This past June, the Fifth International Conference on Self-Healing Materials took place in Durham, North Carolina. The conference attracts a diverse group of top scientists and engineers from around the world to discuss advances in this fascinating field that are based on the healing properties of the human body. Since its first edition in 2007, the mission of this conference has been to provide a high-level international platform for this rapidly developing field. The developments in self-healing materials are such that many are starting to take notice, due to the impact that this would have not only in the industry, but for personal items such as sports equipment, cell phones and even nail polish.
“Self-healing materials” are materials that have the structurally incorporated ability to repair damage. A material is only considered “self-healing” when there is no direction from a human. In the early 2000s, scientists began developing materials that repaired failures and stopped them from worsening. Today, self-healing materials include those with embedded healing agents, those with an internal circulation system similar to blood, shape-memory materials, and reversible polymers.
A professor in the Netherlands, Henk Jonkers, has patented a concrete mixture that contains bacteria as a self-healing agent to repair microcracks. According to a June 5 article in the Smithsonian.com (“With This Self-Healing Concrete, Buildings Repair Themselves”), when concrete cracks, air and moisture trigger the bacteria to “munch” on the calcium lactate, converting it to calcite, an ingredient in limestone, which would thus seal off the cracks. With this self-healing technology, micro-cracks can be sealed immediately, avoiding leakage and more expensive damage. This bacteria can lie dormant for as long as 200 years, well beyond the lifespan of most modern buildings. According to the article, the technology will hit the market for the first time this year, albeit as three separate products: self-healing concrete, a repair mortar and a liquid repair medium.
Other types of self-repairing concrete are under development:
• Researchers in the UK have developed a material similar to Jonkers’ that they hope could be used to repair roads and other infrastructure. They estimate it could reduce costs by up to 50 percent.
• MIT scientists have been working on a concrete healing system that uses sunlight to activate polymer microcapsules to plug cracks.
• A University of Michigan engineer has invented a concrete that expands and reinforces itself with calcium carbonate. This product actually recovers the original load-bearing capacity of the concrete rather than simply filling in gaps.
Self-healing concrete is not only convenient, but sustainable. Any technology that makes concrete structures longer lasting has the potential to not just to cut costs, but to also reduce our carbon footprint.
Since the cost of the technology is still quite high, it will initially only be viable for projects where leakage and corrosion are particularly troublesome, such as in underground and underwater structures. The price of the calcium lactate needed for the bacteria to produce calcite is part of the problem, but Jonkers and his team are working to create a cheaper, sugar-based alternative. And as demand for the concrete increases, the price should fall.
Other self-healing materials have built-in microcapsules filled with a glue-like chemical that can repair damage. If the material cracks, the capsules break, exposing the healing agent and sealing the crack. It is possible that such a mix will be viable on a large scale within the next five years.
Researchers have actually invented a substance that can be incorporated into airplane wings that allows them to heal themselves, in effect placing a scab over small breaks. The down side is that the capsules must be very small or they weaken the material in which they are embedded, which is obviously counterproductive. The capsule is only good for one use; if the material cracks again, the same capsule cannot repair it.
Looking to the Future
Ultimately, it is not difficult to imagine all kinds of applications for self-healing materials, from bridges and buildings that repair their own cracks to car fenders made from shape-memory polymers that automatically flex back to shape after low-speed collisions. The real necessity is artificial materials that behave like the human body: sensing failure, stopping it from getting worse, and then repairing. One day, we might even have replacement parts for the human body that can heal themselves as well as their natural equivalents. At that point, the science of self-healing will truly have come full circle. Trust scientists to reinvent nature.