Stratum corneum conductance

Pruritus of the elderly is another common dermatosis associated with itch.5 One study has shown that elderly patients with generalized pruritus had a higher degree of skin dryness than in age matched control subjects.6 This study also demonstrated that skin surface conductance, a marker of stratum corneum water content, was decreased in elderly patients with generalized pruritus. The study was also successful in demonstrating an acquired abnormality in keratinization in these patients. This abnormality manifested with increased intracorneal cohesion compared with the controls. Another study showed an increase in histamine release and hypersensitivity in patients with senile pruritus.7 Paradoxically, oral anti-histamines are not very efficacious in the treatment of senile pruritus. 

The in vivo flux values for fentanyl agree closely with in vitro flux values obtained with human cadaver skin. In vitro transdermal experiments with fentanyl were conducted using two-compartment permeation cells. Heat-stripped human epidermis was oriented so that the stratum corneum surface contacted the fentanyl hydrochloride solution in the donor compartment. 

While water content of the stratum corneum affects permeability of the tissue, hydration also impacts various physical properties of the membrane such as tensile strength and elasticity, modihes the microenvironment for microorganisms on the tissue surface, alters the thermal conductivity of the tissue and also affects skin appearance. Further, increasing hydration also alters the thickness of the stratum corneum as shown in Figure 4, again taken from data provided by Blank et al. (1984). 

Since percutaneous permeation studies are frequently conducted using laboratory animal models such as rats, mice, and guinea pigs, it should be understood that wide dilferences exist between these models, including the thickness of the stratum corneum, the number of sweat glands and hair follicles, and the distribution of the papillary blood supply. These factors affect both the routes of transport and the resistance to penetration. In addition, the human skin differs from different animal species in biochemical composition and permeability. The subtle biochemical differences between human and animal skin may alter the reaction between permeant molecules and the skin [37]. 

The model to the left in Figure 10.3 is often used to model skin impedance. For instance, R may represent the deeper viable parts and the parallel R and C components represent the poorly conducting stratum corneum (SC). The model to the right may be used for tissue as shown in Figure 10.2. Then G models the extracellular electrolyte, C, the cell membranes, and R the intracellular resistance. Fixed component values in the two models can be found so that they have exactly the same impedance spectrum. 

Proteins in the body liquids may be considered as a colloidal electrolyte solute in a water solvent. Contact with water is the natural state of a protein. In more or less dry form, a protein powder loses some of its electrolytic character it loses the charged double layer on the surface and behaves electrically very differently from protein with water. Such materials may well be mixed conductors—electronic in the dry state and ionic with water content. Keratin is a more or less dry protein found in the natural state of no longer living biological materials such as hair, nails, and the stratum corneum. The water content of such materials is dependent on the relative humidity of the ambient air. The question of ionic or electronic conductivity in proteins is important, and an electronic conduction mechanism must be considered in many cases. 

Thus tissue may be regarded as a conductor or a dielectric, the choice is ours. An electrolytic conductor is characterized by immittance, a dielectric by permittivity or capacitance. However, as we shall see, the conductivity may be complex and thus take care of a capacitive component as well, and permittivity and capacitance may be complex and also take care of a conductance. Muscle tissue is more a conductor with certain capacitive properties stratum corneum is more a dielectric with certain conductive properties. We are in a situation that is confusing for users of bioimpedance data (e.g., medical doctors). Some bioimpedance groups characterize tissue by immittance terminology, others by dielectric terminology. A dermatologist is confronted with skin... 

The stratum corneum layer of nonwetted human skin is strongly variable and may be very poorly conductive. As we shall see, the skin is often dominating the electrical properties of an electrode used for recording endogenic signal sources such as the ECG. This skin influence is therefore treated here. For additional information on skin electrical properties, see Chapter 4. 

Skin sites are preferably palmar or plantar, using bipolar or monopolar leads. SCR is largely determined by the low conductivity stratum corneum shunted by the high but very variable conductance contribution of the sweat content in the ducts in parallel (Boucsein, 1992 p. 59 Edelberg, 1968 Venables et al., 1980). Although SC mainly is a sweat duct—filling variable, the SP is a skin membrane parameter (Venables et al., 1967). 

A prerequisite for using electrical measurements in this way is a detailed knowledge of how the different parts of the skin influence electrical impedance. Furthermore, the current and potential distribution in the skin will also be determined by the electrode geometry, which must be taken into account. As explained in Section 4.2.6, the complex conductance of the stratum corneum and the viable skin converge as the measuring frequency is increased. Measurements at high frequencies will hence normally be largely influenced by... 

Hatzis J, Marks R. The relationship between conductance and capacitance measurements and skin surface contour during hydration of the stratum comeum. In Marks R, Plewig G, eds. Stratum Corneum. Berlin-Heidelherg Springer-Verlag, 1983 257-261. 

Morgan CJ, Renwick AG, Friedmann PS. The role of stratum corneum and dermal microvascular perfusion in penetration and tissue levels of water-soluble contribution of pores to the overall skin conductance. Pharm Res. 1993 10 1699-709. 

Chain length of sodium alkyl sulphate Human skin irritancy Positive reactions at 22.5 mM sol-utions (%) Denaturation of protein Proportion of total sulphydryl groups liberated from ovalbumin (%) Extraction of stratum corneum Increase relative to water proteins amino acids (%) Ion conductance of stratum corneum Rate of change of conductance (Mohm/cm /min xlO ) Cell lysis in vitro Cone. (mM) to release histamine from mast cells Toxicity to mice Intraperitoneal injection mg/kg... 

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