design based on the insect cuticle
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?
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.
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
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.