Self-healing materials take cue from nature

When a crack breaks the channels, the healing agent flows in and hardens

The development of self-healing materials has surged forward as scientists have taken inspiration from biological systems.

Researchers at the University of Illinois in the US have found a way to pump healing fluids around a material like the circulation of animal's blood.

Materials that could repair themselves as they crack would have uses in civil engineering and construction.

Their results are published in the Journal of the Royal Society Interface.

Self-healing materials have been researched for nearly a decade, with a view to reducing the risks and costs of cracking and damage in a wide range of materials.

Different approaches have been taken to creating such materials, depending on the kind of material that needs to be repaired: metals, plastics, or carbon composites.

These methods include creating materials which have micro-capsules containing a healing agent embedded within them, which are broken open when the material is damaged, releasing the healing fluid that hardens and fills the crack.

While effective, this method and others are limited by the small amount of healing agent that can be contained within the material without weakening it.

But new developments in self-healing technology have been pioneered by Prof Nancy Sottos and her team at the University of Illinois Urbana-Champaign, involving the impregnation of plastics with a fine network of channels, each less than 100 millionths of a metre in diameter, that can be filled with liquid resins.

These "micro-vascular" networks penetrate the material like an animal's circulation system, supplying healing agent to all areas, ready to be released whenever and wherever a crack appears.

Limitations still blight this technology however, as the healing process relies on the slow wicking action and diffusion of the healing agent into a crack.

The researchers have therefore taken another lesson from biology to improve on the self-healing material's performance.

Cracking experiment

"In a biological system, fluids are pumping and flowing," said Prof Sottos, so they have devised a way to actively pump fluids into their micro-vascular networks.

Syringes on the outside of the material put healing fluids under pressure so that when a crack appears, a constant pressure drives the fluid into the cracks.

In the experiments that Prof Sottos' team carried out, two parallel channels are created in a plastic and pumped with a liquid resin and a hardening chemical that triggers the resin to solidify.

The micro-vascular healing system is inspired by the circulation of blood in animals

When a crack forms, both micro-channels are breached and the two liquids are pumped into the damaged area.

The researchers experimented with pumping the liquids in pulses so that first the resin was pushed into the crack, and then the hardener, in repeating cycles.

This, they found, was the most efficient way of filling large cracks and ensuring the widest spread of the healing agents.

"Micro-capsule technology will enable damaged openings around 50-100 [millionths of a metre] to be filled, whereas pumping healing agents through a micro-vascular network can fill major cracks up to a millimetre across," said Prof Sottos.

Double duty

Having demonstrated the improved repair that an actively pressurised system provides, the researchers hope that the technology can be utilised in engineering and construction applications with a little further development.

The method of constructing the materials is already well refined, using 3-D scaffolds of "sacrificial fibres" that mould the network of channels within a synthetic material, that are then destroyed in the final stage of production.

In the experimental work that Prof Sottos and her group have carried out, the pumps have been on the outside of the material, but she explained: "We would like to incorporate pumps into the material itself, perhaps pressure or magnetically driven."

Many large-scale structures where self-healing materials would be most useful, for example in aeroplanes and spacecraft, already have hydraulic systems built into them.

Prof Sottos envisaged these hydraulic systems being harnessed to perform a "double duty" in providing pressure for their self-healing materials.

The team are next looking into how the self-healing system can be integrated seamlessly into large-scale civil infrastructures, and how it can be optimised to provide the most healing potential.