1530-437X (c) 2015 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/JSEN.2016.2519392, IEEE Sensors Journal > 1 Abstract— A conventional clinical biopotential measurement system requires the use of wet electrodes that are in contact with skin using conductive gel. Those systems can acquire high quality signals but may not be feasible for a long term purpose due to the skin irritation and allergic contact dermatitis. The emerging of human-centered monitoring calls for alternative solutions. Recent technological advances enable capacitive measurement of electrophysiological measurement which can acquire signals through cloth or even with an air gap; however, difficulties still exist. This paper systemically reviews the recent progress in capacitive measurement from electrode design, analog front end to high level system architecture, aiming to provide comprehensive and practical instructions for entire system design. The current development serves as the solid fundamental, raising new solutions for future improvement. In addition, the challenges and strategies are highlighted in order to demonstrate a technical guide. Some perspectives based on our experience are also discussed hoping to inspire new idea and technique in this area. Index Terms—Physiological measurement; capacitive electrode; non-contact; review; ECG; EEG. I. INTRODUCTION hronic diseases are the leading causes of deaths and disability in the United States. As of 2012, about 117 million people-half of adults-have one or more chronic health conditions [1]. The high cost of prolonged in-hospital care and the annually high expenditure on chronic diseases facilitate the transformation from hospital-centered to proactive, human-center healthcare. Patient comfort, biocompatibility, and operability calls for special attention in the self-monitoring technology. The ultimate goal is to assist people out of hospital without disturbing their daily life. The electrophysiological signals such as Electrocardiography (ECG), Electroencephalography (EEG), Electromyography (EMG), are widely accepted diagnostic tools and standards for medical and research use. The traditional clinical systems with wet electrodes (i.e., Ag/AgCl electrodes) can acquire high quality signals; however, they have limitations in that they causes skin irritation and allergic contact dermatitis; and requires careful skin preparation [2]. To overcome the limitation of traditional wet electrode, capacitive electrode provides an alternative way of surface potential measurement without direct contact with skin. This non-intrusive electrode can sense the bioelectrical signals with Y.Sun is with the Department of Mechanical Engineering-Engineering Mechanics and Department of Biomedical Engineering, Michigan Technological University, Houghton, MI, USA (correspondence email: yes@mtu.edu). X. Yu is with the Department of Civil Engineering and Department of Electrical Engineering and Computer Science, Case Western Reserve University, Cleveland, OH, USA (correspondence e-mail: xxy21@case.edu ). a gap between the skin and the sensor plate, such as hair, cloth, insulation layer or air. It enables long-term monitoring without skin irritations. First introduced by Richardson [3], the potential of capacitive electrophysiological measurement has driven significant efforts in improving the design and performance of the electrode and system in the recent decades [4]-[10]. Various applications have been explored that range from personal health care with sensors placed on bed [11], chairs [12], bathtub [13] and wheelchair [14], exercise assistance [15], to automotive applications [16]-[19]. Clinical experiments are also carried out by some researchers [20]. To acquire high quality electrophysiological signals by capacitive electrodes, a variety of studies have proposed their protocols to address the problem. The proposed solutions can be boiled down to two categories: passive electrode design and the subsequent electronics-analog front end (AFE). First, the design of electrode, including the types of interface [21], material selection [22], geometry [23], polarization and dimension [24], will directly affect the property of skin- electrode interface. The interface has great impact on both signal quality and biopotential instrumentation systems. To evaluate and compare the performance of electrodes, several parameters have been posted including impedance, noise performance, motion sensitivity, susceptibility to moving Capacitive Biopotential Measurement for Electrophysiological Signal Acquisition: A Review Ye Sun, Member, IEEE, and Xiong Yu, Member, IEEE C Power Line/EM Interference Cb idb A B ZinA ZinB ZeA ZeB Zg id1 SW1 SW2 C1 C2 id2 Fig. 1 Equivalent model of different recording situation for capacitive measurement