Innovations in Self-Healing Materials

There is a lot of potential benefit for industry to have self-healing materials, particularly because the key benefits at the top of the pyramid would be that it leads to a longer lifecycle of the material. AZoM speaks to Professor Nancy Sottos from the University of Illinois at Urbana-Champaign about her discoveries in the field.

Can you explain the role of the University of Illinois at Urbana-Champaign in BP’s International Centre for Advanced Materials (BP-ICAM)?

The University of Illinois is one of four academic partners that are involved with the BP-ICAM, and the only university based in the US. The BP-ICAM is formed of The University of Manchester, the University of Cambridge, Imperial College London and the University of Illinois, all backed by BP’s expertise and $100m investment. There are a number of faculties at Illinois that are engaged in materials-based research and across them there are a wide variety of BP-ICAM projects. The project that I'm leading is in the area of smart coatings, which is a key focus for Illinois. At the University we do a lot of work on imparting new properties to coatings. There's also a faculty that looks at surfaces and membranes, as well as different characterization tools for materials which is also part of the BP-ICAM.

What was your inspiration to discovering self-healing materials?

There is a research group here at the University of Illinois called the ‘Autonomous Materials System Group’, which is made up of members from different faculties, so it makes for a good interdisciplinary group. In the late 1990s, we started thinking about self-healing materials. But instead of just examining how we could make things more damage tolerant, we were interested how we could actually heal damage that occurs in polymers and polymer-based composites before it becomes significant or catastrophic.

Polymers and polymer-based composites suffer from having small cracks that can lead to trouble down the road, so if you could heal them quickly before they become bigger cracks, or environmental attack can occur in those small cracks, you would be able to extend the lifetime of the polymer or polymer composites.

The inspiration comes partially from biological systems, as all living systems have the ability to heal to a certain extent, and so that's a huge inspiration. How to do that in a synthetic material, of course, was a challenge. Some synthetic materials at that time have always been reported to have some self-healing properties.     

Concrete is the one material that you will read about the most. More than 2000 years ago, Roman concrete, also called opus caementicium, had the ability to self-seal small cracks so that water could not get inside of it and this material has been studied in detail more recently. There have also been other reports of mending in polymers, which you can heat up and weld them back together. So, there was quite a bit of literature on that area in the polymers world. It inspired our thinking about polymers and how in theory you really should be able to heal cracks in a variety of engineering plastics.

What are the basic principles behind how self-healing materials work?

There are three major strategies for self-healing, but I’m going to focus on self-healing polymers, which is where my expertise lies. There are also three basic strategies for creating self-healing in polymeric materials, two of which are strategies where you add in the self-healing functionality in an extrinsic way.

The first strategy involves the addition of small micro-capsules into the polymer. These micro-capsules contain what we call ‘healing agents’, or materials that are chemically active. What's good about this strategy is that you can make the micro-capsules quite small.  They hold this healing chemistry in a very stable state until it's needed. Then, when a crack occurs, the capsules rupture, the healing agents are released where they then polymerize or do something else that can help repair or prevent further crack damage.     

The second strategy is also an extrinsic one and involves the introduction of microvascular networks, so again, taking a bio inspired philosophy and adding a circulatory network to the material system. But instead of having compartmentalized healing agents in capsules, you can have the healing agents in a vascular network that flows throughout the material. The principal is the same. When a crack occurs, you're able to bring healing agents to the crack or the damaged site from the vascularized network.      

The third approach is an intrinsic approach where you alter the molecular structure of the polymer to give it self-healing properties by introducing a type of dynamic bonding. This could be dynamic hydrogen bonding, or it could be dynamic ionic bonding. There's some interesting work on dynamic covalent bonding in vitrimer networks currently, however they all are involved in adding a dynamic bonding character to the polymer, thus creating a new self-healing polymer.

What are the industrial benefits of having self-healing materials?

There are lots of potential benefits for industry, particularly in extending the lifecycle of the materials. That's a key thing for us here at Illinois; we're trying to study how to extend the entire lifecycle of the material. Another key benefit is that it improves safety and reliability of materials, which is an important goal because if something is in service it will last longer and it won't fail.

Which industries do you see benefitting from your discovery?

There are a lot of different industries that use polymers and polymer-based composites. For BP-ICAM, the main industrial application area is the oil and gas sector. We've been working on protective coatings that can be applied to either the external surfaces of facilities, pipelines, or even on internal surfaces of vessels. Typical examples include epoxy-based coatings that prevent corrosion. These are the same coatings that you use to coat bridges, trains, cars and ships.

What our team has done is to impart self-healing properties to these coatings so when they're scratched or damaged or abraded, the self-healing occurs. Therefore there is a minimum amount of bare steel substrate exposed and you prevent or significantly delay corrosion. This greatly extends the lifetime of the coating on the steel and paint, so you don't have to apply it as often. It also contributes to a much safer environment if you don't have a corroding region of a pipe.

Another industry that's looking at this is the aerospace sector. The idea of having self-healing high performance composites and adhesives is also attractive in that industry.

Are there any limitations to self-healing materials as they currently stand?

I think each of the three different approaches that I talked about have certain limitations and also certain advantages. For example, the micro-capsule based systems that we're using for the coatings and other applications are nice because they are easy to incorporate with existing commercial polymers. As you may know, lots of companies and lots of people don't like to change the systems that they're using because they've passed all these qualifications and standards. So just putting another additive into an existing polymer is often very desirable and in the manufacturing process it can be accomplished easily. You also don't really have to change your manufacturing process a lot when you use capsules as they behave like any other particulate additive that goes into polymers.

The limitation, is that once you open a capsule it's a one-time healing process and can't be refilled thus it is unlike a biological system as it can't heal again. Fortunately, damage doesn't always occur in the same exact spot twice and the capsules are located throughout the coating or the structure, but it is still just a one-time response. The vascular networks, were developed to address the issue of having repeatable healing.

Just like a biological system, the vascular system can heal repeatedly, which has numerous advantages. The disadvantage is that it's a little bit harder to manufacture as it is more complex to add a vascular network into a material. The addition of the vascular network adds void space into the material as well. This means you must carefully engineer where you place the vascular networks and how you introduce them, so that you don't reduce the properties of the initial material. If the vascular network is significantly damaged, it can become clogged and therefore unable to access all points in the material; however, there's still much that can be done to optimize the vascular network strategy.

The third strategy uses intrinsic materials. These are very interesting materials, but they have some limitations as they work due to molecular bonding. As a result, the damage must be quite small in order for it to be healed as the two cracked surfaces have to be very close together. This means that in molecular intrinsic systems you're limited to very small cracks within the polymer. It’s also limited to polymers that are soft and have lots of mobility, meaning they have a low glass-transition temperature (Tg). The polymers can sometimes be too soft or not durable enough to be an industrial material for use on a pipeline or a bridge, where they will be subject to environmental attack.

How are you planning to overcome these limitations as you move forward with your research?

The limitations that I’ve mentioned have possible solutions, which are slowly progressing. People are addressing these issues of trying to make the material more durable and possess dynamic bonding. Each system has certain limitations, but I’m certain there are windows where it can be applied. For example, if you have very large damage that you're trying to address, a vascular system would be the way to go. However, if you have very localized damage and you're looking at a biomedical application, the intrinsic healing might would be a good strategy for that type of application.

About Professor Nancy Sottos

Professor Nancy Sottos is the Donald B. Willet Professor of Engineering in the Department of Materials Science and Engineering and the Beckman Institute at the University of Illinois Urbana-Champaign.  Sottos started her career at Illinois in 1991 after earning a Ph.D. from the University of Delaware. Her research interests include self-healing polymers and advanced composites, mechanochemically active polymers, tailored interfaces and novel materials for energy storage. Sottos’ research and teaching awards include the ONR Young Investigator Award, Scientific American's SciAm 50  Award, the Hetényi Best Paper Award in Experimental Mechanics, the M.M. Frocht and B.J. Lazan Awards from the Society for Experimental Mechanics, the Daniel Drucker Eminent Faculty Award, an IChemE Global Research Award and the Society of Engineering Science Medal. She is a Fellow of the Society of Engineering Science and the Society for Experimental Mechanics.