Wafer-scale integrated micro-supercapacitors on an ultrathin and highly flexible biomedical platform Jimin Maeng & Chuizhou Meng & Pedro P. Irazoqui # Springer Science+Business Media New York 2015 Abstract We present wafer-scale integrated micro- supercapacitors on an ultrathin and highly flexible parylene platform, as progress toward sustainably powering biomedical microsystems suitable for implantable and wearable applica- tions. All-solid-state, low-profile (<30 μm), and high-density (up to ~500 μF/mm 2 ) micro-supercapacitors are formed on an ultrathin (~20 μm) freestanding parylene film by a wafer-scale parylene packaging process in combination with a polyaniline (PANI) nanowire growth technique assisted by surface plasma treatment. These micro-supercapacitors are highly flexible and shown to be resilient toward flexural stress. Further, direct in- tegration of micro-supercapacitors into a radio frequency (RF) rectifying circuit is achieved on a single parylene platform, yielding a complete RF energy harvesting microsystem. The system discharging rate is shown to improve by ~17 times in the presence of the integrated micro-supercapacitors. This result suggests that the integrated micro-supercapacitor technology described herein is a promising strategy for sustainably powering biomedical microsystems dedicated to implantable and wearable applications. Keywords Energy storage . Flexible . Implantable devices . Micro-supercapacitors . Wafer-scale integration 1 Introduction The trends in modern portable consumer electronics and bio- medical devices (small, lightweight, flexible, and sustainable) call for development of an ultrasmall (millimeter- to microme- ter-sized) energy storage system that can be integrated in the final product. In recent years, micro-supercapacitors have attracted significant attention as a new class of miniature energy storage devices to meet this demand. This can be attributed to their traits of fast charging, long cycle life, and compact volume (Chmiola et al. 2010; Pech et al. 2010; Meng et al. 2010; Wang et al. 2011; El-Kady and Kaner 2013; Si et al. 2013). Micro- supercapacitors can store anomalously large quantities of charge either by charge accumulation in the electric double layer formed at the electrode/electrolyte interface (electric double lay- er capacitors; EDLC), or by fast and reversible redox reactions on the surface of the electrode material (pseudocapacitors). In addition, exploration on a wide variety of new electrode mate- rials (Wang et al. 2007; Sharma et al. 2008; Zhang et al. 2008; Zang et al. 2008; An et al. 2010; Mujarwar et al. 2011) as well as electrode morphology (Gupta and Miura 2005; Wang et al. 2006; Wang et al. 2010; Li et al. 2010; Xiong et al. 2012; Tan et al. 2013) have significantly extended the energy density of micro-supercapacitors, making them an attractive candidate for energy storage applications in replacement of conventional ca- pacitors or microbatteries. Unfortunately, such advancements in micro-supercapacitors have mostly been limited to material or device level, while this field is wanting for a study that explores their integration and practical utilization in microsystems. One conceivable reason for this is the difficulty in establishing a compatible wafer-scale integration process to incorporate heterogeneous materials comprising a microsystem: a base substrate (rigid or flexible), system circuitry (printed metals and surface-mount compo- nents), and micro-supercapacitors (electrode and electrolyte). In particular, it is generally even more difficult for flexible micro-supercapacitors to be integrated into a microsystem due to the lack of mature process technology regarding the flexible materials that can enable their monolithic integration. For this reason, flexible micro-supercapacitors usually tend to demon- strate fabrication and characterization only at device level (Meng et al. 2010; Yuan et al. 2012; Si et al. 2013; Niu et al. J. Maeng (*) : C. Meng : P. P. Irazoqui Center for Implantable Devices, Weldon School of Biomedical Engineering, Purdue University, West Lafayette, IN 47907, USA e-mail: jmaeng@purdue.edu Biomed Microdevices DOI 10.1007/s10544-015-9930-4