'Self-Healing' Material Seen In Movies Is Real Possibility
The 1991 film "Terminator 2: Judgment Day" introduced the world to a robot assassin that could instantly repair itself from shotgun blasts and molten lava bursts.
More than a decade later, researchers aren't far off in their quest to design self-healing materials. Instead of developing killing machines, scientists are pursuing self-healing plastics, metals, and even cement. The hope is to build lighter airplanes that use less fuel, to construct buildings and bridges with stronger, more flexible steel, and to pave a road with cement that heals itself at the first sign of a crack. Researchers believe this technology could save industries and governments billions of dollars if they can harness it in an inexpensive way.
"We're moving towards a time when the physical objects around us will be made of active materials that can change shape, heal themselves, and have built-in computation," said Christine Peterson, president of Foresight Institute, a nonprofit think tank. "Right now most objects are stupid objects with no ability to respond to us. One day we'll be able to signal them to change in some way we desire."
The research is being done mainly in the field of nanotechnology, the study of objects at the molecular level. Molecules are independent of one another but are drawn together by things like electrical pulses. The idea is that by changing the properties of individual molecules, researchers can change the overall properties of the structure the molecules form.
To date, the farthest advances in nanotechnology have been made in developing tiny sensors that can detect movement or a change in temperature, among other things. But recently more scientists have begun to study the mechanical aspect of nanotechnology with a view to making stronger products that cost less to maintain.
Most of the research is government funded while some is done at the university level. Individual industries have shied away from the research because commercial applications are more likely to be achieved in five years than a year or two.
"Most industries want the solution yesterday," said Surendra Shah, director of the Center for Advanced Cement-based Materials at Northwestern University.
Much of this research is being done at the National Aeronautics and Space Administration. One of the goals is to have self-healing structures in outer space.
"What we're after is a material that will heal itself if you penetrate it," said Mia Siochi, assistant head for advanced materials and processing research at the NASA Langley Research Center. "In space, if a meteorite hits a structure and goes through it, we want the structure to be able to close the hole itself. We've made materials that show a lot of promise."
Spending Days on the Golf Course
NASA scientists pursuing this research are spending much of their time on the golf course. Instead of playing 18 holes, they are studying the outer-coating of golf balls -- a plastic called surlyn. Surlyn is found in everything from bowling pins to dog-chew toys because of its ability to take a beating. Surlyn, Ms. Siochi said, has the ability to heal itself at the nano-level when damaged. For reasons still being studied, surlyn's molecules unite once they are separated.
NASA doesn't believe surlyn is the answer for its program because it likely won't be able to handle the ravages of outer space -- including massive radiation, Ms. Siochi said. Still, researchers are studying it because they believe any self-healing material they develop will have many of the same properties as surlyn.
Much of the research at NASA and other labs involves creating individual spheres less than a millimeter across, but larger than molecules. The spheres, which aren't visible to the naked eye, are embedded into a certain object -- say a basketball. Each sphere contains materials that essentially have the same properties as the molecules of the basketball. On the outside of each sphere is a coating of a powder-like substance.
If the surface of the basketball is punctured, the molecules and spheres that make up the surface of the basketball are also punctured. Once an individual sphere is cut, the materials inside are released. Those materials react with the sphere's powder coating and fill the hole created by the puncture. This would all happen in a matter of seconds.
"It's pretty wild stuff and yet it's not talked about that much," Ms. Peterson said.
Cutting Through Water
Draper Laboratories is also studying ways to make molecules instantly come back together once they are separated. One area of research is in nanovelcro, said Amy Duwel, group leader for Draper's Micro-Electro-Mechanical Systems division. Draper is an independent nonprofit lab that was spun off from the Massachusetts Institute of Technology in the 1970s but often partners with the university on projects.
"The concept of how it works is no different than saying you can't cut through water," Ms. Duwel said. "When you slide a knife through water you are separating it, but as soon as the knife is removed the water comes back together like nothing happened."
Nanovelcro acts much like regular velcro. The nanovelcro is added to the surface of molecules to make them stick together and, when momentarily separated, to make them stick together again.
The National Science Foundation and the Federal Highway Administration recently took part in a conference to discuss ways to engineer stronger cement and steel. These groups, among others, see a world of promise with nanotechnology and the nation's infrastructure. Much like the sphere research, some of the research here involves having a layer of chemicals inserted at a nanolevel underneath the surface of say, Portland cement, which is used to pave roads. Once the surface or outer level cracks, the chemicals underneath would act to spur the molecules to bind together more tightly -- thereby limiting the crack to a small area. If harnessed, this technology could make the pothole a thing of the past.
Research is also being done to change the properties of the cement at the molecular level. Scientists could add or take away properties in hopes of designing more durable concrete with better traction, said Mikhail Roco , chairman of the National Science & Technology Council's subcommittee on nanoscale science, engineering, and technology.
Researchers are also testing ways to add new properties to metals. This includes making metals impervious to temperature changes which now lead to expansion or contraction and even improving a metal's ability to act as a semiconductor, Mr. Roco said.
The U.S. military is conducting much of its own research into self-healing materials and engineering molecules of certain objects to have improved properties. One area of study involves the gecko -- a tropical lizard that is able to walk on virtually any horizontal or vertical surface, and even stand upside down on a ceiling.
It was long thought that geckos are able to do this because of some biochemical property. But researchers have found the hairs on the tips of a gecko's foot are small enough to fill the cracks between individual molecules. With millions of hairs on each foot, the gecko essentially plugs itself into the gaps between the molecules.
The military would like to design boots that allow its soldiers to do the same thing. Think of an army of spider-men.
自我修复材料不再是科幻
1991年拍摄的影片《终结者-II：审判日》中介绍了一种机器人杀手，这种机器人能在瞬间修复自己遭受的枪伤及熔岩侵袭造成的伤害。
而10多年以后，研究者们在研制自我修复材料方面也与影片不遑多让。不同的是，科学家们研究的不是机器人杀手，而是具有自我修复功能的塑料、金属，甚至是水泥。研究者希望能研制出消耗更少燃料、质量更轻的飞机，或者研制出可用于建造楼房和桥梁的更坚固、更有柔韧性的钢材，或者研制出能在爆裂后自我复原的铺路水泥。研究者认为，如果他们能通过不算昂贵的方式掌握这种技术，那么这种技术可以为企业和政府节约数十亿美元。
一家非赢利性的咨询机构Foresight Institute的总裁克里斯汀?彼得森(Christine Peterson)表示，我们的世界正在走向一个新时代，周围的实物将被改造成活性材料，可以变形、自我复原，并具有内在的智能。目前，许多物体都是非智能的物体，无法对人类做出回应。但将来，人类将能够指令物体按照我们的愿望进行改变。
这项研究主要是在纳米技术的领域内进行，纳米技术是在分子这个层面上研究物体的科学。分子之间是相互独立的，但被电子脉冲之类的东西吸引著聚合在一起。自我复原材料研究的主要思想是，研究者通过改变个体分子的特性，来改变分子所形成的固定结构的整体特性。
迄今为止，最先进的纳米技术成果一直是在开发微型传感器的过程中取得的（这种传感器可用于检测运动或者温度的改变），但最近，越来越多的科学家已经开始研究纳米技术在机械领域的应用，希望研制出维护成本更低却更坚固的产品。
多数这类研究都是由政府资助的，也有一些在大学校园里进行。各类企业对这种研究却一直避而远之，因为要实现研究的商业用途很可能需要5年的时间，而不是一两年。
西北大学(Northwestern University)高级水泥材料研究中心(Center for Advanced Cement-based Materials)的主管苏伦德拉?沙(Surendra Shah)表示，很多企业希望得到现实的解决方案。
有很大一部分研究是在美国航空航天局(National Aeronautics and Space Administration)进行的，目标之一在于研制应用于外太空的自我复原材料。
美国航空航天局朗里研究中心(Langley Research Center)负责高级材料及处理研究的助理主管米阿?西奥奇(Mia Siochi)表示，他们正在研究一种在被刺穿后能自我修复的材料。他说，在太空中，一旦陨石击中了飞船就会穿透而入，他们希望飞船自己能够修补留下的洞。他们已经研制出很有希望符合要求的材料。
美国航空航天局负责这项研究的科学家们正把大量的时间花费在高尔夫球场上，他们不是在打高尔夫球，而是在研究高尔夫球外层上包著的材料，这是一种被称为"舍林"(surlyn)的塑胶。由于舍林具有良好的抗击打性，所以从保龄球到某些玩具上都有舍林的痕迹。西奥奇表示，在纳米这个层次上，舍林在被损害后具备自我修复的能力。舍林的分子在被分开后仍能够聚合，但其中的奥秘仍在研究中。
但西奥奇表示，美国航空航天局并不相信舍林就是解决其研究的金钥匙，因为它可能根本无法抵挡外太空的各种侵袭，包括强烈的辐射。然而，研究者们仍在继续研究它，因为他们相信自己研制出的任何自我复原材料都将和舍林有许多相同的特性。 美国航空航天局及其他实验室的许多研究都与创建小于毫米，但大于分子的单个球体有关。这些球体用肉眼是无法看到的，它们将被置入一个物体中，比如说篮球。每个球体中包含的材料在本质上与篮球的分子具有相同的性质，每个球体外覆盖著粉末状物质组成的外衣。
如果篮球的表面被刺穿了，那么组成篮球表面的分子和球体也同时被刺穿。一旦某单个球体被刺破了，其中包含的材料就会释放出来，那些材料与球体外衣上的粉末状物质发生反应，就填补上了刺穿的洞。这整个过程将在数秒中完成。
Draper Laboratories也在研究分子被分开后迅速重新聚合的途径。该实验室的Micro-Electro-Mechanical Systems 部门负责人艾米?杜威尔(Amy Duwel)表示，他们的研究领域之一就是纳米塑胶。Draper是一个非赢利性的独立实验室，70年代从麻省理工学院(Massachusetts Institute of Technology)分离出来，但现在双方仍经常进行合作。
杜威尔说，此研究的理念与你无法用刀切断流水的道理完全一样。当你用刀劈过去，水就分开了，但一旦刀抽出来，水就迅速弥合得完好如初了。纳米塑胶和普通的塑胶作用相似，把纳米塑胶涂在分子的表面上，一旦分子被瞬间分开后，纳米塑胶能让它们重新黏合。
美国国家自然科学基金(National Science Foundation)和联邦公路局(Federal Highway Administration)最近参加了一个会议，研讨制造更坚固的水泥和钢筋的途径。这些及其他与会的组织都认为纳米技术和国家的基础建设有光辉的前景。他们的研究与前面提到的独立球体研究颇为相似，就是在纳米范围内的物体表面底下插入一层化学物质。比如说铺路用的Portland水泥，一旦水泥的外层表面破裂了，底下的化学物质将发生反应，让分子更紧密地结合在一起，因此就会限制路面局部的破裂。一旦掌握了这一技术，路面坑凹不平将成为历史。
研究者还对在分子层面上改变水泥特性进行了研究。美国国家科学技术委员会(National Science & Technology Council)下属的纳米科技、工程、技术研究委员会主席米克黑尔?罗柯(Mikhail Roco)表示，科学家可以增加或者减少物质的特性，研制出更耐久、牵引力更好的水泥。
罗柯说，研究者也在尝试寻找改变金属特性的方法，让金属不再受到温度变化的影响，不再像现在这样热胀冷缩，甚至于提高金属作为半导体的能力。
美国军方也正在进行他们对自我复原材料的研究，改变某些物体的分子以增强其属性。研究领域之一与壁虎有关，壁虎是热带的一种蜥蜴类动物，能够在垂直和水平表面上行走，甚至头朝下粘在天花板上。
长期以来，人们认为壁虎能够有如此独特的本领是因为一些生物化学特性。但研究者现在发现壁虎脚趾顶端的毛发非常纤细，能够伸到单个分子之间的缝隙里。壁虎的每个脚上都有百万计的毛发，因此它能使自己抓住墙壁分子间的空隙。
美国军方希望设计一种靴子，能让士兵在墙壁上行走，就象是蜘蛛侠组成的军队。
