Active optical navigation of individual microswimmers

(Nanowerk Spotlight) Living systems as templates for engineering designs have inspired countless research projects. One area of particular interest to the nanotechnology community is the design and construction of micro- and nanoscale propulsion systems.
The fabrication of artificial micro- and nanomotors is a high priority in the nanotechnology field owing to their great potential for applications that range from targeted drug delivery, precision nanosurgery, on-chip diagnostics and biosensing, pumping of fluids at the micro- and nanoscale to environmental remediation.
These 'microswimmers' propulsion systems are powered by the conversion of external chemical, acoustic, or electromagnetic energy to swimming motion. Recently, fuel-free microswimmers have been developed that are driven by light, magnetic, electric and ultrasonic fields.
In new work coming out of Prof. Yuebing Zheng's research group at the University of Texas at Austin, the team exploited the opto-thermoelectric effect, i.e., electrolyte Seebeck effect or thermoelectricity, to develop a new type of all-optical microswimmers with active navigation functionality.
By generating light-induced thermoelectric fields, this novel type of microrobots can be manipulated purely by light in a fuel-free environment. Furthermore, the demonstration of coordinated rotating and swimming behaviors for active navigation could benefit the development of a new generation of light-driven microswimmers.
The findings have been published in Light: Science & Applications ("Opto-thermoelectric microswimmers").
The type of microswimmers reported in this work consist of individual Janus particles – polystyrene beads half coated with gold nanoparticles – that can be directionally delivered over 22 times of their own diameters in 39 seconds (specifically, a 5 µm microswimmer can directionally transport over 110 µm).
"Previously, active navigation of individual optical swimmers in a fuel-free fluidic environment has been challenging," Zhihan Chen, a co-first author of the paper, tells Nanowerk. "In this work, we have exploited the opto-thermoelectric field to trigger two working states, i.e., swimming and rotating, under different laser inputs."
He elaborates: "The rotating state can be specifically used for swimming direction positioning, since the swimming direction is same as the vector from the particle's center to the gold coating's center. Once the particle is aligned to the target, the swimming direction would be launched for directional swimming."
By developing a home-built feedback control algorithm to coordinate swimming and rotating states of the swimmers based on real-time imaging, precise all-optical navigation and target delivery becomes possible.
Conceptual design for optical driving and steering of opto-thermoelectric microswimmers
Conceptual design for optical driving and steering of opto-thermoelectric microswimmers. a Under light fields, PS/Au Janus particles are set to swim and rotate alternatively to follow a predefined path. b Upon light irradiation on a Janus particle, a temperature gradient ∇T pointing from the PS side to the Au side is generated on the particle surface due to the asymmetric absorption of PS and Au. c Once the Janus particle is dispersed in a 0.2 mM CTAC solution, a thermoelectric field is induced to drive the Janus particle along the temperature gradient. The white "+" symbols indicate the positively charged surface. In b, c, the asymmetric heating and thermoelectric field under a defocused laser beam are shown in the X–Z plane. d Schematic illustration and e asymmetric heating of the Janus particle when set to rotate (as shown by the maroon arrow) in the X–Y plane by another focused laser beam (indicated by the green region surrounded by a dashed circle). In d, e, the defocused laser beam is switched off. (Reprinted under Creative Commons Attribution 4.0 International License) (click on image to enlarge)
To convert the thermal energy into mechanical energy, the researchers dispersed Janus particles in a water solution with a cationic surfactant called cetyltrimenthylammonium chloride (CTAC), where the Janus particles become positively charged due to the adsorption of CTAC surfactant, while also introducing CTAC micelles and Cl-ions into the solution.
Since the Janus particles have asymmetric shapes – one half is polystyrene while the other half is gold – the particles have different optothermal responses under laser illumination, leading to a temperature gradient on the particles' surface and their surroundings.
The temperature gradient redistributes the CTAC micelles and Cl-ions based on thermophoresis, building an electric field around the charged Janus particles. The resultant opto-thermoelectric force plays an essential role in both swimming and rotating states of the particles.
Upon an expanded laser illumination, the optical force could be neglected, and the Janus particles would swim along the optothermally generated fields. When switched to a focused laser beam, stable in-plane rotation of the Janus particles could be maintained owing to the balance among the opto-thermoelectric force, optical force, and Stokes drag force.
"Since directional swimming of Janus particles under an expanded laser was always disturbed by the inherent rotational Brownian motion, we found it necessary to develop a feedback control algorithm to switch the laser inputs, i.e., the swimming or rotating states of Janus particles, based on real-time imaging, which can actively navigate the Janus particles," Chen points out. "Specifically, whenever the Janus particles deviate from the path toward the target, the algorithm will automatically launch the rotating state. Once the Janus particle is realigned toward the target, the swimming state will be activated again. Through repeated switching the states, the Janus particle can be efficiently delivered to the target."
Swimming of 5 µm PS/Au Janus particles in different directions in 0.2 mM CTAC solution and driving the directional swimming of Janus particles within twelve pre-designed angle ranges (at an interval of 30°).
The remote actuation of the microswimmers requires only low-intensity light (∼0.03•mW•µm-2), making this platform suitable for non-invasive therapeutic target delivery: Target drugs or any other type of biomolecules can be loaded on the polystyrene side of the Janus particle since it has negligible thermal effect, which won’t thermally damage the drug. When the Janus particle is delivered to the living target cell, the strong optothermal effect of the gold coating can open a hole in the cell membrane, facilitating the injection of the drug molecules.
The team is confident that it can double the speed of their microswimmers by improving the response time of the imaging camera and laser shutter. Moreover, the control algorithm can be further optimized to enable navigation of multiple particles simultaneously and add non-collision and path optimization functions to the system.
"One challenge for us is how to balance the manipulation efficiency and accuracy," Chen points out. "At present, global fields are usually applied in order to manipulate multiple swimmers simultaneously. However, since all the swimmers are under the same field, the motion behavior of each swimmer cannot be manipulated individually. This makes it so difficult to construct a parallel microswimmer platform for multitasking."
Going forward, the team will make the swimmers adaptive to more complex environments, especially in vivo, and make their control algorithm more intelligent so that they can autonomously change the paths, speeds or working modes according to their surroundings.
Michael Berger By – Michael is author of three books by the Royal Society of Chemistry:
Nano-Society: Pushing the Boundaries of Technology,
Nanotechnology: The Future is Tiny, and
Nanoengineering: The Skills and Tools Making Technology Invisible
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