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Balance of power – medical-grade power sources

Unlike other, showier, elements of the design process, medical device power supplies do not grab the headlines. However, choosing the appropriate power source is an indispensable aspect of product development, and is critical for determining the safety and reliability of the device. After all, no matter how sharp the cutting edge of a device’s technology, it all counts for nothing if the juice stops flowing. Medical Device Developments runs through some key considerations for OEMs looking to power up.

When choosing a power supply for a medical device, it’s important to tread with caution: compared with industrial power supplies, medical-grade power sources are subject to different risks and more rigorous safety standards. After all, when the device will be coming into such close proximity with a patient, it is imperative that the power does not fail or the current exceed certain specifications.

Of course, this task poses significant challenges. Faced with compressed development cycles and evolving regulatory requirements, a medical device manufacturer may find power supply one of the least intuitive parts of the development process.

It’s crucial to track down a vendor with experience in this field, preferably one that is accustomed to designing medical-grade power supplies as opposed to simply adapting a commercial product. Following are some points to bear in mind when selecting the optimal power source.

"Medical devices are designed for hospital patients, who are typically in a weakened condition – they are likely to be sensitive to minuscule currents that would not affect their healthy counterparts."

Power-supply requirements

Not all medical devices are made alike, and it stands to reason that different equipment will have very different power-supply requirements. Compare, say, a stationary device such as a medical computer workstation with a handheld ultrasound scanner, and it’s clear that a whole range of factors come into play.

  • Battery-powered devices: We are moving towards an era of small, handheld devices that has created a thriving market for high-density medical-grade batteries. As discussed in the last edition of Medical Device Developments, rechargeable batteries have become the preferred option in most instances, with a particular movement towards lithium-ion technologies. These have clear advantages over their predecessors, including a high capacity and a slow loss of charge when not in use. Of course, battery failures remain a grave concern. The Association for the Advancement of Medical Instrumentation (AAMI) identified battery management as one of the top ten challenges for hospitals’ biomedical departments, and the onus is on designers to minimise the risk of unexpected failure or depletion.
  • Spacial constraints: Is it possible to accommodate the necessary power demands within a lightweight device? And, if they can’t scale-up the battery size, is it possible to scale-down those power demands? There are always trade-offs to be made, and as devices become ever more compact, this looks set to remain a question in point.
  • Plugged into the mains: Faced with size and weight limitations, many medical devices will eschew the option of a high-density embedded battery in favour of an external power source. In most instances, they will be plugged directly into a wall outlet, but it’s crucial to keep adequate power in reserve, especially where there are high portability requirements. For instance, a battery backup might be used that can continue to supply power for a specified period of time after the AC source fails. Medical devices of this kind (powered by AC current with battery backup) will require a medically approved converter with low leakage current, depending on the specifics of use.It goes without saying that, if the device incorporates cables and connectors, these will need to be properly systemised to ensure they are easy to store and use. Where electrical equipment is being transported around a hospital, it should be compact, robust, reliable and easy to keep clean.
  • Switch mode power supplies (SMPSs): Most portable medical devices use switch mode, as opposed to linear, power. SMPSs are smaller than conventional transformers and generate less heat while also being relatively inexpensive.
    While they are widely used in imaging devices, patient monitors, diagnostic equipments and pharmaceutical dispensers, there are some conspicuous drawbacks. Since not all SMPSs were created for use in medical applications, the leakage current may be too high. It may be necessary to add in additional isolation (i.e. electrically separating two parts of a circuit by using electromagnetic field coupling between the two) to ensure optimal patient safety.

Safety concerns

Ahead of sizing or pricing considerations, safety is invariably the number-one concern. Medical-grade power supplies must be carefully configured to protect the patient and the operator, ensuring any unintentional current is kept at bay.

  • Leakage currents: An obvious point, but one that is worth reinforcing: medical devices are designed for hospital patients, who are typically in a weakened condition. If gravely ill, they are likely to be sensitive to minuscule currents that would otherwise not affect their healthy counterparts. Acceptable leakage is therefore lower than in industrial equipment. This is particularly the case where the device will come into direct physical contact with patients or even be applied to their internal organs. For instance, if the device will come into contact with a patient’s heart, the limits will be lower than for a device that is simply used within the room. Even a very low voltage can be fatal if the necessary precautions aren’t observed.
  • Electromagnetic interference: Hospital equipment tends to operate on very low-level electronic signals that are highly sensitive to electromagnetic interference. Particularly in devices such as portable ultrasound equipment, electromagnetic compatibility is therefore a key concern.
  • System failures: With critical care applications especially, it is imperative there is no interruption to the power supply. If the battery power ran out or the hospital experienced a power cut, the results could be hugely detrimental. Manufacturers must therefore ensure that adequate back-up systems are in place.

IEC 60601-1 standards

As of June 2013, US medical device companies are required to meet the new IEC 60601-1 standards. Published by the International Electrotechnical Commission, these standards follow on from the first edition (published in 1977) and the second (1988), and fall under the banner ‘Medical electrical equipment – Part 1: General requirements for basic safety and essential performance’. There are equivalent standards in Europe and Canada that came into force in 2012.

"With critical care applications especially, it is imperative there is no interruption to the power supply."

While this third edition was ratified as early as 2005, it took a while to make any kind of impact. For several years, it existed in conjunction with the second edition, and this was liable to cause confusion given the discrepancies between the two. In the new edition, risk management is more of a priority, and there is a heightened emphasis on essential performance (EP). Defined as ‘the performance necessary to achieve freedom from unacceptable risk’, this slightly nebulous-sounding term needs to be clarified by the individual manufacturer. There are also various requirements for leakage current, with maximum permissible limits depending on the application.

All these standards apply to any medical equipment that will be used within the vicinity of a patient and do not so much establish clear-cut rules as provide a framework for analysing risk. Given the wide range of devices in question, their precise implementation is application-dependent – manufacturers will need to assess the proximity to patients and operators, the location and environment of the equipment, and its probable conditions of use.

In all cases, though, the ultimate aim is to reduce electrical hazards under normal and single-fault conditions.

A final point

OEMs are well advised to start thinking about their power supply from the outset. If the decision is left to the last minute, they are unlikely to benefit from much flexibility of choice and may be forced to err on the conservative side.

While, in some cases, an off-the-shelf power source may be suitable for their needs, in most instances, a custom option will be necessary. This means that, ideally speaking, the OEM would leave time to test out various options. Gradually, they can home in on the best and most reliable solution, and make a decision that will ensure an optimally functioning device.