Jianan Li

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.

Comparison of the effects of Mg–6Zn and titanium on intestinal tract in vivo

To evaluate the ability of Mg–6Zn to replace titanium nails in the reconstruction of the intestinal tract in general surgery, we compared the Mg–6Zn and titanium implants with respect to their effects on rat’s intestinal tract by biochemical, radiological, pathological and immunohistochemical methods. The results indicated that Mg–6Zn implants started to degrade at the third week and disintegrate at the fourth week. No bubbles appeared, which may be associated with intestinal absorption of the Mg–6Zn implants. Pathological analyses (containing liver, kidney and cecum tissues) and biochemical measurements, including serum magnesium, creatinine, blood urea nitrogen, glutamic–pyruvic–transaminase and glutamic–oxaloacetic–transaminase proved that degradation of Mg–6Zn did not harm the important organs, which is an improvement over titanium implants. Immunohistochemical results showed that Mg–6Zn could enhance the expression of transforming growth factor-β1. Mg–6Zn reduced the expression of tumor necrosis factor at different stages. In general, our study demonstrates that the Mg–6Zn alloy had good biocompatibility in vivo and performed better than titanium at promoting healing and reducing inflammation. It may be a promising candidate for stapler pins in intestinal reconstruction.

Effects of biodegradable Mg–6Zn alloy extracts on cell cycle of intestinal epithelial cells

In this study, intestinal epithelial cells (IEC)-6 were cultured in different concentration extracts of Mg–6Zn alloys for different time periods. We studied the indirect effects of Mg–6Zn alloys on cell cycle of IEC-6 cells. The cell cycle of IEC-6 cells was measured using flow cytometry. And, the cell cycle of IEC-6 cells was evaluated by investigating the expression of cyclin D1, CDK4, and P21 using real-time polymerase chain reaction (PCR) and Western blotting tests. It was found that the IEC-6 cells displayed better cell functions in 20% extract of the Mg–6Zn alloy extracts, compared to the 100% or 60% extract. The in vitro results indicated that the conspicuous alkaline environment that is a result of rapid corrosion of Mg–6Zn alloys is disadvantageous to cell cycle of IEC-6 cells.

Shape and site dependent in vivo degradation of Mg-Zn pins in rabbit femoral condyle.

A type of specially designed pin model of Mg-Zn alloy was implanted into the full thickness of lesions of New Zealand rabbits’ femoral condyles. The recovery progress, outer surface healing and in vivo degradation were characterized by various methods including radiographs, Micro-CT scan with surface rendering, SEM (scanning electron microscope) with EDX (Energy Dispersive X-ray analysis) and so on. The in vivo results suggested that a few but not sufficient bridges for holding force were formed between the bone and the implant if there was a preexisting gap between them. The rapid degradation of the implantation in the condyle would result in the appearance of cavities. Morphological evaluation of the specially designed pins indicated that the cusp was the most vulnerable part during degradation. Furthermore, different implantation sites with distinct components and biological functions can lead to different degradation rates of Mg-Zn alloy. The rate of Mg-Zn alloy decreases in the following order: implantation into soft tissue, less trabecular bone, more trabecular bone, and cortical bone. Because of the complexities of in vivo degradation, it is necessary for the design of biomedical Mg-Zn devices to take into consideration the implantation sites used in clinics.

The Bioactivated Interfacial Behavior of the Fluoridated Hydroxyapatite-Coated Mg-Zn Alloy in Cell Culture Environments

A partially fluorine substituted hydroxyapatite- (FHA-) coated Mg-Zn alloy was prepared to investigate the interfacial behavior of degradable Mg-based biomaterials with degradable bioactive coatings in a cell culture environment. Peaks from the results of X-ray diffraction (XRD) were characterized and compared before and after cell culture. It was found that Ca-P, including poorly crystalline ion-substituted Ca-deficient HA (CDHA), was formed in greater amounts on the interface of coated samples compared with the uncoated ones. A thermodynamic mechanism for Ca-P formation on biodegradable Mg alloys in a cell culture environment is proposed. Combined with improved cell calcification, the-FHA coated Mg alloys have the ability to promote CDHA formation, as expected thermodynamically. It is suggested that the specific cell culture environment and the bone-like FHA coatings together facilitate the observed behavior. Moreover, cell culture environment probably increased the biomineralization to a detectable level by affecting the kinetics of apatite formation.