Penn engineers have developed a "self-healing" metal. While we're not about to see self-regeneration as impressive as Marvel's Wolverine, the findings could signal a big step in the field of mechanical engineering.
The engineers call their technique "healing" as it shows real similarities to the healing process of bones - our bodies get energy from an external source, i.e. food.
Typically, the repair process for metal involves melting it with welding torches that can reach 6300 °F. The new process, realized by two Penn engineers, allows for metal to be "healed" at room temperature.
The study, conducted by James Pikul, assistant professor in the Department of Mechanical Engineering and Applied Mechanics and Zakaria Hsain, a graduate student in his lab, was published in the journal Advanced Functional Materials.
In some cases, melting metal to repair it isn't a viable option. For example, melting compromises the complex internal structure of metal foams - metals made with internal pockets of air.
While looking for ways to repair these types of metals, Pikul and Hsain drew inspiration from existing “self-healing” materials. These are typically made from relatively soft polymers and plastics.
“The way people do self-healing today is they impregnate these polymers with different chemicals that, when that polymer is ruptured, are released and mix like an epoxy, gluing the material back together,” Pikul said in a Penn Engineering press release.
“That approach works for polymers because polymers can flow and are relatively easy to reshape at room temperature, but that means they have limited strength as a result.”
In order to heal metal foams, Pikul and Hsain started by finding a way for them to “sense” where they had been damaged.
Rather than encapsulating additional chemicals used in the repair, the researchers realized it was possible to use the breaking of a polymer layer as a chemical signal of sorts.
The two researchers used chemical vapor deposition to give each strut of a nickel foam an even coating of Parylene D. Parylene D is a chemically inert, stretchy polymer. The material's damage tolerance is slightly lower than that of nickel, meaning it breaks first when the sample is damaged, revealing the metal under the coating.
Using this as a signal, the researchers were then able to use electroplating to build new nickel struts only on the exposed nickel where needed.
“Unlike polymers, metals don’t flow at room temperature,” Pikul says. “The nice thing with electrochemistry is that metal ions can easily move through the liquid electrolyte. We then use electrochemistry to convert the ions to solid metal. The polymer acts like a lithography mask and only allows the ions to turn into metal where the metal foam was broken."
The team successfully healed three types of damage in their experiments on centimeter-scale samples of their polymer-coated nickel foam. These included samples with cracks, samples that had been pulled apart, and samples that had been cleaved into two pieces.
Imitating self-healing in the body
It can't be said that this method truly shows self-regeneration; it does, after all, require an external power source as well as raw materials. However, Pikul claims that the process is in line with the way that self-healing occurs in the human body.
“I think most people would say bone is a self-healing material,” Pikul explains, “and I think, in practice, our material is very similar to bone. Bone is not fully self-contained either; it needs an energy source and nutrients to heal, both of which come from eating food. In our system, these function similarly to the voltage and electroplating bath.”
As per the Penn Engineering release, Pikul hopes the similarities to biological healing will grow as the research continues.
We might not see X-Men-like abilities anytime soon, but the method shows great promise in repairs for objects like car doors, robotic arms, and even space-station components.