Listen to Your Heart

Cardiac disease is a major cause of death. Wearable heart monitoring sensors could become an important tool for cardiac patients, ensuring continuous monitoring during daily life. This is essential for an accurate diagnosis of heart problems and for life-saving interventions.

This ECG prototype combines ultra-efficient electronics with standard wireless communication (Bluetooth Low Energy). When streaming heart rate, it has one-month autonomy on a single battery charge.

Due to this large market potential, companies and research institutes are developing products and research prototypes of wearable heart monitoring sensors. The aim is to make wearable, easy-to-use and ultralow- power (ULP) sensor systems with a long battery lifetime.

One-month lifetime
Imec (Leuven, Belgium; www2.imec.be) and the Holst Centre (Eindhoven, Netherlands; www.holstcentre.com) are developing technologies for wearable sensor systems. Recently, an ECG patch research prototype was released that combines a ULP ECG system-on-chip (SoC) with a Bluetooth Low Energy (BTLE) radio. The integration of a standardised BTLE radio is important for a commercial breakthrough of sensor systems. Newly released smartphones will include a BTLE radio, which makes it possible to communicate with BTLE-enabled sensor devices.

The innovative design of the new prototype ensures low power consumption. The ECG patch can compute beat detection and transmit heart rate during one month on a 400 mAh Li-Po battery.

System building blocks
The main components of the ECG patch demonstrator are a dedicated mixed-signal ECG SoC (imec) and a commercial BTLE SoC (Texas Instruments).

The ECG patch demonstrator makes use of a dedicated mixed-signal ECG system-on-chip that consists of an analogue front-end (AFE), analogue-to-digital converter (ADC) and ultralow-power digital signal processor (DSP).

The ECG SoC has three main building blocks. First, the analogue front-end supports concurrent 3-channel ECG monitoring with 1-channel impedance measurement and band-power extraction. The second component is the 12-bit analogue-to-digital (ADC) converter. This ADC is capable of compressing the ECG data by a factor of 5. This reduces the power consumption related to data processing and transmission. Third, a dedicated ultralow-power digital signal processor (DSP) is used for on-board signal processing. It uses an SIMD processor architecture, a hardwired accelerate unit, effective duty cycling, instruction cache and clock gating scheme. This DSP performs multichannel ECG processing with additional signal filtering, ECG feature extraction, analysis and motion artifact removal.

A tri-axial accelerometer is also added to the system. The accelerometer provides additional information that can be used to infer the type or level of activity of the user. The ultimate goal is a health patch that combines different relevant sensor measurements and provides an overall picture of the user’s health status.

The BTLE SoC used in the demo is from Texas Intruments. It retrieves data from the ECG and accelerometer sensors and sends it to a BTLE-enabled device such as the newest smartphones (e.g. iPhone 4S). In addition, a MicroSD card can be used for data logging on the system. A 3.7 V/400 mAh Li-Po battery powers the system. The subsystems are connected through SPI interfaces.

A reconfigurable ECG chip for different applications
The ECG SoC can be configured for different modes of operation and different processing needs, providing a versatile platform that can be used in multiple application domains.

Power consumption breakdown of the ECG patch.

At this time, the SoC implements different modes of operation, for example data collection and beat detection. For the data collection mode only the analogue front-end is running. In beat-detection mode, the QRS complex is detected using an algorithm based on derivative or band-power extraction.

In the ECG patch demonstrator two different operation modes were implemented. In the first mode, beat detection is performed on the ECG SoC and the heart rate is transmitted through the BTLE SoC. Average current consumption is 280 µA at 3.7 V. One month lifetime is achieved on a 400 mAh Li-Po battery.

In the second mode, the ECG is sampled at 256 Hz and 3D-accelerometer at 100 Hz (each acceleration axis), and the data is streamed wirelessly using BTLE. In this mode, power consumption is 5.9 mA at 3.7 V, and an autonomy of 2.5 days is achieved.

ePatch technology
The ECG patch electronics mentioned in this article have been integrated with an ePatch platform from Delta (Hørsholm, Denmark; www.madebydelta.com). The ePatch technology offers a biocompatible, modular and robust mechanical housing for integrating the sensor electronics. The resulting ECG patch demonstrator consists of a disposable patch with ECG electrodes and a cap that contains the electronics and two batteries.

Future work

The use of standard wireless communication such as BTLE makes it possible for sensors to send data to smartphones.

Researchers from imec and the Holst Centre chose to integrate Bluetooth Low Energy in the ECG patch demonstrator because this standardised protocol provides connection to the most recent smartphones. This is an important asset for both the user and the doctor, because it allows them to have the data at their disposal anytime, anywhere. In the future, apps can be developed both for fitness and health applications tailored to the needs of the user.

The combination of the BLE radio and the customised ultralow-power ECG SoC ensures extremely low power consumption and thus long-term monitoring without requiring the user to change batteries.

Further work on the ECG patch concentrates on circuits and algorithms for real-time artifact reduction, arrhythmia detection and health status monitoring. After this, new functionality will be added to the patch to turn it into a complete health patch. In this context skin temperature, bio-impedance and ion concentration monitoring will be explored. Furthermore, an ultralow-power radio compatible with relevant standards will be developed to further reduce power consumption, allowing even longer battery lifetime and smaller size for the system. Finally, new electronic integration technologies will lead to conformable systems, significantly enhancing comfort of use and acceptance for wearable sensors.

The collaboration among companies and research institutes is essential to realise breakthrough solutions for wearable sensor applications and ensure their successful deployment as health monitoring products of the future. For this reason, imec and the Holst Centre work together with many industrial partners in their research on ULP wearable sensors.