A team of Massachusetts Institute of Technology (MIT) researchers has developed a nanofibrous hydrogel-based material that mimics the structure of a lobster’s underbelly, and is potentially strong and stretchy enough to be used to make replacement tissues such as artificial tendons and ligaments. The researchers ran the material through a battery of stretch and impact tests, and showed that, similar to the lobster underbelly, the synthetic material is remarkably “fatigue resistant,” and able to withstand repeated stretches and strains without tearing. “For a hydrogel material to be a load-bearing artificial tissue, both strength and deformability are required,” explained co-author Shaoting Lin, PhD, a postdoctoral associate at MIT’s department of mechanical engineering. “Our material design could achieve these two properties.”
Lin and colleagues report on their new material in Matter, in a paper titled, “Strong fatigue-resistant nanofibrous hydrogels inspired by lobster underbelly.” The paper’s MIT co-authors include postdocs Jiahua Ni, PhD, and Shaoting Lin, PhD; graduate students Xinyue Liu and Yuchen Sun; professor of aeronautics and astronautics Raul Radovitzky, PhD; professor of chemistry Keith Nelson, PhD; mechanical engineering professor Xuanhe Zhao, PhD; and former research scientist David Veysset, PhD, now at Stanford University; along with Zhao Qin, PhD, assistant professor at Syracuse University, and Alex Hsieh of the Army Research Laboratory.
A lobster’s underbelly is lined with a thin, translucent membrane that is stretchy and surprisingly tough. This marine under-armor, as one team of MIT engineers reported in 2019, is made from the toughest known hydrogel in nature, which also happens to be highly flexible. This combination of strength and stretch helps to protect the lobster as it scrabbles across the seafloor, while also allowing it to flex back and forth to swim.
“The soft membrane in the underbelly of the American lobster is a hydrogel that contains 90 wt% water and exhibits extremely high fracture toughness (i.e., 24.98 MJ/m3) and tensile strength (i.e., 23.36 MPa) under cyclic loading,” the team explained. “Recent studies further revealed that the extraordinary mechanical properties of the lobster underbelly are mainly attributed to its unique multi-layered nanofibrous structure which consists of aligned chitin nanofibers in each layer.”
Back in 2019, Lin and other members of Zhao’s group developed a new kind of fatigue-resistant material made from hydrogel—a gelatin-like class of materials made primarily of water and cross-linked polymers. They fabricated the material from ultrathin fibers of hydrogel, which aligned like many strands of gathered straw when the material was repeatedly stretched. This workout also happened to increase the hydrogel’s fatigue resistance. “At that moment, we had a feeling nanofibers in hydrogels were important, and hoped to manipulate the fibril structures so that we could optimize fatigue resistance,” said Lin.
Nanofibrous hydrogels are found in both animal and plant bodies, and are also seen in engineering applications, the authors noted. “Owing to the merits of high porosity, high water content, and biocompatibility, nanofibrous hydrogels have been explored in diverse applications, including tissue regeneration, ionic skin, hemostatic dressings, cartilage repair, imperceptible textile sensors, printable electrodes for flexible implants, tissue adhesives, and small-scale bio-robots.”
For their newly reported development, the researchers combined a number of techniques to create stronger hydrogel nanofibers. Their process starts with electrospinning, a fiber production technique that uses electric charges to draw ultrathin threads out of polymer solutions. Electrospinning is one of the most widely used methods for fabricating nanofibrous hydrogels. However, as the scientists pointed out, “Existing electrospun nanofibrous hydrogels are typically weak and fragile because of the low strength of nanofibers and the weak interface between nanofibers.” And while scientists have introduced chemical crosslinks to boost the strength and toughness of nanofibrous hydrogels under a single cycle of mechanical load, “such reinforced nanofibrous hydrogels are still susceptible to fatigue failures under multiple cycles of mechanical loads.”
The team’s fabrication method used high-voltage charges to spin nanofibers from a polymer solution, to form a flat film of nanofibers, each measuring about 800 nm—a fraction of the diameter of a human hair. They placed the film in a high-humidity chamber to weld the individual fibers into a sturdy, interconnected network, and then set the film in an incubator to crystallize the individual nanofibers at high temperatures, further strengthening the material.
They tested the film’s fatigue-resistance by placing it in a machine that stretched it repeatedly over tens of thousands of cycles. They also made notches in some films and observed how the cracks propagated as the films were stretched repeatedly. From these tests, they calculated that the nanofibrous films were 50 times more fatigue resistant than the conventional nanofibrous hydrogels.
Around this time, the team also read a study by Ming Guo, PhD, associate professor of mechanical engineering at MIT, who characterized the mechanical properties of a lobster’s underbelly. This protective membrane is made from thin sheets of chitin, a natural, fibrous material that is similar in makeup to the group’s hydrogel nanofibers.
Guo found that a cross-section of the lobster membrane revealed sheets of chitin stacked at 36-degree angles, similar to twisted plywood, or a spiral staircase. This rotating, layered configuration, known as a bouligand structure, enhanced the membrane’s properties of stretch and strength. The investigators noted that while a bouligand structure is common for mineralized natural materials, and has been reproduced to generate synthetic hard materials, “ … engineering bouligand structures in soft hydrogels has been challenging.”
Lin commented, “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.” Ni, Lin, and members of Zhao’s group teamed up with Nelson’s lab and Radovitzky’s group in MIT’s Institute for Soldier Nanotechnologies, and Qin’s lab at Syracuse University, to see if they could reproduce the lobster’s bouligand membrane structure using their synthetic, fatigue-resistant films.
“We prepared aligned nanofibers by electrospinning to mimic the chitin fibers that existed in the lobster underbelly,” Ni said. After electrospinning nanofibrous films, the researchers stacked each of five films in successive, 36-degree angles to form a single bouligand structure, which they then welded and crystallized to fortify the material. The final product measured nine square centimeters and about 30 to 40 microns thick—about the size of a small piece of Scotch tape.
Stretch tests showed that the lobster-inspired material performed similarly to its natural counterpart, and was able to stretch repeatedly while resisting tears and cracks, a fatigue-resistance attributed to the structure’s angled architecture. “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 explained.
The team also subjected the material to microballistic impact tests with an experiment designed by Nelson’s group. They imaged the material as they shot it with microparticles at high velocity, and measured the particles’ speed before and after tearing through the material. The difference in velocity gave them a direct measurement of the material’s impact resistance, or the amount of energy it can absorb, which turned out to be a surprisingly tough 40 kilojoules per kilogram. This number is measured in the hydrated state.
“That means that a 5-mm steel ball launched at 200 meters per second would be arrested by 13 mm of the material,” Veysset said. “It is not as resistant as Kevlar, which would require 1 mm, but the material beats Kevlar in many other categories.”
And while it’s no surprise that the new material isn’t as tough as commercial antiballistic materials, the lobster belly-inspired nanofibrous hydrogel is significantly sturdier than most other nanofibrous hydrogels such as gelatin and synthetic polymers like PVA. The material is also much stretchier than Kevlar. This combination of stretch and strength suggests that, if their fabrication process can be scaled up and more films stacked in bouligand structures, nanofibrous hydrogels may serve as flexible and tough artificial tissues. “Here, we report a bioinspired design of strong and fatigue-resistant nanofibrous hydrogels that can closely mimic the bouligand structure of soft membranes in lobster underbelly,” the team concluded. “In this work, we provide a general strategy to design fatigue-resistant nanofibrous hydrogels by engineering nanofibers and nanocrystalline domains across varying length scales … This work suggests an avenue toward the next generation of nanofibrous hydrogels for diverse emerging applications, including lightweight physical protection, textile electronics, smart clothing, and tissue engineering scaffolds.”
