Small, autonomous robots that can access cramped environments could help with future search-and-rescue operations and inspecting infrastructure details that are difficult to access by people or larger bots. However, the conventional, rigid motors that may robots rely on are difficult to miniaturize to these scales, because they easily break when made smaller or can no longer overcome friction forces.
Now, researchers have developed a muscle-inspired elasto-electromagnetic system to build insect-sized “soft” robots made of flexible materials. “It became clear that existing soft robotic systems at this scale still lack actuation mechanisms that are both efficient and autonomous,” says Hanqing Jiang, a professor of mechanical engineering at Westlake University in Hangzhou, China. Instead, they “often require harsh stimuli such as high voltage, strong external fields, or intense light that hinder their real-world deployment.”
Muscles function similarly to an actuator, where body parts move through the contraction and relaxation of muscle fibers. When connected to the rest of the body, the brain and other electrical system in the body allow animals to make a range of movements, including movement patterns that generate disproportionately large forces relative to their body mass.
Muscle-Inspired Actuator Technology
The new actuator is made of a flexible silicone polymer called polydimethylsiloxane, a neodymium magnet, and an electrical coil intertwined with soft magnetic iron spheres. The researchers fabricated the actuators using a 2D molding process that can manufacture them at millimeter, centimeter, and decimeter scales. It is also scalable for larger, more powerful soft devices. “We shifted focus from material response to structural design in soft materials and combined it with static magnetic forces to create a novel actuation mechanism,” says Jiang. The researchers published their work in Nature Communications.
The new actuator is able to contract like a muscle using a balance between elastic and magnetic forces. When the actuator contracts, it generates an electrical current to create a Lorentz force between the electrical coil and the neodymium magnet. The actuator then deforms as the iron spheres respond to the increased force, which can be used to provide movement for the robot itself. The flexible polymer ensures that the system can both deform and recover back to its original state when the current is no longer applied.
The system tested by the researchers achieved an output force of 210 newtons per kilogram, a low operational voltage below 4 volts, and is powered by onboard batteries. It can also undergo large deformations, up to a 60 percent contraction ratio. The researchers made it more energy efficient by not requiring continuous power to maintain a stable state when the actuator isn’t moving—a technique similar to how mollusks stay in place using their catch muscles, which can maintain high tension over long periods of time by latching together thick and thin muscle filaments to conserve energy.
Autonomous Insect-Sized Soft Robots
The researchers used the actuators to develop a series of insect-sized soft robots that could exhibit autonomous adaptive crawling, swimming, and jumping movements in a range of environments.
One such series of bug-sized bots was a group of compact soft inchworm crawlers, just 16 by 10 by 10 mm in size and weighing only 1.8 grams. The robots were equipped with a translational joint, a 3.7 V (30 milliampere-hour) lithium-ion battery, and an integrated control circuit. This setup enabled the robots to crawl using sequential contractions and relaxation—much like a caterpillar. Despite its small size, the crawler exhibited an output force of 0.41 N, which is 8 to 45 times as powerful as existing insect-scale soft crawler robots.
This output force enabled the robot to traverse difficult to navigate terrains—including soil, rough stone, PVC, glass, wood, and inclines between 5 and 15 degrees—while keeping a consistent speed. The bug bots were also found to be very resilient to impacts and falling. They suffered no damage and continued to work even after a 30 m drop off the side of a building.
The researchers also developed 14 by 20 by 19 mm legged crawlers, weighing 1.9 g with an output force of 0.48 N, that crawled like an inchworm. These used rotational elasto-electromagnetic joints to move the legs backwards and forwards and weighed just 1.9 g. The researchers also built a 19 by 19 by 11 mm swimming robot that weighed 2.2g with an output force of 0.43 N.
Alongside testing how the bots move on different surfaces, the researchers built a number of obstacle courses for them to navigate while performing sensing operations. The inchworm bot was put into an obstacle course featuring narrow and complex paths and used a humidity sensor to detect sources of moisture. The swimming bots were tested in both the lab and a river. A course was built in the lab where the swimmer had to perform chemical sensing operations in a narrow chamber using an integrated miniature ethanol gas detector.
Jiang says the researchers are now looking at developing sensor-rich robotic swarms capable of distributed detection, decision-making, and collective behavior. “By coordinating many small robots, we aim to create systems that can cover wide areas, adapt to dynamic environments, and respond more intelligently to complex tasks.”
Jiang says they’re also looking into flying and other swimming movements enabled by the elasto-electromagnetic system, including a jellyfish-like soft robot for deep-sea exploration and marine research.
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