In vivo recovery of mechanical properties in rat skin after repeated strain.

A new method was developed to study the in vivo recovery of mechanical properties of rat skin after repeated strain. Full recovery, i.e., restitutio ad integrum, can be observed only in in vivo experiments but not in in vitro conditions. For the in vivo studies, tabs were fastened on the back skin of rats to test stress-strain behaviour both perpendicular and longitudinal to the body axis. Under anesthesia, skin was extended repeatedly 30 times to an elongation of up to 50% of the distance between the tabs. The experiment was repeated in the same animals at either 0.5, 1, 6, or 16 h. Differences depending on the direction of stretching versus body axis were observed during the first run. A decrease in stress values depending on the logarithm of the number of cycles was found, in both directions, to be attributable to the relaxation phenomenon. The measured stress values after 0.5, 1, and 6 h were considerably lower compared to the first run, thus indicating an incomplete recovery. After 16 h, an almost complete recovery was observed, so that, in perpendicular samples, even higher values were observed, thus indicating an overshooting of the repair mechanisms. The difficulties associated with obtaining appropriate physical and mathematical models for the mechanical properties of skin are discussed.