materials

The insect cuticle: (5) Mild synthesis conditions
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The insect cuticle: (5) Mild synthesis conditions

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.

In Nature Easy Does It

Most biological materials, including the insect cuticle, are produced at ambient temperature and pressure. In addition, biological material synthesis often occurs in an aqueous environment. This is very different from how engineered materials are fabricated. Engineered materials are too commonly fabricated with the heat, beat and treat method.

Machine Meltdown by Darkday from flickr.com

Machine Meltdown by Darkday from Fickr.com

A lot of energy is added to the process of fabricating engineered materials. The material is usually forced to conform by adding pressure to it from the top down, and then (not always benign) chemicals are added to give the material additional properties. Nature teaches us that fabrication under mild synthesis conditions is possible too.

In general, most physiological processes related to growth in insects, including the fabrication of new cuticle, occur at temperatures between 40-100°F (5-40°C) and at standard atmosphere (~760 mmHg). This fabrication occurs from the ground up, no big machinery is needed, and only benign chemicals are used.

In my bioinspiration courses I ask students to create an Ashby plot (comparing tensile strength vs Young’s modulus) for engineered and biological materials.

ashby1

Engineered materials plot.

Ashby Plot Complete

Biological materials overlaying engineered materials plot. (By P. Karnstedt)

The activity reinforces that, at least for these two variables, biological materials are often as good, if not better, as engineered materials. Yet biological materials are fabricated at far milder synthesis conditions than engineered materials.

Some notes on how “extremophile insects” are able to survive, and maybe even grow, in extreme conditions.

Insects live in just about every habitat on Earth, from hot deserts and thermal springs to cold caves and frigid mountain streams. Clearly insects and other arthropods can survive some of the most inhospitable conditions on Earth. But some of extremophiles listed below will not molt under these extreme conditions.

  • Desert ants such as Cataglyphis bicolor, which live in extremely hot deserts (Sahara), use behavioral tactics to function at midday temperatures. The stages that will have to produce new cuticle in order to grow, the larval stages, are present underground where temperatures are not as extreme.
  • The flat bark beetle (Cucujus clavipes) larvae and adults survive arctic northern Alaskan winters by way of some clever blood chemistry that results in anti-freeze properties and prevents inter- and intra-cellular fluids from freezing. The larvae do not molt until the warmer periods and will do so under bark.
Red Flat Bark Beetle by Tom Murray via BugGuide.

Red Flat Bark Beetle by Tom Murray via BugGuide.

Cucujus clavipes larva by Lynette via Flickr.com

Cucujus clavipes larva by Lynette via Flickr.com

  • The largest purely terrestrial animal on Antarctica is the tiny midge Belgica antarctica. This insect can survive extreme freezing temperatures, and dry habitat, by using its immediate environment as a barrier and through the management of body water as temperatures drop and come back up. Most of the larval growth however, including molting, only occurs during the warmest weeks when temperatures are well above freezing.
Belgica antarctica larva by Richard Lee. Courtesey: National Science Foundation

Belgica antarctica larvae by Richard Lee. Courtesy: National Science Foundation.

  • Other extreme conditions that some arthropods deal with are lower pressures or very acidic water. The Himalayan jumping spider (Euophrys omnisuperstes) is able to survive at 20,000 feet (6000 meters) despite the lower air pressures (video). But the range of pressures that are encountered on Earth are not as great as the pressure difference humans create during manufacturing of engineered materials, when materials are often pounded into the desired shape.
Himalayan jumping spider by Gavin Maxwell via Arkive

Himalayan jumping spider by Gavin Maxwell via Arkive

  • In one of the most inhospitable places on earth, Cueva de Villa Luz, a cave located in the southern Mexican state of Tabasco, arthropods thrive despite the fact that the waters have an extremely high concentration of hydrogen sulfide. Insects such as predatory belostomatid bugs, have been shown to live within these caves during all life stages. How physiological mechanisms, such as molting, are possibly altered in these acidic waters has not yet been studied in detail.
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The insect cuticle: (4) hydration
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The insect cuticle: (4) hydration

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).

By Rob Mitchell

Various Sphingid moth life stages and species – with different cuticle characteristics. Art by Rob Mitchell, used with permission.

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.

Art by Rob Mitchell

Various beetle life stages and species (plus some of their natural enemies) – all with different cuticle characteristics. Art by Rob Mitchell, used with permission.

The insect cuticle: (3) self-assembly
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The insect cuticle: (3) self-assembly

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.

Self-assembly: manufacturing the future

The materials and structures that we humans create for ourselves generally involve large factory manufacturing systems where we “Heat, Beat and Treat” materials into the structure we require.

Motor Manufacturing Gardiner

“Motor Manufacturing” By Clive Gardiner for the Empire Marketing Board. c1930 National Archives UK

In contrast to most engineered materials, biological materials are assembled from the bottom up, rather than from the top down. The formation of materials like chitin, which is the main component of the insect cuticle, is the result of self-assembly and well ordered biosynthesis.

In engineering the material used is relatively cheap but the shape of the structure is expensive, since it results of very energy-intensive processing (the heating, beating and treating parts).

“A Blast Furnace” By Clive Gardiner for the Empire Marketing Board. c1930 National Archives UK

In biology it is actually the other way around. The instructions for manufacturing processes are stored in the genes of all living things. The information contained in an organism’s DNA determines the sequence of molecules that will be expressed by the organism’s cells. It is the characteristics of these molecules that will determine how they interact and ultimately determine the shape of the final material and resulting structure (=self-assembly). In other words, energy is replaced by information, which is cheap. In biology it is the building blocks that make the structure that are relatively expensive to obtain (through capture, consumption, assimilation, etc.) and organisms cannot afford to waste these building blocks.

The biological manufacturing process is constrained by a range of external factors, which then may result in different structures under different circumstances. These external factors are, for instance, temperature (variable on different time-scales), mechanical loading (variable at different life stages and during different seasons) and the presence or absence of water, light and appropriate nutrition (which all three vary over the lifetime of an organism).

“Kreeping It Real” (Graffiti art on boxcar). Picture by Rob Swatski. Flickr.com.

A characteristic of biological materials is that they grow relatively slowly. At first this may seem like a disadvantage, but since these materials as they grow are exposed to a range of forces they can adapt to situations that may alter over the course of the structure’s lifetime. Engineers, using traditional manufacturing techniques, do not have this luxury, they have to preempt and anticipate changing conditions – or ignore them resulting in a reduced effective lifetime.

Self-assembly happens at all scales in nature – it starts at the molecular scale at which molecules are formed into globs which are formed into fibers. Manufacturing at the nano- or micro-scale may still be in its infancy but great strides are being made. And in the meantime we can apply nature’s lessons regarding biosynthesis and self-assembly at a larger scale – creating hybrids of multiple materials with the functional variety that is required of the material.

As Carl Hastrich points out on his Bouncing Ideas blog:

[the] future is fibre. Everything in nature is a fibre. From beetle exoskeletons, to the incredible structures found in the botanical world, they are all fibres. The results are complex, malleable, repairable materials that integrate a vast array of functions. Have a look at a cross section of human muscle and you’ll see what I mean. Humans on the other hand build with large brittle mono-materials and achieve complex function by mechanically integrating multiple components.

Over the past few decades we have tried to incorporate more manufacturing techniques that are non-toxic, water-based, and do not require high-energy inputs – life-friendly chemistry. 3-D printing techniques open up such an avenue since it is by definition builds from the bottom-up, rather than top-down, thus already mimicking nature in that regard, and new non-toxic base materials for these printers are being developed.

The future of manufacturing. (Art based on pictures by M. Alleyne, Yogendra Joshi and MKZero)

The future of manufacturing biological materials. (Art based on pictures by M. Alleyne, Yogendra Joshi and MKZero)

Further Reading

For this post I looked at a lot of graffiti/street art since I wanted my insect images to have an “industrial” feel. I came across some great art. For examples click here and here and (my favorite) here.

Self-assembly lesson for high-school level students.