Non-invasive and ambulatory monitoring of body parameters is receiving much interest from the medical, sports and entertainment world. Possible applications are the monitoring of brain waves to detect epilepsy, monitoring of muscle activity during an athlete's training, and monitoring of heart rate during gaming. The idea is to develop small, low-power, autonomous biomedical monitoring systems that collect and process data from human body sensors and wirelessly transmit the data to a central monitoring system.
For clinical applications, electroencephalogram (EEG, brain signals), electrocardiogram (ECG, heart signals), and electromyogram (EMG, muscle signals) are common bio-potential signals that are monitored routinely. Patients are typically connected to a bulky, mains-powered device through cables that reduce their mobility and create discomfort.
Recently, researchers in Eindhoven, The Netherlands (IMEC-NL) developed a flexible and stretchable ECG patch to monitor heart activity. All of the components are embedded in a flexible electronic patch that is covered with textile and standard ECG electrodes are used to attach it to the body. Wearable, wirefree and easy to set-up, the system removes the disturbances and discomfort caused by current cardiac monitoring systems. The ECG patch can fit any body curves and allows optimal, personalized placement of the electrodes. It can, therefore, be used to monitor cardiac activity "on-the-move" in daily-life conditions. Placed on the arm or on the leg, the same system can also be used to monitor muscle activity (EMG).
The core of the wireless ECG patch is a miniaturized wireless sensor node integrated on a flexible, polyimide substrate. A unique asset of the sensor node is the ultra-low-power bio-potential ASIC for monitoring the ECG signal. It is a generic programmable readout ASIC which can extract different bio-potential signals (EEG, ECG and EMG). It efficiently deals with the interference and noise problems correlated with these types of signals, and it exhibits very low power consumption, necessary to enable longterm power autonomy. The bio-potential ASIC consists of an analog readout front-end, an analog-to-digital converter (ADC), and a digital signal processing (DSP) unit. The analog readout frontend is responsible for signal conditioning such as amplification and filtering, and it is the most important building block in terms of signal quality. The ADC converts the output of the analog readout front-end into digital domain, so that the DSP unit can perform the signal analysis.
The configurability for different biopotential signals is achieved by adding a stage with configurable gain and filter characteristics after the AC-coupled chopped instrumentation amplifier. While the low-power AC-coupled chopped instrumentation amplifier filters the DC electrode offset and flicker noise, and rejects common mode signals, the configurable back-end stage can be used to configure the gain and filter characteristics of the readout frontend for the needs of different bio-potential signals.
Further, the sensor node includes a commercial microprocessor enabling local digital signal processing, a 2.4-GHz radio link and a miniaturized rechargeable Lithium-ion battery. The battery is placed under the electronic components to ensure the local rigidity required for long-term functioning of the electronic components. In addition, the sensor node features a fork-antenna and a snap-on connector (for connection to the electrode). The total size of the flexible core part is 60×20mm2. Two additional snap-on connectors are coupled to the central part with short wires. The complete system is then integrated into textile to form the ECG patch.
Using the ECG Patch
The wireless ECG patch can work in continuous monitoring mode, in which the ECG — or EMG — data is continuously transmitted to the receiver (sampling frequency between 250 and 1000Hz). For cases in which only heart rate information is required, the heart rate can be computed locally on the node and then sent over the air to the receiver. This allows drastic reduction of the use of the radio and hence increases the autonomy of the system. The embedded miniaturized rechargeable battery offers a capacity of 175mAh, which allows for an optimal autonomy of the system varying from one day in continuous monitoring to several days for simple heart rate monitoring.
Further improvement of wireless sensor systems targets the improvement of autonomy. The idea is to harvest energy from the environment, which then is used to charge an energy-storage device such as a battery. Ultimately, these small autonomous sensors will be linked into a network paving the way to new applications for therapeutics and diagnostics.