Stability Of Lipid Vesicles And Preliminary Study On Their Mediated Calcium Phosphate Mineralization Process

The initial deposition of bone mineral occurs within matrix vesicles?MVs?.MVs have obvious regulatory effects on the crystal form,size,morphology and orientation of minerals.But the formation mechanism and phase transformation condition of calcium phosphate crystals in MVs need to further investigate.Lipid vesicles can be used an in vitro mineralization model to study the formation process of calcium phosphate crystals in vesicles,which is beneficial to fully understand the bone formation process in vivo.However,the size,morphology and stability of lipid vesicles are susceptible to the external environment.The stability of lipid vesicles is a prerequisite for the successful design of in vitro models.Herein,the lipid vesicles were constructed using dipalmitoylphosphatidylcholine and cholesterol as materials to study the effects of environmental factors on the stability of lipid vesicles in static and flowing environments.And lipid vesicles were used as an in vitro mineralization model to preliminarily investigate the mineralization process of calcium phosphate in biomimetic vesicles.The effects of environmental factors such as temperature,pH,salt solution concentration and cationic species on the stability of lipid vesicles in static environment were investigated.It was found that the synthesized vesicles were hollow spheres with a bilayered or multilayered structure,and environmental factors had certain effects on the morphology,particle size and stability of lipid vesicles.When the storage temperature below the phase transition temperature of the lipid vesicles,the size and morphology of vesicles remained good.Within the experimental pH range,lipid vesicles had the best stability under micro-acid conditions.A low concentration of salt solution benefited the formation of lipid vesicles,and when the concentration exceeded a certain value,the vesicle structure was destroyed.The order of the initial size of the lipid vesicles was trivalent cations>divalent cations>monovalent cations>ultrapure water.Divalent and trivalent cations had a certain destructive effect on lipid vesicles,among which Cu²⁺and Al³⁺were more obvious.The zeta potential of lipid vesicles in ultrapure water was negative.The sign and value of the zeta potential depended on the net charge accumulated on the particle surface,which was affected by pH,solution concentration and ion species.At the same ion concentration,the zeta potential increased in the order of Al³⁺>Fe³⁺>Cu²⁺>Mg²⁺>Ca²⁺>Na⁺>K⁺.The effects of pH and cationic species on the stability of lipid vesicles in flowing environment were investigated.The adsorption behavior and deformation of lipid vesicles on the gold surface under different influencing factors were monitored by quartz crystal microbalance with dissipation?QCM-D?.The results showed that lipid vesicles in ultrapure water formed supported lipid bilayers on the gold surface.The morphology and size of the lipid vesicles on the gold surface were affected by pH change.Strong acid or alkaline conditions could destabilize lipid vesicles,and the stability of vesicles was better under slightly acidic or neutral conditions.The deformation degree of lipid vesicles was related to electrostatic interaction between vesicles,vesicle–surface interactions and spatial hydration.The size and uniformity of vesicles on the adsorbed vesicle layer were related to cationic type.The same qualitative kinetics were observed for cations that had the same valence at the same ionic strength.However,different types of cations gave different quantitative effects.The order of the ability to destroy the vesicle morphology was Al³⁺>Cu²⁺>Mg²⁺>Ca²⁺>Na⁺>K⁺,where cation-induced intervesicular bridging might play an important role in the effect of cations on vesicle stability.The mineralization process of calcium phosphate in biomimetic vesicles was preliminarily studied by using lipid vesicles as an in vitro model.The structure of the lipid vesicles was stable for 24 h in the mineralization experiment,and the precipitate was only formed inside the vesicles.The lipid vesicle-mediated calcium phosphate mineralization process was a heterogeneous nucleation process in which calcium phosphate underwent a phase transition of ACP-OCP-HA or ACP-HA.The confined space formed by the lipid vesicles effectively inhibits the growth of calcium phosphate crystals.The combination of QCM-D and AFM could detect the formation of calcium phosphate in lipid vesicles.