Dual-Purpose Skin Care.

Vitamin C skin-care products, which are proven to help fade dark spots and even out skin tone, are more popular than ever. Combining the two is harder than it sounds: both SPF and vitamin C require specific pH environments in order to stabilize, Thaggard explains, which makes them difficult to formulate together. The Daily Dose Vitamin C + SPF 40 Serum ($46) took more than three years and 15 iterations to perfect--but it's since become the brand's most successful direct-to-consumer product launch to date. [Extracted from the article]