@Pope John Peeps II: Doesn't require heavy metals or radioactives, your definition of nutrient is too narrow.

What about the artificial polymers we make? There are no natural processes in Earth's biosphere that make nylon or acrylics or PVC. Nylon doesn't exist out in the wild unless we throw it away.

If we build these bugs so they need to eat nylon, for example, in addition to whatever else they do for us, this cripples them in a key way. If they escape, they'll soon die out because the nylon is not being rapidly replenished by natural sources in the wild—aside from us that is.

You could also build these bugs in such a way that they can't reproduce. We strip out their reproductive machinery. We could could get quite clever with that one. For example we build two species of bacteria that can't reproduce unless they are in close proximity to each other so bacterial genetic conjugation can take place. If either species escapes, they are essentially sterile.

We could also build them to be extremophiles. That they require high or low temperatures, very salty environments or high ambient radiation (Like radiococcus durans) to thrive. That would limit their ability to spread.

Of course there is the jumping gene problem. I don't have an easy answer for that one but, I think if we think these things through there are lots of precautions we can take to reduce the likelihood of runaways.