Strong and Stiff Sapindus saponaria Seed Shell with Gradient Structure

Plants often develop robust lignocellulosic seed shells to protect their embryos. These seed shells with exceptional mechanical properties provide inspiration for bioinspired design and fabrication strategies. Here, a strong and stiff Sapindus saponaria (S. saponaria) seed shell is described, which consists of sclerenchyma cells with varying morphologies. The seed shell is composed of vertically arranged fiber bundles that provide modulus and hardness, equiaxed sclereids that provide toughness, and tissue that plays a transitional role. The channels in the multilayered cell wall are thought to enhance the connection between adjacent units. These features endow the seed shell with good specific fracture strength (540.4 ± 99.2 kN cm² g^-1), which is comparable to the macadamia nut (531.7 ± 145.1 kN cm² g^-1), and high surface hardness (91.7 ± 12.1 HV0.2), which surpassing all other nuts. This unique structure of the S. saponaria seed shell may provide a bioinspired strategy for the design of high-performance structural materials.