Preparation Of Bioactive Molecules-Mineral Functional Composite Layer On Titanium Surface

Titanium and its alloys are extensively used for clinical implants, because of their excellent mechanical properties, and good biocompatibility. The long-term clinical researches show that titanium implant is confronted with the following problems:poor bone regeneration or slow regeneration, poor integration with surrounding bone tissues, and limited service life. The events after implantation include interactions between tissues and implant surfaces. Therefore, it is extremely important to achieve the implant with good comprehensive performance through surface modification. In recent years, further immobilizing the bioactive molecules, such as therapeutic agents or growth factors, with bone-like mineral layer on titanium implants is becoming the research focus in the biomaterials fields. Such surface modification technology could incorporate osteoinductivity and osteoconductivity into the design of the supporting biomaterial, thus improving the osseointegration and extending the service life.In this study, we chose fibronectin (FN) and osteogenic growth peptide (OGP) to prepare a series of bioactive molecules-mineral functional composite layers on titanium surfaces. Firstly, the precipitation of bone-like mineral layer on titanium surfaces was studied, and a detailed insight into bone-like mineral nucleation and growth was discussed. Secondly, a coprecipitation method to build FN-mineral or OGP-mineral composite layers on titanium was developed. The phase composition, micro-morphology and fine structure of composite layers were investigated. The mechanisms underlying the coprecipitation of biomolecules into the mineral layer were also discussed. Finally, the fibronectin and osteogenic growth peptide were immobilized with bone-like mineral layer on titanium with varying immobilizing sequences through the methods of coprecipitation and/or physical adsorption. There are three parts in this dissertation, including:1. A uniform, homogeneous apatite layer was biomimetically precipitated on hydroxyapatite thin film coated titanium through combing IBAD with biomimetic precipitation. XRD patterns indicate that, during the bone-like apatite precipitation, with the increase of immersion time, the density of apatite’s characteristic peak increased and the peak became narrower. The calcium concentration in DPBS solution increased with the dissolution of hydroxyapatite thin film. And then the calcium concentration decreased with the precipitation of apatite. Bone-like apatite was nucleated earliest on the hydroxyapatite thin film with the lowest degree of crystallinity. The rate of nucleation growth increased with increasing crystallinity of the coated hydroxyapatite thin film.2. FN-mineral and OGP-mineral composite layers were biomimetically induced on an active hydroxyapatite thin film through coprecipitation method. FESEM and confocal observations show that, the biomimetic coprecipitation process allowed biomolecules to be incorporated uniformly throughout the mineral layer. Rietveld refinement results suggest that, the mineral layer was indexed of a mixture of apatite and OCP. The phases of FN-mineral and OGP-mineral were partially transformed from OCP to apatite. The lattice parameter c of apatite was higher in the bioactive molecules-mineral composite layers than in the mineral layers, and the parameter a of apatite was changed when the bioactive molecules were coprecipitated on the surfaces. The biomimetic coprecipitated bioactive molecules slowed down the rate of mineral growth and influenced the ultimate structure of mineral. Primary physicochemical characterization and fine structure analysis indicate that the coprecipitation process was mainly mediated by electrostatic attraction among bioactive molecules, material surfaces and the ions in the solutions, and the bioactive molecules participated in the formation of the crystal latticework.3. The coprecipitation and/or adsorption approaches were used to immobilize FN and/or OGP with biomimetic mineral layer onto titanium substrates. FESEM and XRD results show that, the adsorption of BMs did not change the morphology of mineral crystal and the main phase of mineral layer. Whereas, the coprecipitation of BMs enlarge the mineral crystal and change the main phase of mineral layer from apatite to OCP. Release studies demonstrated that either coprecipitation or adsorption approach provided sustained delivery of bioactive molecules. The dual immobilizing of FN and OGP had a significant impact on bMSCs attachment, spreading, proliferation and osteogenic differentiation compared to immobilizing of FN or OGP alone. The multi-immobilizing mineral with various bioactive molecules through adsorption approach has the potential to be used for orthopedic and dental applications.