Page 346 - Advances in Biomechanics and Tissue Regeneration

17.2 BIOMECHANICS IN THE CONTEXT OF THE SKIN 345

the dermis, provides most of the mechanical strength of skin [33]; elastin fibers form a network in dermis and works
like an energy storage device, bringing stretched collagen back to a relaxed position, which means they provide to
the skin the ability to recoil after deformation [3]; and glycosaminoglycans, such as dermatan sulfate, hyaluronic
acid, and chondroitin sulfate, provide to the skin their viscoelastic nature at low loads [34]. Glycosaminoglycans
are covalently linked to peptide chains that form high-molecular-weight complexes called proteoglycans. The vis-
coelastic properties of connective tissue are strongly correlated with the type and the number of
glycosaminoglycans [35].
Skin, as the outer shield of the human body, has often their mechanical integrity threatened and therefore needs to
hold appropriate mechanical properties to respond correctly to the external mechanical forces. To understand the
mechanical properties of skin, one must understand the main terms used to describe them. The following subsections
highlight the basic definitions of the physics terms important to the understanding of the skin mechanics.
Stress and Strain
Most of the times materials are in a state of stress meaning that a force is being applied, and this will cause a change
in its dimensions. Thus stress can be defined as the ratio of the applied force F (in newtons, N) to cross-sectional area A 0
2
(in square meters, m ) of a material. Depending on the force and the direction that is applied, three basic types of stress
can be made on a material: tensile (σ), compressive (σ), and shear (τ) stress (Fig. 17.2).
Strain, the change in material dimensions produced by a force that varies on its nature (tensile or compressive (Ɛ), or
shear (γ) force), is calculated by the ratio of the change in length (ΔL) to the original length (L 0 ). Strain is dimensionless
and sometimes represented as a percent change. The relation between the applied force and strain, or their rates, allows
the assessment of material mechanical properties.
Elasticity, Inelasticity, and Viscoelasticity
Materials display different mechanical behaviors when an external load is applied and then removed (Fig. 17.3).
Elasticity describes the material ability to deform instantaneously when a load is applied and to return immediately
to its original state once the load is removed. On the other hand, inelasticity is the property of a material that keeps it
permanently deformed by a force, which means that the material does not return to the original configuration when
unloaded, remaining in the deformed state. Inelastic materials included viscous (fluids) and plastic materials, while
viscoelastic ones combine the characteristics of both elastic solids and viscous fluids when undergoing deformation
[36]. The viscoelastic materials exhibit time-dependent strain showing a “fading memory.” Such behavior can be linear
(stress and strain are proportional) or nonlinear.
Stiffness, Young’s Modulus, and Strength
Stiffness describes the property of a material to resist the deformation under applied loads. The stiffness of a mate-
rial is commonly defined by Young’s or elastic modulus (E), determined by the slope of the linear region of a stress-
strain curve when tested under tensile or compressive loads. Strength is a measure of the material resistance to failure
by fracture or excessive deformation and can be defined as the maximum stress that the material supports before
breaking. The strength of the materials varies with the nature of the applied force. For instance, there are materials
with higher strength under compression but lower strength under tension.

2
FIG. 17.2 Schematic description of the different types of applied stresses, expressed in newton per square meter (N/m ) or Pascal (Pa), where
2
2
1Pa¼1N/m in the international system (SI). The units used for molecular, cell, or tissue levels are usually nanonewton per square micron (nN/μm )
2
or kPa (where 1 nN/μm ¼1kPa).
II. MECHANOBIOLOGY AND TISSUE REGENERATION