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New Understanding of Natural Silk's Mysteries Could Lead to Stronger, Lighter Materials
By Clay Dillow
Natural silk, as we all know, has a strength that manmade materials have long struggled to match. In a discovery that sounds more like an ancient Chinese proverb than a materials science breakthrough, MIT researchers have discovered that silk gets its strength from its weakness. Or, more specifically, its many weaknesses. Silk gets its extraordinary durability and ductility from an unusual arrangement of hydrogen bonds that are inherently very weak but that work together to create a strong, flexible structure.
Most materials -- especially the ones we engineer for strength -- get their toughness from brittleness. As such, natural silks like those produced by spiders have long fascinated both biologists and engineers because of their light weight, ductility and high strength (pound for pound, silk is stronger than steel and far less brittle). But on its face, it doesn't seem that silks should be as strong as they are; molecularly, they are held together by hydrogen bonds, which are far weaker than the covalent bonds found in other molecules.
To get a better understanding of how silk manages to produce such strength through such weak bonds, the MIT team created a set of computer models that allowed them to observe the way silk behaves at the atomic level. They found that the arrangement of the tiny silk nanocrystals is such that the hydrogen bonds are able to work cooperatively, reinforcing one another against external forces and failing slowly when they do fail, so as not so allow a sudden fracture to spread across a silk structure.
The result is natural silks that can stretch and bend while retaining a high degree of strength. But while that's all well and good for spiders, bees and the like, this understanding of silk geometry could lead to new materials that are stronger and more ductile than those we can currently manufacture. Our best and strongest materials are generally expensive and difficult to produce (requiring high temperature treatments or energy-intensive processes).
By looking to silk as a model, researchers could potentially devise new manufacturing methods that rely on inexpensive materials and weak bonds to create less rigid, more forgiving materials that are nonetheless stronger than anything currently on offer. And if you thought you were going to get out of this materials science story without hearing about carbon nanotubes, think again. The MIT team is already in the lab looking into ways of synthesizing silk-like structures out of materials that are stronger than natural silk -- like carbon nanotubes. Super-silks are on the horizon.
探索蠶絲的奧祕，製造更加結實而輕盈的材料
克雷·迪洛 著
我們都知道，蠶絲具有的韌性是人造織物長期奮力追求的目標。在一項研究中(該項研究成果聽起來更像一則古代中國諺語，而不是材料科學的突破)，麻省理工學院的研究人員發現，蠶絲的力量源於其脆弱，或者，更具體地說，是它的多方面的脆弱。蠶絲的異常耐久性和延展性來自一種特別的氫鍵結構，這些氫鍵本質上非常脆弱，但它們共同創造了一種強壯而富有彈性的結構。
大多數材料(特別是那些要求硬度很高的材料)的韌性來自脆性。因此，和蜘蛛製造的蛛絲類似的蠶絲，因其重量輕，延展性強和韌性高，長期以來引起了生物學家和工程師的興趣(同樣重量，蠶絲比鋼要壯，也不那麼脆)。但表面上，蠶絲看起來卻不那麼強壯;從分子結構上看，它們是由氫鍵組成的，氫鍵比其他分子中發現的共價鍵要脆弱得多。
為了更好地瞭解蠶絲如何以如此脆弱的化學鍵產生這麼強壯的力，麻省理工學院的研究小組創造了一套計算機模型，這種模型能夠讓他們在原子層次上觀察蠶絲的'活動方式。他們發現，微小蠶絲奈米晶體的結構使氫鍵能夠齊心協力地合作，相互增援，對抗外力，同時，當外力減弱時也隨之慢慢減弱，這樣就不至於在蠶絲的整體結構上出現突然的斷裂。
這樣，天然絲能夠既伸縮和彎曲，又能夠保持極高的韌力。對於蜘蛛和蜜蜂之類的昆蟲來說這也沒什麼，但對於蠶絲幾何形狀的這種瞭解，可能幫助人們製造出比我們面前能夠製造的材料更結實而又更柔軟的新材料。最好和最結實的材料通常是很昂貴而又難以製造的(需要高溫處理，或者高能耗處理)。
通過研究蠶絲作為一個例子，研究人員有可能設計出製造材料的一種新方法，即用廉價材料和弱鍵，製造不那麼堅硬而又柔軟，但比目前所用的任何材料都結實的材料。如果你認為不研究碳奈米管的理論，就能從這一則材料學資訊中獲取製造方法，那請三思。麻省理工學院研究小組已經在實驗室利用比蠶絲還結實的材料(比如碳奈米管)研究合成類似蠶絲一樣的結構。超級蠶絲即將出現。
