The mechanical properties of the skin are of importance for various cosmetic and clinical applications. The mechanical properties of the skin are age-, gender-, and race- and region dependent and are influenced by the use of different skin care products and by numerous other parameters and different physiologic conditions as well as skin disorders. Therefore, monitoring alteration in the mechanical properties of the skin as reflected in changes in its viscoelasticity help to diagnose skin lesions or diseases as well as evaluate cosmetic treatment results.

A number of techniques have so far been introduced in experimental and clinical medicine for quantitative in situ evaluation of the mechanical properties of skin. These assays, based on different physical principles, include indentometry, uniaxial tensiometry, torsion measurements, skin compliance to suction, and measurements of speed of elastic shear wave propagation in the skin. The measurement of the speed of propagation of elastic shear waves in viscoelastic materials may be the most accurate noninvasive assay method for the evaluation of their mechanical properties. The portable and user-friendly viscoelasticity skin analyzer (VESA) is the advanced device based on this principle.

Skin is a heterogeneous anisotropic (directional variations in viscoelasticity), non-linear viscoelastic material due to the nature and organization of skin structure. Skin has both viscous and elastic characteristics when undergoing deformation. Elasticity is the ability to retract back to the original shape after deformation. Elastic materials strain instantaneously when stretched and just as quickly return to their original state once the stress is removed. The viscous component of skin’s viscoelasticity is due to the viscous resistance fibers experience while moving through the ground substance which account for the time dependent behavior (hysteresis) of dermis. The viscous component is associated with energy dissipation and the elastic component is associated with energy storage.

Skin is multilayered (epidermis and dermis) and has different mechanical properties in each layer. Generally, the nonlinear anisotropic viscoelasticity of skin depend mainly on the structural organization of the dermal collagen and elastic fibers network, and water, ground substances in the extracellular matrix with less importance, and with less or neglected contribution by epidermis layer. Although the epidermis is stiffer than the dermis, usually the contribution of the epidermis to the mechanical properties of full thickness skin is neglected. Collagen fibers (type I and type III): and elastic fibers arrangement give out different material properties. Applying mechanical stress to skin changes collagen fibers and elastic fibers arrangement and thereby producing skin’s unique stress-strain curve – nonlinear, hysteresis, creep. Skin has non-linear viscoelastic properties and exhibit hysteresis loop effect with energy loss when deformation occurs. Purely elastic materials do not dissipate energy when a load (stress) is applied, then removed. However, a viscoelastic substance loses energy when a load is applied, then removed. Hysteresis is observed in the stress-strain curve, with the area of the loop being equal to the energy lost during the loading cycle. Viscoelastic biomaterials show a time-dependent elastic behavior – that is – the stress-strain curve is not the same for loading and unloading, leading to hysteresis. Creep is a skin mechanical failure-the result of water molecules displacement from collagen fibers network. Creep is the tendency of a solid material to deform permanently under the influence of stresses.

In order to understand the mechanical behavior of the skin, the mechanical behavior of the dermal components are described followed by the illustration of the stress-strain curve of skin:

1) Collagen fibers – the main constituent of the dermis – form an irregular network of wavy coiled fibers that run almost parallel with the skin surface. The fibers are separate from each other along most of their length and held together by the ground substance. Collagen is characterized by high tensile strength of 1.5-3.5 MPa, high stiffness. The width of the bundles is 1-40 μm. Upon stretching, collagen fibers straightens and realign parallel to one another. Collagen types self-assembly, i.e. tilt angle of collagens (orientation), fiber length, volume fraction of the fibers, collagen molecular stretching contribute to the biomechanical behavior of skin in response to stress (tensile or compressive loading). Energy applied to skin is partially dissipated through viscous sliding of collagen fibrils during alignment with the direction of the stress. Changes in the collagen fibril orientation during deformation of the dermis are critical to maintaining the large extensibility of the skin.

2) Elastin fibers are the second main component of the dermis. They are less stiff than collagen and show reversible strains of more than 100%. The fiber width is 0.5-8 μm. The elastin fibers are responsible for the elastic mechanism (shape recovery after deformation) when stress or deformation is released.

3) The ground substance is responsible for the viscoelastic behavior of the dermis and they do not contribute to the tensile strength of the dermis. Fibers experience viscous resistance while moving through the ground substance.

The relationship between the stress and strain that a particular material displays is known as that material’s Stress-Strain curve. It is unique for each material and is found by recording the amount of deformation (strain) at distinct intervals of tensile or compressive loading (stress). The stress-strain curve of skin for uniaxial tension (uniaxial tensiometry) is nonlinear due to the non-uniformity of its structure, as can be seen in the figure. Four stages of deformation (stretching) can be identified:

I. stage I is the initial stretch: the effect of undulated collagen fiber on the strain (extension) is neglected, elastin is responsible for the skin stretching, and the stress-strain relation is approximately linear. The extension is immediate and reversible when tension is applied, and is therefore called an elastic extension – The elastic component dominates the deformation.

II. Stage II is a gradual straightening of an increasing fraction of the collagen fibers causes an increasing stiffness.

III Stage III , In the third phase all collagen fibers are straight and the stress-strain relation becomes linear again. is the subsequent continuous extension while tension is maintained and is gradual and irreversible and has therefore been termed viscous extension, viscous slip, or creep.

IV Stage IV, is where yielding and rupture of the fibers occur beyond the III stage. Tensile strength can be used to measure at which stress point yielding and rupture occurs.

Tensile strength measures the force required to pull (stretch) something. Tensile strength is the opposite of compressive strength and the values can be quite different. Ultimate Tensile strength (UTS), often shortened to tensile strength (TS) or ultimate strength, is the maximum stress that a material can withstand while being stretched or pulled before necking, which is when the specimen’s cross-section starts to significantly contract. Typical human skin’s ultimate tensile strength (UTS) is ~ 20 MPa

The yield strength or yield point of a material is the stress at which a material begins to deform plastically. Plasticity describes the deformation of a material undergoing non-reversible changes of shape in response to applied forces. Typical human skin’s yield strength is ~ 15 MPa

For strains corresponding to the initial large deformation part of the stress-strain curve (phase I), skin shows elastic behavior. For stage II and III skin shows visco-elastic behavior. Skin exhibits all three features of a viscoelastic material. If the dermis is subjected to consecutively applied load cycles, slightly different curves are obtained and significant hysteresis can be observed. Furthermore skin shows stress-relaxation under constant strain and creep under constant stress. The dermal thickness in vivo is less thick than the excised dermis indicating that skin is in a state of pre-tension (prestress). The extent of prestress varies with age, body location and direction relative to that of the fibers. The network of elastic fibers is believed to be mainly responsible for the prestress.