But armed with a newfangled (and environmentally safe) gel, Appel and his colleagues have done just that. Writing today in the Proceedings of the National Academy of Sciences, they detail how their goo can act as a delivery medium to coat vegetation with flame retardant, and keep it there for the whole fire season. If adopted widely (Appel has founded a startup to commercialize it) the gel could become a sort of vaccine against wildfires, applied around the roads and utility infrastructure where 84 percent of California’s 300,000 fires in the last decade have ignited.“The funny thing is that the engineering requirements for delivering a drug in a body for a very long period of time are pretty darn similar to the engineering requirements for maintaining a fire retardant on target vegetation for months,” says Appel. “It needs to be safe, it needs to be totally non-toxic, it needs to not harm the function of the thing you're encapsulating.”
The gel’s main components are cellulose polymers (derived from plant material) and colloidal silica particles, which are chemically identical to sand. “What happens is the polymers cross-link between the particles—I often refer to it as sort of a molecular velcro,” says Appel. The polymers build bridges between the particles, creating a gelatinous structure.
A typical fire retardant dropped from a plane, like the inorganic salt ammonium polyphosphate, or APP, works by sticking to the surface of vegetation and binding to carbon, creating a char layer that is resistant to burning. When APP burns, it produces water, which helps quench the flames. The problem, though, is that APP won’t stick to vegetation for long, so it’s only used to fight fires, not as a widely sprayed prophylactic to prevent them in the first place.
But by mixing APP with this new gel, the researchers could get 50 percent more of the fire retardant to stick to vegetation. Not only that, they found that the concoction continued to work even when exposed to half an inch of rain. And in field tests supervised by CalFire, grass treated with the gel simply refused to ignite.“Just watching the tests firsthand, it created a fuel bed that would not burn,” says Alan Peters, a CalFire division chief in San Luis Obispo. “So creating a non-combustible area of material would obviously have applicability in a variety of uses around homes or along roadsides.” In particular, CalFire sees a good amount of fires on steep inclines, where tractor trailers and other heavy vehicles struggle, overheat, pull over, and set the brush on fire with their undercarriage. And because the researchers designed the gel to be sprayable from standard equipment like hydroseeders, which are usually used to spew a slurry of fertilizer and seeds onto renovated roadsides to bring back vegetation, application would be a low lift.