Light but tough bio-inherited materials: Luffa sponge based nickel-plated composites

Natural structural materials featuring fine hierarchical architectures often display remarkable mechanical properties. To inherit the microstructures of biological materials, nickel-plated luffa sponges were fabricated through electrochemical deposition using natural luffa sponges as templates. Four groups of samples were processed based on nickel electroless and electroplating, and then characterized by X-ray diffraction and optical/scanning electron microscopy. Axial compression tests were performed to characterize the mechanical properties of the nickel-plated samples to compare with those of the original natural sponges. Results showed that a uniform layer of nickel was formed on the luffa fibers by electroless plating; conversely, by electroplating the nickel only minimal deposits were found on the inner luffa wall due to the uneven current distribution over the surface of sponge. Accordingly, electroless plating was deemed to be far more effective for metal deposition of materials with complex structures, such as luffa sponge. Alkali treatments prior to plating were found to be critical for subsequent mechanical performance and energy absorption capacity. The mechanical properties of nickel-plated samples surpass those of original luffa sponges, with the enhancement efficiency, i.e., the ratio of specific stiffness and strength, being highest for electroless-plated samples with a prior alkali treatment. Specifically, their energy absorption capacity was far superior to that in other comparable materials. Using a power scaling law, an empirical relationship was derived which indicated that the bending-dominated behavior of the nickel-plated luffa sponges was similar to that of open-cell foams. We believe that other artificially "bio-inherited materials" could be successfully processed and developed in this manner. The superior properties of bio-inherited materials that we obtained in this work may provide inspiration for future research efforts on bio-inspired structural materials.