Shape-shifting liquid metal robots might not be limited to science fiction anymore.

Miniature machines can switch from solid to liquid and back again to squeeze into tight spaces and perform tasks like soldering a circuit board, researchers report January 25 in Matter.

This phase-shifting property, which can be controlled remotely with a magnetic field, is thanks to the metal gallium. Researchers embedded the metal with magnetic particles to direct the metal’s movements with magnets. This new material could help scientists develop soft, flexible robots that can shimmy through narrow passages and be guided externally.
Scientists have been developing magnetically controlled soft robots for years. Most existing materials for these bots are made of either stretchy but solid materials, which can’t pass through the narrowest of spaces, or magnetic liquids, which are fluid but unable to carry heavy objects (SN: 7/18/19).

In the new study, researchers blended both approaches after finding inspiration from nature (SN: 3/3/21). Sea cucumbers, for instance, “can very rapidly and reversibly change their stiffness,” says mechanical engineer Carmel Majidi of Carnegie Mellon University in Pittsburgh. “The challenge for us as engineers is to mimic that in the soft materials systems.”

So the team turned to gallium, a metal that melts at about 30° Celsius — slightly above room temperature. Rather than connecting a heater to a chunk of the metal to change its state, the researchers expose it to a rapidly changing magnetic field to liquefy it. The alternating magnetic field generates electricity within the gallium, causing it to heat up and melt. The material resolidifies when left to cool to room temperature.

Since magnetic particles are sprinkled throughout the gallium, a permanent magnet can drag it around. In solid form, a magnet can move the material at a speed of about 1.5 meters per second. The upgraded gallium can also carry about 10,000 times its weight.

External magnets can still manipulate the liquid form, making it stretch, split and merge. But controlling the fluid’s movement is more challenging, because the particles in the gallium can freely rotate and have unaligned magnetic poles as a result of melting. Because of their various orientations, the particles move in different directions in response to a magnet.

Majidi and colleagues tested their strategy in tiny machines that performed different tasks. In a demonstration straight out of the movie Terminator 2, a toy person escaped a jail cell by melting through the bars and resolidifying in its original form using a mold placed just outside the bars.
On the more practical side, one machine removed a small ball from a model human stomach by melting slightly to wrap itself around the foreign object before exiting the organ. But gallium on its own would turn to goo inside a real human body, since the metal is a liquid at body temperature, about 37° C. A few more metals, such as bismuth and tin, would be added to the gallium in biomedical applications to raise the material’s melting point, the authors say. In another demonstration, the material liquefied and rehardened to solder a circuit board.
Although this phase-shifting material is a big step in the field, questions remain about its biomedical applications, says biomedical engineer Amir Jafari of the University of North Texas in Denton, who was not involved in the work. One big challenge, he says, is precisely controlling magnetic forces inside the human body that are generated from an external device.

“It’s a compelling tool,” says robotics engineer Nicholas Bira of Harvard University, who was also not involved in the study. But, he adds, scientists who study soft robotics are constantly creating new materials.

“The true innovation to come lies in combining these different innovative materials.”

Birds that dive underwater — such as penguins, loons and grebes — may be more likely to go extinct than their nondiving kin, a new study finds.

Many water birds have evolved highly specialized bodies and behaviors that facilitate diving. Now, an analysis of the evolutionary history of more than 700 water bird species shows that once a bird group gains the ability to dive, the change is irreversible. That inflexibility could help explain why diving birds have an elevated extinction rate compared with nondiving birds, researchers report in the Dec. 21 Proceedings of the Royal Society B.
“There are substantial morphological adaptations for diving,” says Catherine Sheard, an evolutionary biologist at the University of Bristol in England, who was not involved with the study. For instance, birds that plunge into the water from the air, such as gannets and some pelicans, may have tweaks to the neck muscles and the bones in the chest.

It’s possible that some diving birds are evolving under an evolutionary “ratchet,” where adaptations to exploit a certain food source or habitat unlock some new opportunities, but also encourage ever more specialized evolutionary tailoring. These birds may become trapped in their ways, increasing their risk of extinction. That’s especially true if their habitat rapidly changes in some negative way, possibly because of human-caused climate change (SN: 1/16/20).

Evolutionary biologists Josh Tyler and Jane Younger investigated the evolution of diving in Aequorlitornithes, a collection of 727 water bird species across 11 bird groups. The team divided species into either nondiving birds, or one of three diving types: foot-propelled pursuit (such as loons and grebes), wing-propelled pursuit (like penguins and auks) and the plunge divers.

Diving has evolved at least 14 separate times in the water birds, but there were no instances where diving birds reverted to a nondiving form, the researchers found.

The scientists also explored the link between diving and the development of new species, or their demise, in various bird lineages. Among 236 diving bird species, 75, or 32 percent, were part of lineages that are experiencing 0.02 more species extinctions per million years than the generation of new species. This elevated extinction rate was more common in the wing-propelled and foot-propelled pursuit divers compared with plunge divers. Bird lineages that don’t dive, on the other hand, generated 0.1 more new species per million years than the rate of species dying out.

“The more specialized you become, the more reliant you are on a particular diet, foraging strategy or environment,” says Tyler, of the University of Bath in England. “The range of environments available for foraging is much larger for the nondiving birds than for the specialist divers, and this may play into their ability to adapt and thrive.”

Within diving bird groups, the less specialized, the better. Take penguins, a group that has become the subject of a fair share of conservation concern (SN: 8/1/18). The researchers point out that gentoo penguins (Pygoscelis papua) — which have a broad diet — have larger population sizes than related chinstrap penguins (P. antarcticus) that eat mostly krill, and may actually be as many as four very recently diverged species.
The International Union for the Conservation of Nature considers both penguin species to be of “least concern” in terms of imminent extinction risk. But chinstrap numbers are declining in some areas, while gentoo population numbers remain generally stable.

If some diving birds are being trapped in their environments by their own adaptations, that doesn’t bode well for their long-term survival, say Tyler and Younger, who is at the University of Tasmania in Hobart.

According to the IUCN, 156 species, or about one-fifth, of the 727 species of water birds are considered vulnerable, endangered or critically endangered. The researchers calculate that of the 75 diving bird species from lineages with heightened extinction rates, 24 species, or nearly one-third, are already listed as threatened.