MIT researchers have demonstrated a technique for depositing a very thin layer of a water-repellent coating that will make it possible to waterproof new kinds of materials and may offer ways to combine waterproofing with antibacterial and other active coatings. The technique has many potential applications, including fabric coatings for soldier uniforms, coatings for fine wire neural probes and insulation for integrated circuits.
Researchers working with Professor Karen Gleason of the Department of Chemical Engineering have used a process called hot filament chemical vapor deposition (HFCVD) to deposit nanolayers of polytetrafluoroethylene (PTFE, also known as Teflon). They have used the process to waterproof ordinary cotton T-shirt fabric, which retains its breathability and is indistinguishable in look and feel from untreated fabric.
Unlike many commercially available waterproofing processes, the HFCVD process deposits the coating from the vapor phase, offering the potential to coat materials that cannot be immersed in a solution. “For example, the U.S. Army is interested in waterproofing bullet-proof panels made of Kevlar,” Gleason said. “Because of Kevlar’s chemical characteristics, some of these other solution-based techniques wouldn’t necessarily even wet it.”
The technique is capable of coating unusual geometries, like fine wires, on which traditional PTFE deposition processes—involving baking a thick layer of powder—do not work. Gleason also points out that the HFCVD process can go beyond the exterior surface of a material to coat interior cavities, such as those in a porous substance like foam. “If we coat a piece of one-inch foam and we cut the foam open, it’s hydrophobic, or water-repellent, in the middle,” she says.
The process involves building up the PTFE coating one molecule at a time, much like beads on a necklace. Gleason says such nanoscale control may allow researchers to tailor the bead at the outer surface to provide additional properties beyond water-repellency. “I could imagine hooking DNA onto the PTFE bead to form a biological sensor,” says Gleason. “We might be able to hook on other biologically active materials—enzymes that could destroy a toxin, for instance.”
Gleason and her team received patents on the process in 1999 and 2000, but are just beginning to investigate the potential for combining the PTFE coating with other active molecules.
The next step, according to Gleason, is to design a generic “tether” that will allow the researchers to attach many different kinds of active molecules to the PTFE chains. Currently, Gleason is collaborating with Professor Alexander Klibanov of the Department of Chemistry to find a way to combine her team’s waterproofing technique with a microbe-killing fabric treatment that Klibanov’s group has invented.