Wollastonite Coatings Research Articles

Comparative Study of the Bioactivity of Hydroxyapatite and Wollastonite Coatings

Comparative Study of the Bioactivity of Hydroxyapatite and Wollastonite Coatings

Fabrication of Wollastonite Coatings on Zirconia by Room Temperature Spray Process.

Wollastonite-based bioceramic powders were synthesized by solid-state reaction between calcite and silica powder. To control the phase and bioactivity of wollastonite, we also prepared four kinds of wollastonite-based bioceramics by adding an enstatite (MgSiO3) powder. After solid state reaction at 1000-1300 degrees C, micron-sized powders were obtained by milling and screening. By using these powders, wollastonite coatings were fabricated on zirconia substrates by a room temperature spray process, and their microstructure and composition under different coating condition were investigated. Wollastonite coatings on zirconia substrates were formed with the homogeneous microstructure and nanoscaled grain size. The phase composition of the resultant wollastonite coatings was similar to that of the starting powders, however, the grain size of the wollastonite particles was reduced to about 100 nm due to their formation by particle impaction and fracture. The wollastonite coating layer exhibited bioactivity in a stimulated body fluid and forming ability of 25 nm sized-hydroxyapatite precipitates of during in vitro test in SBF solution.

Effect of Heat Treatment Temperature on Microstructure of Sol-Gel Derived Wollastonite Coating

Wollastonite coatings on titanium alloys substrates were prepared by sol-gel with different heat treatment temperatures. Microstructures of the specimens were analyzed by XRD. SEM was used to observe surface morphologies of wollastonite coatings. The results show that with the heat treatment temperature increasing, the amorphous coating transforms to a crystalline coating. There are many pores in coatings prepared by sol-gel when heat treatment temperature higher than 900°C with the PH value to 2.5. There are no cracks in coating under such conditions.

Dissolution of Plasma-Sprayed Wollastonite Coatings: The Effects of Microstructure Coupled with Stress

Wollastonite coatings are deposited on the U-shape titanium alloy coupons by atmospheric plasma spraying at substrate temperatures of room temperature and 400 °C, respectively. The effects of applied stresses and microstructure on the dissolution behavior of wollastonite coatings have been investigated. The dissolution rate is characterized by the ion concentration changes of Ca, Si, and P in the SBF solution. The coatings deposited at room temperature show higher porosity and lower crystallinity, and further higher dissolution rate is observed, compared with the coatings deposited at a substrate temperature of 400 °C. Applied tensile stresses promote the coating dissolution. The effect of compressive stresses on the dissolution of coatings depends on the balance between the hindrance effect and the promotion action caused by the debonding and/or peeling off. The stress condition cannot change the phase transformation process when the substrate crystal structure is adverse for the apatite precipitation.

Mechanical Properties and Dissolution Behavior of Plasma Sprayed Wollastonite Coatings Deposited at Different Substrate Temperatures

The effect of substrate temperature on the microstructure, mechanical properties and dissolution behavior of wollastonite coatings is investigated. The crystallinity of as-deposited coatings increases with increasing substrate temperature, whereas the porosity shows only a little variable tendency. The Knoop hardness and elastic modulus increase with the substrate temperature up to 400 °C firstly, and then a decrement is observed with the temperature further increasing to 600 °C. The dissolution rate characterized by the pH changes and the ion concentration changes of Ca, Si and P in SBF decreases with the increase in the substrate temperature. It can be concluded that increasing the substrate temperature is a feasible method to improve the mechanical properties and to decrease the dissolution rate of wollastonite coatings. However, the bioactivity of coatings deposited on preheated substrates is superior to those on non-preheated ones, which presumably results from their different phase compositions.

Characterization of wollastonite coatings prepared by sol–gel on Ti substrate

Wollastonite coatings were prepared by sol–gel on Ti substrate and their microstructures have been studied. The phase compositions and the surface morphologies of these coatings were examined by X-ray diffraction and scanning electron microscopy. Thermal behavior of dried gel was examined by differential scanning calorimetry (DSC) and thermogravimetry (TG). There are many cracks among coatings and particles with size about 200–300 nm distributing inside cracks. DSC and TG results show that the glass transformation temperature of dried gel is about 850°C. After calcined at temperature 900°C, the phase of coatings consists of wollastonite, SiO2, and CaSi2O5.

Preparation and Characterization of CaO-ZrO2-SiO2 Coating for Potential Application in Biomedicine

In this paper, chemically synthesized CaO-ZrO2-SiO2 (CZS) powder was plasma sprayed onto a Ti-6Al-4V alloy substrate for its potential applications in biomedicine. Surface morphology and phase composition were analyzed by scanning electron microscopy (SEM) and x-ray diffraction (XRD). The degradation behavior of the coating in biological environment was in vitro appraised in Tris-HCl buffer. The mass loss was much lower than that of the wollastonite coating (WC) and close to that of the hydroxylapatite coating (HC). The bond strength with titanium alloy substrate was 31.8 ± 4.7 MPa. In vitro bioactivity appraisement results showed that apatite could be formed on the surfaces after soaking in simulated body fluid (SBF) for 14 days. Canine marrow stem cells (MSCs) also showed well adhesion and proliferation on the coating surfaces. In summary, results suggest that the coating possesses well cytocompatibility and may be an appropriate candidate for application in biomedicine.

In vitro comparison of antibacterial properties of plasma sprayed wollastonite coatings loading silver and gentamicin

To develop antibacterial coatings for orthopedic implants with a sustained release of drugs. Wollastonite coatings were deposited on the titanium substrates by an atmospheric plasma spray system. After soaking in weight percent of 5% AgNO(3) solution for 24 h, the wollastonite coatings loading silver were obtained. Gentamicin were loaded on the wollastonite coatings by collagen grafting process. The release rates of drugs from wollastonite coatings were investigated by the in vitro solution soaking test. One strain of S. aureus was used in zone of inhibition test to evaluate the antibacterial properties of drug loaded wollastonite coatings, and the cell culture test was used to evaluate their cytotoxicity. Silver and gentamicin loaded wollastonite coatings were successfully prepared. The release of silver ions from the silver loaded wollastonite coatings lasted 50 d in deionized water, effectively inhibiting the growth of S. aureus for 40 d. While an initial burst release of gentamicin was found during the in vitro solution soaking test. The gentamicin released from gentamicin loaded wollastonite coatings can inhibit the growth of S. aureus for 18 d. Both the two kinds of antibacterial wollastonite coatings showed no adverse effect on cellular adhesion, proliferation and alkaline phosphatase expression. Compared with gentamicin loaded wollastonite coatings, silver loaded wollastonite coatings may have more promising clinical applications due to the even and long-time antibacterial agent release.

Biological and antibacterial properties of plasma sprayed wollastonite coatings grafting gentamicin loaded collagen

In this work, gentamicin loaded collagen was grafted on the surface of plasma sprayed wollastonite coatings to obtain an implant having excellent bioactivity and cytocompatibility as well as antibacterial property. The bioactivity and cytocompatibility of the wollastonite coatings grafting gentamicin loaded collagen were examined by simulated body fluid (SBF) soaking test and in vitro cell culture test. The release rate of gentamicin from collagen was measured using UV spectrophotometer in phosphate-buffered saline (PBS) and the antibacterial activity against Staphylococcus aureus (S. aureus) was evaluated by zone of inhibition test and bacterial counting method. The results showed that a composite layer with collagen and silicon-rich particles was formed on the surface of the coating after the graft of collagen. The grafted collagen layer mixed with silicon-rich particles could induce the precipitation of apatite after soaking in SBF for 14 days and improved the cellular proliferation on wollastonite coatings. The release of gentamicin from the collagen layer sustained 11 days in PBS and effectively inhibited the growth of S. aureus. In conclusion, the wollastonite coating grafting gentamicin loaded collagen had excellent bioactivity and cytocompatibility as well as good antibacterial properties, which would be beneficial for the long term stability and surgical success rate of implants.

Electrophoretic deposition of bioactive wollastonite and porcelain–wollastonite coatings on 316L stainless steel

Wollastonite and porcelain–wollastonite coatings on stainless steel were obtained by electrophoretic deposition using acetone as dispersive medium. A direct electric current of 800 V for 3 min was used for obtaining the single wollastonite coating. A well-sintered layer was observed after heat treatment at 1050 °C for 1 h in air. The two-layer coating was obtained by depositing dental porcelain at 400 V for 30 s followed by the deposition of wollastonite at 400 V for 3 min. After forming the two layers, this complex coating was heat treated at 800 °C for 5 min. Under these conditions, strong bonds of both the interface wollastonite–porcelain and that of porcelain–metallic substrate were observed. The in vitro bioactivity assessment of the coatings was performed by immersing the deposited substrates in simulated body fluid (SBF) for 21 days. All the materials showed to be highly bioactive through the formation of a homogeneous apatite layer.

Electrophoretic Deposition of Wollastonite on 316L Stainless Steel from Different Dispersing Media

Natural wollastonite was electrophoretically deposited on 316L stainless steel in order to promote a bioactive surface. The effect of the disperse media and the deposition time on the deposit weight and microstructure of the wollastonite coatings was evaluated. The disperse media were methanol, acetone, ethanol, propyl alcohol and the deposition time was in the range of 1 to 180 s. Suspensions were prepared by using wollastonite powder with a mean particle size of 2 μm at a concentration of 1 g/L. The deposition was performed under a DC field of 800 V. The coated substrates were sintered at temperatures ranging from 900 to 1050oC in air for 2 to 4 h. Dense, homogeneous and crack-free coatings were obtained by using methanol and acetone. No wollastonite coating was obtained by using ethanol at these processing conditions and few wollastonite particles were deposited on the metallic substrates by using propyl alcohol. The deposit weight increases as the deposition time is increased in all the cases.

EPD-Sintering of Hydroxyapatite, Porcelain and Wollastonite on 316L Stainless Steel

Hydroxyapatite, porcelain and wollastonite coatings on stainless steel 316L were produced by electrophoretic deposition (EPD) in ethanol and acetone using a voltage between 200 and 1000 V during 0.5 to 60 s. The particle size distribution of the starting suspension was 0.3 to 4.9 microns with an average size of 1.5 microns. The coatings were analyzed using scanning electron microscopy. The amount of ceramic material on the surface of the metallic samples was evaluated by determining their difference in weight before and after the electrophoretic deposition process. The conductivity and zeta potential of the dispersing media were also evaluated. Dense, homogeneous and crack-free green coatings were obtained. The deposition rate was higher by using acetone as dispersing media. The higher zeta potential and the lower viscosity were attributed to be the cause of the better electrophoretic deposition of the acetone and methanol ceramic suspensions. Submicron particle coatings were then sintered between 800 and 1000 C during 2 h. The sintered coatings presented a very homogeneous polycrystalline structure free of cracks. The results show that the application of high voltage during short periods of time is an effective method to obtain ceramic coatings with good sinterability.

Plasma-Sprayed Wollastonite Coatings for Biomedical Application

A new bioceramic coating based on wollastonite was prepared by plasma spraying. The coatings exhibited good mechanical properties. The bond strength of the coating on substrate was about 40 MPa, which is higher than that of HA coatings used in orthopedics and dentistry. The bioactivity of wollastonite coatings was evaluated in vitro and in vivo. After immersed in simulated body fluid, a bone-like apatite layer was formed on the surface of wollastonite coatings. Osteoblast could survive and proliferate on the surface of coatings. After implanted in dog’s cortical bone, histological observation demonstrated that bone tissue could extend and grow along the surface of wollastonite coatings. The coating bonded directly to bone without any fibrous tissue, indicating good biocompatibility and bone conductivity. The wollastonite coatings also showed good bone inductivity property, inducing new-bone formation on their surface after implanted in marrow. The results obtained indicated that the plasma-sprayed wollastonite coatings possessed good mechanical properties and excellent bioactivity in vitro and in vivo. It appears that a wollastonite coating may be suitable for the repair and replacement of living bone, especially for load-bearing situations.

Comparison in In Vitro Evaluation of Wollastonite and Dicalcium Silicate Coatings

In-vitro evaluation of plasma sprayed wollastonite and dicalcium silicate coatings was carried out by SBF soaking test and osteoblasts seeding test. Apatite layers were formed on the surfaces of the two coatings indicative of the excellent bioactivity. The formation rate of apatite is higher on dicalcium silicate than on wollastonite. The cause is believed to be the presence of orthosilicate species in the dicalcium silicate coating promoting easier leaching by exchanging H3O+ ions from the solution with calcium ions concentrated in the orthosilicate positions. At the same time, loss of soluble silicon occurs, and it is supposed to enhance the repolymerization of the silica gel layer and provide the active sites for the nucleation of apatite. The outcome is that apatite forms faster on dicalcium silicate than on wollastonite. The data obtained from the osteoblasts seeding test indicate that the wollastonite and dicalcium silicate coatings promote the proliferation of osteoblasts and possess excellent biocompatibility.

Study on Plasma Sprayed Calcium Silicate Coatings

Wollastonite and dicalcium silicate coatings have been prepared on Ti-6Al-4V substrate via plasma spraying. Bond strength test, simulated body fluid (SBF) immersion, in vitro cell culturing, and in vivo implantation were carried out to evaluate their mechanical and biological characteristics. The results obtained showed that both coatings possess higher bond strength as compared with hydroxyapatite (HA) coating. In the meanwhile, the good bioactivity and biocompatibility were confirmed in this study.

In vivo evaluation of plasma-sprayed wollastonite coating

Wollastonite coatings were prepared by plasma spraying. The bioactivity of wollastonite coatings was investigated in vivo by implanting in dog's muscle, cortical bone and marrow, respectively. The behaviour of bone tissue around wollastonite coatings were examined by histological and SEM observation. After 1 month in the muscle, a bone-like apatite layer was found to form on the surface of the wollastonite coating. When implanted in cortical bone, histological observation demonstrated that bone tissue could extend and grow along the surface of the wollastonite coating. The coating bonded directly to the bone without any fibrous tissue, indicating good biocompatibility and bone conductivity. SEM and EDS analysis revealed that bone did not bond to wollastonite coating directly, but through a Ca/P layer. This suggested that the formation of bone-like apatite layer was very important for bonding to the bone tissue. The amount of bone–implant contact was also measured. Wollastonite coating was shown to stimulate more bone formation on its surface than titanium coating after implantation for 1 month, enhancing the short-term osseointegration properties of implant. The test in marrow indicated that wollastonite coatings could induce new bone formation on their surface showing good bone inductivity property.

Dissolution and mineralization of plasma-sprayed wollastonite coatings with different crystallinity

Wollastonite coatings were prepared by plasma spraying and their behaviour during heat treatment was investigated. The phase compositions and the surface morphologies of these coatings were examined by XRD and SEM. While the as-sprayed coating contained plentiful amorphous phase, the heat-treated coating showed high crystallinity. The dissolution behavior of coatings was evaluated by immersion in TRIS-buffer solution. The surface and cross-section morphologies of the coatings after immersion were observed. The results indicated that the preferential dissolution of the amorphous phase led to the disintegration of the coating. In contrast, the heat-treated coating exhibited lower solubility and remained integer. After immersion, pH changes and the concentrations of Ca and Si in the solutions were also measured. It was found that the wollastonite dissolved incongruently, whereby Ca was released at a higher rate than Si, resulting in the presence of a reactive layer enriched in silica at the coating surface. The difference in bioconductivity of wollastonite coatings with different crystallinity was investigated by immersion in simulated body fluid (SBF) solution. While apatite granules appeared on the as-sprayed coating surface after 1 day of immersion, they could not be found at the surface of the heat-treated coating.

Biocompatibility of Plasma-Sprayed Biomedical Coatings

In this paper, the biocompatibility of plasma sprayed Al 2 O 3 , ZrO 2 , hydroxyapatite and wollastonite coatings were discussed. Osteoblasts isolated from neonatal rat calvaria were cultured onto the surface of coatings. Scanning electron microscopy was used to observe the variations of cell morphologies to reflect the biocompatibility of the coatings. Osteoblasts were difficult to grow and proliferate on the surface of alumina coating, indicating plasma sprayed Al 2 O 3 coating possesses poor biocompatibility. The osteoblasts can grow and proliferate on the surface of ZrO 2 coating. However, the growing and proliferating rate of osteoblasts on the surface of ZrO 2 coating is lower than that on the surface of bioactive ceramic coatings, including HA coating and wollastonite coatings. Therefore, the biocompatibility of bioactive ceramic coatings is better than that of bioinert ceramic coatings.

Mechanism of apatite formation on wollastonite coatings in simulated body fluids

The formation mechanism of apatite on the surface of wollastonite coating was examined. Plasma-sprayed wollastonite coatings were soaked in a lactic acid solution (pH=2.4) to result in the dissolution of calcium from the coating to form silanol (Si–OH) on the surface. Some calcium-drained samples were soaked in a trimethanol aminomethane solution (pH=10) for 24 h to create a negatively charged surface with the functional group (Si–O −). These samples before and after treatment in a trimethanol aminomethane solution were immersed in simulated body fluids (SBF) to investigate the precipitation of apatite on the coating surface. The results indicate that the increase of calcium in the SBF solution is not the critical factor affecting the precipitation of apatite on the surface of the wollastonite coating and the apatite can only form on a negatively charged surface with the functional group (Si–O −). The mechanism of apatite formation on the wollastonite coating is proposed. After the wollastonite coatings are immersed into the SBF, calcium ions initially exchange with H + leading to the formation of silanol (Si–OH) on the surface of the layer and increase in the pH value at the coating–SBF interface. Consequently, a negatively charged surface with the functional group (Si–O −) forms on the surface. Due to the negatively charged surface, Ca 2+ ions in the SBF solution are attracted to the interface between the coating and solution, thereby increasing the ionic activity of the apatite at the interface to the extent that apatite precipitates on the coating surface.

Bioactivity of Plasma Sprayed Wollastonite Coatings in Simulated Body Fluid

Bioactivity of Plasma Sprayed Wollastonite Coatings in Simulated Body Fluid