Youtao Xie

Graphene-reinforced calcium silicate coatings for load-bearing implants

Owing to the superior mechanical properties and low coefficient of thermal expansion, graphene has been widely used in the reinforcement of ceramics. In the present study, various ratios of graphene (0.5 wt%, 1.5 wt% and 4 wt%) were reinforced into calcium silicate (CS) coatings for load-bearing implant surface modification. Surface characteristics of the graphene/calcium silicate (GC) composite coatings were characterized by scanning electron microscopy. Results show that the graphene plates (less than 4 wt% in the coatings) were embedded in the CS matrix homogeneously. The surfaces of the coatings showed a hierarchical hybrid nano-/microstructure, which is believed to be beneficial to the behaviors of the cell and early bone fixation of the implants. Wear resistance measured by a pin-on-disc model exhibited an obvious enhancement with the adoption of graphene plates. The weight losses of the GC coatings decreased with the increase of graphene content. However, too high graphene content (4 wt% or more) made the composite coatings porous and the wear resistance decreased dramatically. The weight loss was only 1.3 ± 0.2 mg for the GC coating containing 1.5 wt% graphene (denoted as GC1.5) with a load of 10 N and sliding distance of 500 m, while that of the pure CS coating reached up to 28.6 ± 0.5 mg. In vitro cytocompatibility of the GC1.5 coating was evaluated using a human marrow stem cell (hMSC) culture system. The proliferation and alkaline phosphatase, osteopontin and osteocalcin (OC) osteogenesis-related gene expression of the cells on the GC1.5 coating did not deteriorate with the adoption of graphene. Conversely, even better adhesion of the hMSCs was observed on the GC1.5 coating than on the pure CS coating. All of the results indicate that the GC1.5 coating is a good candidate for load-bearing implants.

Porous Tantalum Coatings Prepared by Vacuum Plasma Spraying Enhance BMSCs Osteogenic Differentiation and Bone Regeneration In Vitro and In Vivo

Tantalum, as a potential metallic implant biomaterial, is attracting more and more attention because of its excellent anticorrosion and biocompatibility. However, its significantly high elastic modulus and large mechanical incompatibility with bone tissue make it unsuitable for load-bearing implants. In this study, porous tantalum coatings were first successfully fabricated on titanium substrates by vacuum plasma spraying (VPS), which would exert the excellent biocompatibility of tantalum and alleviate the elastic modulus of tantalum for bone tissue. We evaluated cytocompatibility and osteogenesis activity of the porous tantalum coatings using human bone marrow stromal cells (hBMSCs) and its ability to repair rabbit femur bone defects. The morphology and actin cytoskeletons of hBMSCs were observed via electron microscopy and confocal, and the cell viability, proliferation and osteogenic differentiation potential of hBMSCs were examined quantitatively by PrestoBlue assay, Ki67 immunofluorescence assay, real-time PCR technology and ALP staining. For in vivo detection, the repaired femur were evaluated by histomorphology and double fluorescence labeling 3 months postoperation. Porous tantalum coating surfaces promoted hBMSCs adhesion, proliferation, osteogenesis activity and had better osseointegration and faster new bone formation rate than titanium coating control. Our observation suggested that the porous tantalum coatings had good biocompatibility and could enhance osseoinductivity in vitro and promote new bone formation in vivo. The porous tantalum coatings prepared by VPS is a promising strategy for bone regeneration.

Chemical stability and antimicrobial activity of plasma sprayed bioactive Ca2ZnSi2O7 coating

Calcium silicate ceramic coatings have received considerable attention in recent years due to their excellent bioactivity and bonding strength. However, their high dissolution rates limit their practical applications. In this study, zinc incorporated calcium silicate based ceramic Ca2ZnSi2O7 coating was prepared on Ti-6Al-4V substrate via plasma spraying technology aiming to achieve higher chemical stability and additional antibacterial activity. Chemical stability of the coating was assessed by monitoring mass loss and ion release of the coating after immersion in the Tris–HCl buffer solution and examining pH value variation of the solution. Results showed that the chemical stability of zinc incorporated coating was improved significantly. Antimicrobial activity of the Ca2ZnSi2O7 coating was evaluated, and it was found that the coating exhibited 93% antibacterial ratio against Staphylococcus aureus. In addition, in vitro bioactivity and cytocompatibility were confirmed for the Ca2ZnSi2O7 coating by simulated body fluid test, MC3T3-E1 cells adhesion investigation and cytotoxicity assay.

Preparation and in vitro evaluation of plasma-sprayed Mg2SiO4 coating on titanium alloy

In this paper, chemically synthesized Mg(2)SiO(4) (MS) powder was plasma-sprayed onto a titanium alloy substrate to evaluate its application potentials in biomedicine. The phase composition and surface morphology of the MS coating were analyzed. Results showed that the MS coating was composed mainly of Mg(2)SiO(4) phase, with a small amount of MgO and glass phases. Mechanical testing showed that the coating exhibited good adhesion strength to the substrate due to the close thermal expansion coefficient between the MS ceramic and the titanium alloy substrate. The measured bonding strength was as high as 41.5+/-5.3MPa, which is much higher than the traditional HA coating. In vitro cytocompatibility evaluation of the MS coating was performed using canine bone marrow stem cells (MSCs). The MSCs exhibited good adhesion, proliferation and differentiation behavior on the MS coating surface, which can be explained by the high protein adsorption capability of the MS coating, as well as the stimulatory effects of Mg and Si ions released from the coating. The proliferation rate of the MSCs on MS coating was very close to that on the hydroxylapatite (HA) coating. Alkaline phosphatase (ALP) activity analysis demonstrated that the ALP level of the MSCs on the MS coating remained high even after 21days, implying that the surface characteristics of the coating are beneficial for the differentiation of MSCs. In summary, our results suggest that MS coating might be a new approach to prepare bone implants.

Improved hMSC functions on titanium coatings by type I collagen immobilization

In this study, type I collagen was fixed onto plasma-sprayed porous titanium coatings by either adsorptive immobilization or covalent immobilization. Surface characterization by scanning electron microscopy (SEM), diffuse reflectance Fourier transform infrared spectroscopy (DR-FTIR) and X-ray photoelectron spectroscopy (XPS) confirmed the biochemical modification of the titanium coatings. The immobilizing effects of type I collagen, including variations in the amount and stability of collagen, were investigated using Sirius red staining. A greater amount of collagen was found on the covalently immobilized titanium coating, and higher stability was achieved relative to the absorptive immobilization surface. Human mesenchymal stem cells (hMSCs) were used to evaluate the cytocompatibility of the modified titanium coatings. Type I collagen immobilized on titanium coating led to enhance cell-material interactions and improved hMSC functions, such as attachment, proliferation, and differentiation. Interestingly, covalently immobilized collagen on titanium coating showed a greater capability to regulate the osteogenic activity of hMSCs than did absorbed collagen, which was explained in terms of the increased amount and higher stability of the covalently linked collagen. The type I collagen covalently immobilized titanium coatings with improved biological function may exhibit better osteointegration in clinical application.

Fabrication and in vitro evaluation of stable collagen/hyaluronic acid biomimetic multilayer on titanium coatings.

Layer-by-layer (LBL) self-assembly technique has been proved to be a highly effective method to immobilize the main components of the extracellular matrix such as collagen and hyaluronic acid on titanium-based implants and form a polyelectrolyte multilayer (PEM) film by electrostatic interaction. However, the formed PEM film is unstable in the physiological environment and affects the long-time effectiveness of PEM film. In this study, a modified LBL technology has been developed to fabricate a stable collagen/hyaluronic acid (Col/HA) PEM film on titanium coating (TC) by introducing covalent immobilization. Scanning electron microscopy, diffuse reflectance Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy were used to characterize the PEM film. Results of Sirius red staining demonstrated that the chemical stability of PEM film was greatly improved by covalent cross-linking. Cell culture assays further illustrated that the functions of human mesenchymal stem cells, such as attachment, spreading, proliferation and differentiation, were obviously enhanced by the covalently immobilized Col/HA PEM on TCs compared with the absorbed Col/HA PEM. The improved stability and biological properties of the Col/HA PEM covalently immobilized TC may be beneficial to the early osseointegration of the implants.

Fabrication of nano-structured calcium silicate coatings with enhanced stability, bioactivity and osteogenic and angiogenic activity.

The bioactivity and stability of coatings on alloy implants play critical roles in the fast osseointegration and maintenance of a long-term life span of the implants, respectively. Herein, nano-sheet surface on bioactive calcium silicate (CaSiO3, CS) coatings on metal substrates was fabricated by combining atmosphere plasma spraying (APS) and hydrothermal technology (HT). The glassy phase in CS coatings generated by APS was converted into crystalline sheet-like nano-structures after HT treatment. Compared with the original CS coating samples, HT treatment decreased the degradation rate of the CS coatings. Moreover, the fabricated nano-structured topography of CS coatings increased the apatite mineralization ability and significantly enhanced the cell attachment, proliferation, differentiation, alkaline phosphatase (ALP) activity and expression of osteogenic genes and angiogenic factors of rat bone marrow stromal cells (bMSCs). Our results suggest that the nano-structured CS coatings have immense potential in improving the clinical performance of medical implants.

Enhanced cellular responses to titanium coating with hierarchical hybrid structure.

In this work, nano/micro hierarchical hybrid structured surface was prepared by fabricating a titania nanotube layer in plasma sprayed porous titanium coating (TC). In vitro human marrow stem cells (hMSCs) were employed for the evaluation of the biological properties of the anodized titanium coating with a hierarchical structure (HSTC). Significantly higher cell adhesion quantity (about 30% more) was found on the HSTC than that on the as-sprayed TC. The expressions of osteocalcin (OC) and osteopontin (OPN) for the HSTC were also detected to be about twice as high as those on the as-sprayed TC. The enhanced cell responses on the HSTC were explained by the improved protein adhesion resulted from the increased surface area and surface energy. Combining the advantages in the mechanical fixation and long-term stability of the plasma sprayed porous TC, the HSTC with a hierarchical structure may be a good candidate for hard tissue replacements, especially for load-bearing implants.

ROCK-regulated synergistic effect of macropore/nanowire topography on cytoskeletal distribution and cell differentiation

Nanotopographical modification of implants has been proved to be a promising approach to inducing osteogenesis, which has also been utilized to promote the biological property of plasma sprayed titanium coating possessing macropores. However, less attention has been paid to the detailed process of cell–surface interaction and its mechanism especially related to the macro/nano structure. In this study, focus has been put on the early MC3T3 cell adhesion behaviors on macro/nano structured surfaces and its correlation with cell differentiation. A nanowire titanium (NT) structure has been produced by a hydrothermal method, and introduced onto a traditional plasma sprayed titanium coating (TPS) to obtain the nano-structured plasma sprayed titanium coating (NTPS). The results showed that the NT surface was beneficial for filopodia generation and cell spreading, while cells on the TPS surface could adapt themselves to the rough macroporous topography but poor spreading could be observed. The hierarchical NTPS surface was able to make a synergistic effect, with better cell spreading attributed to nano topography and multi-dimensional cytoskeleton distribution caused by the macroporous structure. Accordingly, different levels of cytoskeleton tension, with moderate Rho-associated kinase (ROCK) activity on NT and TPS but higher expression on NTPS could be observed. Consequently, NTPS had better differentiation performance compared to the NT and TPS surface. After ROCK was inhibited, the difference between all the groups diminished. It can be concluded that the better performance of NTPS in triggering cell differentiation may attribute to its higher cytoskeleton tension than NT and TPS, which was obtained by the synergistic effect of macro/nano-topography on cell spreading and cytoskeletal distribution.

Effects of Zn Content on Crystal Structure, Cytocompatibility, Antibacterial Activity, and Chemical Stability in Zn-Modified Calcium Silicate Coatings

In our previous study, Zn-modified calcium silicate coatings possess not only excellent chemical stability but also well antibacterial activity. Still, effects of zinc content on these properties and cytocompatibility remain unclear. In this paper, two kinds of Zn-modified calcium silicate coatings (ZC0.3, ZC0.5) were fabricated on Ti-6Al-4V substrates via plasma spraying technology. X-ray diffraction results and transmission electron microscopy observations showed that the ZC0.5 coating was composed of pure hardystonite (Ca2ZnSi2O7) phase, while, besides Ca2ZnSi2O7 phase, the amorphous CaSiO3 phase was also detected in the ZC0.3 coating. Chemical stability in Tris-HCl buffer solution and antibacterial activity of the Zn-modified calcium silicate coatings increased with an increase in zinc content. In vitro cytocompatibility evaluation demonstrated that the proliferation and alkaline phosphatase activity and collagen type I (COLI) secretion of osteoblast-like MC3T3-E1 cells on Zn-modified coatings were significantly enhanced compared to the Zn-free coating and Ti-6Al-4V control, and no cytotoxicity appeared on Zn-modified coatings. The better antibacterial activity and the enhanced capability to promote MC3T3-E1 cells differentiation of Zn-modified coatings should be attributed to the slow and constant Zn2+ releasing from the coatings.