The use of wave propagation techniques to evaluate age-related changes in viscoelastic properties of skin

An analysis of an experimental procedure used to determine age related changes in mechanical properties of the skin was performed. The experiment is based on the propagation of a shear wave through the skin and was termed the "shear wave experiment". The analysis was used to determine the effects of parameters such as the properties and thickness of the different skin layers on the propagation of the wave. A separate experiment, which was termed the "strip experiment", was developed and used to measure viscoelastic properties of excised strips of rabbit skin. The measured properties were used in the analysis of the shear wave experiment.

The analysis showed that if the stiffness of the stratum corneum was very high relative to that of the dermis, a situation which may exist under dry conditions, experimental measurements were strongly effected by the properties of the stratum corneum. However, if the stiffness of the stratum corneum was less than about 50 times that of the dermis, results were not significantly effected by the presence of the stratum corneum. In this case, the analysis showed that at lower frequencies ($<$1000 Hz) measured results depended strongly on skin thickness, while at higher frequencies (2000 Hz) the results were relatively insensitive to both the thickness of the dermis and the properties of the subcutaneous fat. Results also depended on the distance between the applied shear disturbance and the measurement point.

The strip experiment developed to measure viscoelastic properties of skin yielded repeatable results for the storage and loss extensional modulus in the frequency range of 200 to 1000 Hz.

The analysis suggests that the shear wave experiment is an appropriate experiment to study age related changes in properties of the skin as long as the stratum corneum is kept moist, results are obtained at higher frequencies (2000 Hz) and distances between the input disturbance and the measurement point are kept greater than 3 mm.