Chemists have gotten a lot of mileage out of the Nobel-Prize-winning, light-activated motor molecules invented in Ben L. Feringa’s lab at the University of Groningen. Feringa has used them to create molecule-sized cars that scoot along a surface, and others have incorporated them into polymers or used them to drill holes in cancer cells.
Now, Feringa’s group managed to get these molecule-sized machines to flex some muscle. His team created a water-soluble version of the motor that assembles into fibers. In the presence of calcium ions, these fibers organize into macroscale strings made mostly of water that flex in response to ultraviolet light. They can even lift a small weight: a 400 mg piece of paper (Nat. Chem. 2017, DOI: 10.1038/nchem.2887).
As with previous versions of Feringa’s motors, these molecules rotate via isomerization around a double bond when hit with UV light. The motor molecules pack closely together in the self-assembled fibers and expand a little bit in the presence of light, causing the string to bend. “You amplify a tiny motion from the molecular level all the way up to the macroscopic level,” Feringa says.
While others have made artificial muscles using molecular machines covalently linked to polymers, this is the first time that such muscular systems have been made entirely from assembled small molecules. “Apparently—and this was really a surprise—once they are organized, they can sustain the motion and they don’t fall apart,” Feringa points out.
The muscular movement is remarkable, says Ray Baughman, an expert in artificial muscles at the University of Texas, Dallas. He’s particularly impressed because the fibers are 95% water and don’t contain polymers, yet the material still can transduce molecular scale motion over macroscale distances. Baughman wonders if similar systems could be designed to move in response to other types of radiation that could be transmitted more easily in biological systems to release drugs at specific rates and sites.
The work demonstrates that a soft hydrogel held together by weak noncovalent bonds can do mechanical work, adds University of Strasbourg’s Nicolas Giuseppone, who has used motor molecules with polymers to create macroscale motion. “This type of approach can lead to low-cost actuating materials that could be of interest for implementations in soft robotics.”