Variations with age in the mechanical properties of human skin in vivo

To quantitatively assess the viscoelastic properties of human skin, an in vivo testing method has been developed and previously reported. In this method, a strain-gauged pretension device is first used to determine the natural skin tension and then to apply a predetermined tension. A suction cup device with a rectangular cross-section and semi-circular ends is then applied to the pretensioned area and the skin deflection versus negative pressure characteristic is recorded. With the basic characteristics of the skin stress field determined by the geometry of the cup as a homogeneous stress state, the stress-strain response curve is computed from the recorded data. Using a constant pressurisation rate testing mode, the back and forearm of 116 healthy male and female subjects ranging in age from two to 67 years were tested. Variations in the shape and average slope (approximate modulus) of the stress-strain curves have been investigated with changes in age and sex. It has been found that the average slope decreases through maturation, reaches a minimum between the ages of 15 and 25, and then appears to increase with advancing age. The data points begin to diverge after approximately 30 years of age. It is hypothesised that this diverging phenomenon may be a result of the cumulative effect of ultraviolet radiation on the collagen and elastin networks. An examination of the shape of the curves appears to indicate that stiffness of collagen in the dermis increases with increasing age. However, it appears that the lower portions of the curves, possibly associated with the elastin and ground substance components, undergo changes not common to every individual. It is also hypothesised that 'scatter' in the data obtained from females and the sudden change noted in the shape of the curves at puberty may be due to hormonal changes.