Come into our web, say the Israeli researchers
Two Technion experts in nanotechnology are imitating the spider to produce threads with potentially amazing applications.
The spider is a strange creature. It traps its prey by spinning a web, whose threads are fine and strong. In fact, scientists have been trying for years to copy the spider's web, and five years ago, Prof. Eyal Zussman and his colleague, Prof Alexander Yarin, both professors of mechanical engineering at the Technion Israel Institute of Technology in Haifa, began to do this, developing a technology for creating tiny artificial spider webs, known as electrospun nanofiber.
"Working with nanometric things simply piqued our curiosity," says Zussman. Since then the two have managed to develop artificial spider webbing that can be used to make sensitive biological sensors, for making huge sails in space and tiny conductors for computer chips. The commercial potential is yet to be realized.
"We have no free time to approach commercial companies," explains Zussman with a smile, but adds that they have begun contacting companies in the hope that the fibers will be integrated into a commercial product.
Nanotechnology, the field in which Zussman and Yarin are involved, uses measurements that are confusing because they are so small. One nanometer is a billionth of a meter. For the sake of comparison, the diameter of a human hair is about 40 microns, or 40 millionths of a meter. In the mid-1990s, then U.S. president Bill Clinton ordered the transfer of millions of dollars for nanotechnology research, leading to tremendous growth in this field. In Israel, too, several centers were established for the study of substances and processes on the nanometric level. Unlike the centers in the U.S., however, not one of them was established with government assistance.
In recent years, a few groups worldwide have begun to develop spider webbing under laboratory conditions in order to harness its unique properties - tremendous strength in an ultra-fine fiber. While spiders use a clear, sticky viscose fluid made of various proteins to make the silk-like fibers for their webs, Zussman and Yarin use a polymer solution, a chemical substance composed of large molecules.
The polymer solution has two important properties: it is flexible and sticky, so that when it is stretched, it forms fibers. Zussman and Yarin sought a sophisticated method for producing the fibers and chose an electrospinning technique. This method was invented in the 1930s, but few applications were found for it. Now, says Yarin, an appropriate use has been found.
This is how it works: The polymer solution is put in a syringe equipped with electrodes that create high tension in the solution. The solution is pushed out of the syringe very slowly until a semi-spherical droplet forms at the end of it. The moment the tension in the syringe is increased to tens of kilowatts, the polymer solution acquires an electrical charge. The electrical forces acting on the droplet pull it downward and at a certain point it is stretched into a conical shape from which an ultrafine fiber is emitted, and which accumulates on the surface beneath the syringe. The diameter of the fibers range from a few dozen nanometers to one micron, depending on the type of polymer being used. The fiber is gathered in a random manner and is not woven.
Improving the UAV
These fibers have been produced in other places, too, and there was no significant innovation in Zussman's and Yarin's laboratory until recently, when they decided to try to organize the fibers. The fibers that pile up beneath the syringe are inside a strong electronic field. Zussman and Yarin built a kind of lens that functions like a lightning rod, or rather a fiber rod. The fibers are drawn to the lens, which can then be used to weave or twist them in any pattern - into a rope or fabric.
"That was our breakthrough," says Zussman. "Most of the developments that preceded us produced a pile of fibers that were used as a filter. We succeeded in weaving the fibers in a systematic manner and created a structure of fibers rather than a formless mass."
The formless mass of nanofibers could be used as a filter because although the fiber itself is tiny, the surface area of the pile is much greater than can be perceived by the human eye. It could therefore be used to filter dust or other biological substances.
Zussman's and Yarin's work makes it possible to control the configuration of the fibers and opens the door to a whole world of applications. In a joint project with Prof. Daniel Weiss of the Technion, Zussman and Yarin made a tiny aerodynamic decelerator, like a parachute, made of nanofibers. The fibers were prepared with a component that is sensitive to a particular biological substance. When the decelerators come in contact with the substance, they change color.
"This is not just a curiosity," note Zussman. "This development means that the security forces could spread dozens or hundreds of these parachutes in the air, and they would monitor a specific area for biological or chemical substances."
Another military application is the protection of soldiers via coating their uniforms with the nanofibers. The fibers would enable soldiers to quickly determine if they had come in contact with a hazardous chemical or biological substance and could even halt or neutralize the biological or chemical contamination.
Another development that interests the security establishment is related to the strength of the nanofibers. "It takes between one and 10 grams of force to tear spider silk, depending on the type of spider that spun the silk," explains Zussman. "The fibers that we have produced so far require only 0.1 gram of force to tear them. This means that we are not even close to producing fibers as strong as real spider silk."
In order to strengthen the fibers, Zussman and Yarin are working with chemical engineer Prof. Yachin Cohen, also at the Technion. Together they have managed to lace the polymer fibers with carbon tubules, which strengthen them significantly. It isn't even a complicated process, as the carbon tubules are simply added to the polymer solution and are drawn into the fibers as they are pushed out of the syringe.
"The stronger fibers can be used to build lighter UAVs (unmanned airborne vehicles) that are just as strong," explains Zussman. "This will save on fuel and the UAVs will be able to remain airborne for longer."
Such projects that have pure military potential are partially financed by the security establishment. Next week representatives of the American army will be visiting the Technion to examine Zussman's and Yarin's developments.
"We have technology and a method for weaving the fibers," says Zussman. "Now we need to find suitable applications." To this end Zussman and Yarin are currently working with a group of doctoral students on experiments with different polymers, some with electrical conductivity, to see what can be done with them.
One set of experiments is being conducted in cooperation with Prof Oded Yarden of the Faculty of Agriculture at Hebrew University in Jerusalem. If the fibers can be permeated with antifungal or antibacterial substances, they can be used in making bandages to reduce infection, or to coat surfaces in order to protect agricultural produce during storage.
Fibers that conduct electricity can be used instead of today's copper wires on computer circuit boards. The electronics industry is approaching the limit of the fineness of the copper wires. Intel, the microprocessor manufacturer, currently uses copper wires that are 130 nanometers thick (0.13 microns).
"With basic technology and simple methods," says Zussman, "we have succeeded in producing conductive fibers hundreds of microns long and only 10 nanometers thick."
Using electrostatic fields, Zussman and Yarin can direct the fibers to fall in patterns that produce electrical circuits. "If we, using our "primitive" methods, can produce such tiny conductive fibers, just think what Intel could do with this technology," says Zussman.
This property actually opens a whole new field - nanoelectronics. If a company like Intel or Applied Materials, considered the world's largest manufacturer of machines used to print electronic circuits, decided it could use Zussman's and Yarin's technology, it could even be the basis for all the electronic circuits in the next decade. That is not the last of Zussman's and Yarin's ideas either. Another application for the nanofibers is out in space, where the fibers could be used to build solar sails that would provide energy to power spacecraft. The sails would be suspended on mechanical arms that would move the sails in relation to the sun, ensuring a constant source of energy.
Zussman and Yarin suggest solving the problem of transporting solar sails the required size (hundreds of square meters) by weaving the sails after the spaceship has been launched. "Our idea is to send the raw materials and the machines into space and then build the sail the required size and shape. Although there are other theoretical problems that have still not been solved, Zussman says he and Yarin plan to try offering the idea to NASA in the near future.
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