Bax Lindhardt

Researchers winning battle against sweat

Tuesday 30 Mar 21


Esben Thormann
DTU Chemistry
+45 45 25 24 39

Polymers are everywhere

A polymer is a large molecule, made up of many identical building blocks that are linked together. Many different kinds of polymers exist in nature, such as cellulose, natural rubber, polysaccharides, and DNA. Various plastics and silicone are examples of synthetic polymers made by humans.

What is a stoma?

A stoma is an artificial opening on the abdomen, whereby a part of the intestine is diverted and attached to the skin.

This allows feces or urine to exit through the stoma and be collected in a stoma bag, which is either attached directly to the skin with adhesive, or connected to a plate that adheres to the skin.

Stomas are created as a result of intestinal disease (typically cancer), urinary tract disease, congenital malformation or an accident. They can either be permanent or temporary. 10-12,000 Danes have a stoma.


Coloplast and Riemann are jointly developing new materials with DTU that prevent sweat from impairing the adhesiveness of stoma bags and sunscreens. 

Stoma bags and sunscreens are two very different products, but have one important thing in common: it is best if they adhere to the skin for as long as is necessary. In the case of stoma bags, it must also be possible to detach them again without too much difficulty. This demands a lot of knowledge about the adhesiveness of materials, and around ten researchers have been examining this area for several years. A large joint project between DTU Chemistry and Coloplast and Riemann has explored new material compositions that ensure products such as Coloplast’s stoma bags and Riemann’s sun protection have the best possible adhesiveness. 

“A good adhesive has to meet many requirements,” notes Esben Thormann, Professor at DTU Chemistry and head of the project. “Adhesion is the most important, of course, but it must also be so cohesive that you can remove a stoma bag without leaving adhesive material on the skin,” says the professor, who has been researching polymers for many years. His research was most recently recognized in summer 2020, when he received the Danish ATV | Elastyrene polymer award.

Moisture impairs adhesiveness

Polymer is the term for substances—both natural and synthetic—made of molecules that contain a large number of iterations of one or more types of atoms or atomic groups joined together. Polymers play a key role in the development of the ideal adhesive material.

Adhesiveness is achieved by a special group of polymers that are good at creating optimal contact with the skin, despite its uneven surface structure. But adhesiveness is particularly difficult to maintain when the skin surface is moist, which happens when people sweat, for example.  

When sweat oozes out of the sweat pores in the skin, it accumulates on the skin surface and pushes the adhesive material away. The skin can also be damaged by the accumulated moisture.

The adhesive has to absorb sweat

The researchers have therefore sought to develop an adhesive capable of absorbing moisture, by adding water-absorbing particles (special super-absorbent polymers). They can absorb sweat as it is produced. But this creates a new problem, notes Esben Thormann: 

“When the water-absorbing particles absorb moisture, they expand many times in size. If a particle like this is sitting on the surface, close to the skin, it expands so much that it pushes the adhesive polymer away from the skin. We could add fewer water-absorbing particles, but only the ones in direct contact with the skin can absorb the sweat. If there are only a few of them, they will quickly fill up. They have limited capacity. So the challenge was to find a way to combine the water-repellent, adhesive polymers with the water-absorbing particles.”

The researchers succeeded. They found a solution whereby an internal structure in the adhesive material makes it possible to transport sweat away from the skin through the surface of the adhesive, deeper into the adhesive material. The solution led Coloplast to apply for a patent for the invention. 

During the process, the researchers had to find new measurement methods, so they could better understand what was happening inside the adhesive material. They ended up passing an electric current through the various adhesive materials and performing measurements using ‘impedance spectroscopy’, under which the electrical resistance of the material and its ability to store charge (capacitance) reveals the water transportation.

The actual path of the liquid through the adhesive was also studied using advanced microscopy at the Danish Molecular Biomedical Imaging Centre in Odense, using equipment designed to examine biological material.

Skin model with artificial sweat

The measurement methods allowed the researchers to gain a good understanding of how water, sweat, or even liquid content from leaking stoma bags are handled by the adhesive. The next question was how a newly developed adhesive would actually perform on sweaty skin.

Nothing beats testing on real people, but everyone has different skin and sweats differently. Even the same subject sweats differently from trial to trial, notes Esben Thormann:

“Clinical studies will result in very large variations and be expensive. They are not a particularly suitable method for screening and developing new types of materials.”

In the past, pig skin has typically been used to test adhesives, but now the researchers wanted to carry out tests on something equivalent to skin that starts to sweat. They therefore began to produce laboratory models that were good imitations of human skin.

Skin model

The research team used gelatin, which is extracted from animal products such as pig skin, as a base. Gelatin resembles skin chemically, but it also had to behave like real skin. It had to have the right structure, the ability to absorb water and, not least, be able to release sweat, just like our skin.

“We were able to make our skin model sweat by drilling small holes in it and pumping water through, but it didn’t work properly because the holes varied in size and all the water came through the biggest hole. Then we heard about a research group at the University of Cincinnati in USA who had developed a model of sweating skin. We sent one of our PhD students over there to learn about the technique,” says Esben Thormann.

“With the new knowledge, we were able to build a model where we can now control how artificial sweat is emitted through a series of small sweat pores that almost resemble the real thing in terms of size and distribution across the skin.”

New knowledge put to use

The researchers now have a model that is physically, chemically, and physiologically similar to human skin, and they can control how much it sweats. For example, they can simulate a person playing sport, who is therefore sweating a lot. They can trial various material compositions in the adhesives, leave them attached for various periods of time, and then measure how easy or difficult they are to remove again.

“Our model gives us a new way to measure how well the adhesive sticks to sweaty skin, and how effectively sweat is absorbed. The new method can be a good supplement to the normal testing methods,” explains Esben Thormann. The researchers are still working to develop optimal adhesive materials, but the adhesives, skin model, and electrical measurement method are already being used at Coloplast. Kristoffer Hansen, a researcher and project manager at the company, reports:

“We’ve developed new adhesives that work better with sweat, and the test methods have been integrated into Coloplast. We’re currently using ideas and technologies from the project in development projects in the company.” He adds: “We’ve gained some tools that allow us to do some things differently than before. We have new understanding, and some testing methods that can be used for many different adhesives for many different purposes,” says Kristoffer Hansen.

No more sunscreen in the eyes

The new testing methods and material understanding that meets the challenge of sweat have also been applied to sunscreen. Sunscreen and sweat are a very bad combination, because the sweat makes the sunscreen simply run off.

Some people refrain from using sunscreen on their forehead because it runs into their eyes. But there is also hope ahead here, notes Esben Thormann:

“We have taken our sweating skin model and stood it up vertically, and then smeared sunscreen on the upper part. So we can simulate what happens to sunscreen on a sweating forehead. Using a UV camera, we can see how the sunscreen’s UV filters move, such as when they run into people’s eyes.

The model allows us to measure how much effectiveness the sunscreen loses when people sweat, and how much it runs into the eyes—things we have not been able to measure before. We can test new compositions of ingredients in sunscreen, and see that we can improve sunscreen by combining water-repellent and water-absorbing particles in new ways.”

Riemann’s Research Director, Shadi Jafarzadeh, reports that the project has given a deep understanding of how the various ingredients in sunscreen behave when it is exposed to sweat. The new test methods have also been taken on board at Riemann:

“We can now test various prototypes faster, cheaper, and in an ethical way. More specifically, we can simulate and study how small beads of sweat can affect how uniformly sunscreen protects the skin, both at a microscopic and macroscopic level,” she says.

“Then we can find the optimal ingredients for an innovative sunscreen with superior sweat resistance, that makes it safer to perform physical activities in the sunshine.”

The major collaboration project between DTU and the two companies will be completed in early 2021, but Coloplast will continue to work with DTU Chemistry to develop adhesive materials with innovative new properties.

About the project

The ‘Smart skin moisture removal for extended adhesion of stoma bags and long-lasting sunscreen’ research project is a joint project between the ‘Polymers and functional surfaces’ research group at DTU Chemistry and the Coloplast and Riemann companies. Through the Grand Solutions programme, Innovation Fund Denmark has contributed DKK 12.2 million of the project’s total budget of DKK 22.5 million.

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