Biologically Responsive Coatings Through A Zinc-releasing Or ECM-mimicking Approach

The research on metallic implants surface modification using bioactive coatings and their cell and tissue response is one of the most attractive aspects of biomaterials.Bioactive coatings with high biological response should be able to modulate materials/cell interfical reactions at the molecular level and initiatively stimulate cell activity to promote bone growth.Hydroxyapatite(HA) coating has been widely used to improve implant/bone tissue bonding.However,the clinical application of HA coatings shows limited bioactivity and relatively large dissolution or resorption rates after implantation,which may result in slow osteointegration in the early stage and poor long-term effectiveness.In this dissertation,based on the interface reactions between implant and bone,we proposed one bioactive functional coating on titanium substrate.Compared with HA,fluoridated hydroxyapatite(FHA) has comparable bioactivity and better dissolution resistance.In this study,a dense and stable bottom layer was first applied on titanium to promise the long-term stability.Then, based on this bottom layer,we designed one bioactive coating with zinc ions release ability to stimulate bone formation in the early stage.Finally,an extracellular matrix(ECM) mimicked layer composed of calcium phosphate/collagen was assembled,which might facilitate osteoblasts adhesion and also act as carrier of bioactive molecules such as bone morphogenetic proteins.The researches are summarized as followings:(1) Ca(NO₃)₂-P₂O₅-HPF₆-C₂H₅OH system was selected as precursors and Zn(YO₃)₂ was added to prepare zinc containing FHA coating.The XRD results demonstrated the crystal phase was still apatite,but the crystalinity decreased after zinc incorporation.The coating was dense and the thickness was about 2μm.The scratch test demonstrated that the bonding strength between the coating and the substrate was strong,which was about 480±13 mN.The XPS results showed the content of zinc on the coating surface was obviously higher than that in the coating, indicating zinc enrichment on the coating surface.(2) Characterization of ZnFHA coating.The ZnFHA coating could induce apatite layer formation on its surface when soaked in SBF solution,indicating the coating was bioactive.The samples were also soaked in Tris buffered solution to investigate their zinc release property.The zinc content tested in the solution increased dramatically in the first 12 hours and then tapered off.In vitro cell viability was tested in term of MTT absorbance.Comparatively,the cell counts on the zinc containing FHA coatings showed statistically significantly higher optical density value than those on titanium substrate and FHA coating.(3) Preparation and characterization of Nano ZnTCP particles.In this study, ZnTCP particles were used as zinc carriers.ZnTCP were successfully synthesized through crystallization of zinc containing amorphous calcium phosphate(ACP) at 800℃.The structural analysis proved that zinc ions had entered the calcium sites in theβ-TCP lattice.The zinc content in ZnTCP increased with increasing zinc concentration in the Ca(NO₃)₂-Zn(NO₃)₂-(NH₄)₂HPO₄-PEG system.SEM and TEM results showed that the particle size of ZnTCP decreased with increasing zinc content in the particle,about 200～500 nm.(4) Preparation and characterization of ZnTCP/FHA coating.The nano ZnTCP particles were added to prepare ZnTCP/FHA biphasic coating on titanium substrate.We used two ways to regulate the zinc content in the coating:add different amount of ZnTCP particles in the colloidal solution(Series 1);add the same amount of ZnTCP particles in the colloidal solution(Series 2).The obtained coating from the colloidal solution is composed of two phases:HA and TCP.All the coatings exhibited a heterogeneous surface with micro-scaled roughness.The EDX results showed that there existed an obvious amount of zinc in the agglomerated ZnTCP particles.The zinc release behavior of ZnTCP/FHA coating was slow and sustained for up to tens of days.Moreover,the zinc release process could be regulated by changing zinc content in the coating.(5) In vitro and in vivo test of ZnTCP/FHA coating.The ZnTCP/FHA coating (Series 2) could support osteoblasts growth and showed statistically increase in cell viability in comparison with the TCP/FHA coating and FHA coating.In vivo results showed new bone formed in the gap created at the implantation site,and was in direct contact with the ZnTCP/FHA coating after 12 weeks implantation. These evaluations demonstrated that the ZnTCP/FHA coating had good cytocompatibility,the capability of accelerating bone formation and osteointegration with the implant. (6) Electrochemical deposition of calcium phosphate/collagen extracellular matrix(ECM) mimicked layer composed of calcium phosphate/collagen was assembled on titanium substrates using electrolytic deposition method.The oxidized titanium substrates were used to simulate those with FHA bottom layers in this research.We systematically investigated the micro-structure of the layer under different parameters and the ECM mimicked layer formation mechanisms were discussed.The crystal phase of Calcium phosphate changed with the deposition parameters:at 3.0 V,pH:3.6,Ca²⁺:4.5 mM,pO₄^3-:9 mM,the crystal was OCP;at 3.0 V,pH:3.6,Ca²⁺:4.5 mM,PO₄^3-:9 mM,forming amorphous phase. The formation of the calcium phosphate/collagen layer involved two different electro-chemical behaviors:electrolytic deposition of Ca-P and electrophoretic deposition of collagen molecules.It is believed that the simultaneously occurrence of these two kinds of deposition was crucial to form a homogenous layer.The calcium phosphate/collagen coating showed good osetoblast cells biocompatibility.