Bioinspiration – crossing interdisciplinary borders

Bioinspiration – crossing interdisciplinary borders

It is really going to happen! We talked about this for YEARS, and now we are finally going to see it come to fruition.

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Kate Loudon and I have known each other for a long time. It was kind of inevitable that two women who were part of the leadership of the Entomological Society of America’s Section B, now the Physiology Biochemistry and Toxicology (PBT) section, would become friends. We are actually from THAT era where female leadership in the ESA was a rarity (not anymore!).

Since we both have an interest in insect physiology (broadly) and biomechanics (specifically) we started talking about organizing a bioinspiration symposium. Fundamental insect biomechanics studies have inspired technologies for some time now. For about 5 years I have been teaching courses on bioinspiration and I use Kate’s research on bed bug-killing materials as an example of innovations that can be inspired by nature and benefit society. So the match seemed natural. Also, we really like each other and would use any excuse to collaborate on something.

It took a while but we managed to put together an awesome symposium with prestigious speakers on the biggest entomological stage ever; the 2016 XXV International Congress of Entomology to be held in Orlando, FL (Sept 25th-30th).

We are so thrilled about the line-up. There is a great variety of speakers (topics, nationality, ethnicity, gender) and we can’t wait for them to interact with each other and other interested entomologists. Some of our speakers have never been to an entomological meeting. We expect to get them hooked, or at least speak well of us entomologists once they are back at their home institutions.

We hope that as a result of this symposium new collaborations will develop, be it to delve into new research questions or to explore educational avenues.

Let me first introduce you to the speakers. Hopefully as the symposium draws closer I can share a little bit more about the topics and speakers in follow-up posts.

  • Our first speaker will be Dr. Robert Wood who is the Charles River Professor of Engineering and Applied Sciences in the Harvard John A. Paulson School of Engineering and Applied Sciences. Prof. Wood is also a founding core faculty member of the Wyss Institute for Biologically Inspired Engineering, a power-house in the field of bioinspiration.  The Wood lab is probably most famous among entomologists for their work on robobee – a miniature flying, and now also sensing, robot inspired by biology.

  • The next speaker will be Chen Li from Johns Hopkins University. Prof. Li coined his own research topic: terradynamics. Similar to how aero- and hydrodynamic principles have shaped our knowledge about animal locomotion in air and water it is Prof Li’s goal to better understand animal locomotion on complex (always shifting) terrains, thus his creation of the terradymics lab at JHU.  Cockroaches feature prominently in his research.

  • Next we switch from robotics to bioinspired materials. Kate (Dr. Catherine Loudon, University of California at Irvine) will share her work on how small structures on bean leaves kill bed bugs and how these structures (and their special characteristics) have spurred interest in the development of physical insecticidal bioinspired materials.
  • Faithful readers of this blog will know by now how enamored I am by the insect cuticle. I am therefore glad that we will have Dr. Stevin Gehrke (Fred Kurata Memorial Professor of Chemical Engineering at the University of Kansas) talk about the physical properties of beetle elytral cuticle and why this type of biomaterial may have many possible applications.
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Tenebrio molitor with characteristic elytra covering the hind wings. By gbohne from Berlin, Germany

  • Next I will discuss a relatively new project from my lab at the University of Illinois at Urbana-Champaign in collaboration with Dr. Nenad Miljkovic from Illinois’s Mechanical Science and Engineering Department and Dr. Donald Cropek who is a chemist from the U.S. Army Corps of Engineers. Over the past few months we have initiated a comparative study of native Illinois cicadas’s wings to determine physical and chemical attributes that make the wings have (super)hydrophobic, self-cleaning and maybe even antimicrobial characteristics. This collaboration has been really fun and I have learned about a lot of new techniques and I hope to share some of my excitement with the symposium’s audience (Bouncing water droplets anyone?!!!!).
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Neotibicen dorsatus at Loda Prairie, July 2016. By Marianne Alleyne

  • Another interesting biological material is, of course, spider silk. Dr. Crystal Chaw from Dr. Cheryl Hayashi‘s lab at the University of California at Riverside will explain how studies on the evolution of spider silk have helped in the engineering of artificial silk production.

  • From biological materials we next move to different types of flow in insects. First Hodjat Pendar from Prof Jake Socha’s lab (Virginia Tech) will be talking about how the insect’s tracheal system (which is actually linked to other physiological systems) can serve as inspiration for novel flow control. It is a fascinating topic which I have touched upon previously in a blog post.

  • Flow sensing at a small scale is definitely a topic that is of interest to engineers. And it is something that insects do very well. Dr. Jérôme Casas from the University of Tours (France) will present some of the work he has been doing with Dr. Gijs Krijnen (University of Twente, The Netherlands) on the fluid dynamics of olfaction in insects.
  • Another amazing sensor found in insects is the IR sensor in pyrophilous beetles such as in the genus Melanophila. Will we ever be able to engineer an IR sensor as sophisticated as the ones found in beetles? Dr. Helmut Schmitz (Institute of Zoology of the University of Bonn) will share his work to explain how an understanding of the active amplification mechanism seen in the beetle’s IR sensor might help bring us closer to a robust and sensitive bioinspired IR sensor.
irorgan

IR organ of Melanophila acuminata. Schmitz & Bousack (2012) PLoS ONE 7(5): e37627.

  • At this point in the symposium we are shifting gears just a little bit to talk about how to actually DO bioinspired design, and how can we best teach our students to come up with successful bioinspired designs. Most people when they first hear about bioinspiration or biomimicry they immediately think this line of thinking makes total sense. Biologists want to contribute and feel even more justified to delve into fundamental biological questions. Engineers are happy to add bioinspiration into their imaginary toolbox. But for bioinspiration to be successful, to actually have as an end result a bioinspired technology that is based on real biological data, biologists and engineers have to work together. And that is not always so straightforward (Writes the entomologist who has been married to a mechanical engineer for 20+ years. Trust me, it is not straightforward.). Prof. Ashok Goel (Georgia Tech, Co-Director of the  Center for Biologically Inspired Design. will discuss some of the cognitive challenges that he has encountered when working with collaborators and students on biologically inspired design projects. What he has learned about how engineers and biologists approach certain problems is fascinating.
  • The symposium will again switch topics somewhat by next delving into social insect behavior. First up will be Dr. Ted Pavlic (Arizona State University, Associate Director for Research at The Biomimicry Center at ASU) who will talk about how social insects make group decisions and how that knowledge can be transferred to create smart and adaptive teams of robots.
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Eciton hamatum workers on the trail, Jatun Sacha reserve, Napo Ecuador. Alexander Wild.

  • We will end the symposium with another social insect talk, this one by Dr. Deborah Gordon (Stanford University). Prof. Gordon will talk about her research on collective behavior in ants and how they have influenced engineered networks.

Kate and I hope you can join us for our symposium, either in person or virtually via Twitter or Instagram (we will use hashtag #ICE2016BioI and #ICE2016) and follow-up blog posts. Feel free to use Twitter to ask questions of the speakers (@Cotesia1).

“See” you on the 29th!

 

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The insect cuticle: (5) Mild synthesis conditions

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.

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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.
The insect cuticle: (4) hydration

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

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.

The insect cuticle: (1) multi-functionality

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.

Cellular and acellular layers make up the insect cuticle. With the most interior layer being comprised of living epidermal cells that secret the outside layers, which then completely cover the insect – even some of the interior surfaces such as the trachea, foregut and hindgut – are lined with cuticle. (The hierarchical characteristics of these layers will be discussed in the next post.). One of the characteristics that makes the insect cuticle, an “inspirational” biological material is its multi-functionality – something that is rarely seen in engineered materials.

Living materials, including the insect cuticle, often exhibit novel properties that are difficult to incorporate (all at once) into engineered materials. Unique physical and chemical interactions of the biomolecules that make up the cuticle (building blocks) at the nanometer scale convey characteristics such as high strength, energy absorption, and flexibility. Currently multi-functionality in engineered materials is limited to different functions due to a hybrid of several distinct phases the material can attain.

Below I have sketched out some of the functions of the insect cuticle. (If I forgot some important functions please leave a comment below and I will add them to the diagram)

insect cuticle functions

Insect Cuticle Functions – probably not complete – click on diagram to enlarge.

Insect Cuticle Functions

Protective Barrier

The insect’s exoskeleton/cuticle/integument is doing the functions of both our skin and our bones. The cuticle completely covers the insect (~armored skin), while at the same time serving as a supportive skeleton (~bones).

The protective covering creates a barrier. Precious water and ions are prevented from freely moving outward, while pathogens, parasites and dangerous chemicals are prevented from moving inward. This function is not at all trivial for insects. Since insects are relatively small they present a large surface area to the outside environment so that loss of water is a greater problem than it is for larger animals such as mammals.

Structure and Form

The insect’s exoskeleton gives the insect structure and form. And over an individual’s lifetime that form can change. In the case of holometabolous insects, such as flies, wasps, bees, beetles, butterflies and moths, this form change is striking. As an immature caterpillar a moth has a cuticle that stretches and is relatively soft, as a pupa the same individual (using the same building blocks, or biomolecules) has a cuticle that is extremely tough and does not change shape easily. Then as an adult moth the cuticle, including the wings, has yet other features that make the insect successful.

The change in structure and form seen in holometabolous insects, and to some extend the growth strategies employed by ametabolous and hemimetabolous insects, enables the animal to exploit different habitats and diets – even during its lifetime.

This is one of the most striking things about the cuticle. During the separate life stages the cuticle has different functions, it therefore has distinct characteristics and appearance. Yet, the biomolecules that make up the cuticle are pretty much the same, and one individual can synthesize all these seemingly very different types of cuticle.

How different is the cuticle from life stage to life stage? I asked family, friends and colleagues to describe, in non-scientific terms, what the cuticle of each form of the hornworm (=moth) feels like. Here are their responses.

Caterpillar or Larva

CaterpillarDrawing

Caterpillar (Drawing by Marianne Alleyne)

Soft, rubbery, squishy, velvety, muscular, cold. Feels like pleather, like a writhing rubber pickle, like play dough that I rolled in my hands to make a snake.

Pupa

PupaDrawing

Pupa (Drawing by Marianne Alleyne)

Hard, smooth, leathery, acrylic, slick. Feels weird & dead.

Adult

Adult Moth (Drawing by Marianne Alleyne)

Adult Moth (Drawing by Marianne Alleyne)

Fuzzy, hairy, soft. Feels like perfumed talc from my grandmother’s vanity. Holding it will probably feel like a fluttering beakless bird but since I haven’t held a fluttering bird, with or without a beak, I can’t be sure.

Movement

The rigidity that the exoskeleton exhibits makes it possible that insects can make rather precise muscle movements since those are due to the insertion of muscles to the integument wall. It is often the cuticle that has important biomechanical features that enables an insect to run, jump, dig, fly or swim. These precise movements are also essential for respiration (creating flow of air in and out of the tracheal system), food manipulation, excretion and osmoregulation.

Sensing the Environment

Maybe surprisingly a rigid integument is not necessarily limiting awareness of the surroundings. The cuticle has been modified in many insects into structures that can sense most of the modalities that we can sense with our skin. Some examples are the trichoid sensilla, the campaniform sensilla and chordotonal organs. Note that all three of these sensors use the cuticle as an integral part of their structure.

The functionality of the trichoid sensilla - a mechanosensory hair - is dependent on the rigidity of the cuticle.

The functionality of the trichoid sensilla – mechanosensory hairs – is dependent on the rigidity of the cuticle. (Drawing by Marianne Alleyne)

The campaniform sensilla is a mechanoreceptor found in insects. When the exoskeleton bends the resulting strain stimulates the sensilla.

The campaniform sensilla are mechanoreceptors found in insects. When the exoskeleton bends the resulting strain stimulates the sensillum. (Drawing by Marianne Alleyne)

The chordotonal organ is a stretch receptor that senses to what degree the cuticle is being deformed. This deformation can then give information about movement of body parts, gravity (proprioception) and vibrations of the surrounding air (hearing).

The chordotonal organ is a stretch receptor that senses to what degree the cuticle is being deformed. This deformation can then give information about movement of body parts, gravity (proprioception) and vibrations of the surrounding air (hearing).

Energy Storage

For many insects the cuticle also represents a temporary food store. The basic building blocks, to some extend, can be withdrawn during times of nutritive stress. Having to molt to be able to grow in size is one of the drawbacks of having an exoskeleton. Molting consumes time, energy and metabolic resources, and makes the insect vulnerable against pathogens, predators and water loss.  Reabsorbing much of the cuticle during molting minimizes some of these costs. (The closed-loop cycle characteristics of the cuticle will be discussed in part 8 of this series.)

Behavior Modulation

The single layer of epidermal cells that secretes the cuticle also secretes and deposits within or on the cuticle hydrocarbons that are involved in behavioral sequences that are important in recognition and mating. Pheromones and pigments are also deposited. The cuticle may also have modified structures that are important in mating or other behavioral processes – such as bumps,  hairs, nano-scale structures that create structural colors.

By further studying the insect cuticle it is my hope that new materials can be created, using similar “manufacturing” steps as employed by insects, that can provide increased function through integrated or further-integrated systems. Insects do it, we should too.

Next: the structural hierarchy of the insect cuticle.

(My apologies to dumpy grey-brown moths everywhere. That is a terrible drawing of an adult moth. Oh, well)

Insects have advanced degrees in Material Science and Manufacturing

There is much to learn from nature about biological materials – materials produced by a biological system. Just think about the materials in nature that you are most familiar with: wood, wool, cotton, shells, skin….consider:

  • Animals and plants are constructed from materials that are readily at hand – no dangerous mining endeavors in geo-politically sensitive areas. Sea shells are made from materials available in the sea water. Bone is made from components available in our (and our mother’s) diet. Insect cuticle is synthesized from maternal sources and from the individual’s diet.
  • The materials found in nature have many of the same attributes as engineered materials, yet avoid the heat-beat-and-treat manufacturing methods that are so costly to us and our environment. We manufacture materials by applying large amounts of energy so we can first homogenize materials, then add even more energy, and dangerous chemicals, to impose the structure that we want.
  • The periodic table categorizes more than 100 elements. Although around 25 types of elements can be found in biomolecules, only six elements are most common. These are carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur. Few metals are used by nature, although iron, zinc, and manganese are essential in various ways.

PeriodicTable6If you create and then overlay* Ashby plots of biological and engineered materials you will learn that nature has many more tricks up its sleeve than we do. Insect cuticle is very similar to skin and bone, and some of the engineered materials, it is also ubiquitous, yet we know still so very little about it.

Collection of live and preserved insects hanging out in the Alleyne-lab. All insects have a cuticle with the same basic building blocks. But the cuticle of different species, or even different life stages, differ in hardness, flexibility. permeability, color, etc. (Photo: Marianne Alleyne)

At first glance the structure that is a characteristic of insects and other arthropods, the cuticular exoskeleton, seems such a simple yet sophisticated material. Upon closer inspection it is actually maddeningly complex and enigmatic, just ask any researcher working on it (needless to say, there aren’t too many). The chemical analysis of the insect cuticle is a frustrating undertaking.  Perform any manipulation to isolate it and “voila” the structure in which you were interested has been changed. The cuticle has been described by biochemists as “intractable.” It is possible to (partially) dissolve cuticle in various solvents and analyze the results, but the results are quantitative, both because it is not easy to break apart the assembled materials, but also because chemical transformations can occur during the process of dissolution. In the future multidisciplinary (including molecular) approaches may give us more insights into the exact composition of different types of cuticle. Maybe we will soon learn ‘why some bugs go “squish” when you step on them, but others go “crunch” even though they made out of same material’

As an old seasoned insect physiologist I am still amazed by the insect cuticle since it can have varied structure at the meso-level (allowing insects as a group to live out many life histories), yet the basic (nano-, micro-) structure and components of the cuticle are not numerous or terribly exotic. The insect cuticle is all around us, it is what makes the insects so successful.

The cuticle is produced at atmospheric pressure and at temperatures that are not extreme. Simple building blocks, mainly chitin, proteins and water, make up the insect cuticle. It can have many different features and characteristics just because the building blocks occur in different ratios or orientations. Large amounts of the cuticle are recycled by the individual insect itself, and the remainder can easily be broken down by other life forms.

Can we humans manufacture such an amazingly versatile material in a similar manner?

To manufacture anything close to the insect cuticle we need to understand the characteristics of the material. The characteristics of biological materials, that set them apart from most synthetic/engineered materials, were recently defined and listed by Meyers et al. (1). (The closed-loop cycle characteristic of the cuticle is rather unique to arthropod cuticle and should be added to the list when considering the insect cuticle).

  1. Multi-functionality
  2. Hierarchical structure
  3. Self-assembly
  4. Hydration
  5. Mild synthesis conditions
  6. Self-healing capabilities
  7. Closed-loop cycle
  8. Evolutionary and ecological constraints

Over the next few days/posts I will discuss each characteristic as it relates to the insect cuticle. At the conclusion of the series I will discuss how they all combine and result in the interesting mechanical properties of the insect cuticle, and how cuticle-inspired materials might become useful.

*Feel free to contact me for Bioinspiration learning module using Ashby plots.

References:

  1. Meyers, M. A., J. McKittrick & P-Y Chen (2013) Structural Biological Materials: Critical Mechanics-Materials. Science V339, pp 773-779. DOI: 10.1126/science.1220854

Next: the multi-functionality of the insect cuticle.

Going Micro-Tubular

Going Micro-Tubular

The ability to manufacture devices at the very small, or micro, scale brings with it interesting applications possibilities and surprising challenges.  For instance, certain physical phenomena may not be generally encountered with doing macro scale experiments on the lab bench but are dominant when experiments are done with a micro scale “lab-on-a-chip.” The field of microfluidics deals with the behavior and manipulation of fluids, both liquids and gases, that are constraint to a small scale (1 to several hundred micrometer). The physical properties of fluids at this scale are particularly interesting and could possibly be exploited.  For instance, fluids at a certain scale are no longer practically compressible.  Also, it is difficult to maintain pressure or pressure-driven flow within a micro scale fluidic system. One example is the phenomenon of back-flow whereby fluid moves in one direction when pressure is applied only to have it reverse and flow back when the pressure is released.  Due to the physics of scaling laws, these various non-intuitive phenomena are less severe when transporting gasses than liquids in a micro scale device.  For instance, gravity has no influence on gasses and diffusion of gasses at the micro scale is VERY fast.  Therefore, gas species can mix through diffusion with simple laminar flows at low Reynolds numbers.

In nature microscale liquid and gas flow is quite common. Maybe engineers should look at insects to see how fluids can most effectively be moved around at the microscale. The insect’s respiratory system, for instance, uses microscale flow transport to achieve an absolutely essential physiological function.  The respiratory system at first glance seems so simple – just some gas-filled tubes. At the same time it seems so foreign – no breathing through mouth or nose, no lungs or blood or hemoglobin involved (at least not at first glance). The respiratory system is also elegant in its simplicity through ingenious feedback loops – fine-tuning through direct nervous innervation or by sensing chemical compositions of the blood (called hemolymph in insects).

This system that is key to maintaining homeostasis is also as varied as insects are varied. Ingenious adaptations are seen in insects that live in different habitats – within soil, in water, in arid climates, in cold climates, within a host, while pupating, while flying, while walking, etc.

In this post I’ll focus mostly on the “generalized” terrestrial insect.

The insect’s tracheal system

The insect’s respiratory system’s main functions are to carry oxygen and carbon dioxide to and from the tissues. The gas-filled tubes, called tracheae, start at the spiracles, which are paired openings in the cuticle that can occur in almost every or only a few segments (depending on the insect species). The spiracles are innervated by the nervous system and can be open and closed through muscular action.

Spiracles

Spiracles of a lepidopteran larva. Two are visible in this picture; they appear as black openings surrounded by yellow ring. (Picture by Geoff Gallice)

Inside the insect the tracheal systems starts out as two or more longitudinal trunks, which further into the insect branch into smaller and smaller channels. Tracheae range in diameter from a few mm to 1 μm. The smallest branches are called tracheoles, which range from 1 to 0.1 μm in diameter. The tracheoles’ tips are in contact with cells and are often fluid-filled. This means that at the tips of the tracheoles oxygen has to cross first a liquid, then the tracheolar walls, then cross the plasma membrane of the cell and finally move through the cytoplasm of the cells to reach the mitochondria. Carbon dioxide makes the same trek, but in the opposite direction.

Tracheal System

Tracheal System (Drawing by Marianne Alleyne)

TrachealSystem

Cross-section of the tracheal system of “general” insect. Other insects may have more than 2 longitudinal trunks and/or air sacs and/or no spiracle functional in a particular segment. (Drawing by Marianne Alleyne)

Cross section of tracheal tube. A tracheal tube is comprised of an epidermal outer layer which bathes in hymolymph, an inner intima layer, and taenidia which reinforce the intima layer much like a car's radiator hose is reinforced by coil of steel. (Drawing by Marianne Alleyne)

Cross-section of tracheal tube. Shown is an epidermal outer layer bathing in hemolymph, an inner intima layer, and taenidia, which reinforce the intima layer (much like a car’s radiator hose is reinforced by coil of steel). (Drawing by Marianne Alleyne)

In order to get oxygen to move into the cells and carbon dioxide to move out of the body, all while limiting energy use, insects rely on diffusion and convection.

  • Diffusion: passive movement of molecules down their concentration gradient.

Oxygen diffusion rate is more rapid in air than in liquid, and because the tracheal system consists mostly of air-filled tubes the gas-exchange by diffusion is extremely rapid and quite substantial.  While the cells use the oxygen, the partial pressure of oxygen decreases near the tips of the tracheoles, which then draws in more oxygen from the tracheae and the air outside of the insect. Diffusion is of critical importance at the tips of the tracheoles, but it can only work at micrometer distances since at this point the oxygen has to move through a liquid. This is why almost every cell in the insect is at close proximity to a tracheole tip. The cells that require high respiratory rates (flight muscle cells, for instance) may even have multiple tracheole tips invaginating the cell wall.

The prominent Danish physiologist August Krogh devised an early model (1920) of grasshopper respiration that was based on oxygen partial pressures and suggested that simple diffusion could provide adequate gas exchange in insects. This claim had a huge impact on the field early and quickly became dogma, despite some contemporary contrary evidence. In 1964 “a next-generation Dane” Torkel Weis-Fogh showed that in larger or more active insects the distance between the spiracles and the tips of the tracheoles was too great and that diffusion was insufficient if the insect was active. So what other mechanisms do insects use to move oxygen and carbon dioxide through the tracheae?

  • Convection: bulk movement of a gas or a liquid driven by differential pressures.

Compared to diffusion, convection can achieve greater gas exchange rates over longer distances. The trade-off is that it requires more work and thus more energy.

    • One type of convective air-movement seen in insects is, confusingly, termed suction ventilation. This movement of air is primarily achieved by opening and closing, or shielding differentially, a subset or all of spiracles at different parts of the body. This creates a pressure difference between the spiracles and the tracheal ends and thus creates passive airflow.
    • Autoventilation is achieved by using wings or legs to pump air through the body.
    • Abdominal pumping, using the muscles in the abdomen to pump air.
    • Another, more important, type of convective air movement is achieved by the tracheae (tracheal compression), and their associated air sacs (tracheal sections that are not reinforced by taenidia), collapsing and inflating.
Silkworm Bombyx Mori Spiracle and Trachea 100X mag

Digital image of silkworm Bombyx mori trachea with taenidea (100X mag) by Paul Joseph, Birmingham, UK.

With novel imaging and measurement techniques researchers have shown that rhythmic tracheal compression is quite common.  Synchrotron x-ray imaging has been used since 2003 to visualize tracheal structures in living insects. We can now see that muscles compress the exoskeleton of the insect, which results in air moving through the tracheae since the tracheae and air sacs are collapsing.  These studies, which are now linked with real-time flow-through respirometry, show that rapid cycles of collapse and re-inflation in a variety of insects’ tracheal systems – especially in the head and thorax regions – results in large changes in tracheal volume and are major components of ventilation in insects. Even the different ways the tracheal system can compress and re-inflate is probably as varied as insects themselves.

Must watch: Beetle Tracheae Collapsing (video 1, video 2, video 3). Supporting materials from Westneat, et al. (2003) Science V299 (5606), DOI: 10.1126/science.1078008

Using the insect’s tracheal system as inspiration for microscale flow devices

The impetus for this blog post was a recent paper I came across entitled: Selective pumping in a network: insect-style microscale flow transport, by Yasser Aboelkassem and Anne E. Staples.  The topic of microscale flow transport is of interest to engineers because there are many applications that need precise flow control of just microliters of gas or fluid. Microscale flow devices or microfluidic devices are already around us and are assumed to change research tools in the life sciences, aid drug discovery and chemical analyses, and revolutionize health diagnostics. Modern DNA sequencing systems (Fluidigm, Illumina, Roche (454), etc.) rely on microfluidics. “Droplet” analyses also rely on microfluidic technologies. Cheap paper-based microfluidic immunoassay tests will become important for healthcare in the developing world (Diagnostics for All). And those are just a few of the applications.

Insect tracheal systems have been of particular interest to engineers now that we are able to visualize this physiological process (moving fluid and air through tubes) in action.  Insects are able to move fluid and air with precision with minimal energy requirements and little input from outside of the body. All characteristics that would work well within a “lab-on-a-chip”.

Insects use contractions at very specific places along the tracheal system, which probably affords them more precise control than if just relying on the more crude pressure difference approach. Through these small, but targeted, compressions the airflow to specific areas of the body can be achieved. This is apparently not an easy task to achieve in small-scale devices, and one can appreciate that by imagining larger tubes.

even tho a mushroom didn't ask me to

Imagine a garden hose…(Image by zen Sutherland)

Imagine a garden hose. By stepping onto the hose at one point the fluid or air inside the hose will be displaced to either side. As soon as you step off the hose the fluid or air will rush back (=back-flow). How can one create directional flow of a particular speed or pressure? By stepping at multiple spots along the hose, sometimes closing the hose off completely and other times only partially. Stepping off of the hose according to a certain sequence will also result in a directional flow. Of course, this is just one hose. Now imagine doing this with hoses that are branching into thinner hoses, that have multiple openings, with many more feet that can step on and off of them…and then think of a miniaturized version of that and you will see the insect’s tracheal system.

This is what Aboelkassem and Staples tried to model in their paper. The fluid dynamics research group headed by Staples at Virginia Tech’s Department of Engineering Science and Mechanics is apparently closely aligned with the biomechanics research lab of Jake Socha at the same departments. The Socha lab is one of the labs that has used synchrotron x-ray imaging to study insect respiration. The theoretical and computational model proposed by Aboelkassem and Staples incorporates rhythmic wall contractions along a network of tubes (in this case an 8-armed network) without the need for valves. The model’s tubes/channels are squeezed at different places along the tube. The contractions were actuated to move with different time lags from each other. In this way the researchers showed that fluids can be transported and flow velocity, pressure and direction can be controlled.

Simplified insect physiological network modeled by Aboelkassem & Staples. The "trachea" can be collapsed at multiple points within the network (red arrows). The collapse motion protocol can be varied by including time phase lags. The result is precise movement of the liquid or gas.

Simplified insect tracheal network modeled by Aboelkassem & Staples. The “tracheae” can be collapsed at multiple points within the network (red arrows). The collapse motion protocol can be varied by including time phase lags. The result is precise movement of the liquid or gas (green arrows).

We have learned from insects that localized rhythmic tracheal contraction is an effective way to move gas through micro-tubes. Aboelkassem and Staples claim to have been inspired by the insect’s tracheal system to model “selective pumping in a network”. They show, by using theoretical (low Reynolds number) flow analyses and computation fluid dynamics, that the network might also enable fluids to be transported precisely into particular tubes without the use of mechanical valves – at least through an 8-armed tracheal system. It will be interesting to see if this research group will continue to follow the insects’ lead by adding more branches/channels, by adding air sacs (which seem to be very important in insects), by making the model 3D, by adding actively opening and closing ports/spiracles to the outside, and by incorporating the diversity of insect respiratory systems into different models. If done correctly, and by using microfabrication techniques that can make flexible micro- and nano-scale tubes, it is likely insects can make a contribution to the rapidly growing technological field of microfluidics.

Resources and References:

For a more detailed description of the insect’s respiratory system:

For description of how synchrotron x-ray imaging is used to study insect respiration:

  • Westneat, M. W., O. Betz, R. W. Blob, K. Fezzaa, W. J. Cooper and W. -K. Lee. (2003) Tracheal respiration in insects visualized with synchrotron x-ray imaging. Science V299, 5606, pp. 558-560. DOI: 10.1126/science.1078008
  • Westneat, M.W., J.J. Socha and W.-K. Lee. (2008) Advances in biological structure, function and physiology using synchrotron x-ray imaging. Annual Review of Physiology V70: 119-142. DOI: 10.1146/annurev.physiol.70.113006.100434
  • Socha, J.J., W.-K. Lee, J.F. Harrison, J.S. Waters*, Fezzaa, K. and M.W. Westneat. (2008) Correlated patterns of tracheal compression and convective gas exchange in a carabid beetle. Journal of Experimental Biology V211: 3409-3420. DOI: 10.1242/​jeb.019877
  • Socha, J.J., T. Förster and K.J. Greenlee. (2010) Issues of convection in insect respiration: Insights from synchrotron x-ray imaging and beyond. Respiratory Physiology and Neurobiology V173S S65–S73. DOI: 10.1016/j.resp.2010.03.013
  • Waters J.S.,W.-K. Lee, M.W. Westneat and J. J. Socha. (2013) Dynamics of tracheal compression in the horned passalus beetle. American Journal of Physiology:Regulatory, Integrative and Comparative Physiology V304(8) R621-7. DOI: 10.1152/ajpregu.00500.2012

Bioinspired microscale flow structures:

Special thanks to Dr. Paul Kenis for taking the time to discuss microscale flow transport with me. I appreciate his patience with me – clearly a novice. Bedankt!