Frontiers of Materials Science (v.4, #2)

A review on magnesium alloys as biodegradable materials by Xue-Nan Gu; Yu-Feng Zheng (111-115).
Magnesium alloys attracted great attention as a new kind of degradable biomaterials. One research direction of biomedical magnesium alloys is based on the industrial magnesium alloys system, and another is the self-designed biomedical magnesium alloys from the viewpoint of biomaterials. The mechanical, biocorrosion properties and biocompatibilities of currently reported Mg alloys were summarized in the present paper, with the mechanical properties of bone tissue, the healing period postsurgery, the pathophysiology and toxicology of the alloying elements being discussed. The strategy in the future development of biomedical Mg alloys was proposed.
Keywords: biomaterials; magnesium alloys; degradation; corrosion

Influence of dicalcium phosphate dihydrate coating on the in vitro degradation of Mg-Zn alloy by Shao-Xiang Zhang; Jia-Nan Li; Yang Song; Chang-Li Zhao; Xiao-Nong Zhang (116-119).
To reduce the degradation rate and further to improve the biocompatibility of magnesium alloy, dicalcium phosphate dihydrate (CaHPO4·2H2O, DCPD) has been fabricated on a kind of magnesium-zinc alloy by way of electrodeposition method. The experimental results of XRD, SEM and EDS showed that the electrodeposited coating on the Mg-Zn alloy mainly contains the flake-like DCPD, along with some octacalcium phosphate (Ca8(HPO4)2(PO4)4·4H2O, OCP). After the in vitro degradation of the coated alloy in modified-simulated body fluid (m-SBF), it was proved that the coating could reduce the degradation rate effectively, and the samples were covered by calcium phosphate salts with a higher Ca/P ratio. Therefore, it indicates that compared with the bare alloy, the DCPD coating rendered a more biocompatible surface, and is a promising coating candidate for biomedical magnesium materials.
Keywords: biodegradable; magnesium; corrosion; coating; DCPD

The corrosion degradation behavior and in vivo test of 4N-Mg after surface modification by heat-self-assembled monolayer (HSAM) were studied in this paper. Comprehensive techniques were used to analyze the corrosion process, corrosion rate and corrosion mechanism. The Mg samples with and without modifications were embedded in the thigh bones of white rabbits, and TC4 was used as the control. The concentration of Mg2+ ion in blood were analyzed after 2, 6, and 12 weeks of implantation. Then the specimens were analyzed by scanning electron microscope (SEM), and tissue slices were observed by optical microscope. The results show that the modified Mg had better corrosion resistance. The in vivo study confirmed that the magnesium concentration in blood was in the normal scale, and the Mg have good bone inducement and showed excellent capabilty of contact with bone. After implantations for a few weeks, the new bone obviously mineralizes on the interface between the modified Mg and bone. The untreated magnesium corroded faster than the HSAM treated magnesium in vivo. All the results indicated that Mg showed better biocompatibility and the capacity of inducing new bone due to HSAM had an effect on controlling the corrsion rate of Mg.
Keywords: magnesium; biomaterials; surface modification; biocompatibility

Effect of Mg2+ concentration on biocompatibility of pure magnesium by Jia-Cheng Gao; Li-Ying Qiao; Ren-Long Xin (126-131).
The aim of the present study is to find the correlation between the Mg2+ concentration degraded from pure magnesium material and the biocompatibility of the material. Hemolysis ratio (HR) of the extracts of pure magnesium with different Mg2+ concentration were measured according to ISO 10993.4 standard. The cytotoxicity tests were carried out by both indirect contact with fibroblast L929 and preosteoblasts MC3T3-E1, and MTT tests were used. Cytotoxicity of the pure magnesium with and without surface modification was further evaluated by direct contact method. Samples were cultured with Osteoblast MC3T3-E1 and the effects of the material on viability and activity of cells were discussed. The results showed that the hemolysis rate and cytotoxicity of the modified Mg could meet the requirement for biomaterials. In our test, the hemolysis rate of the extracts was qualified when the concentration of Mg2+ ⩽ 42 mg/L; the extracts with 202 mg/L Mg2+ met the cytotoxicity requirement, and the extracts with 156 mg/L Mg2+ promoted cell proliferation. Therefore, the biocompatibility of magnesium-based materials can be improved by suitable surface modification.
Keywords: magnesium; biomaterials; surface modification; biocompatibility

The samples made of a Mg-2.5wt.%Zn-0.5wt.%Zr alloy were immersed in the 20% hydrofluoric acid (HF) solution at room temperature for different time, with the aim of improving the properties of magnesium (Mg) alloy in applications as biomaterials. The corrosion resistance and in vitro biocompatibility of untreated and fluoride-coated samples were investigated. The results show that the optimum process is to immerse Mg alloys in the 20% HF solution for 6 h. After the immersion, a dense magnesium fluoride (MgF2) coating of 0.5 μm was synthesized on the surface of Mg-Zn-Zr alloy. Polarization tests recorded a reduction in the corrosion current density from 2.10 to 0.05 μA/cm2 due to the MgF2 protective coating. Immersion tests in the simulated body fluid (SBF) also reveal a much milder corrosion on the fluoride-coated samples, and its corrosion rate was calculated to be 0.05 mm/yr. Hemolysis test suggests that the conversion coated Mg alloy has no obvious hemolysis reaction. The hemolysis ratio (HR) of the samples decreases from 11.34% to 1.86% with the HF treatment, which meets the requirements of biomaterials (HR < 5%). The coculture of 3T3 fibroblasts with Mg alloy results in the adhesion and proliferation of cells on the surface of fluoride-coated samples. All the results show that the MgF2 conversion coating would markedly improve the corrosion resistance and in vitro biocompatibility of Mg-Zn-Zr alloy.
Keywords: Mg-Zn-Zr Alloy; hydrofluoric acid (HF); corrosion resistance; in vitro biocompatibility; biomaterials

Fabrication and corrosion behavior of HA/Mg-Zn biocomposites by De-Bao Liu; Ming-Fang Chen; Xin-Yu Ye (139-144).
The thermal-treated hydroxyapatite (HA) particles, Mg and Zn powders were used to prepare the HA/Mg-Zn composites with different HA contents by means of powder metallurgy technology. The microstructures, formation phases, and corrosion behaviors in simulated body fluid (SBF) were studied in comparison with pure magnesium and HA/Mg composites fabricated by the same preparation technology. As a result, no evident reaction happened between HA particles and Mg matrix during sintering process, and Zn atoms diffused into Mg matrix to form a single phase Mg-Zn alloy matrix. The addition of HA particles changed the corrosion mechanism of Mg matrix. During the corrosion process, HA particles would adsorb PO 4 3− and Ca2+ ions efficiently and induce the deposition of Ca-P compounds on the surface of composites. HA could improve the corrosion resistance of magnesium matrix composites in SBF and restrain the increase of pH of SBF. Furthermore, the addition of Zn was favorable to improve the corrosion resistance of HA/Mg composites due to the densification of composites and the formation of Mg-Zn alloy matrix.
Keywords: composites; hydroxyapatite (HA); Mg-Zn alloy; fabrication; corrosion behavior

Molecular self-assembly is ubiquitous in nature and has recently emerged as a new bottom-up approach in constructing biomaterials. Synthetic peptides assemble through specific molecular recognition and form diverse nanostructures. The resulting versatile peptide self-assemblies may be used in a wide range of biological and medical applications. Examples of two self-assembling peptide systems are presented and techniques for self-assembly control are discussed.
Keywords: peptide; biomaterial; self-assembly

An extremely simple approach is described here to synthesize bulk quantities of conducting polymer microspirals assembled from nanofibers by in situ chemical oxidative polymerization in the presence of a conventional surfactant. It is worth noting that the surfactant used in our approach is in crystalline state, which is quite different from micellar state in emulsional polymerization reported previously. The growth mechanism of the conducting polymer is proposed.
Keywords: conducting polymer; surfactant; spiral; crystal defect

A mesoporous silica modified conjugated polymer film: Preparation and detection nitroaromatics in aqueous phase by Feng Liu; Li-Juan Zhang; Jian-Hua Xiao; Jun Hu; Hong-Lai Liu (158-163).
The linear conjugated polymer of polyfluorene/poly(p-phenylenevinylenes) (PFO/PPV) was synthesized and selected as the matrix film. The incorporation of a small fraction of inorganic mesoporous silicas into the polymer matrix resulted in a significant increase in overall detection efficiency of nitroaromatics. The structure of the obtained mixed films was characterized by 1H-NMR, IR, and XRD. The optical and fluorescence properties of the mixed films were demonstrated by ultraviolet and visible spectroscopy (UV-Vis) and photoluminescence (PL) spectra in ethanol/water solution. Compared with the pure conjugated polymer film, the mixed films had high fluorescence quenching sensitivity toward nitroaromatic compounds, especially 2,4,6-trinitrophenol (TNP), in the aqueous phase. The fluorescent emission quenching is quantitative and can be analyzed on the basis of the Stern-Volmer model relation with the quenching process. The results revealed that such mixed matrix films of conjugated polymer/mesoporous silicas can be a promising strategy of designing chemosensory materials for detecting nitroaromatic compounds.
Keywords: nitroaromatic compounds; Heck coupling; fluorescent quenching; aqueous phase

Synthesis and characterization of a thermo-sensitive poly(N-methyl acryloylglycine methyl ester) used as a drug release carrier by Kui-Lin Deng; Hai-Bin Zhong; Yi-Suo Jiao; Ting Fan; Xiao Qiao; Peng-Fei Zhang; Xiao-Bo Ren (164-170).
In this article, poly(N-methyl acryloylglycine methyl ester) (PNMAME) was prepared as a novel thermosensitive material with a lower critical solution temperature (LCST) at around 49.5°C. The chemical structures of the monomer NMAME and PNMAME were characterized by 1H NMR and IR measurements. The LCST was investigated systematically as a function of PNMAME concentration, inorganic salt solution and pH value. The results indicated that LCST of PNMAME was obviously dependent on PNMAME concentration and pH. The LCST was increased with a decrease in pH value and PNMAME concentration. To obtain a thermo-sensitive hydrogel with the phase transition temperature close to human body temperature, the copolymerization was conducted between NMAME and N-acryloylglycine ethyl ester (NAGEE). The release behavior of caffeine was evaluated at different temperatures and contents of cross-linkers (N, N-methylenebis(acrylamide) (NMBA)). The increase of cross-linker content led to a decrease in the release rate of caffeine due to higher crossing density in the hydrogel network. In addition, a faster release of caffeine from the hydrogel with 3% NMBA at 37°C was found in contrast to that at 18°C.
Keywords: thermo-sensitive; N-methyl acryloylglycine methyl ester (NMAME); lower critical solution temperature (LCST); drug release

Design and characterization of bioceramic coating materials for Ti6Al4V by Qiang Wei; Zhen-Duo Cui; Xian-Jin Yang; Lian-Yun Zhang; Jia-Yin Deng (171-174).
Novel bioceramics used as coating materials for Ti6Al4V were designed and characterized by adjusting the thermal expansion coefficient. The results show that the thermal expansion coefficient (α) of 6PM-B5-F4 coating is 10.1 × 10−6/°C, which matched that of Ti6Al4V. The bonding strength between the alloy and 6PM-B5-F4 coating was further measured by the longitudinal pull-off test. The in vitro response of the bioceramic was studied by immersing the specimens in simulated body fluid (SBF). The bioceramic morphology and structure were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD) and Fourier transformed infrared spectroscopy (FTIR).
Keywords: Ti6Al4V; bioceramic; bonding strength; thermal expansion coefficient; bioactivity

The purpose of this study is to clear the difference of histology and elemental composition of the cuspal enamels among the human deciduous teeth. The reground surfaces at the cuspal enamels of them were observed under SEM. The contents of seven elements were analyzed quantitatively with EPMA. It is thought that not only the first zone but also the second zone of the inner enamel in the deciduous molar causes the lateral enlargement of the dental caries along the dento-enamel junction because the second zone of the inner enamel in the deciduous molar was more porous than that in the permanent premolar. It is thought that the cuspal enamel in the deciduous teeth is low calcified than that in the permanent teeth because the contents of calcium and phosphorus in the former were significantly lower than those in the latter. It is considered that the cuspal enamel in the deciduous teeth may include less magnesian whitlockite than that in the permanent teeth because the content of magnesium in the former was significantly lower than those in the latter. A relationship was recognized between the complexity level in the histological structure and the elemental composition of the cuspal enamels among the human deciduous teeth.
Keywords: histological structure; scanning electron microscopy; elemental composition; cuspal enamel; human deciduous teeth

Superhydrophobic and transparent coatings have been prepared by self-assembly of dual-sized silica particles from a mixed dispersion. The desirable micro/nano hierarchical structure for superhydrophobicity is constructed simply by adjusting the size and ratio of the dual-sized particles without organic/inorganic templates. The transparency of the prepared coatings is also researched, and the light scattering can be reduced by lowering the ratio of big sub-micro particles while the superhydrophobicity maintains unchanged. When nano particles with a diameter of 50 nm and sub-micro particles with a diameter of 350 nm are assembled, a superhydrophobic property with a water contact angle of 161° is achieved. Additionally, the coated glass is also very transparent. The highest transmittance of the coated glass can reach 85%. Compared to traditional colloid self-assembly approach, which often involves dozens of steps of layer-by-layer processing and organic/inorganic templates, the present approach is much simpler and has advantages for large-scale coating.
Keywords: superhydrophobicity; transparency; dual-sized particles; self-assembly; silica coating

Investigation on the stability of Li5La3Ta2O12 lithium ionic conductors in humid environment by Wei-Guo Wang; Xian-Ping Wang; Yun-Xia Gao; Jun-Feng Yang; Qian-Feng Fang (189-192).
In this paper, the stability in humid air of Li5La3Ta2O12 lithium ionic conductors synthesized by conventional solid-state reaction was investigated by internal friction, conductivity, weight variation, X-ray diffraction, and thermogravimetric analysis methods. It was found that when the Li5La3Ta2O12 samples were aged in open air at room temperature, the internal friction peaks associated with the short-distance diffusion of lithium vacancies gradually shift toward higher temperature and increase in height, while the weight of the sample increases and impurity phases of LiOH·H2O appear. These results reveal that the Li5La3Ta2O12 compounds are unstable against moisture in open air at room temperature. It was suggested that the protons from the moisture substitute the lithium ions in Li5La3Ta2O12 samples to form Li2O and new protonic derivatives, Li5−x La3Ta2O12−x (OH) x (0<x<2.15), and the resultant Li2O may react further with water to form LiOH·H2O.
Keywords: lithium ionic conductor; Li5La3Ta2O12 ; internal friction (IF)

The effect of HfO2 second phase in Fe films upon ion irradiation by Na Zhang; Zheng-Cao Li; Zheng-Jun Zhang (193-196).
Ferrum of BCC crystal structure is a typical kind of matrix in structural alloy steels which could be strengthened by introducing some second phase. In the present study, BCC Fe thin films with hafnium oxide (HfO2) second phase have been synthesized in an electron beam evaporation system. Multi-layered and glancing angle deposition (GLAD) techniques were taken to form some HfO2 second phase in Fe films. Ion irradiation was conducted to investigate the irradiation resistance of the obtained samples with and without HfO2 second phase.
Keywords: hafnium oxide (HfO2); second phase; ion irradiation; glancing angle deposition (GLAD)

Studies on softening kinetics of niobium microalloyed steel using stress relaxation technique by Cheng-Liang Miao; Cheng-Jia Shang; Guo-Dong Zhang; Guo-Hui Zhu; Hatem Zurob; Sundaresa Subramanian (197-201).
Stress relaxation was studied in a series of low carbon, high Mn microalloyed steels containing 0.012 wt.%, 0.06 wt.% and 0.1 wt.% Nb. The stress-relaxation curves were modeled using a physically-based model that takes into account the time evolution of precipitation, recovery and recrystallization as well as their interactions. The results confirm that the high Mn, high Nb design can offer distinct advantage over the low Mn design for the application of near-net shape processing.
Keywords: high Nb; high Mn; stress relaxation; recovery; recrystallization; softening

Welding technology is widely used to assemble large thin plate structures such as ships, automobiles, and passenger trains because of its high productivity. However, it is impossible to avoid welding-induced distortion during the assembly process. Welding distortion not only reduces the fabrication accuracy of a weldment, but also decreases the productivity due to correction work. If welding distortion can be predicted using a practical method beforehand, the prediction will be useful for taking appropriate measures to control the dimensional accuracy to an acceptable limit. In this study, a two-step computational approach, which is a combination of a thermoelastic-plastic finite element method (FEM) and an elastic finite element with consideration for large deformation, is developed to estimate welding distortion for large and complex welded structures. Welding distortions in several representative large complex structures, which are often used in shipbuilding, are simulated using the proposed method. By comparing the predictions and the measurements, the effectiveness of the two-step computational approach is verified.
Keywords: buckling; finite element method (FEM); inherent deformation; prediction; twisting deformation; welding distortion

Numerical simulation of mechanical controlling parameters for Type IV cracking on the welding joints of martensitic heat-resistant steel by Jian-Qiang Zhang; Bing-Yin Yao; Tai-Jiang Li; Fu-Guang Liu; Ying-Lin Zhang (210-216).
The maximum principal stress, von Mises equivalent stress and equivalent creep strain in the welding joint of martensitic heat-resistant steel (9Cr1MoVNb) are simulated by finite-element method (FEM) under the condition of 600°C and applied stress of 80 MPa. The results show that the maximum principal stress and von Mises equivalent stress are high on the curved points of two sides of the groove face near the fine-grain heat-affected zone (HAZ). The creep strain mainly concentrates in the fine-grain HAZ; the maximum creep strain locates in the bottom of fine-grain HAZ of specimen. The stress triaxiality in the fine-grain HAZ is maximum, and creep cracking occurs because of the intensive constrain of base metal and weld. The simulation result is good in agreement with those of crack initiation site and propagation path by using the stress triaxiality as the mechanical controlling parameter of weld joint of martensite heat-resistant steel. Therefore, it is reasonable that the stress triaxiality is used for analysis initiation and propagation of Type IV cracking in the fine-grain HAZ.
Keywords: martensitic heat-resistant steel; Type IV cracking; von Mises equivalent stress; equivalent creep strain; numerical simulation

It is a very important phase in resin transfer molding (RTM) process that resin is cured. The result of the curing process determines the quality of a part, including mechanical properties, lifecycle of the part under high temperature and chemical properties. Therefore, it is very meaningful to discuss the curing process. In our work, the code is prepared based on unstructured mesh using divergence theorem. A case is used to verify properness of the code and the results are in good agreement with the published experiment data. In the paper, some factors of materials and numerical calculation, e.g., time step, reaction heat, the whole heat conductivity of fiber and resin and fiber initial temperature, which affect result of simulation, are emphatically investigated and carefully revealed. The conclusion shows that time step, the reaction heat and heat conductivity have an important effect on the curing process, while fiber initial temperature has very little impact. These are helpful to understand and adopt the curing process in order to produce good products.
Keywords: resin transfer molding (RTM); cure; simulation; finite volume; unstructured mesh