Scientists have developed innovative microrobots that operate at the diffraction limit of light, combining programmable nanomagnets, rigid panels, and ultra-flexible hinges. These robots move using magnetic fields, mimicking the movement of a caterpillar and being able to change direction. In addition to locomotion, they also adjust optical networks to manipulate light, making them useful in areas such as microscopy, sensors, and medical imaging.
Researchers have recently developed an innovative microrobot that can move in a controlled manner using magnetic fields. It is so small that it operates at the “diffraction limit,” or at the wavelength of visible light, making it useful for advanced optical applications. Let’s explore how it works.
First, the team fabricated tiny cobalt magnets called nanomagnets, which are about 100 nanometers across (a nanometer is one billionth of a meter). These magnets are specially shaped and arranged in arrays, meaning they are arranged in specific patterns. Each nanomagnet acts as a magnetic dipole, meaning it has a “north” and a “south” that align along its length.
This alignment is determined by the shape of the magnets, and the longer the magnet is relative to its width, the stronger the magnetic field needed to shift it.
These nanomagnets are mounted on small panels of rigid glass. To connect these panels, the researchers used hinges made using a special technique called atomic layer deposition (ALD), which allows for extremely thin and flexible hinges.
Figure 1: (A) Scanning Electron Microscopy (SEM) image of the magnetic microbot. (top right inset) Arrays of low/high aspect ratio (AR) nanomagnets are used to program the microbot. (bottom right inset) Final magnetization of the microbot (bottom left magnification). False-color SEM image of the atomic layer deposition (ALD) hinge.
These hinges are both strong enough to hold the microbot in one piece, but also flexible enough to bend when subjected to magnetic torques. This enables the robot to move and transform.
The microbot’s movement is based on simple concepts inspired by origami paper folding, such as the mountain-valley pattern. When an external magnetic field is applied, the glass panels bend, creating a basic motion.
To make the robot crawl like a caterpillar, the scientists used sinusoidal magnetic fields (which vary in gentle waves) applied at different angles. This causes the robot’s body to alternately stretch and contract, pushing it forward.
This gradual movement, shown in Figure 2A, allows the robot to move forward about half its length with each step. The speed of movement can be adjusted simply by changing the frequency of the magnetic fields.
In addition to crawling, the microrobot can also turn and change direction, as shown in Figure 2C, by adjusting the applied magnetic fields. This makes it highly versatile for navigating different surfaces.
Figure 2: (A) Magnetic microrobot stepping motion controlled by external sinusoidal magnetic torque. (B-C) Caterpillar-like locomotion with a combination of sinusoidal magnetic field in the z-axis (1 Hz) and the x-axis out of phase by 90 degrees. (C) Controlled turning locomotion.
The researchers extended the basic idea of mountain-valley folding to create magnetically adjustable diffraction gratings. Diffraction gratings are structures that scatter light in specific patterns, useful in a variety of optical applications.
In Figure 3A, for example, a diffraction grating was created using rigid panels connected by flexible hinges. When an external magnetic field is applied, the hinges bend, and the panels move, changing the spacing of the lines in the grating. This changes how light interacts with the structure, an effect visible in the observed diffraction patterns.
Figure 3: (Above, left) (A) Diffraction grating with magnetically adjustable periodicity. (B) Schematic illustrating the mountain-valley fold of the grating. (C) Image (left) and diffraction image (right) of the grating before and after actuation.
This technology could be applied to optical metamaterials, materials that control light in novel ways, with potential in areas such as sensors, microscopy, and medical imaging.
The researchers also created more complex versions of these robots, as shown in Figure 4. They built a 50-micrometer-wide structure (a micrometer is one-millionth of a meter) made of 25 tiny, connected panels.
Figure 4: (Above right) (A) Micrograph of optical metamaterial with magnetic structure. (B) Programmed directions of magnetic dipoles. (C) Microrobot actuation with an external out-of-plane magnetic field, increasing from left to right. (D) Microrobot locomotion, including a 90-degree change in direction.
These robots can contract and move using the same principle as magnetic fields. In addition, they can perform more elaborate movements, such as changing direction at 90-degree angles.
These microrobots represent a breakthrough in science because they combine mechanical and optical properties at extremely small scales. They can be used to precisely manipulate light, probe the microscopic world, and even perform sophisticated medical applications. For example, they could be used to create tunable optical elements or to perform high-resolution imaging at submicroscopic scales.
In addition, the researchers hope to expand this technology to create “optical meta-atoms”, even smaller elements that can manipulate light in even more advanced ways. This opens up new possibilities in light science, with implications for spectroscopy, medical imaging, and other areas.
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Magnetically programmed diffractive robotics
CONRAD L. SMART, TANNER G. PEARSON, ZEXI LIANG, MELODY X. LIM, MOHAMED I. ABDELRAHMAN, FRANCESCO MONTICONE,
ITAI COHEN, AND PAUL L. MCEUEN
SCIENCE. 28 Nov 2024. Vol 386, Issue 6725. pp. 1031-1037
DOI: 10.1126/science.adr2177
Abstract:
Microscopic robots with features comparable with the wavelength of light offer new ways of probing the microscopic world and controlling light at the microscale. We introduce a new class of magnetically controlled microscopic robots (microbots) that operate at the visible-light diffraction limit, which we term diffractive robots. We combined nanometer-thick mechanical membranes, programmable nanomagnets, and diffractive optical elements to create untethered microbots small enough to diffract visible light and flexible enough to undergo complex reconfigurations in millitesla-scale magnetic fields. We demonstrated their applications, including subdiffractive imaging by using a variant of structured illumination microscopy, tunable diffractive optical elements for beam steering and focusing, and force sensing with piconewton sensitivity.
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