Self-Healing Materials Move From Repairing the Microscopic to Bullet Holes

The ability to heal a wound has to be among the more miraculous things the human body does. Think about it: a slip of a paring knife leaves an open bleeding mouth on the palm of your hand. You might make a trip to the emergency room for stitches, but that’s not healing; stitches just serve to facilitate healing and ensure that healing doesn’t go off-track. Your body does the real work.

Let's review. The very first thing after an injury is that the body will send a deluge of platelets to the wound, where they become sticky and adhere, eventually forming a plug that stems the bleeding from the inside. Once the plug is in place, the blood’s clotting factors, various proteins that are always circulating through the body, form fibers that become integrated into a mesh. The mesh then catches red and white blood cells, making it stronger. The net effect is of the blood turning from a liquid into a gel around a sort of lattice or scaffold structure; the new material fills in the wound and the bleeding stops.

More things happen after this first step, which is known as hemostasis. White blood cells haul off invading bacteria, while fibroblasts move in to produce collagen and extracellular matrix (the filler material between cells). Hair follicles produce the first wave of keratinocytes—the outermost skin cells comprising the epidermis—which provide a first sheen of scar tissue. Eventually, those cells will be made by the newly formed epidermis itself. Toward the end, the wound gets visited by a strange sort of cell known as a myofibroblast. Myofibroblasts are related to muscle cells and their job is to line a healing wound and contract, as smooth muscle cells do, shrinking the overall size of said wound by up to 80 percent. Once this work is done, the mytofibroblasts kill themselves via the process of programmed cell death known as apoptosis. At this point, the wound is basically healed and all you had to do was watch.

Imagine if we didn’t have this process, if when we get hurt, we just stay hurt like a crack on a smart-phone screen or gash in a tire. Life on Earth would be considerably more terrifying. Now follow that thought in the opposite direction: imagine if smart-phone screens and tires could heal on their own, just like we do. It’d be a future in which we no longer have to ceaselessly replace the things that we build. The cracked screen doesn’t go in the trash, it becomes once again intact. The space suit tear 230 miles above the planet’s surface? It seals itself up just in time (about 30 seconds, for the record). These are the promises of self-healing materials, a very new subfield of materials science tasked with the development of synthetics that fix themselves just like human tissue, or at least very close to it. It would be a world at least much closer to one without waste.

In 2000, Scott White, an aerospace engineering professor at the University of Illinois, published a study describing a plastic-like polymer that was manufactured with microscopic capsules containing a liquid healing material. When the polymer was ruptured, the capsules ruptured as well, releasing the monomer dicyclopentadiene, which then underwent a chemical reaction with the polymer material itself, binding to it and sealing the wound. 2005 saw a study from a new team, based at the University of Bristol, that replaced the capsules with hollow fibers filled with not just a repair agent, but a fluorescent dye as well. In 2011, White returned with a study shifting the basic capsule idea to electronic circuits, a matter of filling the capsules with liquid metal rather than monomer. This week, White has a study out in the journal Science demonstrating self-healing at never before seen scales, filling gaps up to 35 mm in diameter in as little as 20 minutes.

The problem with self-healing large holes is that the repairing agent is released as a liquid, which gets pulled in unwanted directions by gravity. The bigger the hole, the more time the agent has to be influenced by gravity and drawn away from where it’s needed. It "bleeds out." This is the problem that White et al overcame in this newest round of research, adding a step that makes the process work a bit more like wound healing does in animals. Before the monomer binding/filling agent undergoes its final reaction with the injured polymer—after which it becomes a rigid polymer itself—it reacts with a second agent, also released by the tear. This second agent causes the monomer to gel, with the increased viscosity working against gravity enough to allow the agent to fill a much larger hole. For comparison's sake, the previous largest repair was 100 microns, about 100 times smaller than achieved by White el al.

Co-author Brett Krull, a materials science and engineering graduate student working with White, took a few minutes last week to help explain the process. “We’ve designed a system that autonomously responds to catastrophic damage events. When damage occurs—in our case, an impact event that both punctures/ejects material and creates radiating microcracks similar to a bullet through glass—the damage ruptures vascular networks contained within the material. [This is] similar to veins and arteries in a biological organism.

“Before damage, the intact vascular networks contain two isolated, stable liquids, but as damage occurs the liquids from each network are released into the damage region where they mix and the mixing event initiates chemical reactions,” Krull said. “The two liquids are formulated such that they only contain a portion of the chemistry needed for the gelation and polymerization reactions, meaning they need to mix in order to undergo transitions to gel and polymer stages. The first reaction is a very rapid gelation, similar to a blood clot, which provides a sort of scaffold for further deposition of more liquid.”

The second reaction is slower, and works to covert the get formed by the quick first reaction into a rigid polymer. All of this continues until the puncture is complete “refilled.” The tiny veins and arteries carrying the two repair agents are once again sealed, and the two reactions stop. The result is essentially plastic scar tissue, with impact tests showing the repair achieving about 60 percent of the original, uninjured material. That’s not bad considering a conventional, non-autonomous repair still only achieves about 70 percent.

The next target for the technology is making it scale upwards. More complex vascular networks would mean being able to heal multiple wounds at the same time, while improved chemistry could allow self-healing to take place under a wider range of environmental conditions. "The ultimate goal is to design engineering materials that continuously regenerate and remodel, similar to biological materials like bone so that they never truly age," Krull said. "This would improve safety and extend the lifetime of components far beyond their current limitations."