Hierarchical interfacial structuring of the calcareous base of the barnacle Balanus albicostatus over different length scales

How the calcareous bases of acorn barnacles are interfacially structured and conserved for underwater attachment function remains unclear, especially given the existence of proteinaceous adhesives in between the bases and substrata. We demonstrate with the barnacle Balanus albicostatus that the base adopts a unique, disc saw-like geometry, densely perforated with radial microchannels that are intelligently built by the animal to enhance adhesion. Meanwhile, the base is shown to be constructed of intergrown calcitic microcrystals with almost no preferred orientation, some varied extent of morphologic change and high level of atomic disorder compared to its geological equivalent. Further, both protein and polysaccharide chemistries are found to be involved in the microstructural development of the base by virtue of their acidic functional groups or confined space, with the latter (behaving as a hydrogel) possibly being dominant in amount. The key microstructural features along with the placement of organic components play crucial roles in the mechanical response of the base to external loads. Our results explain how the barnacle base has been evolutionarily adjusted over different length scales to enhance adhesion, which would lay the foundation of research into antifouling strategy as well as the synthesis of organic-inorganic hybrid interfacial materials.