Accounting for natural tension in the mechanical testing of human skin.

A low-pressure suction device has previously been used to obtain quantitative information on the mechanical properties of human skin. Reducing the raw data from this technique was complicated by the fact that the skin is already biaxially loaded in the natural state and a rational basis on which to compare results obtained from different subjects was not available. Using a strain gauged pretension device, a procedure for determining the natural state tension and extension fields in the skin has been developed. The natural tension was then relaxed in the direction of testing and the suction device used to determine the two-dimensional skin tension-extension ratio response of the skin at a constant testing rate. The data from a number of subjects have been used to develop a new multidimensional stress-strain theory in terms of two material constants that are related to basic material characteristics of the dermis and are uniquely determinable regardless of the natural state stress field. Tests performed on the upper backs of 23 healthy adult males were used to investigate the variations in the material constants with age and sun-exposure habits. Age variations were found to corroborate earlier studies, and variations with sun exposure habits were related to known dermal collagen and elastin changes due to ultraviolet radiation. The apparent sensitivity of the testing procedure to physiologic state variables holds out the hope that mechanical properties characterization will be a useful tool in the evaluation of the severity of certain pathologic states and the effect of therapy.