fig1

Figure 1. Working principles, operating modes, and system architecture of PENGs. (A) Direct piezoelectric effect; (B) Converse piezoelectric effect. (C) Schematic illustrations of the d₃₁ and d₃₃ strain modes in piezoelectric materials. (D) Schematic illustrations of the shear piezoelectric strain modes associated with d₁₄ and d₁₅. (E) Structural components of a PENG based on KNLN particles and Cu, adapted with permission from Ref.^[34]. Copyright © 2014 Wiley-VCH. (F) Multilayer structure of an implantable piezoelectric nanogenerator (i-PENG), adapted from Ref.^[35] under CC BY 4.0 license.