The A*STAR heart sensor allows electrocardiogram data to be monitored remotely for up to a week at a time.
© iStockphoto/dip2000
The continuous monitoring of vital signals represents a great opportunity for the early detection and treatment of chronic disease. However, the usability of electronic health monitoring devices has been hampered by poor battery life, high cost, and the bulkiness of devices. Now, Hui Teo, Minkyu Je and co-workers at the Institute for Microelectronics of A*STAR, Singapore, have built a compact, low-power, low-cost heart sensor that is capable of continuous monitoring and broadcasting for an entire week.
Approximately 99% of the total power used by a continuous-time monitoring system is devoted to the wireless broadcast of data to a remote detector. The team therefore concentrated on making this process as efficient as possible. A critical step was choosing the broadcast frequency. For example, in the popular Bluetooth and ZigBee wireless technology standards, a 2.4 GHz frequency is often used. While this frequency allows for high data transmission rates, it also requires a substantial amount of power. At the same time, high data rates are unnecessary for the broadcast of electrocardiogram (ECG) data. Teo’s team therefore chose to broadcast at 434 MHz, which significantly reduced the power needed. This frequency was generated on-chip using spiral inductors specifically designed for the lower frequency.
The researchers also considered the non-broadcasting components of the wireless sensor in their efforts to reduce overall power consumption. They implemented a type of encoding for their broadcasted data called ‘on–off keying’, which allowed them to completely shut down parts of the circuit when they were not in use, thus reducing power requirements. The processing of the ECG data before transmission was also made more efficient, by adopting an approach called ’delta-sigma modulation’. This allowed the efficient digitization of the analog data gathered by the heart monitor into higher-resolution data for broadcast.
The fabricated wireless sensor occupied an area of only 2.3 × 2.3 mm2, and using a 0.7 V standard button-cell battery, it monitored and broadcasted an ECG signal to a server 10 m away for over 200 hours. This compares favorably with existing continuous-monitoring sensors, which generally broadcast for two days or less. Because most of the components required for the entire data collection and broadcast process are integrated into a single chip, and because of the small chip size, the sensor costs less than US$20. The researchers report that they plan to extend the design to monitoring other vital body signals, such as temperature, blood pressure, oxygen levels and blood sugar.
The A*STAR-affiliated authors in this highlight are from the Institute for Microelectronics.
