Fabrication Of A HA Nanorod Patterned Pore-sealed MgO Coating On Magnesium And Evaluation Of Osteoblast And Osseointegration Responses

In the last decade,magnesium?Mg?and its alloys have been widely investigated as the third generation of biodegradable metals for orthopedics applications.Compared to traditional metals used for endosseous applications,Mg-based alloys have similar elastic modules with that of natural bone and excellent mechical properites,moreover,Mg-based alloys can degrade safely in vivo without toxicity to the human body.However,the major obstacle inhibiting their clinical use is a relatively high degradation rate in the physiological environment especially containing lots of Cl^-,which would lead to loss of mechanical integrity prematurely and mismatch in the time for fractured bone to sufficiently heal.To improve their corrosion resistance and simultaneously enhance their biocompatibility to shorten the healing time of fracture bone,in this paper,we used micro-arc oxidation?MAO?and hydrothermal treatment?HT?to fabricate a novel bilayer coating?termed as HT24h?on Mg.This coating comprises an outer layer of narrow interrod spaced?<70nm?hydroxyapatite?HA?nanorods and an inner layer of MgO containing HA/Mg?OH?₂-sealing pores,in which the roots of the HA nanorods are embedded in the MgO matrix and the Mg?OH?₂ nanoplates are among the HA nanorods.The hydrothermal formation mechanism of HA formed on MAO-derived MgO was explored.Also,evolution of structure and bonding integrity of HT24h coating after immersion in physiological saline?PS?for 0⁹0days,regulation of osteoblst functions in vitro and osseointegration in vivo of the coating were investigated,together with MgO containing Mg?OH?₂-sealing-pores?HT2h?and porous MgO?MAO₀?coatings.Sebsquently,we modified the MAO-HT method to shorten the formation time of HA nanorods and eliminate additional phase component of Mg?OH?₂.Using the modified approach,we herein fabricated another bilayer coating composed of HA nanorods as an outer layer and HA nanorods-sealed MgO as an inner layer within 5 hours of HT,and also investigated the adhesion strength,protective efficacy and cytocompatibility of the coating,together with the MAO₀ coating and bare Mg.Main results obtained are as follows:1)Formation mechanism of HA nanorods during HT in HT24h coating can be described as follows:the hydroxyl groups and water molecules in the hydrothermal solution attacked the outermost layer of the MgO to generate Mg?OH?₂ nanoplates according to the following equation:MgO+H₂O?Mg?OH?₂.In parallel to the formation of Mg?OH?₂,the migration of the PO₄^3-and Ca²⁺incorporated in the MgO layer occurred,having a tendency from the interior of MgO to its surface.Once the local Ca²⁺and PO₄^3-in the outermost layer of the MgO reached critical concentrations due to migration,becoming supersaturated with respect to HA,they reacted with hydroxyl groups in the hydrothermal solution to form HA nuclei on MgO and then became nanorods with prolonging HT time.Moreover,we have determined that the migration of Ca²⁺ions from the MgO layer played a predominant role in HA nucleation,while Ca²⁺ions supplied by the thermal dissociation played a predominant role in the growth of HA nanorods.2)Immersion tests of MAO₀,HT2h and HT24h coatings in PS solution indicated that cracking rather than delamination is a common feature in most areas of the coatings up to 90days.Degradation is the cause of the thinning of the coatings,depending on the phase components of the coatings.MAO₀ and HT2h coatings exhibit significant thinning owing to fast degradation of MgO;however,HT24h coating keeps structure integrity for longer time,which can be attributed to the fact that the HA nanorods underwent quite slow degradation while the underlying MgO only underwent conversion to Mg?OH?₂ without dissolution of the Mg?OH?₂.Scratch tests reveal that after 90 days of immersion,MAO₀ and HT2h coatings exhibit a relatively fast drop in bonding integrity,and the failure positions under critical loads change from the interiors of the MgO to the MgO/Mg interface for MAO₀ coating or the upper Mg?OH?₂/MgO interface for HT2h coating.However,HT24h coating retains relatively high bonding integrity,although the failure position under critical load changes from the interior of the MgO to a point between HA and MgO layers.Compared to MAO₀and HT2h coatings,HT24h coating is shown to greatly reduce the degradation and thereby maintain sufficient mechanical integrity of Mg.3)Compared to MAO₀-and HT2h-coated Mg,HT24h-coated Mg greatly enhanced osteoblast functions,which ascribed to the HA nanorods derived up-regulated intracellular Ca²⁺and down-regulated intracellular Mg²⁺concentrations as well as the lightened H₂evolution and alkalization of surrounding medium.Owing to the enhanced osteoblast functions,HT24h-coated Mg greatly enhanced osteogenesis,bone-implant contact and push-out force characterized osseointegration.According to the microstructure of push-out disrupted surfaces of the coated Mg,the bonding mechanism of the newly formed bone to the HA nanorods can be described as follows:HA nanorods can significantly simulate osteoblasts to secrete noncollagenous proteins such as bone sialoprotein and osteoprotein to wrap each of the HA nanorods,which can form cement line by mineralization.Cement line can overlay the HA nanorods to form mechanical or chemical bonding,and then osteoblasts secreted collagen overlying the cement line matrix and the collagen fibers were further mineralized to form bony matrix.Due to the high-bonding stability at bone/HA nanorods interfaces,pushed-out failure of HT24h-coated Mg occurred predominantly within the peri-implant bone rather than within the coating or at the coating/Mg interface.4)Using the modified approach,we herein fabricated HT5h coating,which processed relatively high cohesive strength.Moreover,HT5h coating appeared more effective than the MAO₀ coating in protection of Mg from corrosion and significantly enhanced he mitochondrial activity,adhesion and proliferation of osteoblasts.