Ragworm inspires shape-shifting material

CHEMICAL ENGINEERS at MIT have developed a material which can change shape and rigidity depending on the conditions, inspired by the jaw of the ragworm, Nereis virens.

The ragworm’s jaw is made of a soft, protein material, but can change to become as hard as human dentin, which makes up teeth. The team, led by Francisco Martin-Martinez, researched how the change occurs, with a view to creating a bio-inspired, artificial material with the same properties. They believe the new material they have created, which is sensitive to pH and ion concentrations, could be used for example in self-powered soft robotics to control movement or in sensors.

The researchers looked at how the protein material in a ragworm’s jaw changes its form, by first looking at the precise molecular make-up of the material. They found that histidine amino acids in the protein bind to ions in its environment at certain pH levels, which changes the shape of the protein, making it more or less flexible.

Martin-Martinez and the team developed a synthesised protein hydrogel material, similar to the material found in a ragworm’s jaw. When there are zinc ions present, at certain pH levels, metal-coordinated crosslinks are created within the protein, which strengthens the material. The bonds, however, are reversible, and changing the conditions changes the material back to its previous form.

“Changing the pH or changing the ions is like flipping a switch. You switch it on or off, depending on what environment you select, and the hydrogel expands or contracts,” said Martin-Martinez.

The MIT team created a model which could predict how the substance will change in the presence of different pH levels and ion mixtures, which allowed them to further develop and refine the material.

“Its ability to contract and expand makes it especially suited to creating devices that work as muscles for so-called soft robots, which are made of polymers. It could also be used in the development of sensors that do not need to use external power supplies and control devices for complex electronic systems,” said Martín-Martínez.

ACS Nano doi.org/f9qg4k