Underneath that webbed outer shell is a second, smooth layer of Vectran that looks like a vinyl shower curtain, protecting the bladder from anything that might somehow make it through the webbing.
"The idea is redundancy and flexibility," Roushey said.
Engineers still must design containers for the plugs, as well as effective inflation and security systems. Tunnels will have to be modified, too, and that will take time and money.
Roushey said those details may take two to three years to fully resolve, but the goal is to create and install plugs that function like air bags in a car. They sit unnoticed -- until they're needed.
The testing also helps researchers determine how much larger the plug should be than the tunnel it's designed to seal and protect. Roushey said the idea is to develop a range - say 5 percent to 20 percent -- to fit most applications.
The plug's design has changed as newer, stronger materials have emerged.
While Homeland Security was primarily interested in protecting transportation tunnels, Verrico said the plugs could also be used to stop smoke, gases and chemicals.
"But also, the real test here is the deployment method," he said. "How do you get this thing installed in the tunnel so that it's able to quickly inflate, be deployed and doesn't kink up and will seal the tunnel? There are a lot of aspects to determine."