Battery Design: The Key to Improving Wearable Medical Devices
By Neil Oliver – 30/10/2013
Neil Oliver at Accutronics outlines the issues involved in achieving optimum battery design for medical devices, particularly wearable and implantable devices.
According to a piece of research published by IMS (www.imsresearch.com) 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 and infusion pumps and wearable devices such as endoscopy recorders and blood pressure and glucose monitors are only a few of the technologies that 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.
Although 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 important issues that need to be considered when designing, testing and manufacturing battery powered wearable medical devices.
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 designers of removable and embedded batteries 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.
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 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 users look-up tables to predict its state of charge. This method is far more accurate than simple voltage based gauges but it is no substitute for a “battery-side” fuel gauge, which allows the power source to constantly tracks its condition.
Remaining benign throughout its lifetime is of pre-requisite importance for any type of battery, but is of particular concern when the battery powers a device worn close to the human body.
Safety standards published by the International Electrotechnical Commission (IEC) and Underwriter Laboratories (UL), in addition to battery transportation standards published by the IEC and the 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.
Safety testing includes altitude simulation, shock, vibration, overcharge, forced discharge, thermal cycling, external short circuit, nail penetration, crush and drop – basically any form of use or abuse that can be expected during the life of the battery.
The golden rule for any designer of portable electronic devices is to involve the battery integrator early in the design process. Drawing on their expertise can help make the final product smaller and less costly than if battery designers are left out of the loop and simply asked to fill a void in the device with a battery.
Unfortunately, the latter is often the case and this can lead to the development of a compromised design in which energy density possibilities are not maximised and costly electrical and mechanical fixes must be put in place to realise the original design expectation.
As an example of this approach, Accutronics recently worked with a medical imaging company to develop a new lithium polymer smart battery. It was involved from the very first concept sketches, because the new device had to be 50% thinner than the existing model.
Using a collaborative approach it developed and produced a battery measuring 7.1 mm thick, but which delivered in excess of 24 Wh. The battery became part of the device, providing both power and mechanical strength. If the battery design had been left until the end of the process the whole design would have been compromised.
Choosing embedded or removable
The current trend in consumer electronics is for a battery to be embedded in a device, meaning they are factory fitted and can only be replaced by trained professionals, not by the users themselves. Embedded batteries are used extensively in tablets, media players and smart phones. However, embedded batteries are not always the perfect solution.
An exhausted embedded battery cannot simply be removed and replaced with a fully charged one. This may be acceptable if the battery is in your media player, but not so convenient if the device is monitoring your vital signs following a major operation. Also, a device containing an embedded battery must be returned to base for a battery replacement, which is time consuming and costly. Because medical devices have a life of up to ten years the battery may need to be replaced five or six times during its life depending on usage.
Removable batteries offer far greater flexibility because they can be easily swapped and a user can carry spare batteries if they need extended runtime. The device manufacturer also has the option to provide additional batteries for extended runtime.
Finally, there is also a third option of fitting a small embedded battery or super capacitor to act as a “bridge” while removable batteries provide the main power source. With this solution the device can run 24/7 if supplied with a sufficient number of charged batteries.
There are literally hundreds of factories manufacturing lithium-ion cells, some producing hundreds of millions of cells on fully automated production lines in humidity controlled factories. Others produce lower quantities through semi-automatic or manual assembly, in ambient temperature conditions. The quality of product from the latter is rarely acceptable for professional devices due to variations in performance and safety.
When contracting a battery integrator, the original equipment manufacturer should ensure that the cells being used come from a source that has a proven track record of providing reliable and safe products. The manufacturer has a further responsibility to ensure the battery integrator can provide reliable, auditable paperwork in relation to safety testing at both a cell and a battery level, and that no changes will be made to either to ensure a diligent evaluation is being made.
Failure by the manufacturer to actively get involved and address these issues with the battery integrator may lead to product performance or safety issues at a later date. This could be extremely damaging to the manufacturer’s reputation and products.
Battery design is rapidly adapting to the development of new technologies such as wearable and implantable devices. Much like Google who became indispensable to Internet users, some of these devices will, once in production, be essential to the day-to-day life and wellbeing of patients.
Many wearable and implantable devices are still at a prototype stage, but the technology is considered essential to the future of healthcare. The key to achieving the best design solution with minimum efforts is for battery design engineers to work alongside device manufacturers from the earliest stages of the project. This will ensure that wearable medical devices, expected to reduce the costs of healthcare, as well as improve or even save peoples’ lives, will become increasingly available in the near future.