Revisiting of δ−PVDF nanoparticles via phase separation with giant piezoelectric response for the realization of self-powered biomedical sensors

Piezo-, pyro-, ferro-electricity in carbon-fluoro polymers such as poly(vinylidene fluoride) (PVDF), its co-polymers and their nanostructures are gaining remarkable technological interest in self-powered flexible electronics. However, to achieve nanostructures of PVDF with preferred electroactive phase and enhanced piezoelectric coefficient is still remains a challenge. In this regard, we have introduced a nanoprecipitation technique to machinate PVDF nanoparticles with predominant piezoelectric delta (δ) phase (which is least studied till date) using the bi-solvent phase separation technique. It is noteworthy that δ-phase of PVDF possess excellent piezoelectric properties which is comparable to β-phase, nevertheless it has been rarely explored because of its ultra-high electric field (~MV/m) based processing conditions adopted so far. In this context, solvent based phase separation approach is most convenient and thus expected to be an industrially viable approach to scale up piezoelectric δ−PVDF nanoparticles which has enormous technological and commercial merits. The piezoresponse force microscopy (PFM) study is performed to confirm the nanoscale piezo- and ferro-electricity in δ−PVDF nanoparticles that manifests the giant piezoelectric coefficient (d33) of ~−43pm/V and optimum phase reversal (∆φ≈ 180°) behavior under very low applied bias (i.e., at ~ ± 5 V). As a proof of concept, a flexible piezoelectric nanogenerator (f-PNG) was fabricated comprising of δ−PVDF nanoparticles. It generates promising electrical output with power density of 2 µW/cm2. Moreover, its ability to track the physiological signal such as arterial pulse detection indicates the potential utility of δ−PVDF nanoparticles based self-power sensors and actuators.