...uses extremely small nanoparticles embedded into the hydrogel lenses. These engineered nanoparticles react with glucose molecules found in tears, causing a chemical reaction that changes their colour.
http://www.nano.org.uk/news/index.php?article=319 via io9 | Image: NASA
I see what you did there.
BTW - that article you linked to was really poorly written. (But scientists just have to do good science, right, not necessarily communicate it.)