Nature Podcast：铁定甲虫坚不可摧的秘密

Host: Shamini Bundell

The diabolical ironclad beetle is, as the name might suggest, pretty tough. There have been reports of these insects being run over by cars and just shrugging it off and continuing about their beetley-business. For that reason, researchers have been very interested to discover the secrets of the diabolical ironclad beetle’s toughness, as Geoff Marsh has been finding out. First, here’s the Natural History Museum’s beetle curator, Max Barclay, speculating on how this hardy bug got its name.

Interviewee: Max Barclay

So, the diabolical ironclad beetle is quite a large beetle that is usually found in the Southwestern United States, and you get them along the Pacific Sea border of places like California. Now, the diabolical ironclad beetle was named by somebody called John Lawrence LeConte, and it’s obviously only speculation as to why he would have given it the name diabolicus, associated with devils or hell. It could be because it’s a sort of black, carunculated beetle that lives in dark places, but it could equally be because the exoskeleton of this species is so hard. Of course, entomologists preserve specimens by putting them on pins, and getting a pin to go through a specimen of the diabolical ironclad beetle is actually very difficult. I have tried to pin them before and it is extremely challenging to actually get a pin through the specimen.

Interviewer: Geoff Marsh

So, for two centuries or more, the extreme toughness of this beetle has been nothing much more than a nuisance to museum creators like Max, but more recently, these bizarre mechanical properties came to the attention of a materials scientist, David Kisailus at the University of California, Irvine.

Interviewee: David Kisailus

My student actually, Jesus Rivera, who’s the lead author on this paper, had visited an entomology museum at the University of California, Riverside, and he found out about these beetles, and we had heard from folklore, if you will, that they can be run over with a car and they don’t die and, of course, we had to test it.

Beetle versus car, take two. Still alive. Mechanically fine. Playing dead, but mechanically looks good.

Interviewee: David Kisailus

And so, we thought it was dead but then after a couple of seconds, it just started walking around again. So, we thought, wow, this beetle is super tough and we have to explore a little bit more about them.

Interviewer: Geoff Marsh

In a sense, you wanted to know, beyond those fun anecdotes you mentioned about the car and the mounting pins, exactly how tough the beetle really was. How do you do that scientifically?

Interviewee: David Kisailus

So, one of the first tests that we actually performed was taking a mechanical test stage and we basically placed the beetle on this metal plate and crushed it, and then measured the force required to crush the beetle. And of course, we had to have our control, so we looked at other beetles.

Interviewer: Geoff Marsh

What are we talking here? Give us some numbers.

Interviewee: David Kisailus

Well, I mean, in the study that we performed, it was 150 newtons of force required to crush the beetle. Other beetles only were able to get to 50 newtons, so three times stronger. So, for example, folks say that you can take your thumb and your forefinger and you can squeeze as hard as you can and you won’t be able to crush these beetles. The force that they can withstand before failing is 39,000 times their own body weight, which is pretty impressive.

Interviewer: Geoff Marsh

So, the first part was to push this beetle to its limits in your lab, but the second part was to understand what it was about the beetle that gave it this incredible toughness. What were its secrets?

Interviewee: David Kisailus

One of them, the first thing that we found, was that the edges around the elytra, these hardened forewings that we call lateral supports, actually provided, if you will, the first line of resistance to this compression, and that these structures were graded where there was more interlocking between the top half and bottom half of the exoskeleton, closer towards its organs, but that interlocking was reduced as you moved towards the tail end, which enabled the beetle to squeeze under rocks or in between bark.

Interviewer: Geoff Marsh

So, it has a bit of sort of suspension at the back, but at the front end where it needs to protect all its internal organs, it’s really rigidly held together.

Interviewee: David Kisailus

Indeed, that’s a perfect way of saying it. It’s more rigid near the organs and more giving near the tail end. And the other architectural feature that we discovered within this beetle was essentially where the two halves of the elytra are joined, the two halves are actually stitched together using this architecture where you have these bulbous structures that are integrated with the female side.

Interviewer: Geoff Marsh

I would have assumed that was a kind of weak spot.

Interviewee: David Kisailus

Yeah, you would imagine the failure would be at the seam, but indeed, that’s actually what we investigated, the sutural region. In fact, when my student first showed me the micrographs, we found that it looks very much like a jigsaw puzzle. So indeed, the reason it doesn’t fail at this interface is that it has this bulbous, jigsaw puzzle-like architecture that really provides a lot of strength to keep it from pulling apart.

Interviewer: Geoff Marsh

What is it about that that gives such toughness and strength to its forewings?

Interviewee: David Kisailus

Yeah, so we actually used some 3D printing to look at the different geometrical features that an interlocking structure might have, where we varied the angle of this ellipsoid, if you will, and what we found was that there was an ideal angle, that if you had two large an angle, it was too bulbous, let’s say, then upon pulling it, the neck region of that bulbous structure would fail. And in fact, it led to our next discovery, which was quite critical. When my student performed some experiments on the elytra in the synchrotron where we pulled it apart in situ, we actually found that those bulbous structures, those sutures, actually were not solid homogeneous pieces of, let’s say, a protein, but rather they have a microstructure within it that allows them to locally fail or slip. They have some give so that they don’t catastrophically fail but rather they fail locally within the blades or the sutures themselves.

Interviewer: Geoff Marsh

And that wasn’t the end of your experiment, was it, because you then wanted to see if you could be inspired by those architectures that you’d found in the beetle and make your own kind of bioinspired material and test your ability to improve toughness, and it worked, didn’t it?

Interviewee: David Kisailus

Indeed, and we realised there’s some issues in, for example, the aerospace industry where the newer aircraft are now integrating a lot of composite materials, carbon fibre reinforced plastics, into their air frames. So, how do you actually fuse carbon composite to, let’s say, an aluminium structure? And so, people will use rivets and fasteners and other types of structures to bind these two structures together and often, those structures will fail. So, we actually fabricated carbon fibre composites that mimicked the bulbous structure from the suture of the beetle, and joined it to an aluminium piece and compared the strength and toughness of this structure versus a standard aircraft fastener, and found that it had similar strength to an aircraft fastener that they actually had 100% more toughness than the aircraft fasteners, and the implication of that is, of course, it means they’re more reliable, that they would not fail catastrophically but they would fail more gracefully, which means that if an aircraft had some issue, people inspect these parts and you might actually see, oh, there’s damage, we should replace this, rather than catastrophic failure occurring.

Interviewer: Geoff Marsh

So, you translated some of those macro-architectural findings, that jigsaw shape of the blades, into your own engineered structures and that seemed to work pretty well, but what about those micro-structural details you saw within the blades in the beetle, those laminated layers that stopped those interlocking structures snapping at the neck? Can you mimic those micro details?

Interviewee: David Kisailus

In fact, that’s the next part of our study, Geoff, is to understand what are the proteins? Are they hyper-elastic? Do they allow for extreme extensibility? So, it could be a polymer, for example, that’s hyper-elastic, but we need to understand what these components are made of within the beetle. I think that’s the next step, to really understand the material components and determine whether or not the material components are, what role they play and whether or not we can translate that into perhaps new materials. That might be a different avenue that would be beyond these macro- and micro-architectures, for sure.