Things break. Such is life. Such is physics. A smart phone is dropped and the screen cracks. The width of a parking space is misjudged and paint is scratched. It's frustrating, frequently expensive, and annoyingly wasteful. Sometimes though, the ramifications of even the smallest fractures are far more serious. A tiny fissure forms in a concrete dam. A sky scraper window cracks and eventually shatters, raining down debris. A microscopic breach in an airplane's fuselage grows and ruptures.
Replacing a cracked tablet screen is one thing. Replacing cracked windows in a high rise is trickier. Identifying one nearly imperceptible fracture in a machine with around six million functional parts is eye-wateringly difficult. Safety standards, rigorous inspections and maintenance save lives, but it would also be nice if broken things could fix themselves.
Fortunately, we are in the midst of a fascinating rise of self-healing materials that promise to do just that.
But what does 'self-healing' mean in this context? The term brings to mind something biological and indeed, self-repair is a hallmark of living things, from DNA and cell wall repair in single-celled organisms all the way to wound and bone repair in our own bodies. Yet neither your windshield nor your smart phone is alive or sentient (well, not yet anyway, we'll set AI aside for the moment).
Some efforts to produce self-healing materials do indeed rely on the use of living microorganisms, such as in the case of self-healing concrete which uses microbes to produce calcite. However, the majority of smart materials involve the use of polymers.
The word polymer is derived from Greek and literally just means 'many parts'. They are essentially large molecules with many repeating smaller molecule units. Natural polymers are all around us, and in us. Plant cellulose is a polymer, so is DNA. The list of synthetic polymers is a long one, too: polystyrene, nylon, PVC, and so on.
When a material breaks, at the molecular level the bonds between some of the molecules break.
The ability of a polymer to self-heal comes down to the nature of those molecular units, and their capacity to reorganize and re-form bonds with one another. When your current phone screen cracks, there's not much energetic encouragement for this to happen. The shape of the molecules are such that they can get tangled, diffuse slowly and don't realign well on their own.
But a self-healing polymer can be tailored to become chemically 'sticky' along break lines. Such is the case here, where polymer strands are linked by many pairs of sulfur atoms forming a tiny bridge (aka a disulfide bond).
The connection between those sulfur atoms is strong, but not as strong as the bonds between atoms in the main strand of each polymer, so when this material breaks, it's the disulfide bonds that break. Importantly, those sulfur atoms are very unhappy going solo and are desperate to get back together. They will quickly re-align with each other.
Polymers can also be designed to respond to a variety of different energetic conditions. Perhaps just a little bit of pressure is needed, such as pressing two broken pieces together. In other cases it might be temperature. NASA has designed a polymer that isn't bullet proof, it's bullet permeable: a bullet will break the polymer but, simultaneously, the temperature of impact causes the polymer to flow and bond, sealing after the bullet passes through. While not ideal for body armor, it's very useful for repairing damage to satellites and spacecraft caused by high-speed debris.
Such smart materials are particularly beneficial for mending the tiny fracture that's hard to find or hard to reach, but other self-healing materials are being designed so that their repair only occurs under extremely specific conditions so that the healing can be controlled.
Some researchers are designing polymers that heal in the presence of certain wavelengths of light. Imagine fixing a paint scratch on your car by simply shining a small UV light on the affected area. The polymers flow into the scratch and bond, leaving no trace of the damage.
Other polymers are being designed to respond to specific electromagnetic fields. This is particularly useful where a polymer isn't made up of the same repeating molecular unit, but instead comprises different segments that each responds to a different electromagnetic field. As such, you can build the material piece by piece by changing which field you use. It sounds painstaking, and probably is, but it could be immensely useful for building nano-circuits.
There are myriad possibilities with self-healing materials, with potential applications in everything from 3D nanostructures to biocompatible body parts to space craft.
Here are just a few recent developments to keep an eye on:
- A polymer for self-healing glass
- Self-healing gold particles
- Self-healing rubber
- Self-healing silicon nanoparticles for longer lasting batteries
- A spray on water-repellant that self-repairs
