14 February 2012
Power management in battery powered medical devices
A fundamental shift in the nature of healthcare delivery is seeing a host of therapeutic procedures, once found only in hospitals, migrating to the doctor's surgery and the home. The driver is the increased costs of treating a growing population and the need for greater efficiency.
As a result, patients can now manage a wider range of conditions (improved monitoring for blood pressure, for example) or control the delivery of specific drugs using 'combination' devices (such as the insulin pen) designed to administer the correct dose in the correct way.
Medical device design has always focused on reliability and safety, but there is now an increasing emphasis on usability, not only to gain regulatory approval – usability is now assessed formally – but also to maximise user acceptance and commercial success. But defining 'usability' can be challenging. Many clinicians and patients are familiar with gadgets such as tablet computers or mobile phones and expect the same type of functionality from a medical device. Although such functionality could, in theory, deliver highly relevant benefits, there are particular constraints that can prohibit their inclusion.
Despite the limitations, there is significant opportunity to increase functionality and usability. Automatic dose counters on drug delivery devices, for example, help patients monitor their dosing regime; instructions on an lcd can 'coach' patients when using a device for the first time; and inbuilt monitors can make sure a device is used correctly, by measuring air flow or breath rates, for example. Wireless comms can relay information to healthcare practitioners if regular observation is required.
Power management strategies
Such functionality requires a power management strategy which ensures the device delivers the functions required while remaining reliable, always ready to use and safe when in operation. As a result, the chief power source for most portable medical devices remains the primary battery. Rechargeable batteries – although commonplace in consumer devices – are not viable. They cannot deliver the consistent levels of power required, not just because of their intrinsic design and discharge characteristics, but also because they depend on efficient recharging by the user, something which cannot be guaranteed.
This need for certainty and guaranteed performance is critically important to medical devices, especially if they could save somebody's life. Without guarantees and evidence that devices will work as required, they won't make it through the regulatory process.
Device design, not surprisingly, determines battery specification. In a small device, for example, the battery must fit between (and not impede) the intricate components which operate within a very confined space; for drug delivery, batteries must power the device for significantly longer than required to deliver the total dosage and with a sufficient margin of safety. Most importantly, however, the power strategy must ensure the device will continue to operate, even if electronic elements are damaged or battery power interrupted.
One solution could be to combine mechanical and electronic functionality. An inhaler could feature a cap which, as it is removed, activates a shuttle mechanism to prime the drug for delivery. Such a 'belt and braces' approach helps to minimise the power requirements and is essential if a drug is potentially life saving.
Increased electronic functionality raises other design concerns. For example, if a refillable device – such as a belt worn insulin pump – needs an electronic connection between the separate parts, designers have to decide how to maintain this connection: do they split the battery between the two components or maintain energy flow using electrical contacts or ultrasound. And how should multifunctional devices show they are 'broken'? If it is vital that patients are warned immediately if a device has malfunctioned, should that device be 'always on' and, if so, how should this be shown? Power management strategies have to reflect device design, the way the device interacts with its user and user habits when operating the device.
Ensuring safety critical operation
One way to ensure patient safety is to separate essential and non essential functionality, a strategy Team Consulting uses when designing 'safety critical' devices – medical devices which can cause death or serious injury if they malfunction. Once again, users of such devices are demanding increasingly sophisticated functionality, but such functionality can also increase the risk of malfunction or user error.
In response, Team's medical systems architecture uses two separate power management regimes: one to drive non essential functionality (such as a touch screen); the other to power essential operations, such as a drug pump. This mean the failure of non essential functionality – say the touch screen freezes – does not affect the device's core operation. This split architecture also minimises the impact of user error on core device operation, an acknowledged problem when functionality increases.
The recycling challenge
With an appropriate power management strategy, current battery technology can readily meet the needs of devices. But, although battery development is not an issue, battery recycling is of growing concern.
The increased availability of multifunctional medical devices means a significant number of short use or disposable devices contains batteries. Originally exempted from the WEEE Directive, medical devices are now covered by the updated WEEE legislation of 2006, so battery disposal is a primary design consideration. For reasons of user safety, removable batteries are discouraged, yet the battery should be removable at end of life to allow recycling. The strategy for resolving these conflicting requirements must be set out early in the product design process, as it places constraints on the electronics, mechanical and product design teams.
Alternative power sources
Although battery disposal is a serious issue, realistic alternatives to primary batteries are not yet on the horizon. Although supercapacitors may have sufficient energy to power single or daily use devices, they require informed patient interaction to charge the device in advance of drug delivery. Allowing the device to be completely passive between doses also restricts usability and is not ideal.
In the longer term, energy harvesting may have a role to play in medical devices, possibly through kinetic energy or solar power. In contrast to devices such as mobile phones, the time of use and energy consumed by a drug delivery device is tiny compared to the time available to harvest sufficient energy before the next dose, but medical devices rely on exactly the right amount of energy being available at the right time, something which harvesting cannot guarantee.
A device used once a day, and in a non critical context, may be able to use harvested energy, but the challenge is to incorporate a harvesting mechanism that works for patients of all lifestyles and in all home environments. Today, this is a long winded strategy when the easiest solution is to plug the device into the mains.
As the drive for energy conservation improves the efficiency of alternative power supplies, such solutions may become more viable. But the emphasis will remain on those developing such technologies to supply reliable data.
Chris Ferris is head of electronics and software with Team Consulting.