I have a complicated relationship with waterproof jackets (mine, to be specific). Every time it rains, I put on the same old jacket and tell myself that this time it will keep me dry. And every time, I am wrong. Inevitably, the rain gets in, and I find myself wondering why I continue to own a jacket that reliably fails to fulfill its primary objective.
All of this to say, the other day as I stomped home soaking wet, my jacket got me thinking about something I had never really considered: how do you actually make something waterproof?
It turns out that the answer has less to do with clever fabrics invented by humans than I expected. In fact, some of the best ideas about how to engineer a waterproof material come from plants.
Now, “nature knows best” is not a phrase that usually fills me with confidence. I am very fond of trees, of course, but I am equally fond of evidence. Nature has also given us poisonous mushrooms, malaria and several species of wasps that lay their eggs inside other animals. Evolution is not exactly a design competition with human comfort in mind. But when it comes to managing water, plants really do have some impressive tricks.
The problem with staying dry
We begin with an engineering predicament (try not to let the excitement overwhelm you): how to make a material both waterproof and breathable.
Consider a rain jacket. You want rain to stay on the outside, obviously. But you also want water vapour from your body to escape. Otherwise, you have solved the problem of getting wet from the rain by getting wet from your own sweat instead. Modern materials such as Gore-Tex can do this remarkably well, but they also come with drawbacks, including cost, environmental concerns and limitations in breathability.
So, researchers have started looking at organisms that have been dealing with water for considerably longer than we have: plants and fungi (anything with a cell wall, really).
Plants are really good at water
Plants have evolved elaborate ways of controlling where water goes, how quickly it moves and how much of it they lose. They can move water through their tissues using passive forces such as capillary and water-potential gradients, while their cell walls help them maintain internal pressure without simply bursting.
Some plants take this to an extreme (and are hence termed “extremophiles”). Desert plants must conserve water. Plants living in salty environments must cope with challenging fluid conditions. Resurrection plants can dry out almost completely and then recover when water becomes available again. Meanwhile, I have yet to evolve a satisfactory response to getting caught in the rain.
Scientists call the practice of borrowing useful biological designs biomimicry. And when it comes to waterproofing, plants offer plenty of inspiration.
Take the lotus leaf. Lotus plants can grow in muddy water while their leaves remain remarkably clean. Their surfaces are covered in tiny structures that cause water droplets to bead up and roll away, taking dirt with them. This is known as the “lotus effect”, and it has inspired superhydrophobic materials designed to repel water and clean themselves.
Copy the trick, not the whole plant
Using techniques such as moulding, photolithography and 3D printing, researchers can recreate microscopic grooves, bumps and other structures found in leaves, cactus spines and pitcher plants, giving them a closer look at the clever ways plants manage water.
Cactus spines are a particularly good example. Their conical shape, combined with tiny grooves and barbs along the surface, helps capture droplets from the air and guide them towards the base of the spine. Researchers have recreated these features in synthetic materials, sometimes simplifying the elaborate structure of the spine down to a series of cones, grooves or microscopic bristles. In some cases, these artificial versions have even collected water more efficiently than the cactus spine itself.
Plants can also manipulate water without simply collecting it. The leaves of some species have microscopic structures that create differences in how readily their surfaces interact with water, allowing droplets to move in a preferred direction. The pitcher plant, for example, has a slippery, intricately structured rim that can encourage water to flow in one direction. Researchers have mimicked this kind of directional transport to design surfaces that could move liquids through microfluidic devices, membranes and other technologies.
The most striking thing that I found in my research is that none of these designs relies on a single magical material. The trick is often in the architecture: the shape, texture and arrangement of tiny structures on a surface. A material can be chemically ordinary and still behave in a remarkably sophisticated way if its surface has been designed correctly. But those microscopic features can become less effective when dirt and grime build up on the surface, which is why a good wash and reproofing treatment can sometimes bring a jacket’s waterproofing back to life and give it a little more time before it needs replacing.
So, perhaps I’ll look at my jacket a little differently the next time it rains. I’ll still probably complain when I get wet. But at least now I know where I’d look for a better solution.
@SophieTsengPellar & @CatWang
Sophie Tseng Pellar is a Masters Student in the Surgical and Interventional Sciences program at McGill. Her research interests include exercise physiology, biomechanics and sports nutrition.
Part of the OSS mandate is to foster science communication and critical thinking in our students and the public. We hope you enjoy these pieces from our Student Contributors and welcome any feedback you may have!