Giga-ohm high-impedence FET input amplifiers for dry electrode biosensor circuits and systems

Recently with rising health costs and an aging population, there is an increased demand for comfortable monitoring and sensing of biosignals in order to enable and encourage the transition of healthcare services into everyday living including the home, workplace, and during exercise. Sensors can be situated in objects that people interact with daily, such as within a computer, chair, toilet, car, telephone, or any portable personal electronic device. Moreover, the relatively recent wide availability of highly integrated, inexpensive microelectronics with embedded software, open-access wireless protocols, and high-power-density batteries has led many research groups to develop wearable, wireless biosignal sensor-based systems that are worn on the body and integrated into clothing, capable of interaction with other devices that are nowadays commonly in our possession such as a mobile phone, laptop, personal digital assistant (PDA), or Ipods. As this wireless biomedical long-term monitoring moves toward personal monitoring, it demands very high input impedance systems capable of extending the reading of biosignal during daily activities offering a kind of "stress-free," convenient connection, with no need for skin preparation. In particular, we highlight the development and broad applications of our own circuits for wearable biopotential sensor systems enabled by the use of a field effect transistor (FET)-based amplifier circuit with sufficiently high impedance to allow the use of passive dry electrodes, which overcome the significant barrier of gel-based contacts. First, we review the recent state of the art in noninvasive biosensor circuits and systems. Then we present the highlights of our own research on long-term monitoring, in particular for the brain computer interface (BCI), which aims to provide a new communication channel to the human brain that is independent of standard pathways such as muscles and nerves. This innovative and exciting research field is in need of a reliable and easy to use long-term recording system for brain signals (electroencephalogram (EEG)). We then discuss sensor impedance measurements and future directions in this exciting and highly active field and we conclude by presenting our own ideas and research plans for future works.