516 SYNTHETIC POLYMER ANALOGS <strong>OF</strong> <strong>PIEZOELECTRIC</strong> <strong>DRY</strong> <strong>SKIN</strong> 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. 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