A new DARPA program called VAPR, for Vanishing Programmable Resources, is seeking to create "transient electronics" that can 'vapr'ize themselves when they're no longer being used:
"Transient electronics developed under VAPR should maintain the current functionality and ruggedness of conventional electronics, but, when triggered, be able to degrade partially or completely into their surroundings. Once triggered to dissolve, these electronics would be useless to any enemy who might come across them."
Apparently, electronics have become so pervasive that after combat they're littered all over the battlefield to the extent that picking them all up would be impractical. The concern is that "the enemy" is able to "repurpose and study" said electronics, compromising our "strategic technical advantage." So great, let's just make 'em all disappear instead!
DARPA, of course, has no idea how these disappearing electronics will actually work, but the concept is that something as complicated as an encrypted radio signal or as simple as a change in temperature would set any VAPR electronics off. The first incarnation is likely to be some sort of cheap sensor that could be deployed in a hostile area, send back data, and then melt into uselessness, but there are a lot of different places you could go if you got something like this to work properly. The danger, of course, is that an evil genius manages to crack your radio signal or whatever, and that your fancy new arsenal melts right before your eyes.