Molecular machines are ubiquitous in nature and have evolved over billions of years to exploit energy from sunlight or complex chemical reactions in the body. They are made up of complicated assemblies of proteins responsible for a host of processes in living organisms, such as ion transport, ATP synthesis and cell division. In fact, our muscles are controlled by the co-ordinated movement of thousands of these machines.

“The organogel material we used to make our new device works out-of-equilibrium by constantly consuming the energy from light and, in this sense, it is very like these biological systems, which produce their motion under the same type of thermodynamic conditions (while usually consuming ATP in their case),” explains team leader Nicolas Guiseppone of the Institut Charles Sadron at the University of Strasbourg.

Organogel that can reversibly contract and expand when irradiated with light

Although artificial nanoengines have come along in leaps and bounds over the last few years, one big challenge remains in that researchers need to be able to amplify the molecular-level mechanical motion produced by these devices to a sizeable motion at the macro-scale. The problem is that, in solution, these molecular machines often move about randomly and it is difficult to control their motion in any one direction. To do this, complicated chemical modifications need to be made to the structure of the actuating motor part of the machine.

Now, Guiseppone and colleagues have developed an organogel that can reversibly contract (move backward) and expand (move forward) when exposed to light. The system consists of two mechanically-active molecular units (a motor and a modulator) linked by polymer chains (see image). When exposed to irradiating UV light, the motor turns and braids the polymer chains. This braiding leads to the material contracting along its entirety.

By then replacing the UV light by white light, the motor stops rotating and the modulator is activated. The modulator can turn as well, but in the opposite direction to the motor, which unbraids the polymer chains. This unbraiding results in the entire material expanding again.

“Even more interestingly, it is possible to turn both the motor and modulator at the same time by using a combination of UV and visible light, explains Guiseppone. We can regulate the speed of rotation (or frequency) by the intensity of light at each wavelength and precisely regulate the contraction/extension of the material by varying this rotation (see video).

A sort of “molecular gearbox”

“In fact, the system can be thought of as a molecular motor linked to a transmission unit (the polymer chains) and to a clutch (the modulator),” he tells nanotechweb.org. It is a sort of molecular gearbox that either transmits or stops the work produced by the motor to the rest of the system to produce something useful (here, it is a muscle-like contraction).

“Our organogel is a very new type of mechanically-active material in which the working unit is embedded at the molecular level in it,” he adds. “What is more, connecting different mechanically-active units into modules in the way we have done is new in the world of molecular machines. This connection provides a general way to reverse a molecular motor’s direction of motion in a system that is normally restricted to moving in a single direction by physical laws.”

The new nanomachine is described in Nature Nanotechnology doi:10.1038/nnano.2017.28.