Biomechanical measurements in microscopically thin stratum corneum using acoustics

Background/aims: This study investigated whether a scanning acoustic micros cope (SAM) could be developed to measure changes in the mechanical properti es of the microscopically thin external layer of skin, the stratum corneum. The adapted microscope was used to determine the effects of various aqueou s reagents, as compared with water, on the thickness, density, compression wave velocity and elastic constant, using 10 mum thin sections of human ski n. Methods: Specimens were exposed to aqueous solutions of glycerol, sodium ch loride and alpha-hydroxy caprylic acid (HCA) to investigate permeation and to investigate the effect of these solutions on the above physical properti es for signal transmission at a centre frequency of approximately 650 MHz i n a path parallel to the epidermal layers. Results: The system of measurement was found to give repeatable results and to provide a reliable indicator of the change in mechanical properties of the stratum corneum that arise from imbibition with different substances. A coustic measurements of untreated specimen thickness and density agreed wel l with optical measurements and with previously published measurements, res pectively. The trend in change of elastic moduli, however, was not in agree ment with the results of the large-scale extensibility tests of Takahashi e t al. and Hall & Hill, which used thicker specimens that contained addition al epidermal or dermal layers. Conclusion: Disparate trends in elastic moduli were believed to be due to t he strain rate dependence of the tissues and the different rates of loading applied during testing.