Darpa, the Pentagon's wide-eyed research arm, is betting big on "metamaterials" -- composites that can seemingly-impossible new properties, thanks to their molecular structure. But even for Darpa, and even for metamaterials, this seems like a long shot: a $15 million program to build shoot-through, one-way-invisible, self-healing shields for soldiers in urban battlefields.
Metamaterials are already showing promise, as the building blocks to real-life invisibility cloaks; that's because the composites let electromagnetic waves flow around them, instead of reflecting 'em back. Darpa's "Asymmetric Materials for the Urban Battlespace" program goes way, way beyond mere invisibility, however.
"Asymmetric, or 'one-way,' materials will support basic unit operations such as raids, cordon and search activities, snap checkpoints, and fire fights," according to military budget documents. "Friendly forces will be able to see through [one of these new materials] and shoot through it, but hostile forces will not." Such shields will also have "the ability to 'self-heal' if necessary. The materials must be lightweight, respond instantly, and be easy to deploy and retract in confined spaces."
Darpa doesn't give much guidance on how this might be done. But the agency does offer a clue, buried in an earlier budget document: "Initial studies have shown optical analogs of secure digital communication hold great promise for providing a 'coded' obscurant system. The optical properties of obscurant can be tailored such that they develop transparency at narrow, tunable wavelengths. This narrow band optical bleaching phenomena could be realized through optical threshold sensitive switching materials akin to some developed for laser protection goggles."
Of course, producing stuff like this won't be easy, Darpa admits. There are "significant technical obstacles," the agency notes, including "the design and fabrication of composite or meta-materials with true one-way capabilities." Which is pretty much the whole program.