A tankful of wriggling proto-fish could one day offer a novel kind of strong, flexible material for buttressing bulletproof vests and reinforcing lightweight automobile parts. Jawless, spineless hagfish might seem an unlikely source for such advances. But the ancient animals, it turns out, exude a slime with extraordinary properties that just might spawn a new class of earth-friendly materials.

A proto-fish with a knack for exuding an extraordinary slime may one day provide the basis for earth-friendly materials. Image courtesy of Andra Zommers and Douglas Fudge.

At least, that’s the great hope of Douglas Fudge, a biologist at the University of Guelph in Canada. Fudge keeps 50–100 hagfish in chilly seawater tanks to harvest their slime. What makes that slime so extraordinary are its peculiar threads, which are both remarkably strong and bendy. Adapting those threads for human use is one example of biomimetic engineering in which scientists seek to develop materials inspired by nature: gecko foot-inspired glues, for example, or buildings based on heat flow in termite mounds.

Ancient Abilities Although they are considered vertebrates, hagfish barely make the cut because they lack true bony vertebrae. Instead, they have a rod-like notochord made of cartilage. Although fossil records have not revealed their exact lineage, hagfish might have diverged from other vertebrates early in the evolution of fishes, about 500 million years ago. Regardless, they seem to have retained much of their primordial form, even as they boast some rather impressive biology. “The slime, in and of itself, is incredibly unique in the animal kingdom,” says Timothy Winegard, a former Master’s student of Fudge’s and now the manager of the Algonquin Wildlife Research Station in Ontario, Canada. When provoked—by a predator or competitor—the hagfish might release just 90 milligrams of milky, concentrated “pre-slime” (what researchers call “exudate”) from glands in its skin. That material combines with seawater to produce nearly a liter of watery slime (1), which scientists believe clogs the gills of any would-be hagfish-eaters. Just how the hagfish makes the slime ingredients—and how it exudes them in less than a second—have been major questions in Fudge’s laboratory. But he is also spinning his own hagfish threads, which he believes might someday replace petroleum-based fibers, such as Kevlar. In principle, hagfish fibers could be easier to produce on a large scale, compared with another prime candidate for biomimetic fibers, artificial spider silk. Genetically engineering bacteria to make large quantities of big, repetitive spider silk proteins has proven difficult. Plus, the natural silk gets much of its strength as it’s shaped by the spider’s spinnerets, fine for a spider but tricky to replicate. Fudge thinks hagfish have an advantage, and it starts with the makeup of their slimy secretion.

Special Ingredients Hagfish slime is like no other biological ooze. The hagfish makes it with two different ingredients: the threads and mucin vesicles. Mucin-coated threads create a 3D sieve: a network of fibers that doesn’t hold water like a gel does, but simply slows it down. It’s squishy and slimy. But if you stretch it, you feel strong fibers pulling back. And if you hold it up for a few minutes, much of the water drains out, leaving behind a handful of slimy fibers, each one to three microns in diameter. Those fibers are bundled intermediate filaments, one of the three types of cytoskeleton in cells. The rubber band-like filaments not only support a cell’s architecture, but also combine with other materials to make the hard keratin in hair and fingernails. The hagfish threads possess another intriguing property: pull them about 33% longer than their original length and they still remain soft and stretchy. But more pulling causes the flexible α-helix–shaped proteins in the filaments to break open and combine with neighboring proteins, creating another shape called a β-sheet. This structure imparts extra strength and stiffness to the thread (2). Once this transition begins, the thread can be stretched farther, to more than triple its original length. It breaks at a stress of around 700 megaPascals, according to Fudge’s measurements. That’s compared with 1,000 megaPascals for spider silk, which also gains strength from β-shaped proteins. The secret to the hagfish’s quickly deployed, untangled slime fibers lies within its slime gland cells, as Winegard discovered from electron microscopy studies of cells at different stages of maturity. The thread starts out as a thin, disorganized fiber in a cell with a large nucleus in the middle. As the thread grows, it butts up against the cell membrane and naturally starts coiling around the nucleus, like a garden hose wrapping around a ball. The nucleus shrinks, adopts a conical shape, and migrates to one end of the cell. There, it acts as a spindle onto which about 500 neat loops of threads eventually nest together (3). “Tim’s work was a big step forward for us,” says Fudge. “It gave us a glimpse into the precise organization.” Timothy Winegard, a former student in the laboratory of Douglas Fudge, displays the special, robust threads that make up hagfish slime. Image courtesy of Dean Palmer (photographer).