Acoustic propulsion of nanomachines depends on their orientation

Date:
March 11, 2022
Source:
University of Mu"nster
Summary:
Scientists have now found answers to central questions which had
previously stood in the way of acoustic propulsion of nanoparticles.



FULL STORY ========================================================================== 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. A solution to the problem could be acoustically propelled particles.

Johannes Voss and Prof. Raphael Wittkowski from the Institute of
Theoretical Physics and the Center for Soft Nanoscience at the University
of Mu"nster (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
Mu"nster 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 Mu"nster 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 Mu"nster 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."

========================================================================== Story Source: Materials provided by University_of_Mu"nster. Note:
Content may be edited for style and length.


========================================================================== Related Multimedia:
* A_conical_nanoparticle_in_water ========================================================================== Journal Reference:
1. Johannes Voss, Raphael Wittkowski. Orientation-Dependent
Propulsion of
Triangular Nano- and Microparticles by a Traveling Ultrasound
Wave. ACS Nano, 2022; DOI: 10.1021/acsnano.1c02302 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/03/220311095306.htm

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