Review on Electromechanical Coupling Properties of Biomaterials.

Electromechanical coupling properties of biological materials, especially cellulose from plant cell walls and proteins from animals, are of great interest for applications in biocompatible sensors and actuators and ecofriendly energy harvesters. On the basis of their anisotropic nanostructures, cellulose and fibrous proteins such as collagen, silk, keratin, etc. are expected to be piezoelectric; however, this property does not necessarily translate to cellulose- or protein-containing bulk materials. In fact, the values of piezoelectric coefficients reported for cellulose and proteins in the literature vary over several orders of magnitude, which raises the question of whether these are truly intrinsic piezoelectric properties of biological materials or whether they are obscured with other electromechanical coupling processes such as electrostriction, flexoelectricity, electrochemical transport, or electrostatic deformation. This critical question about intrinsic and extrinsic electromechanical coupling mechanisms is reviewed in this article. The origin of piezoelectricity of cellulose and collagen (the most widely studied protein for piezoelectricity) is discussed based on their molecular structures. Key requirements to construct macroscopic piezoelectric biocomposites are addressed in terms of packing orders or arrangements of polar domains in composites. On the basis of this structural argument, truly piezoelectric responses of macroscopic materials fabricated with or containing cellulose and collagen are found to be extremely difficult to observe or quantify; most values reported in the literature as piezoelectric coefficients of such materials appear to originate from other electromechanical coupling mechanisms. Clarifying these mechanisms is important to properly design electromechanical devices using biobased materials.