Skin is the outermost and largest organ in living organisms; it is comprised of
three layers and several sublayers. The three main layers are the epidermis, the
dermis and the hypodermis. The function of this organ is twofold: on the one
hand it provides protection from potential harmful factors (e.g. excessive
thermal and mechanical stimuli), while on the other hand it allows for crucial
interactions with the outside world (e.g. tactile sensations, friction, thermal
balance and transpiration). Skin is also the center of several aesthetic
concerns, due to its role of “external façade” for each individual. Thus,
special interest is devoted by the scientific community to matters as aging,
growth and re-growth of the tissue, as well as to the occurring and development
of skin diseases.
Skin owes most of its peculiarities to its microstructure, which is
characterized by the presence of a dense collagen network, located in the dermal
layer and intertwined with a network of elastin fibers (Fig. 1). Not only the
stress-strain behavior, but also phenomena as skin aging and the insurgence and
evolution of the related diseases can be characterized with reference to this
microstructure (Fig. 2).
Figure 1 – Multiphoton microscopy
pictures of porcine skin. Second harmonic generated images of skin collagen and
elastin networks in the a) unstretched and b) stretched (λ=1.5) configuration.
The collagen network is shown in green, the elastin network is in red, and the
stained cellular nuclei appear as red dots. The stretched configuration shows a
collagen network which tends to align with the direction of the applied
displacement, thus contributing to the tissue mechanical behavior.
Figure 2 – Example of the influence
of collagen on the skin global behavior. The typical curve resulting from a
uniaxial test is comprised of three regions: a toe region where the elastin
structure is mainly responding and the collagen fibers are progressively aligned
according to the external loading direction, a heel region where the collagen
fibers, already aligned, straighten in response to the external load, and a
third region, approximately linear, where these fibers are fully straight and
aligned and are directly loaded. The curve shown has been reproduced by means of
the material parameters published by Barbarino et al. [6t]
Our research focuses on the contribution of the skin microstructure to the
global characteristics of the tissue. The investigation is carried out by
combining in vivo and in vitro experimental techniques for structure
visualization and for the assessment of the tissue mechanical properties (e.g.
confocal and multiphoton microscopy, aspiration test) with finite elements
simulation of the tissue behavior under specific loading conditions (Fig. 3).
Figure 3 – Examples of our investigative effort: a) in vitro multiphoton
microscopy of porcine skin (green: collagen, red: elastin, red dots: stained
nuclei); b) in vivo aspiration tests of facial tissues [13t]; c)
finite element modeling of the face [6t].