How do plants build strong shells?

Seed encapsulation in hard shells is common for protection and dispersal. In their article "Topological Interlocking and Geometric Stiffening as Complementary Strategies for Strong Plant Shells", the research group led by Notburga Gierlinger from Institute for Biophysics reveal previously unknown functional interrelations between the structure, geometry, and mechanics in nutshells. They found that simple geometric modifications, such as the development of puzzle-shaped instead of round cells, and small spherical shells instead of large ellipsoidal shells, increase the fracture resistance of hard plant shells resource-efficiently.

Many organisms encapsulate their embryos in hard, protective shells. While birds and reptiles largely rely on mineralized shells, plants often develop highly robust lignocellulosic shells. Despite the abundance of hard plant shells, particularly nutshells, it remains unclear which fundamental properties drive their mechanical stability. This multiscale analysis of six prominent (nut)shells (pine, pistachio, walnut, pecan, hazelnut, and macadamia) reveals geometric and structural strengthening mechanisms on the cellular and macroscopic length scales. The strongest tissues, found in walnut and pistachio, exploit the topological interlocking of 3D-puzzle cells and thereby outperform the fiber-reinforced structure of macadamia under tensile and compressive loading. On the macroscopic scale, strengthening occurs via an increased shell thickness, spherical shape, small size, and a lack of extended sutures. These functional interrelations suggest that simple geometric modifications are a powerful and resource-efficient strategy for plants to enhance the fracture resistance of entire shells and their tissues. Understanding the interplay between structure, geometry, and mechanics in hard plant shells provides new perspectives on the evolutionary diversification of hard seed coats, as well as insights for nutshell-based material applications.


"Topological Interlocking and Geometric Stiffening as Complementary Strategies for Strong Plant Shells"

A study paper by: Jessica C. Huss, Sebastian J. Antreich, Jakob Bachmayr, Nannan Xiao, Michaela Eder, Johannes Konnerth, Notburga Gierlinger

Full article in: Advanced Materials, 20 October 2020,