In the early 2000s, while wristwatch sales declined with the advent of cell phones and their timekeeping capability, demand for Citizen watches in North America remained robust. Eco-Drive models were well received, generating a third of Citizen's North American revenues by 2000. In 2002 the VITRO technology (Eco-Drive VITRO) came on the market, where the solar cells were no longer even slightly visible under the dial. During the mid-2000s, wristwatch sales improved for Citizen thanks to further development of the Eco-Drive line and integration of radio-controlled timing with the 2002 Eco-Drive line. Later specialized tool watch designs were introduced like the Promaster Eco-Drive Professional Diver 1000M Titanium BN7020-09E in 2017. This is a very large watch, the titanium watch case has a diameter of 52.2 mm and thickness of 22 mm and features a helium release valve, designed for mixed gas saturation diving at great depths.