Intrafibrillar Collagen Mineralization Of Inorganic Functional Materials Controlled By Biological Confinement

As billions of years of natural selection,biological systems evolved into highly integrated,but methodical and low energy consumption systems.In contrast to our artifical technologies where harsh conditions are necessary,natural structure-forming processes in biological systems can efficiently and accurately mineralize biomaterials under mild environmental conditions.Recently,the concept of“bioprocess-inspired fabrication”was put forward.It advocates to study the biological processing or the relation between biological processing and biological structure,for inspiring the design and exploitation of novel synthesis technologies,and leading to the development of new techniques of materials fabrication.Bone is widely attracted attentions due to its relation between good mechanical properties and hierarchical structure.The building blocks of bone are mineralized collagen fibrils,their organized assembly from nanoscale to macroscale endows the bone with a multiscale structure.Collagen fibrils present periodic structures,which provide space for intrafibrillar growth of oriented hydroxyapatite nanocrystals in bone and contribute to the good mechanical properties of bone.Therefore,inspiring from the intrafibrillar mineralization during bone formation can provide new ideas for materials synthesis and fabrication.However,there are not many reports focused on mineralization of non-native inorganic materials inside collagen fibrils and detailed forming processes of crystals inside collagen fibrils remain poorly understood.Especially,the exploration of the structure of mineralized collagen fibrils and their related functions have not yet been reported.Therefore,our thesis will reveal the growth process of calcium fluoride nanocrystals within collagen fibrils,achieve the controllable calcium carbonate mineralization of collagen films by adopting the principle of collagen fibrils mineralization,and explore the piezoelectric properties of strontium carbonate mineralized collagen films.Firstly,the rapid intrafibrillar mineralization of calcium fluoride nanocrystals with periodical patterned nanostructures is demonstrated.The negatively charged calcium fluoride precursor phase infiltrates collagen fibrils through the gap zones creating an intricate periodic mineralization pattern.Later,the nanocrystals initially filling the gap zones only expand gradually into the remaining space within the collagen fibrils.Atomic probe tomography reconstruction demonstrates that the ratio of minerals in unit volume of gap/overlap is 1.3.The organized structure of mineralized collagen fibrils contributes to its high bending strength.Mineralized tendons with organized calcium fluoride nanocrystals acquire mechanical properties（indentation elastic modulus～25.1GPa and hardness～1.5 GPa）comparable or even superior to those of native human dentin and lamellar bone.Secondly,the controllable mineralization of collagen films and tendon is realized.The intrafibrillar mineralization of calcium carbonate is similar to calcium fluoride.The precursor phase infiltrates gap zones firstly,and diffuses into overlap zones.Finally,it converts to a calcite mineralized collagen with single-crystal like structure.The process of collagen film mineralization is closely related to the concentration and size of precursor in mineralizing solution through the observation of optical microscope and the model construction of“heterogeneous nucleation”.The size distribution of mineralized areas in collagen films shows a self-similar evolution.The slopes（kinetic rate）of mineralized areas in p H 9.0,9.25 and 9.5 are 21.4±0.3,36.1±0.97 and10.5±0.44μm/h,respectively.The in situ Raman results clearly reveal a gradual diffusion of ACC precursor along the surface of tendon,and a fast transformation process of amorphous phase to vaterite.Finally,we achieved the intrafibrillar mineralization of oriented strontium carbonate nanocrystals in vitro,which exhibits good piezoelectric properties.The amorphous strontium carbonate precursors penetrate from the gap zones and fills gradually into the whole space within the collagen fibrils,and transform into co-oriented crystalline phase.Isolated mineralized collagen fibrils with organized ASC nanocrystals acquire good flexible properties and inverse piezoelectric responses with an effective piezoelectric coefficient of 3.45 pm·V^-1,much higher than individual collagen(1.12 pm·V^-1)and strontium carbonate crystals(0.092 pm·V^-1).These results might indicate that the organic and inorganic components synergistically contribute to the piezoelectric effect of bone.Furthermore,devices of flexible strontium carbonate mineralized collagen films exhibit a regular open-circuit voltage of 1.2 V under a compressive stress and a stable cycling short-circuit current of 80 n A under a bending mode.