Frontiers of Materials Science (v.9, #1)

Central nervous system (CNS) presents a complex regeneration problem due to the inability of central neurons to regenerate correct axonal and dendritic connections. However, recent advances in developmental neurobiology, cell signaling, cell-matrix interaction, and biomaterials technologies have forced a reconsideration of CNS regeneration potentials from the viewpoint of tissue engineering and regenerative medicine. The applications of a novel tissue regeneration-inducing biomaterial and stem cells are thought to be critical for the mission. The use of peptide nanofiber hydrogels in cell therapy and tissue engineering offers promising perspectives for CNS regeneration. Self-assembling peptide undergo a rapid transformation from liquid to gel upon addition of counterions or pH adjustment, directly integrating with the host tissue. The peptide nanofiber hydrogels have mechanical properties that closely match the native central nervous extracellular matrix, which could enhance axonal growth. Such materials can provide an optimal three dimensional microenvironment for encapsulated cells. These materials can also be tailored with bioactive motifs to modulate the wound environment and enhance regeneration. This review intends to detail the recent status of self-assembling peptide nanofiber hydrogels for CNS regeneration.
Keywords: self-assembling peptide; hydrogel; central nervous system (CNS); nerve regeneration

Semiconductor metal oxide compounds based gas sensors: A literature review by Sunil Jagannath Patil; Arun Vithal Patil; Chandrakant Govindrao Dighavkar; Kashinath Shravan Thakare; Ratan Yadav Borase; Sachin Jayaram Nandre; Nishad Gopal Deshpande; Rajendra Ramdas Ahire (14-37).
This paper gives a statistical view about important contributions and advances on semiconductor metal oxide (SMO) compounds based gas sensors developed to detect the air pollutants such as liquefied petroleum gas (LPG), H2S, NH3, CO2, acetone, ethanol, other volatile compounds and hazardous gases. Moreover, it is revealed that the alloy/composite made up of SMO gas sensors show better gas response than their counterpart single component gas sensors, i.e., they are found to enhance the 4S characteristics namely speed, sensitivity, selectivity and stability. Improvement of such types of sensors used for detection of various air pollutants, which are reported in last two decades, is highlighted herein.
Keywords: gas sensor; semiconductor metal oxide (SMO); sensitivity; air pollutant; gas response

In vitro and in vivo evaluation of bone morphogenetic protein-2 (BMP-2) immobilized collagen-coated polyetheretherketone (PEEK) by Ya-Wei Du; Li-Nan Zhang; Xin Ye; He-Min Nie; Zeng-Tao Hou; Teng-Hui Zeng; Guo-Ping Yan; Peng Shang (38-50).
Polyetheretherketone (PEEK) is regarded as one of the most potential candidates of biomaterials in spinal implant applications. However, as a bioinert material, PEEK plays a limited role in osteoconduction and osseointegration. In this study, recombinant human bone morphogenetic protein-2 (rhBMP-2) was immobilized onto the surface of collagen-coated PEEK in order to prepare a multi-functional material. After adsorbed onto the PEEK surface by hydrophobic interaction, collagen was cross-linked with N-(3-dimethylaminopropyl)-N′-ethyl carbodiimide hydrochloride (EDC) and N-hydroxysuccinimide (NHS). EDC/NHS system also contributed to the immobilization of rhBMP-2. Water contact angle tests, XPS and SEM clearly demonstrated the surface changes. ELISA tests quantified the amount of rhBMP-2 immobilized and the release over a period of 30 d. In vitro evaluation proved that the osteogenesis differentiation rate was higher when cells were cultured on modified PEEK discs than on regular ones. In vivo tests were conducted and positive changes of major parameters were presented. This report demonstrates that the rhBMP-2 immobilized method for PEEK modification increase bioactivity in vitro and in vivo, suggesting its practicability in orthopedic and spinal clinical applications.
Keywords: polyetheretherketone (PEEK); spinal implantation; bone morphogenetic protein-2 (BMP-2); collagen; surface modification

To search for new bioactive materials which can be used as the substitute of bone repairing and drug carriers, Sema3A-loaded chitosan microspheres (SLCM) and silk fibroin (SF) were mixed with calcium sulfate cement (CSC). SEM, particle size analysis and swelling rate determination were performed to study properties of the microspheres. The drug loading, encapsulation efficiency and drug release rate were determined by ELISA. Microspheres with different SLCM weight contents (0.5%, 1% and 5%) were prepared to determine which one has the strongest mechanical properties and the appropriate setting time. It was revealed that CSC/SF/0.5SLCM has satisfactory mechanical properties, and its in vitro biocompatibility was assessed by MTS. Chitosan microspheres (5–18 μm) were globular, the surface was smooth, and the swelling rate is (77.02 ± 5.57)%. With this formula, the setting time was increased with the addition of SLCM in CSC/SF, and the cumulative drug release rate is 44.62% in 28 d. XRD results demonstrate that the main component is calcium sulfate. Also it was found that CSC/SF/0.5SLCM supports the growth of MC3T3 cells. Thus the preparation of CSC/SF/0.5SLCM was reliable, and the products had good structures, physical properties and biocompatibility, appearing to be a promising bone substitute material.
Keywords: Sema3A; chitosan microsphere; sustained release; silk fibroin (SF); bone cement

Mechanical and corrosion properties of Al/Ti film on magnesium alloy AZ31B by Rong-Chang Zeng; Ke Jiang; Shuo-Qi Li; Fen Zhang; Hong-Zhi Cui; En-Hou Han (66-76).
Preparation of titanium film on magnesium substrate faces a challenge due to non-Fickian inter-diffusion between titanium and magnesium. Aluminum can build a bridge between titanium and magnesium. Al/Ti duplex coatings were deposited on magnesium alloy AZ31B using magnetron sputtering (MS). The low temperature diffusion bonding behavior of the Mg/Al/Ti coating was investigated through SEM and its affiliated EDS. The phase structure and critical load of the coatings were examined by means of XRD and scratch tests, respectively. The results demonstrated that the bonding strength was significantly improved after a post heat treatment (HT) at a temperature of 210°C. The diffusion mechanism of the interfaces of Mg/Al and Al/Ti in the coating was discussed based on the analysis of formation energy of vacancies and diffusion rates. The Al/Ti dual layer enhanced the corrosion resistance of the alloy. And the HT process further increased the corrosion resistance of the coated alloy. This result implies that a post HTat a lower temperature after MS is an effective approach to enhance the bonding strength and corrosion resistance of the Al/Ti film on Mg alloys.
Keywords: magnesium alloy; aluminum/titanium coating; magnetron sputtering (MS); diffusion; bonding strength

W-Cr-C coatings with different chromium contents (0–3 wt.%) were fabricated on the tungsten substrates by spark plasma sintering (SPS) method from the graphite and chromium mixed powders. SEM and XRD were exploited to analyze the effect of Cr contents on the microstructure of coatings. It was found that the abnormal hollow WC grains disappeared with addition of Cr less than 2%, and the microstructures were largely refined and densified. With further increase of Cr addition, the grains changed slightly but the densification was reduced. The most dense coating was achieved at 1 wt.% Cr. Corrosion behavior of the W-Cr-C coatings were investigated by impedance spectrum and potentiodynamic polarization tests. Results suggested that the W-1Cr-C coated W sample exhibited the lowest corrosion current density and highest corrosion potential due to the most densified microstructure, indicating that the addition of Cr at 1 wt.% was optimal for WC coating against corrosion.
Keywords: W-Cr-C coating; spark plasma sintering (SPS); corrosion resistance; electrochemical measurement

Influence of microstructures on thermal fatigue property of a nickel-base superalloy by Peng-Cheng Xia; Feng-Wen Chen; Kun Xie; Ling Qiao; Jin-Jiang Yu (85-92).
Effect of microstructures such as the distribution and shape of carbide and γ′ phase on thermal fatigue property of a superalloy was investigated experimentally. The resistance of thermal fatigue of the studied alloy decreases with the rising upper temperature. For the as-cast alloy, the thermal fatigue crack mostly origins from carbide at low upper temperature and results from oxidation at high upper temperature. The thermal fatigue crack of the heat treated alloy is mainly initiated by the oxidized cavity and then propagates through the join of the oxidized cavity. The orientation of crack propagation and direction of dendrite growth of alloy have the angle of 45°. There is γ′ denuded region near the thermal fatigue crack because of oxidation.
Keywords: nickel-base superalloy; thermal fatigue; microstructure; crack

Thermal barrier coatings (TBCs) with Y2O3-stabilized ZrO2 (YSZ) top coat play a very important role in advanced turbine blades by considerably increasing the engine efficiency and improving the performance of highly loaded blades. However, at high temperatures, environment factors result in the failure of TBCs. The influence of calcium-magnesium-alumino-silicate (CMAS) is one of environment factors. Although thermo-physical effect is being paid attention to, the thermo-chemical reaction becomes the hot-spot in the research area of TBCs affected by CMAS. In this paper, traditional twolayered structured TBCs were prepared by electron beam physical vapor deposition (EBPVD) as the object of study. TBCs coated with CMAS were heated at 1240°C for 3 h. Additionally, 15 wt.% simulated molten CMAS powder and YSZ powder were mixed and heated at 1240°C or 1350°C for 48 h. SEM and EDS were adopted to detect morphology and elements distribution. According to XRD and TEM results, it was revealed that CMAS react with YSZ at high temperature and form ZrSiO4, Ca0.2Zr0.8O1.8 and Ca0.15Zr0.85O1.85 after reaction, as a result, leading to the failure of TBCs and decreasing the TBC lifetime.
Keywords: thermal barrier coating (TBC); yttria-stabilized zirconia (YSZ); calcium-magnesium-alumino-silicate (CMAS); thermo-chemical reaction; high temperature