Biomimetic hierarchical composites inspired by natural pomelo peel for mechanical-damage resistance and storage of fruits

Mechanical damage to fruits with a fragile texture during storage and transportation accelerates the process of fruit spoilage and causes considerable economic loss. Herein, a biomimetic hierarchical composites inspired by natural pomelo peel were developed to achieve mechanical-damage resistance and fresh preservation of fruits. The mesocarp of the biomimetic composites included a hierarchical cellular sponge comprising absorbent cotton with microsized fibers and chitin with nanosized fibers, which rendered the mesocarp soft and elastic (only with a 12.25 % unrecoverable strain after 5000 compressions), consequently aiding the absorption of external forces and effectively protecting the fruits. The exocarp was a dense silk hydrogel that resisted puncture and distributed external forces acting on it. Furthermore, the mesocarp and exocarp were firmly bound together through mechanical interlocking and strong interface interactions induced by hydrogen bonds, which supported the transfer of external forces through the exocarp to the mesocarp and the subsequent absorption of forces, avoiding the disintegration of the composites during use. Therefore, the biomimetic pomelo peel composites could protect the test fruit (strawberry) from almost all mechanical-force damage, including impact, vibration, compression, and puncturing. The excellent antibacterial property and hydrophilicity of the mesocarp (mesocarp comprising cellulose and chitin and mesocarp in situ modified with silver nanoparticles) and the physical barrier of the exocarp effectively extended the shelf life of strawberries to 21 days (the longest strawberry preservation record to date), which is considerably longer than the 9 days in the control group. Thus, this study used only natural polymers (cellulose, chitin, and silk) to achieve favorable shock resistance and preservation of strawberries and provide a new design strategy for developing multifunctional fruit preservation material.