Development Biomimetic Functionally Graded Materials That Mimic Insect Self-Cleaning Organs

Functionally graded materials (FGMs) are the materials with varying composition and structure gradually over volume, resulting in corresponding changes in the properties of the materials. Unlike traditional composites of homogeneous materials, FGMs can compromise between the desirable properties of the component materials in the desirable places in the materials. The exoskeleton of insects is a notable example of FGMs in nature. Cuticle, the constituent material of insect exoskeleton, is characterized by a wide range of elastic moduli, which can be as high as metals and as low as rubbers, are integrated without boundaries to achieve high strength, lightweight, and multifunctions.<br/>The purpose of this study is to analyze the correlation between the composition and functionality of exoskeletons developed through evolution and to apply the insights to the engineering of novel biomimetic FGMs.<br/>Honeybee (<i>Apis mellifera</i>) was selected as a model organism in this study. The antennae of honeybees have many sensors like the five human senses, and when the antennae become contaminated, dirt accumulates on the surface sensors and interferes with the sensing function. Therefore, in order to maintain their sensing function, honeybees clean their antennae using antenna cleaners on their forelegs. We focused on antennae and its specific body parts, antenna cleaners, to investigate the correlation of its material composition and functionality.<br/>A confocal laser scanning microscope (CLSM) was used to analyze the morphology and composition of antennae and antenna cleaners. The CLSM images showed that both antennae and antenna cleaners were not homogeneous in composition, but had a combination of soft and hard parts. The antenna cleaner was also found to have a sandwich structure with soft parts sandwiched between hard parts by observation at high magnification.<br/>We analyzed the correlation between composition and functionality by quantitatively analyzing the efficiency of the antenna cleaners in removing contaminants from the antennae. The results showed that most of the contaminants were removed in a single cleaning and that contaminants were more easily removed closer to the tip than at the base of the antennae. Based on these results, we thought that the high removal rate of antenna cleaners can be attributed to the sandwich structure, and will try to fabricate biomimetic FGMs based on these structures. In this study, we attempted to develop the FGMs with a sandwich structure having a soft and a hard layer by using vitrimers which are a special type of cross-linked material with associative dynamic covalent bonded cross-links. Vitrimers can enhance interfacial adhesion and help fabricate FGMs because the bond exchanges are occurred by heat treatment. At last, we evaluated the interfacial adhesion of the fabricated materials by atomic force microscope (AFM) and the cleaning efficiency of the fabricated materials by independent test.