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How wearable medtech is changing batteries

05 March 2018

Michele Windsor, global marketing manager at Ultralife Corporation, looks at what the rise in wearable medical devices means for power requirements.

At CES 2018, one of the world’s largest beauty companies unveiled a wearable UV patch to monitor sun exposure. The sensor, connected to a smartphone app, is an example of how the wearable device market has expanded. Previously only focused on monitoring chronic diseases, the wearable trend has now expanded to preventative systems. 

Millions of people across the globe are already using devices to monitor their health and fitness. While they may vary in sophistication, keeping count of steps, tracking heart rate and monitoring blood sugar levels all display the trend for patient engagement, of individuals taking increased control over their own health. Data from these devices can also help medical practitioners monitor existing conditions or be alerted in the case of emergencies.

What can wearables do?

In a clinical trial at Poole Hospital in the UK, patients with epilepsy combined wearable monitoring bands and inputted data into a smartphone app to track diet, sleep, social activity and medication. They then noted when they suffered a seizure, allowing doctors to access the combined data and analyse it. Doctors were able to identify patterns of stress, behaviour and lifestyle factors that would increase the risk and frequency of seizures. 

Trials like this are the future of medical treatment. The US Agency for Health Research and Quality (AHRQ) reports that more facilities are implementing chronic disease management (CDM) programmes that use this new technology to reduce hospital stays and manage conditions day by day. 

While wearable devices may not treat conditions, they will allow doctors and patients to manage conditions proactively, rather than reactively. For example, rather than a diabetic patient visiting A&E regularly for dangerous blood sugar levels, doctors are now supporting the use of wearable devices that promote activities linked to healthy blood sugar levels, such as changing diet, regular exercise and staying on top of medication intake. 

In particular, developments such as smart contact lenses can also measure glucose levels in a patient’s tears that fall between the layers of the contact lens. The lenses can then provide a continuous monitoring of blood sugar levels, in case the patient forgets to take their blood sugar through fingerstick blood tests. 

When medical devices were only found in hospitals or other medical facilities, design engineers had a different approach to power management than they do now. Devices were larger, didn’t necessarily need to be portable or provide anything more than backup power. 

The new approach to power

If devices are now being used for constant monitoring of health conditions, the batteries used need to be extremely reliable. While previously the devices may have only been used for recreational purposes, if they are used for monitoring chronic conditions, battery technology must be specifically adapted to the needs of the medical market.

One particular drawback to some modern batteries, such as sealed lead acid batteries, is a gradual increase in internal resistance which leads to battery impedance and a drop in power quality. To ensure this does not happen, design engineers for medical devices should consult with experts in the battery industry who can advise on batteries that contain materials with low resistance. 

Electronic and ionic resistance are the two factors that make up a battery’s internal resistance. All the components in a battery have their own resistance, which is the electronic resistance. This includes the cell cover, can and current collectors, as well as the battery-level components such as wiring, fuses and sense resistors. 

Electrochemical factors are the reason for ionic resistance, such as electrolyte conductivity, ion mobility and electrode surface area. Ionic resistance and electronic resistance combined makes total effective resistance, which if it is too high, the voltage will drop when the battery is under load. Obviously, this is not ideal for a medical device. 

Therefore, any battery used for a medical device must have a low internal impedance, so it can provide a high-power output with a stable voltage. 

Safety

In batteries used to power medical devices, safety is naturally a high priority. This is not just in terms of their reliability, but also due to their extensive use. Battery failures in the past have proven that, when using lithium-ion cells, effective shutdown separators need to be chosen to protect the user of the device. 

Medical device manufacturers must ensure that, when choosing a battery for use, it has suitable safety features such as shutdown separators. This is a thin porous membrane that separates the anode and cathode, while still allowing ion transport in the cell. 

Ultralife’s next generation 9V lithium battery has three cells, which are each constructed with an internal shutdown separator. If the internal temperature rises excessively, the separator closes the pores of the battery and shuts down. This protects the user of the device, who may be wearing it while asleep. 

Size

As more medical monitors become wearables and devices get smaller, with more features included in them, every millimetre of space is important. However, medical device manufacturers must remember that, when designing a device, the power requirements are one of the most important things to consider. 

This means that it is far too late in the design process to create a small space for the battery to fit in and then ask the battery manufacturer to fill the space. While it may be possible to do this with increasingly small batteries, it may not be the best use of the space available.

Thin cell batteries can have exceptionally small dimensions, such as a thickness of 0.4mm and a length and width of 20mm. However, it is only by consulting with a battery manufacturer at the start of the design process that OEMs can ensure that the battery will fit into the device and fulfil the necessary power requirements. 

This is the key to batteries in medical devices. As the wearable market is ever growing, diversifying and changing, consulting battery manufacturers with experience in the medical market is the best way to ensure that OEMs receive a battery that is safe, reliable and suitable for the device they are powering. With different applications being developed every day for the wearable medical market, battery research and development will continue to innovate to power these potentially life-saving devices. 


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