SciCafe: Technology Inspired by Nature with Tak-Sing Wong
TAK-SING WONG (Mechanical engineer, The Pennsylvania State University): Today I'm going to take you on a journey to a world of nature-inspired technology. I'm a mechanical engineer and a material scientist. I'm interested to look at natural or biological systems, take their best ideas to create new materials, and use these materials for different applications.
Today I'm going to show you two examples–two stories–and hopefully for these two stories you will gain an appreciation of how simple observations in nature–adding a little bit of imagination–can lead to advanced materials technology that can impact our everyday life. So are you guys ready to listen to the story and take a journey with me?
WONG: Great. So let me begin the journey with an ant. What you're looking at here is a carpenter ant. Like many other ants, they’re exceptional climbers. They can climb on any surface that you can imagine. So what contributes to their exceptional climbing abilities? If you take out a very powerful microscope and look at the feet, this is what it will look like. This is the feet of an ant, okay? And you look at the scale bar, 10 micrometers, this is about one-tenth of the diameter of a hair. Really tiny.
So on the feet of an ant, it is equipped with these powerful claws that help them to grab on to rough terrain. And not only that, it just equipped with an adhesive pad that secrets oily fluid that helps them to attach on a smooth surface by capillary forces. So this ant can walk on both rough and smooth surface. It seems that they can conquer any land and mountains. But not to this specific plant.
What you're looking at here is a Nepenthes pitcher plant. Anyone of you has seen a pitcher plant before? Some of you, excellent. The reason why it is called pitcher plant is because this shape looks like a pitcher. And it is also a carnivorous plant, meaning that it eats insects. But unlike Venus flytrap, which has active movement to capture insects–like a mouth that opens and closes to capture the insect–this pitcher plant just sits there, it doesn't move at all. So the question is, how can they capture insects if they don't move at all? The secret lies at the surface of the rim. Their rim can get really slippery when it is wet. Curious about how it works? Let me show you a video.
When the surface of the rim is really dry, it's not slippery at all. As you can see, ants can walk on this surface no problem. However, after a rainy day when the surface becomes fully wet, the surface becomes so slippery that none of the ants can stick on it and just slide off from the slippery slide. Look at those, so sad. So why is that? Why does the surface become so slippery when it is wetted?
Let's go back to look at this pitcher plant, and let's take out our powerful microscope to look at the surface of the rim again. So if we take a microscopic picture at the surface of the rim, this is how it would look like. The rim of the pitcher plant consists of a microscale landscape. And you see the scale bar there is 200 microns, it means that it's about twice the size of a hair’s diameter. They are really tiny. And this microscale landscape actually acts like a sponge, it helps to trap a thin layer of water very stably on the surface. If you still remember how ants attach on surfaces through their oily feet, and if those oily feet are stepping on this water-infused sponge, what happens? We know that water and oil don't mix, right? So they are kind of like sitting or walking on this ice skating ground, right? It's like us, we are doing ice skating but down the slope. The ants just slide down to the plant.
So, inspired by this concept, a few years ago when I was still at Harvard University working with Joanna Eisenberg, we have developed this material cause SLIPS. It stands for Slippery Liquid-Infused Porous Surfaces. This material can be made in three simple steps. First, we start with a porous or textured material, something like a sponge. And then we add lubricant to it. This lubricant can be a water or oil some liquids that have a very strong chemical affinity with the solid matrix. And afterward, that is your SLIPS. It can repel anything that comes into contact.
So this is great. So now we have a synthetic material that might act like a pitcher print surface, but we still need to prove it. In order to prove it, we really do put an ant with a piece of jam on its leg on this SLIPS. So what you're going to see in this video–I'm going to flip this bottle over and see what happened. This ant in the jam just cannot grab on anything, just slides right down. Okay, so you might ask, this ant might start as immobile, right? That's why it falls down, why it's not moving. So in order to answer this question, we have done another experiment showing that this ant is completely healthy.
What you're looking at in this video, at the top half–the white collar part–is a Teflon coating. And we all know that Teflon is super non-sticky because you put it on your cooking pan. At the bottom panel is a SLIPS coating. So let's see what this ant, how this ant will react. So this ant is warming up, and then walk in this Teflon surface, so far it’s doing pretty good until when it starts to move on this SLIPS coating, it just gives up and walks back out. There's nothing he can do. Too bad. So we have shown that we can make a synthetic material that can do what a pitcher plant can do. So this is great, but we can engineer this surface such that it can do what a pitcher plant cannot do. Indeed, SLIPS has a number of very exceptional property that I'm going to show you next.
First of all, SLIPS is super non-sticky, and one of the best comparison to compare the non-stickiness of SLIPS is by using Teflon. In this video, what you're going to see is, I'm going to put [a piece of] scotch tape on [one piece of Teflon and one piece of SLIPS] and then peel it off. The one who doesn't get attached to the scotch tape, it's the one who's the winner. So let's take a look at that. So we have two scotch tape on the SLIPS, on the Teflon: one, two, three, boom. So SLIPS is really non-sticky. Even Teflon can get picked up on a scotch tape, but not SLIPS. So this is great. Also, we can engineer SLIPS so that it can repel any kind of liquids. In this case, I use crude oil as an example. Most of you have played with cooking oil before if you cook. Crude oil is something that is even stickier than cooking oil, okay? In this example, you see that the crude oil doesn't stick on SLIPS, but it’s staying on everything else including the Teflon and aluminum. And you can engineer this surface such that it can repel a broad range of fluid. So this is great. So far we have shown that SLIPS is a super non-sticky liquid repellent, and SLIPS is also self-cleaning.
What you're looking at in this example, if we contaminate this surface with some kind of dust–in this case, carbon dust–just by putting a drop of liquid on top of the surface, sliding this material back and forth, all the dust gets magically picked up by water droplet, and the surface is rendered clean again. So SLIPS is liquid-repellent, non-sticky, and self-cleaning. So this would be a slick solution to a broad range of sticky problems that we see in our daily life. Let me give you a few examples.
In the biomedical space, you might know that blood coagulation and biofouling of medical devices and implants lead to significant mortality worldwide. So this is a bad problem. In order to solve this problem, we can engineer SLIPS such that it is blood-repellent, as shown in this video. As sticky as blood is, it doesn't stick on SLIPS. But it sticks on everywhere else, including Teflon and glass material. Just a few years ago, my colleagues from Harvard University have shown that by putting this slippery coating inside medical devices such as a catheter, it can effectively prevent blood coagulation for up to eight hours or beyond. So this is very important.
So far I've shown you that SLIPS can repel blood. How about bacteria? I think many of us don't like bacteria, right? Because they cause a lot of problems. Indeed, bacteria cause over 700,000 hospital-related infections, which kills about 75,000 people just in the United States per year. So this is bad, and we can engineer SLIPS such that it is bacteria-repellent. In this video, what you're seeing–again, on the left-hand side we used Teflon as a control, and on the right-hand side is SLIPS. We put two drops of bacteria on them and let them sit in there for 24 hours. So after 24 hours, a bacterial biofilm will be formed on the Teflon surface. As you can see, that drop is just stuck there. But on SLIPS, the drop has maintained its mobility, indicating that bacteria doesn't really accumulate on the surface.
If this doesn't convince you, we have further tried to put SLIPS and the Teflon coating inside a catheter and then flow bacterial solution inside this catheter over extended period of time. After 24 hours, this is what you're going to see. On the left-hand side is a Teflon control surface. So we take our optical microscopy image to look at the interface between the fluid and the solid. Each individual green dot you see here are individual bacteria. So you see that there are so many bacterias colonized on the surface just right after 24 hours [on the Teflon]. But SLIPS is still fairly clean. Ok, let's do something more extreme. Let's keep doing this experiment for up to 7 days. This is what happened. On the control surface, you can't even see individual bacteria because what happened is this individual bacteria connects together to form a three-dimensional community, what we call a biofilm. So they’re clusters of bacteria, you can't even count them individually. But on SLIPS, is still very, very clean, it is antifouling. So we have shown that in the laboratory SLIPS can actively reduce bacteria accumulation up to 96-99 percent. And we have tried many kinds of bacteria, including bacteria that cause food poisoning, pneumonia, and sepsis, and SLIPS just works.
Okay. So far we have shown that SLIPS can be used as a non-fouling coding for medical devices. Indeed, SLIPS can also be used as a sensor. So imagine in this situation you have a very large water droplet, and you just want to detect a few molecules inside this droplet. It is a very challenging task because it is analogous to finding a needle inside a haystack, it's very difficult to do. One way we can detect the molecules is by concentrating all the molecules into one single spot, hopefully, that increases the probability of finding [the molecules]. But that is not as easy as it sounds, because for those of you who like to drink tea or coffee, you're probably aware of this coffee ring effect. If you put a drop of coffee with coffee particles inside this droplet and let it evaporate, what happened is that this coffee particle, instead of concentrating in one spot, it just spread everywhere. So that's the coffee ring effect that you see. However, if you put the same coffee droplet on a SLIPS surface, this is what happens. As the droplet evaporates, all the molecules and particles are going to be concentrated into one single spot. And you can see those light-emitting molecules individually until at the end when the droplet is fully evaporated away, you can finally find a cluster of those particles or molecules.
So by combining the slippery surface and advanced optical detection method, my research group has recently successfully detected chemical and biological molecules down to a single-molecule level. Imagine using this technique to detect the disease-associated molecules inside a drop of blood or saliva, for early cancer detection. Isn't it wonderful?
So, so far I've shown you that this slippery coating or slippery surface can be used for non-fouling coating for medical device and sensor. This SLIPS can also be able to solve our water scarcity problem. You may or may not know that four billion people are under severe water scarcity at least one month in a year. This is a big problem. And this is going to be a bigger problem because as the time goes, we have more people. Indeed, it is projected that by 2050 we'll have close to 10 billion people on earth. So water is going to be a big problem. But how can we solve this problem? One solution we have is to create a low-cost, decentralized water supply. And one technology to enable that would be to collect water directly from the air. You may have a question: do we even have sufficient water in the air for us to drink? The answer is yes. We have so much water that it is sufficient to support every single one on the planet for ten years if we can capture all the water in the air. And this source is renewable because water evaporates and gets circulated across the globe.
But the problem is, how can we effectively collect this water onto a surface and transport it away? So just recently my research group has developed this new material called slippery rough surface that can effectively collect water from the air and transport them away. As the name implies–slippery rough surface–it is not only slippery but also has a very high surface area such that it can collect water very effectively. In this video, you're seeing that I'm putting slippery rough surface and SLIPS side-by-side. You can see that slippery rough surface actually can collect way much more water than the control SLIPS surface. In the laboratory, we have quantified that slippery rough surface can collect around ten times more water than a typical fog-harvesting material, and we are still working on this. So this is great, collecting water from the air could be one potential solution to end this water scarcity problem. But there are also other solutions.
But before I show you the next solution, let me ask you this question. This is a pitcher plant. Does the shape of this pitcher plant remind you of something that you use every day, and that something consumes a lot of water? What is that, any guesses? Toilet, excellent. Fantastic. They look alike, this pitcher plant and the toilet, they look quite similar, right? So now we think that some of the most water efficient toilets we use in the United States takes about 1.6 gallons per flush. Every one of us flushes a few times a day, and now times this number to the global population. We flush a lot of water down the drain every day. And how much water we flush, it's over 141 billion liters of water. That’s a lot of zeros, but how much water is it? It's equivalent to the water consumption for over 300 million people per day. That's the population of the United States. So we flush a lot of water down the drain.
So to solve this problem, my research group has recently developed this new slippery surface coating called Liquid Entrenched Smooth Surface, LESS for short. This coating you can apply on any smooth surfaces, including toilet surfaces, and the most important thing is that it’s super non-sticky. Even the stickiest solid that you find–I mean, you guys know what I'm talking about. And I'm going to show you this video next. Okay, on the right-hand side is a typical toilet. That's actually the best toilet you can get commercially. And on the left-hand side is the toilet with our coating. So this white substance here, it's not what you think, this is a synthetic poop that we make in our lab. We actually got this recipe from our collaborator from Cranfield University who’s making their own waterless toilet. Okay, so you see in this video that this substance is really sticky. It sticks on the regular toilet, but on the slippery toilet, it just slides like the ant sliding off from the pitcher plant.
So in the laboratory, we have shown that by putting our coating, you can actually reduce the water consumption for the cleaning by up to 90 percent. Isn't it great? So not only can you reduce your water footprint, the most important thing–this material is super non-sticky, non-fouling–you can actually reduce the number of times you need to clean your toilet. Who doesn't want that, right?
So with that, let's move on to the part two of the story, which is related to filtration. Are you guys ready? Okay, let's go.
I'm sure most of you probably drink tea or coffee, and for those of you who do, I'm sure that you know how a coffee filter or tea infuser works. It works by having this membrane with some holes in it which lets small particles go through, and blocking large particles, right? This filtration mechanism has been used for hundreds of years, with some of the earliest examples such as sieving. But have you ever imagined that we can create a material that can do a reverse filtration? Meaning that you let large particles go through, but blocking small objects. That sounds really counterintuitive, right? You don't see that in our daily life because they don't exist. However, in nature, in the microscopic world, some microorganism actually use this filtering mechanism to consume food, which I'm going to show you in the next video.
In this beautiful video, what you're going to see is two microorganisms. What's happening is that the large organism is going to eat the small organism, okay, and the way that it does it is by opening up a cell membrane and conformably wrapping around this smaller organism. Look at that, this small micro-organism tries to escape. It couldn't, right? And at the end of this process, the cell membrane of this large microorganism just closes itself up and self-heals. Look at the end of this process, it just self-heals. Throughout the whole process, no small particle or small molecule inside the cell is leaking out to the external. So this cell membrane kind of shows you that large particle can enter without leaking out small molecules or particles. And the key to this process is that the material itself has to be self-healing, and has to have the ability to conformally wrap around the object.
So, inspired by this concept, my research group has recently developed a self-healing liquid membrane that can do just that. This is a video demonstration. What you're looking at here are two particles: one small particle, and one large particle. They release at the same height and look at when the small particle interacts with the membrane–it just stops there. And when a large particle passes through, look at the self-healing of the membrane. So this membrane allows you to capture a small particle but lets a larger object go through. So now we have the materials that do exactly what our cell membrane does. But that's not enough, because as engineers we want to know the working principle of how this works so that we can engineer a better membrane. So to do that one of my group members–Dr. Birgitt Boschitsch, who is also in the audience there–so, you study the governing mechanism of this reverse filtration, so what you discover is as follows.
In order for the particle to pass through this membrane, it has to carry sufficient kinetic energy so that it can break through the surface tension of the membrane. And for smaller particles, if it doesn't have sufficient kinetic energy, it doesn't break through the membrane, okay, since kinetic energy is related to the size of the particle as well as the traveling speed of the particle. So larger or faster particles would go through the membrane, but smaller or slower-moving particles wouldn't.
This is analogous to a liquid trampoline. If you have a person which is bigger jumping on this trampoline, he can break through this trampoline and the trampoline self-heals. But if you put a baby or a small kid jumping on a trampoline, he or she can never break through the trampoline. So that's the mechanism. And to quantify this mechanism, Birgitt has defined this “E*” parameter, which is simply the ratio between the surface energy of the film and the kinetic energy of the particle. So with this E* parameter, so if [the particle] starts larger than 1–meaning that the kinetic energy of the particle is smaller than the surface energy of the film–the particle will be retained. On the other hand, when E* is smaller than 1–meaning that the kinetic energy of the particle is now larger than the surface energy of the film–the particle will go through. Knowing this quantitative relationship is important because now we can engineer the membrane for specific size cell activity.
So, in our world a lot of the harmful objects are typically on a smaller size if you think about virus, bacteria, dust, allergen, or disease-carrying insects, they're typically a smaller size. And with this membrane, now we can engineer it so that we will block out all these harmful objects, and letting larger and more useful object to pass through. In this case, we use a bee as an agricultural example. To show that this membrane can actually retain live insects, we have done this experiment here. What you're seeing here are many flies inside this jar, and on top of it is a membrane. So look at this particular fruit fly in the center of the bottle. It's flying towards the membrane, and then it got captured. The reason is that this fruit fly doesn't have sufficient kinetic energy to break through the surface tension of the membrane, and that's why it is retained.
So this reverse filtration mechanism is very powerful because it allows applications that were previously unattainable using any solid-based membrane. One example for that is for surgery. Imagine putting this membrane on top of an open wound, so doctors can push the surgical tube through the membrane while blocking dust or bacteria, as indicated by the pink color here as an illustration. So you see the surgery can go through the membrane while blocking all the small particles. And also, this membrane is transparent, it doesn't block the visibility. And also, you can freely move this tube through the membrane, and you just saw an incision procedure just carried out. So this is very important because it allows surgery to be carried out in places where high-quality surgery facilities are not available. So you can create a clean surgery environment anywhere you go.
The next application is related to the waterless toilet. I know right, we do toilet application a lot, yeah. But imagine putting this membrane on a waterless toilet, where it lets the solid waste to pass through and blocking the unpleasant odor. That is important because we know that waterless toilets or latrines, they don't smell good. In this example, you see there's some white foggy gas coming out on one end–this is just for flow visualization–and we use Tic Tacs to simulator solid waste this time. Okay, so this Tic Tac can pass through the membrane very nicely, and you see there's no gas coming out because the membrane is blocking the gas particles. So this membrane can be used as an auto-barrier for waterless toilet or latrine, which is a big problem because there are still 1.1 billion people practicing open defecation, and one of the reason is because these toilets do not smell good.
So with that, that concludes the second part of the story. I hope these two stories show you the power of nature inspiration. With more than eight million species on the planet, there are plenty of novel ideas or technologies that can be derived from these species. All you need to do is to look at nature carefully, study them, and exercise your creativity. You might be the next one who can come up with a novel nature-inspired technology that can help to make the world a better place. So with that, I will end here, and thank you very much for listening.
What does a carnivorous plant have in common with the design for a water-saving toilet? What about a hungry cell with surgical equipment? Tak-Sing Wong, a professor of engineering at Pennsylvania State University, introduces two cutting-edge technologies that have been directly modeled after natural phenomena.
This SciCafe took place at the Museum on November 7, 2018.
The SciCafe series is proudly sponsored by Judy and Josh Weston.