Scanning electron microscopy images of the inorganic material in the two different states. Image: RIKEN.
Scanning electron microscopy images of the inorganic material in the two different states. Image: RIKEN.

By embedding titanium-based sheets in water, a group led by researchers from the RIKEN Center for Emergent Matter Science (CEMS) in Japan has created an inorganic material that can be converted from a hard gel to soft matter by changing the temperature.

Inorganic life forms often appear in science fiction stories, but in reality, systems that respond to stimuli such as temperature changes are nearly always based on organic materials. Research in the area of 'adaptive materials' has therefore almost exclusively focused on organic substances. There are, however, potential advantages to using inorganic materials such as metals, including better mechanical properties.

With this in mind, the RIKEN-led group decided to try recreating behavior displayed by organic hydrogels, but using inorganic materials. Their inspiration came from an aquatic creature known as a sea cucumber. These fascinating animals are related to starfishes and have the ability to morph their skin from a hard layer to a kind of jelly, allowing them to expel their internal organs – which are eventually regrown – to escape from predators. Chemicals released by the sea cucumber's nervous system trigger a change in the configuration of a protein scaffold, causing the skin to morph from hard to soft.

To replicate this ability inorganically, the researchers experimented with arranging nanosheets – in this case, thin sheets of titanium oxide – in water, such that the nanosheets made up 14% of the resulting material by weight and water made up the remaining 86%.

"The key to whether a material is a soft hydrogel or a harder gel is based on the balance between attractive and repulsive forces among the nanosheets," explained Koki Sano of RIKEN CEMS and first author of a paper on this work in Nature Communications. "If the repulsive forces dominate, it is softer, but if the attractive ones are strong, the sheets become locked into a three-dimensional network, and it can rearrange into a harder gel. By using finely tuned electrostatic repulsion, we tried to make a gel whose properties would change depending on temperature."

The group was ultimately successful in doing this, finding that the material changed from a softer repulsion-dominated state to a harder attraction-dominated state at a temperature of around 55°C. They also found the material could be switched between the two states repeatedly without significant deterioration.

"What was fascinating," Sano adds, "is that this transition process is completed within just two seconds, even though it requires a large structural rearrangement. This transition is accompanied by a 23-fold change in the mechanical elasticity of the gel, reminiscent of sea cucumbers."

To make the material even more useful, the researchers doped it with gold nanoparticles that could convert light into heat. This allowed them to change the structure of the material by simply shining laser light on it to heat it up.

"This is really exciting work as it greatly opens the scope of substance that can be used in next-generation adaptive materials, and may even allow us to create a form of 'inorganic life'," said Yasuhiro Ishida of RIKEN CEMS, one of the corresponding authors of the paper.

This story is adapted from material from RIKEN, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.