It’s been said for quite some time that silks, especially spider silks, are pound-for-pound stronger than steel. Silk is also famously flexible. Now there is a study from the Massachusetts Institute of Technology (MIT, Cambridge, USA) that explains how this strength is the product of something that shouldn’t be that strong – simple hydrogen bonds between proteins.
Whether it’s silkworm silk (the textile silk) or spider silk (that stuff of horror movies), silk is a natural protein fiber. The fiber is put together something like a stack of pancakes, the analogy carrying as far as being soft and flexible. What has puzzled scientists for some time is that the bonds holding the stack together (technically, beta-sheets of a fibroin protein) are simple hydrogen bonds, which are not known for their strength.
The MIT researchers used computer models to probe the structures of silk, principally the beta-sheets of fibroin, for their failure mechanism – the point where the bonds break. They found that unlike many other very strong substances, for example ceramics, the hydrogen bonds of silk fail slowly and unevenly, not all breaking at once like ceramics. This gives silk considerable elasticity – it bend (or stretches) before it breaks. This accounts for some of the strength.
Looking at the silk material in greater depth, the researchers observed the tiny (nano-scale) beta-sheet crystals (the fibroin is formed into crystalline shape) and that these crystals were in alternating alignment from sheet to sheet. In a sense, the alternating shapes of the crystals are ‘interlocking’ and allow for more hydrogen bonds – there’s more strength in numbers. The result is a silk thread, a ‘column’ or stack of crystalline sheets, that can actually support the hydrogen bonding as the material stretches or bends. The hydrogen bonds work together, reinforcing each other against outside forces. This is what gives silk such extensibility and strength. Think of the strands of spider silk stretched across more than a meter of space and able to withstand strong winds and fierce struggles as spider engulfs prey.
One of the more important findings of the MIT study was the critical relationship between the size of the beta-sheet crystals and the properties of the silk. With a size of about 3 nanometers (3 billionths of a meter), the silk is ultra-strong and ductile. Make those crystals just above 5 nanometers and the silk becomes weak and brittle. As is often shown to be the case, materials behave differently at very small sizes, particularly at the nanoscale.
There are plenty of implications for this study, not the least of which is a better understanding of fibers, hydrogen bonding crystalline structures, and tensile strength. Markus Buehler, Associate Professor at MIT’s Department of Civil and Environmental Engineering and leader of the research team, puts it this way:
He notes that the findings could be applied to a broader class of biological materials, such as wood or plant fibers, and bio-inspired materials, such as novel fibers, yarns and fabrics or tissue replacement materials, to produce a variety of useful materials out of simple, commonplace elements. For example, he and his team are looking at the possibility of synthesizing materials that have a similar structure to silk, but using molecules that have inherently greater strength, such as carbon nanotubes.
The long-term impact of this research, Buehler says, will be the development of a new material design paradigm that enables the creation of highly functional materials out of abundant, inexpensive materials. This would be a departure from the current approach, where strong bonds, expensive constituents, and energy intensive processing (at high temperatures) are used to obtain high-performance materials.