Papers by Author: Yang Leng

Abstract: ab initio simulations were employed to investigate the crystal structure of carbonated apatite (CAp). Two possible sites for the carbonate ions in the apatite lattice were considered: carbonate substituting for OH^- ion (type-A) and for PO₄^3- ion (type-B). A combined type-AB substitution was also proposed and numerous possible charge compensation mechanisms were treated. The results show that the most stable type-A CAp had its carbonate triangular plane almost parallel to c-axis, making an angle of about 2° at z = 0.46. In the most stable type-B CAp structure, the nearest Ca (2) ion was replaced by a sodium ion and the carbonate group was lying almost flat in b/c-plane. Of all the models considered, mixed substitution type-AB where two carbonate ions replacing one phosphate group and one hydroxyl group shows the most stable structure.

Abstract: Carbonated apatite, the basic mineral component in human hard tissues and an important bioceramic material, has been extensively studied. However, its atomic arrangements in apatite crystal structure and its experimental characterization are still not lack of debating. We analyzed infrared (IR) vibrational spectroscopy for carbonated apatite determinations, by comparatively studying the IR spectra of hydroxyapatite and of surface carbonate absorption, biological apatites (human enamel, human cortical bone, and two animal bones) and carbonated apatite. The carbonated apatite samples were sythesized by various methods, including precipitation method, hydrothermal reaction and solid-gas reaction at high temperature. The comparative study indicates that the bands at ~880 cm^-1, ~1413 cm^-1, and ~1450 cm^-1 should not be used to identify carbonated apatite since they may result from carbonate absorption on surfaces of apatite crystals or separated carbonate phase present with apatite crystals. The IR characteristic bands of carbonate substitution in apatites should be: ν₃ at ~1465 cm^-1 for type-B (CO₃ substituting for PO₄) and ν₃ band at ~1546 cm^-1 for type A (CO₃ substituting for OH). These signature IR bands are further confirmed by the ab initio simulations.

Abstract: Percutaneous type of orthopedic and dental implants requires not only a good adhesion with bone, but also the ability to form good attachment and seal with connective tissues and skins. Currently, the skin-seal of such implants still remains as a problem to be resolved. Electrochemical processing was used to modify the surface of titanium implants in order to improve the ability of anti-bacteria infection and skin seal around the implants by synthesizing a fluoridated calcium phosphate thin film on titanium substrate. The surface of titanium was cathodically treated in an electrochemical cell. A thin film of about 80 nm thickness was deposited on the titanium surface by controlling the treatment parameters. The dense and gel-like film was composed of calcium phosphate and fluorine ions. Fluorine ion has the anti-bacteria property and could help to improve the skin seal around the percutaneous device. The electrochemical method of fluoridated calcium phosphate thin film synthesis will provide an alternative method for surface treatment of orthopedic and dental implants.

Abstract: Nanocrystalline Zn-substituted calcium hydroxyapatite (HA) powder was synthesized by wet chemical method. Detailed characterization was carried out with both experimental techniques and numerical simulation method. X-ray diffraction (XRD) patterns show the calcium phosphate maintains as the apatite phase when the atomic ratio of Zn/(Zn+Ca) is less than 17% Zn in aqueous solutions. The calcium phosphate crystallinility decreases with the Zn concentration increase. The morphological changes with Zn substitution in HA were investigated by TEM. Lattice parameters of the apatitic samples were determined by XRD Rietveld refinement method. A computational study using ab initio generalized gradient approximation density functional theory was performed on Zn-substituted HA. Comparison of the experimental and computer simulation results provides our insights of Zn substitution in apatite structure.

Abstract: This study reports a novel method for the preparation of several biologically important calcium phosphate (Ca-P) phases such as hydroxyapatite (HA), dicalcium phosphate dihydrate (DCPD) and dicalcium phosphate anhydrous (DCPA). X-ray diffraction (XRD) results showed that phase pure DCPD, DCPA and HA nano-crystals could be produced in the Ca2+/PO4 3- solutions with the presence of EDTA at 120 oC, 180 oC and 210 oC, respectively. Transmission electron microscope (TEM) micrographs revealed that all the Ca-P precipitates were needle-like or rod-like. Most of the precipitates ranged from 100 ~ 200 nm in length. Selected area electron diffraction confirmed that the longitude direction of the rod-like HA precipitates were along c-axis and the flat surface was (110). Thermal gravimetric analysis of the DCPD precipitates revealed that phase transformations of DCPD to DCPA and DCPA to HA occurred at 139 oC and 195 oC, respectively, which resulted in the different Ca-P phases during hydrothermal synthesis at different temperature ranges.

Abstract: Osteoinductivity of hydroxyapatite (HA) was investigated using uncommitted pluripotent mouse stem cells, C3H10T1/2 in an in vitro differentiation assay. HA exhibited impressive ability to induce expression of osteo-specific genes in C3H10T1/2, including alkaline phosphatase (ALP), type I collagen (COL1) and osteocalcin (OCN); compared with its insignificant up-regulation of the same genes in osteoblast-like cells, Saos-2. HA osteoinductivity exhibited in C3H10T1/2 was comparable to that of a bone morphogenetic protein (BMP) with reference to up-regulating osteo-specific genes except the core binding factor 1 (Cbfa1, Runx). This result implies a difference in osteogenic induction pathway initiated by HA and BMP. HA osteoinductivity was also demonstrated in the stem cells culture using conditioned medium derived from cells cultured on HA substrates. The medium exhibited excellent ability to up-regulate ALP without the presence of HA and BMP. The result suggests that the HA can interact with the cells and generate potent inductive substance released into the medium. Such substance in turn is able to induce uncommitted cells to differentiate into the osteolineage.

Abstract: Bioactivity of hydroxyapatite reinforced ultrahigh molecular weight polyethylene (HA/UHMWPE) nanocomposites with HA volume content of 10~50 % was evaluated by simulated body fluid (SBF) immersion. The effect of HA content on the capability for calcium phosphate (Ca- P) induction was studied. It was found that Ca-P deposition covered the whole surface of the composite with 30 vol. % of HA after immersion for 1 day and the layer grew to around 10 0m thick in one-week immersion, while there was few nucleus formed for composites with HA content lower than 30 vol. % after one-week immersion. The Ca-P structure was identified as octacalcium phosphate (OCP) by SEM, TEM, and ToF-SIMS.

Abstract: Previously, various surface treatments have been used to enhance the ability of titanium surfaces to induce calcium phosphate (Ca-P) formation in simulated body fluid (SBF). However, there were no systematic investigations on the key factors of the surface treatments to control such ability. In this work, three factors are studied for their roles in controlling Ca-P precipitation in SBF: surface roughness, surface chemistry and surface energy. Four kinds of single-step chemical treatments on Ti surfaces were carried out, including alkali treatment (AT), nitric acid treatment (NT), hydrogen peroxide treatment (HPT), and heat treatment. The experimental results show that the surface energy is the most likely controlling factor of Ca-P formation ability.

Abstract: Hydroxyapatite reinforced ultrahigh molecular weight polyethylene (HA/UHMWPE) nanocomposites with HA volume fraction 0.1~0.5 are processed by twin-screw extrusion compounding and compression molding followed by hot drawing. SEM micrographs show that HA nano-particles are homogeneously dispersed in the highly oriented UHMWPE inter-fibrils. Tensile tests show that the modulus increases, while the strength and ductility decrease, with the increase of HA content. A good combination of mechanical properties can be obtained in the composite with HA nano-particles volume fraction 0.3.

Abstract: Porous tricalcium phosphate ceramics were immersed in static and dynamic revised simulated body fluid (RSBF) at 37°C. Morphology, composition and phase of precipitates on TCP were identified by SEM, FTIR and TEM methods. FTIR and TEM results indicated the deposits on the inner pore walls of TCP were OCP, and SEM results implied that the deposited way of precipitates in static system was different from that in dynamic system.