This is an image of a temporary gel made from Fmoc tripeptides, which could hold something in place and then automatically disintegrate when no longer needed. Image: Benedikt Rieß/TUM.
This is an image of a temporary gel made from Fmoc tripeptides, which could hold something in place and then automatically disintegrate when no longer needed. Image: Benedikt Rieß/TUM.

Materials that assemble themselves and then simply disappear at the end of their lifetime are quite common in nature. Researchers at the Technical University of Munich (TUM) in Germany have now successfully developed supramolecular materials that do the same thing, disintegrating at a predetermined time – a feature that could find use in numerous applications. They report this work in a paper in Nature Communications.

Although an increasing amount of man-made waste is being recycled, the process is often expensive. "So far, most man-made substances are chemically very stable: to decompose them back into their components, one has to spend a lot of energy," explains Job Boekhoven, professor of supramolecular chemistry at the TUM. Inspired by biological processes, the chemist is now pursuing another path.

"Nature does not produce garbage dumps. Instead, biological cells are constantly synthesizing new molecules from recycled ones. Some of these molecules assemble into larger structures, so-called supramolecular assemblies that form the structural components of the cell. This dynamic ensemble inspired us to develop materials that dispose of themselves when they are no longer needed. "

One of the key differences between man-made substances and most living biological materials is their energy management: man-made materials are in equilibrium with their environment. That means that they don't exchange molecules and energy, and so remain static and stable.

Nature works according to another principle: living biological materials, like skin, bone and cells, are not in equilibrium with their environment. A constant input of energy and building blocks is necessary for their construction, maintenance and repair.

"A typical example of an energy source is adenosine triphosphate, ATP for short," explains Boekhoven. "As long as enough energy is available, damaged components and entire cells can be broken down and replaced by new ones, otherwise the organism dies and disintegrates into its basic building blocks."

The new materials that Boekhoven is exploring, in conjunction with an interdisciplinary team of chemists, physicists and engineers at the TUM, are based on this natural model. The molecular building blocks are initially freely mobile, but if energy is added in the form of high-energy molecules, the blocks spontaneously assemble to form supramolecular structures.

These structures then autonomously disintegrate once the energy is exhausted, meaning their lifetime is predetermined by the amount of ‘fuel’ added. In the laboratory, the materials can be set to degrade autonomously after defined periods that range from several minutes to several hours. Moreover, following a cycle, the degraded material can be reused by simply adding another batch of high-energy molecules.

Employing organic molecules known as anhydrides, the scientists designed different versions that assemble into colloids, supramolecular hydrogels or inks. In these materials, a chemical reaction network converts dicarboxylates into the metastable anhydrides, driven by the irreversible consumption of carbodiimide as ‘fuel’. Because of their metastable character, the anhydrides automatically hydrolyze back to their original dicarboxylates, with half-lives in the range of seconds to several minutes.

The molecules form very different structures depending on their chemical composition, allowing for various possible applications. Spherical colloids, for example, can be loaded with water-insoluble molecules – these could be used to transport drugs against cancer directly to the tumor. At the end of their mission, the colloids would autonomously dissolve, thereby releasing the drugs locally.

Other building blocks assemble into long fibrous structures that transform fluids into gels and might be used to stabilize freshly transplanted tissue for a predetermined time, after which the body would take over this function. In addition, inks with precisely defined durability could be produced from molecules that assemble into star-shaped assemblies.

Will it be possible to build supramolecular machines or mobile phones that simply disappear when they are no longer needed? "This might not be completely impossible," says Boekhoven, "but there is still a long way to go. Right now we are working on the basics."

This story is adapted from material from the Technical University of Munich, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.