Multi-Gradient Bone-Like\nNanocomposites Induced by\nStrain Distribution

The heterogeneity of bones is elegantly adapted to the\nlocal strain\nenvironment, which is critical for maintaining mechanical functions.\nSuch an adaptation enables the strong correlation between strain distributions\nand multiple gradients, underlying a promising pathway for creating\ncomplex gradient structures. However, this potential remains largely\nunexplored for the synthesis of functional gradient materials. In\nthis work, heterogeneous bone-like nanocomposites with complex structural\nand compositional gradients comparable to bones are synthesized by\ninducing strain distributions within the polymer matrix containing\namorphous calcium phosphate (ACP). Uniaxial stretching of composite\nfilms exerts the highest strain in the center, which ceases gradually\ntoward the sides, resulting in the gradual decrease of polymer alignment\nand crystallinity. Simultaneously, the center with high orientation\ntraps most ACP during stretching due to the nanoconfinement effect,\nwhich in turn promotes the formation of aligned nanofibrous structures.\nThe sides experiencing the least strain have the smallest amounts\nof ACP, characteristic of porous architectures. Further crystallization\nof ACP produces oriented apatite nanorods in the center with a larger\ncrystalline/amorphous ratio than the sides because of template-induced\ncrystallization. The combination of structural and compositional gradients\nleads to gradient mechanical properties, and the gradient span and\nmagnitude correlate nicely with strain distributions. Accompanying\nbone-like mechanical gradients, the center is less adhesive and self-healable\nthan the sides, which allows a better recovery after a complete cutting.\nOur work may represent a general strategy for the synthesis of biomimetic\nmaterials with complex gradients thanks to the ubiquitous presence\nof strain distributions in load-bearing structures.