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The key to improving wearable medical electronics

Posted: 23 Apr 2014  Print Version  Bookmark and Share

Keywords:wearable  implantable  battery  medical devices  fuel gauging 

Google is still in its teens, yet it has dramatically changed the way most people see and use the internet and has become an integral part of daily life.

In similar ways, new technologies which were the reserve of Sci-fi less than ten years ago are currently being developed for both consumer markets and the professional world. What Google has done for the internet, wearable and implantable technologies may well do for medicine.

According to a piece of research published by IMS in 2012, the wearable technology market is expected to reach $6 billion by 2016. IMS defines wearable technologies as products that are worn on the user's body for an extended period of time, contain advanced circuitry, as well as wireless connectivity, that can process data.

The medical field is one of the leading professional sectors when it comes to investments in research and product design. Portable devices such as nebulisers or infusion pumps and wearable devices like endoscopy recorders or blood pressure and glucose monitors are only a few of the technologies which are now available to the public.

The development of wearable medical devices is aimed at reducing patients' stays in hospitals and the implied costs. The continuous flow of information provided by wearable patient monitors can also lead to earlier detection of problems and can result in better clinical results.

While the benefits of these devices are obvious, their design has brought to light several technical challenges, one of which is the continuous tug of war between reduced size and weight and suitable battery life. Reliability and safety are other key issues which need to be considered when designing, testing and manufacturing battery powered wearable medical devices.

Battery solutions
An essential component of wearable technologies is the power source which usually comes in the form of a battery. Because the device must be transported on the body of the user and must function continuously, a long battery life is essential. The battery must be as small and light as possible, so as not to impede the user's daily activities.

Innovative battery design for wearable medical devices is a constant challenge for manufacturers everywhere. The perpetual battle between size and weight on one side, and device runtime on the other, is one of the most daunting tasks that removable and embedded battery designers currently face.

Rechargeable Lithium-ion batteries are an ideal choice for wearable medical devices because of their high volumetric and gravimetric energy density. Simply put, the batteries allow devices to run for longer between charging, with minimal weight and volume. Today's Lithium-ion batteries have an energy density of approximately 500 Watt-hours per litre and 180 Watt-hours per kilogram and can be judged as the 'best in class' for commercially available rechargeable battery technologies.

Fuel gauging
Accurate fuel gauging is vital for all battery powered medical devices. The ability of the device to reliably predict its remaining runtime regardless of temperature, age, or usage profile is essential in order to avoid runtime anxiety. If users cannot trust the runtime prediction, then they may feel the need to carry additional batteries or not leave home for prolonged periods during the time the wearable medical device is operating.

Most single cell batteries in smaller devices use 'device side' fuel gauges. A circuit in the host device measures the temperature, voltage and impedance of the battery and uses look-up tables to predict its state of charge. This method is far more accurate than simple voltage based gauges but it's no substitute for a 'battery-side' fuel gauge, which allow the power source to constantly tracks its condition.

Safety
Remaining benign throughout its lifetime is of pre-requisite importance for any type of battery. This is of particular concern when the battery powers a device worn close to the human body. Safety standards published by both the IEC (International Electrotechnical Commission) and UL (Underwriter Laboratories), in addition to battery transportation standards published by the IEC and UN (United Nations), provide a regulated framework for testing. This can be certified by a third party body and used as documented evidence when applying for certification of a complete product.

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