Lips are perhaps the most sensual part of the face and play a major role in perceived beauty. They are also in constant motion and exposed to physical and chemical changes that alter and impact their normal form, as evidenced by wrinkles and dryness. The histology of the lips is well-described, and the vermilion area where the lips end and facial skin begins is covered by a thin stratum corneum made up of orthokeratotic cells that have a shorter turnover rate than the normal stratum corneum.
Unlike other skin, the lips lack epidermis, and with a thickness of just three to five cellular layers, they are very thin compared with typical facial skin, which has up to 16 layers. Lighter colored lip skin also contains fewer melanocytes, which protect lips and impart color—as do blood vessels, as they appear through the thin lip skin. Further, the lip skin is not hairy and does not have sweat glands, therefore it does not have the usual protective layer of sweat or body oil to keep the skin smooth, inhibit pathogens or regulate warmth. For these reasons, the lips dry out faster than other skin and become chapped more easily. Preliminary studies have shown that lips lose five times more water than body skin.
To improve the appearance of dry and wrinkled lips, it is therefore necessary to hydrate them with emollients and moisturizers—and a lipstick designed to highlight the beauty and color of lips as well as moisturize them meets both fashion and function needs. To this end, several ideas were considered before the author began work on a final product concept; this process is described here. The resulting lipstick was then tested for immediate and long-lasting moisturization using a corneometer, and a quantitative study was conducted with female users to rate the usability of the lipstick.
As noted, a lipstick concept was envisaged to provide high levels of hydration to the lips. Moisturization can be accomplished by occlusion, i.e., preventing water loss by coating the skin to trap moisture in—essentially, replicating what a healthy sebum balance already does. This approach is common among many brands, and staple ingredients include petroleum, mineral oil and lanolin.
Thus, previous attempts were made by the author to improve the moisture efficacy of traditional lipsticks (data not shown) by incorporating ceramides; oil components; products with beeswax, shea butter and vitamin E; or oils such as jojoba, olive and coconut. These provided lips with adequate moisture via occlusion. While these are somewhat effective, such products can also clog pores and cause contact irritation due to their oil content.