The medical device and engineering industry continues to grow and evolve year over year. As technology improves, the population expands, and the depth and breadth of medical knowledge and treatments grow, so does our need for better devices and equipment. As such, current trends spanning all industries, like electronics getting smaller and more powerful, as well as increased focus on improved customer experience, have greater implications and challenges for the medical industry. Should a device fail, a patient's well-being could be endangered. Additionally, medical devices need to be portable, costs need to be judiciously managed so that treatment can be made available to the populace, and reliability is crucial.
Major design flaws can lead to poor thermal management and product performance. Not planning for a thermal solution can cost time, money, and resources as well as cause a large amount of rework or even massive device failures. Planning ahead during the initial product design to allow for appropriate cooling makes a significant difference to the device's success. Part 1 of this article evaluates key thermal factors to consider during the development process to ensure that the design is safe and effective. Part 2, which will run in a future issue, will address additional considerations, including regulations and manufacturing costs.
Start with the End User in Mind
As with most electronics, the number one priority is the end user. This is especially true for the medical device industry, as the health and safety of both care-givers and patients are at stake. Engineers must determine exactly who will be using the device and how. Usage dictates requirements, which heavily impacts thermal management choices. For this, there are five critical questions to address.
Will the patient or caregiver be in direct contact with the device? If so, will they be near enough to the heat source to feel it? This is perhaps the most obvious consideration. If they touch the device or are exposed to it while running, will it be uncomfortably hot or even burn them? Finding a safe heat dissipation path and moderating touch temperatures are imperative if the end user will be in close contact with the device. In these cases, requirements must be set to account for safety and comfort rather than the device's operating limits. For example, if a device were to feature a power-intensive sensor that comes into proximity with the patient, heat pipes might be a requirement to quickly and safely remove the heat from the device, even if the device itself can handle the heat load.
For higher power electronics, no matter where the heat is being dissipated, one must account for the user's safety. If using an enclosure, heat must be dissipated through one of the sides or through a heat exchanger. The user should be prevented from contacting these surfaces by use of a guard that still allows airflow but prevents wayward fingers from contact.
How often and for how long will the equipment be in use? Equipment use must be carefully considered in conjunction with the device's rejected heat. Devices that are used for longer stretches of time or with more frequency will likely need more powerful cooling. Constant use devices are likely going to need active thermal solutions because the device will consistently be generating heat with little to no cool down period. Devices used often but only intermittently have a very different set of thermal requirements as will devices used infrequently but with high heat loads that need to be cooled quickly.
With smaller heat loads, even if used regularly, a thermal mass might be all that is necessary to conduct the heat away from the device and dissipate the heat slowly and naturally. If the heat load is considerable, the device will require a more complex system to dissipate the heat as quickly as possible. High, instantaneous loads may be too much for a typical thermal solution to handle before the device reaches maximum junction or case temperatures. Heat pipes or higher power cooling systems, such as liquid solutions, may be required to safely dissipate these loads.
With intermittent use devices, frequency is also a key consideration. When the heat load is infrequent, the effective heat load could be absorbed into the thermal mass of the heat sink and slowly dissipated to the environment. Phase change materials, such as paraffin waxes, can also be used to absorb heat temporarily. However, when a device is operated so frequently that it does not have time to fully cool down between uses, inconsistent power dissipation must be considered. This is when the heat essentially adds up over time, causing a higher average operating temperature than what the device should be exposed to. These transient loads are often overlooked in the design phase and result in inadequate cooling. Additional analysis may be required prior to design and product layout if there is the potential for high transient heat loads. The final cooling solution may be drastically different when these transient loads are taken into account.
Does the device need to be portable? How portable does it need to be, and how will it be powered? Although size and weight requirements are always important, there are varying degrees of limitations based on portability. For stationary equipment, larger or heavier solutions may be used and trade-offs between forced convection and passive cooling can be considered. If a caregiver needs to transport the device, its weight, shape, and size must be controlled but may be less of a priority. It is essential to also consider whether the device will need to be stackable and if so, how this affects the locations of power supply within the device and the associated ventilation.
If the patient or caregiver must carry or wear the device, these factors are crucial. Weight requirements will heavily impact material choices. For example, while more thermally conductive, copper is three times heavier than aluminum. A possible alternative would be to use a graphite heat sink which can be up to four times more conductive than copper and lighter than aluminum. Device shape dictates heat sink shape and vice versa. Heat sink shape then impacts fin type and geometry. Size, weight, and shape all affect each other and technology choices. Further analysis is often required to examine the design trade-offs between these key factors in determining the ideal thermal solution.
Additionally, fully portable devices are powered by batteries, which generate their own waste heat and require charging. Therefore, if active cooling is to be used, it is important to note the balance between how much it is cooling and how much power the thermal solution is consuming. Using too much power to cool the device drains the battery and results in constant recharging and potential field use failure.
What sort of environment will the device be used in? All environments pose their own unique thermal challenges, including the controlled environment of a lab. In any environment, engineers need to consider touch temperature, ambient temperature, noise considerations, and robustness.
Touch temperature tends to be a bigger issue when the device is used in operating rooms where surgeons perform delicate procedures and must not be distracted or fatigued by hot instruments. Since patients are usually under anesthesia during these procedures, instruments just a few degrees above body temperature can damage patient tissue without any indication until after the procedure. Finger guards or plastic shrouds can help protect staff from accidental contact with hot surfaces. Skin can withstand higher touch temperatures on plastic than metal, due to plastic's lower thermal conductivity, which therefore transfers heat more slowly to the skin. Guidelines for allowable touch temperatures for different materials over different durations are specified in IEC 60601 and should be reviewed early in the design process.
Ambient temperatures are typically more easily controlled in labs than in other environments where it becomes a particularly limiting factor. Ambient temperature effects the operating temperature of the device, but the device can also increase the room temperature causing the environment to become uncomfortably hot for the user. This is especially true if the user is close to the device. Proximity makes the location of ventilation openings extremely important, to ensure that hot air is not dispelled directly onto the patient or caregiver.
Noise considerations are also key to patient comfort, especially for a device that is used on children or is in constant or frequent use. Loud noises often cause patient distress, even if infrequent. Noise level is especially important in the operating room (OR), where surgeons communicate with a team of people and listen for subtle sounds that may indicate a problem during their procedure. Fans, while essential to providing higher airflows and heat transfer in smaller spaces, can produce noise. Noise may not be a concern in some environments, such as labs. But for a device that is with a patient at night either at home or a hospital room, even lighter noise can be irritating or disruptive to sleep and the healing process. Larger heat sink surface areas are required to increase heat transfer if fans are not a viable option.
Environment plays the largest role in how robust the device and solution must be. The equipment may need to operate in a stationary, climate controlled lab, in an unstable outdoor environment, or anywhere in between. Devices operated by the patient daily outside of a controlled environment require a certain amount of ruggedness to withstand day-to-day usage. Dust, shock and vibration, foreign debris, and liquid ingress are all potential issues that must be accounted for in the thermal solution.
If the device will be carried around in a case, it does need to withstand higher temperature fluctuations or compensate for other limiting factors like reduced air flow. This will affect technology choices, especially in regard to fans. The electronics enclosure may be embedded in alternative materials, such as foam, which may totally block any airflow to the devices. In these cases, initial brainstorming and creative problem solving may be required to find a solution to keep device at safe and reliable temperatures.
Is the device critical to sustaining life? Life-sustaining medical devices go through much more rigorous testing and certification processes. They also typically require redundant systems. Based on design, the redundant system may require its own thermal solution, or the thermal solution must be developed to accommodate both systems. For example, heat pipes or thermosiphons can be utilized as a reliable way to transfer heat from the redundant system to the cooling solution rather than maintaining an entirely separate solution for the system or relying on an active switch.
There is a high correlation between how cool electronics are kept and the longevity of the device. Keeping the electronic device as cool as possible increases its safety, reliability, accuracy, and service life. This is crucial in cases when a patient's life is supported by the device.
The thermal solution has a significant impact on a device's safety and success, but far too many engineers consider cooling late in the design process. The sooner thermal needs are evaluated, the better they can be accommodated, and a fully optimized and cost-efficient solution and end product can be designed. From small portable devices to huge equipment, all medical electronics require proper thermal management to ensure that they meet the high standards of the industry. The safety, health, and comfort of patients, families, caregivers, and healthcare professionals rely on these devices every day.
This article was written by Julie Strachan, Thermal Systems Design Engineer, Aavid, Thermal Division of Boyd Corporation, Laconia, NH. For more information, visit here.