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Masters of Physics in the Living World
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Throughout the living world, cells, plants and animals display proficiency in dynamics — the physics of movement. Pulling off these successful moves often involves precise coordination of different physical laws.

Damselfly Elasticity

We all know about dragonflies (Günter Bechly’s specialty), those brilliant hovering insects that delight children and adults. Damselflies are similar, except slightly smaller and more slender. We could spend time talking about these amazing insects’ use of optical flow navigation, predictive hunting techniques, or other capabilities that inspire human engineers. Today, let’s learn about another trait requiring mastery of physics: the flexibility of the joints in their wings. A new paper in  PLOS ONE revealed an amazing protein that makes the wings springy and flexible. In “Morphological and mechanical properties of flexible resilin joints on damselfly wings,” they introduce the protein resilin:

The main focus of this study is on the flexible element that has been found on the wings of damselflies, called resilin. Resilin is the rubber-like protein found in specialized regions of the cuticle of most insects that gives low stiffness, high strain and efficient energy storage that functions in insect flight. Weis-Fogh first described resilin from the flight systems of locusts and dragonflies, it was described to be similar to swollen isotropic rubber, but its elastic behaviour is unlike any other natural or synthetic polymer. Additionally, resilin was shown to have remarkable mechanical properties where it is two decades [i.e., orders of magnitude] higher than for elastin, suggesting that resilin is a more mobile bio-polymer. 

An insect wing needs to achieve the optimum tradeoff of elasticity and durability. It cannot tear too easily in the wind, but it cannot be too stiff, either. The resilin protein has the ideal properties for flexibility and energy storage. It can deform and then pop back into position with a spring-like response. “Resilin functions as an elastic spring that demonstrates extraordinary extensibility and elasticity,” the four authors say. The springiness comes from the precise placement of the correct amino acids in the sequence.

Using atomic force microscopy and other techniques, the researchers measured the physical properties of resilin that give it such ideal properties, allowing wings to flex and get the most lift for amount of energy consumed. There’s more than just the availability of resilin to the insect. The researchers also found that the distribution of resilin varies throughout the wing, putting the flexibility where it is most needed. That’s a separate design principle from the material alone:

The structural analysis revealed that the flexibility of the wings varied from one area to another, and the resilin distribution pattern was the mechanism that controlled the characteristics of the wing…. Additionally, the AFM images revealed resilin nanostructures of varied sizes and enabled the calculation of elasticity values at each section of the wing; membrane, mobile and immobile joints in Rhinocypha spp…. While studies on silks and elastin received a lot of attention in the past decade, this has now change [sic] to focus on recombinant resilin; structure-mechanical properties of the resilin with potentially greater application in a variety of fields.

As is common in papers dealing with natural designs, biomimetics has overtaken evolution as the focus of interest. The authors are excited to think about how their discovery could help engineers use this “remarkable” springy protein for “greater application in a variety of fields.” Flubber, perhaps? Nature had it first!

High Jump Champs

Who could forget the remarkable gears of the planthopper we wrote about five years ago that store elastic energy for its rapid jumps? Well, Current Biology published an interesting “Quick Guide” to “Insect Jumping Springs” that shows there’s more than one way to leap. Although Sutton and Burrows don’t mention the gears, they do share some “Wow!” facts about planthoppers:

A planthopper can accelerate in less than 1 millisecond to a take-off velocity of 5 [meters per second], requiring a power output (energy per given time) of tens of thousands of Watts per kilogram of muscle.

How can they get such superpowers, when muscle can only generate 300 Watts per kilogram? There’s another physical limitation, too: the faster a muscle contracts, the less power it can generate, “exacerbating the problem.” Furthermore, for a jumping insect, the power can only operate when it’s in contact with the ground. A poor planthopper seems to have three strikes against it.

How do these insects do it? They jump by using springs; devices that allow energy to be stored gradually in mechanical deformations and then released abruptly.

In insects that use springs to jump, the legs are first moved into the same cocked position and the joints locked. The power-producing muscles then contract slowly over periods of 100 milliseconds to a few seconds without moving the legs; instead, the force generated distorts parts of the skeleton, which store mechanical energy. The sudden release of these loaded skeletal springs then powers the rapid propulsive movements of the legs. The elastic recoil of the spring returns the stored energy very quickly. The power is amplified because almost all the energy produced by the slow contraction of muscle is returned to the leg in a much shorter time, delivering the thousands of W kg–1 of mechanical power required for jumping.

The authors compare this to stretching an archery bow and releasing the stored energy rapidly. “Compare how far you can throw an arrow with how far you can fire it with a bow,” they comment. Known insects that use springs for jumping include fleas, grasshoppers, froghoppers, planthoppers, flea beetles, and “even in a cockroach,” but many more are likely to be discovered. If you see an insect achieving more than 300 Watts per kilogram of muscle, it’s probably using a spring mechanism. And now, once again, we find that protein resilin we just learned about:

These springs are a composite of hard, highly-sclerotized cuticle and the highly-elastic protein resilin. A hard material such as sclerotized cuticle can store considerable energy even if deformed by only a small amount, but is susceptible to fracture. Resilin, on the other hand, is much softer, stores much less energy when deformed a similar amount, but is resilient and able to strain large amounts if necessary. It returns quickly and reliably to its original shape upon repeated deformations.

What a remarkable thing: the same protein is used for completely different functions in the damselfly wing and in the froghopper leg. Notice, too, that resilin has to cooperate with the structures around it. By itself, it could neither fly nor jump. As with the previous article, the authors end with biomimetics — a design focus. Imitating the jumps of insects will require detailed research, showing that design is not a science stopper, but an inspiration for scientific understanding and application:

The shape and material composition of these biological springs are very different from man-made springs. If we wish to apply biological lessons to modern spring design we need to address three outstanding questions. First, how does the geometry of the biological springs affect their ability to bend, store and release energy? How are the different springs adapted to meet the specific needs of different insects? Finally, what contributions do the hard and soft component materials make to the properties of the springs that enable such reliable storage and release of energy?

That’s another example of what we said at the beginning: achieving these feats of dynamics “involves precise coordination of different physical laws.” In this case materials science, energetics, elasticity, acceleration, and more were involved. Even then, nothing would work without the brain programming to use it.

Plant Gyroscopes

Plants seem so passive, just blowing in the wind, with nowhere to go. We know, however, that certain plants can move rapidly by storing turgor pressure, such as the sensitive plant and the Venus flytrap. There’s one area in botany that really puts the dynamite in dynamics: seed dispersal. A fascinating new study from Pomona College and the Rancho Santa Ana Botanic Garden, reported by  Phys.org,  reveals a highly unusual example: a plant that has mastered the Frisbee toss! And it’s a most unlikely contender: the wild petunia Ruellia ciliatiflora — “not actually very closely related to petunias, though it does produce pretty flowers.” (Hey, it’s time to plant your garden annuals for spring color.) So how does this humble little flower disperse its seeds over large distances? Get ready for some more “Wow!” expressions, because we have another world record to share:

What is most striking about the plant is the way it disperses its seeds — by flinging them great distances when its fruit is exposed to water. But until now, little research has been done to find out how the flower flings them so far. To learn more, the researchers brought some of the plants into their lab and filmed seed dispersal using a high-speed camera.

The researchers discovered that there are multiple factors at play. One is the glue-like material that holds the seeds in place, another is the disc shape of the seeds. Little hooks behind the seeds also assist in launching them. Perhaps most importantly, spin develops due to the way the seed is flung. In slowing down the action, the researchers observed that the seeds spin up to 1,660 times per second, making them the fastest spinners known in nature.

Faster than a spinning Frisbee, this plant accelerates its seeds’ angular momentum to this incredible speed, faster than any animal can (including a human Frisbee champ), giving the seed more lift and farther distance. How does it do it? The authors did not say. They did find that the rapid spinning has a purpose: a surprising gyroscopic effect:

The videos offered evidence of the assist the seeds get from spinning — some of those ejected did not spin, and only traveled half as far as those that did. And those that did were able to travel as far as seven meters and were launched at speeds up to 22 miles per hour. Oddly, they spin vertically and counterclockwise, like a Frisbee on its side. The spin, the researchers found, resulted in a gyroscopic effect, keeping the seeds stabilized while the backspin produced less drag, keeping the seeds aloft longer and thus allowing them to fly farther.

That’s amazing. Who would have thought a plant could launch seeds at 22 mph with a spin of 1,660 rotations per second? Those seeds must have seemed like a blurry bullet passing by before the high-speed camera revealed the secret. Once again, we see that this plant had to utilize multiple laws of physics to succeed: properties of materials, gyroscopic effects, aerodynamics and hydrodynamics (since water is involved). How many mutations did that take?

These three examples show that there’s a world of design out there needing to be revealed. If you earn a degree in biomimetics or physics, never lose your inspiration at the ingenuity of living designs. They are the real PhDs. Scientists are their students.