ABSTRACT
The study of the piezoelectric and pyroelectric properties of biopolymers began with materials of natural origin such as hair, wool, bone, collagen, DNA, protein, and wood [1-8]. More recently, synthetic biopolymers such as the biodegradable and optically active poly-$-hydroxybutyrate (PHB), poly-L-lactic acid (PLLA), useful for biomedical applications, and the poled aromatic polyurea for industrial applications have been subjects of intense research efforts [9,10]. Piezoelectric polymeric materials may also find applications as transducing elements in biosensors and immunosensors because of their good piezoelectric properties when immersed in the liquid phase. Briefly, the majority of crystals without a center of symmetry (noncentrosymmetric) exhibit the piezoelectric effect. The term piezoelectric derives from the ability of a noncentrosymmetric crystal to become polar after the application of a stress. When a piezoelectric crystal has a spontaneous polarization, i.e., a unique polar axis that shows properties at one end different from those at the other end, it is termed pyroelectric. Altering the temperature of these polar crystals produces an electrical charge on the crystal surface perpendicular to the axis of polarization. When pyroelectric crystals show a reversal in the direction of spontaneous polarization upon the application of an electric field, they are further classified as ferroelectrics. Hence a ferroelectric crystal is both pyroelectric and piezoelectric. A through review of the fundamentals of piezoelectricity, pyroelectricity, and ferroelectricity in crystals can be found in the texts by Jona and Shirane [11] and Xu [12].
