Lobster underbellies inspire spiraling material for artificial tendons
A newfound understanding around the intricate architecture of lobster underbellies has provided MIT engineers with a model for a tough and stretchy new hydrogel. The material mimics the spiraling structure of the creature's under-armor that affords it great durability and resistance to tearing, with the creators hopeful it could find use as artificial ligaments and tendons.
The new material is the result of parallel lines of research, one of which centers on the ongoing development of hydrogels under mechanical engineering professor Xuanhe Zhao. Researchers in Zhao's group had been working on fatigue-resistant hydrogels, which are made from water and cross-linked polymers and consist of ultra-fine fibers aligned like a bunch of straw. This enables them to endure repeated bouts of stretching and straining without tearing.
Meanwhile, another group of MIT scientists recently published research describing the mechanical properties of a lobster underbelly. Cross-sections of this protective membrane revealed sheets made of the natural polymer chitin, which were stacked on top of each other at 36-degree angles much like a spiraling staircase. This is referred to as a bouligand structure, and the researchers say it is key to the membrane's natural stretchability and strength.
“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,” says study author Shaoting Lin.
The scientists collaborated to recreate this structure using the fatigue-resistant hydrogels. This involved using electrospinning to create ultrafine threads about 800 nanometers in diameter, which were bunched together to form flat films and welded in a high humidity chamber, and then crystalized in an incubator.
Five of these films were than stacked on top of one another, each at a 36-degree angle to form the spiraling bouligand structure. This stack was again welded and crystalized to strengthen the material and form a hydrogel around the size of a small piece of Scotch tape.
Stretch testing showed the hydrogel to be just as resistant to tears and cracks as the natural lobster underbelly membrane. The scientists also made some notches in the film to see how cracks might propagate as it underwent stretching, with the angled architecture seeming to contain the damage and result in a material 50 times more fatigue-resistant than conventional 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,” Lin explains.
The scientists also conducted impact testing using tiny microparticles, fired at the material at high velocity. This revealed a high impact resistance, or energy absorption, of 40 kilojoules per kilogram of material.
“That means that a 5-millimeter steel ball launched at 200 meters per second would be arrested by 13 millimeters of the material,” says study author David Veysset. “It is not as resistant as Kevlar, which would require 1 millimeter, but the material beats Kevlar in many other categories.”
The stretchiness is one category where this hydrogel outperforms Kevlar. The scientists believe that this, combined with its excellent strength, could one day see it put to use as flexible and durable artificial tissues, like ligaments and tendons. That would require that the fabrication process to be dramatically scaled up, though the team is excited about the possibilities.
“For a hydrogel material to be a load-bearing artificial tissue, both strength and deformability are required,” Lin says. “Our material design could achieve these two properties.”
The research was published in the journal Matter.