Christopher Weder at Case Western University has developed a new polymer fishing line that would, when viewed under UV light, change colors under excessive stress. The idea is that you can check your line for stress that could lead to breaking using a small UV lamp before fishing. The line is currently in the proof of concept phase, but they are already working on line that would change color under visible light. UV lights aren’t hard to come by though, in fact there are plenty water-resistant UV LED keychains on the market.
The polymer may also have a future in tamper resistant packaging, changing colors to indicate wether a package has been open and resealed.