A cut above

US researchers borrow from human skin to develop self-healing polymer they hope can eventually be applied to mechanical fatigue in aircraft. Siobhan Wagner reports.

A polymer developed by engineers at the University of Illinois is claimed to be able to mimic human skin by healing itself time after time. Its creators are hoping that the material could eventually be used in aircraft, microelectronics or anywhere polymers are subject to thermal or mechanical fatigue.

One of the project’s researchers, Scott White, an aerospace engineering professor, explained that the materials work by using embedded, 3D micro- vascular networks that emulate biological circulatory systems.

‘In the human body, if you get a cut in the skin, you are able to re-supply the necessary chemical building blocks to heal yourself from the capillary bed that is underneath the surface,’ he said. ‘The networks we have developed work in the same manner. There is an epoxy coating — the epidermis — and a capillary bed underneath, which is the substrate that re-supplies healing agent.’

The circulatory nature of the network allows it to promote healing to a damaged area multiple times.

White’s partner on the project, Nancy Sottos, a materials science and engineering professor, said this latest innovation comes after years of research into self-healing materials.

In 2001, her research team developed a self-healing material that consisted of a micro-encapsulated healing agent and a catalyst distributed throughout a composite matrix. When the material cracked, microcapsules would rupture and release healing agent. This then reacted with the embedded catalyst and polymerised in the crack plate.

‘It works great, but one drawback is once you’ve broken all the capsules open in a particular location, you can’t heal any more,’ she said.

So the researchers took inspiration from the human body to develop a material that can continuously heal faults. To create this, White said they use a special assembly robot that writes with a concentrated polymeric ink, dispensed as a continuous filament, to fabricate a 3D structure, layer by layer.

Once the structure has been produced, it is surrounded with an epoxy resin. After curing, the resin is heated and the ink — which liquefies — is extracted, leaving behind a substrate with a network of interlocking microchannels.

In tests, the coating and substrate are bent until a crack forms in the coating. The crack propagates through the coating until it encounters one of the fluid-filled capillaries at the interface of the coating and substrate. The healing agent moves from the capillary into the crack, where it interacts with catalyst particles embedded in the coating. If the crack re-opens under additional stress, the healing cycle is repeated.

Sottos said in the current system, the healing process stops after seven healing cycles.

‘The seven healing cycles comes from the fact that we’re using a catalyst and then delivering just one component, the healing agent, in the vascular network,’ she said.

‘After we heal it seven times in a row, there is a build-up of healed material in the crack. A bio-analogy would be that basically you are forming almost a scar and you’re losing availability of the catalyst. The new healing agent cannot get to the exposed catalyst any more, which is a necessary element for that thing to work.’

Since the recent unveiling of their materials, the researchers have developed a new ‘two-network’ design that has shown healing cycles between 20 and 30 times.

‘Now, we’ve put two networks in the material,’ she said. ‘The two components — the epoxide or the resin and the hardener — are in separate networks and only meet and mix in the crack.’

There are other improvements the team would like to make. Currently, the material can only heal cracks in the epoxy coating — analogous to small cuts in skin. The next step is to extend the design to where the network can heal lacerations that extend into the material’s substrate.

The way to improve on that, Sottos believes, is to again use nature as a model and make the material’s microvascular system more closely resemble those in the human body.

‘We’re working on designs where the network sort of branches — large and small channels — just like arteries and veins and capillaries,’ she said.

While there is much promise for these microvascular networks with material fatigue, White said he sees the system being used in other ways in the future like self-heating and self-cooling materials.