Flexible Circuits Unfold

June 2009
By Henry S. Kenyon


Researchers working with the U.S. Air Force Office of Scientific Research have developed a method to spray down a film of carbon nanotubes to form thin, flexible electronics. These pliable circuits will be applied to a variety of soft materials such as cloth and plastic.

Nanotech process allows systems to be installed on soft, non-rigid surfaces.

Warfighters one day may have electronics literally painted onto their uniforms thanks to a new technology for printing circuitry. The process involves spraying a film composed of carbon nanotubes onto a surface to form thin, flexible circuits. This capability potentially can be applied to cloth, plastics or other soft materials, opening the possibility for communications devices built into clothing or solar panels sprayed onto the tops of tents.

Funded by the U.S. Air Force Office of Scientific Research (AFOSR), the flexible electronics project is a partnership between researchers at DuPont and CornellUniversity. The scientists have discovered a way to produce a semiconducting ink that can be sprayed to form thin, flexible electronics. This solution-based process is based on an inkjet deposition method, and it would allow electronics manufacturers to access markets such as clothing and medical implants that currently are beyond the realm of traditional semiconductors, says George Maliaras, director of the Cornell University Nanoscale Science and Technology Facility in Ithaca, New York. Maliaras is a participating researcher in the AFOSR program.

Carbon nanotubes are the key feature of this ink. Composed of a single carbon molecule formed into a tube, these structures have been studied for years because their tiny size and ability to efficiently move electric currents make them very useful in microelectronics. Nanotubes are cylindrical carbon structures a nanometer in diameter. They can be metallic or semiconducting depending on the “roll-up” direction of the carbon atoms. Both types have very useful electrical properties. Metallic nanotubes are highly conductive, while the semiconducting tubes move electrons more efficiently than silicon.

But a fundamental hurdle faced by electronics designers is that when nanotubes are made, or synthesized, an undifferentiated mix of metallic and semiconducting tubes is produced. Maliaras explains that when this combination is sprayed or layered onto a surface, it acts as a metal because the metallic tubes short-circuit the semiconductor tubes. “Electrically speaking, it has limited functionality because it behaves more like a metal than a semiconductor,” he says.

Currently, this unfiltered mix of nanotubes can be used in electrodes and connections, but not in circuits. The main development challenge has been in removing or separating the metallic nanotubes from the semiconductor tubes. The AFOSR researchers discovered a way to chemically change the metallic nanotubes so that they would not interfere electrically with the semiconductor tubes. Known as cycloaddition, the process attaches fluorinated molecules to the nanotubes that eliminates the short-circuiting properties of the metallic tubes.

However, Maliaras contends that scientists do not really understand whether the process eliminates the metallic tubes or converts them to semiconductors. He notes that the result of cycloaddition is that the metallic tubes no longer interfere with the existing semiconductor tubes. He adds that the chemical separation process was the major technical hurdle. Although this has been accomplished, scientists still want to understand what is happening to the nanotubes. “Are we killing the metal tubes or are we converting them to semiconductors? There is a lot of science to be uncovered there,” he observes.

Maliaras explains that the process now allows nanotube-based solutions to be inexpensively created and spread across very large surfaces, beyond the realm of what currently is available. For example, researchers may soon develop semiconducting inks that can be sprayed on surfaces such as cloth.

In terms of electronics, the process allows the development of very-large-area solar cells or flexible displays that can be rolled up for storage. Maliarias notes that the U.S. Army is interested in field-deployable solar cell arrays. He notes that sensors or other electronic systems could be sprayed onto buildings or aircraft wings to monitor the entire structure in real time. “You could get real-time feedback about how an aircraft’s wings are bending in flight,” he says.

Another area of application is in medicine, where devices must be able to conform to internal body cavities or other spaces. Flexible circuits could be used for retinal implants or in other applications where sensors or other devices are attached to, or must conform to, an organ’s shape. Maliaras adds that flexible circuitry also could be sprayed onto bandages or other sensors to be wrapped around parts of the body, such as arms or legs.

The next part of the program will determine if the process can be used to control the doping of the nanotubes. Because nanotubes are still a relatively new technology, Maliaras explains that much of the current research has focused on processes for efficiently growing the structures for use on microchips and in electronic components such as integrated circuits. He remarks that many researchers can spend months or years studying the particular properties of a single type of nanotube and not how they might interact with each other. “This research is perfectly fine because we want to understand the performance limits of nanotubes. But if you want to have a technology, you’re not going to assemble things one tube at a time; you’re going to need a massively parallel technique for laying down film. Separating metallics from semiconductors is the number-one technological challenge,” he says.

For the laboratory bench-top work, the nanotube film was laid down on a heavily doped silicon oxide base. Maliaras says that this base provides a stable, insulated base on which to conduct tests. But materials such as cloth must be able to interact with the printed circuitry. He notes that for every new material that may serve as a substrate for the spray-on circuitry, a great deal of research must be conducted to fully understand that material’s properties.

The four-year flexible electronics project is now in its second year. Over the next two years, the research team will focus on two areas. The first is to understand what happens during the conversion of the metallic nanotubes, and whether it is possible to use this understanding to control the doping process. The second area is to explore the technology’s application in solar cells.

Maliaras notes that at the end of the program, his team will ask to renew its project. He explains that because it is a government program, the Air Force will have its choice of how it will apply the technology to other programs. There also is the potential for Cornell or DuPont to spin off any technologies or processes, but this probably would happen some years from now, he says.

U.S. Air Force Office of Scientific Research: www.wpafb.af.mil/AFRL/afosr/
Cornell University Nanoscale Science and Technology Facility: www.cnf.cornell.edu
DuPont: www2.dupont.com