Piezo- and Triboelectric Effects

In Chapter 3, we have learnt that an applied electric field induces dipole moments in a dielectric. Such a displacement of charges generally generates a dimensional change in the material. This is called electrostriction. 

Vice versa, mechanical stress changes the dimensions of the material. This does not usually result in an electrical polarization of the material because most materials have a so-called center of symmetry canceling opposite charge displacements. However, crystals lack such a center of symmetry, and they generate an internal polarization, P, when mechanically deformed. These materials are called piezoelectric, with a direct conversion from mechanical to electrical energy. 

Piezoelectric properties have been found in human hair and other keratinized materials. This is also the case for bone and tendon. Bone remodeling has been attributed to piezoelectricity. The theory is that the mechanical stress on a bone generates bioelectricity that in turn influences bone growth. Because most biomaterials exhibit piezoelectric properties, it is not strange that there are many theories postulating piezoelectric effects in tissue for example, that the transducer mechanism in the inner ear, in the hair follicles, and of touch and vibrational sensitivity is piezoelectric. Results in the literature are often with dry sample, and the question remains as to the importance of piezoelectricity in living, highly conductive tissue. 

Presumably human hair, and perhaps dry SC, has a position not too far from wool. 

From data found in Section 4.1, what is the approximate permittivity of water at 37 °C  

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