Jackfruit: Composition, structure, and progressive collapsibility in the largest fruit on the Earth for impact resistance.

The jackfruit is the largest fruit on the Earth, reaching upwards of 35 kg and falling from heights of 25 m. To survive such high energy impacts, it has evolved a unique layered configuration with a thorny exterior and porous tubular underlayer. During compression, these layers exhibit a progressive collapse mechanism where the tubules are first to deform, followed by the thorny exterior, and finally the mesocarp layer in between. The thorns are composed of lignified bundles which run longitudinally from the base of the thorn to the tip and are embedded in softer parenchymal cells, forming a fiber reinforced composite. The mesocarp contains more lignin than any of the other layers while the core appears to contain more pectin giving rise to variations in compressive and viscoelastic properties between the layers. The surface thorns provide a compelling impact-resistant feature for bioinspiration, with a cellular structure that can withstand large deformation without failing and wavy surface features which densify during compression without fracturing. Even the conical shape of the thorns is valuable, presenting a gradually increasing surface area during axial collapse. A simplified model of this mechanism is put forward to describe the force response of these features. The thorns also distribute damage laterally during impact and deflect cracks along their interstitial valleys. These phenomena were observed in 3D printed, jackfruit-inspired designs which performed markedly better than control prints with the same mass. STATEMENT OF SIGNIFICANCE: Many biological materials have evolved remarkable structures that enhance their mechanical performance and serve as sources of inspiration for engineers. Plants are often overlooked in this regard yet certain botanical components, like nuts and fruit, have shown incredible potential as blueprints for improved impact resistant designs. The jackfruit is the largest fruit on Earth and generates significant falling impact energies. Here, we explore the jackfruit's structure and its mechanical capabilities for the first time. The progressive failure imparted by its multilayered design and the unique collapse mode of the surface thorns are identified as key mechanisms for improving the fruit's impact resistance. 3D printing is used to show that these structure-property benefits can be successfully transferred to engineering materials.