*2.3. Measurements and Stimulation of Piezoelectric Nanofiber Nanogenerators (NGs)*

The generated voltage, current, and charge of the NGs generated via dynamic mechanical stimulus from a speaker (8 Ω, 1 W) were assessed by a Tektronix Keithley electrometer 6514. The speakers were driven by sinusoidal signals includinga4V amplitude at different frequencies (2 Hz, 3 Hz, 4 Hz, and 5 Hz) generated by function generator (DS345, Stanford Research Systems, Sunnyvale, CA, USA).

To quantitatively analyze the deformation and potential distribution of P(VDF-TrFE) nanofibers based on NG by a uniaxial stress (~0.5 Mpa), we performed finite element modeling (FEM) by COMSOL multiphysics software (5.3, COMSOL Inc., Stockholm, Sweden).

From the analysis of electrical characteristics, the cell membrane can be equivalently modeled as a resistor capacitor with electrical properties such as extracellular medium resistance (Rcm), membrane resistance (Rm), membrane capacitance (Cm) and ion equilibrium potential (Vm). According to these four characteristics, an equivalent circuit can be constructed [40,41]. The external electrical stimulation is generated by a P (VDF-TrFE) piezoelectric nanofiber NG under a dynamic mechanical vibration, which can be represented as a voltage source. By using the equivalent circuit model, the expected behavior of the effective voltage and current applied to the cell membrane by NG was evaluated. The circuit was simulated by Multism software (14.0, National Instruments Co., Austin, TX, USA).
