Changli Zhao

Research on an Mg–Zn alloy as a degradable biomaterial

In this study a binary Mg-Zn magnesium alloy was researched as a degradable biomedical material. An Mg-Zn alloy fabricated with high-purity raw materials and using a clean melting process had very low levels of impurities. After solid solution treatment and hot working the grain size of the Mg-Zn alloy was finer and a uniform single phase was gained. The mechanical properties of this Mg-Zn alloy were suitable for implant applications, i.e. the tensile strength and elongation achieved were approximately 279.5MPa and 18.8%, respectively. The results of in vitro degradation experiments including electrochemical measurements and immersion tests revealed that the zinc could elevate the corrosion potential of Mg in simulated body fluid (SBF) and reduce the degradation rate. The corrosion products on the surface of Mg-Zn were hydroxyapatite (HA) and other Mg/Ca phosphates in SBF. In addition, the influence caused by in vitro degradation on mechanical properties was studied, and the results showed that the bending strength of Mg-Zn alloy dropped sharply in the earlier stage of degradation, while smoothly during the later period. The in vitro cytotoxicity of Mg-Zn was examined. The result 0-1 grade revealed that the Mg-Zn alloy was harmless to L-929 cells. For in vivo experiments, Mg-Zn rods were implanted into the femoral shaft of rabbits. The radiographs illustrated that the magnesium alloy could be gradually absorbed in vivo at about 2.32mm/yr degradation rate obtained by weight loss method. Hematoxylin and eosin (HE) stained section around Mg-Zn rods suggested that there were newly formed bone surrounding the implant. HE stained tissue (containing heart, liver, kidney and spleen tissues) and the biochemical measurements, including serum magnesium, serum creatinine (CREA), blood urea nitrogen (BUN), glutamic-pyruvic transaminase (GPT) and creatine kinase (CK) proved that the in vivo degradation of Mg-Zn did not harm the important organs. Moreover, no adverse effects of hydrogen generated by degradation had been observed and also no negative effects caused by the release of zinc were detected. These results suggested that the novel Mg-Zn binary alloy had good biocompatibility in vivo.

Electrodeposition of Ca-P coatings on biodegradable Mg alloy: in vitro biomineralization behavior.

Preparing stabilized apatite on biodegradable Mg alloy may improve biocompatibility and promote osteointegration. In the present work, three kinds of Ca-P coatings, brushite (DCPD, CaHPO(4).2H(2)O), hydroxyapatite (HA, Ca(10)(PO(4))(6)(OH)(2)) and fluoridated hydroxyapatite (FHA, Ca(5)(PO(4))(3)(OH)(1-)(x)F(x)) are fabricated by electrodeposition on a biodegradable Mg-Zn alloy. The crystalline structures, morphologies and compositions of these Ca-P coatings have been characterized by X-ray diffrection, scanning electron microscopy and energy-dispersive spectoscopy. The effects of these coatings on the degradation behavior and mineralization activity of the Mg-Zn alloy have also been investigated. The experimental results showed that these coatings decreased the degradation rate of Mg-Zn alloy, while the precipitates on the uncoated and DCPD-coated Mg-Zn alloy in modified simulated biological fluid had low Ca/P molar ratios, which delayed bone-like apatite formation. Both the HA and FHA coating could promote the nucleation of osteoconductive minerals (bone-like apatite or beta-TCP) for 1month. However, the HA coating transformed from DCPD through alkali heat treatment was fragile and less stable, and therefore its long-term corrosion resistance was not satisfactory. Instead, the FHA was more stable and had better corrosion resistance, and thus it should be better suited as a coating of Mg implants for orthopedic applications.

In vitro responses of human bone marrow stromal cells to a fluoridated hydroxyapatite coated biodegradable Mg—Zn alloy

Bone-like fluoridated hydroxyapatite (FHA) coatings were prepared on Mg-6 wt.%Zn substrates using electrochemical method. Human bone marrow stromal cells (hBMSCs) were utilized to investigate the cellular biocompatibility of Mg-6 wt.%Zn alloy after surface modification. The adhesion of hBMSCs was evaluated using scanning electron microscopy (SEM) and laser scanning confocal microscopy (LSCM). The proliferation of the cells was also measured by carrying out the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) test. And the alkaline phosphatase activity (ALP) was assessed to evaluate the early stage of differentiation. Lastly, reverse transcription-polymerase chain reaction (RT-PCR) test was taken. It was found that the hBMSCs displayed better cell functions on the bioactive FHA coated alloy, compared to the bare Mg-6 wt.%Zn alloy. The in vitro results indicated that the bioactive FHA coating can improve the interfacial bioactivity of Mg-6 wt.%Zn substrate, specifically, both on biodegradation behavior control and good cellular proliferation and differentiation.

Interaction between a high purity magnesium surface and PCL and PLA coatings during dynamic degradation.

In this study, polycaprolactone (PCL) and polylactic acid (PLA) coatings were prepared on the surface of high purity magnesium (HPMs), respectively, and electrochemical and dynamic degradation tests were used to investigate the degradation behaviors of these polymer-coated HPMs. The experimental results indicated that two uniform and smooth polymer films with thicknesses between 15 and 20 µm were successfully prepared on the HPMs. Electrochemical tests showed that both PCL-coated and PLA-coated HPMs had higher free corrosion potentials (E(corr)) and smaller corrosion currents (I(corr)) in the modified simulated body fluid (m-SBF) at 37 °C, compared to those of the uncoated HPMs. Dynamic degradation tests simulating the flow conditions in coronary arteries were carried out on a specific test platform. The weight of the specimens and the pH over the tests were recorded to characterize the corrosion performance of those samples. The surfaces of the specimens after the dynamic degradation tests were also examined. The data implied that there was a special interaction between HPM and its polymer coatings during the dynamic degradation tests, which undermined the corrosion resistance of the coated HPMs. A model was proposed to illustrate the interaction between the polymer coatings and HPM. This study also suggested that this reciprocity may also exist on the implanted magnesium stents coated with biodegradable polymers, which is a potential obstacle for the further development of drug-eluting magnesium stents.

In vitro and in vivo studies on the degradation of high-purity Mg (99.99wt.%) screw with femoral intracondylar fractured rabbit model

High-purity magnesium (HP Mg) takes advantage in no alloying toxic elements and slower degradation rate in lack of second phases and micro-galvanic corrosion. In this study, as rolled HP Mg was fabricated into screws and went through in vitro immersion tests, cytotoxicity test and bioactive analysis. The HP Mg screws performed uniform corrosion behavior in vitro, and its extraction promoted cell viability, bone alkaline phosphatase (ALP) activity, and mRNA expression of osteogenic differentiation related gene, i.e. ALP, osteopontin (OPN) and RUNX2 of human bone marrow mesenchymal stem cells (hBMSCs). Then HP Mg screws were implanted in vivo as load-bearing implant to fix bone fracture and subsequently gross observation, range of motion (ROM), X-ray scanning, qualitative micro-computed tomography (μCT) analysis, histological analysis, bending-force test and SEM morphology of retrieved screws were performed respectively at 4, 8, 16 and 24 weeks. As a result, the retrieved HP Mg screws in fixation of rabbit femoral intracondylar fracture showed uniform degradation morphology and enough bending force. However, part of PLLA screws was broken in bolt, although its screw thread was still intact. Good osseointegration was revealed surrounding HP Mg screws and increased bone volume and bone mineral density were detected at fracture gap, indicating the rigid fixation and enhanced fracture healing process provided by HP Mg screws. Consequently, the HP Mg showed great potential as internal fixation devices in intra-articular fracture operation.

Hierarchical titanium surface textures affect osteoblastic functions

This study investigated the surface characteristics and in vitro cytocompatibility of hierarchical textured titanium surfaces with nanograins and microroughness, produced by surface mechanical attrition treatment (SMAT). The surface characteristics were evaluated by scanning electron microscopy, X-ray diffraction, transmission electron microscopy, contact angle, and surface energy measurements. The in vitro cytocompatibility of the SMAT processed surfaces (hereafter Ti-SMAT surfaces) were assessed in terms of cellular attachment, morphology, viability, alkaline phosphatase (ALP) activity, and mRNA gene expression. Two other titanium surfaces were compared: well-polished Ti6Al4V surfaces (hereafter Ti-polish surfaces) and thermally sprayed rough surfaces (hereafter Ti-spray surfaces). The Ti-SMAT surfaces showed a higher hydrophilicity and increased surface energy compared with the Ti-polish and Ti-spray surfaces. Consequently, these Ti-SMAT surfaces demonstrated enhancement of cell attachment, spreading, viability, and ALP activity. Reverse transcriptase polymerase chain reaction (RT-PCR) analysis showed significantly higher ALP activity and stronger expression of mRNA levels of key osteoblast genes in cells grown on the Ti-SMAT surfaces than the other two surfaces. These results reveal a synergic role played by nanostructure and microtopography in osteoblastic functions and demonstrate the more promising cytocompatibility of the hierarchical textured surfaces. It is suggested that the SMAT process may provide a novel method of surface modification to the currently available metallic biomaterials.

Ag-Incorporated FHA Coating on Pure Mg: Degradation and in Vitro Antibacterial Properties.

Fluoridated hydroxyapatite (FHA) coating can help retard the degradation of magnesium, and possess good biocompatibility. However, the antibacterial property of FHA is very limited. In this work, we aimed to incorporate silver into FHA structure to fabricate biocompatible and antibacterial coatings with enhanced anticorrosion property. The results showed that the Ag-FHA coating prepared by electrochemical deposition and subsequent immersion in AgNO3 solution was superior to the Ag-FHA coating prepared by coelectrodeposition in terms of crystal structure, surface morphology and corrosion resistance. The release of Ag(+) ion causing high antiplanktonic bacterial rate and excellent antiadherence property to MRSA. Meanwhile, good cell compatibility of MC3T3-E1 including cell viability, cell adhesion, and cell morphology was achieved under the controlled degradation. The balance of degradation and antimicrobial property of Ag-incorporated FHA coating made it an alternative in the application of surface modification for biodegradable Mg.

Guided proliferation and bone-forming functionality on highly ordered large diameter TiO2 nanotube arrays.

The significantly enhanced osteoblast adhesion, proliferation and alkaline phosphatase (ALP) activity were observed on TiO2 nanotube surface in recent studies in which the scale of nanotube diameter was restricted under 100 nm. In this paper, a series of highly ordered TiO2 nanotube arrays with larger diameters ranging from 150 nm to 470 nm were fabricated via high voltage anodization. The behaviors of MC3T3-E1 cells in response to the diameter-controlled TiO2 nanotubes were investigated. A contrast between the trend of proliferation and the trend of cell elongation was observed. The highest cell elongation (nearly 10:1) and the lowest cell number were observed on the TiO2 nanotube arrays with 150 nm diameter. While, the lowest cell elongation and highest cell number were achieved on the TiO2 nanotube arrays with 470 nm diameter. Furthermore, the ALP activity peaked on the 150 nm diameter TiO2 nanotube arrays and decreased dramatically with the increase of nanotube diameter. Thus a narrow range of diameter (100-200 nm) that could induce the greatest bone-forming activity is determined. It is expected that more delicate design of orthopedic implant with regional abduction of cell proliferation or bone forming could be achieved by controlling the diameter of TiO2 nanotubes.

Comparison of the effects of Mg–6Zn and Ti–3Al–2.5V alloys on TGF-β/TNF-α/VEGF/b-FGF in the healing of the intestinal tract in vivo

To evaluate the different effects of Mg-6Zn alloy and Ti-3Al-2.5V alloy implants in intestinal tract healing, we compared these two different alloys with respect to their effect on a rats intestinal tract, using serum magnesium, radiology, pathology and immunohistochemistry in vivo. It was found using the scanning electron microscope that the Mg-6Zn alloy began to degrade during the first week and that the Ti-3Al-2.5V alloy was non-degradable throughout the process. The Mg-6Zn alloy did not have an impact on serum magnesium. Superior to the Ti-3Al-2.5V alloy, the Mg-6Zn alloy enhanced the expression of transforming growth factor-β1 in healing tissue, and promoted the expression of both the vascular endothelial growth factor and the basic fibroblast growth factor, which helped angiogenesis and healing. The Mg-6Zn alloy reduced the expression of the tumor necrosis factor (TNF-α) at different stages and decreased inflammatory response, which may have been related to the zinc inhibiting TNF-α. In general, the Mg-6Zn alloy performed better than Ti-3Al-2.5V at promoting healing and reducing inflammation. The Mg-6Zn alloy may be a promising candidate for use in the pins of circular staplers for gastrointestinal reconstruction in medicine.

Magnesium inference screw supports early graft incorporation with inhibition of graft degradation in anterior cruciate ligament reconstruction.

Patients after anterior cruciate ligament (ACL) reconstruction surgery commonly encounters graft failure in the initial phase of rehabilitation. The inhibition of graft degradation is crucial for the successful reconstruction of the ACL. Here, we used biodegradable high-purity magnesium (HP Mg) screws in the rabbit model of ACL reconstruction with titanium (Ti) screws as a control and analyzed the graft degradation and screw corrosion using direct pull-out tests, microCT scanning, and histological and immunohistochemical staining. The most noteworthy finding was that tendon graft fixed by HP Mg screws exhibited biomechanical properties substantially superior to that by Ti screws and the relative area of collagen fiber at the tendon-bone interface was much larger in the Mg group, when severe graft degradation was identified in the histological analysis at 3 weeks. Semi-quantitative immunohistochemical results further elucidated that the MMP-13 expression significantly decreased surrounding HP Mg screws with relatively higher Collagen II expression. And HP Mg screws exhibited uniform corrosion behavior without displacement or loosening in the femoral tunnel. Therefore, our results demonstrated that Mg screw inhibited graft degradation and improved biomechanical properties of tendon graft during the early phase of graft healing and highlighted its potential in ACL reconstruction.