Material by Nature

In the natural world, Mother really does know best.

The Grande Dame perpetually has trumped mankind’s best efforts to replicate her masterful and oftentimes miraculous designs. Steel, for instance, is considerably stronger than bone—Magna Mater endowed the body’s latticework with a measly 150 MPa tensile strength, 2 percent strain to failure and a 4MPa (m)1/2 fracture toughness (most definitely solid, but rather weak for a structural material). Most, if not all, alloy metals are decidedly tougher: Structural steel has a 400 MPa (megapascal) tensile strength score, while carbon steel enjoys an MPa grade of 841. 

Yet even the mightiest man-made metal is no match for spider silk, which on an ounce-for-ounce basis, boasts a far superior tensile strength (five times higher than steel).

Homo sapiens’ attempts to build a better hip socket similarly have fallen short. Though polyethylene and ceramic are both anatomically sound substitutes for the ball and acetabulum cavity, neither is a viable long-term solution. Titanium, stainless steel and cobalt chrome, of course, are undeniably strong—all three metals easily can support the body’s weight and withstand the natural force from hip and leg muscles—but they also are corrosive and can generate nasty ions that lead to infection, swelling and osteolysis.

Mother Nature prevails. Again.

“Nature already has found simple, elegant, sustainable solutions to some of our most daunting problems,” University of Pennsylvania chemistry professor Virgil Percec, Ph.D., noted during a 2011 trade show lecture. “Using nature as a model and mentor offers great promise for developing new commercial products, launching new industries, and for basic progress in science and technology. The models are there…waiting for us to fathom and mimic.”

The models have always been there but scientists only recently have begun to unlock the secrets to Mother Earth’s handiwork. Their research is leading to discoveries that are narrowing humanity’s learning gap and eventually could spawn some remarkable new materials, many of which may revolutionize medicine. A few of the more promising breakthroughs over the last several years include:
 
Shrilk. Made from discarded shrimp shells and silk proteins (hence its name), this thin, clear, flexible material is as strong as aluminum at half the weight. Harvard University researchers strictly followed Dame Nature’s recipe in layering the material’s two main ingredients—the silk protein fibroin, and chitosan, a variation of the substance chitin that comprises much of an insect’s tough outer layer. The way in which both materials are layered creates the stiff but flexible design that one day might be used to create dissolvable sutures for hernia repair, protective coverings for burns and wounds, and a biological scaffold for tissue regeneration. “Much of the structural properties found in nature are not just chemistry, they’re architecture,” Wyss Institute Director Donald Ingber said.
 
Bone glue from sea worms. The human body can be an unforgiving environment for uninvited guests, particularly those bearing the hallmarks of human intervention. The metal pins, nails, rods and screws currently used to fix severely fractured or shattered bones are subject to corrosion from bodily fluids, and don’t always achieve perfect alignment. Hence the quest to find the Holy Grail of bone repair—a biocompatible, bioabsorbable adhesive capable of reconstructing a splintered skeleton within the body’s hot, chemically harsh environment. Scientists may have found such mythical perfection in the natural glue secreted by the sandcastle worm (Phragmatopoma californica), a tiny creature that builds its home out of sand, shells and other sea detritus. A synthetic version of the worm’s glue created by University of Utah scholars has passed toxicity studies in cell culture, and is twice as strong as the natural adhesive it mimics. “We recognized that the mechanism used by the sandcastle worm is really a perfect vehicle for producing an underwater adhesive,” Russell Stewart, Ph.D., a bioengineer at the University of Utah in Salt Lake City, told the American Chemical Society. “This glue, just like the worm’s glue, is a fluid material that, although it doesn’t mix with water, is water soluble.”     
 
Moth eye-influenced radiographic imaging. Moths are more intelligent than they appear. One species can jam bat sonar with their ultrasonic clicking sounds, and all varieties use transverse orientation to fly in a straight line (except around artificial light). The moth’s non-reflective eyes allow the insect to use minimal amounts of light to navigate its way in the dark, a process that has inspired researchers from the City University of New York and Tongji University in Shanghai, China, to develop a new class of materials that improves the light-capturing efficiency of X-ray machines and similar medical imaging devices. The team’s work focused on scintillation materials—compounds that re-emit absorbed energy from incoming particles like X-ray photons in the form of light. In radiographic imaging devices, scintillators convert the X-rays leaving the body into visible light signals picked up by a detector to form an image. Improving the output (image) requires increasing the incoming X-ray dosage, which can be detrimental to human health. A much safer alternative involves improving the efficiency in which the scintillator converts X-rays to light, achievable with a 500 nm-thin film made of cerium-doped lutetium oxyorthosilicate. Researchers encrusted the crystals with pyramid-shaped bumps (protuberances) made of silicon nitride; each bump, or “corneal nipple,” was modeled after the structures in a moth’s eye, designed specifically to extract more light from the film. Between 100,000 and 200,000 protuberances fit within a 100 by 100 µm square. The research team roughed up the device sidewalls to improve its ability to scatter light and enhance the scintillator’s efficiency. Lab tests showed the X-ray mammographic unit with the lutetium oxyorthosilicate-coated scintillator increased emitted light intensity by as much as 175 percent compared with a traditional scintillator.
 
Silkworm ligaments and tendons. Spiders make for lousy farmers. Their crop—the stronger-than-a-Boeing, collagen-like silk—is a valuable commodity but the insects are too territorial and cannibalistic to produce the item en masse. Fortunately, they don’t have to: University of Notre Dame scholars have created transgenically engineered silkworms capable of producing fibers tougher than typical silkworm thread and as strong as the dragline silk strands made by spiders. Such properties make the silk an ideal medical material, particularly in orthopedics, where the fibers can be used to spin artificial ligaments, tendons and tissue scaffolds.
 
Medical resilins from dragonfly wings. Scientists discovered resilin—a natural protein more resilient than any synthetic rubber—about a half-century ago in the wing hinges of locusts and tendons of dragonflies. The material has remarkable properties, the most extraordinary of which may be its pliability: Resilin can stretch to three times its original length and bounce back to its original shape without ever losing its elasticity, despite repeated stretching and relaxing cycles. Over the last five decades, researchers have made major strides toward practical uses of resilin in medicine. Scientists have modified the material with gold nanoparticles, for example, for use in diagnostics, they’ve engineered mosquito-based resilin to behave like human cartilage and developed a hybrid material for cardiovascular applications. “This increasing amount of knowledge gained from studies on natural resilin and resilin-like polypeptides continues to inspire new designs and applications of recombinant resilin-based biopolymers in biomedical and biotechnological applications,” University of Delaware professor and Deputy Dean of Engineering Kristi L. Kiick wrote.