This is one in a series presenting news on technology and innovation, made possible with generous support from the Lemelson Foundation.
Makers like to combine unusual items to create new things. To do so, they need super-strong adhesives — tapes and glues — to hold it all together. But sometimes they want to be able to take the items apart again. That’s been a problem, because reversible adhesives usually are not very strong. Sticky stuff can be super-strong and permanent — like superglue. Or it can be less sticky but easily removed — think of a sticky note. Now, though, researchers have created an adhesive that’s both reusable and super-strong.
Shu Yang works at the University of Pennsylvania in Philadelphia. Her team described their new superglue July 9 in the Proceedings of the National Academy of Sciences.
As a materials scientist, Yang uses physics, chemistry and engineering to create new types of items. In her work, Yang often finds inspiration for new materials based on structures that exist in nature.
For years, she has been working to create an adhesive that not only holds well but also can be undone and reused. Earlier work mimicked the tiny hairs on gecko feet. Although the material came unstuck easily, it didn’t have a strong grip. So the scientists in Yang’s lab kept searching for something better.
One day, a student in her lab was playing with a substance known as a hydrogel. A polymer, it’s made up of repeating chains of smaller chemicals. This particular gel turns soft when wet — in fact, it’s what makes contact lenses so flexible. Yang’s lab had been using it to make various structures for about 10 years. The student, Gaoxiang Wu, made patterns with it on a glass slide and then left it there.
When Wu returned, the hydrogel had hardened and was seriously stuck. He pulled, pried and scraped, but nothing separated the gel from the glass slide. Then he added water — and it came right off.
That finding made Yang and her team curious. Why had this dried gel been so hard to remove? They also wondered whether anything in nature might also work that way. And before long they discovered that snails make a similarly sticky goo.
During the heat of the day, snails are at risk of drying out. To prevent this, a snail finds a good spot near the ground with plenty of moisture. There, it pumps lots of mucus through the opening on its shell.
The mucus oozes over the ground, filling in any gaps. As it dries, the mucus hardens. This creates a structure that is both protective and adhesive. Called an epiphragm (EP-ih-fram), it seals the moist snail inside its shell, protecting it from predators that would readily munch on it if they could get to the meat inside. When temperatures fall in the evening and humidity rises, the mucus loosens. Now free to move, the snail continues on its way.
A sticky situation
Yang and her team reached out to Anand Jagota. He is a bioengineer at Lehigh University in Bethlehem, Penn. Jagota specializes in soft materials and their adhesive properties. Together, the researchers studied the hydrogel and found that it worked the same way as a snail’s slime. When the gel was wet, it oozed into every little nook and cranny, just like snail mucus. When it dried, the material turned hard and glassy. Now it was almost impossible to pull off.
The key, Yang says, is that the gel is very soft when wet. It’s a lot like those squishy splat balls that stick to the wall, she says. Almost every surface has tiny imperfections, she observes. So with just a small bit of pressure, the gel can squeeze into all of those the tiny pores and spaces on the surface it’s sticking to.
For most adhesives, that surface roughness is a problem. It reduces the amount of contact between the adhesive and surface, she notes. That makes the adhesive less sticky. But the hydrogel fills in those itty-bitty gaps, forming a tight connection.
What’s more, the gel doesn’t shrink much as it dries. So it doesn’t pull away from those rough patches. Instead, it holds its shape as it turns glassy. In fact, that’s key to why it sticks so well — it’s locked on to the tiny cavities. “We couldn’t separate the gel from the substrate,” Yang says. No matter what tool they tried.
Strong adhesives are important for making things — building cars or airplanes, for example. So the team wanted to figure out just how strong their new adhesive was.
To find out, they made the gel on one side of a sheet of silicon. They cut the gel-coated silicon into small chips no bigger than a postage stamp. Then they superglued a piece onto each side of a strip of heavy-duty metal. They glued other squares of bare silicon to one end of each of two Kevlar ribbons. (Kevlar is a tough material used to make bulletproof vests. It wouldn’t fall apart during testing.)
Finally, the team added water to the hydrogel and pressed the ribbons’ bare silicon wafers onto either side of the gel-coated metal strip. When it dried, the Kevlar ribbons were firmly attached to the metal.
The team used the hydrogel to attach the metal strip to the front of a filing cabinet. They then hung a 4-kilogram (8.8-pound) weight from the Kevlar ribbon. It held firmly. Only when the team added water did the hydrogel soften, letting the weight fall to the ground.
The hydrogel was even stronger than the superglue. Sometimes when the weight was added, the superglue used in the setup broke free. But the hydrogel always held fast.
In a final test, the team built a large metal frame and stuck the gel-coated strip with the Kevlar ribbons to the top. They attached the Kevlar to a harness worn by a student in the lab. He carefully lifted his feet off the ground — and waited. The strip held. Just two postage-stamp-sized pieces of gel held up an 87-kilogram (192-pound) man!
The new adhesive “beautifully imitat[es] the reversible bonding mechanism used in the snail’s epiphragm,” says Diana Kay Hohl. She is a materials scientist at the University of Fribourg in Switzerland. Water is a good choice to make the new adhesive non-toxic, she says. But water is slow to dry. The hydrogel takes longer to dry than many other strong glues, Hohl notes. And water won’t work in many types of manufacturing, she points out.
“It will be interesting to see if, in next-generation materials, the adhesion can be tuned by triggers other than water,” she says. Heat or light, for example, might make the super-adhesive more suitable for manufacturing.
bioengineer Someone who applies engineering to solve problems in biology or in systems that will use living organisms.
chemical A substance formed from two or more atoms that unite (bond) in a fixed proportion and structure. For example, water is a chemical made when two hydrogen atoms bond to one oxygen atom. Its chemical formula is H2O. Chemical also can be an adjective to describe properties of materials that are the result of various reactions between different compounds.
chemistry The field of science that deals with the composition, structure and properties of substances and how they interact. Scientists use this knowledge to study unfamiliar substances, to reproduce large quantities of useful substances or to design and create new and useful substances. (about compounds) Chemistry also is used as a term to refer to the recipe of a compound, the way it’s produced or some of its properties. People who work in this field are known as chemists.
engineering The field of research that uses math and science to solve practical problems.
epiphragm A type of mucus secreted by a snail or mollusk prior to hibernation or when access to moisture dries up. Once the mucus dries, it safely seals the animal its body inside its shell. In most species, the dried material remains elastic. But some species reinforce this material with calcium carbonate, creating an epiphragm that is both strong and hard to break.
gecko A small to medium sized reptile found in warm to equatorial regions of the world. Some 2,000 different species of this lizard exist, in a wide range of colors. These reptiles eat insects, worms and even the occasional small bird. But they are best known for being able to climb slick surfaces, owing to special structures on the bottom surfaces of their feet.
gel A gooey or viscous material that can flow like a thick liquid.
generation A group of individuals (in any species) born at about the same time or that are regarded as a single group. Your parents belong to one generation of your family, for example, and your grandparents to another. Similarly, you and everyone within a few years of your age across the planet are referred to as belonging to a particular generation of humans. The term also is sometimes extended to year classes of other animals or to types of inanimate objects (such as electronics or automobiles).
glass A hard, brittle substance made from silica, a mineral found in sand. Glass usually is transparent and fairly inert (chemically nonreactive). Aquatic organisms called diatoms build their shells of it.
glue A sticky substance that attaches one material to another.
humidity A measure of the amount of water vapor in the atmosphere. (Air with a lot of water vapor in it is known as humid.)
hydrogel A “smart” material that can change its structure in response to its environment, such as the local temperature, pH, salt or water concentration. The material is made from a polymer — a chain made from links of identical units — that have free, water-attracting ends sticking out. So in the presence of water, it may hold (bond) those water molecules for quite a while. Some hydrogels are used in baby diapers to hold urine, in potting soils to hold water near to plants until they need it and in wound dressings to keep a sore from drying out.
hydrogen The lightest element in the universe. As a gas, it is colorless, odorless and highly flammable. It’s an integral part of many fuels, fats and chemicals that make up living tissues. It’s made of a single proton (which serves as its nucleus) orbited by a single electron.
Kevlar A super-strong plastic fiber developed by DuPont in the 1960s and initially sold in the early 1970s. It’s stronger than steel, but weighs much less, and won’t melt.
manufacturing The making of things, usually on a large scale.
materials science Someone who studies how the atomic and molecular structure of a material is related to its overall properties. Materials scientists can design new materials or analyze existing ones. Their analyses of a material’s overall properties (such as density, strength and melting point) can help engineers and other researchers select materials that are best suited to a new application.
mechanism The steps or process by which something happens or “works.” It may be the spring that pops something from one hole into another. It could be the squeezing of the heart muscle that pumps blood throughout the body. It could be the friction (with the road and air) that slows down the speed of a coasting car. Researchers often look for the mechanism behind actions and reactions to understand how something functions.
moisture Small amounts of water present in the air, as vapor. It can also be present as a liquid, such as water droplets condensed on the inside of a window, or dampness present in clothing or soil.
mucus A slimy substance produced in the lungs, nose, digestive system and other parts of the body to protect against infection. Mucus is made mainly of water but also includes salt and proteins such as mucins. Some animals use mucus for other purposes, such as to move across the ground or to defend themselves against predators.
physics The scientific study of the nature and properties of matter and energy. Classical physics is an explanation of the nature and properties of matter and energy that relies on descriptions such as Newton’s laws of motion. Quantum physics, a field of study that emerged later, is a more accurate way of explaining the motions and behavior of matter. A scientist who works in such areas is known as a physicist.
polymer A substance made from long chains of repeating groups of atoms. Manufactured polymers include nylon, polyvinyl chloride (better known as PVC) and many types of plastics. Natural polymers include rubber, silk and cellulose (found in plants and used to make paper, for example).
pore A tiny hole in a surface. On the skin, substances such as oil, water and sweat pass through these openings.
predator (adjective: predatory) A creature that preys on other animals for most or all of its food.
pressure Force applied uniformly over a surface, measured as force per unit of area.
Proceedings of the National Academy of Sciences A prestigious journal publishing original scientific research, begun in 1914. The journal's content spans the biological, physical, and social sciences. Each of the more than 3,000 papers it publishes each year, now, are not only peer reviewed but also approved by a member of the U.S. National Academy of Sciences.
silicon A nonmetal, semiconducting element used in making electronic circuits. Pure silicon exists in a shiny, dark-gray crystalline form and as a shapeless powder.
slide In microscopy, the piece of glass onto which something will be attached for viewing under the device’s magnifying lens.
substrate The base on which an organism lives (in biology) or onto which atoms settle (in chemistry).
toxic Poisonous or able to harm or kill cells, tissues or whole organisms. The measure of risk posed by such a poison is its toxicity.
Journal: H. Cho et al. Intrinsically reversible superglues via shape adaptation inspired by snail epiphragm. Proceedings of the National Academy of Sciences of the United States of America. Vol. 116, July 9, 2019, p. 13774. doi: 10.1073/pnas.1818534116.