Mechanical Experiments on Skin

A lot of experiments have been conducted to establish the mechanical behaviour of dermis. Mostly the experiments include tensile testing, indentation, suction testing and torsion testing of human skin. Other tests such as elastography, wave propagation and normal traction are also conducted. Skin layers are closely connected with each other. Therefore it is difficult to separate the mechanical behaviour of only dermal layer with epidermal and subcutaneous tissue layer.

2.3.2.1 Tensile Testing

Tensile testing has been used widely in the mechanical characterization of the human skin. The skin is loaded parallel with the surface during a tensile test and two modes used: uniaxial and biaxial. Usually two strips are attached to the skin and pulled apart. These strips have effects on the deformation of the skin due to its adhesive property.

An uniaxial test has been conducted on the human skin (calf area). A pair of strips (10×10 mm) were fixed to the skin at a distance of 5 mm then 12 N load was applied at a interval of 10 and 20 sec. Ultrasound technique was used to measure the deformation of 1.2 mm thick skin. During the test, a non-linear stress-strain graph was obtained. The Young’s Modulus of 4 MPa was found from the test with strain of 0.32 (Manschot and Brakkee 1986). In order to obtain the skin deformation, the skin was mechanically separated from its surrounding area by a frame. To obtain the strain, markers were applied to the skin. Displacement of the strips and amount of force applied were used to solve a skin model (Lanir 1983). In that skin model, the structure of human skin is classified as the direction and distribution of collagen and elastin fibres.

2.3.2.2 Indentation Testing

Indentation test consists of a rigid indenter that is used to apply known load to the skin. a low-pressure indentometry on a 0.2 cm2 forehead skin is used to observe the effect of the stratum corneum due to indentation (Dikstein and Hartzshtark 1983). They have shown that the stratum corneum does not provide any influence to the indentation. However, they have suggested that the condition of ground substance and elastin network has influence on the deformation of skin due to indentation. Another study has shown a use of an indenter to identify the mechanical characteristics of human skin and soft tissues (Bader and Bowker 1983). In that study, measurement of the tissue thickness was carried out. A constant load was applied for 120 sec then removed. The recovery of the soft tissue was observed. The experiment was conducted by varying the load and indenter diameter. Instant deformation, creep, and a long recovery was observed in the skin.

2.3.2.3 Skin Suction

Suction tests are normally carried out to identify the skin elevation. A probe with circular aperture is used for suction. The skin deformation is an output of optical system. Darmaflex and Optical Cutometer are the two commercially available system for suction testing. Two studies show the use of Dermaflex (Gniadecka and Serup 1995; Pedersen et al. 2003). In one of the studies the measuring probe was attached to the skin. The size of the aperture was 10 mm. The elevation measured was 1.8-4.0 mm (on different location of body) (Gniadecka and Serup 1995). Elasticity for different group of subjects were also measured during the test. This type of system only provides information about dermal properties.

On the other hand, Cutometer measures the mechanical characteristics of epidermal layer along with dermal layer. They have used Cutometer to establish the mechanical properties of human skin. A study have shown that a probe with 90 g of weight was used with variable apertures. Variable apertures provide deformation in deeper skin layer by the suction (Barel et al. 1995). They have shown the deformation vs. time and pressure vs. deformation graphs from a suction test.

Cutometer provides mechanical properties of epidermis and dermis but this type of system is very expensive and very sophisticated.

2.3.2.4 Skin Torsion

Another popular method is known as torsion. In this type of test, a mediator disc with a guard ring is attached to the skin. Mediator disc generates the torque. This type of method has very few advantages.

• Anisotropic effect of the skin is reduced

• Underlying soft tissues do not have any impact on measurements

The stiffness of the skin was studied in with an equipment and 28.6 × 10−3 Nm torque was applied for 2 min (Agache et al. 1980; Leveque et al. 1980). The diameter of the mediator disc was 25 mm and the guard ring diameter was 35 mm. They were attached to the forearm skin using adhesive. Then 12.6 kPa pressure was applied to the mediator disc to ensure the contact of skin constantly. The mathematical equation used to identify the Young’s Modulus, E is given below (Agache et al. 1980).

E = M

2 ×Π× 0.4 × e × r1× r2×θ

(2.1)

where M is the torque applied in Nm, e is thickness of the skin, r1and r2 are the radius of

mediator disc and guard ring respectively andθ is the angle of rotation in radians. Young’s Modulus for less than 30 years old group was calculated 4.2 × 105Pa and 8.5 × 105Pa for over 30 group. Using the same group, another test was carried out. They have established the effect of aging on mechanical properties of the skin (Escoffier et al. 1989). In that study, a 25 MHz ultrasound with 70 m resolution was used to measure the thickness of the skin. The diameter of the mediator disc was 18 mm and 24 mm for the guard ring. The torque was 2.3 × 10−3 and 10.1 × 10−3 Nm respectively. Time period was 60 sec. The instant skin deformation was described as Uealong with Uv(viscous).

Also a recovery Urwas found due to removal of torque (shown in figure 2.5). Mathematically

the deformation was defined as:

U(t) = Uv× (1 − e

−t

τ _{) (2.2)}

where τ is relaxation time. Figure 2.5 shows the skin elasticity and recovery due to age. Results from that experiment strongly shows a linear relationship between skin elasticity

Fig. 2.5 Skin deformation curve (Escoffier et al. 1989) and recovery with age.