I have to admit that even as someone who is fascinated by most insects, the earwig freaks me out. Upon seeing one, I'm typically too busy trying to squash it to notice any details about its anatomy. So it was a bit of a surprise to find out that not only do they have wings, but their wings are world record holders in a specific aspect of insect winginess: they take up the least space when folded compared to their extended size. The ratio between these states can reach as high as 18-to-one.

With that fact in mind, I was less surprised to find out that researchers have decided to study this bit of biology to see if they can derive any insights from what evolution has done with the earwig. In today's issue of Science, there is a report on what has been learned by three researchers: Jakob Faber and André Studart of ETH Zurich and Andres Arrieta of Purdue University. They find that, to mimic the earwig's wing, an origami-style folding approach won't do. Instead, they have designed and 3D-printed a selection of meta-stable designs that, with a small input of energy, rapidly flip between folded and unfolded states.

When many people, including most materials scientists, think of folding, their first thought is origami. But the research team found that the earwig's "exquisite natural folding system" behaves in a way that "cannot be sufficiently described by current origami models." Part of the issue is one of materials science: there are certain folding patterns in the wing that just can't be done by creating a crease in a single material or using the straight lines of origami. In addition, the wing is bi-stable, holding itself in place during flight with minimal input from muscles and folding up entirely without any muscular energy being expended.

The secret to this is partly that biology is not limited to either straight lines or a single material. In fact, the joints on an earwig's wing are rich in a protein called resilin, which forms a flexible polymer that can store and release energy as it is bent and relaxed.

Dr. Jakob Faber, ETH Zürich

Dr. Jakob Faber, ETH Zürich

Dr. Jakob Faber, ETH Zürich

Dr. Jakob Faber, ETH Zürich

To find out more about how the system works, the insect wing was subjected to finite element analysis, which models the behavior of a system's individual components, then builds up more complicated equations by combining these elements. This revealed some basic principles about the wing's behavior. To give one example, if a joint between two segments of wing is evenly spaced across the entire junction, unfolding will result in a simple, linear expansion. If the joint is asymmetrical, then the pieces will also rotate relative to each other as the joint unfolds.

4D? What? Throughout the paper, the researchers refer to their process as "4D printing." This isn't the first time I've heard the term, so there's a chance that people are going to try to popularize it. Let's hope they fail, because it's a very confusing term, and one that is hard to find a good definition for. In physics, the first three dimensions are space, and the fourth is time. 3D printing involves creating an object that extends across all three spatial dimensions. But what could printing time possibly look like? Do you go back and add new parts by reprinting? Nope. Instead, 4D printing refers to printing an object that can adopt more than one shape. The form you print might not be the form that an object is stuck in for the rest of its existence. Since this could be true for anything at all printed with a flexible material, it seems like an excessively broad definition. And because the confusion about printing time means that anyone who comes across this term will have to look it up instead of it being self-evident, it's probably best that everyone forgets it has ever been used.

A simplified version of the results of the finite element analysis were then programmed into a design tool, allowing the researchers to make foldable surfaces that mimic those of the earwig's wings: compact folding and stable folded and unfolded states. The resulting designs could then be sent to a 3D printer that used different materials for the segments and the joints and could create arbitrary curves for any of the joints. In fact, the researchers generally found it easiest to print the folded form of whatever they were making, since the print head had to travel less.

The system worked incredibly well. A simple pattern with four squares demonstrated the metastability of the earwig wing. With a small bit of energy input, it could snap between its open and closed states in as little as 80 milliseconds. But they also printed more complex structures, like a reproduction of the earwig wing. And they also produced some novel designs, like a pair of pincers that could flip between open and closed states, with the closed state being able to grip objects. The entire closure process is controlled by a small collection of panels where the two arms of the pincer meet; changes there simply propagate down the arms.

The authors say that this proves the strength of their design software. "By designing the materials and geometrical parameters of this cell," they write, "the inherent energy landscape of the entire gripper mechanism can be programmed." Clearly, the system is also flexible enough to make everything from a mechanical device to a foldable wing.

While this work is unlikely to change my reaction to coming across an earwig in the real world, it does give me a newfound appreciation for the forces that drove their evolution. It has even helped me appreciate 3D printing a bit more.

Science, 2018. DOI: 10.1126/science.aap7753 (About DOIs).