PIEZOELECTRIC PROPERTIES OF DRY HUMAN SKIN

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5 I 2 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: i 6 =1 i _j IEEE Transactions on Electrical Insulation Vol. EI-211 No3` June 1986 i (l When studying piezoelectric properties of biopolymers it is customary [7] to define a cartesian coordinate system where the z-3 axis coincides with the assumed prevailing orientation of electric dipoles, which for a-helical proteins, is colinear with the helix long axis. The x_1 axis is assumed to be perpendicular to to the skin surface. Following these definitions, d14 represents the shear piezoelectric coefficient obtained by measuring the electrical charges on electrodes deposited on skin surfaces, generated by applying an uniaxial mechanical stress in the skin plane directed 450 from the preferential orientation of piezoelectric protein fibers. Because skin shows relaxational dispersion [8], the d coefficients are in fact complex quantities having an in-phase d' real part and an out-of phase d" complex part. A frequency dependence of the piezoelectric coefficients in this case has to be expected. Angular dependence in the plane of the samples (dermis, true epidermis and horny layer) of d' and of the real part of the elastic modulus c' have been measured at 25°C and at the frequencies of 1, 10, and 30 Hz using a Rheolograph Piezo (Toyo Seiki Seisaku-sho, Tokyo, Japan). The dependence of real d' and imaginary part d" of shear piezoelectric coefficient on temperature from -100°C to 1000C at 10 Hz have also been measured, using the same apparatus. Thickness-compression piezoelectric measurements have been performed by applying square wave stress pulses generated by a flat circular plastic pin, driven by an electromagnetic shaker. Load readings have been performed by means of a miniature load cell (Type 9201, Kistler Instrumente AG, Winterthur, Switzerland) mechanically in series with the sample, while charge generated by skin samples were detected by means of a charge amplifier (Type 5007, Kistler Instrumente AG, Winterthur, Switzerland). Two different methods have been used to determine the eventual pyroelectricity of skin samples. The first method, also called the static method [9], is the only one capable of giving absolute values of the pyroelectric coefficient. The measurements have been implemented using a Thermo Controller Unit (Toyo Seiki, Seisaku-sho, Tokyo, Japan) where the temperature has been raised at a rate of 2°C per minute. The voltage appearing at the electrodes was measured with a very high input impedance (1015 Q) voltmeter built around a FET-input electrometer operational amplifier (AD 515 J, Analog Devices, Norwood, MA, USA). This method, however, is known to be effected by errors related to voltages not of pyroelectric origin which are spontaneously generated by the sample and by voltages generated by any piezoelectric material when it is nonuniformly heated (tertiary pyroelectric effect). To provide an alternative measuring techniques, the rectangular pulse heating method [9] has been used, although this method is capable of providing only relative values of the pyroelectric coefficient. A sharply focused 50 W incandescent lamp was used as light source and light pulses of 1 s duration were delivered to the skin samples to change the temperature. However, both the static method and the classical rectangular pulse heating method are only capable of providing pyroelectric informations integrated over the sample thickness. To probe the spatial distribution of polarization across the thickness of the epidermis sample, as required to evaluate the hypotesis of a pyroelectric activity in the basal cell layer tonofibrils, the Collins' thermal pulse method [10] has been used. A very short (1 ms) duration light pulse, generated by an electronic flash lamp, was delivered to the sample located in a shielded, hermetic cell through a glass window; voltage at the electrodes was amplified with the Ad 515 J operational amplifier and recorded by means of a 7912AD Tektronix transient recorder. The physical basis of this method resides in producing, by means of thermal diffusion, a transient inhomogeneous strain across the sample, in order to measure its inho-moc,neous pyroelectric res-ponsc. Altlhough the information about the polarization distribution is convoluted with a time-dependent temperature distribution, a deconvolution process is typically applied to the experimental data to obtain the first few Fourier coefficients of the polarization distribution [11]. For the purpose of the present work, however, we were only interested to detect a limiting case of spatial distribution of polarization inside epidermis where the pyroelectric response may be eventually concentrated only at one very superficial layer of the sample. In this simple case, qualitative inspection of the volatage-time responses obtained by alternating the side of the sample in respect to the impinging light is sufficient to determine eventual strong asymmetry in the pyroelectricity spatial distribution [11]. Authorized licensed use limited to: University of Texas at Austin. Downloaded on June 7, 2009 at 20:31 from IEEE Xplore. Restrictions apply.
De Rossi et al.: Piezoelectric properties of dry human skin Dermis RESULTS Typical recordings of the angular dependence of the real part of the piezoelectric shear coefficient and elastic modulus are shown in Fig. 1 for dermal samples. Fig. 1: In-pZane anguZar dependence of the reaZ part of eZastic modulus (c') and piezoeZectric shear coefficient (d') for a sanple of dermis from the breast. The values of d'(max) at room temperature have been found to be of the order of 0.05 to 0.1 pC/N. The presence of one or two maxima in the in-plane polar behavior of c' obtained for different samples from different anatomical sites indicates slight preferential uniaxial or biaxial orientation of collagen fibrils in the dermal three-dimensional structural network. The invariably observed bisinusoidal behavior of d' polar response is similar to the one observed in oriented collagen structures and uniaxially oriented synthetic polypeptides [7]. This kind of anisotropy can be generally expressed through the relation [12] d' = 0.5 dl4sin(2e) (2) In the case of prevailing uniaxial anisotropy, such as shown in Fig. 1, for a composite piezoelectric material where the degree of orientation is low, the following expression can be used [12], assuming collagen to originate piezoelectric activity: d' = 0.5 d04 1cF sin[2(O - 0 )] (3) where dC'4 is the shear piezoelectric coefficient for the single crystallite, b the volume fraction of the paracrystalline piezoelecYric component (collagen) in the sample, Fc the degree of orientation of collagen fibers, and 0c the angle of average direction of collagen crystallites orientation. Referring to Fig. 1, if we assume the angle of average direction of collagen crystallites orientation corresponds to the maximum of angular response of the real part of elastic modulus, 8c=90 '. The relationship d(e) should be hence represented by the function: d' = 0.5d1c v F sin[2(O - 2J] (4) As the maa itude of d 4 has been measured to be 6 pC/N (18.2xlO- cgsesu) [13], the volume fraction of collagen 4v in dried dermis is approximately 0.75 [14] and the measured d'(O) for dermis has been found to be 3.3xlO09 cgsesu, it follows from Eq. 4 that F..= 0.05. This low value of F. would indeed imply an almost random distribution of collagen fiber orientation in the plane of the dermis. This finding is indeed compatible with ultrastructural observations [15]. To obtain further evidence of collagen being at the origin of piezoelectric properties of dermis, the temperature dependence of the real and imaginary components of d of dried dermis have been measured and compared with the temperature dependence of df4 and d'c4 of pure, uniaxially oriented collagen at the relative humidity of 39% [16]. As shown in Fig. 2, the pattern of the two curves is very similar, although a shift is present along the temperature axis. It is known [17] that increasing water content shifts the d'(T) curve toward lower temperature, so we ascribe this discrepancy to the higher water content of the pure collagen sample. Epidermis Typical recording of c' and d' angular dependence are shown in Fig. 3 for samples of whole epidermis (horny layer was not removed). The value of d'(max) at room temperature has been found to be typically of the order of 0.01 to 0.03 pC/N. As in the case of dermis, piezoelectric response can be described in terms of in plane shear stress piezoelectricity originating from slight preferential orientation (uniaxial or biaxial) of fibrous proteins in the tissue. In Fig. 4 the temperature dependences of d' and d" are reported for true epidermis and horny layer, together with rhinoceros horn samples. Close similarities can be observed. It appears that, in analogy with rhinoceros horn samples, the piezoelectric response of human epidermis can be ascribed to keratinlike a-helical fibers. True piezoelectric response (polar) to uniaxial thickness compression has not been observed in epidermal samples, although erratic stressgenerated potentials usually were observed. Thermal pulse experiments were performed on samples of epidermis to eventually verify the suggested hypothesis [5] of basal cell layer tonofibrils being responsible for the suggested piezo- and pyroelectric properties of epidermis. Thermally generated electric signals were not clearly pyroelectric in nautre, and sample reversal in respect to the radiant heat source did not show any marked asymmetry in charge response. Horny Layer 513 The characteristic features of piezoelectric response of the horny layer have been found to be very similar to those of true epidermis. However, d'(max) of the horny layer has been found to be of the order of 0.1 Authorized licensed use limited to: University of Texas at Austin. Downloaded on June 7, 2009 at 20:31 from IEEE Xplore. Restrictions apply.
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PIEZOELECTRIC PROPERTIES OF DRY HUMAN SKIN

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