Some natural materials, like spider silk, demonstrate extraordinary tensile strength; others, like elastins (proteins found in the tissue of vertebrates), are so resilient that they can stretch dramatically yet still return to their original size. Few materials, however, boast both properties-great strength and flexibility. That hasn’t stopped materials scientists from looking for such substances, since these materials would be tremendously valuable in everything from artificial ligaments to tires. Now, in an unlikely place (the common marine mussel), researchers have found a threadlike substance that may fit the bill.
J. Herbert Waite, a University of Delaware marine biochemist who is studying the mussel’s so-called byssal threads, says he was drawn to the material in his quest to develop “artificial scaffolding” for use in repairing human tendons and ligaments. Materials strong enough to withstand the strain of a human joint often fail to bounce back to their original size. Waite compares them to the plastic used to hold a six-pack of aluminum cans together. “The material is tough,” he says, “but when you pull that can out, it has no elastic recoil.”
Mussels have long attracted the attention of marine biologists because of the super-strong adhesive they excrete at the end of their thread-like tethers to attach themselves to rocks, docks, and the underbellies of ships. But Waite says that equally impressive is the ability of the fine threads to hold the substantial mollusks while withstanding the constant motion of the tide.
Waite’s graduate student and research team member Kathryn Coyne says that the byssal threads feature “a stiff tether” at the end closest to the organism and a “shock absorber” on its protruding end. What gives the byssal threads their remarkable mix of abilities, she explains, is the gradual transition from relative stiffness to relative elasticity along the length of the thread’s core.
Close inspection of the molecular construction of byssal threads by Waite’s team has revealed a seamless gradient between two types of collagen, a fibrous protein commonly found in bone, cartilage, and connective tissue. High concentrations of the stiffer collagen at the end of the thread closest to the mussel gradually give way to a similar but more elastic collagen, which rises to its peak concentration at the thread’s far end.
According to Coyne, this is the first material known to combine proteins in such a manner. By using this technique in mussels, says Coyne, nature has created a type of collagen (a tough material usually capable of stretching only 10 percent beyond its original length before breaking) that can stretch to 160 percent of its original length while still retaining an overall strength 5 times greater than a person’s Achilles tendon.
Both the protein’s molecular structure and the way the tendon-like material is seamlessly woven to achieve a gradient between stiffness and elasticity are of keen interest to materials designers. But Waite cautions that it is not yet feasible to manufacture materials featuring such gradual transitions from one type of protein to another. And so far the unique properties of the proteins in byssal threads offer no more than tantalizing clues about how uniform materials can be made more elastic without greatly diminishing their tensile strength.
Still, Waite adds, “it’s fun to dream about versatile new materials, from steel-belted radials to shoe soles, that might draw upon byssal threads’ secrets, being both soft and flexible yet tough enough to pound the pavement.” At the very least, he says, the new findings “offer a completely new way of looking at the potential properties of structural collagen-the human body’s most abundant protein.”