Energy materials for transient power sources

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Introduction Recent advances in materials science and consumer electronics have witnessed a growing demand for transience— the ability for technology to function without leaving a permanent mark. For biomedical implants, where the purpose is to monitor and restore biological performance through functions such as nerve/muscle regeneration, wound healing, repairing bone fractures, or smart controlled release of drugs, transient power is essential.1 Biodegradable implantable medical bionics (BIMBs) are expected to provide diagnostic or therapeutic function and to gradually be dissolved, absorbed, or excreted while releasing biologically benign or beneficial products. In this respect, they preserve the sophisticated capabilities of diagnosing and treating disease while eliminating the secondary surgical retrieval and reducing chronic inflammation.2 An energy source is needed to drive BIMBs. The development of implantable bionics is limited by current power sources, including remote wireless power or a traditional lithium battery.3,4 On-board biodegradable power sources can be used to power BIMBs, yet challenges remain with additional demands placed on the materials used. Recently, many reviews have appeared that focus on the deployment of transient electronics for clinical and environmentally

relevant applications.5–7 Advances in materials and structures of implantable biodegradable power sources (e.g., energystorage devices and energy-harvesting systems) are the primary focus of this article.

Materials for biodegradable batteries Bioresorbable metals in primary batteries Local electrochemical batteries are the most common source of energy for implantable medical devices due to their reliable and robust operation. A galvanic cell composed of two metallic electrodes with different electrochemical potentials was clinically used in the first ingestible electronics as a diagnostic tool in clinical work.8 This strategy can be expanded to the use of biocompatible and bioresorbable metals for biodegradable batteries. Mg, Mo, Fe, and Zn are bioresorbable metals possessing the merits of biocompatibility, acceptable degradation rates, and nontoxic soluble dissolution products. Among them, Mg and its alloys are promising electronegative element metals with a standard electrode potential of –2.34 V (versus saturated calomel electrode [SCE]), and a high theoretical capacity of 2200 mAh g–1. Moreover, as most body fluids (i.e., gastric acid, blood, saliva, and urine) function as an ion-transport medium, they are predestined for use as an electrolyte.9

Xiaoteng Jia, State Key Laboratory of Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, China; [email protected] Caiyun Wang, Intelligent Polymer Research Institute/ARC Centre of Excellence for Electromaterials Science, University of Wollongong, Australia; [email protected] Chong-Yong Lee, Intelligent Polymer Research Institute/ARC Centre of Excellence for Electromaterials Science, University of Wollongong, Australia; [email protected] Changc