Biophysical Characteristics of Lip Vermilion among Healthy Individuals in Southern China -- Correlations between Age and Vermilion Physiology

Abstract

Background: The vermilion of the lips plays an important role in the aesthetic appreciation of the lower face. However, there were few human subject studies on the vermilion of the lips in China. This study aimed to investigate the physiological characteristics of lip vermilion of healthy individuals living in China.

Methods: Volunteers aged 18-55 years were divided into three age groups, namely the younger, the medium, and the older. Lip physiological indicators, including stratum corneum hydration (SCH), epidermal layer hydration (ELH) at a depth of 0.5 mm below the vermilion surface, transepidermal water loss (TEWL), instant skin elasticity (ISE), glossiness, and color were measured through objective and noninvasive measurement instruments and optical imaging techniques. Data comparison and analysis among tested physiological indicators and across various instruments were also conducted.

Results: Statistically significant negative correlations were identified between age and several parameters, including ISE, glossiness, the L*, a*, and b* values, as well as the erythema index (EI) for the lower lip vermilion (p < 0.05). In contrast, a positive correlation was observed between age and ELH (p < 0.05). The lower lip vermilion in the younger group exhibited markedly higher glossiness and color metrics (L*, a*, EI) compared to the older group (p < 0.05). Additionally, a clear negative relationship was noted between TEWL and both elasticity and SCH for the lower lip vermilion (p < 0.05). Futhermore, the upper lip vermilion demonstrated a higher SCH and a lower TEWL (p < 0.05) than the lower lip vermillion. Image analysis by VISIA-CR proved to be more effective for evaluating lip color than SkinColorCatch due to their different detection mechanisms.

Conclusion: This study revealed the influence of age on lip physiological indicators, and demonstrated the correlations among non-color physiological parameters. It also highlighted the physiological disparities between the upper and lower lips. These insights may enhance our comprehension of lip vermilion physiology and offer a theoretical foundation for the efficacy assessment of lip care products.

Keywords: TEWL; age; color; hydration; lip vermilion; physiology.