Natural gradient composite made of aragonite nanofibers: the ligament of bivalve Acesta marissinica

Here we investigate the nanostructure of the fibrous ligament (FL) in bivalve Acesta marissinica, using scanning electron microscopy (SEM) and an image processing method. We find this FL is mirror symmetrical in its transverse section and consists of aragonite nanofibers and organic materials, the former of which are generally arranged in a featherlike pattern along the mirror plane. Further, we find this FL has a unique graded architecture. From its inner (near the mirror plane) to lateral part (near the ligament–shell junction), the nanofiber angle gradually increases from about 20° to 45°, the nanofiber volume fraction remarkably decreases from 70% to 14%, and the nanofiber diameter also changes from about 137 nm to 85 nm. This novel design allows this FL to be both resilient and strong to meet the biofunctional requirements. We expect the present findings may help us to develop new functionally graded materials (FGMs) and further understand bivalve life processes.