Material design based on the insect cuticle

responsibility for wording of article: Akira Takashima (OIST)

The insect exoskeleton (cuticle) supports the body, maintains the shape, and provides waterproofing. It also has mechanical properties such as high plasticity, adhesiveness, wear resistance, and gas diffusion control capacity. In addition, because adult insects need to fly, it is very light. These properties lead to the following questions:

1. What is the secret of the excellent mechanical properties of the insect cuticle despite being lightweight?

2. How do the mechanical properties of the insect cuticle contribute to its functions?

We suggest that the answers to these questions will be very useful in developing new materials. Below, we outline the general chemical and mechanical properties of the insect cuticle. 

Chemical properties of the cuticle

The insect cuticle is a fiber-reinforced composite material composed of chitin nanofibers and protein as the base material. Chitin is a polysaccharide resembling cellulose. The crystals show high thixotropic properties in suspension (when allowed to stand in suspension, a gel is formed; however, if shaken vigorously, the suspension acquires the properties of a sol), becoming liquid crystals. This observation suggests that it can easily form a stable structure required for self-organization.

The base material (protein) binds to chitin, providing a mechanical property that can maintain the stability of the cuticle. It is thought to have the same function as the resin, which is the base material in glass fiber composite materials. The protein binding to chitin fits into the chitin-binding site, showing a molecular configuration similar to that of the β sheet in silk. In addition, the cuticle contains a β-turn-containing protein called resilin, which is similar to the wettable elastic protein elastin typically found in vertebrates. Resilin is an important component of the wettable cuticle. Protein surrounds the chitin nanofibers in a regular manner. The proteins are attached to only 1 specific surface of the nanofibers and not to the other surfaces. The surface area of the chitin-binding surface per unit volume of the cuticle is 106-fold higher than that of carbon fiber composites. The interfacial shear strength of the protein and chitin is approximately 30 MPa, representing approximately half the adhesive strength between the carbon fiber and the base material. 

When an insect grows, a new cuticle forms under the old cuticle, and the insect must break the old cuticle and get out of it. In the cuticle, various substituted ortho-dihydroxy phenols secreted by epidermal cells are converted to highly reactive quinone compounds, which presumably help to harden the cuticle through protein crosslinking. At the same time, a large volume of water is lost from the cuticle. The rigidity of the cuticle is because of a secondary reaction on the removal of water from the protein, which is more important than the crosslinking by phenols (covalent bonding). This enables rigidity control by manipulating the water content. The mechanical properties of the cuticle are very sensitive to water content, suggesting that the hydrogen bond is an important element in cuticle stability. For example, in the soft cuticle of the fly larva, if water is reduced slowly, the rigidity is increased up to 10 times by a change in the water content of only 2%–3%. 

Mechanical properties of the cuticle

The mechanical properties of fiber-reinforced composite materials are influenced by the fiber volume content and orientation of fibers (nanofibers). In the insect cuticle, the dry weight of chitin is low in the hard cuticle and high in the soft cuticle. Most of the remaining portion comprises the base proteins. The orientation pattern of the chitin nanofibers is similar to that observed in liquid crystal materials. All the nanofibers are lined up in the same direction; the tips of the fibers are aligned or irregular and are organized in thick layers with regularly alternating directions. The orientation of the fibers reflects the mechanical properties of composite fiber materials. For example, the orientation of fibers in the plate-like material covering the insect body is changed to a spiral orientation and is responsible for high tensile strength. In the tendon of the jumping muscle on the hind limb of the grasshopper, which experiences a large amount of strain, the chitin fibers are oriented in the direction of the maximum principal stress.

The cuticle on the insect mandibles, which bite, cut, and rasp at food, are reinforced. Their hardness is similar to that of the tooth enamel. This hardness is achieved by incorporating heavy metals such as zinc, manganese, and sometimes iron. These metals account for up to 16% of the total dry weight of the cuticle and increase the hardness from 25 to 80 kg f/mm2.

Observing the relationship between Young’s modulus and the density of the cuticle, the density range is narrow (1–1.3 Mg/m3); however, the Young's modulus range is very wide. The cuticle is used over a wide range of conditions due to its characteristic properties mentioned above. Its resistance to bending deformation is comparable with that of wood, aluminum, and carbon fiber-reinforced polymers (CFRPs) typically used in airplanes.

Insects sense the outside world through mechanical sensors such as sensory hair and campaniform sensilla. The cuticle can be seen as a barrier featuring these sensors. These sensors represent an excellent example of the optimal combination of shape and material. These sensilla are essentially holes in the cuticle. The strain on the cuticle is monitored by the deformation of the hole. The area surrounding the hole is flexible so that the load to a wide area of the cuticle surface can be amplified by a factor of 10 or more. Each hole is surrounded by a fibrous chitin matrix positioned in such a manner that the strain can be safely dispersed. Little is known about insect joints; however, the chitin and cuticle may be made up of elastic hinges with the ability to tolerate a high degree of deformation. 

Structural rigidity depends on the properties of the constituent material and its shape. It is possible to increase the rigidity of the cuticle to hardness similar to that of structural elements on aircrafts, such as bends, corrugated sheets, beams, webbing, stringers, and flanges. Depending on the site, the cuticle can also have a sandwich structure. This reinforcement is useful for resisting force and slowing down the destruction of the cuticle. An optimal design for materials based on all aspects of the insect cuticle has not yet been established.

Biomimetics of the cuticle

The cuticle, with its diverse structures and functions, is a major source of inspiration for innovative engineering ideas. Examples include double-sided attachment systems without the use of adhesives, friction-proof joints that can tolerate various frictional conditions, and functional surface dynamic sensors. The structure and properties of the cuticle as a fiber-reinforced composite material meeting all the complex requirements in the real environment also provide a good example. Nanofibers similar to chitin are being considered for the design of composite materials of a high quality. Understanding the properties of the insect cuticle as a material is comparatively easy; however, evaluation and comparison of cuticle production and design are difficult. To resolve this challenge, a new problem-solving process, such as the TRIZ theory, is required.

Further Reading:

昆虫ミメティックス Insect Mimetics (2008),針山孝彦,下澤楯夫 pp. 631–640

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