I heard a report on the radio the other day about a new safety device for table saws that can sense the difference in density between wood and flesh, and will stop the blade immediately if you try to push some of the former (like your thumb) into the saw blade. Since it was on the radio, I don't have any idea how it works, but I suspect it's some sort of light or laser beam that the object being sawn has to pass through on its way to the blade. Obviously, this is expensive, sophisticated technology that's probably impractical for most users at the moment, but the report said it is available commercially and is being successfully used in high school shops and vo tech programs. Anyway, it got me thinking that someday we may see something like this on chain saws. A better approach than a flesh-sensor might be a device that detects a specific material (woven into the fabric of wood-cutting clothing), engaging the chain brake when the saw gets too close.