Wounds as probes of electrical properties of skin

Wahlsten, O.; Apell, P.

doi:10.5617/jeb.130

Figures & Tables

Electric field pattern at the edge of a semi-infinite capacitor as a very rough model of a “wound” situation. The “wound” is to the left and the intact skin (the capacitor corresponds to the epidermis) is to the right. Field lines go from the bottom plate of the capacitor (potential V0) to the upper (at zero potential; just below the stratum corneum). The dashed line (upper) indicates a path along which the potential is studied and is presented in the lower figure as we move into the “wound”. Notice how well the potential is approximated by an exponential form with a length-scale being the capacitor plate separation (a).

Model cross section of skin with a wound (W) as used in our computational scheme. The relevant length-scales and dielectric permittivities are given in Table 1 for the wound (W), stratum corneum (SC), living epidermis (E), dermis (D), and hypodermis (H). The depth (z) and radius (r) of the wound is set to 2 mm in order to comply with the wound size in the measurements we compare with [14]. The model is rotationally symmetric around the vertical r=0 axis. The actual calculations extend further to the right than shown in the figure (r=50 mm) to assure convergence.

Calculated electrical field around a wound (W). The electric field is directed towards the wound in its lower part. At the top of the wound it points in the other direction. The colors represent the electric potential according to the scale to the right (mV). There is a vanishingly small penetration of the field into dermis and hypodermis owing to their large dielectric permittivities.

Figure 4

Experimental surface potential scanning across a human wound [14]. 0.0 mm corresponds to the middle part of the wound, where the potential has its minimum value. The curve that has a maximum at 0.0 mm (red) is the topographical profile of the wound (swelling makes it peak at wound center). The curve that has a minimum at 0.0 mm (blue) represents the measured potential. Probe size is 0.5 mm.

Calculated wound potential at different probing depths for a wound 2 mm deep with 2 mm radius. The different curves represent different probing depths (see curve legend), starting from the skin surface (bottom curve) going deeper into the wound. Notice the change in curvature, from convex to concave as we go from a region dominated by the upper epidermis charges to the region dominated by the lower epidermis charges. For an intermediate range of depths we have a more intricate signature related to the high dielectric screening in the dermis.

Experimental potential variation in a wide wound [14]. (radius much greater than depth). The over-all potential shape resembles that of a narrow wound (Figure 4). However it has in addition a local maximum in the middle of the wound.

Calculated potential for wounds of different sizes at a constant probing depth of 1.25 mm. The wound depth is in all cases 2 mm (Figure 2). We go from a 0.5 mm radius (narrow wound) to a 3.5 mm radius (wide) wound. Notice that we can reproduce the experimentally found local potential maximum in the center for wider wounds appearing approximately when the radius exceeds the depth.

Summary of modeling parameters used in the different areas_ εr is the static relative dielectric permittivity for wet tissue_

Area	Block	Thickness (mm) [1]	εr
Wound	W	2.0	80 [31]
Stratum Corneum	SC	0.05	10⁴ [32, 33]
Epidermis	E	1.0	10⁶ [32]
Dermis	D	2.0	10⁸ [30]
Hypodermis	H	3.0	10⁷ [30]

Wounds as probes of electrical properties of skin

Figures & Tables

Figure 1

Figure 2

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Figure 4

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Figure 7

Summary of modeling parameters used in the different areas_ εr is the static relative dielectric permittivity for wet tissue_

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