PIEZOELECTRIC PROPERTIES OF DRY HUMAN SKIN

516
SYNTHETIC POLYMER ANALOGS OF
PIEZOELECTRIC DRY SKIN
In the process of investigating the microscopic origin
of piezoelectric properties of biological materials,
comparison with artificial piezoelectric material
of inorganic and organic nature, has often been of
valuable help [3]. To provide further insight into the
nature of the piezoelectric activity of true epidermis,
two synthetic polymer analogs have been prepared and
characteri zed.
The first physical model is made of a film of uniaxially
oriented polyhydroxybutyrate (PHB), a semicrystalline
biopolymer of bacterial origin which is known to
assume a D (c22) symmetry upon mechanical drawing [20].
PHB samples were supplied by Marlborough Ltd., Stocktonon-Tees,
U.K. in the form of 200pm thick films, uniaxially
stretched to 3.7 times.
The second analog is made of a 120pm thick polyvinylidene
fluoride (PVDF) film which has been purposely
poled under a temperature gradient of 40%C and at a low
electric field (0.5 MV/cm) following the method indicated
by Marcus [211 to concentrate its piezo- and pyroelectric
activity in a very thin layer close to one
of the electrodes. This would in some aspect model the
given picture of basal cell layer piezo- and pyroelectric
properties. Comparative evaluation of the two
polymers with true epidermis samples, performed by the
experimental procedures used in the present work, has
shown a very marked phenomenological analogy between
PHB and epidermis piezoelectric properties.
CONCLUSION
Human dermis, true epidermis, and horny layer, all
show piezoelectric activity. The preponderant contribution
appears to originate from shear piezoelectricity
of collagen network in dermis, and a-helical keratinlike
fibrils in epidermis and horny layer. The horny
layer typically shows the highest activity.
No experimental evidence has been found for the suggested
hypothesis [5] of pyroelectric response in the
epidermis arising from uniaxially oriented tonofibrils
in the basal cell layer being perpendicular to the dermal-epidermal
junction. This hypothesis, however, cannot
be totally discarded because more refined measuirement
techniques are possibly needed to detect the very
low level of pyroelectric activity and thickness compression
piezoelectric response originating from a
single cell layer. Strong contribution to electromechanical
or thermomechanical response of human skin
in the dry state has been found to arise from trapped
charges, as is to be expected, because of the high ionic
content of skin tissue. Finally, we observed piezoelectric
decay with increasing moisture content. This
fact does not corroborate the hypothesis [5] of piezoand
pyroelectricity being a major effect in moist skin
electromechanical properties.
REFERENCES
[1] A.J. Grodzinsky, "Electromechanical and physicochemical
properties of connective tissue" CRC
Crit. Rev. Biomed. Eng. Vol 9, pp. 133-197, 1983.
IEEE Transactions on Electrical Insulation Vol. EI-21 No.3, June 1986
[2] C.T. Brighton, J.Black and S.R. Pollack, Electrical
properties of bone and cartilage: experimental
effects and clinical applications,
Grune & Stratton NY, 1979.
[3] W.S. Williams, "Piezoelectric effects in biological
materials", Ferroelectrics Vol 41, pp. 225-
246, 1982.
[4] M.H. Shamos and L.S. Lavine, "Piezoelcetricity as
a fundamental porperty of biological tissues",
Nature Vol 213, pp. 267-269, 1967.
[5] H. Athenstaedt, H. Claussen and D. Schaper,
"Epidermis of human skin: pyroelectric and piezoelectric
sensor layer", Science Vol 216, pp.
1018-1020, 1982.
[6] J.F. Nye, Physical properties of crystals, The
Clarendon Press, Oxford 1957, Chapter 7.
[7] E. Fukada, "Piezoelectric properties of biological
polymers", Quart. Rev. Biophys. Vol 16, pp.
59-87, 1983.
[8] D. De Rossi, P. Dario and C. Domenici, "The
electret nature of human skin: a model for artificial
tactile sensor arrays" (Abstr.), IVth
Meeting of the European Society of Biomechanics,
Davos (Switzerland), September 1984.
[9] S.B. Lang, "Pyroelectric effect in biological materials."
in Electronic conduction and mechanoelectrical
transduction in biological materials,
B. Linpinski Ed., M. Dekker Inc. NY, 1982.
[10] R.E. Collins, "Measurement of charge distribution
in electrets", Rev. Sci. Instrum. Vol 48, pp. 83-
91, 1977.
[11] A,S. De Reffi, C.M. Guttman, F.I. Mopsik, G.T.
Davis and M.G. Broadhurst, "Determination of
charge or polarization distribution across polymer
electrets by the thermal pulse method and
Fourier analysis", Phys. Rev. Lett. Vol 40, pp.
413-416, 1978.
[12] T. Furukawa and E. Fukada, "Piezoelectric relaxation
in poly(y-benzyl-gluamate)", J. Polym. Sci.
Phys. Ed. Vol 14, pp. 1979-2010, 1976.
[13] E. Fukada, "Piezoelectricity of biological materials."
in Electronic conduction and mechanoelectrical
transduction in biological materials,
B. Lipinski Ed., M. Dekker Inc. NY, 1982.
[14] R.D. Harkness, "Mechanical properties of skin in
relation to its biological function and its chemical
components" in Biophysical properties of the
skin, H.TR. Elden Ed., Wiley & Sons NY, 1971.
[15] W. Montagna and P.F. Parakkal, The structure and
function of skin, 3rd Edition, Academic Press NY,
1974.
[16] E. Fukada, H. Ueda and R, Rinaldi, "Piezoelectric
and related properties if hydrated collagen",
Biophys. J. Vol 16, pp. 911-918, 1976.
[17] H. Maeda and E. Fukada, "Effect of water on piezoelectric,
dielectric and elastic properties of
bone", Biopolymers Vol 21, pp, 2055-2068, 1982.
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