The finding was published recently in Small magazine, a weekly publication on nanotechnology. The created nanorobots are capable of moving self-sufficiently. There are two aspects to achieve this: create a molecular robot that can be reversibly deformed and convert this deformation into propulsion for the robot itself. Yoshiyuki Kageyama is the one who leads the group and had already solved the first step in a previous job, that is, the creation of robots that can deform reciprocally. According to Edward Purcell’s (1977) scallop theorem, tiny objects cannot convert their reciprocal motion into progressive motion. A reciprocal movement is any repetitive movement in a straight line. In the current study, the scientists took the next step, that is, they managed to self-propel the molecular robot, in a movement limited to two dimensions, inside a water tank, under negligible viscosity resistance conditions. In this sense, the Reynolds number is important and the authors cite it in their article, whose title is Self-Propulsion of a Light-Powered Microscopic Crystalline Flapper in Water . The Reynolds number is defined as the relationship between the inertial or connective forces between the viscous forces. It is essential to be able to describe a laminar (small number) or turbulent (large number) flow. The authors worked with a low Reynold number, in which the scallop theorem is framed.
For microscopic matter to show dynamic activity, energy is needed and this can arrive in the form of light. In the present case, the micro-robot works with blue light. Intermittent movements were achieved, not continuous. The reciprocal flipping motion of the crystals mimics that of a fish moving its tail fin, although many nanocrystals swam in the opposite direction that fish would. A small glass with a less deformable part in its fin shows a jerky swimming motion, while a glass with a uniformly deformed fin achieves push-type movements. They have actually known three nation styles of the micro submarine which they have called the “hit” style, the “kick” style and the “sideways” style.
Molecular switches are molecules that have more than one stable state and that can change from one to another according to certain stimuli: pH, light, temperature, electric current, micro-contact or by the presence of a ligand. A combination of various stimuli can be used to make a molecule that generates on / off analogous states. This makes them suitable for nanotechnology research for molecular computers. Although it is not something invented by the human being, because in the world of biology it is also present. For example, in allosteric regulation.
Photochromic molecules represent a characteristic example of a molecular switch. These compounds are capable of switching between two or more states when irradiated by a specific wavelength. Azobenzenes are photochromic, due to the double bond that join two nitrogen atoms linked to two benzene rings. The so-called cis-post photoisomerization occurs. The trans isomer of azobenzene absorbs light at 313 nm and the cis isomer does so at 460 nm.
The crystals are prepared from a derivative of azobenzene and oleic acid, by complex procedures described in the article. The self-repeating movement of elongation and shrinkage occurred through the crystalline phase transition triggered by photoisomerization, a property that gives the crystal the ability to switch between the two forms (cis-trans), as if it were a molecular switch. The photoisomerization of azobenzene was already known since 1937, by GS Hartley. This change in the position of a single bond is the trigger for the movement of the entire crystal. It is an intermittent swimming movement in which a part of the glass is the closest thing to a tiny fin. Until now, as stated in the article, the expected efficiency is not shown, as there are flaws in the directionality. However, “this study shows a basic design for creating small artificial active motion devices.”
The article is signed by Kazuma Obara, Yoshiyuki Kageyama and Sadamu Takeda, all scientists from the University of Hokkaido, Japan. Indeed, Kageyama is not the first time that he has published something about it, in fact he has been researching the movement of molecular robots for a long time. In 2019 he published an article in ChemPotoChem, Light-Powered Self-Sustainable Macroscopic Motion for the Active Locomotion of Materials . Here I did a review of the work on photosensitive molecules that show mechanical functions. He introduced mathematical models in order to open doors to go from discontinuous to continuous motion, a challenge that continues to exist. Already in March 2020 he published Light-Driven Flipping of Azobenzene Assemblies — Sparse Crystal Structures and Responsive Behavior to Polarized Light , in Shemistry A Eurpean Journal, where he analyzed the use of azobenzene in crystals as a possible reaction center before light to achieve a response mechanics. As we can see, a step-by-step directed study. Perhaps soon we will finally see the continuous movement, without kicking, hitting or shoving. The Hokkaido University itself has echoed the news of the new article on its website: Mobile molecular robots swim in water. It shows a photo of the authors and other members of the Kageyama team.
