1900783 (1 of 10) © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.advmattechnol.de FULL PAPER Broadband, Tunable, Miniaturized Vibration Energy Harvester Using Nonlinear Elastomer Beams and Stretchable Interconnects Zining Yang, Nishana Ismail, ChangHee Son, Placid M. Ferreira, and Seok Kim* DOI: 10.1002/admt.201900783 deal of research has gone into the develop- ment of vibration energy harvesters (also known as vibration power generators) over the past decade. Based on the structural architecture, vibration energy harvesters can be categorized as either resonant gen- erators (or inertial force generator) or direct force generators. [4] Instead of har- nessing the deformation of a material (e.g., pressing, flexing, or stretching motion) induced by direct forces, a resonant gen- erator typically consists of a rigid casing with a seismic mass suspended inside. [4] To date, most of ambient vibration energy har- vesters adopt the resonator design, because it only requires one point of attachment to the vibration source which makes the mounting more convenient. [4] The mass moves relative to the casing in response to external vibration and simultaneously, con- verts kinetic energy into electricity through electrostatic, electromagnetic, piezoelectric, or triboelectric trans- duction mechanisms. [5–8] Despite the considerable research effort on vibration energy harvesters, several challenges still exist, which limit the wide- spread usage of resonant vibration energy harvesters. For a practically useful resonant generator in the context of wireless sensor nodes, at least four criteria need to be satisfied. (1) The device needs to be highly miniaturized (e.g., footprint <1 cm 2 ) to fit in a typical wireless sensor node. [3] This goal can be achieved by the adoption of microelectromechanical systems (MEMS) technology. [1] (2) A low operating frequency which matches the frequency range of the target vibration sources is needed to obtain maximum efficiency. [9] However, typical resonant gen- erators (such as MEMS energy harvesters) are based on springs made of rigid materials which intrinsically tend to exhibit high resonant frequencies. [10] (3) Broadband response and (4) reso- nance tunability which can significantly improve the efficiency as most real-world vibrations have widely distributed frequency spectra or time variant frequency peak. [11–13] However, typical resonant generators are based on linear resonators which have narrow bandwidth and a single resonant frequency. [11,14–16] Here, we demonstrate a design construct and materials for a soft-rigid hybrid device enabling broadband, resonance tun- able vibration energy harvesting and self-powered motion sensing. The key concept of the reported design, which is inspired in part by recent works on soft electronics and soft A miniaturized vibration energy harvester, a small yet sustainable power source that converts ambient mechanical vibration into electricity, is consid- ered as a key technology to advance wireless sensor networks for the internet of things. Conventional chip-scale vibration energy harvesters, such as micro- electromechanical systems devices that are mostly based on rigid materials (e.g., silicon), inherently exhibit high resonant frequency, narrow bandwidth, and a single peak frequency. Therefore, they are often unsuitable for many real-life applications, as most ambient vibrations have low frequency, broad spectrum, and time variant resonance. Here, an unconventional, soft-rigid hybrid architecture for vibration energy harvesting, which is inspired by soft electronics, is presented to overcome these limitations. By harnessing soft materials undergoing large deformation, the reported device is designed and tested to demonstrate its energy harvesting performance with high min- iaturization, low operation frequency, broadband spectrum, and resonant frequency tuning. Dr. Z. Yang, Dr. N. Ismail, C. Son, Prof. P. M. Ferreira, Prof. S. Kim Department of Mechanical Science and Engineering University of Illinois at Urbana-Champaign 1206 W. Green St., Urbana, IL 61801, USA E-mail: skm@illinois.edu The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/admt.201900783. 1. Introduction The continuous advancements in electronics have opened the door for exciting applications such as the Internet of Things, which involves a large number of wireless sensor networks. However, the realization of this vision is hindered by a lack of long-lasting wireless power source. Since a wired power supply is not an option, each wireless sensor node has to rely solely on the stored energy from batteries, which introduces a prohibitive cost associated with the inevitable battery replacement and recharging events. [1] To reduce the need for battery maintenance, harvesting wasted energy from the ambient environment as a method to power up wireless sensors has obtained great attention from both academia and industry. [2] Among forms of ambient energy sources, vibration energy has gained the most interest because of its abundance in various environments. [3] Consequently, a great Adv. Mater. Technol. 2019, 1900783