Molecular flip in crystals driven by light creates microrobotic propulsion

Could lead to bio-inspired microrobots that deliver drugs to target tissues
July 8, 2016

(a) Sequential micrographs of one cycle of self-oscillation observed under 435-nm light and schematic illustrations. (b) Schematic illustration showing setup. (credit: Tomonori Ikegami et al./Angewandte Chemie)

Hokkaido University researchers have designed a crystal material that continually flips between two positions like a paddle, propelling an attached structure, when stimulated by blue light. It could lead to bio-inspired microrobots that deliver drugs to target tissues, for example.

The team made azobenzene-oleic acid crystals, composed of an organic compound called azobenzene, commonly used in dye manufacturing, and oleic acid, commonly found in cooking oil. Azobenzene molecules take two structurally different forms: cis and trans, and they were found to switch back and forth when stimulated by the light.

The frequency of the motion also increased with increased light intensity. Some crystal complexes they created even exhibited swimming-like motions in the water, the researchers report. Previously reported light-responsive materials have been limited in their ability to deform, the researchers noted.

“The importance of this study lies in the realization of macroscopic self-oscillation by the repeated reversible reaction of a molecular machine with the cooperative transformation of a molecular assembly,” the researchers note in a paper published in the journal Angewandte Chemie. “These results provide a fundamental strategy for constructing dynamic self-organizations in supramolecular systems to achieve bioinspired molecular systems.”


Ikegami T. et. al. | A crystalline assembly of azobenzene derivative and oleate showed oscillatory bending-unbending motion under continuous 435-nm light irradiation.


Abstract of Dissipative and Autonomous Square-Wave Self-Oscillation of a Macroscopic Hybrid Self-Assembly under Continuous Light Irradiation

Building a bottom-up supramolecular system to perform continuously autonomous motions will pave the way for the next generation of biomimetic mechanical systems. In biological systems, hierarchical molecular synchronization underlies the generation of spatio-temporal patterns with dissipative structures. However, it remains difficult to build such self-organized working objects via artificial techniques. Herein, we show the first example of a square-wave limit-cycle self-oscillatory motion of a noncovalent assembly of oleic acid and an azobenzene derivative. The assembly steadily flips under continuous blue-light irradiation. Mechanical self-oscillation is established by successively alternating photoisomerization processes and multi-stable phase transitions. These results offer a fundamental strategy for creating a supramolecular motor that works progressively under the operation of molecule-based machines.