# Uniaxial mechanical stretch properties correlated with three-dimensional microstructure of human dermal skin

## TL;DR

This study explores how the mechanical properties of human skin relate to its 3D microstructure, focusing on collagen and elastin fibers.

## Contribution

The novel contribution is the correlation of mechanical stretch properties with 3D microstructural parameters of collagen and elastin in human dermis.

## Key findings

- Age correlates negatively with Young’s modulus and collagen density.
- Elastin fibers align significantly during stretching in both the heel and linear regions.
- Collagen bundles mainly engage and orient in the linear region of stress-strain curves.

## Abstract

The intact and healthy skin forms a barrier to the outside world and protects the body from mechanical impact. The skin is a complex structure with unique mechano-elastic properties. To better direct the design of biomimetic materials and induce skin regeneration in wounds with optimal outcome, more insight is required in how the mechano-elastic properties emerge from the skin’s main constituents, collagen and elastin fibers. Here, we employed two-photon excited autofluorescence and second harmonic generation microscopy to characterize collagen and elastin fibers in 3D in 24 human dermis skin samples. Through uniaxial stretching experiments, we derive uni-directional mechanical properties from resultant stress-strain curves, including the initial Young’s modulus, elastic Young’s modulus, maximal stress, and maximal and mid-strain values. The stress-strain curves show a large variation, with an average Young’s modules in the toe and linear regions of 0.1 MPa and 21 MPa. We performed a comprehensive analysis of the correlation between the key mechanical properties with age and with microstructural parameters, e.g., fiber density, thickness, and orientation. Age was found to correlate negatively with Young’s modulus and collagen density. Moreover, real-time monitoring during uniaxial stretching allowed us to observe changes in collagen and elastin alignment. Elastin fibers aligned significantly in both the heel and linear regions, and the collagen bundles engaged and oriented mainly in the linear region. This research advances our understanding of skin biomechanics and yields input for future first principles full modeling of skin tissue.

The online version contains supplementary material available at 10.1007/s10237-023-01813-3.