Lobster underbellies’ intricate architecture has provided MIT engineers with an ideal model for a strong and stretchy new hydrogel. The material mimics the lobster underbelly’s spiraling structure for its outstanding durability and resistance to tearing. The researchers are confident the hydrogel could be used for artificial tendons and ligaments.
The new material is the outcome of corresponding lines of research, which focuses on the ongoing development of hydrogels led by mechanical engineering professor Xuanhe Zhao. Scientists in Zhao’s group had been developing fatigue-resistant hydrogels made from water and cross-linked polymers and consist of ultra-fine fibers aligned like a bundle of straw. This allows them to endure constant stretching and straining without tearing.
Previously, another group of scientists published a study describing the mechanical properties of a lobster underbelly. Cross-sections of this protective membrane showed polymer chitin sheets stacked on top of each other at 36-degree angles similar to a spiraling staircase. This is known as a bouligand structure, and MIT researchers point out that it’s key to the membrane’s strength and stretchability.
Shaoting Lin, a study author, said:
We learned that this bouligand structure in the lobster underbelly has high mechanical performance, which motivated us to see if we could reproduce such structures in synthetic materials.
The scientists used fatigue-resistant hydrogels to recreate the bouligand structure. This involved using electrospinning to produce ultrafine threads approximately 800 nanometers in diameter. These threads were subsequently assembled to create flat films and welded in a high humidity chamber and then crystallized in an incubator.
Five of these films were fixed on top of one another, each at a 36-degree angle to form the spiraling structure. Finally, this stack was again welded and crystalized to strengthen the material and form the hydrogel.
Stretch tests revealed that the hydrogel has equivalent resistance to tears and cracks as the lobster underbelly membrane. The researchers made some indentations in the film to see how cracks might spread as it experienced stretching. The angled architecture seemed to contain the damage and resulted in a 50-times more fatigue-resistant material than current nanofibrous hydrogels. “Intuitively, once a crack in the material propagates through one layer, it’s impeded by adjacent layers, where fibers are aligned at different angles,” added Lin.
To conduct impact testing, the scientists fired tiny microparticles at the material at high velocity. The results showed that the material has high impact resistance (or energy absorption) of 40 kilojoules per kilogram. “That means that a 5-millimeter steel ball launched at 200 meters per second would be arrested by 13 millimeters of the material. It is not as resistant as Kevlar, which would require 1 millimeter, but the material beats Kevlar in many other categories,” explained study author David Veysset.
One category where this hydrogel outperforms Kevlar is stretchiness. The scientists are confident that when this stretchiness is combined with the materials’ excellent strength, it could be used for flexible and durable artificial tissues, such as tendons and ligaments. “For a hydrogel material to be a load-bearing artificial tissue, both strength and deformability are required. Our material design could achieve these two properties,” said Lin. The findings were published on June 2, 2021, in the journal Matter.
Earlier this year, researchers from Australia’s RMIT University also turned to lobsters for inspiration. The team developed a new 3D printing technique for concrete that makes it more robust by mimicking the curvy patterns found on hardy lobster shells.
