The giant flexoelectric effect in a luffa plant-based sponge for green devices and energy harvesters

Abstract

Soft materials that can produce electrical energy under mechanical stimulus or deform significantly via moderate electrical fields are important for applications ranging from soft robotics to biomedical science. Piezoelectricity, the property that would ostensibly promise such a realization, is notably absent from typical soft matter. Flexoelectricity is an alternative form of electromechanical coupling that universally exists in all dielectrics and can generate electricity under nonuniform deformation such as flexure and conversely, a deformation under inhomogeneous electrical fields. The flexoelectric coupling effect is, however, rather modest for most materials and thus remains a critical bottleneck. In this work, we argue that a significant emergent flexoelectric response can be obtained by leveraging a hierarchical porous structure found in biological materials. We experimentally illustrate our thesis for a natural dry luffa vegetable-based sponge and demonstrate an extraordinarily large mass- and deformability-specific electromechanical response with the highest-density-specific equivalent piezoelectric coefficient known for any material (50 times that of polyvinylidene fluoride and more than 10 times that of lead zirconate titanate). Finally, we demonstrate the application of the fabricated natural sponge as green, biodegradable flexible smart devices in the context of sensing (e.g., for speech, touch pressure) and electrical energy harvesting.

Keywords: energy harvester; flexible sensor; flexoelectricity; green device; luffa plant-based sponge.

Fig. 1.

The LS can be utilized as a flexoelectric generator for charging smart devices.

Fig. 2.

Morphological characteristics of luffa. Photos of the (A) fresh luffa, (B) dry LS, (C) natural untreated luffa (Left) and chemical-treated LS (Right), and (D) LS placed on the stamen to illustrate its lightweight characteristic. SEM images of the (E) macroscopic LS with fibers and pores, (F) and (G) cross-section of a fiber, (H) and (I) oblique section of a luffa fiber.

Fig. 3.

The performance of the LS and comparison with piezoelectric materials. (A) The current (left axis, black) of the LS and the real-time applied displacement (right axis, red) at 2 Hz. (B) The relationship between current and applied strain of the LS of three samples. (C) The mass- and deformability-specific flexoelectricity-mediated current W_sec of the treated LS, untreated luffa, porous PDMS, and porous PDMS/calcium copper titanate (CCTO) composite (23). (D) The density-specific μ₁₁₃₃ of the LS, porous PDMS, porous α-PVDF, and porous PDMS/CCTO composite (23). (E) The equivalent piezoelectric coefficient $d_{33}^{\mathit{eq}}$   of the treated LS, untreated luffa, porous PDMS, PZT, porous β-PVDF, and porous PDMS/CCTO composite. (F) The density-specific $d_{33}^{\mathit{eq}}$  of the treated LS and common piezoelectric materials, including PZT-3, PZT-4, PZT-5H (47, 48), PZT-5A, Ba(Zr_0.2Ti_0.8)O₃-50(Ba_0.7Ca_0.3)TiO₃ (BZT-50BCT) (49), PZT (50), ZnO, LiNbO₃, Nylon, Rochelle salt, triglycine sulfate (TGS), KTiOPO₄ (KTP) (51), PVDF, PVDF-TrFE (52), diisopropylammonium bromide (DIPAB) (53), (K,Na)NbO₃ (KNN)-Based (48, 54), BaTiO₃ (BT), PbNb₂O₆ (PN) (55), Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ (PMN-PT) (56), and SiO₂ (57).

Fig. 4.

Theoretical model description. (A) A picture of a sample. (B) A unit cell representing the macroscopic porous structure. (C) Half unit cell with vertical applied force F [blue ones in (B)] and (D) half unit cell with horizontal applied force F [orange ones in (B)]. (E) The schematic plot representing the macroscopic LS with layers of unit cells. (F) A unit cell showing through holes porous structures along a fiber, and (G) the cross-section of a fiber. (H) Comparison of experimental and theoretical values of equivalent piezoelectric coefficients of the LS.

Fig. 5.

Application of flexoelectricity of the LS. Sensor applications for (A) bending angle of fingers (the left y axis represents the real-time current waveform, and the right y axis represents the corresponding peak current.), (B) movement of the throat, (C) pressure sensor, and (D) arrays of the pressure sensor. (E) Circuit diagram of the SGG. (F) LS SGG lights up six LEDs.