The rate of collagen maturation in rat and human skin

Hydrothermal Isometric Tension (HIT) relaxation was observed at the boiling point in rat, cat and human skin samples of different ages, after raising the temperature from 37 degrees to 100 degrees C at a rate of 1.15 degrees C/min. A Maxwellian relaxation to nearly zero was observed at the boiling point. A given velocity constant characterized a given tissue whether experiments were performed under pressure or under tensile stress. However, the velocity constant of the relaxation decreased as a function of age for all three species during the period studied. Moreover, the velocity constant decreased at the same constant rate for skin samples of cats aged 1 to nearly 5 years, of rats aged from 9 to 28 months and of children from birth to 10 years. We propose a model, based on the well known rubber-elastic properties of the denatured collagen network. In this model (1) HIT decreases as an exponential function of time during bond scission along the polymeric chains; (2) the velocity constant of the relaxation process is proportional to the rate of bond rupture and inversely proportional to the number of stable polymeric chains originally present per unit volume; (3) the evolution of the velocity constant as a function of aging finds an explanation. The HIT test should find useful applications in pathology and pharmacology, since it provides rapid, precise information on the stable state of collagen reticulation in small biopsies.