Scientists at ETH Zurich have developed new forms of tiny magnetic actuators with new materials and microscopic 3D printing technology.
Research has been ongoing into micrometre-sized actuators, which could eventually make it possible to transport drugs or chemical sensor molecules to specific locations throughout the human body. (Remember the sci-fi movie Fantastic Voyage?)
Development of such micro-devices has taken a decisive step forward with a new production technology and new materials that have made it possible to manufacture tiny actuators in any form and optimise them for future applications. These lengthened actuators elements, which are able to travel through liquids, have a helical shape and are magnetic.
They are driven by an external rotating magnetic field; they align themselves along the magnetic field lines and rotate about their longitudinal axis. Due to their helical shape, they are able to swim forward through liquids.
“Previously, these elements wobbled as they moved forward, and they were less efficient because their magnetic properties were not ideal”, doctoral student Christian Peters says. “We have now developed a material and a fabrication technique with which we can adjust the magnetic properties independent of the object’s geometry.”
3D Printing at Microscopic Levels
The scientists employed a light-sensitive, bio-compatible epoxy resin, which included magnetic nanoparticles.
In the first phase of the curing stage, they exposed a thin layer of this material to a magnetic field. This field magnetised the nanoparticles, leading to a particle re-arrangement in form of parallel lines. The orientation of these lines establishes the specific magnetic properties of the material.
Then researchers fabricated the tiny elongated structures out of the modified epoxy film using two-photon polymerisation.
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 enabled the researchers to manufacture helical structures 60 micrometres in length and nine micrometres in diameter, and with a magnetisation perpendicular to the longitudinal axis.
A conventional manufacturing method would not have allowed the production of an object with such magnetic properties, as the preferred magnetisation is usually in the direction of the longitudinal axis of an object, like a compass needle. The new actuators can be precisely controlled, they swim nearly four times as fast as previous elements, and do not wobble.
If similar micro-elements are to someday carry medications or chemical sensor molecules to specific locations in the body, the actuators will need to be coated with the corresponding molecules. Not only that, but the larger the element’s surface, the larger the quantity of materials that can be transported.
The researchers showed the theoretical feasibility for coating 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.”
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 that swim as fast as corkscrew-shaped actuators The new shapes differ from the latter in that their surface is two to four times larger.
Peters C, Ergeneman O, Wendel García PD, Müller M, Pané S, Nelson BJ, Hierold C:
Superparamagnetic Twist-Type Actuators with Shape-Independent Magnetic Properties and Surface Functionalization for Advanced Biomedical Applications.
Advanced Functional Materials 2014. 24: 5269-5276, doi:10.1002/adfm.201400596
Image Credit: Peters C et al. Advanced Functional Materials 2014, reprinted with permission of Wiley