A new fibre that are both strong as metal and elastic as rubber

Materials World magazine
25 Mar 2019

A new fibre with high elastic properties and metallic strength makes a tough material, which could be used in soft robotics or packaging. Idha Valeur reports.

Merging the elasticity of rubber with the strength of a metal into a single fibre has resulted in the creation of a tough material, which researchers said could be incorporated into soft robotics, packaging materials or next-generation textiles.

North Carolina State University, USA, Department of Chemical and Biomedical Engineering, Professor Michael Dickey, said the material combines ‘the best of both worlds’. A rubber band can stretch over a long distance with minimal force, but a metal requires a lot of force to stretch and will snap before it has been pulled very far.

The interest in tough materials focuses on their ability to dissipate energy before they fail. Dickey told Materials World, the human body is composed of several tough materials, such as skin and cartilage. ‘These materials have to be tough because failure would be disastrous. Energy dissipation mechanisms are also important safety features in more mundane things like car bumpers or safety nets,’ he said.

Consisting of a gallium core with an elastic polymer sheath – a thermoplastic elastomer created by melt spinning – wrapped around, the fibre maintains the core strength under stress but as the metal breaks, the sheath embodies the strain between the breaks in the metal and sends the stress back to the core again. This is similar to how human tissue keeps bones in place when they are broken.

‘In addition to being tough, these fibres are electrically conductive to 100% strain, useful for sensors, heaters and also electronics.
[They] can heal – so they can be used again, and have tuneable mechanical properties.

‘Specifically, they can be rendered to be more like skin in the sense that skin is very stretchy to a point, and then becomes stiff when stretched to its limit. These fibres can be tuned to have this same property, which is called J-shape behaviour because of the shape of
the stress-strain curve.’

When asked why gallium was the preferred metal, Dickey said, ‘we chose it because it is a material we use routinely in our laboratory. It is easy to form the samples by injecting the metal into the hollow fibres because gallium has a low melting point. The low melting point also made it easy to ‘heal’ the broken portions of the core of the fibre. Gallium is also relatively soft, so it required less force to break relative to most metals. It will be interesting to try this with other core materials.’

According to Dickey, using metals other than gallium should be possible. To make it work, the team predicts the force required to break the shell must be greater than the force needed to break the core.

‘Thus, if a stronger metal like copper is used, for example, the metal will need to have a smaller diameter or a stronger shell material to ensure the metal core will break before the surrounding shell,’ he said.

The team intends to try other low melting point alloys, with a slightly higher melting point than gallium to make the core more thermally stable. ‘Alloys based on bismuth tend to have melting points that are above that of gallium, but still relatively low compared to other metals,’ Dickey said.

Tough as skin

The desire to create a material as tough as skin is increasing, as a need to have a material which is able to deform with ease up to a certain point, before stopping without failing. ‘For example, there are emerging types of electronics that are soft and stretchable, but nearly all of them fail if they are stretched too far,’ Dickey said.

‘Likewise, there are new types of robots being explored that are built entirely from soft materials to mimic creatures like the octopus. Doing so provides greater degrees of freedom of motion and dexterity, as well as the ability to safely interact with humans. Making materials that go into these structures more robust could lead to robotics that are less prone to fail.’

When developing the tough fibre, the team was inspired by two fields – new molecular approaches for making tough materials, and mechanical metamaterials that have new properties resulting from the interplay of their geometric structure. ‘These types of metamaterials are interesting because they have properties that are not typically found in bulk materials,’ Dickey said. ‘My hope is that the concept of using the interplay between the materials in a core and shell structure may inspire other types of fibres or materials with new properties.’

Today, the development is at a proof-of-concept stage.

The paper, Toughening stretchable fibres via serial fracturing of a metallic core, published in Science Advances can be read here: bit.ly/2UlUI2T