This post is part of a series on the insect cuticle as a biological material that can inspire novel engineered materials. The characteristics of the cuticle, setting it apart from most synthetic/engineered materials, will be discussed in this series. The introduction to the series can be found here.
Hydration: Slippery when wet-ish
Traditional manufacturing of engineered materials tries to avoid, at all cost, incorporating water. Yet, the properties of biological materials are determined to a large degree by the absence or presence of water molecules located within the extracellular spaces (=hydration level). The presence of water affects mechanical properties such as toughness or strength. It serves as a plasticizer, keeping a material flexible and resilient. Once an engineered material is manufactured we don’t really want to be able to manipulate these types of properties; materials are to remain static. But wouldn’t there be advantages to having bioinspired materials that are more adaptive depending on their surroundings? Certainly materials that are able to alter their function within a structure at different times will result in some very innovative applications.
Insects go through different life stages, most pronounced in holometabolous insects; egg, caterpillar, pupa, adult. All these life stages exhibit different cuticle characteristics, maybe even on the same body (e.g. tough thorax, soft abdomen).
It is not well understood how hydration influences one of the most impressive features of the insect cuticle: the ability to sclerotize (also called tanning or hardening). The chitin fibers that are a major component of the cuticle are hydrophylic and are thus surrounded by water molecules. Chitin becomes dehydrated when it becomes part of the protein matrix since the proteins within this matrix are generally hydrophobic. This dehydration contributes to cuticle stiffening and de-plasticization. The most commonly referenced mechanism that results in sclerotization is the cross-linking between proteins and phenols that are present in the cuticle. There is certainly some experimental evidence for covalent interactions between proteins and phenols that help stabilize the cuticle, but it is not clear if these interactions directly result in a stiffening of the cuticle. Now, all these interactions between chitin, proteins, phenols and water result in a type of hydrophobic coating of chitin fibrils that increases the mechanical load the proteins can carry. Softening of the cuticle also becomes less of an issue. Just which of these interactions have an hardening effect on the cuticle AND are actually resulting in sclerotization, has yet to be precisely determined
It has been shown that water content of the cuticle has an important role in determining cuticle mechanical properties. At the same time it also clear that sclerotization is not just simply dehydration. In other words, the cuticle minus water becomes hardened, no matter if the cuticle is sclerotized or not. Sclerotization is an irreversible pathway, the bonds cannot be broken. The presence or absence of water in an unsclerotized cuticle means that some fine-tuning of mechanical properties can still occur even after the cuticle has been fully formed.
To me this seems like a feature that we might want to consider using in engineered materials. Fabricating materials using chemistry in water is a different approach, but this is how biological materials are synthesized in nature. To be able to create materials that can adapt to their environment, or are multifunctional, just by varying the degrees of adsorption and absorption of water, could lead to exciting innovations.