Linear shear response of the upper skin layers

Area of Science:

• Biomedical Engineering
• Materials Science
• Dermatology

Background:

• Understanding the mechanical properties of the epidermis is crucial for various applications, including drug delivery and prosthetic design.
• Previous studies often relied on tensile testing, which may not fully capture the complex shear behavior of skin tissues.
• An accurate in vitro method for determining shear properties is needed to complement existing knowledge.

Purpose of the Study:

• To establish a reliable in vitro experimental method for quantifying the shear properties of human epidermis.
• To investigate the dynamic shear modulus of stratum corneum and viable epidermis.
• To assess the influence of environmental factors like relative humidity on epidermal shear properties.

Main Methods:

• Utilized a parallel plate rheometer for shear testing on ex vivo stratum corneum and viable epidermis samples.
• Validated the experimental setup using thin silicon sheets to ensure accuracy and reproducibility.
• Conducted preliminary tests to define the linear viscoelastic range and assess the impact of normal loading and environmental equilibration.

Main Results:

• The developed in vitro method demonstrated reproducible measurements of epidermal shear properties.
• The dynamic shear modulus for stratum corneum was determined to be in the range of 4-12 kPa.
• Shear modulus decreased with increasing relative humidity, and values were significantly lower than those predicted from tensile Young's moduli, suggesting anisotropic material behavior.

Conclusions:

• The study successfully established a reproducible in vitro method for measuring epidermal shear properties.
• Results indicate that stratum corneum exhibits anisotropic mechanical behavior, with shear modulus being humidity-dependent.
• The findings provide valuable data for biomechanical modeling of skin and inform applications requiring an understanding of skin's mechanical response.