Elaborate architecture of the hierarchical hen’s eggshell

Eggshells are one of the most common and well-studied biomaterials in nature and exhibit unique properties of gas conduction. However, the morphologies of eggshells at the submicro-/nano-scale and their impact on eggshell functions remain unclear. In this work, the architecture of hen's eggshell at different length scales has been systematically investigated by scanning electron microscopy (SEM) and environmental SEM (ESEM). It is found that the skeleton of calcium carbonate (CaCO3) has hierarchical structures at nano- to micro-scales: primary nano-particles of ~10 nm loosely congregate giving a porous and rough texture, and compose the upper-level morphologies including submicro spheres, nano-rods, rhombohedral-cleavage pattern and slices, which are elaborately arranged in a surface layer, palisade layer and mammillary layer along the radial direction. Accordingly, the pore system exhibits a three-level hierarchy, namely nano-scale pores (between nano-rods and primary nano-particles), submicro-scale pores ("bubble pores") and micro-scale pores (opening of "gas pores"). Further investigation shows that hen's eggshell regulates gas conduction through adjusting the sizes and numbers of submicro-scale "bubble pores". Based on our observations, a new description of hen's eggshell is presented, which amends the conventional view of micro-scale, straight and permeating "gas pores", and reveals the role of hierarchical pores in gas conduction and contamination resistance.