countermeasure for curing postoperative tissue lesion from an osteosarcoma excision.

Photo-controlled degradation of PLGA/Ti3C2 hybrid coating on Mg-Sr alloy using near infrared light

A PLGA/Ti₃C₂ hybrid coating was successfully deposited on the surface of magnesium-strontium (Mg-Sr) alloys. Compared with the corrosion current density (i_corr) of the Mg-Sr alloy (7.13 × 10⁻⁵ A/cm²), the modified samples (Mg/PLGA/Ti₃C₂) was lower by approximately four orders of magnitude (7.65 × 10⁻⁹ A/cm²). After near infrared 808 nm laser irradiation, the i_corr of the modified samples increased to 3.48 × 10⁻⁷ A/cm². The mechanism is that the local hyperthermia induced the free volume expansion of PLGA, and the increase in intermolecular gap enhanced the penetration of electrolytes. Meanwhile, the cytotoxicity study showed that the hybrid coating endowed the Mg-Sr alloy with enhanced biocompatibility.

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PLGA/Ti₃C₂ hybrid coating enhanced the corrosion resistance of the Mg-Sr alloy.

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PLGA/Ti₃C₂ hybrid coating exhibited outstanding photothermal performance.

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The degradation rate was adjusted by near infrared light.

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Providing a promising strategy for remote controlling degradation of Mg-based alloy.

Effect of strain on degradation behaviors of WE43, Fe and Zn wires

The biodegradable metallic devices undergo stress/strain-induced corrosion when they are used for load-bearing applications. The stress/strain induced-corrosion behavior causes differences in corrosion rate, corrosion morphology, strain distribution and mechanical performance of the devices. One representative example is the biodegradable stent. Biodegradable stents undergo complex inhomogeneous deformation that can cause dramatic non-uniform stent degradation, resulting in stress concentration and stents failure. The degradation of biodegradable devices requires special attention to the mutual effect between the applied strain and degradation. The quantitative relationship between strain and corrosion of the sample alloys (WE43, Fe and Zn), selected from three typical biodegradable metals, is firstly investigated and compared in this study. The in vitro degradation and the strength retention of WE43, Fe and Zn wires were investigated under different elastic and plastic strain levels ranging from 0.1% to 30%. The results indicated that the applied strain could bring down the corrosion potential, increase corrosion current and accelerate the degradation of three biodegradable metals. Specifically, remarkable enhanced localized corrosion was observed for plastic strained WE43 compared with those with elastic strains. This localized corrosion morphology significantly accelerated the strength decline at first, while the differences diminished with longer immersion period. Fe and Zn exhibited increased degradation with plastic strain applications than those under elastic strains. However, the degradation was not further increased with the increasing magnitude of plastic strains. Moreover, the bended wires were subcutaneously implanted in the dorsal aspect of the rats and the effect of bending deformation on in vitro and in vivo degradation of three metallic wires were also compared. The U-bended WE43 wires suffered more severe in vitro degradation at the stress concentrated region. Surprisingly, the early fracture of the undeformed regions was observed in the in vivo test. In conclusion, the corrosion rate, corrosion morphology and mechanical properties of WE43, Fe and Zn was sensitive to magnitude of the applied strains. The quantification results provided new insights into understanding the strain-dependent corrosion of three biodegradable metals both in vitro and in vivo. STATEMENT OF SIGNIFICANCE: Biodegradable implants are subjected to various mechanical environment during the deployment and subsequent physiological activity. It is necessary to have a clear understanding of the effects of the applied stress on degradation. This study addresses the quantitative effects of applied strain/stress on the in vitro and in vivo degradation of three typical biodegradable metals (Mg, Fe and Zn). These quantification results provide new insights into understanding the strain-induced corrosion of three metals.

Degradation and osteogenic induction of a SrHPO4-coated Mg-Nd-Zn-Zr alloy intramedullary nail in a rat femoral shaft fracture model

Magnesium and Mg-based alloys are promising biomaterials for orthopedic implants because of their degradability, osteogenic effects, and biocompatibility. However, the drawbacks of these materials include high hydrogen gas production, unexpected corrosion resistance, and insufficient mechanical strength duration. Surface modification can protect these biomaterials and induce osteogenesis. In this work, a SrHPO₄ coating was developed for our patented biodegradable Mg-Nd-Zn-Zr alloy (abbr. JDBM) through a chemical deposition method. The coating was characterized by in vitro immersion, ion release, and cytotoxicity tests, which showed a slower corrosion behavior and excellent cell viability. RNA sequencing of MC3T3E1 cells treated with SrHPO₄-coated JDBM ion release test extract showed increased Tlr4, followed by the activation of the downstream PI3K/Akt signaling pathway, causing proliferation and growth of pre-osteoblasts. An intramedullary nail (IMN) was implanted in a femoral fracture rat model. Mechanical test, radiological and histological analysis suggested that SrHPO₄-coated JDBM has superior mechanical properties, induces more bone formation, and decreases the degradation rate compared with uncoated JDBM and the administration of TLR4 inhibitor attenuated the new bone formation for fracture healing. SrHPO₄ is a promising coating for JDBM implants, particularly for long-bone fractures.

Electrodeposited Assembly of Additive-Free Silk Fibroin Coating from Pre-Assembled Nanospheres for Drug Delivery

Electrophoretically deposited (EPD) polymer-based coatings have been extensively reported as reservoirs in medical devices for delivery of therapeutic agents, but control over drug release remains a challenge. Here, a simple but uncommon assembly strategy for EPD polymer coatings was proposed to improve drug release without introducing any additives except the EPD matrix polymer precursor. The added value of the proposed strategy was demonstrated by developing a novel EPD silk fibroin (SF) coating assembled from pre-assembled SF nanospheres for an application model, that is, preventing infections around percutaneous orthopedic implants via local delivery of antibiotics. The EPD mechanism of this nanosphere coating involved oxidation of water near the substrate to neutralize SF nanospheres, resulting in irreversible deposition. The deposition process and mass could be easily controlled using the applied EPD parameters. In comparison with the EPD SF coating assembled in a conventional way (directly obtained from SF molecule solutions), this novel coating had a similar adhesion strength but exhibited a more hydrophobic nanotopography to induce better fibroblastic response. Moreover, the use of nanospheres as building blocks enabled 1.38 and 21 times enhancement on the antibiotic release amount and time (of 95% maximum dug release), respectively, while retaining drug effectiveness and showing undetectable cytotoxicity. This unexpected release kinetics was found attributable to the electrostatic and hydrophobic interactions between the drug and nanospheres and a negligible initial dissolution effect on the nanosphere coating. These results illustrate the promising potential of the pre-assembled strategy on EPD polymer coatings for superior control over drug delivery.

Enhanced osteoinductivity and corrosion resistance of dopamine/gelatin/rhBMP-2-coated β-TCP/Mg-Zn orthopedic implants: An in vitro and in vivo study

Magnesium-based biomaterials are attracting increasingly more attention for orthopedic applications based on their appropriate mechanical properties, biodegradability, and favorable biocompatibility. However, the high corrosion rate of these materials remains to be addressed. In this study, porous β-Ca3(PO4)2/Mg-Zn (β-TCP/Mg-Zn) composites were fabricated via a powder metallurgy method. The β-TCP/Mg-Zn composites with 6% porosity exhibited optimal mechanical properties, and thus, they were selected for surface modification. A novel dopamine/gelatin/recombinant human bone morphogenetic protein-2 (rhBMP-2) coating with demonstrated stability was prepared to further improve the corrosion resistance of the composite and enhance early osteoinductivity. The homogeneously coated β-TCP/Mg-Zn composite showed significantly improved corrosion resistance according to electrochemical and immersion tests. In addition, extracts from the dopamine/gelatin/rhBMP-2-coated β-TCP/Mg-Zn composite not only facilitated cell proliferation but also significantly enhanced the osteogenic differentiation of Sprague-Dawley rat bone marrow-derived mesenchymal stem cells in vitro. Furthermore, in vivo experiments were performed to evaluate the biodegradation, histocompatibility, and osteoinductive potential of the coated composite. No obvious pathological changes in the vital visceral organs were observed after implantation, and radiography and hematoxylin-eosin staining showed strong promotion of new bone formation, matched composite degradation and bone regeneration rates, and complete absorption of the released hydrogen gas. Collectively, these results indicate that the dopamine/gelatin/rhBMP-2-coated β-TCP/Mg-Zn composite offers improved corrosion resistance, favorable biocompatibility, and enhanced osteoinductive potential for use in the fabrication of orthopedic implants.

Green Tea Polyphenol Induced Mg2+-rich Multilayer Conversion Coating: Toward Enhanced Corrosion Resistance and Promoted in Situ Endothelialization of AZ31 for Potential Cardiovascular Applications

As a promising biodegradable metallic material, magnesium (Mg) and its alloys have attracted special attention in the recent decade. However, challenges still remain due to its high corrosion rate and insufficient biocompatibility after implantation. In this work, we prepare a simple and versatile green tea phenol-metal induced multilayer conversion coating (Mg²⁺ incorporated epigallocatechin gallate (EGCG) coating) on magnesium alloys' (AZ31) substrate by layer-by-layer (LBL) method. The surface morphology results revealed that, with the incorporation of Mg²⁺, the as-formed EGCG/Mg coating was rich in phenol-Mg complex and presented more homogeneous and dense morphology, with far less cracks than the pure EGCG coating. The in vitro degradation rate and corrosion resistance were studied by electrochemical corrosion tests and monitoring of the changed pH value and hydrogen evolution, respectively, which revealed that the corrosion rate was effectively decreased compared to that of bare AZ31 after it was protected by EGCG/Mg coating. In vitro and ex vivo thrombogenicity test demonstrated the EGCG/Mg coatings presented an impressive improvement in decreasing the adhesion and activation of platelets and erythrocytes, in activated partial thromboplastin time (APTT), and in antithrombogenicity compared to those of bare AZ31. Owing to the mild degradation rate, in combination with the biological function of EGCG, enhanced endothelial cells' (ECs') adhesion and proliferation, suppressed smooth muscle cells' (SMCs') adhesion/proliferation, and inhibited cytokine release were observed on EGCG/Mg coated AZ31 alloy. Besides, the in vivo subcutaneous embedding experiment suggested that the EGCG/Mg coating performed more mild tissue response due to the improved corrosion resistance to the surrounding microenvironment. Moreover, for in vivo abdominal aorta assay, the EGCG/Mg coated AZ31 wire presented better corrosion resistance and enhanced re-endothelialization compared to bare AZ31 wire. These results suggested the potential of using green tea polyphenol induced Mg²⁺-rich multilayer conversion coating for enhanced corrosion protection and desired biocompatibility of biodegradable cardiovascular implants.

Dual Function of Magnesium in Bone Biomineralization

Magnesium (Mg²⁺ ), as a main component of bone, is widely applied to promote bone growth and regeneration. However, Mg²⁺ can chemically inhibit the crystallization of amorphous calcium phosphate into hydroxyapatite (HA). The underlying mechanisms by which Mg²⁺ improves bone biomineralization remain elusive. Here, it is demonstrated that Mg²⁺ plays dual roles in bone biomineralization from a developmental perspective. During embryonic development, the Mg²⁺ concentration is enriched in the early stage from embryonic day 13.5 (E13.5) to E15.5, but gradually decreases to a stable state in the late phase, after E15.5. Appropriate concentrations of Mg²⁺ can promote the mineralization of bone marrow mesenchymal stem cells, while excessive Mg²⁺ impairs their osteogenesis. The earlier the Mg²⁺ is added, the stronger the observed inhibition of mineralization. In particular, less Mg²⁺ is present in fully mineralized collagen than in poorly mineralized collagen. Furthermore, a high concentration of Mg²⁺ changes the crystalline morphology of HA and inhibits collagen calcification. Functionally, a high-Mg²⁺ diet inhibits bone biomineralization in mouse offspring. Taken together, the results suggest that appropriate regulation of Mg²⁺ concentration over time is vital for normal biomineralization. This study is significant for the future design of bone substitutes and implants associated with Mg²⁺ content.