09 August 2002
09 August 2002
Researchers at UCLA believe that the secret to creating stronger, better materials may be solved by studying an unlikely source: the common spider.
Engineers can improve the design and processing of materials by emulating some of the spider's web-spinning abilities, enhancing the functionality of a medley of products, from tennis rackets to stealth bombers, according to Thomas Hahn, mechanical and aerospace professor at UCLA's Henry Samueli School of Engineering and Applied Science, and Frank Ko, a materials engineering professor from Drexel University.
"Spiders are remarkable in a number of ways," said Ko, who is spending a sabbatical at UCLA. "The silk they produce has a rare combination of strength and toughness, meaning that not only can it hold relatively heavy objects, it can stretch great lengths without snapping."
Lab tests by Ko revealed that spider silk is also highly resistant to degradation and can be spun in air or underwater.
Both the fibrous form of spider silk and its extremely fine nature - as fine as .02 microns - also hold distinct advantages.
For engineers asked to design materials for today's consumer and industrial markets, reproducing the properties inherent in spider silk is very attractive. "Normally we can make material very strong, but at the expense of toughness. And we can make things very tough, but at the expense of strength. Combining the two characteristics - as the spider does - is our challenge," Hahn said.
In January, Ko, who has spent years examining the engineering properties of spider silk, joined his longtime friend and colleague Hahn at UCLA to conduct a number of research projects, influenced in part by their fascination with the spider.
Hahn, for example, is taking nanoparticles provided by UCLA chemistry professor Richard Kaner and putting them into polymers to make stronger and more functional nanocomposites. Starting with a basic polymer - similar to the biological material the spider uses to spin its web - Hahn adds nanoparticles with certain properties to tailor composites for different functions. "A spider has the impressive ability to change the properties of the silk it produces for different tasks," Ko said. "There is a similarity to what we are trying to do."
For example, by adding graphite nanoplatelets, Hahn can create a material with greater electromagnetic capabilities, including high conductivity, an important property for aircraft. "You would no longer have to worry about electromagnetic waves and electrostatic charges that can interfere with the performance of electronic components," Hahn said. And the alternative could have dangerous consequences. "If lightning strikes a wing that is made of poorly conducting materials, it will leave a big hole in the wing," he said.
The ability to add functionality to a composite benefits a host of industries. Hahn, who has worked closely with the U.S. Navy and Air Force for 30 years, points out that in the aerospace industry in particular there is a strong incentive to use high-performance materials. Space applications, satellites and stealth aircraft all require high precision, temperature control, stiffness control, stability and radar absorption.
Though such tailoring of materials has long been common using micron-sized particles, Hahn uses nano-sized particles to add functionality. "When you use micron-sized particles, strength is decreased," Hahn said. "Using nanoparticles may allow us to add function such as electromagnetic properties without sacrificing strength."
Ko explained further. "In general, nanotechnology allows us to get down to something called the quantum effect. This effect describes how performance can be enhanced exponentially, chemical reactions can occur much more quickly, electrons move faster, heat is conducted much better. Because of the fineness of the material, and the cohesion between the atoms, the material is much stronger," he said.
In other words, things are enhanced. Sensors can detect parts per billion instead of million, batteries last longer without recharging and a stiffer tennis racket can improve a player's game.
While Hahn experiments with adding functionality to nanocomposites, Ko is focused on fibers and nanocomposites in fiber form. Ko believes that an important aspect of spiders' silk is the fibrous form it takes. While a spider is able to manufacture its strands of silk with seemingly no effort, humans must use processes such as electrostatic spinning, or "electrospinning," to make nano-scale fibers.
Electrospinning is a process capable of producing fibers less than 100 nanometers in diameter - 1000 times smaller than a human hair. An electric charge is used to "spin" a liquid polymer from a needle-shaped device onto a ground plate. The resulting nanofibers are of substantial scientific and commercial interest because the ultra-fine spun materials provide unusually high porosity and surface area, with very small pore sizes.
The advantage of the fiber form, Ko said, is that fibers can make very conformable, flexible structures. "A solid sheet, for example, cannot be conformed in as many ways as a fiber-shaped structure can," Ko said. "Think of a piece of paper compared to a string. In fiber form we can shape it into more geometric shapes."
For the same reasons Hahn inserts nano-sized particles into a polymer to enhance its properties, Ko says nano-scale fibers are in many ways better than larger micron-sized fibers. "Nanofibers have more surfaces to work with," Ko said. "When you have very small-diameter material in a linear form, you get a lot of surfaces and more reaction sites for a chemical to attach to, and to react with. So you get a lot more interaction from the material for the same amount of mass."
Potential applications for materials made with nanoparticles are well-known and wide-ranging, including notebook computers, hydrogen energy storage, building construction and drug delivery. The field of electronics is being affected as well, with wires and electronic devices shrinking in size but growing in power and speed. Consumer and industrial product manufacturers are even using nanotechnology as a marketing tool, with one Los Angeles sporting goods shop selling tennis rackets with "carbon nanotubes" in the handle.
Spiders may hold yet more answers for engineers working to make better materials and ultimately, improved products, suggests Ko, who considers the arachnid's silk to be one of nature's most impressive materials.
"We can learn much from the spider," Ko said. "There are still mysteries left to solve."