Ultra-low power wireless body monitoring

Demands on healthcare throughout the world are changing. The global demographic trend towards ageing populations, coupled with less active lifestyles and fast-food diets, is leading to higher probability and earlier onset of chronic conditions such as Type 2 diabetes and cardiovascular disease. This, in turn, is translating to a substantial increase in the proportion of resources required for long-term, continuous care and a growing burden on healthcare infrastructures.

Pro-active, continuous monitoring has been shown to enable much more effective treatment of chronic disease and deliver improved patient outcomes. Healthcare information systems in use today, however, are mainly designed to manage acute illness, such as infections and injury, making them ill-equipped to cope with the growing requirement for pervasive monitoring of long-term conditions. With healthcare budgets already overstretched and fewer carers – professional and non-professional – available to meet these increased needs, the forecasted additional demands are simply unsustainable using current practice. Factoring in the growing patient demands for personalized treatment, medicine and services, the result is huge cost pressures on healthcare. A new paradigm is required.

The new wave: pervasive, personalised healthcare
Today, 70-78 per cent of health budget expenditure is on chronic disease. (Ref 1) As care is moved increasingly from traditional care environments such as hospitals to the home and community care settings, better and more cost-effective ways of looking after patients are needed.

The “state-of-the-art” body-worn telecare systems in use today are typically wired, bulky and powerintensive. These home monitoring solutions not only negatively impact upon a patient’s quality of life; crucially, they also fail to deliver the continuous monitoring and direct wireless reading of vital signs, such as blood pressure and heart rate, that would enable effective diagnosis, therapeutics and patient care within a personalised healthcare scenario.

With the prevalence of chronic conditions set to escalate dramatically in coming years, the ability to harness non-intrusive, pro-active healthcare monitoring and 24/7 diagnostic and intervention capabilities – at acceptable system cost levels – is becoming a key priority for healthcare providers.

Major disruptive technology available today could provide the answer to cost-effectively managing tomorrow’s chronic disease burden. Intelligent, nano-powered, real-time wireless body monitoring systems that effectively bring the economies of scale of the semiconductor industry to healthcare are poised to deliver a new generation of low-cost, disposable healthcare solutions. In order to deliver the low cost and disposability required to enable truly lifestyle-compatible, chipscale wireless body monitoring, key manufacturing challenges are now being addressed through combining the best characteristics of the digital and analogue processing worlds.

Ultra-Low Power Analogue Processing: Sub-threshold CMOS
Digital processing devices and architectures are used extensively throughout the communications and computing industries. However, the computational complexity and low power consumption demanded by many new products – including the latest body-worn monitors – cannot be achieved by simply making bigger and faster digital chips.

By contrast, analogue processing is capable of achieving very high levels of computational complexity at significantly reduced power levels. However, the multi-dimensional nature and perceived difficulty of analogue integrated circuit design has generally inhibited the development of analogue processors and architectures.

The growing requirement for ultra-low power signal processing, coupled with advances in CMOS process technology, has led to the development of ultra low power analogue processing circuits which exploit the little-used "subthreshold" region of the transistor.

CMOS transistors operating in the sub-threshold region consume very little power since they are barely turned-on, meaning that the current through the transistor is extremely low: a few nanoamps or less. In this region, the voltage/current characteristics provide a well-defined exponential (Figure 1) that can be exploited in such a way that the physical properties of the transistors themselves can be used as mathematical building blocks to carry out a wide range of functions.

Analogue signal processing circuits are often able to operate extremely efficiently due to the relatively small number of devices required. When the circuits are designed using CMOS transistors operating in the sub-threshold region, ultralow power operation (that is, total system power in microwatt or nanowatt range) is possible. In certain signal processing applications, for example, real-time spectral analysis, power savings of between 100 and 1000 times have been shown over digital equivalents on a like-for-like basis.

Analogue signal processing operations are carried out almost instantaneously: we simply have to wait for the transistor’s output to settle.

A digital signal processing operation, on the other hand, typically takes a number of clock cycles to produce the desired result, due to the sequential nature of digital circuits. Analogue circuits are therefore often the only solution when high-speed, real-time signal processing is a requirement.

Clearly, both analogue and digital processing paradigms bring particular strengths and weaknesses: analogue offers the capability for high-speed processing at very high levels of efficiency, while digital processing is robust, reproducible, and reconfigurable.

A new patented approach developed by Toumaz Technology – Advanced Mixed Signal (AMx) technology – combines the best of both these processing techniques, exploiting the physics of silicon to produce nano-power analogue computing elements.By using digital elements to dynamically reconfigure, control, monitor and calibrate functional analogue processing blocks on a chip, processing blocks can become re-useable design elements or IP blocks in signal processing and low-power radio systems, enabling much wider use of analogue processing techniques.

Analogue to assist digital, digital to assist analogue
This “best of both worlds” methodology is particularly applicable for system-on-chip applications where very low power operation combined with system flexibility at low cost is critical.

In particular, this approach to system-on-chip integration is uniquely viable for healthcare and human monitoring applications by addressing two key areas. Firstly, to achieve savings in power and/or silicon area, computationally-intensive algorithms are implemented using efficient low power analogue semiconductor processors based on ‘analogue sub-routines’. Selected signal processing algorithms can then be implemented using low power analogue hardware, rather than digital hardware or software-based sub-routines.

Secondly, to maintain the necessary system accuracy without the requirement for post-manufacture trimming of the analogue sections (or the use of expensive process options), on-chip digital closed-loop control is used to implement features such as selfcalibration and re-tuning. In addition to correcting for manufacturing variations to increase yields, these tuning algorithms can also be re-run at intervals during the operation of the system. This means that drift, due to factors such as environmental changes or ageing, can be compensated for. Moreover, for many applications, the analogue sections may be reconfigured under digital control; for example, an analogue filter may be reconfigured to change the type or order of the filter, or a discrete cosine transform block may be switched to vary the input and output vector sizes.

Example system: The Sensium
AMx technology has been leveraged in the development of the Sensium – a System-On-Chip generic wireless sensor platform to address the new wave of personalised care applications in medical and professional healthcare.

Designed to be able to work with a wide range of body-worn physical and bio-chemical sensors, the Sensium comprises the complete integrated wireless infrastructure for intelligent, continuous monitoring of key physiological parameters. Together with appropriate external sensors, the Sensium can be configured to detect vital signs such as ECG, blood oxygen and glucose, body temperature, and even motion and mobility, through the integration of 3-axis accelerometers and pressure sensors.

The first-generation system includes a reconfigurable sensor interface, digital block with low power microprocessor core, and an RF transceiver block. The on-chip program and data memory allows local processing of signals, which significantly reduces the transmit data payload. The device is manufactured using an advanced 130 nanometre RF CMOS process, ensuring that the predictability and accuracy of the sub-threshold analogue processing block is maximised.

The provision of local intelligence enables the Sensium to capture, dynamically process and filter “problem” event data – such as irregularities in heartbeat or blood pressure – and send it wirelessly to a PC, mobile phone or a PDA (for transmission to another network node) via an ultra low-power NSP (Nanopower Sensor Protocol) short-range radio telemetry link. NSP is specifically optimised for application in the “human space”, operating at significantly lower power than alternative short-range technologies such as Bluetooth or ZigBee.

Due to its AMx processing algorithms, the nano-powered Sensium requires only a very small battery, enabling it to be body-worn with complete freedom of movement.

The system can even be attached to a disposable “digital plaster”, offering weeks’ to month’s lifetime and requiring no battery change.

In a typical deployment, wearable sensor nodes (target stations) support a range of sensors generating data at rates up to 50kbps. Very low level analogue signals from the sensors are pre-processed by nanopower circuitry (amplification, filtering, data conversion, data compression, modulation, and so on), before being transmitted as an RF signal. AMx technology allows these signals to be processed in either or both analogue or digital domains, as appropriate, to optimise power consumption.

The low power RF signal is then transmitted to the basestation. The basestation can be linked to up to 8 target stations, each monitoring multiple physiological signals on the body. By providing a complete wireless infrastructure to cover “the last metre” – including body worn sensor platforms, base station units, PHY, MAC and API – the Sensium system delivers an open platform on which to run other applications and services.

System Architecture for Wireless Body Area Networks
Leveraging ultra-low power signal processing techniques to keep area and cost as low as possible, intelligent microchip-sized wireless body monitoring systems are set to enable a wealth of new healthcare applications, offering quality of life for users and providing critical physical, bio-chemical and genomic data for healthcare professionals.

As adoption of these technologies accelerates, a number of different models may emerge, although a fully integrated system infrastructure to deliver wireless body monitoring services is likely to comprise the following key elements: a suitable lowpower connection from the sensor interface devices to the network; a PDA, smartphone or bedside unit incorporating a network gateway or modem to connect to the network provider; and a data store/server – including decision-making engine capabilities – to provide added-value data and information to the healthcare service provider or service purchaser.

With demand for personalised healthcare and continuous body monitoring set to experience exponential growth, we could well see the emergence of a new breed of service provider organisations set up specifically to deliver patient information services to the professional healthcare market.

These services could incorporate a wide range of solutions, including monitoring, clinical data or alarm services, links to dedicated call centres, or SMS and email functions.

New platforms for healthcare and lifestyle management
As the fundamental infrastructure for an end-to-end telemedicine system, intelligent integrated platforms have the potential to revolutionise the architecture of healthcare information systems. By providing a platform for healthcare analysis and decision-making based on real-time data, continuous wireless body monitoring systems offer the potential to deliver greatly improved healthcare outcomes at dramatically reduced cost.

Today, Toumaz Technology is collaborating with Oracle and The Institute of Biomedical Engineering, Imperial College, London, for the pilot of a major new mobile chronic disease monitoring system based on the Sensium. Initially being trialled on heart failure patients in several London hospitals, the Pervasive Monitoring System combines the Sensium platform with an advanced central transactional database capabilities.

Through the incorporation of messaging, automated reporting and online medication capabilities, the Pervasive Monitoring System has the potential to integrate with existing patient information systems, such as the UK’s National Spine and the worldwide standard HL7 system, to become a total patient care package and clinical repository of data.

In future scenarios, by allowing two-way flows of information (for example, an uplink of raw or processed data and a downlink of requests or activation signals), intelligent sensor interface platforms could offer further use in closed loop systems to control drug delivery and maintain key physiological parameters, such as blood pressure, within an optimum range.

By bringing the economies of scale of the semiconductor industry to healthcare, ultra-low power wireless body monitoring applications such as “digital plasters” may even ultimately become over-the-counter items at consumer cost levels. Broadening the application beyond prescription healthcare, these technologies are set to enable an explosion of product opportunities in the rapidly growing lifestyle management sector.

With the promise of enhanced quality and efficiency of treatment through more timely and personalised care, and unprecedented freedom and flexibility for patients, ultra-low power wireless body monitoring has potential to transform healthcare and lifestyle management for millions of people.