Tiny magnetic actuators can swim through liquid, and scientists would like to use them one day to deliver drugs or sensors inside the human body.
The helix-shaped actuators are driven by an external rotating magnetic field; they align themselves along the magnetic field lines and rotate about their longitudinal axis.
When applying conventional fabrication techniques, the magnetic properties of these micro-objects depend on the shape of the devices themselves. This restriction made it difficult for researchers to develop actuators with precise control and directional stability, says Christian Peters, a doctoral student at ETH Zurich.
“Previously, these elements wobbled as they moved forward, and they were less efficient because their magnetic properties were not ideal. We have now developed a material and a fabrication technique with which we can adjust the magnetic properties independent of the object’s geometry,” explains Peters.
Microscopic 3D printer
Peters and colleagues utilized a light-sensitive, bio-compatible epoxy resin, in which they incorporated magnetic nanoparticles. In the first part of the curing stage, they exposed a thin layer of this material to a magnetic field.
This field magnetized the nanoparticles, leading to a particle re-arrangement in form of parallel lines. The orientation of these lines determines the magnetic properties of the material.
The researchers then manufactured the tiny elongated structures out of the modified epoxy film via two-photon polymerization.
This technique is similar to a microscopic 3D printer: a laser beam is moved in a computer-controlled, three-dimensional manner within the epoxy resin layer, thus curing the resin locally. Uncured areas can then be washed away with a solvent.
This technique allowed the researchers to manufacture helical structures 60 micrometers in length and nine micrometers in diameter, and with a magnetization perpendicular to the longitudinal axis—something not possible with a conventional manufacturing method.
The new actuators can be controlled precisely, they swim nearly four times as fast as previous elements, and they do not wobble.
Spirals and double-twists
Previous micro-actuators usually took the shape of a corkscrew (helix), but thanks to the microscopic 3D fabrication technology the ETH scientists were able to produce modified shapes.
In the study, published in Advanced Functional Materials, they fabricated structures similar to spiral-shaped, twisted strips, and double-twisted wires. Tests show that these forms swim as fast as corkscrew-shaped actuators, but the new shapes differ from the latter in that their surface is two to four times larger.
“This makes the actuators more interesting for certain applications,” says Salvador Pané, research associate in the group led by Bradley Nelson, professor of robotics and intelligent systems.
If such elements are to carry medications or chemical sensor molecules to specific locations in the body, the actuators must be coated with the corresponding molecules. And the larger the element’s surface, the larger the quantity of materials that can be transported.
The researchers demonstrated that it is possible, in principle, to coat the structures with interesting biomedical materials by connecting antibodies to the surface of the spiral motors.
“But it is not just about swimming micro robots,” says Peters. “The new technology can also be used when other micro-objects have to be manufactured with specific magnetic properties.”
Source: ETH Zurich