Can we make self-healing materials?

Once they’re made, materials can’t grow, or adapt, or repair themselves if they get damaged. But, perhaps, future materials will be able to heal themselves. Richard Trask from Bristol University is developing self repairing materials and he began by explaining to Izzie Clarke the struggle confronting a budding biomimicist.

Richard - For an engineer or a materials scientist as myself, I think that one of the greatest challenges is trying to have synthetic materials that behave in the same way as a biological material. So if you think about nature, we kind, of grow materials which adapt to the environment they find themselves in. We explore different material systems that branch into, whether it's a tendon or even your eye. We're a long way away from being able to achieve that with our synthetic materials. Presently we would probably take different material systems and bolt them together which is often very immature in terms of its processing and also not very elegant.

Izzie - This is a problem that a lot of researchers find themselves in. In nature whether it's us humans birds or plants, if we get injured or cut, our clever body is able to heal itself. But technology and engineering don't have this luxury. That's something Richard is hoping to solve.

Richard - The research that we've been doing is looking at self-heating materials for aerospace structures. So if you could imagine, you're flying along in an aircraft, something strikes the wing or the fuselage, and you initially, you wouldn't see the damage but internally in these sort of advanced materials there will be a lot of fractures of the matrix material or fracture of the fibers. So the idea for the self healing network is to be able to restore the performance of the internal structure. We would do this by looking at vascular networks and embedding them within the structure. So a vascular network is something similar to the veins and arteries that you and I have within within the human body, and equally the vascular networks that you see within plant-based systems as well, where the sole purpose is to move a fluid to a damage event to allow the damage to then be healed for the structure to the back to 100 percent. Aircraft as they are designed today aren't necessarily designed
for any form of damage. So as we stand here, or as we sit here at the moment then you won't find a vascular healing network system on an aircraft structure but it is certainly something that all the aircraft manufacturers are looking at possibly for inclusion in the future.

Izzie - So that's the scenario, if there's an internal break in the aircraft. Something that big players like the European Space Agency have looked into. But what if there's a hole that goes all the way through?

Richard - There are colleagues of mine over at University of Illinois in America who are looking at how you can get a fluid to, sort of, bridge a gap. So if we were to punch a hole through a structure then actually the vascular network could deliver a resin such that it could slowly progress across the surface and then completely fill over that hole to ensure that you had continuity of the material.

Izzie - Let's take a look at our own vascular system. Thanks to our heart, blood is pumped all around our body and researchers are exploring a similar system for these aircraft. There would be a central location that would be able to detect when any damage occurs. Pump a resin over to that damaged area which would then fill up a hole or a crack in the system. But one advantage of us, over rigid systems, is that we can move and adapt.

Richard - The way that the materials community is moving now is it's looking at how a material system could react to the environment that it finds itself in so that it could actually remodel. So at the moment there's a great interest in the world of 4D printing. So this is taking 3D printing and introducing a smart material that after it's been printed, can react to an environment to change its shape or to change its properties.

Izzie - Now some of us have just about wrapped our heads around 3D printing: The ability to print 3D structures. So what's 4D printing?

Richard - 4D printing is, in essence, you've taken a smart material, so this smart material could be a shape memory polymer or a shape memory alloy or even something that's sort of a little bit softer which is sort of a hydrogel based system. But the idea is that you would introduce them by using a 3D printer. So there are some changes that you will have to do to the printing process, but once you place these materials into your architecture you then have the ability afterwards to either use hydration or dehydration or temperature to actually trigger the material to change itself. So the architecture that you print will then change and evolve according to the way these materials change and evolve.

Izzie - And using this process you can add or remove water, change the temperature of the material. And with that it will move or change shape. But why do we need this technology in the first place?

Richard - If you think about what we might need for the human body, then we would like some soft structures that could basically go through keyhole surgery and then expand and fill a void or a cavity inside us to, sort of, repair it. So that in some ways is similar to what you would have with a hip replacement. But at the moment that's done with a very hard metallic structure that pushed through and, sort of fused it into your bone. But 4D materials as a soft construct could actually move in and permeate into the bone structure.

Another example would be how it would be used in soft robotics. So again thinking about how the future of soft robotics are moving away from these articulated joints. We could use soft materials that could then adapt and remodel themselves depending on where you want them to actually fold and bend. With us humans you're looking at specific joints, which is the wrist and the elbow, but in the future perhaps robotics wants the ability to have a flexible joint. As you can imagine it could actually be something like an octopus, it has the ability to sort of change where it's folding and where its joints are so 4D printing would allow you to make a construct where you could actually tailor and design the joint to occur in any location and then it could actually move or remodel itself to form a joint in a different area. So it could be applied in the field of soft robotics.