Special report: Students of the insects - Lessons from nature

Materials World magazine
,
5 Feb 2013
The locust’s wings, legs, back and joints are all made from the same material. Insect cuticle’s remarkable properties are being closely studied by engineers.

Some fly across oceans with tiny wings. Others walk up walls. Their
exoskeletons can be as strong as antler while remaining flexible. There
is much to learn from insectkind. 

The pygmy mole cricket uses the water as a trampoline. A species of flat bug has infrared sensors that detect forest fires, and when the mosquito bites, its serrated proboscis nimbly avoids nerves, leaving many of its victims oblivious.

Insects are ingenious. Humans imitate them, but never with the same efficiency. This should not come as a surprise. They have existed for more than 400 million years. They outlasted dinosaurs and will outlast us. The best we can do is replicate their methods to make our stay on their patch more comfortable.


Six legs stick. Six legs detach.
The pygmy mole cricket lives next to waterways in South Africa and in the south of the USA. While it can jump far and high, it often misjudges its leaps and ends up in the water. To protect against snapping fish jaws, it can also jump from the water’s surface using spring-loaded paddles and spurs on its hind legs. At the University of Cambridge, UK, researchers observing this Inspector Gadget-type phenomenon, believe this system could be used on the propellers of sub-aquatic robotic vehicles.

The soles of some insect’s feet have also held scientists in thrall, especially those working with adhesion. Dr Jan-Henning Dirks, of Trinity College Dublin, in Ireland, explains, ‘Insects can stick to almost all types of substrate, whether it’s glass, plastic or wood. Not only can they stick to something very firmly, they can detach. Humans can use super glue or other adhesives to attach things to other parts, but how do you detach these parts? That’s a complicated mechanism, but insects do it with their feet many times a second.’

The soles rely on one of two systems to adhere. The first is a pattern of fine hairs that optimises contact, even with the roughest of surfaces. The second system incorporates smooth, pillow-like pads. ‘The material is so soft that it makes very close contact with the substrate,’ he says. ‘Although the systems look different, they both secrete an adhesive fluid. That’s one of the secrets of how they attach to surfaces.’

While at the University of Cambridge, Dirks used these insights to give insects the slip. Together with two colleagues, he invented the InsectiSlide polymer, which reduces the adhesive properties of insect soles. ‘It’s a plastic with special properties, like a sponge. Insect feet secrete an adhesive emulsion. We know that for many insects this emulsion is made up of water and oil. It has a very high viscosity, like ketchup. The adhesive emulsion changes its viscosity when a force is applied. [So] we came up with a surface coating called the InsectiSlide. When the insect puts its foot onto the surface, the polymer selectively removes most of the water from the adhesive emulsion. What’s left is the oil. As you can imagine, standing in oil is not so easy. That’s when the insect slips down.’ The coating can be applied to air conditioners and in wooden house bases, where termites gnaw.

In recent times, Dirks’ research interests have shifted from the sole to the body, specifically the insect’s exoskeleton. ‘There hasn’t been much work done on insect cuticles, even though it’s the second most common natural material in the world. What we know so far is that it’s a very versatile material. If you think about it – the whole insect is made of one type of material. Imagine trying to build an aeroplane or car from a single type of material. Insects actually do that – their joints, legs, protective back plates and wings are all made from cuticle. This versatility makes the insect cuticle special. Some body parts can be very stiff and very tough. At the same time, it can be flexible in the joints, [so it] moves easily and expands.’

That said, many of the material’s secrets remain elusive. Researchers know it has a plywood structure, that chitin filaments are embedded in its protein matrix, and that water content plays an important role, but a better understanding of the principles that underlie its properties is needed before artificial cuticle can be built.

‘We identified body parts, such as the legs and the wings, which have fascinating properties,’ he says. ‘We can go into more detail [with them now and ask] – is it the layering of the cuticle that makes it so tough or the combination of water inside the polymers and proteins? We also want to look at how the cuticle behaves in terms of fatigue. [Desert] locusts can fly across the Atlantic Ocean with their wings flapping hundreds of thousands of times. The wing cannot self heal, so once there is a crack or a notch in it, it will ultimately fail. [So], how do these wings withstand crack propagation?’

‘We found that the veins in locust wings act like watertight compartments in a ship,’ he adds. ‘The vein-like pattern captures cracks in the membrane before they reach a critical length.’ This principle could be used to build more durable, lightweight micro-air vehicles.

Nanoscale analysis and fabrication will accelerate development. He believes that recreating materials on a small scale will allow scientists to apply different principles that weren’t previously possible. ‘If you build something bigger for a human-sized robot, for example, you can’t necessarily use the same physical principles that insects use. They are small and the robot is big, but the technology will allow us to build things on the same scale as insects.’

However, material development is only relevant if the correct insect is chosen to fulfil a specific function. He explains, ‘Dragonflies fly fast and are very manoeuvrable. So, if you are interested in building a faster, more manoeuvrable flying robot, then you might look at a dragonfly. If you are interested in building a small flying robot that can travel long distances, then you might look at the wings of the locust, and if you are studying adhesion you would look at insects that are good at sticking to surfaces, such as the weaver ant. For most of the questions the engineer asks, the entomologist will find an insect that helps answer the question.’

Fiery love affairs
Some flat bugs of the Aradus genus follow forest fires. Their wood-boring larvae can only mature in burnt trees, so if they can’t find fires, they die. Thankfully, nature has equipped them well. The bodies of the fire-loving creatures incorporate sensors for smoke and infrared radiation. The smell receptors are located on the antennae and the infrared (IR) receptors are on the thorax or abdomen. When the information from both sensing systems converges, the insect knows exactly where to go to find old flames. Inspired by this, a University of Bonn team, in Germany, has used the insect as the model for uncooled IR receptors.

Dr Barbara Webb is also drawn to insect sensory systems. ‘If we can understand these smaller brains first,’ she says, ‘it can give us some useful insights into more complicated brains’. As Director of the Insect Robotics Group at the University of Edinburgh, UK, she oversees projects that investigate how insect brains work and how they can be implemented into robotics.

On reflection, insects and robots make good bedfellows. As Dr Webb explains, insect brain circuits can be simple enough to replicate, and they are robust enough to survive in inhospitable places. This makes them suitable in robots that are built for messy tasks, such as in nuclear reactors and oil spill clean up.

‘One [example] we’re looking at is ants, because they have pretty spectacular navigation capabilities that seem quite complex,’ she says. ‘[These capabilities] relate well to current robotics, where a lot of work is about navigation, effectively autonomous cars. The methods people use for that [autonomous cars] are very computationally intensive. Insects can do many of the same things with massively less computation.’

The Insect Robotics Group is interested in how different sensory systems interact within the brain. To understand these interactions better, the researchers observed fruit flies flying in a one metre arena, where they were exposed to odours and visual cues. They also studied how crickets walk to sound sources and the way ants follow visual routes.

However, the process from behavioural observation to a completed robot is exhaustive. ‘We do observational experiments on the animal itself to quantify more precisely what it does. Then we need to think about the sensory systems. Can you buy sensors that give you the same information the animal gets? Sometimes you can get better information than the animals, and [other] times you can’t get anything that is as good. For instance, there are no chemical sensors available that are comparable to insect chemical sensors.

We may need to design a system to make it more like the animal’s. Then we need to think about what the computation is – what happens to that sensory input and how it gets transformed into the behaviour of the animal. We look at what information is available – at the neurophysiology and the known processing. However, we almost never know the whole brain circuit, so what we implement is not a copy but a hypothesis.’

After several rounds of testing and much circuit tweaking, the team chooses a robot base that suits the insect’s behaviour. ‘Then we put the circuits on the robot, see what it does and compare the behaviour of the robot to the animal.’

Light on the wing
The mosquito proboscis is a nifty appendage. It slips through flesh, touching few nerve endings on its way. The insect fills its belly with blood and whirrs away, leaving its victim oblivious. According to researchers at the Kansai University, in Japan, the proboscis’ jagged outline is responsible for its subtle mastery. The serrated shape reduces the resistant force when it pushes forwards and the anchoring effect when it is pulled out. Inspired by this, they have developed a hypodermic needle from a crystal silicon material that is said to touch fewer nerve endings, making injections less painful.

As with many biomimetic projects, the researchers focused on a narrow aspect of the creature’s behaviour and physical design. At first, Shu Yang’s focus was equally narrow. She was smitten by the brilliant blue of the Morpho butterfly’s wings and wondered how they came to be that way. On closer investigation, the University of Pennsylvania researcher learned that the wing contains remarkable properties. ‘The colours are bright, shiny and angle independent,’ she says, noting that this is due to a combination of structural colour and pigmentation.

She says the structural colour, which is also present in some beetle scales and opals, is caused by the interference, diffraction, or scattering of light by periodic arrays of transparent materials. This lends it a colour that is more intense than those found in pigments.

The Morpho wing surface is also built to slip the attention of hanging droplets. Yang explains, ‘On some butterfly scales, there are periodically undulated, micron and submicron-sized ridges consisting of a multilayered lamellar structure [made] from chitin and air. On some other butterfly scales, there are spongy periodic holes. Between the ridges or holes, air is trapped. This lowers the surface energy of the wing and repels water.’

To recreate the properties of the butterfly wing, the team fabricated a periodically porous 3D structure with a rough texture. To do this, they applied holographic lithography, where the laser creates a crosslinked 3D network to a material known as photoresist. ‘The 3D periodic structure is responsible for the displayed colour,’ she says. ‘The microporosity – together with the nanoroughness and hydrophobic coating on the 3D lattice – contributes to the overall super-hydrophobicity.’

The hydrophobic surface could be useful in anti-icing applications, shower doors and in packaging applications (to avoid leaving residual waste in containers). For the structural colour, Yang and her colleagues have more audacious plans. ‘We are working on dynamically tuning colours for display, specifically from coloured films to transparent windows (like chameleon camouflage), which could be integrated with CMOS technology and imagers used as optical sensors in energy efficient buildings.’

There are hundreds of research groups around the world borrowing engineering and design methods from our insect brothers. Yang’s team, for example, is also studying the compound eyes of flies, the dry adhesion of ant feet and the water harvesting ability of Namib beetles in arid environments.

So, what if these projects grow wings? Will our reactions improve fivefold with our artificial fly eyes? Will we crawl up walls with sticky hands and harvest rainwater on our backs?

The answer is yes. We just need to wait for nanotechnology to get a move on.  

 

Insect inspiration


Waterboatman submarines
It seems odd for an aquatic creature to have hydrophobic properties, but when the water boatman’s hind wings stop swinging, their superhydrophobicity floats the creature to the surface. The collaborative Chinese team that conducted the research believes this feature can be used in submarines.  

Desert locust compass
The locust can navigate over long distances using the polarisation pattern of the sky. Having studied the creature’s dorsal rim, a group at the University of Bielefeld, in Germany, has produced a global compass that can orientate with limited spectral information.  

Earthworm robots
The Insect’s entomological cousin, the earthworm, squeezes and stretches its muscles to move. MIT researchers have now imitated this movement in its Meshworm robot using artificial muscle made from nickel and titanium. The robot is designed to negotiate tough terrain and squeeze into hard-to-reach places.