Spheres do not naturally become worms as they are two distinct shapes with different properties. Spheres are three-dimensional shapes with a round surface, while worms are long, thin, and cylindrical in shape. A previously unknown form of hydrogel formation has been elucidated: chemists found unusual interactions between polymers.
Hydrogels? Many people unknowingly use these substances. Hydrogels absorb a lot of liquid when used as superabsorbents in nappies, for example. The initially dry material becomes Jelly-like during the process, but it does not wet. Some people apply the swellable material to their eyes; soft contact lenses are simply hydrogels. The same is true for jelly and other commonplace materials.
Hydrogels are also used in science. They are chemically long, three-dimensional cross-linked polymer molecules that form cavities. They can absorb and hold water molecules on the inside.
In the working group of former Würzburg chemistry professor Robert Luxenhofer, the suitability of hydrogels for biofabrication is being tested: For example, hydrogels can be used for 3D printing as scaffold structures, on which cells can be attached. In this way, for example, artificial tissues can be produced for medical research and regenerative therapies.
If gel formation does occur, it is usually due to hydrogen bonds, which are attractive forces between polar functional groups involving hydrogen atoms that have a stabilizing effect. Such interactions are of central importance for the structure and function of proteins, for example.
Robert Luxenhofer
Hydrogel formation posed a puzzle
Dr. Lukas Hahn of Luxenhofer’s team noticed an unusual type of hydrogel formation during this research. He discovered it in polymers used in nanomedicine, specifically drug delivery.
In water at 40 degrees Celsius, these polymers form spherical nanoparticles. When the water cools below 32 degrees, the spheres form worm-like structures and a gel forms. It re-dissolves when heated.
“This behavior is very unusual in synthetic polymers and was completely unexpected,” says Robert Luxenhofer, who now teaches and conducts research at the University of Helsinki. If gel formation does occur, it is usually due to hydrogen bonds, which are attractive forces between polar functional groups involving hydrogen atoms that have a stabilizing effect. Such interactions are of central importance for the structure and function of proteins, for example.
However, things are quite different with the polymers we are dealing with here. In terms of their chemical structure, they are not capable of forming hydrogen bonds with each other. Apparently, the researchers had stumbled upon an unknown mechanism of gel formation.
Breakthrough with NMR spectroscopy
To solve the puzzle, Robert Luxenhofer sought a cooperation with chemistry professor Ann-Christin Pöppler at Julius-Maximilians-Universität Würzburg (JMU), an expert in the characterisation of nanoparticles made of polymers. In cooperation with other research groups, her team took a closer look at the peculiar form of gel formation — a complex puzzle that took a good two years to solve.
“Because we used a wide range of analytical tools, we were able to elucidate the unknown mechanism. In the end, however, various methods of NMR spectroscopy provided the breakthrough,” explains the JMU chemist. Her doctoral student Theresa Zorn discovered what causes gel formation in this case: specific interactions between amide groups of water-soluble polymer building blocks and phenyl rings of non-water-soluble polymer building blocks. The spherical nanoparticles condense and restructure into worm-like structures as a result of these interactions.
The findings could be confirmed by theoretical calculations: Dr. Josef Kehrein, a former Ph.D. student of JMU professor Christoph Sotriffer, an expert in computer-aided modelling of three-dimensional interactions between molecules, succeeded in doing so. He, too, is now working in Helsinki.
The results have been published in ACS Nano, a journal of the American Chemical Society (ACS). The German Research Foundation (DFG), the Academy of Finland and other supporters funded the work.
Which research steps follow
Where do we go from here? The researchers are convinced that the newly discovered mechanism of hydrogel formation is applicable to other polymers and their interactions with biological tissues.
As a result, the team intends to chemically modify the polymers to see how this affects their properties and hydrogelation. It may be possible to specifically influence the gelation temperature, as well as the gel’s strength and durability. The modified materials could be chosen for use in biofabrication.
