Design strategies and structure simplification methods of self-propelled micro-/nanomotors
2017
Self-propelled micro-/nanomotors (MNMs), which are defined as micro-/nanodevices
capable of converting various energy into autonomous motion, can be
used to pick up, transport, and release various cargoes within a liquid
medium. They have important potential applications, for example, in
drug delivery, biosensors, protein and cell separation, microsurgeries
and environment remediation. This review comprehensively introduces
the design strategies and structures of self-propelled MNMs along
with an outlook for their future development. It starts with the summary
of the propulsion mechanisms of self-propelled MNMs of bubble recoiling
and self-phoresis induced by the asymmetric release of products or
heat. For bubble recoiling propulsion, the continuous momentum change
is caused by a jet of bubbles, while for self-phoresis propulsion,
the MNMs move in a local electric field, concentration gradient, surface
tension gradient, or temperature gradient, etc. After systematically
and in-depth understanding these propulsion mechanisms, it has been
pointed out that the key to design self-propelled MNMs is to construct
an asymmetric field across micro-/ nanoparticles. Following this clue,
the structures evolution and simplification methods of self-propelled
MNMs are reviewed. Janus structures and multilayer-tubular structures,
which are prepared through asymmetric modification process, electrochemical
synthesis, template-assisted method, rolled-up nanotech, etc., have
been firstly proposed to construct asymmetric fields across micro-/nanoparticles
for their propulsion. However, the complicated structure and preparation
process hinder the application of MNMs. Anisotropic single-component
irregular particles, tubes and bowl-like MNMs, which are obtained
by dry spinning method, “growing-bubble”-templated self-assembly,
etc., have been subsequently achieved by utilizing their anomalous
morphology and the nucleation preference of bubble molecules on different
surfaces. This kind of MNMs show somewhat simple structure and can
be easily fabricated, but the motion direction is still difficult
to control because of the Brownian motion. Isotropic semiconducting
MNMs have been recently developed by taking advantage of the limited
light penetration depth in the isotropic photoresponsive particles,
of which the motion is independent of the rotational Brownian motion.
This suggests a remarkable breakthrough in design strategy of MNMs
due to the simple isotropic structure of the motor and the controllability
in both motion direction and speed by light. Besides the evolution
of self-propelled MNMs from the complicated structure to the simplified
one, some remarkable progresses have also been made on the motion
control, functionalization, etc. For example, the speed and state
of MNMs can so far be easily adjusted by the concentration of fuels,
the intensity of external fields, etc. The direction can be controlled
accurately by magnetic field, electric field, light, etc. Numerous
complex tasks can also be performed effectively, such as protein separation,
drug delivery, environmental detection and remediation, etc. Lastly,
an outlook is also provided on the future development and main challenges
of self-propelled MNMs. The future development of MNMs should be focused
on improving energy conversion efficiency through optimization of
structures, exploring new propulsion mechanisms and endowing MNMs
with environmental responses for self-navigation, detection, and specific
operations. In this way, MNMs will approach to the practical applications
in biomedicine, environment treatment, microengineering, etc.
Keywords:
- Correction
- Source
- Cite
- Save
- Machine Reading By IdeaReader
0
References
3
Citations
NaN
KQI