Y.L. Chan

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.

Relationship between osseointegration and superelastic biomechanics in porous NiTi scaffolds

The superelastic nature of bones requires matching biomechanical properties from the ideal artificial biomedical implants in order to provide smooth load transfer and foster the growth of new bone tissues. In this work, we determine the biomechanical characteristics of porous NiTi implants and investigate bone ingrowth under actual load-bearing conditions in vivo. In this systematic and comparative study, porous NiTi, porous Ti, dense NiTi, and dense Ti are implanted into 5 mm diameter holes in the distal part of the femur/tibia of rabbits for 15 weeks. The bone ingrowth and interfacial bonding strength are evaluated by histological analysis and push-out test. The porous NiTi materials bond very well to newly formed bone tissues and the highest average strength of 357 N and best ductility are achieved from the porous NiTi materials. The bonding curve obtained from the NiTi scaffold shows similar superelasticity as natural bones with a deflection of 0.30-0.85 mm thus shielding new bone tissues from large load stress. This is believed to be the reason why new bone tissues can penetrate deeply into the porous NiTi scaffold compared to the one made of porous Ti. Histological analysis reveals that new bone tissues adhere and grow well on the external surfaces as well as exposed areas on the inner pores of the NiTi scaffold. The in vitro study indicates that the surface chemical composition and topography of the porous structure leads to good cytocompatibility. Consequently, osteoblasts proliferate smoothly on the entire implant including the flat surface, embossed region, exposed area of the pores, and interconnected channels. In conjunction with the good cytocompatibility, the superelastic biomechanical properties of the porous NiTi scaffold bodes well for fast formation and ingrowth of new bones, and porous NiTi scaffolds are thus suitable for clinical applications under load-bearing conditions.

Use of focused ion beam milling for investigating the mechanical properties of biological tissues: A study of human primary molars

In this paper, the usefulness of the specimen shaping ability of focused ion beam (FIB) milling in the micrometer scale and the high force resolution of the nanoindentation technique are demonstrated on human primary teeth. Micro-cantilevers, with a triangular cross-section <5 microm in width and 10 microm in length, were produced within 50 microm of the dentin-enamel junction (DEJ) using FIB milling, and were point-loaded at their free ends at 20 microN/s until failure using a nanoindenter. The elastic modulus and flexural strength of such micro-samples of human enamel, and their variation with respect to prism orientation, were studied and compared to data from bulk enamel measured using nanoindentation and three-point bend tests. The elastic modulus of the micro-cantilever samples was found to be comparable to that obtained by nanoindentation on bulk samples, but it demonstrated significant anisotropy commensurate with the microstructure of enamel which was not measurable using nanoindentation on bulk samples. The flexural strength of the enamel micro-cantilevers also exhibited strong anisotropy, and was about one order of magnitude higher than that of bulk specimens measured by three-point bending. Through a Weibull analysis, this size dependence of the strength was found to be similar to the normal behaviour in brittle materials. The flexural strength of the enamel samples was also found to be sensitive to changes in the degree of mineralization of the samples.

Degraded prism sheaths in the transition region of hypomineralized teeth

OBJECTIVES Failure of the enamel adjacent to the defects in teeth with molar incisor hypomineralization (MIH) limits the success rate of the restorations placed in these teeth and this frequently leads to their ultimate extraction. To understand the cause, a state-of-the-art combination of focused ion beam (FIB) and nanoindentation techniques was used to evaluate the fracture properties and microstructure of enamel from specific regions of two MIH teeth. METHODS Nanoindentation, bend tests on micro-cantilevers and transmission electron microscopy (TEM) were employed to compare the microstructure and mechanical properties of the unaffected, opaque and transitional region in two MIH teeth. Special attention was paid to the transitional region in all the experiments in an attempt to identify its role in affecting the overall integrity of the MIH teeth. RESULTS The enamel in the transitional region, despite its translucent appearance under the naked eye, was found, under TEM, to have prism sheaths that were significantly less mineralized than unaffected enamel and were proved to be weaker in holding the prisms together when measured using bend tests on micro-cantilever samples machined from the region. CONCLUSION The enamel in the transitional region adjacent to the demarcated defects in MIH has notable alterations in their prism sheaths which likely contribute to their lowered mechanical properties.

Nickel release behavior and surface characteristics of porous NiTi shape memory alloy modified by different chemical processes.

As a non-line-of-sight surface modification technique, chemical treatment is an effective method to treat porous NiTi with complex surface morphologies and large exposed areas due to its liquidity and low temperature. In the work described here, three different chemical processes are used to treat porous NiTi alloys. Our results show that H(2)O(2) treatment, NaOH treatment, and H(2)O(2) pre-treatment plus subsequent NaOH treatment can mitigate leaching of nickel from the alloy. The porous NiTi samples modified by the two latter processes favor deposition of a layer composed of Ca and P due to the formation of bioactive Na(2)TiO(3) on the surface. Among the three processes, H(2)O(2) pre-treatment plus subsequent NaOH modification is the most effective in suppressing nickel release. Small area X-ray photoelectron spectroscopy reveals that the surfaces treated by different chemical processes have different structures and compositions. The sample modified by the H(2)O(2) treatment is composed of rough TiO(2) on the outer surface and an oxide transition layer underneath whereas the sample treated by NaOH comprises a surface layer of titanium oxide and Na(2)TiO(3) together with a transition layer. The sample processed by the H(2)O(2) and NaOH treatment has a pure Na(2)TiO(3) layer on the surface and a transition layer underneath. These results help to elucidate the different nickel release behavior and bioactivity of porous NiTi alloys processed by different methods.

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.

Cerebral motor cortical mapping: Awake procedure is preferable to general anaesthesia

Objective:  To investigate the outcome of surgical resection of cerebral lesions near or at the motor cortex under general anaesthesia or in the awake condition.

Formation of CaPO 4 and suppression of Ni leaching in nitinol using oxygen and sodium plasma immersion ion implantation

This study investigates the feasibility of apatite formation and the enhancement of Ni corrosion resistance in nitinol using combined sodium and oxygen plasma immersion ion implantation. X-ray photoelectron spectroscopy, scanning electron microscopy, and energy dispersive spectroscopy are used to determine the elemental depth profiles, surface morphology, chemical composition, and reactivity of the specimens. Immersion tests in simulated body fluids and cell cultures, used to assess the bioactivity and cytotoxicity of the plasma implanted samples, show better osteoblast adhesion which indicates enhanced biological performance.

Bioactivity and Corrosion Resistance of NiTi After Calcium Plasma Immersion Ion Implantation

Summary form only given. Plasma immersion ion implantation (PHI) is an effective approach to enhance the surface properties of various types of biomaterials. In order to enhance the surface bioactivity and corrosion resistance of NiTi shape memory alloy, calcium ions were implanted into NiTi alloys that have been pre-plasma-implanted with oxygen and nitrogen. The results are compared to samples without pre-treatment. The bioactivity of calcium-implanted NiTi was evaluated by immersion tests in simulated body fluid (SBF). The surface of NiTi before and after immersion tests was characterized by X-ray photoelectron spectroscopy (XPS). The XPS results reveal that the structure of the calcium-implanted layer in all the samples is composed of calcium oxide and gradually becomes a Ca-P layer after a period incubation in SBF. This Ca-P film also can be detected by scanning electron microscopy (SEM). To evaluate the anti-corrosion performance of NiTi, electrochemical potentiodynamic polarization tests were conducted on the NiTi samples in SBF. NiTi samples with pre-oxygen and nitrogen plasma implantation exhibit better corrosion resistance than single calcium-implanted samples. Our results indicate that calcium implantation can enhance the bioactivity of NiTi alloy due to the formation of calcium phosphate on the surface of the alloy. Plasma ion implantation can also improve the corrosion resistance of NiTi.