A self-healing material that can sense when there’s trouble

I’m not ashamed to admit this, I loved watching Terminator. The cyborg assassin (a.k.a Arnold Schwarzenegger) always recovered from damage, no matter what happened. It turns out, the ability for material to really do this, isn’t that far-fetched after all.

Engineer Henry Sodano brings a little of James Cameron’s science fiction movie into his lab at Arizona State University.

Sodano told me about how his self-healing material can sense when there’s trouble.

Essentially, Sodano thinks his polymer should be as smart as our bones. The material must know when it has a problem, know how to stop the crack from causing more damage and be able to fix it.

It’s the sensors embedded into the polymer material and external computer system that give the self-healing material the brains to know what’s going on.

Sodano talked to SmartPlanet about his self-healing polymer.

SmartPlanet: So what exactly does your material do?

HS: When a crack forms in the material, it can recover the plastic deformation and heal itself. It does that without anyone in the loop.

I am working with shape memory polymers, which have the ability to return to the geometry they are initially cast into. In the particular system, I have devised a network of fiber optic cables which are distributed throughout the material. They are used to sense damage and deliver stimulus to allow it to adapt and heal. For instance, as the damage propagates through the material, the fiber optics fractures allowing the light to be emitted into the polymer directly at the sight of damage. The light gets absorbed by the polymer and creates heat which causes a local reduction of the polymer stiffness causing the cracks to blunt and stop propagating. The thermal energy is then used to induce the shape memory effect to recover the original shape and properties of the material.

SmartPlanet: This material seems cool, but what can it be used for?

HS: I generally perceive its integration into fiber reinforced composites, typically used in aerospace structures, wind turbines and satellites. It can be used essentially in a diverse set of applications. The material without my feedback system is brittle, so it there’s a crack moving through, the material readily breaks.

The material essentially acts like bone which can sense a fracture, adapts to try to stop its propagation through a range of mechanisms and then heals.

That’s the terminator aspect. He was able to reform his body. This polymer can return to its original geometry. When we take away the heat from it, it will be as strong as it was originally.

It is a complex series of events: sense the crack, stop its propagation, recover any deformation  and reload it. The material has 96 percent of its original strength.

SmartPlanet: Can you talk about some of the possible applications?

HS: It can be used in satellites or aircrafts. If a crack starts moving through material, you want to stop it immediately. Or else some accident might occur. We’d like to stop propagation of damage when you don’t have access. It can also be used to fix civil infrastructure and be used in prosthetics so a simple crack won’t result in another surgery.

These fiber optic cables run through the polymer. One of the biggest challenges is how do you identify where the damage is? You might have a small crack. It’s smaller than a dime in a structure the size of an aircraft. How do you identify where the crack is without a human or prior knowledge? How do you find the damage? What do you use for regeneration?

We solved that through this fiber optic network. It needs some interface to perform the sensing [so there's an external computer at work].

SmartPlanet: Aren’t other people trying to make self-healing material too?

HS: Self-healing materials have been explored. Ours is the first one that has a sensing component in it thus making it a closed loop system. It can actively heal damage, instead of passively healing it. Existing materials uses a similar concept to epoxy. I’m sure you are familiar with epoxy, mix part A and B such that it polymerizes and acts as glue.

In prior forms, as damage passes through the materials it cause part A and B to come into contact thus reacting and essentially gluing the material back together.

[This technique is] not very advanced. There’s and uncured polymer in the material, so it leads to a reduced of the virgin material’s strength. In our material, we sense where the damage is, then apply controlled levels of energy to heal the damage. There’s a computer doing the brain work.

SmartPlanet: What’s the importance of this research?

HS: The failure of structural materials is a huge problem. We couple structural health monitoring with self-healing. No one has combined the two before. We can sense damage and then respond to it. It only takes seconds to stop the damage.

SmartPlanet: Who funded the research?

HS: The research was funded by the National Science Foundation. There’s a lot of work to do to find out how we can optimally do this. How do you utilize the system? In terms of demonstrating that it can be used, the material is fairly well demonstrated now. However, the fundamental mechanics of how it returns its strength is less known. It makes sense why it works, although some of the healing mechanisms are a little less known.

We don’t actually recreate the polymer bonds that were broken. We believe we are changing the shape of the crack. What happens is when you have a crack, it’s very sharp. It has a lot of strain energy at its tips. It tries to fracture the next molecule. If it’s a blunt crack, the energy is distributed over a larger area. We think we change the geometry of the tip. This modifies the energy of the tip. But we don’t know the exact mechanism and still need to investigate more.