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

Area of Science:

• Biomechanics
• Materials Science
• Plant Biology

Background:

• Organisms use protective shells for embryos, with plants utilizing robust lignocellulosic structures.
• The fundamental properties governing the mechanical stability of hard plant shells remain largely unelucidated.

Purpose of the Study:

• To identify the geometric and structural mechanisms responsible for the mechanical stability of prominent nutshells.
• To understand the interplay between cellular and macroscopic features influencing shell fracture resistance.

Main Methods:

• Multiscale mechanical analysis of six distinct nutshell species (pine, pistachio, walnut, pecan, hazelnut, macadamia).
• Evaluation of cellular-level interlocking and macroscopic geometric factors (thickness, shape, size, sutures).

Main Results:

• Walnut and pistachio shells exhibit superior strength due to topological interlocking of 3D-puzzle cells, surpassing fiber-reinforced structures.
• Macroscopic strengthening is achieved through increased shell thickness, spherical shape, smaller size, and reduced suture length.
• Cellular and geometric features interact to enhance overall shell mechanical stability.

Conclusions:

• Simple geometric modifications represent a resource-efficient strategy for plants to significantly improve shell fracture resistance.
• Findings offer insights into the evolution of seed coat structures and potential applications in nutshell-inspired materials.