Contents lists available at ScienceDirect Nano Energy journal homepage: www.elsevier.com/locate/nanoen Full paper A fully enclosed, 3D printed, hybridized nanogenerator with flexible flux concentrator for harvesting diverse human biomechanical energy Pukar Maharjan, Hyunok Cho, M. Salauddin Rasel, Md. Salauddin, Jae Yeong Park ⁎ Department of Electronic Engineering, Kwangwoon University, Seoul, Republic of Korea ARTICLE INFO Keywords: Fully-enclosed Hybrid nanogenerator Wrist-wearable energy harvester Self-powered sensor Biomechanical energy ABSTRACT Human body motion is highly regarded as a promising source of energy for powering body-worn electronic devices and health monitoring sensors. Transforming the human biomechanical energy into an electrical energy provides a sustainable energy to drive those devices and sensors, reducing their battery dependency. This work presents a fully-enclosed wrist-wearable hybridized electromagnetic-triboelectric nanogenerator (FEHN) for effectively scavenging energy from the low-frequency natural human wrist-motion (≤ 5 Hz). The FEHN in- corporates the rolling electrostatic induction and electromagnetic induction using a freely moving magnetic ball inside a hollow circular tube. The materials used in 3D printing technology are used as energy harvesting ma- terial for easy, quick and worthwhile fabrication of the FEHN. A thin flexible flux concentrating material is introduced to increase the emf and enhances the electromagnetic output performance. The FEHN can harvest energy under the diverse circumstances and irregular wrist-motions, such as swinging, waving, shaking, etc. Following the experiments, the FEHN achieves an average power density of 0.118 mW cm -3 and can drive a commercial wrist-watch continuously for more than 23 min from just 5 s of wrist motion. This successful de- monstration renders an effective approach for scavenging wasted biomechanical energy and provides a pro- mising solution towards the development of sustainable power supply for wearable electronic devices and self- powered healthcare monitoring sensors. 1. Introduction The express growth of the wearable electronics has shown the in- tense attention of developing a sustainable power supply for them. Specially, wrist-wearable electronic devices are rapidly booming in the fitness and healthcare monitoring application. To date, such electronic devices are powered by either rechargeable or replaceable batteries, which are environmentally hazardous, bulkiness, complicate to replace and expensive. As a result, extensive efforts have been devoted for scavenging the human biomechanical energy [1–7], in order to re- charge the batteries and eventually towards the development of the self-powered wearable electronic devices and sensors [8–16]. Formerly, variety of energy harvesting mechanisms have been utilized for scavenging the biomechanical energy such as triboelectric [17–28], piezoelectric [29–31], and electromagnetic [32–36]. Among these, electromagnetic mechanism shows better output power performance in comparison to other mechanism as it generates higher amount of cur- rent. Since, the output power of nanogenerators alone still remains not enough to significantly power wearable electronics, hybridizing mul- tiple mechanism shows better performance in harvesting energy than the individual mechanism [37–52]. For the first time, Yang et al. in- troduced the hybridized electromagnetic-triboelectric nanogenerator to scavenge mechanical energy, which can obtain more electricity as compared with the individual electromagnetic generator or tribo- electric nanogenerator [33,41,45,48]. These investigations are very important for developing new mechanical energy harvesters. Although the previously reported works are very interesting and innovating, we believe that the wearability of the wearable harvester is also important which we found missing in those works. Also, in our previous work [40], we reported a hybrid nanogenerator which is solely designed and dedicated for harvesting human locomotion. But in reality, we found other different human activities such as working, entertainment activ- ities, computing, education, indoor activities, and so on, are dominant than locomotion in our daily activities, especially when focusing on human wrist motion. Since the previous one was too much specialized for the human locomotion, we further studied, redesigned and ad- vanced a generalized form of wrist-wearable harvester using difference materials and flux concentrators which can scavenge energy at any kind of wrist motion such as shaking, swinging, twisting, waving, etc. along with high output performance. https://doi.org/10.1016/j.nanoen.2018.08.034 Received 4 July 2018; Received in revised form 14 August 2018; Accepted 14 August 2018 ⁎ Corresponding author. E-mail address: jaepark@kw.ac.kr (J.Y. Park). Nano Energy 53 (2018) 213–224 Available online 17 August 2018 2211-2855/ © 2018 Elsevier Ltd. All rights reserved. T