Like anyone who’s nuts for nuts, Notburga Gierlinger is always frustrated by the two or three uncracked pistachios that inevitably wind up at the bottom of a bag. Pistachio shells are so hard to crack, “I’m always afraid I’ll break my teeth!” says Gierlinger, a biophysicist at the University of Natural Resources and Life Sciences, Vienna. Now, she and her colleagues have discovered why the shells are so strong. The velvety nuts are encased in an “ingenious” microscopic structure of interlocking cells so tightly bound, they never let go of each other. The material is strong enough that it might one day be used for shock-absorbing items such as safety helmets.
“These tissues achieve a holy grail of materials science,” says Naomi Nakayama, a bioengineer at Imperial College London who was not involved in the new work. Stiff materials like glass tend to be brittle and break relatively easily, she says, whereas tougher materials tend to be flexible, like spider silk. “But these [shells] have achieved stiffness and toughness in combination.”
To find out what makes nuts so hard to crack, Gierlinger and her colleagues have spent years exploring the biomechanics of pine, pistachio, and walnut shells, among others. Two years ago, they discovered the secret of a walnut shell’s toughness: It consists of 3D, puzzle-shaped cells with interlocking lobes. Last year, they found that the cells in pistachio shells have those lobes, too. And whereas most nutshells include several kinds of cells, walnuts and pistachios only have one kind.
To figure out just how tough those cells are, Gierlinger’s team ran a new experiment using state-of-the-art equipment. They examined broken fragments of walnut and pistachio shells at the nanoscale with a micro–computed tomography scanner and electron, atomic force, and infrared microscopes—a “materials science candy shop,” of equipment, Nakayama says.
Like those in walnut shells, the interlocking cells in pistachio shells hook up with 14 neighboring cells, Gierlinger and her colleagues found. The cells in both shells also have tough cell walls packed with spiraling coils of microfibers. But tensile strength testing showed the pistachio shell material is far stronger, likely because its cells have three times as many lobes as walnut cells. That gives them 30% more surface area to lock on to one another, the researchers report today in Royal Society Open Science. And unlike walnut cells, the pistachio cells connected via ball-joint structures, similar to human hip joints, as seen in a 3D animation (below). That makes pistachios the “master of geometrical cell interlocking,” Gierlinder says.
When the researchers looked at the edges of the fractured shells under their microscopes, they saw that many of the walnut cells had separated from neighboring cells, with cell walls sticking out like the round bumps on the top of Lego bricks. In contrast, the pistachio cells put up a much bigger fight. Most never separated from their neighbors. Instead, splitting the pistachio shell material required slicing through individual cells—including their tough, coil-packed walls.
Karl Niklas, a biophysicist at Cornell University who wasn’t involved in the study, says he was struck by the different ways in which walnut and pistachio shells responded to mechanical forces at different levels within the shells’ structures. Their properties could make them ideal for creating shock-absorbing devices such as safety helmets and car bumpers. That’s because they can take the energy of an impact and, by bending or stretching instead of breaking, redirect it away from the object to be protected. “Have you ever tried to break a pistachio shell with a hammer?” he says. “I have, and it’s not easy!”
For Gierlinger, the study also provides a valuable lesson: If you use your teeth to crack that unopened pistachio, aim for the one weak spot—the seam. Or better yet, use a nutcracker.