Facial expression under stiff stratum corneum leads to strain concentrations, followed by residual wrinkle formation.

Objective Computer simulation studies of skin models, which indicate skin compression in the same manner as facial expressions, have suggested that stratum corneum could control skin-folding patterns, which may play an essential role in wrinkle formation. However, it is not clear to what extent the mechanics of stratum corneum influence wrinkle formation in vivo. The aim of this study was to verify that stratum corneum could control strain distribution during facial expressions, which in turn leads to wrinkle formation. Methods In experiments in vivo, volunteers were instructed to smile under 10% or 80% relative humidity (dry or humid conditions, respectively). Skin movement around their eye corners during smiling was captured by a high-speed video camera. Particle-tracking velocimetry was applied to video recordings to analyse skin strain distribution. Also, wrinkle volumes before or after smiling were measured using replicas. Results With smiling under dry conditions, high strain was localized to form crease-shaped wrinkles whereas, under humid conditions, localized strain was dispersed. Furthermore, increased wrinkle volume after smiling was promoted under dry conditions. Conclusion Because exposure to dry or humid conditions in the short term could affect only stratum corneum mechanics, the present results indicated that stratum corneum could be considered to be responsible for localized strain during facial expressions. This strain is followed by residual wrinkle formation. Accumulation of residual wrinkles will produce permanent wrinkles in the long term. Improving the mechanics of stratum corneum might be an effective approach in wrinkle formation prevention.