Scientists have created a special kind of elastic polymers that are highly reactive to the ultraviolet radiation in sunlight. The newest prototype is designed so that when it is broken, the separated ends chemically bond together with just a minute’s worth of exposure to UV rays. The light energizes ions in the polymer, making them react together extremely quickly. The downside is that the polymer gets to extremely high temperatures of 220 degrees Celsius (428 degrees Fahrenheit) during this process, making it potentially unsafe.
For the scientifically inclined, you can learn more about this polymer works here.