Before we have self-healing cars or buildings, we need strong materials that can fully self-repair in water-free environments. Self-healing materials work very well if they are soft and wet, but research groups have found that the ability to self-repair diminishes as materials dry out. Scientists at Osaka University are beginning to bridge this gap with rigid materials that can repair 99% of a cut on the surface in semi-dry conditions.
“The combination of physical and chemical self-healing enables materials to exhibit rapid and efficient self-healing even in a dried, hard state,” says senior author Akira Harada, a supramolecular polymer chemist at Osaka University. “Only a small amount of water vapor is needed to facilitate self-healing in the dried film state. In other words, water serves as a non-toxic glue in the self-healing process,” adds co-author Yoshinori Takashima, an associate professor at Osaka University.
Material engineers use several strategies to generate self-healing materials. They can physically embed the material with microcapsules or pathways filled with healing agents or build the material by using molecules, such as polyrotaxane, that change shape in response to damage—also called stress relaxation. Chemical self-healing materials use reversible bonds ranging from reversible chemical reactions to intermolecular interactions such as hydrogen bonding.
Harada’s lab combined physical and chemical self-healing mechanisms in their materials by using polyrotaxane as a backbone structure cross-linked by reversible interactions, in this case between boronic acid and diols. The polyrotaxane structure enables stress relaxation in recovery from a shallow dent, and the reversible nature of the bonds enables chemical self-healing from a deep cut. The combined approach allowed the materials to recover up to 80% of their strength within 10 minutes (without the combination, the materials could repair only up to 30% of their strength after an hour).
Someday, chemically protective suits made of fabric coated in self-healing, thin films may prevent farmers from exposure to organophosphate pesticides, soldiers from chemical or biological attacks in the field and factory workers from accidental releases of toxic materials, according to a team of researchers.
“Fashion designers use natural fibers made of proteins like wool or silk that are expensive and they are not self-healing,” said Melik C. Demirel, professor of engineering science and mechanics. “We were looking for a way to make fabrics self-healing using conventional textiles. So we came up with this coating technology.”
The procedure is simple. The material to be coated is dipped in a series of liquids to create layers of material to form a self-healing, polyelectrolyte layer-by-layer coating.
This coating is deposited “under ambient conditions in safe solvents, such as water, at low cost using simple equipment amenable to scale-up,” the researchers report today (July 25) online in ACS Applied Materials & Interfaces.
Polyelectrolyte coatings are made up of positively and negatively charged polymers, in this case polymers like those in squid ring teeth proteins.
“We currently dip the whole garment to create the advanced material,” said Demirel, who is also a member of the Huck Institutes of the Life Sciences. “But we could do the threads first, before manufacturing if we wanted to.”
A drop of water self-heals a multiphase polymer derived from the genetic code of squid ring teeth, which may someday extend the life of medical implants, fiber-optic cables and other hard to repair in place objects, according to an international team of researchers.
“What’s unique about this plastic is the ability to stick itself back together with a drop of water,” said Melik Demirel, professor of engineering science and mechanics, Penn State. “There are other materials that are self healing, but not with water.”
Demirel and his team looked at the ring teeth of squid collected around the world — in the Mediterranean, Atlantic, near Hawaii, Argentina and the Sea of Japan — and found that proteins with self-healing properties are ubiquitous. However, as they note in a recent issue of Scientific Reports, “the yield of this proteinaceous material from natural sources is low (about 1 gram of squid ring teeth protein from 5 kilograms of squid) and the composition of native material varies between squid species.”
So as not to deplete squid populations, and to have a uniform material, the researchers used biotechnology to create the proteins in bacteria. The polymer can then either be molded using heat or cast by solvent evaporation.
Read more: Water heals a bioplastic
What if mending a ripped garment, or repairing a leaky storage container, was as easy as shining a light on the damage?
We’re not there yet, but such materials could be possible in the future—researchers have now demonstrated a new way to produce light-healed polymers. In the April 21 issue of Nature, a group from Case Western Reserve University, the U.S. Army Research Laboratory and the University of Fribourg in Switzerland reported the creation of polymers that heal their own wounds under brief exposures to ultraviolet light. (Scientific American is part of Nature Publishing Group.)
Many materials can self-repair with the help of heat, which liquefies the material and allows it to fill cuts, cracks, or gaps. The light-healed polymers work in much the same way: ultraviolet light excites the material, causing it to heat up and temporarily liquefy. When the light source is switched off, the polymer cools and resolidifies, having filled its cracks and gashes.
“We’ve shown that cuts or scratches in such materials can be healed by exposure to lamps—simple lamps such as those dentists use, for example, to cure fillings,” study co-author Christoph Weder of the University of Fribourg said in an April 19 teleconference. Some of Weder’s group’s experimental materials, which include metal ions such as zinc or lanthanum in a rubbery polymer, were able to regain their original toughness after light-induced repairs.
The polymers are composed of small building blocks assembled “to basically mimic the molecular architecture of normal polymers,” Weder explained. “We have small building blocks with sticky end groups, and those sticky end groups are kind of glued together. We heal these materials by exposure to light, and what light does is it unglues these sticky end groups and liquefies the material.”
The healing process is relatively quick—two exposures of 30 seconds each did the trick for one experimental polymer, which quickly heated up to 220 degrees Celsius and liquefied. A 2009 study from another group reported a similar self-repair effect for polyurethanes, but that effect required much longer UV exposures. Weder said the researchers were now investigating how to get the same results from other wavelengths of light—for instance, a polymer that repairs itself under illumination by blue visible light.