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Molecular Robotics and molecular machines propulsion technologies

Molecular machines can be defined as devices that can produce useful work through the interaction of individual molecules at the molecular scale of length. A convenient unit of measurement at the molecular scale would be a nanometer. Hence, molecular machines also fall into the category of nanomachines. Molecular machines depend on inter- and intramolecular interactions for their function. These interactions include forces such as the ionic and Van der Waal’s forces and are a function of the geometry of the individual molecules.


Microscopically tiny nanomachines which move like submarines with their own propulsion — for example in the human body, where they transport active agents and release them at a target: What sounds like science fiction has, over the past 20 years, become an ever more rapidly growing field of research. However, most of the particles developed so far only function in the laboratory. Propulsion, for example, is a hurdle. Some particles have to be supplied with energy in the form of light, others use chemical propulsions which release toxic substances. Neither of these can be considered for any application in the body.

LMU chemists have developed the first molecular motor that can be powered by light alone.

Chemists from LMU in Munich have developed a molecular motor that is powered by light. This means that molecules can carry out specific rotary movements by means of an external energy supply. They thus form an important basis for applications in nanotechnology. Molecules that change their structure under the influence of light are particularly suitable for nanoscale motors. Up to now, light-driven molecular motors have required additional reactions driven by heat. This is why they were previously dependent on the ambient temperature.


Henry Dube, LMU chemist, together with the student Aaron Gerwien, has now achieved the decisive breakthrough. They have developed a molecular motor that functions independently of temperature. It uses light as the driver and runs even faster at low temperatures. Until now, this was not possible. Molecular motors reacted to the ambient temperature. This also limited the field of application in nanotechnology.


In order to generate a complete 360-degree rotation with the rotary motion, a certain part of a molecule is to perform several rotary steps around another. The problem here is that the molecule must be prevented from turning back again. To do this, molecular motors require so-called ratchet steps. These are intermediate steps that change the molecule after a rotation in such a way that a back rotation is excluded. Normally, this is achieved by heat. Disadvantage: If the ambient temperature drops, the molecular motor runs all the slower – and it stops at cold temperatures.


Like earlier motor systems developed by Dube and his colleagues, the new motor is based on an organic substance called hemithioindigo. This molecule is made up of two different carbon skeletons, which are connected by a mobile double bond. “We have succeeded in modifying the molecule such that a complete rotation of one of the structural modules relative to the other requires only three reaction steps,” says Dube. Each rotational step is activated by visible light and there is no need for intermediate, thermally driven ratchet steps. Indeed, all three steps involved in the full rotation are promoted by a reduction in temperature, so that the rotation rate of the new molecules actually increases at lower temperatures.


“Each rotation step is made up of three different photoreactions, two of which we experimentally demonstrated directly for the first time only this year,” Dube explains. The researchers are confident that their motor’s novel driving mechanism and unique behavior will make it possible in the not too distant future for researchers to synthesize molecular machines which, thanks to their relative insensitivity to the precise environmental temperature, will enable unique applications not possible with hitherto known motors.


Acoustic propulsion of nanomachines

Johannes Voß and Prof. Raphael Wittkowski from the Institute of Theoretical Physics and the Center for Soft Nanoscience at the University of Münster (Germany) have now found answers to central questions which had previously stood in the way of applying acoustic propulsion. The results have been published in the journal ACS Nano.

Travelling ultrasound waves are suitable for propulsion

Ultrasound is used in acoustically propelled nanomachines as it is quite safe for applications in the body. Lead author Johannes Voß sums up the research carried out so far as follows: “There are many publications describing experiments. However, the particles in these experiments were almost always exposed to a standing ultrasound wave. This does admittedly make the experiments considerably simpler, but at the same time it makes the results less meaningful as regards possible applications — because in that case travelling ultrasound waves would be used.” This is due to the fact that standing waves are produced when waves travelling in opposite directions overlap one another.

What researchers also did not previously take into account is that in applications the particles can move in any direction. Thus, they left aside the question of whether propulsion depends on the orientation of the particles. Instead, they only looked at particles aligned perpendicular to the ultrasound wave. Now, for the first time, the team of researchers in Münster studied the effects of orientation using elaborate computer simulations.

They came to the conclusion that the propulsion of the nanoparticles depends on their orientation. At the same time, the acoustic propulsion mechanism in travelling ultrasound waves functions so well for all orientations of the particles — i.e. not only exactly perpendicular to the ultrasound wave — that these particles really can be used for biomedical applications. Another aspect the Münster physicists examined was the propulsion the particles exhibited when they were exposed to ultrasound coming from all directions (i.e. “isotropic ultrasound”).

A basis for the step towards application

“Our results showed how the particles will behave in applications and that the propulsion has the right properties for the particles to actually be used in these applications,” Johannes Voß concludes. As Raphael Wittkowski adds, “We have revealed important properties of acoustically propelled nanoparticles which had not previously been studied, but which need to be understood to enable the step to be made from basic research to the planned applications involving the particles.”

The two Münster researchers examined conical particles, as they can move fast even at a low intensity of ultrasound — i.e. they have efficient propulsion — and also they can easily be produced in large numbers. The particles are almost one micrometre in size — almost a thousand nanometres. In comparison, a red blood cell has a diameter of around 7.7 micrometres. This means that the nanoparticles could move through the bloodstream without blocking up the finest blood vessels. “The particle size can be selected in line with what is needed in the particular application intended, and the propulsion mechanism also functions in the case of smaller and larger particles,” Johannes Voß explains. “We simulated the particles in water, but the propulsion is also suitable for other fluids and for tissue.”

By means of computer simulations, the team investigated systems and their properties which could not be studied in the many preceding experiments. Looking into the future, Raphael Wittkowski says, “An important step would be for experiment-based research to move on to looking at these systems.”


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