Self-disposing — got to be simpler than recycling

Nature doesn’t make garbage dumps — many materials self-assemble and then simply de-assemble at end of their lifetime. But with plastic bottles and cans made by humans it is a very different story. Currently, the disposal method of choice is recycling, but this is an expensive undertaking. Most man-made substances are chemically very stable and so to decompose them back into their components requires a lot of energy.

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, thus remaining the way they are.

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

Job Boekhoven, professor of Supramolecular Chemistry at the Technical University Munich (TUM), has been inspired by biological processes and, along with an interdisciplinary team, is looking at supramolecular structures that can autonomously disintegrate.

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

Because the molecules form very different structures depending on their chemical composition, numerous application possibilities arise. Spherical colloids, for example, can be loaded with water-insoluble molecules — these could be used to transport drugs against cancer directly to the tumour cell. 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 stabilise freshly transplanted tissue for a predefined time, after which the body would take over this function.

Will it be possible to build supramolecular soft drink bottles and beer cans that simply disappear when they are no longer needed? “This might not be completely impossible,” stressed Boekhoven, “but there is still a long way to go. Right now we are working on the basics.”