Zinc Ferrite Nanoparticle-Based Wearable Piezoelectric Nanogenerators as Self-Powered Sensors to Monitor Human Motion

With technological advancements in healthcare, the demand for wearable sensors to monitor human movements has become increasingly popular. However, the limited lifetime of the energy sources that power these devices remains a challenge. In this work, we report on a low-cost, flexible, and skin-conformal wearable piezoelectric nanogenerator (PENG) based on hydrothermally grown zinc ferrite nanoparticles, i.e., ZnFe 2 O 4 (ZFO nanoparticles), and poly(dimethylsiloxane) (PDMS) composite as a self-powered sensor to monitor patients’ activities. The FCC cubic spinel crystal structure of ZFO nanoparticles with Fd 3̅ m space group is confirmed by X-ray spectroscopy (XRD) and Raman spectroscopy. The non-centrosymmetric structured ZFO nanoparticles exhibit exceptional piezo-properties, including a d 33 value of 333 pm/V obtained from the piezoresponse force microscopy (PFM) study, which is significantly higher than that of similar class of oxide-based piezoelectric materials. To fabricate the device, the ZFO nanoparticles and PDMS-based composite solution is spin coated on a Cu foil and sandwiched between a pair of Cu foils after drying it. The as-fabricated PENG with an optimized 5 wt % of ZFO nanoparticles exhibits an open-circuit voltage ( V oc ) of 40 V and a short-circuit current ( I sc ) of 0.6 μA upon finger tapping. Further, the PENG is used as a self-powered sensor to monitor various human activities, including human gait, fall detection, and joint movement. The fabricated PENG demonstrates excellent stability with 95% voltage output retention after 60 days. The work demonstrated here has enormous potential in the healthcare sector, which involves patient rehabilitation for musculoskeletal abnormalities, leg injuries, and elderly assistance including geriatric rehabilitation, etc.