Sticky and slippery science for biomedical applications

[MUSIC: Drum solo used with permission of Jed Irvine, senior faculty research assistant of computer science at Oregon State University, and Engineering Out Loud listener]

KEITH HAUTALA: How does an insect climb straight up walls or windows without losing its grip? And how does a snake slither across sharp rocks, without slicing itself open? Can engineers use this knowledge to improve biomedical technology? Stay tuned for some sticky and slippery science.

[MUSIC: The Ether Bunny by Eyes Closed Audio used with permission of a Creative Commons Attribution License] From the College of Engineering at Oregon State University, this is Engineering Out Loud.

HAUTALA: I'm Keith Hautala. This season we are focusing on research that keeps us healthy and safe. I'm going to take you on a journey that will follow the tiny, wet footprints of a ladybug, and we’ll go inside the mouth of a frog. But it all starts with the secret language of lobsters.

JOE BAIO: So, actually, my very first project in this field was when I had just finished undergrad and I started working in a lab doing biomechanics work. We were looking at fast animal movements,

[SFX: Spiny Lobster Sound, California Academy of Science, used with permission under Creative Commons Attribution-NonCommercial]

Lobsters actually make noise, you know? They talk to each other; they make these little scratching noises.

HAUTALA: That’s Joe Baio. He’s an assistant professor of bioengineering here at Oregon State.

BAIO: And if you look at that mechanism, it’s a stick-and-slip. So they have this little, like, violin bow on their antennae. And it sticks, and then it slips, and that's what makes a little noise, like a scratching noise.

[SFX: Spiny Lobster Sound, California Academy of Science, used with permission under Creative Commons Attribution-NonCommercial]

And so we’re looking at that stick-and-slip mechanism. And then that kinda got me in. Like, “Oh, that's what's going on with a slug or snail!” It sticks and then slips, sticks and slips, and that’s how it kind of slides.

HAUTALA: If you’ve ever sat and watched a snail or a slug moving, it is kind of fascinating. That trail of slime it leaves behind actually plays a key role in how it gets around.

BAIO: So there's something about the chemistry there that's interesting, right? It for some reason can, under certain environmental conditions, get sticky and provide a lot of friction for the animal movement. And then, all of a sudden, it turns off and allows the animal to move freely. So it's slippery. So that's kind of how I got into the adhesion business, or the interest in that. And then you start looking at all kinds of animals, right? I mean they can stick to all kinds of things.

HAUTALA: If you were going to try to come up with the First Law of Sticky Science, it might be that for every thing that wants to stick  to something, there’s something else that doesn’t want things sticking to it.

BAIO: And then on the opposite end there’s all kinds of plants. So, plants don't want insects walking on the leaves, so they come up with crazy waxy surfaces that prevent insects from walking on them, prevent insects from eating them and destroying the plant. So there's just tons of different systems out there that rely on these dynamics between sticky and slippery, and we're just starting to look at them at the molecular level.

HAUTALA: These molecular mechanisms evolved over very long periods of time, basically through trial and error.

BAIO: Yeah. So you know, if you look at the natural world, it's all these “extreme properties” materials — whether it's skin or a plant leaf — that really just kind of adapted to these really extreme temperatures, or to deter predators, or to be able to stick to something, or to be able to prevent their skin from falling off when they slide on a rocky surface. Evolution has gone through hundreds of millions of years to find solutions to these problems, and we as engineers are just starting to look at them, and look in the natural world for solutions to some of our problems.

HAUTALA: There are all sorts of applications in medicine, particularly, for getting things to stick where you want them to. And to not stick where you don’t want them to.

BAIO: Trying to come up with coatings that don't wear down, like a coating that you could put on an artificial hip so that it doesn't wear down, or that prevents bacteria from adhering to it, or a new type of adhesive that can basically turn on and off … Can we get away from stitches, right? Or staples, and glue tissue together, right? Or glue dental implants in. And maybe, do it in a way that's biocompatible, right? It's — we're using kind of biological molecules — so maybe it's less likely to kind of cause a problem in your body when you put it in. And also, looking at these adhesives from nature, it’s that they turn on and off.

HAUTALA: That’s kind of a key selling feature of these natural adhesives: The ability to turn the stickiness on and off. Imagine a bandage that would stick to your elbow all day without falling off. But then, it peels off easily, without ripping off two layers of skin!

BAIO: So let's say a fly or some spiders even — or ladybugs is what we actually study in our lab. And they have these little hairy feet, and they look similar to a gecko foot, where they're full of hairs. And, unlike the gecko, these insects also have a little fluid. So these hairy feet bend and they make, like, a really nice contact to whatever substrate they're trying to adhere to. And then this fluid also gets, you know, basically emitted from their feet and kind of also helps make contact. And kind of the theory was that this fluid can adapt, right?

HAUTALA: So this mysterious ladybug foot juice changes properties, depending on what kind of surface the ladybug is walking on. If that sounds like there’s some complicated chemistry going on, well … there is.

BAIO: So, it's super complicated. It's like an emulsion of proteins. All kinds of biomolecules. So: lipids, fatty molecules, proteins, sugars, water, other acids. And we were thinking maybe, like, different parts of these things are important depending on what substrate they're walking on, right? Evolution has over-designed it. And so we were looking at: OK, let's let an animal walk on all kinds of different substrates and look how this fluid actually adapts, and adapts at the chemical level.

HAUTALA: When it comes to sticking and slipping, all of the important action takes place where the rubber hits the road, so to speak. At the surface.

BAIO: We are kind of surface scientists in my group. And so we are really interested in what's going on at the outer few molecules of an interface, right? Whether it's the surface of a silicon wafer on a computer chip or the surface of a ladybug’s foot.

HAUTALA: To figure out what happens on the surface of a ladybug’s foot, Joe and his colleagues created a whole bunch of designer surfaces with different properties for ladybugs to walk on.

BAIO: And when the ladybug walks on it, it leaves actually a little wet footprint. And then we immediately take that footprint and do our chemical analyses on it. And we kind of look through what are the chemical components that actually interact with that substrate. And then we maybe look at how that changes as it walks on different substrates. And so then we can look at the physics in the mechanism of how different parts of this fluid come down and bind to the substrate versus they bind to the foot edge of the ladybug. And then we can take all of what we know about these components, maybe put something that's similar into a beaker and then make our own kind of sticky ladybug fluid. And, and we've been mildly successful. We're at the stage now where we know what's going on, kind of like the different chemistries, and what's actually interacting, and we're still in the process of making a mimic and then applying that mimic to, to biomedical applications.

[SOUND EFFECT: Ambience, Florida Frogs Gathering, A, used with permission of Creative Commons Public Domain]

HAUTALA: Another sticky mystery concerns the tongues of frogs. When a frog sees something it wants to eat — say a tasty cricket — it shoots out its tongue. The tongue sticks to the cricket, and the frog pulls it back in.

BAIO: So something about the frog tongue chemistry makes it sticky, but it can't be sticky when it's in their mouth, right? They'll just stick to the roof of their mouth. So when a frog hits a prey item, the frog extends its tongue, it has a bunch of this fluid on its edge of its tongue, it hits a fly or something it wants to eat, sticks to it, and retracts it back in. So there's some sort of chemical change going on where, all of a sudden, how does this become sticky as it hits the fly surface? And so, we're interested in looking at that mechanism too. And so, my graduate student made a bunch of substrates with different chemistries on it and allowed the frog, to hit it. We basically, actually … These little substrates were clear and when you stick a fly behind it and trick the, the frog into hitting, whacking, our sample with its tongue.

HAUTALA: So, what they get is a frog tongue-print on a glass slide.

BAIO: And what's left there is the spit of a frog. And if you look at the chemical structure there, you notice that it's different when it's on his tongue versus at the surface. So when it’s at the surface as it sticks, there's a bunch of fluid between the tongue and the substrate as the tongue is retracted. It again applies this kind of force, this kind of sheer force. And then that causes the basic molecular structure to change. It almost causes these little molecules to form fibrils, almost like a rope. And it gets really, really strong and sticky, and that allows the frog to retract its prey. And so again, this idea of on and off, right? Why, by applying a certain force you can turn the stickiness on or off or adjust it in different ways. And what's really interesting me is just this crazy idea that force, like something physical, can change the chemical structure of these kind of fluids.

HAUTALA: In the frog’s saliva is a kind of mucus, which contains these big molecules called mucins. These are basically just proteins with lots of sugars on them. What Joe and his collaborators found out is that in their “off state,” these mucins are just a disordered mess, pointing in all directions. But when they stick to a surface and get stretched, they interact with each other. They start to stack up, and form these fibrils that are very strong and very hard. And nobody really knew how this worked before. It took some really smart bioengineers to figure it out.

BAIO: And the way we kind of did this, this, we do a lot of surface analytical work. So again, going back to my students, we had this frog print, right? So then we have this goo on a slide or on a substrate and then we take it to the synchrotron, and so this is where, you know, a synchrotron, we're, we're using tunable X-rays. So they spin electrons around very fast, and create tunable X-rays that we use. So we hit this surface, we have our surface with our spit on it, we hit it with X-rays and what comes off are other light photons, so pieces of light, and then we measure the kinetic energy of this light that comes out and that can tell us what's at the surface, what molecules are there. And the cool thing is it's also sensitive to order. So the way we set up the experiment is we changed the way the light interacts, with polarization, the way the light hits the surface at certain angles, and we look for changes in the, in the response and that can tell us: Oh! All of these molecules are well-ordered! Or all of them are disordered. And so that's how we could tell that this, this fluid turns on after it hit the surface, right? Hits the surface, it undergoes an ordering process, all these molecules order a very distinct way. And we were able to see that with our spectroscopy.

HAUTALA: So the frog tongue mucus is basically a pressure-sensitive adhesive that turns on when force is applied to it.

BAIO: The interesting thing about it, though, is just how fast that process is. It's super fast, right? It becomes sticky almost immediately, right? It has to, right? Otherwise the prey item can just disappear or fly away or escape. And so that's … I think the next step is understanding the kinetics. So how fast is this kind of chemical re-formation process, right? How it can go from disorder to order that quickly is super impressive.

HAUTALA: This kind of pressure-sensitive adhesive could find some novel, if not downright revolutionary, applications in biomedical fields.

BAIO: Yeah, I mean, so yeah. Again, gluing,really crazy tissues together, right? During surgery, so let's say you cut open a heart and you need a glue that works really fast, right? To prevent the patient bleeding out. Or something that you could, you just want a temporary adhesion to something, right? You know, you want to be able to hold pieces of tissue together and then be able to reopen them, you know, maybe in later surgery. These sorts of things. Or just a way, you know, you don't have to put a lot of foreign things in your body. These are proteins. These are, fat molecules, maybe some water and a little bit of fatty acids, and things like this. And so the body might not see i t the same way as it see like a huge piece of metal staple, right?

HAUTALA: So, ladybug feet and frog tongues are examples of two completely different mechanisms on the sticky side of things. On the other end of the spectrum, we get into the slippery stuff. Things like snakes. What sort of useful tricks can they teach us?

[MUSIC: “Curb Stomp,” by Underbelly. Used with permission of YouTube Audio Library.]

BAIO: One example I give is the ball of an artificial hip, right? It's rubbing against the other part, the cup of your hip, or artificial cup, and over time it wears down, right? And so then you have to replace that hip. And so could you come up with a way to make it slippery, so you prevent this abrasive kind of process. And, I was talking to a colleague, and he's like, “Oh, you know, snake skin has to deal with this all the time.” So we studied the king snake from California. And they're on all kinds of crazy sharp rocks, and while they do shed their skin, it's pretty anti-abrasive. There are scales, and so they can slide over and over on something that's really sharp, or really rough, and their skin maintains its integrity. It's also self-cleaning, which is a whole ’nother thing is, is just fouling, so it prevents stuff from adhering to it. If you can create something that's self-cleaning or non-fouling you’re going to make like a billion dollars, you know?

Catheters that you stick into a body: If you make them non-fouling, bacteria can't stick to them. Or a stent. When you stick in a stent in your bloodstream, preventing blood clots and things like that. So this idea of having something really anti-abrasive and non-fouling or self-cleaning is super impressive, this material.

HAUTALA: Snakes are a lot more slippery on their bellies than on their backs. Joe and his colleagues figured there was some kind of specially adapted biomolecular mechanism at work.

BAIO: Again, we took the samples and kind of chemically imaged them with our surface analytical tools and found that on the belly of the snake, where it's slipperier, there are these basically fat molecules, kind of like these really long-chain carbon molecules. And they form a really beautiful monolayer at the snakeskin surface. And if you look at the top of the snake where there's higher friction forces, they have these same molecules, but the way they're interacting with the surface, the way they're pointing, the way they're ordered is way different.

So on the bottom, it's well ordered, well stacked together, like all lined up. And on the top they’re kind of disordered again. And so this well packed layer, of oil basically, allows these snakes to slide on a substrate or a surface without a lot of abrasion. And then this layer also provides, I think, some non-fouling capabilities. And so we've started to identify what molecules are actually at these surfaces, and then trying to come up with a way to easily coat, like, a biological implant or an artificial hip, or these sorts of things. Is there an easy way to create a coating that exactly mimics the chemistry of it, like a king snake skin surface.

HAUTALA: Joe says he gets a lot of inspiration from nature, whether he’s reading a biological science paper about it or just living in it, taking hikes with his young son or exploring his own backyard. And it’s something he says all kinds of engineers can benefit from.

[MUSIC: “Quiet Nights,” by Nate Blaze. Used with permission of YouTube Audio Library.]

BAIO: There's all this research that just being out in nature, you know, really calms nerves and inspires you and, and I think just being able to look at biology, I think more engineers need to look at biology instead of just looking at what other have done, right? So when most engineers take a product — whether you're designing a new engine for a plane or a, a new type of diabetic glucose sensor — and look at what's previously been done and kind of make iterations on that. But I think this idea of kind of going back and looking at what sort of problems nature has already solved, and how did they do it, and can you kind of take any inspiration from that — is key. I think by studying nature we can make some, or probably lead to some really cool discoveries that can have a huge impact in medical field, or in all kinds of different fields.

HAUTALA: This episode was produced by me, Keith Hautala, with additional audio help from Molly Aton and Rachel Robertson. Our intro music is “The Ether Bunny” by Eyes Closed Audio on Soundcloud, used with permission under a Creative Commons Attribution license. Other music and sound effects were also used with appropriate licenses. You can find the details in our show notes, which are online, along with other episodes, at engineeringoutloud.oregonstate.edu. Subscribe on our website, or by searching for “Engineering Out Loud” on your favorite podcast app.