Scientists Invent Self-Healing Material with Engineered Squid Proteins
New study finds that genetically-engineered squid proteins, found off the coast of Spain, can rapidly self-heal and biodegrade.
What do the U.S. military, SpaceX, and Amazon have in common? They use robots to automate repetitive tasks. Humans are prone to error, after all, and need to eat and sleep. Robots, on the other hand, do not eat or sleep. They are made of thick metal, often with futuristic, brutalist designs. But they are prone to break and tear — especially when they jump 30 feet in the air.
Though robots are traditionally made of hard metal to protect sensitive components, a growing number of engineers are shifting their attention to soft robots, built from flexible materials similar to those found in nature. Soft robots have many advantages compared to their clunky ancestors; they are typically lighter and more adaptable, at least in terms of what materials they can be constructed with. These attributes make soft robots ideal for navigating unpredictable surroundings and for physically interacting with people.
Last year, scientists at the University of Southern California developed an insect-like robot, weighing just 95 milligrams, with wings made from an ultra-light, flexible material. The soft robot, called the Bee+, is “capable of perching, landing, following a path, and avoiding obstacles”, according to an article in the MIT Technology Review. Watch the Bee+ in action.
But what happens when a soft robot like the Bee+, which lacks a tough, metal exterior, crashes and breaks?
Ideally, soft robots would be able to heal themselves. Some progress has been made towards this goal; engineers have created self-healing materials out of cross-linked, rubber-like polymers, for example, and have integrated them into “transparent” electronics, soft robots, and other devices.
Unfortunately, most self-healing materials have serious drawbacks. They are heavy, require a fresh supply of hazardous catalysts to repair, the self-healing takes a long time (a day or more), or the material, after being repaired, loses functionality.
A new study, published in Nature Materials, has solved many of these shortcomings, offering a protein-based biomaterial that is both light and able to self-repair damage in about one second. Inspiration for the new material came from an unexpected place: a squid, captured in the azure waters off the Spanish coast.
“I went fishing close to my hometown and collected some of this interesting material,” said Abdon Pena-Francesch, lead author of the new study, in an email. “Playing around with this material, we soon realized that it has very interesting properties.”
As a post-doctoral fellow at the Max Planck Institute for Intelligent Systems, in Stuttgart, Germany, Pena-Francesch has been working with this “material” for years. Harvested from Loligo vulgaris, the common European squid, the protein is a critical component of the suction cups that squid use to capture prey.
L. vulgaris has habitats throughout the eastern Atlantic, off the coast of Africa, throughout the Mediterranean, and further north, in the frigid waters of the North Sea.
“I went fishing close to my hometown and collected some of this interesting material…”
But harvesting a protein from squid, and implementing it into soft robotics, is not a simple task. First, the team of engineers — hailing from laboratories at both Penn State University and the Max Planck Institute for Intelligent Systems — had to understand how the protein, at the molecular level, achieves its unique, self-healing feature. In other words, which specific amino acid “building blocks” are responsible for its unique property?
To find out, the researchers sequenced material from the ‘suction cups’ of multiple different species, and slowly built a collection of protein sequences to test.
“We were investigating this material and sequenced several species of squids to look into how the material is produced in nature,” says Pena-Francesch. “We then analysed different species and came up with a ‘master sequence’. From there, we developed these materials capable of self-healing with high strength and speed, and integrated them into soft robotic platforms.”
After testing each protein and assessing its properties, like flexibility and strength, the team figured out which “building blocks” in the proteins were responsible for conferring optimal properties. By tweaking and refining what nature had provided, a ‘master sequence’ emerged that checked all of the boxes: it was lightweight, strong, and could heal itself.
They put the engineered material through a battery of tests.
In one test, they coated a thin film with the synthetic proteins, and then scratched the film via laser micro-etching. By heating the protein-coated film, either locally or in a temperature-controlled room set to 50 °C, the researchers were able to completely repair the scratch in two seconds.
In another test, a protein film 50 microns thick (a bit thinner than a human hair) was punctured with a tiny hole. Again, heating of the damaged site — for just one second — was enough to completely repair the material.
In a final test, the researchers cut the protein material, splitting it into two halves, and then healed the cut in about one second. The repaired materials completely “recovered their elastomeric properties, and exhibited stretching deformations greater than 200% strain and post-healing strength of up to 23MPa”. For context, the yield strength of aluminum is 15–20 MPa.
Since the synthetic biomaterial is built from proteins — the fundamental building block for every living organism — it also biodegrades. The researchers demonstrated that, by placing the material in a low pH solution, it can fully degrade in about five minutes. On-demand degradation of biomaterials could be used to remove medical implants, for example, without the need for a retrieval surgery.
But the biomaterial does have its drawbacks. It has to be hydrated to function, reducing its applicability for long-term, outdoor environments. Additionally, there must be a way to heat the site of the damage. Perhaps soft robots, in the future, could travel to a “heating room” autonomously to repair damage after a long day of work.
Future applications for this biosynthetic, self-healing material may come sooner than expected. According to the U.S. Army, which funded the study, the self-healing, biodegradable material can “be used to repair materials that are under continual repetitive movement such as robotic machines, prosthetic legs, ventilators and personal protective equipment like hazmat suits.”
Pena-Francesch, A., Jung, H., Demirel, M.C. and Sitti, M. “Biosynthetic self-healing materials for soft machines” Nature Materials. Published online July 27, 2020.
Previous studies on self-healing, protein-based materials:
“Segmented molecular design of self-healing proteinaceous materials” in Scientific Reports (2015).
“Squid-Inspired Tandem Repeat Proteins: Functional Fibers and Films” in Frontiers in Chemistry (2019). Open Access Review.
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