| Duo to the biodegradability,bio-safety,excellent mechanical properties and others,magnesium are becoming the most promising biodegradable biomaterials.They may help to solve the problems that are faced by permanent materials in human body,such as physical irritation,toxic ions release,mechanical mismatch and others.However,the key issues hindering clinical applications of magnesium-based materials are the too fast corrosion rate in human body and the uncertain biocompatibility.In addition,their evolving corrosion/degradation behavior plays an important role in their clinical usage as well as in guiding the surface modification of magnesiurn-based biomaterials.Concerning these problems,this dissertation aims to study the evolving degradation behavior of Mg-based compared to other biodegradable metals via long-term immersing in phosphate buffered saline,and based on this fundamental understanding to further surface-modify magnesium-based materials for corrosion controlling and biocompatible/bio-functional improvement.The evolving corrosion behavior of pure Mg compared with pure Fe and pure Zn was investigated by means of long-term immersing in phosphate buffered saline(PBS).The investigation focused on corrosion rate,mode,products and surface characteristics as a function of immersion time up to 21 days.From transient results,the open circuit potential increased for Mg,Zn and Fe,while their corrosion rate decreased in reverse oder.However,in a long-term immersion period the corrosion behavior of these three metals present different tendency.Mg showed a continuously decreasing corrosion rate with the protective corrosion products accumulation;Fe presented a continuously decreasing corrosion rate with little corrosion products;Zn displayed a decrease in an initial short time immersion and then increased markedly in later stage.Such discrepancy is dependent upon their different corrosion modes and corrosion products changing with immersion time,which provides a fundamental guideline to modification of these biodegradable metals.To surface modify Mg,a polydopamine(PDA)layer was sandwiched between a TiO2 coating and Mg substrate to enhance the corrosion protection.The PDA layer was covalently immobilized on Mg,which can extremely enhance the deposition of TiO2 by liquid phase deposition subsequently on it.The hybrid TiO2/PDA coated Mg exhibited significantly smaller free corrosion current density as well as a remarkably lower degradation rate in vitro(up to 21 days)in PBS compared to direct TiO2 coated and untreated Mg.The efficacy is ascribed to the organic PDA layer which suppresses the electric pathway of galvanic corrosion cell and therefore corrosion rate of magnesium.Further,small organic molecule phytic acid was coated on magnesium to modify its corrosion controlling as well as biocompatibility,which is advantageous over conventional inorganic or polymeric coatings with regards to efficacy of corrosion controlling and biocompatibility.The small molecules can be covalently immobilize on the alkaline pre-treated Mg surface and further in-situ deposited accompanied with chelating Mg ions released from magnesium,resulting in an effective corrosion controlling layer.It was ascertained that the phytic acid coating appeared dense and homogenous,serving as an efficiently protective layer on Mg with smaller corrosion current density and degradation rates compared to bare Mg.Based on the above scheme,an enhanced corrosion-controlling and osteo-compatible phytic acid coating integrated with more added Mg ions(Mg-PA)was constructed on Mg through an alternating dip-coating approach.In this strategy,PA molecules were covalently immobilized on a chemically converted Mg(OH)2 base layer,and more PA molecules were deposited subsequently via chelating reactions with the help of additive Mg ions.The covalent immobilization and the Mg ion chelating-assisted deposition made the coating even denser and homogenous relative to direct PA coating,which guarantees therefore an improved corrosion protection.Moreover,the Mg-PA coating performed osteo-compatible promoting not only bioactivity of bonelike apatite precipitation,but also induced osteoblast cells adhesion and proliferation.This is ascribed to its nature of PA molecule and the biocompatible Mg ion,both of which mimic partly the compositional structure of bone.Themagnesium ion-integrated PA-coated Mg might bode well for the future of biodegradable bone implant application.Furthermore,drugs of heparin(Hep)or bivalirudin(BVLD)was incorporated into an phytic acid(PA)coating on magnesium for corrosion-control and bio-therapeutic surface modification.Electrochemical corrosion and in vitro immersion degradation investigations revealed that PA&Hep-and PA&BVLD-coated Mg had the same effect or even slowed-down corrosion/degradation rate in PBS compared to PA-coated Mg.Moreover,Hep-or BVLD-loaded PA coatings on Mg showed relative good blood biocompatibility with prolonged clotting time,inhibited platelets adhesion as well as reduce hemolysis compared to untreated Mg.In addition,both PA&Hep and PA&BVLD coatings restrained smooth muscle cell proliferation.In vivo assay indicated that PA&Hep-coated Mg exhibited significant different in mass loss compared to untreated Mg and better histocompatibility than other samples.These results demonstrated that the present coating strategy shows great potential in surface modifying biodegradable Mg.Finally,by exploiting mechanical and biological merits of Mg,an Mg alloy mesh reinforced polymer/ECM hybrid scaffold was fabricated for potential bone-related regeneration applications.The objective is to enhance both mechanical supportive and osteogenic properties of the current bone regenerative scaffolds.An Mg mesh was embedded within polymer/extracellular matrix(ECM)scaffold by concurrent electrospinning of poly(lactic-co-glycolic)(PLGA)polymer and electrospraying of a solution of demineralized bone matrix(DBM)suspended into sodium hyaluronate(HA).According to bone marrow stem cells(BMSCs)culture results,the hybrid scaffold(Mg-PLGA@HA&DBM)showed a similar BMSCs proliferation rate compared to TCPS and the composite w/out Mg mesh in 7 d culture.Alizarin red staining results of BMSC seeding on samples' surface for 21 d showed that PLGA@HA&DBM and Mg-PLGA@HA&DBM samples presented statistically more red color generation compared to the other surfaces.Also,Mg-PLGA@HA&DBM significantly exceeded PLGA@HA&DBM in this parameter.This dissertation deals with first the evolving corrosion behavior of Mg compared with Fe and Zn in a long-term immersion course,which guides further surface modification of Mg in terms of corrosion control as well as biocompatibility.One organic electric insulating buffer layer sandwiched between TiO2 coating and Mg substrate is able to enhance corrosion protection of magnesium,proposing a dual corrosion protective and biocompatible layer to modify Mg.In addition,optimizing process is feasible to improve the efficacy of immobilization and deposition of such small organic phosphoric acid by chelating with Mg ions to improve further the quality of coating as well as its biocompatible/bio-functional performance.More important,drugs loading can be easily achieved on this kind of coating simply by chemical dip-coating method to create corrosion controlling,biocompatible and even therapeutic hybrid surface layer on magnesium.Last,magnesium alloy mesh enhanced polymer/ECM hybrid scaffold provides a new way to make use of Mg with aspects to mechanical and osteogenic merits for bone regenerative scaffold. |
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