A Sticky Problem

by James Morris

At Brandeis University, three undergraduate students – Rebecca Korn, Tahlia Quartin, and Saqib Hashim – had a problem. The door to the lab where they worked wouldn’t stay open, and was propped open with an old computer, like this:

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You may not think about this very much, but the problem of how to hold two things together, like the door and floor, or the door and wall, is not a simple one. For clothing, we have buttons, zippers, and ties. But these don’t create a tight seal. Then there is tape and glue, but there are problems here too. Some are strong (like duct tape and crazy glue), but can’t be removed easily, while are others can be removed easily (like sticky notes), but aren’t very strong. This is a classic example of a trade-off.

Of course, there are doorstops. But the students were taking a class taught by Dr. Maria Miara called Bio-Inspired Design. They asked themselves the question: Can we look to nature to solve our problem? Many people have done just that.

Consider Velcro. Velcro was invented by George de Mestral, a Swiss electrical engineer, in the 1940s. He realized that the burs of plants – which seem to stick everywhere – might provide an effective and easy way to hold things together. Taking a closer look, he noticed that their “stickiness” is due to small hooks, which grab on to the loops of fabric, like his pants or the fur of passing animals.

Why not use this same structure to build something that holds firmly, but releases easily? Taking his cue from nature, he constructed a fastener made up of two parts, one with small hooks and the other with small loops.

VelcroDe Mestral patented his invention in 1955, calling it Velcro, a combination of the French words velours (velvet, with small loops) and crochet (hook). Ever since its invention, Velcro has had many practical applications, used as fasteners for clothes, picture hangers, and package closures, to name a few.

Today, using nature to tackle problems goes by the name biologically inspired engineering, also called biologically inspired design or biomimetics. It is a new and exciting area of research, drawing from many fields, including biology, engineering, physics, and materials science. It provides an interdisciplinary way to solve all kinds of practical problems.

Velcro has many uses, but it won’t work for the door. The door is much too heavy and would easily pull apart the small hooks and loops. So the students asked if there other models in nature they could turn to. Instead of loops and hooks, how about a suction cup?

Suction cups are found on the legs of octopuses, squids, and some caterpillars. And suction cups also have lots of practical applications, from Nerf darts to toilet plungers to cups used to attach navigation devices to car windshields. Suction cups work by creating less pressure inside the cup compared to the pressure outside the cup. The higher pressure outside holds the cup firmly in place. But it can be released if the seal is broken and the pressure is equalized on the two sides.

One of the strangest examples of a suction cup in nature comes from a type of fish called a remora. Remoras are known for hitching a ride on sharks, whales, rays, even small boats and occasionally scuba divers. A remora has a modified fin along its back that acts like a suction cup, allowing it to stick to other fish and get a free ride, catch scraps of food, and aerate its gills as the host swims along.

The cup creates a tight seal, but it can also be released quickly and easily. As a result, many researchers are studying it to understand how it functions and mimic it for all kinds of applications, like medical bandages and underwater adhesives. It also seemed like a perfect solution for the door. Rebecca, Tahlia, and Saqib decided to construct a suction-cup device modeled after the remora’s modified dorsal fin, like this:

Photo credit: Rebecca Korn

Photo credit: Rebecca Korn

Unfortunately, the device did not generate enough force to hold the door open.

Perhaps there is something else about the remora’s suction cup that allows it to stick firmly. Inside the cup, the remora has plates of tissue made of many small hairs called spinules, which increase friction and help the remora stick. Drs. Michael Beckert, Brooke Flammang, and Jason Nadler recently studied the geometry of the spinules and measured their contribution to remora adhesion as a way to better understand how remoras stick so well.

It turns out that many animals, including some lizards, frogs, insects, and spiders, use small hairs to stick to smooth surfaces. Take geckos. They seem to defy gravity. They are able to run up walls and even scurry across ceilings. How do they do this? They have toe pads made up of many small hairs called setae. These hairs are thought to adhere to surfaces by weak interactions. Although these interactions are weak individually, the large number of setae (about 6.5 million in total!) results in a strong force overall.

In Mission Impossible: Ghost Protocol, Ethan Hunt (played by Tom Cruise) uses gecko-like gloves to ascend the outside of the world’s tallest building, Burj Khalifa, in Dubai. Sound impossible? It’s not.

In 2014, researchers at Stanford University’s Biomimetic Dexterous Manipulation Laboratory engineered synthetic setae made of silicone that allow humans to scale a glass wall. By examining their adhesive properties, the researchers were able to copy and then modify them to support humans.

Inspired by the remora’s spinules and the gecko’s setae, Rebecca and Tahlia came up with a new model – a plate with many small projections to help it grab on to the floor. They then used 3D printing to build a device to hold the door open. They call it a “Redoora” and it looks like this:

Photo credit: Maria Miara

Photo credit: Maria Miara

Problem solved!

Biologically inspired engineering holds great promise to tackle all kinds of problems, using solutions provided by nature. What we build and what we find in nature may even end up looking the same. But it’s important to remember that they got there along two very different paths. We usually build things from scratch, with a clear goal in mind, like engineering adhesive gloves or a doorstop. What we find in nature, by contrast, evolved over time without a goal or direction. Evolution works essentially by “tinkering,” as the biologist François Jacob famously said, using what it has and sometimes even modifying existing structures for new functions. After 4 billion years of life on this planet, it has stumbled across some beautiful and clever solutions that are worth mimicking.

© James Morris and Science Whys, 2016.

7 thoughts on “A Sticky Problem

  1. Mary Quartin

    I think your students are geniuses! Good for you for sticking at it until you solved the problem!

    Reply
  2. James Morris Post author

    In an article in The New York Times (“That Long Winter’s Nap” Tuesday July 5, 2016), Dr. Heiko T. Jansen, referring to the study of hibernating bears to solve human problems, is quoted as saying, “We have to learn and relearn and relearn that nature has solved these problems, and it’s our job as primates to figure this out.”

    Reply

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