C.L. Chu

A Biomimetic Hierarchical Scaffold: Natural Growth of Nanotitanates on Three-Dimensional Microporous Ti-Based Metals

Nanophase materials are promising alternative implant materials in tissue engineering. Here we report for the first time the large-scale direct growth of nanostructured bioactive titanates on three-dimensional (3D) microporous Ti-based metal (NiTi and Ti) scaffolds via a facile low temperature hydrothermal treatment. The nanostructured titanates show characteristics of 1D nanobelts/nanowires on a nanoskeleton layer. Besides resembling cancelous bone structure on the micro/macroscale, the 1D nanostructured titanate on the exposed surface is similar to the lowest level of hierarchical organization of collagen and hydroxyapatite. The resulting surface displays superhydrophilicity and favors deposition of hydroxyapatite and accelerates cell attachment and proliferation. The remarkable simplicity of this process makes it widely accessible as an enabling technique for applications from engineering materials treatment including energy-absorption materials and pollution-treatment materials to biotechnology.

Biodegradable poly-lactic acid based-composite reinforced unidirectionally with high-strength magnesium alloy wires

Biodegradable poly-lactic acid (PLA)--based composites reinforced unidirectionally with high-strength magnesium alloy wires (MAWs) are fabricated by a heat-compressing process and the mechanical properties and degradation behavior are studied experimentally and theoretically. The composites possess improved strengthening and toughening properties. The bending strength and impact strength of the composites with 40 vol% MAWs are 190 MPa and 150 kJ/m(2), respectively, although PLA has a low viscosity and an average molecular weight of 60,000 g/mol. The mechanical properties of the composites can be further improved by internal structure modification and interface strengthening and a numerical model incorporating the equivalent section method (ESM) is proposed for the bending strength. Micro arc oxidization (MAO) of the MAWs is an effective interfacial strengthening method. The composites exhibit high strength retention during degradation and the PLA in the composite shows a smaller degradation rate than pure PLA. The novel biodegradable composites have large potential in bone fracture fixation under load-bearing conditions.

Microstructure, nickel suppression and mechanical characteristics of electropolished and photoelectrocatalytically oxidized biomedical nickel titanium shape memory alloy

A new surface modification protocol encompassing an electropolishing pretreatment (EP) and subsequent photoelectrocatalytic oxidation (PEO) has been developed to improve the surface properties of biomedical nickel titanium (NiTi) shape memory alloy (SMA). Electropolishing is a good way to improve the resistance to localized breakdown of NiTi SMA whereas PEO offers the synergistic effects of advanced oxidation and electrochemical oxidation. Our results indicate that PEO leads to the formation of a sturdy titania film on the EP NiTi substrate. There is an Ni-free zone near the top surface and a graded interface between the titania layer and NiTi substrate, which bodes well for both biocompatibility and mechanical stability. In addition, Ni ion release from the NiTi substrate is suppressed, as confirmed by the 10-week immersion test. The modulus and hardness of the modified NiTi surface increase with larger indentation depths, finally reaching plateau values of about 69 and 3.1GPa, respectively, which are slightly higher than those of the NiTi substrate but much lower than those of a dense amorphous titania film. In comparison, after undergoing only EP, the mechanical properties of NiTi exhibit an inverse change with depth. The deformation mechanism is proposed and discussed. Our results indicate that surface modification by dual EP and PEO can notably suppress Ni ion release and improve the biocompatibility of NiTi SMA while the surface mechanical properties are not compromised, making the treated materials suitable for hard tissue replacements.

Electrochemical Stability of Orthopedic Porous NiTi Shape Memory Alloys Treated by Different Surface Modification Techniques

The complex surface morphology and large exposed surface area induce electrochemical instability on porous NiTi shape memory alloys in human body fluids. Consequently, leaching of toxic nickel ions from the alloys impede wider applications of the materials in the biomedical fields, especially as bone implants. Electrochemical impedance spectroscopy (EIS) is a useful tool to evaluate the electrochemical stability of surface film in simulated body fluids (SBF) and to identify the most effective surface modification techniques for porous NiTi alloys. In the present work, EIS is employed to characterize porous NiTi alloys that have been modified by various processes in SBF at 37°C to evaluate the relationship between the surface film structure and electrochemical stability. Two different equivalent circuits involving a dual oxide film model with a porous outer layer and an inner barrier layer are proposed to model the experimental data acquired under open-circuit conditions for the control sample (dense NiTi) and porous NiTi alloys, respectively. The modeled results reveal that both chemical treatment and oxygen plasma immersion-ion implantation are effective surface modification techniques to form a protective film with higher electrochemical stability on the surface of porous NiTi alloys.

BIOMIMETIC DEPOSITION OF APATITE ON SURFACE CHEMICALLY MODIFIED POROUS NiTi SHAPEMEMORY ALLOY

Porous NiTi shape memory alloy (SMA) with 48% porosity and an average pore size of 50–800 μm was synthesized by capsule-free hot isostatic pressing (CF-HIP). To enhance the surface bioactivity, the porous NiTi SMA was subjected to H2O2 and subsequent NaOH treatment. Scanning electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy analyses revealed that a porous sodium titanate (Na2TiO3) film had formed on the surface of the porous NiTi SMA. An apatite layer was deposited on this film after immersion in simulated body fluid at 37°C, while no apatite could be found on the surface of the untreated porous NiTi SMA. The formation of the apatite layer infers that the bioactivity of the porous NiTi SMA may be enhanced by surface chemical treatment, which is favorable for its application as bone implants.

Cytocompatibility and haemocompatibility of Zr, ZrC and ZrCN films

Abstract The cytocompatibility and haemocompatibility of three zirconium based films, i.e. Zr, ZrC and ZrCN, deposited on NiTi shape memory alloy by magnetron sputtering are investigated and compared to those of electropolished NiTi shape memory alloy. The Zr(C,N) series films have deteriorated wettability but have positive effects on the blood compatibility and cytocompatibility of NiTi. Better haemolysis resistance and thromboresistant properties are observed. There are more living cells on the Zr(C,N) series films, and the cells show a higher relative growth rate value than those on the electropolished NiTi. The Zr(C,N) series films act as barrier layers and promote the proliferation of fibroblasts by blocking the leaching of toxic nickel ions from NiTi.

Microstructural characteristics and biocompatibility of a Type-B carbonated hydroxyapatite coating deposited on NiTi shape memory alloy

Microstructural characteristics and biocompatibility of a Type-B carbonated hydroxyapatite (HA) coating prepared on NiTi SMA by biomimetic deposition were characterized using XRD, SEM, XPS, FTIR and in vitro studies including hemolysis test, MTT cytotoxicity test and fibroblasts cytocompatibility test. It is found CO(3)(2-) groups were present as substitution of PO(4)(3-) anions in HA crystal lattice due to Type-B carbonate. The growth of Type-B carbonated HA coating in SBF containing HCO(3)(-) ions is stable during all periods of biomimetic deposition. The carbonated HA coating has better blood compatibility than the chemically-polished NiTi SMA. There was a good cell adhesion to this HA coating surface and cell proliferation in the vicinity of the coating was better than that for the chemically-polished NiTi SMA. Thus biomimetic deposition of this carbonated HA coating is a promising way to improve the biocompatibility of NiTi SMA for implant applications.

Microstructure and properties of ZrTiC(N) composite films on NiTi alloy

Abstract The microstructure, blood biocompatibility and corrosion resistances, and mechanical properties of multinary ZrTiCN and ZrTiC composite films are investigated and compared to those of ZrCN film, ZrC film and NiTi shape memory alloy. The quaternary ZrTiCN and ternary ZrTiC composite films with ZrC and TiC as the predominant phases have a poor crystallinity but a higher adhesion strength with the NiTi substrate than both ZrCN and ZrC films due to the increased deposition temperature and the presence of TiC/TiN phases. Both coatings can improve the surface mechanical properties of biomedical NiTi shape memory alloy while simultaneously offering distinct advantages such as improved blood biocompatibility and corrosion resistances.

Nanoindentation study on mechanical properties of Zr(C)N films deposited on NiTi shape memory alloy

Surface nanomechanical properties of ZrN and ZrCN films deposited on NiTi shape memory alloy (SMA) by magnetron sputtering were investigated by nanoindentation study. And their surface deformation mechanism under nanoindentation load was also proposed and discussed by comparison with that of the electropolished NiTi SMA.

Surface hardening of NiTi shape memory alloy induced by surface nanocrystallization via surface mechanical attrition treatment

In terms of grain refinement mechanism induced by plastic deformation, NiTi shape memory alloy (SMA) was processed by means of surface mechanical attrition treatment (SMAT) in order to achieve surface nanocrystallization for surface hardening. The cross sectional microhardness of the SMAT treated NiTi SMAs was measured, in comparison with those of the residual stress relaxation and recrystallization annealled NiTi specimen. The results show that surface nanocrystallization induced by grain refinement enhanced the surface hardness of NiTi SMAs.