Tiny Transformers

Image Courtesy of Court Johnson.

Fans of the cult classic film franchise Terminator remember the iconic scene where the evil robot T-1000 easily passes through a metal grate by partially turning into liquid. Well, machines can do that now, and they don’t have to be evil! Researchers from Sun Yat-Sen University, The Chinese University of Hong Kong, and Carnegie Mellon University have created a robot that can turn from solid into liquid and vice versa, at will. 

The scientists drew inspiration for this innovation from the most mysterious of places: the ocean. The unassuming sea cucumber has a remarkable ability to rapidly change its stiffness to adapt to its environment. To do so, the animal internally manipulates the millions of tiny fibers embedded within its tissues to link together in a tight mesh for hardening, and unlink for softening.

The sea cucumber employs this tactic in many ways, including stiffening to navigate hostile environments that would otherwise pierce and tear soft tissue, and softening to move through obstacles and fit into efficient hiding spots. Scientists were especially inspired by the sea cucumber’s ability to fit through tight spaces and sought to develop this process for use in machinery.

In order to replicate this ability, scientists used magnetoactive liquid-solid phase transitional matter, which is a magnetic substance that can quickly switch between liquid and solid states. Since the process of changing states between solid and liquid is tied to temperature, the scientists had to find a metal that melted and froze at relatively warm temperatures. They chose gallium, which is a nonmagnetic metal that melts at 29.8 degrees Celsius. This means that it is a solid at room temperature and exhibits the strength typical of a solid metal, but it melts when held in your hand for a while. Once they had fast-transition matter, they had to turn it into a responsive machine.

This next step was done by embedding ferromagnetic neodymium-iron-boron microparticles into the internal structure of gallium. These micromagnets were held in fixed positions by the strong solid matrix of gallium so they all synchronized appropriately with the magnetic field. This process produced a gallium alloy that could respond to magnetic fields and enabled the researchers to control its movement. These magnets and the physical properties of gallium contribute to most of the functionality of the shape-shifting robots.

In its solid state, the shape-shifting machine is very responsive to magnets and can easily be controlled by manipulating the magnetic field around it. These properties allow the machine to move through a given path, jump over obstacles and move up to speeds of 1.5 meters per second. When the machine encounters a space too narrow for a solid, it turns into a liquid through internal heating that melts the gallium.

This heating is achieved by manipulating the magnetic field around the machine to cause its micromagnets to form a specific pattern that induces a current within the metal. This current encounters resistance as it flows through the robot, causing it to produce enough heat and raising the temperature to about thirty-five degrees Celsius, which is above the melting point of gallium. This process, known as electromagnetic induction, enabled scientists to dictate when and where the material changed from solid to liquid.

As a liquid, the properties of the alloy notably change. It no longer responds as well to magnets because the solid matrix holding and aligning the micromagnets falls apart during melting. As a result, the micro-magnets respond independently to the magnetic field and to each other, creating many shifting incohesive magnetic alignments within the material, reducing the complexity of the material’s mobility. Nonetheless, while it loses its ability to jump, the liquid still responds enough to magnets that it can split into smaller blobs, elongate, reshape itself, and merge from smaller parts, just like water.

In order to turn back into a solid, the matter simply cools to room temperature and solidifies.  You may wonder why scientists can’t cool the material the same way they heated it up, but cooling using an electric current is difficult unless special thermoelectric materials are used, which would interfere with the functionality of the liquid-solid machine. Nonetheless, senior author and mechanical engineering professor Carmel Majidi of Carnegie Mellon University is working with another group to implement similar functionality, so they may eventually be able to dictate when it solidifies as well.

A shape-shifting machine sounds great for escaping through the bars of a prison cell, but these malleable machines have tangible real-life applications as well. “The medical sector has the greatest potential to benefit from applying this technology,” Majidi said. The scientists demonstrated these applications by using the machine to remove a foreign object from a model stomach. In real life, a person would swallow the machine as a pill, and it would be guided to the foreign object using a magnet. At this point, it would change into a liquid and envelop the object in a process similar to a white blood cell consuming harmful cells. It would then solidify to trap the object and carry it out under the guidance of a magnetic field. Since the body is warm, scientists would add compatible metals such as bismuth or iron to the alloy in order to raise the melting point above the average body temperature. 

The machine could also be used for drug delivery: it could be inserted into the body as a solid containing the medicine to be delivered and guided to a specific location. Once there, it would melt and release the medicine before solidifying and exiting.

Furthermore, the machine could be used in the assembly of circuits by carrying tiny components to specific points in the circuit, changing to liquid around the connectors, and then solidifying to form a firm weld that conducts electricity through the component. In addition, the material can act as a universal screw for construction by pouring the liquid machine into a screw hole and using a magnet to guide and fit it snugly into every crevice, before solidifying and fixing itself as the perfect screw. Moreover, the machines could be used in the remote repair of most engineering structures. Imagine a future where you need only drop a pin-shaped machine inside a malfunctioning computer to allow a hardware specialist working from home to diagnose and repair your device in mere moments. 

These applications are possible because the machine, in its solid state, is designed to carry up to ten thousand times its own weight, which was demonstrated as the machine lifted and supported a two-hundred-gram weight. The ability to bear this weight is more than sufficient when applied to the minuscule scale that the machines are expected to operate on, and more weight can be supported by using swarms of these machines to carry heavier loads through weight distribution.

“The materials required to produce a machine are about as costly as a kitchen magnet,” Majidi said. This affordability has the potential of reducing surgery costs when manufactured and applied en masse. 

With more research being conducted focusing on the development of nanomachines, we can expect even more interesting, quality-of-life improvements through inventions like this one.