| Among all the currently used biomaterials, bioactive glasses are of great interest in medical applications for their high bioactivity and good biocompatibility. Compared with hydroxyapatite (HA), another important bioactive ceramic, the bioglass composition can be varied in a wide range without losing its bioactivity. Thus, by changing the composition, the desired physical, mechanical and biological properties for the bioglass can be realized. Besides, as one kind of Class A bioactive materials, bioglass reacts much faster to tissue than HA (in a few hours instead of several days) and can bond to both hard and soft tissue. However, bioactive glasses are brittle and exhibit low mechanical strength. These drawbacks limit its application as bulk material in load bearing sites. Coating metallic prostheses with bioactive materials could be a route to combine the good mechanical properties of base materials with the biological properties of biomaterials. Various coating techniques, such as plasma spraying, sputtering, sol-gel and electrophoresis deposition have been used to obtain bioactive glass films. Among them, pulsed laser deposition (PLD) is a feasible method for producing adherent and uniform films with controlled stoichiometry. In this work, bioactive glass films were prepared on titanium alloy by pulsed laser deposition. The experimental parameters of PLD and composition of bioactive glass films were adjusted in order to abtain high-quality bioactive thin films for medical applications.Up to now, how to select the experimental parameters of PLD still need attempted. Effect of experimental parameters of PLD, including Ar pressure, substrate temperature, energy density and deposition duration on the composition, morphology, bonding configuration and adherence of the films is systematically studied. The purpose is to optimize the experimental parameters of PLD.Effect of the pressure of Ar atmosphere on the morphology, bonding configuration and deposition rate of the bioglass thin films was studied.(1) micron-sized and submicron-sized particles were found on the surface of the thin films, which is the typical morphology of the PLD films. It can be observed that the particles on the surface of the film deposited in the vacuum are mainly in the ball-like shape. The percentage of particles on the films increases with Ar pressure and their shapes become irregular. A great number of droplets are first correlated with the structural feature of the targets. The phenomenon that the quantity of particles increases with the pressure can be interpreted as a consequence of condensation from vapor species under high gas pressure. (2) Compared with the target, the Si-O-NBO/Si-O-Si (s) intensity ratio decreases for the films. Besides, this tendency becomes obvious with the increase of the Ar pressure. The above results demonstrated that the bonding configuration of the target is not correctly transferred to the films. This effect is associated to the network rearrangement during the film growth, originated by the complex physical mechanisms in the PLD process. (3) As the pressure increases, the film growth decreases following an linear dependence.Effect of the temperature on the morphology, bonding configuration and adherence of the bioglass thin films was also investigated.(l) High substrate temperature could supply more kinetic energy for the ablated atoms to diffuse sufficiently, leading to smooth surface. On the country, at low substrate temperature, low energy could be supplied for the mobility of the adatoms on the substrate surface. Thus, the film with high roughness and some structural defects was observed. (2) It can be observed that compared with the target, the Si-O-NBO (s)/Si-O-Si (s) intensity ratio decreases for the films. Besides, this tendency becomes obvious with the decrease of the temperature. (3) The bonding stress between the film and the substrate decreases with the temperature. The reason is that lower temperature of the film means higher temperature gradient, leading to the higher heat stress.Energy density also has an effect on the deposition rate, composition, morphology and adherence of the films. (1) In the low energy density, there are obvious differences of elemental composition between the films and substrate. Compared with the target, the film has higher content of Na and Ca, as well as lower content of Si and P. (2) The higher the energy density is, the higher attenuation depth is, and the more particles are produced accordingly. Besides, these particles impinge against the substrate with high kinetic energy, leading to the rough films even with the holes. (3) The deposition rate as a function of the fluence (Fig) shows a threshold of about 3J/cm2, below which film growth is barely observable. Above the threshold value, the film deposition rate increases with the laser fluence. (4) As the energy density increases, bonding stress between the film and substrate decreases due to the poor quality of the film.Deposition duration decides the thickness of the film, and thus the bonding stress between the film and the substrate. The longer the deposition duration, the thicker the film, and the lower bonding stress between the film and the substrate. The reason is the critical load decreases with increasing coating thickness due to the accumulation of stresses in the coatings.The physical mechanism of PLD is very complex. According to the S-N model, the target is considered as very large plane without thickness. It is difficult to relate the experimental results with the parameters of PLD, and thus optimize the processing of PLD. In this work, the first and second processing of PLD, that is the ablation of the target and the movement of the plasma, were investigated. The equation of ablation rate was deduced on the base of the model of the massive target. Results showed the ablation rate is decided by the physical parameters of the laser and target. In the equation, the relation between the deposition rate and the energy density is consistent with the experimental results. On the base of the dynamic model, the evolutionary process of the plasma in the stage of isothermal expansion and adiabatic expansion was discussed. The equation of the anisotropic distribution of the plasma was deduced by making the equation of the ablation rate as the boundary condition of the plasma movement. Therefore, the processing of the ablation of the target and that of the expansion processing of the plasma were associated.The in vitro bioactivity process of bioglass was studied by detecting the variation of the morphology and structure of the film in the simulated body fluid (SBF), as well as the ion concentration and PH value of the SBF. It shows that the in vitro bioactivity process of bioglass follows the five stage sequence: (1) Rapid exchange of Na+ or K+ with H+ or H3O+ from solution; (2) Breakage of Si—O—Si bonds and formation of Si—OH (silanols) at the glass solution interface; (3) Condensation and repolymerizationof an SiO2 rich layer on the surface; (4) Migration of Ca2+ and PO43+ groups to the surface and growth an amorphous CaO-PaOs-rich film; (5) Crystallization of theamorphous CaO-P2O5 film by incorporation of OH- and CO32- anions from solutionto form a apatite layer.Generally, the thermal expansion coefficients (a) of the bioglasses in the first demonstrated system Bioglass? (4545, originally developed by Hench) are higher than that of Ti6A14V, the a mismatch between the substrate and the coatings would cause a residual tensile stress at the interface, resulting in cracks propagation and poor adherence. Therefore, tailoring the a to the substrate through regulating the composition of the bioglass was studied. Another important factor that affects the adhesion is the thickness of the coatings. It is known that the critical load decreases with increasing coating thickness due to the accumulation of stresses in the coatings. It is the precipitation of the crystal apatite layer that assures the successful bonding of the coating to host tissue, indicating the bioactivity of the bioglass. Therefore, the thickness of the glass coatings should exceed a threshold value to develop the complete bioactive process before it is dissolved in vitro. Thus, decreasing the critical thickness of the coatings is also needed to ensure the good adhesion to the substrate.In this work, a kind of bioactive glass coating is deposited on Ti6A14V by pulsed laser. The composition is designed by partial substitution of Mg for Na in Bioglass(?). The aim is to obtain a bioactive glass coating with good adhesion to Ti6A14V substrate by adjusting its a to the substrate and decreasing the critical thickness of the coating. According to the information provided from the in vitro bioactivity, the onset time for the specimen 1 to precipitate HA is 70h, while for specimen 2, it is 90h. This means that the bioactivity index of specimen 2 is lower than that of specimen 1 and requires more time for bone bonding.Recent investigations proposed a "charged surface" theory to explain the mechanism of the bioactivity for the different bioactive materials. It is believed that the complex process of apatite formation describe above is well interpreted in terms of the electrostatic interaction of the functional groups with the ions in the fluid. The bioactivity mechanism for different biomaterials is similar in essence. The formation of bone-like apatite is induced by functional groups that have a specific arrangement. In the body environment, these functional groups assume a negative charge, and induce apatite formation via the formation of amorphous calcium compound and the subsequent formation of an amorphous calcium phosphate that finally transforms into bone mineral-like apatite.Previous work shows that the effect of MgO on the bioactivity of the bioglass is somewhat controversial. This behavior is thought of as "anomalous property of Mg" and is related to its physical property. It is showed that when the content of MgO for specimen 2 is 10mol%, part of Mg ions would become network former. This result equals to increasing the amount of SiO2. Thus, Q2 groups can not be detected. The role of Q2 groups in the dissolution rate of the silica through the formation of Si-OH groups at the glass surface has been proved. Thus, the reason that the 10mol% content of MgO can decrease the bioactivity and the thickness of the films was revealed by discussing the characterization of the bioglass structure.
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