Study On The Hydroxyapatite/bioglass Composite Films Produced By Pulsed Laser Deposition

Hydroxyapatite (HA, Ca₁₀(PO₄)₆(OH)₂) which has the similar chemical composition and structure to the nature Ca phosphate mineral present in a biological hard tissue, is one of the most attractive materials for human hard tissure implant due to its excellent biocompatibility and bioactivity properties. However, the brittle nature of HA ceramic limits its clinical use in a load-bearing situation. It is the only advantage if it is used as a film material based on a metal substrate such as titanium and its alloy, and this kind of implants combines the mechanical benefits of metal alloy with the biocompatibility of bioceramic. Nowadays, HA films have been widely studied by many methods including plasma spraying, sol-gel, magnetron sputterin, electrophoretic deposition and so on, and a certain successful application has been obtained by these traditional methods especially by plasma spraying. However, there are many drawbacks of the films produced by these methods: decomposition of HA, low crystallinity, poor mechanical bonding between the film and the substrate. Pulsed laser deposition (PLD) offers a new technique for preparing HA film because PLD can deposite the films which have good mechanical bonding and similar stoichiometry of the target. In addition, the difference of the coefficient of thermal expansion between HA and Ti alloy induces the formation of cracks which decreases the service life of the implant. One of the solution way is to mixture HA with 45S4 bioglass which has better bioactivity, low coefficient of thermal expansion and rapid dissolution rate in simulated body fluid (SBF), and this kind of HA/45S5 composite films would have higher bioactivity and bonding strength between film and substate. In this paper, HA/45S5 composite films were deposited by PLD on Ti-6A1-4V, the microstructure, phase constitution, functional groups characteristic and bonding strength of the deposited films were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), electron probe microanalysis (EPMA), Fourier transform infrared spectroscopy (FTIR), atomic force microscopy (AFM) and so on, the bioactivity was studied by immersion in SBF, and biological safety was evaluated by hemolysis test, in vitro cytotoxicity test and acute systemic toxicity test.The experimental results show that there are many global and irregular particles with a diameter of 0.2-3μm on the PLD HA/45S5 composite film surface. The compositions in 45S5 remain amorphous state after deposition, part of HA decomosite intoβ-Ca₃(PO₄)₃ and the crystallinity is influenced the deposited parameters greatly. Hydroxyl groups in HA crystal lose, and the tetrahedron network structure of Si-O-Si is rearranged which results in the shift of peak position of Si-O vibration towards lower values in infrared absorbance spectra, simultaneously non-bridging silicon-oxygen (Si-O-NBO) bonds form.There are great influences of deposition parameters on the microstructure, phase constitution, bonding strength of the HA/45S5 composite films. High substrate temperature can compensate for the energy loss of vaporized species and contribute to deposition process of more species on the substrate surface, consequently the particles density and the surface roughness increases. High substrate temperature also increases the crystallinity of the film which is helpful for the increase of the bonding strength between film and substrate. However, high substrate temperature accelerates the loss of P element, thus increases the Ca/P atomic ratio of the film.The reduction of the atmosphere pressure means the decrease of the gas molecule density in the reaction chamber, which induces the decrease of the collision probability between the species in plumes and gas molecule, hence decrease the energy loss. So the species which reach the substrate surface have more kinetic energy, consequently the bonding strength between film and substrate increases. With the decrease of the atmosphere pressure, the surface roughness and the crystalinity of the film increase, and the cone angle become larger which diminish the restraint for the species and accelerate the P loss. In micro-oxygen atmosphere, the P element with low melting point is easily oxidized, and the P loss can be decreased during cooling processing which can decrease the Ca/P ratio. What's more, phenomenon of the CO₃^2- substitution for PO₄^3- was observed. The first deposited species, which have the epitaxial growth characteristic along the substrate surface, have good lattice matching with the substrate, so the bonding strength increases with the pulses number decreasing.The target composition has little influence on the surface morphology, but has great influence on the P and Na loss degree. Generally, the higher the relative content of 45S5 in target material is, the more serious P and Ca lose. The addition of 45S5 decreases the crystallinity of the whole film and affects the crystal orientation of HA. When the additive quantity of 45S5 is 20 wt.% or 50 wt.%, c-axis preferred orientation occurs. As we all know, the OH^- has the strict [0001] orientation in HA crystal, and OH^- easily loses during PLD process which induces the formation of cavity channel with the [0001] direction. During the film forming process, apatite maybe grows along the [0001] direction to form c-axis orientation with the action of foreign particles. The addition of 45S5 also can decrease the difference of the coefficient of thermal expansion between HA and Ti alloy which is helpful to enhance the bonding strength between the film and the substrate. However, excessively high addition quantity of 45S5 enhances the non-crystallization trend of the film, consequently decrease the bonding strength. In this experiment, when the addition quantity of 45S5 is 20 wt.%, the bond strength reaches maximum value, and the critical load is 18.7N.50 wt.% HA+50 wt.% 45S5 film deposited with a substrate of 200℃at 45 Pa Ar atmosphere is amorphous, and proper post-heat treatment can increase the crystallinity and the bond strength of the film. The critical load of the film treated at 600℃for 1h is 17.5N, and with the increase of temperature and prolongation of holding time, the integrality of the film is destroyed by the heat treatment, thus the bond strength has the downward trend. When temperature remains constant, the crystal grows up with the prolongation of time. When the holding time is 2h, the film treated at 700℃has the characteristics of a-axis preferred orientation, and fine granular particles precipitate, while the film treated at 800℃has the characteristics of c-axis preferred orientation, and acicular structure was observed in the surface. With the increase of temperature and prolongation of holding time, the degree of lattice distortion of the tetrahedron network structure of Si-O-Si induced by the foreign particles decreases, which weakens the absorption intensity and even eliminates its existence of Si-O-NBO bonds. Furthermore, the bend vibration absorption peak of Si-O at 739 cm^-1 shifts towards high values in infrared absorbance spectra.The in vitro immersion tests indicate that the PLD composite films have good bioactivity, and the dissolution and reprecipitation rate are influenced by the film crystallinity. After immersion in SBF, the dissolution behavior of the PLD films will dominate at the beginning and then reprecipitation behavior dominates.During the immersion process of the films contained 45S5 composition, ions exchange occurs immediately between the Na⁺, Ca²⁺ ions in film and the H⁺ or H₃O⁺ in solution, P element dissolves into the solution by the forms of PO₄^3-, HPO₄^2-, H₂PO₄^-, and Si element dissolves by the forms of Si(OH)₄ after Si-O-Si bonds are destroyed. When the concentration of Si(OH)₄ near the film surface is saturated, [SiO₂] polymer easily forms after condensation reaction, and the [SiO₂] polymer can precipitate on the film surface to form a SiO₂-rich layer which has a porous network structure. With the action of zeta potential, Ca²⁺ and PO₄^3- ions will reprecipitate onto the SiO₂-rich layer to form amorphous CaO-P₂O₅ layer. After incorporation of OH^-, CO₃^2- from solution, crystallization of amorphous CaO-P₂O₅ leads to the formation of HA or carbonated apatite. For HA film, after the dissolution of Ca²⁺, PO₄^3- ions into solution, the reprecipitate process is mainly controlled by the zeta potential. Once the Ca²⁺, PO₄^3- ions in the solution near the film surface is saturated, they will reprecipitate onto the film surface alternatively to form new CaP film.After immersion in SBF, the FTIR results show that the characteristic absorption peak intensity of the as-deposited films decreases gradually and even vanishes, afterward the characteristic absorption peak of newly formed apatite appears and then enhances. By the way, carbonated apatite easily forms by B-type substitution of CO₃^2- ion. The XRD results show that with the prolongation of the immersion time, the crystallinity of the amorphous films increase gradually, while for the films with high crystallinity, the crystallinity decreases first and decreases afterward.The biological safety of PLD films was evaluated by hemolysis test, in vitro cytotoxicity test and acute systemic toxicity test. The results indicate that the films deposited in our experiments have no hemolysis action, no cytotoxicity and no acute systemic toxicity. No obvious inhibitive effect was observed on the growth and proliferation of L929 cells, and no poisoning effect was observed on the tested animals, which means the PLD films deposited in our experiments have good biocompatibility and exhibite good biological safety for clinical trial.