PIEZOELECTRIC PROPERTIES OF DRY HUMAN SKIN

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5;14 i 13 z e1 >1 -Dermis -Oriented Collagen Fag. 2: Temrperature dependence of reaZ (dLf) and imaginary (d"' ) parts of shear stress piezoeZectric coefficient ot human dermis and pure coZZagen (coZZagen data from [13]). to 0.2 pC/N, a factor 5 to 10 times higher than the typical value of true epidermis. Neither pyroelectri c response nor thickness compression piezoelectricity were detected in the horny layer samples. A MODEL OF SKIN PIEZOELECTRICITY BASED ON MICROSTRUCTURAL ORGANIZATION OF FIBROUS PROTEINS Fibrous proteins constitute a major component of skin. IEEE Transactions on Electrical Insulation Vol. EI-21 No.3, / T(C) / June 1986 They are, in the epidermal cells or in the form of fully developed keratin in the horny layer or finally as a three-dimensional network of collagen filaments in dermis (also containing elastin and reticulin) [15]. M _- 0.5 0 -0.5 E 4 z Fig. 3: In-pZane anguZar dependence of the reaZ part of elastic moduZus (c') and piezoeZectric coefficient (dT) for samples of epidermis from the thigh. The organization and orientation of fibrous proteins show distinctive features in the three cutaneous sections which may prove to be related strictly to the particular tensorial form assumed by the piezoelectric coefficients and to the eventual presence of local symmetry in the orientational arrangement, which may be compatible with pyroelectric activity. Fig. S shows a schematic drawing of skin, where the arrangement of fibrous proteins, as determined from X-ray analysis or ultrastructural observations [15], is clearly evident. The collagen network of dermis, as compared to the collagenous structural frame of other connective tissues such as bone and tendons, appears to be less ordered and oriented. Some degree of orientation, however, exists and it varies from one area to the next. For the sake of formulating a structural model of piezoelectricity in dermis, a loose, slightly oriented collagen network is compatible with shear piezoelectricity, but does not support any pyroelectric activity. These findings have been indeed observed in the present study as previously described. A more complex picture needs to be analyzed for what fibrous proteins arrangement is applicable to human epidermis. It appears that piezoelectric properties in epidermis originate from a-helical keratin-like tonofibrils, and their orientational arrangement is crucial to infer possible structures of their corresponding coefficient tensor. An almost uniaxial orientation of tonofibrils perpendicular to the plane of the epidermis is indeed found in the basal cells layer [18], these tonofibrils being seen as precursors of fully developed keratin. The uniaxial arrangements of tonofibrils in the basal cells is progressively lost in upwards epidermal areas to assume a more isotropic, star- Authorized licensed use limited to: University of Texas at Austin. Downloaded on June 7, 2009 at 20:31 from IEEE Xplore. Restrictions apply. 2 o
De Rossi et al.: Piezoelectric properties of dry human skin 0 E 10 'N 1 E - .5 -.5 -1L - - - Rhinoceros horn 14 _ - Skin horny layer o\\\ - - True epidermis -100 -50 0 50 T(C) d4 --- Rhinoceros horn - Skin horny layer - - True epidermis -100 -50 -\ 0 50 T(T) _. \E . '\, /''\ ~~~~~~I Fig. 4: Temperature dependence of real (df4) and imaginary (dr ) parts of shear stress piezoeZectric coefficient of human true epidermis and horny Zayer and rhinoceros horn (rhinoceros horn data from [13]). like arrangement in the spinous layer t15]. In the horny layer the cells become fully cornified and flattened, and filaments account for almost half of the total protein content [15]. These keratin filaments are predominantly parallel to the plane of skin [19]. In the case of uniaxially oriented tonofibrils in the basal cell layer, assuming isotropy in the plane perpendicular to the helical axis, the kind of symmetry Fig. 5: Schematic diagram showing the Layering of skin where the fibrous proteins organization is evident. ExtraceZZuZar coZZagenous mesh of dermis is shown in the bottom, while the intracelZular tonofibriZs in the epidermis are shown to progressiveZy change their orientation from an aZmost uniaxiaZ arrangement perpendicuZar to skin surface in the basaZ ceZZ Zayer to an horizontaZ, aZmost uniplanar arrangement in the horny Zayer. which can be hypothesized is C.co(). This symmetry calls for the following form of the piezoelectric coefficient matrix [7]: II0 0 0 d14 dl 5 0 d = O O d1 5 -d1 4 ° d31 d31 d33 0 This sort of symmetry would also support pyroelectric activity [9]. This reasoning is the basis of Athenstaedt's speculations on the origin of pyroelectric activity he claimed to exist in epidermis [5]. Except for a layer of one cell thickness (the basal layer) a more comples form of d should be postulated, were probably all the 18 d coefficients should be non-zero. The keratin organization in the horny layer allow to suggest a Dm (X22) form of symmetry and consequently [7] a very simple form of the d matrix. 0 0 d14 0 0 d - 0 0 0 0 -d14 0 0 00 0 0 This kind of symmetry supports neither pyroelectric nor thickness-compression piezoelectric properties. The higher degree of in-plane orientation and the larger fractional content of fibrous proteins in the horny layer in respect to true epidermis would suggest a higher piezoelectric activity in the horny layer. This fact has indeed been observed in the present study, Authorized licensed use limited to: University of Texas at Austin. Downloaded on June 7, 2009 at 20:31 from IEEE Xplore. Restrictions apply. 0 (5) (6) 515
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PIEZOELECTRIC PROPERTIES OF DRY HUMAN SKIN

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