Boosting Energy-Harvesting Performance of PVDF Nanofiber-Based Piezocomposite using Metal-Free Organohalide Crystals

Polyvinylidene fluoride (PVDF) has been widely studied in the field of self-powered active implantable medical devices (AIMDs). Because it has decent piezoelectricity, which is required for sensing various body signals, biocompatibility, and mechanical flexibility. To enhance the polar <i>β</i>-phase of PVDF, which is associated with its piezoelectric behavior, the incorporation of inorganic piezoceramics into PVDF has been a commonly considered approach. The physicochemical interfacial interaction between PVDF and these piezoceramics enables the non-polar <i>α</i>-phase of PVDF to be transitioned to the polar <i>β</i>-phase, resulting in the improved piezoelectric response with a high piezoelectric coefficient (<i>d</i>₃₃). In recent studies, although metal-based piezoceramics, such as lead zirconate titanate (PZT), barium titanate (BTO), and lead magnesium niobate (PMN), have been effective in enhancing the piezoelectric properties of the resulting PVDF-based piezocomposite, they showed limits in terms of mechanical flexibility, biocompatibility, and cytotoxicity, which are important for developing industrially viable AIMDs. In this study, we develop an ultra-thin and flexible piezocomposite by incorporating the metal-free organohalide crystals into the PVDF matrix using the electrospinning process. The electrohydrodynamic spinning process facilitates the interfacial attraction between the polymer chain and the organohalide crystals, which promotes the alignment of the <i>β</i>-phase of PVDF. Herein, 1,4-diazabicyclo[2.2.2]octane (DABCO)-NH₄I₃, which is the metal-free organohalide crystals having ABX₃ molecular structure with hexagonal crystal system and consists of one-dimensional chains of face-sharing (NH₄)X₆ octahedra separated by organic cations, was used. Compared to the bare PVDF electrospun nanofibers, DABCO-NH₄I₃-incorporated PVDF electrospun nanofiber exhibits a significantly improved piezoelectric response with showing a high <i>d</i>₃₃ value of 202 pC/N. The enhanced <i>β</i>-phase of the PVDF nanofibers was examined using the grazing-incidence wide-angle X-ray scattering (GIWAXS) technique. Its high biocompatibility was also demonstrated by the cell viability test using fibroblast cells. The piezoelectric nanogenerator (PENG) fabricated using the piezoelectricity-enhanced nanofibers yields an outstanding energy-harvesting performance with a maximum output voltage of ~ 80 V, which is 10-fold higher than that observed from the bare PVDF nanofiber-based PENG. Since the DABCO-NH₄I₃-incorporated PVDF nanofibers developed here not only have high biocompatibility and flexibility but also show enhanced piezoelectric behaviors, it is expected to show great potential in the field of self-powered AIMDs.