Bouncing bubbles drive micro-submarines

Powerful tiny things, precisely controllable: Nanotechnologists have developed sophisticated micro-floats that can arrange silicate spheres into letters, push cells into position and even climb stairs. The tiny submarines are driven by the movements of a bubble under the spell of ultrasound waves and steered by magnetic fields. This drive concept could open up new opportunities in medicine, chemistry and nanotechnology, say the scientists.

Delicate tweezers or thin needles are traditionally used when researchers and medical professionals want to manipulate something on a microscale. However, in addition to these techniques, another contact-free process has emerged in recent years: lasers can form optical tweezers and thus enable precise interventions at the microscopic level. This technology was honored with the Nobel Prize in Physics in 2018. But it has disadvantages: lasers can only generate very little power – it is not sufficient for certain mechanical manipulations. Laser light can also damage organic materials.

Under the spell of sound and magnetic force

As an alternative, concepts have been developed in recent years that are based on the movement of tiny objects using ultrasound or magnetic force. So far, the power development and controllability of micro-swimmers based on these techniques have left something to be desired, reports the team of US and Chinese scientists in the journal “Science Advances”. According to them, their new hybrid system now offers more opportunities to manipulate objects at the micro level.

The microswimmers of the scientists are structures that are reminiscent of hollow cartridge cases that are open at the bottom. They consist of a polymer structure produced by 3D printing processes, which is coated with gold. In addition, the capsules have a layer of nickel in the front area, which allows them to be aligned using magnetic fields. The inner surface of the sleeve is chemically treated to repel water. As a result, a tiny air bubble forms in the structure when immersed in water.

Controllable quickly and precisely

This effect becomes the key element of the system, the scientists report: the bladder acts as the motor of the tiny vehicle. To do this, it is resonated by ultrasonic waves, causing it to move up and down quickly inside the capsule. This creates forces that drive the structure forward. This drive system enables the mini-submarines to reach high speeds: they overcome their own length more than 17 times per second. External magnetic fields, to which the nickel element in the capsule reacts, are used for control. “These micro-floats offer new possibilities to manipulate individual particles precisely and three-dimensionally without the need for special sample preparation or further modifications,” says co-author Joseph Wang from the University of California in San Diego.

In order to demonstrate the capabilities of the tiny things, the researchers used them to move individual silicate spheres and body cells in aqueous media. In the tests, they pushed particles around to arrange them into letters within a few minutes. The micro-swimmers are also able to master three-dimensional obstacles, the scientists emphasize: They can be steered up micro-sized stairs. This shows that they can also be used in complex environments.

The team now sees the concept as potential for further developments in the field of nanotechnology. “An important aspect is that ultrasound and magnetism are biocompatible, which makes this micromotor system interesting for use in biological applications,” says co-author Fernando Soto from the University of California at San Diego, In this context, he and his colleagues now want to work on making the material of the micromotors biocompatible: Biodegradable polymers could be used and the nickel could be replaced by less toxic magnetic material, the scientists say.

Source: University of California – San Diego, video: Liqiang Ren and Fernando Soto, professional article: Science Advances, 5: eaax3084

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