Keeping babies dry: Superabsorbers are used mainly in baby diapers


Manfred Wilhelm, a scientist at the Karlsruhe Institute of Technology, has a dream: going where no one has been before. Today he’s using the superabsorbers that are normally inside baby diapers to make polluted water drinkable—and potentially change the world

When Manfred Wilhelm needs time to himself, he goes down to the basement. He has three teenage daughters and holds a professorship at an internationally renowned university, so he rarely has uninterrupted time for his work. In his house near Karlsruhe, he writes expert opinions, edits scientific articles, and thinks about his specialist field, polymer chemistry. He jots down his best ideas for research projects on a whiteboard behind his desk at the Karlsruhe Institute of Technology (KIT), where he has taught and researched polymer materials for the past 11 years. Laypeople would simply call them plastics.

»Science is the power of anticipation. I want to create something that still hasn’t been created«

Manfred Wilhelm

teaches and researches at the Institute for Chemical Technology and Polymer Chemistry at the Karlsruhe Institute of Technology (KIT)

Students looking for topics for their bachelor’s, master’s, or doctoral theses often find what they’re looking for on Wilhelm’s whiteboard. Three theses are currently devoted to the topic of water. Depending on their conclusions, they might be able to change the world—with a type of plastic that so far has mainly been used as a toilet.

Superabsorbers are plastics that can absorb huge volumes of liquid. Industrial companies primarily use this white granulate to make baby diapers. The first Pampers were lined with superabsorbers in 1987. Since then, practically all modern baby diapers use superabsorbers, which can easily soak up 500 times their own volume of water and retain it securely. Drier baby bottoms, happier infants, and free-spending parents are the three pillars of the diaper business.

“I actually had a brainstorm while changing a diaper,” says Wilhelm, who is now 51. During a family vacation on the Baltic Sea he noticed how fast his young daughter’s diapers also swelled up as she played in the salt water. He had previously been reading up on polyelectrolytes for his inaugural lecture at the university, and a few years before that he had worked at the renowned Weitzman Institute in Israel, commuting between the Max Planck Institute in Mainz and the Rehovot research Institute south of Tel Aviv. “I still remembered Israel’s difficulty with securing freshwater,” he says. “Every day there was something in The Jerusalem Post about the current level of water in the Sea of Galilee.” All of these thoughts and memories coalesced one day at the changing table. Wouldn’t it be possible to use superabsorbers to desalinate seawater? The answer is yes, as Wilhelm’s student Lukas Arens is demonstrating in his doctoral thesis. All the same, it’s not very easy.

A question of energy

To understand why not, we first have to know how superabsorbers soak up water. Wilhelm explains the process by having visitors stir a teaspoon of grainy white superabsorber powder into a beaker of water. Within seconds, it forms a tough gel that can’t be removed by tipping or shaking the beaker. The secret lies in the powder’s molecular structure. “This is a polymer—a long molecular chain of acrylic acids that forms a three-dimensional network that acts like a molecular sponge,” Wilhelm explains. He jumps up and grabs from a shelf a brightly colored plastic ball that could have come from a toy store. It’s a Hoberman sphere, which can be pulled in every direction via clever hinges to form ever larger spheres. “The superabsorber polymer can do this even better. In contact with water, it grows a thousandfold,” says Wilhelm. “It packs the water molecules into the interior of its web of cross-links and holds them there by means of ionic interactions.” The tighter the meshes of the cross-links—that is, the greater the degree of cross-linking—the more firmly the superabsorber holds the water, and the less it swells up.

»The problem of desalination has basically been solved, but we want to prove that there’s another way to do it«

Lukas Arens,

a doctoral candidate at KIT, is developing systems to make desalination by means of superabsorbers measurable

For his thesis, Arens is now exploiting the fact that freshwater is quickly soaked up by the superabsorber, whereas salt and salt water take longer. “And vice versa,” he says, meaning that when he presses a superabsorber that is full of salt water, the salt water comes out first, followed by the freshwater. “By applying the necessary pressure, we can separate freshwater from salt water,” he concludes. In a long series of trials at the laboratory in Karlsruhe, he is using a specially made press to find out how effective and efficient this method is, how much energy it requires, and how strongly cross-linked it has to be.

It’s ultimately a question of energy. “Theoretically, the smallest amount of energy needed for desalination is one kilowatt-hour per cubic meter of the resulting freshwater,” says Wilhelm. “Present-day facilities need around ten times that much energy. We’re already in the same order of magnitude.” He’s thinking of the huge desalination plants in Saudi Arabia and the United Arab Emirates, where gigantic power plants produce hundreds of millions of liters of drinking water every day, partly by turning the seawater to steam and partly through reverse osmosis, in which water is pressed through special membranes. “Just the fact that we’re in effect using a three-dimensional membrane in our superabsorber offers this process advantages,” says Wilhelm.

A look through the Hoberman sphere: Manfred Wilhelm poses with the model developed by the US designer Chuck Hoberman

Demonstrating potential

Two rooms over, the bachelor’s degree candidate Ilona Wagner is exploiting another selective behavior of superabsorbers. “Depending on the degree of cross-linking, superabsorbers also soak up water polluted with different metals at different rates,” she explains. She is experimenting with arsenic, cadmium, lead, and chromium—extremely toxic metals that are present in untreated industrial wastewater. Unfortunately, in underdeveloped countries and emerging markets they end up in the drinking water of millions of people. “There are processes for removing these toxins from the water, but they are complicated and very expensive,” says Wagner. Her initial experiments have shown that carcinogenic chromium salts, which are used in the tanning industry in India and Bangladesh, for example, can soon be removed from water by superabsorbers at rates as high as 99 percent. Because chromium salts are colorful, even laypeople can see this effect. In Wagner’s beakers, the superabsorber flocculates steadily, causing the greenish color of the chromium to disappear from the overlying water. Wagner’s idea is that even very small businesses could put fleeces containing superabsorbers, which are merely oversized diapers, into their tanning brew to remove the chromium and dispose of it separately. However, the road from her bachelor’s thesis to the implementation of her idea will be a long one.

The implementation of another one of Wilhelm’s ideas, the “osmosis motor,” seems even further away. In this device, the osmotic pressure of a swelling superabsorber is used to transform energy. The basic principle has been demonstrated in the KIT laboratory, where various devices and weights constantly move up and down, driven by the differing salt content in the water. Christopher Pfeifer is finishing up his doctoral thesis on this topic. Things are looking good, he says: “Now we just have to optimize the energy yield by finding the right degree of cross-linking in the superabsorbers.”

liters per second

was the global capacity for producing drinking water by desalination in 2015. About one percent of the world’s drinking water is produced via desalination, but this figure is increasing rapidly.


main technical processes

are used today for desalination: distillation and membranes. Energy-intensive distillation dominates only in the Mideast, where almost half of the world’s desalination for drinking water takes place. Cheap petroleum makes that possible.
3 kWh

per 1,000 liters

is today a very good rate for the energy needed for reverse-osmosis desalination. Distillation requires around three times as much energy. Prof. Wilhelm believes his method requires even less.

Will the superabsorber ideas on Wilhelm’s whiteboard ever be able to replace established processes? Wilhelm himself is skeptical. It’s true that he has already commercialized about a dozen of his ideas, because his institute specializes in developing new measuring methods and innovative measuring devices that help plastics manufacturers understand their polymers better. However, he concedes that “the industry is basically resistant to innovation.” He adds that this isn’t so bad: “The resistance against completely new technologies also generates pressure to continue improving the tech- 3 kWh nologies that already exist.” The research done at KIT is part of this effort. Wilhelm’s graduates are entering the industry to teach plastics new tricks such as powering innovative soles for athletic shoes at Nike. “If my devices are used and my graduates get good jobs, that’s success enough for me,” Wilhelm says. He was the first member of his family of winegrowers to go to college, so he knows how far one can get with education, brains, and good ideas. But he also knows the limits of his craft. “Our task as researchers is not to put living men on the moon,” he says. “Engineers are better at doing that. We want to be the first to come up with these new ideas.”

Tom Rademacher

is regularly amazed by the absorptive power of today’s diapers, now that he has children of his own. He lives, works, and changes diapers in Cologne