DEVELOPMENT OF AN ULTRA LOW POWER WIRELESS SENSIUM PLATFORM PROVIDING PERSONALISED HEALTHCARE FOR CHRONIC DISEASE MANAGEMENTPantelakis Georgiou,1,2 Leila Shepherd,1,2 Panu Kwansuwan,1,3 Damitha Arachchige,3

Matt Addis,1 Timothy G. Constandinou,1 Tor Sverre Lande1 and Chris Toumazou.1

1Institute of Biomedical Engineering, 2Department of Electrical & Electronic Engineering and 3Department of Bioengineering, South Kensington Campus, Faculty of Engineering, Imperial College London.

ACKNOWLEDGEMENTS

The authors would like to acknowledge the Oracle Corporation and Toumaz Technology Limited for supporting this work.

THE NEED FOR REMOTE HEALTHCARE MONITORING

With our ageing global population and chronic disease such as heart failure and diabetes on the rise in the developed world, technology is turning to the needs of society in the development of biomedical circuits and systems. There is a growing recognition that disease management is improved by continuous monitoring of critical parameters for a full assessment of a patient’s medical condition.

Currently such data is acquired through scheduled hospital appointments which are disruptive to the patient’s lifestyle. Measurements taken in this way are subject to diurnal variations and adverse changes are only recognised and treated once they have occurred with a potentially detrimental impact on the patient’s condition.

The clear need for real-time continuous monitoring and analysis of vital signs has lead to research into small, body-worn or implantable biosensors. Our research in this field focuses on the electronic circuitry required to drive these sensors in a low-power and intelligent manner.

Why low-power?

The more power consumed by the sensium chip, the more the user will be disturbed by the battery: either by a physically large device or the need for regular recharging.

Why intelligent?

Continuous transmission of all biosensor data is power-hungry. Communication bandwidth should be reduced through selective transmission, requiring signal processing to:

i) filter noise and other sensor non-idealities from data before wireless transmission.

ii) learn a patient’s normal operating range for a given parameter and send data with greater frequency or transmit alarms when that parameter appears abnormal.

Why real-time continuous?

Circadian rhythms and day-to-day changes in activity and diet can mean that one-off blood tests do not always detect problems.

THE SENSIUM

The Sensium is an ultra low power wireless sensor interface with embedded local intelligence intended for use in the medical and professional healthcare area. Combined with complimentary external sensors, the Sensium can provide ultra low power monitoring of blood glucose, pH levels and ECG signals. Multiple embedded-Sensium “digital plasters” can be used to continuously monitor key physiological parameters on the body. Subsequently, either the raw data or processed information can be wirelessly transmitted to a Sensium server incorporated into a PDA or smart phone. The data can be further filtered and processed there by application software and inserted into a data repository using the ORACLE Healthcare Transaction Base (HTB). The idea is that this database is directly accessible by medical staff to be aware of a patient’s vital signs at any one time. This network also allows for the feedback of diagnostic data from the clinicians to the patients.

ELECTROCARDIOGRAM (ECG)

The raw ECG signal is obtained from 1 or 2 electrodes on the body. Pre-processing tasks such as signal amplification and conditioning are performed on the Sensium chip. In addition, ECG filtering including QRS-wave extraction, peak detection, and aberrant beat classification can all be performed locally in the embedded Sensium processor. Upon detection of aberrant beats, the Sensium typically transmits an alarm to both the PDA or smart phone and the patients health monitor. After synchronization, the Sensium streams a short burst of raw ECG data containing information prior to the alerted incident. This information is useful to the health monitor in assessing whether the patient is in need of immediate assistance.

BIOCHEMICAL MONITORING

The Ion Sensitive Field Effect Transistor (ISFET) is a silicon-based biosensor which allows low power, real time monitoring of pH, blood gas and glucose levels. Combined with the Sensium, pH and blood glucose trends can be monitored alerting the patient proactively on the onset of danger. Also using the PDA the patient may check his blood glucose levels and be informed when to take his insulin dose.

WIRELESS CONNECTIVITY

Information transferral from sensor nodes to the PDA or smart phone is established using short range wireless technology local to each node. Currently Bluetooth™ is used for the prototype devices, soon to be substituted by the Toumaz Falcon™ chipset. This uses a proprietary technology to provide a secure, duplex communication channel optimized for power consumption within the body area network.

Bluetooth NORDIC ANSEM CHIPCON AMIS TOUMAZ

Frequency Band 2.4GHz 2.4 GHz 900 MHz 2.4 GHz 400 MHz 900 MHz

Data Rate 1Mbps 1 Mbps 115 kbps 250 kbps 16 kbps 160 kbps

Modulation GMSK GFSK 2-FSK BPSK ASK GMSK

TX Current 25mA 19 mA 14 mA 17 mA 25 mA 3 mA

RX Current 37mA 25 mA 19 mA 20 mA 7.5 mA 3 mA

Voltage supply 2.7-5.4V 1.9 -3.6V 2.7V 2.1 -3.6V 2.4-3.6V 1-1.5V

Package size 6.3x8mm 5x5mm 6x6mm 7x7mm 5x5mm 5x5mm

RX Power 100mW 47 mW 51 mW 42 mW 18 mW 3mW

Figure 1. Using the ISFET biased in the sub-threshold region of operation it is possible to sense pH (left) and glucose (right) with a current consumption of nanoamps.

The Sensium Vision… The Digital Plaster

Table 1: ISM Band Transceiver Performance Comparison

Figure 2. Designed for low cost, miniature and ultra low power implementation, the Sensium is aimed to being embedded within a “digital plaster”; distributed through prescription .