The summer movie season is already in full swing, with “Avengers: Endgame” already earning more than $2.7 billion worldwide and many others hoping to cash in on global superhero excitement.
“Spider-Man: Far from Home” is the latest cinematic telling of the story of Peter Parker, the friendly neighborhood superhero who can climb up walls and is incredibly strong and agile. Parker is also a scientifically-savvy hero, making his own costumes and gear, who continues to inspire audiences 57 years after he was originally brought to life by Stan Lee and Steve Ditko.
And while radioactive spiders might not be lurking in labs giving unsuspecting students the ability to climb walls, Spider-Man’s superpowers and his synthetic webbing might not be completely out of reach. Penn Today talked with materials scientist and engineer Shu Yang to learn more about the real-world versions of these “super” materials, and how engineers in her field are inspired by biology to create manmade materials with unique functions.
When engineers see the incredible materials that come from biology, things like strong-yet-flexible muscle tendons, wall-climbing geckos, or spider silk, how do they approach the process of creating manmade materials with similar properties?
Biologists and engineers definitely have to work together. Many years ago, we made iridescent opal-like colors and said, ‘We’re mimicking the biology; they have these interesting colors and we’re mimicking butterfly wings,’ but without actually knowing how bioorganisms work and why they do so.
Then we started to work with Dan Janzen and Alison Sweeney, and now we’re trying to understand convergence of biology and asking deeper questions: If you see the color, where’s the color coming from? Is it because of the morphology, or because of the chemistry? Within the same family of butterflies, why do they have different colors? Why do some plant leaves or seeds and butterfly wings have similar colors? Are they due to the same mechanism?
If we can understand why they behave this way, we can design a structure or design a chemistry to have similar functionality without taking hundreds of millions of years to make them, or taking laborious steps to make the same very sophisticated structures as biology does.
What makes spider silk such a “super” material?
There are different kinds of alignment of the silk protein strands, which is very critical. It’s similar to your tendons, which all have directionality. You can bend an arm in one direction but not in another. Spider silk has multileveled structures, or hierarchy; they are not made of a single type of proteins. They are made of different proteins, which have different morphologies and orientations. And there are seven different types of glands that spiders produce to spin their silks.
Furthermore, spiders are knitting these silks into orb webs of different kinds of geometry, and that’s also enhancing the strength. If you have spider web in the wind, it can move around, but it doesn’t actually break part. Having this kind of geometry is really important. Some researchers even argue that wind induces variations in spiderweb geometry.
This is something we are trying to do right now with origami/kirigami structures: You’re not changing the material’s intrinsic property, just using cutting and folding as a tool, which brings an extra, previously unattainable level of design, dynamic, and deployability that make an initially rigid, unstretchable panel stretchable and foldable at any scale.
Spider-man’s synthetic web fluid is described as “a shear-thinning liquid” that “on contact with air, the long-chain polymer knits and forms an extremely tough, flexible fiber.” Is this a material that sounds realistic?
Absolutely. Shear thinning means it is originally a stiff or highly viscous material, but if you shear it, it becomes less viscous and can be easily aligned in the shear direction. That’s why you create a certain kind of alignment of molecules.
For example, ketchup is a shear thinning material. It’s difficult to get out of the bottle, so you shake it to shear the molecules in the ketchup, which decreases the viscosity. The biopolymer chains will stretch so they become disentangled and slip away from each other, and that’s why they can come out of the bottle.
In 3D printing, shear thinning is very important because when you print, you want the material to go through as a liquid, otherwise it will clog the nozzle. But once it comes out you want it to be solidified, immediately, otherwise the material collapses, and you won’t be able to have a good print.
One of Spider-Man’s powers is the ability to climb walls and buildings. There are a few animals in addition to spiders, such as geckos, who can also do this. How does it work?
In the field of robotics, the gecko excited people because their ability to climb walls is not based on the capillary or on the vacuum. With frogs, they have liquid coming out, so it’s kind of sticky and messy, plus you would have to carry the liquid for the robot. Likewise, if it’s based on a vacuum, you need to carry the vacuum pump, so it’s not energy efficient.
I showed this movie in my class: This guy was using vacuum pads to climb, and even though it’s just a five-level building, it takes him several hours. At the end he saw rain and he started to worry, because if you have water your vacuum doesn’t work anymore.
That’s why people are interested in the gecko because it has none of this. It’s based on the Van der Waals interaction through microscopic hairs, called setae, on their toe pads and at the end of each seta there are about 1,000 nano hairs, called spatulae. They have many, many rows of setae. When setae/spatulae are straight, there’s very little contact of them with a flat surface, so it’s very easy to come off. However, when the setae bends, the contact area increases significantly, considering there are millions of spatulae, so you have better adhesion. The setae can open and close this way, so that’s how they change the adhesion.
We can fabricate those structures that can mimic the structural adhesion, but the problem is that the gecko is only 50 grams and they have millions of these setae on their palm. It’s actually over-engineered. A human is at least 50 kilograms, so 1,000 times bigger.
For comparison, super glue adhesion is about 1,000 newtons per square centimeter strong, duct tape is 50, and the gecko setae is only about 10. So researchers face this dilemma: If you want super strong, like superglue, it’s not reversible, and if you want it reversible, you can’t go super strong.
How close are we to developing a material that will let people climb up walls without relying on a vacuum or wet, sticky adhesive that’s not reversible?
We just got a paper accepted about this adhesive. It’s super strong and reversible, and my student used 2-centimeter-square strips to hold himself up. It’s made from a hydrogel that has been used in contact lenses. As you know with a contact lens, if you put it into the water it’s soft, bendable, but if you forget to put it in water, it becomes dry and brittle or glassy. Our material is rubbery in the water and glassy when dry.
When its rubbery, you put it on a substrate, and no matter what kind of substrate it is, the material squeezes into it, and during the drying they become rigid. The beauty is that while drying they don’t shrink that much. Why is this important? On the surface there are many grooves, so the material squeezes into these grooves and makes perfect contact. However during drying, if they shrink, they delaminate, and pop out, thus, losing the contact with the substrate.
Our hydrogel material doesn’t shrink much, and it dries very fast, so it keeps the deformed shape. And within minutes the material actually changes the elastic modulus by 1,000 times, so all of a sudden it goes from squishy, squeezing into the cavity, to very rigid, like plexiglass. Importantly, it remains in the cavity, which means it’s difficult to pull them apart from the substrate. But if you add water, they swell and become rubbery and slippery, so you can separate them.
Peter Parker is known for being a superhero, but he’s also a student with a strong interest in science and engineering. Do you think Parker has characteristics that make him a good scientist as well as being a masked crusader?
The most important thing about being a scientist or an innovator is you need to have curiosity. Peter Parker was curious. He saw this spider, he was wondering what was going on, then he was bitten by it.
It’s also not afraid of taking a risk. It’s always OK to make a mistake because if you make the mistake, you learn, and you can keep trying new things.
In the meantime, I would like my students to do their homework first and anticipate the risk. I told them that before you do the experiment, give me a plan. Think about what is a potential outcome, what should I pay attention to, so your research will be more productive. If something comes out unplanned, it will be easier for you to trace back and find out why. Then it will lead to a new round of discovery and understanding.
So, if you see something different, don’t just ignore it; it could be something more interesting or a breakthrough. A lot of science could go into a different direction, so being open minded and being curious is really critical.
Source: University of Pennsylvania