Researchers from the Indian Institute of Science Education and Research in Kolkata have synthesised bendable and highly stiff cocrystals by mixing caffeine with some organic small molecules in an organic solvent1.

These crystals are exceptionally stiff, like some low-density metals, with remarkable elastic flexibility. Such a combination of properties makes them potentially useful for designing flexible electronic devices.

Stiff and hard crystals are usually brittle. They are also not flexible. It is difficult to combine stiffness with elasticity in a single organic crystal.

To make a stiff and elastic organic crystal, the scientists prepared the cocrystals by mixing solutions of caffeine and chloro-nitrobenzoic acid in methanol. They then explored the stiffness, elasticity and other mechanical properties of the cocrystals by exposing them to increasing loads that exert stress.

The researchers, led by C. Malla Reddy, found that the crystals owe their elasticity and stiffness to different types of intermolecular interactions. Such interactions also redistribute the stress, allowing the crystals to withstand a large amount of stress.

The individual components of the crystals, when crystallised separately, do not show elastic flexibility. However, the components, when combined in cocrystals, display elasticity.

This, the researchers say, suggests that the arrangement of molecules in the crystals is an important contributor to their elasticity.

“The cocrystals could be used to make flexible semiconducting single crystals which would help fabricate durable and flexible transistors,” says Reddy. Endowing the crystals with additional properties such as fluorescence will also help design smart biomedical devices or sensors, he adds.


1. Dey, S. et al. Mechanically interlocked architecture aids an ultrastiff and ultra-hard elastically bendable cocrystal. Nat. Commun.10, 3711 (2019)