Medical Charged for life A tall order
Doctors, surgeons, and healthcare practitioners are calling for medical technology (MedTech) original equipment manufacturers (OEMs) to address problems with the battery life of implantable medical devices (IMDs). Concerns include the risk of infection for patients needing to undergo frequent surgery to replace batteries and a need for battery life to be raised to a minimum of 25 years alongside calls for the industry to incentivise longer product development lifecycles (PDLCs). Here Neil Oliver, technical marketing manager at Accutronics, looks at how to increase the longevity of medical device batteries
E
vidence of the demand we place on the humble battery is evident if you take a look at your smartphone. The battery is expected to provide a continuous, often high-discharge, power supply for components including a backlit capacitive touchscreen, front and rear cameras and LED flash, speakers, graphics card, and the CPU. Your phone battery essentially powers a device with more computing power than the original Apollo rocket that took men to the moon. With the demand for smaller and lighter batteries for shrinking consumer devices, it shouldn’t be surprising that most phone batteries don't last more than a day per charge. The trend for frequent evolutionary, rather than revolutionary, hardware updates has resulted in shorter product development lifecycles (PDLCs) of one year or less in the consumer industry, something that is also beginning to affect medical device development. The push to cut costs and improve
efficiencies in the medical industry means that products are expected to last anywhere from 10 to 15 years to lower the total cost of ownership and increase return on investment. However, increased use of portable, wearable, and implantable devices means that the batteries need to step up, as they will be required to last for up to 25 years or more.
MedTech challenges
Portable and wearable medical devices are often designed to include removable, rechargeable batteries making it possible
to replace a battery without replacing the device. However, the same rules clearly do not apply when it comes to implantable devices, where the embedded battery cannot be replaced, meaning the entire device needs to be surgically removed and replaced. Doctors in the British Medical Journal
(BMJ) recently highlighted this problem. Cardiologists John Dean and Neil Sulke have argued that the battery life of implantable medical devices (IMDs) needs to be longer to avoid the frequency of surgery currently required and to remove the subsequent risk of infection. They have also called for IMDs to have greater accuracy when it comes to monitoring remaining battery life, and argue that devices could be made bigger to hold more charge. Although Accutronics specialises in batteries for portable rather than implantable medical devices, these problems affect the industry across the board. Improvements in battery life, battery density, protection circuits, smart features, and sustainable design engineering are all necessary if we want to increase longevity. However, many OEMs leave battery selection as an afterthought in the design and development process. The US Food and Drug Administration (FDA) has expressed concerns about batteries in medical devices in recent years, stating problems in three key areas; insufficient quality assurance in medical devices, a lack of knowledge integrating batteries into medical devices, and limited knowledge about when to replace the battery.
Batteries in medical devices are expected to last five to 25 years and remain safe throughout. They must provide accurate state-of-charge information, be highly reliable with predictable discharge profiles, and use battery chemistry suitable for the application. They must also be able to handle high temperatures, both inside and outside the body. To understand how to increase longevity, we have to understand specific device and requirements. External devices such as portable ventilators and anaesthesia machines are designed to replace the need for mains power, so they typically use Lithium-ion (Li-ion) cells that deliver continuous high-discharge power, with batteries that need to be charged weekly, if not daily.
Our own CMX battery is designed for this purpose. The battery is available in three versions, using eight, twelve or sixteen '18650' sized cells, with continuous discharge rates of up to 300W. Smart features, like those in our CMX series, ensure delivery of core requirements. These include active and passive protection circuits that prevent over-temperature, over and under-voltage, overload and short circuit. Smart power management means the battery only requests charge when needed and shuts down when not being used and accurate fuel gauging, to within one per cent, further enhances reliability.
A lesson in chemistry
On the other end of the spectrum, we have implantable medical devices (IMDs) with power ratings less than one ampere, often in the milliamp and microamp range. As well as requiring batteries that can last for years without being recharged, IMDs include a range of devices, such as neurostimulators, cardiac resynchronisation therapy (CRT) pacemakers and drug delivery systems.
IMDs typically use lithium/iodine batteries in the microampere range, lithium/manganese oxide in the milliampere range, and silver vanadium oxide (SVO), in the one ampere range and over. The majority of these battery chemistries have been around since the early 1970s and have been refined over the years by tweaking the use of
electrolyte in liquid and solid forms and by using hybrid cathode materials such as coating SVO with carbon monoflouride for higher power delivery. Despite these improvements, it is important to remember that a battery is a consumable part that will degrade over time. This is why choosing the right battery chemistry is vital. The discharge characteristics of a battery change over time with changes to the electrolyte's conductivity and impedance.
Design engineers don't usually have to be concerned with these problems because Lithium-ion batteries typically deliver between 300-500 charge cycles before performance drops. However, this isn't viable with implantable devices. Here, design engineers must examine the affects of chemistry on the discharge profile and calendar life of the battery to predict the end of the battery's life.
The future
Continued innovation by medical device OEMs will create incentives to develop longer life batteries. New research into nano-materials and charging technology is paving the way for flexible, self-healing batteries with reversible chemistries that will offer long lifespans and fast, wireless inductive charging.
While this technology is still in the early stages of development and requires many years of reliability testing before it can be used, it will open up sectors beyond the medical industry into wearable, healthcare and patient environments, providing OEMs with further profitable avenues. Maybe then our phones can focus on being phones rather than pseudo-medical devices and make it through more than a day on a full charge.
www.accutronics.co.uk/index.php
26 October 2016
Components in Electronics
www.cieonline.co.uk
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