Changes in mechanical properties of skin as a marker of biological age

The development of the methodology of biological age assessment belongs to the main direction of the research in contemporary gerontology. The sets ("batteries") of tests for biological age assessment involve also large number of typically biophysical measurements. For example measurements of accommodation range, visual acuity, audiometry, vital capacity, blood pressure, pulse frequency and many others. Measurement of skin elasticity is also included in batteries for biological age assessment in many laboratories. As mechanical properties of the skin belong to the quantities strongly modified by aging, they are potential biomarkers of biological age. Unfortunately, the currently applied methods are dubious from metrological and biophysical points of view for many reasons. As a rule, the time of diminishing of some type of deformation is used for this purpose. The subjective errors are present in considerable extent and interpretation of results is also difficult. Above it, this type of measurements is usually called measurement of skin elasticity but, in fact, the process of diminishing of deformation does not depend only on elasticity but depends significantly also on plastic properties of the skin. In our laboratory we suggested and verified methodology which may eliminate the problems discussed above. The methodology is based on theory of viscoelastic bodies. The principle of the methodology is as follows. The human skin (in vivo) is exposed to constant force for limited time. During the same time and after the deformation of the skin is measured. The resulting curve (creep curve) is dependency of deformation on time. The rheological model of the mechanical properties of the skin can be derived from this curve. The model consists of serio-parallel combinations of two elastic (Hooke's) and two plastic (Newton's) bodies. Rheological parameters of these bodies depend on age. The best marker of biological age prove to be modulus of Hooke's body, which is responsible for rapid phase of elastic response. The creep curves were measured in group of 69 persons of both sexes. The age range was between 25 and 82 years. The elastic modulus of Hooke's body responsible for rapid response was calculated in all cases. Further the following measurements was performed in all persons in experimental group: range of accommodation, threshold of hearing of high frequencies of sound (pitch ceiling), vital capacity of lungs, systolic and diastolic pressures of blood (in rest and after exercise), reaction times on visual and acoustic signals. All these markers where evaluated and compared. The following two parameters were taken into account: relative change per one year and coefficient of correlation. The elastic modulus proved to be marker of biological age of the similar quality as accommodation range, pitch ceiling and vital capacity and better than blood pressures and reaction times. The suggested methodology of measurement of mechanical properties of skin seems to be new, well defined biophysical marker of biological age.