Dry Human Skin

Mechanoelectrical conversion properties of dry human skin have been examined and their origin investigated in dermis, epidermis and horny layer. Shear stress piezoelectricity is observed in all three cutaneous sections. Piezoelectric properties of dermis can be ascribed to its collagen structural network, while the piezoelectric properties of epidermis appear to originate from partially oriented a-helical keratin-like tonofibrils. The highest piezoelectric coefficients have been found in horny layer samples. The values of thickness-compression piezoelectric coefficients were so low that they were not discernable from the intrinsic noise of the measuring system. Although thermally generated currents have been observed in the course of thermal pulse experiments, no evidence of relevant contributions from pyroelectric (polar) responses has been noticed in all cutaneous compartments. INTRODUCTION Different biological tissues show the ability to convert mechanical and thermal stimuli into electric signals [1]. Extensive studies are available [2], examining mechanoelectrical properties of bone both in the dry and in the moist, physiological state. The hypothesis ascribing bone mechanoelectric transduction to piezoelectric properties of the collagenous structure in the dry state and to electrokinetic phenomena (streaming potentials) in the moist state nowadays has a large consensus [3]. The eventual role of these phenomena on bone regrowth and healing is presently largely debated. However, the mechanoelectrical properties of human skin have been very little studied and their origin is largely unknown. Shamos and Lavine [41 have reported observations of direct piezoelectric effects in dry, whole human skin samples from the forearm and callus from the sole. These authors, by analogy with the behavior of bone and tendons, ascribe the piezoelectric properties of human skin to oriented collagen fibrils in dermis. Athenstaedt et al. [5] have examined fresh human skin samples both in vitro and in vivo, reaching the conclusion that the observed electrical signals obtained in response to mechanical or thermal stimuli are intrinsically of piezoand pyroelectric nature. The same authors reported measurements performed on dry skin observing a reduced (20 to 60% less) mechanoelectrical activity compared to moist samples. It is worthwhile noting that, while Shamos and Lavine ascribe the piezoelectric properties of human skin to dermal collagen network, Athenstaedt et al. suggest that these properties originate-from uniaxially oriented polar keratin filaments in the basal cells layer of epidermis. To clarify the nature of mechanoelectrical transduction phenomena in human skin and to comprehend better their microscopic origin, a systematic study of these properties both in the dry and moist state has been undertaken. We report here results obtained on dry human skin samples. SAMPLE PREPARATION Several samples of human skin from breast, thigh, pre0018-9367/86/0600-0511$01.00 @ 1986 IEEE 511 Authorized licensed use limited to: University of Texas at Austin. Downloaded on June 7, 2009 at 20:31 from IEEE Xplore. Restrictions apply. IEEE Transactions on Electrical Insulation Vol. EI-211 No3` June 1986 puce, and callus from sole have been obtained from patients undergoing plastic surgery. Prepuce samples were used because they are almost entirely composed of true epidermal tissue (the horny layer is virtually absent). The samples were rinsed in distilled water, cut into circular shape (2 cm diameter) and separated into their dermal, epidermal and horny layer components by microsurgical operations. The samples were dehydrated in vacuum for 12 h. Typical dehydrated sample thickness was 300 to 400 vim for whole epidermis, 100 to 200 vm for horny layer and 300 to 500 vm for dermis. Electrodes made of conducting silver-loaded paint (Acheson Colloiden B.V., Scheemda, Holland) were applied by brushing on the surfaces of the samples used in piezoelectric measurements. Colloidal graphite electrodes were sprayed on the samples used in thermal pulse experiments. After application of the electrodes the samples were again vacuum dried for two hours. EXPERIMENTAL METHODS Dry skin is an anisotropic material which contains oriented polar crystallites in an amorphous organic matter, and its piezoelectric characterization would in principle require determination of 18 components of the piezoelectric coefficient third-rank tensor d.. (the reduced matrix notation [6] has been used in iie present work). di are the proportionality constants between the applied mechanical stress T and the generated electrical polarization P: