Condition monitoring in a heartbeat

An imminent breakthrough could radically alter both low-cost condition monitoring of healthcare patients and the performance of engineering structures, as Tom Shelley discovers

Single chips, powered by printed batteries, are set to revolutionise the way in which human health and engineering structural performance are monitored – aided and abetted by a strip of disposable sticking plaster.
The monitoring system, developed to improve dramatically the care of patients with chronic conditions, the elderly and expectant mothers in their own homes, is already arousing interest in mainstream engineering. And with the device held in place by a modest strip of sticking plaster, its findings can be reported wirelessly to a base station and database.
The system is the brainchild of Toumaz Technology in Abingdon, Oxfordshire, originally spun out of Imperial College by Professor Chris Toumazou, Keith Errey and Dr Alison Burdett. The business started as a ‘fabless’ semiconductor company designing integrated circuits, with a speciality in very low power signal processing and wireless communications. At the end of 2005, they decided the time was ripe to apply their expertise to the healthcare market.

“Our vision is in developing products for monitoring vital signs and bio-markers,” says Burdett. “Our chip sensor interface and wireless platform are general purpose, but we want to focus on the healthcare market. Healthcare in the UK has to become more efficient. There are many instances where, if patients could be continuously monitored, they would recover better at home than in hospital.”
This was the driving force that led to the company developing a small module capable of monitoring the human heart, using two ECG electrodes and a temperature sensor. The tiny device (measuring a mere 30mm x 45mm x 4mm thick) was powered by a single zinc oxide hearing aid battery. “Size is everything in this market,” Burdett explains. “Our vision is the digital plaster. This should be disposable, with a three- or four-day lifetime. We wanted to stay away from lithium batteries. Zinc oxide has a very high energy density and is kinder to the environment, so you can throw it in the bin. But the output is typically only about 1.4V, which affects chip design. The module has allowed us to do trials with early stage customers, while we worked on the integrated circuit, which measures only 6mm x 6mm.”
Eureka was shown a mock-up of the chip-based device, which includes a printed battery and printed antenna, all on a small flexible PCB that would also, in practice, support a few tiny capacitors, plus a crystal and sensors. For the wireless communications, states Burdett, “the printed zinc oxide battery produces only about 2mA peak - not enough to drive a Bluetooth or Zigbee radio. So, for further power savings, we have developed our own wireless protocol, which is simple, but very programmable. The radio has a range of typically 5m, but it is possible to increase this to 10m in a clear space. We want to use it to get data to a network through a standard wireless network terminal, such as a PDA version or mobile phone, with a simple plug-in card communicate with the body-worn module.” A partner, she adds, is developing a pre-prototype demonstrator that sits inside the mobile phone.
Each chip can accept input from up to three sensors - accommodating low-level voltage signals, such as might be produced by ECG measurements - and has a Wheatstone Bridge interface. It can also receive three accelerometer inputs and has interfaces for specific chemical sensors, as well as having an on-chip temperature sensor. Chemical sensors can be amperometric, biasing the sensor with a voltage and measuring current, or biased with current, using the chip to detect voltage drop.
The way the device makes and manages ECG measurements is particularly clever. First, the combination of sensor, chip and local base station has to learn what the heartbeats from a particular patient are like. The sensing device, therefore, begins by streaming all data to the local base station. After a few beats – the actual number is programmable – the base station constructs a template of the complete waveform and passes this back to the sensor chip. That becomes the first normal beat. Subsequent beats are then compared locally in the sensor chip to this template. If an unfamiliar beat is seen – ie, one that cannot be matched in the sensor chip – this is passed to the base station and another template generated, which is sent back to the sensor chip. Up to 16 different beats may be classified, or programmed and stored. Every 10 minutes or so – again, the interval is programmable – the sensing system communicates with a remote database and sends a summary file of the data.

Continuous history
In this way, a complete and continuous history can be built up of the patient’s ECG, yet only a minute amount of data is passed through the network. Additionally, and critically, if a beat or series of beats is seen that cannot be matched with any of the known templates, or if a beat is detected that fits the shape of a previously programmed beat – say, for a known arrhythmia suffered by the patient - the system is able to send high-resolution digitised data through the network in real time. This data represents a period leading up to the triggering event, as well as subsequent to the event.
The other striking part of the whole process comes through a collaboration with Oracle, which has developed healthcare products based on enterprise solutions that are now in service in the NHS and overseas. At present, nearly all patient data is entered manually, but the healthcare providers would like this to be automated. Toumaz and Oracle have put together a demonstration system that uses a local Oracle Lite database on a PDA or other small device that, by communicating with the main Oracle database, is able to alert a doctor, medical service and/or patient when an action needs to be taken.
In the medical field, apart from monitoring the elderly and chronically sick, there are other areas of application. “We are just now defining a trial around children and young people with early stage type 2 diabetes to find what measures are successful in getting people to modify their activities to maintain and improve health,” says Burdett. “Accelerometer data can, for example, be used to monitor exercise activity.
“In another trial in May, we will start monitoring women in the last month of pregnancy, as an alternative to ultrasonic monitoring every day, which is invasive. In the last month of pregnancy, you get a strong foetal ECG signal. We have been talking to various parties about Parkinson’s disease, monitoring the effects of anti-tremor drugs. It would be so much easier to establish a personalised dose, if you can exactly measure its effects. Blood flow, related to blood pressure, is something we are looking at. There are some nice time-of-flight techniques for monitoring flow - or just looking at rising or falling blood pressure is enough for some conditions.”
Sports colleges and sports training organisations are also showing great interest, particularly for elite athletes training for national Olympic teams. And it goes way beyond the strictly medical as well. For example, a wind turbine blades manufacturer has approached the company, looking for a means to monitor tip vibrations by using a mechanism that is small, light and wireless. The range of potential applications for this technology seems vast.
At present, the system is available as a small PCB-based unit, forming part of a reasonably priced development kit. Once it goes into large volume production, however, the disposable sensing systems are likely to cost only a few pounds each to manufacture – making them indispensable additions to both the engineer’s toolbox and the doctor’s bag.
“Development kits are in the region of a few thousand pounds, depending on customisation,” says the company. “Modules are 300-500 and – when the chip goes into production – the volume price for the chip is initially expected to be less than $30 (15).”
As for timing, Toumaz intends to start supplying development kits to customers this month. “We can produce the modules and are developing the chip,” says Burdett. “We are now pretty much coming to the end of testing.” The chip device is built around the Toumaz radio transceiver and baseboard controllers, a set of intelligent sensors interfaces and drivers, an 8-bit 8051 RISC processor, 32K of program memory and 32K of data memory. These amounts were apparently dictated by the need to be able to run the ECG algorithm. “We have strategies for mitigating against interference. If you go out of range, it will store its data and transmit it when you get within range.” For the PCB module, this has already been available for some while.
“There are so many applications we can supply for, if you can make devices small enough and low enough in power consumption,” Burdett concludes. “We are only limited by our imagination.”

Design Pointers

*l Devices can accept input from up to three sensors and report wirelessly to a PDA or mobile phone base station
* The PCB module version is powered by a zinc oxide hearing aid battery
* The 6mm square chip-based version sits on a small, flexible PCB with a printed zinc oxide battery and printed antenna. It should eventually be cheap enough to be disposable. The concept is to have complete sensing systems on sticking plasters

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