This Clever Gel Works Like a Vaccine for Wildfires

Spray the flame-retardant goo on vegetation, and it’ll protect the plants from igniting for months.

grass burning around a pole
A utility pole was treated with viscoelastic fire retardant fluids and subjected to control burns. The treatments protected pole from fire damage.PHOTOGRAPH: JESSE D. ACOSTA

Stanford materials scientist Eric Appel didn’t set out to help save people from wildfire, but from disease. Usually he works on developing gels that can ferry drugs into the human body. So if you want to bestow a patient with, say, antibodies to fight off HIV infection, you’d inject them with a gel loaded with the stuff, where it might persist in the patient for perhaps a year. If used widely across an at-risk population, theoretically you can better face down an epidemic.

It wasn’t until Appel’s brother in law—Jesse Acosta, formerly a fire prevention forester for the state of Hawaii, now at Cal Poly San Luis Obispo—said hey, what about loading these gels with fire retardants and applying them to the body that is Mother Nature? That would be the same red stuff you see planes dropping on wildfires, which is effective but fleeting: The material will blow away in the wind or wash away in a rainstorm, meaning you can’t proactively treat an area long-term to be more resistant to fire.

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.

The lifespan of the gel fits in perfectly with California’s unique brand of fire problem. The state’s biggest fires tend to rage in the fall before the first rains, when seasonal winds tear through the state, pushing blazes with incredible speed through dry brush. Once the rains come, the threat subsides. The idea with the fire-retardant gel would be to spray before the summer and forget about it. Only a good rain will wash it away, and as soon as that happens you don’t need the gel anymore because the vegetation is now well hydrated.

Naturally, you might be worried about toxicity, but this stuff is benign in the environment, Appel says. The silica component is essentially sand, and the cellulose is from plant material, which you might also find in personal lubricants and cosmetics for added viscosity. The researchers went further with toxicity tests with bacteria. It’s particularly important that the stuff doesn’t degrade aerobically—that is, bacteria feeding on it and consuming oxygen. If a bunch of the material washes into a river, you don’t want an outbreak of bacteria that in turn suck all the oxygen out of the water and kill the organisms in there. While the gel checks out in the lab, we’ll probably want more safety testing before wide-scale deployment. MORE

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