Biomineralization Mechanism Of Mg-calcite Mesocrystals And Bioinspired Fabrication

Mg-calcite materials are commonly found in marine echinoderm skeletons,bird eggshells,and other animal skeletons such as foraminifera.A transient amorphous calcium carbonate(Mg-ACC)precursor firstly deposits during biomineralization of these skeletons,and subsequently transforms into the stable crystalline phase Mg-calcite.Previous studies reported that organic molecules such as aspartic acid played an important role in stabilizing ACC precursors and regulating calcite crystallization.However,the influence of Mg2+ions on the composition and microstructure of Mg-calcite crystals remains poorly understood.Mg-calcite biominerals can be used to fabricate tricalcium phosphate(Mg-TCP)by hydrothermal reactions.However,the effects of Mg2+ ion substitution on formation of Mg-TCP crystals are still unclear.In order to solve these problems,the following five sections have been studied in this doctoral dissertation:Part 1:Biomineralization of Mg-calcite mesocrystalsThe effect of Mg2+ ions on the calcite lattice was investigated by studying the composition and microstructure of natural calcite materials with different Mg content.The SEM and TEM results proved that the echinoderm Mg-calcite was mesocrystals formed by the ordered assembly of nanocrystals.The particle size of the nanocrystals in the mesocrystals was measured to be 5-20 nm after tilted by exposure to high energy electron beams in the high resolution TEM.Part 2:Bioinspired synthesis of Mg-calcite mesocrytalsMg-ACC nanoparticles were prepared using a biomimetic method,which were progressively dehydrated under the gradient pressure,and further assembled to form Mg-calcite mesocrystals.Mg2+ ion substitution can inhibit crystallization by forming the metastable structure of Mg2+-H2O-CO32-,which allowed the ordered assembly of precursor nanoparticles to form Mg-calcite mesocrystals,even without any organic additives.Part 3:Hydrothermal synthesis of Mg-substituted tricalcium phosphate(Mg-TCP)from Mg-calciteMg-calcite with a high Mg content(Mg/(Mg+Ca)≥ 10 mol.%)can be hydrothermally converted to Mg-TCP through ion-exchange reactions,while calcite without Mg can be hydrothermally converted to hydroxyapatite(HA).This is due to the extremely high energy increase for Mg substitution of Ca sites in HA with high coordination numbers,while Mg substitution of Ca sites in β-TCP with low coordination numbers requires much lower energy.The preference for Mg2+ion substutition in β-TCP is spetulated as following:Ca(5)>Ca(4)>Ca(1,2,3).Moreover,Mg-calcite blocks as prepared following the biomimetic way can be hydrothermally converted to Mg-TCP.Thus,we developed a novel method for hydrothermal synthesis of Mg-TCP using Mg-calcite at relatively low temperature～180℃,which reduced energy cost compared to traditional high-temperature(above 800℃)calcination method.Part 4:Hydrothermal synthesis of biphase calcium phosphate(BCP)from chicken eggshellsChicken eggshells comprise of Mg-calcite with a low Mg content((Mg/(Mg+Ca)～2 mol.%),which can be hydrothermally converted to BCP,a mixture of HA andβ-TCP.Beta-TCP crystals formed in the interior of eggshells through ion exchange reactions,whereas HA nanoparticles formed on the outer surfaces by dissolution and reprecipitation reactions in the hydrothermal system.Mg substitution and particle size synergistically adjusted the ion exchange and dissolution and reprecipitation reactions,which allowed to produce BCP with different β-TCP content(28.6-77.8wt.%).The in vivo degradation of BCP ceramics increased with the increase of TCP content,when implanted in rabbit femoral condyle defects.The residual amount of BCP ceramics with 77.8%TCP was smaller and more new bone formed after implanted for 8 weeks,compared to BCP samples with 28.6%TCP.Part 5:Preparation and characterization of hybrid materials of amorphous magnesium-substituted calcium phosphate/polyacrylic acid(Mg-ACP/PAA)Following the biomineralization strategy,Mg-ACP/PAA hybrid materials were prepared through synergistically stabilizing of Mg-ACP by PAA chelation and Mg2+ion substitution,which can be stable for more than one year at ambient condition.The pH-sensitive Mg-ACP/PAA hybrid nanoparticles can be efficiently degraded in the lysosome of the cell within 24 hours,which may avoid the enrichment of nanoparticle in organs and thereby prevent the risk of biotoxicity.