As the medical device and engineering industry grows, there is also demand for better devices and equipment. Trends such as smaller and more powerful electronics, as well as increased focus on improved customer experience, affect the medical device industry as much as any other. Medical devices, though, also need to be portable, and their costs must be judiciously managed. Above all, reliability is crucial.
Planning ahead during the initial product design to allow for appropriate cooling is essential to a device's success. Part 1 of this article (MDB, November 2018) evaluated key thermal factors to consider during the development process to ensure that the design is safe and effective. Part 2 now addresses additional critical considerations, including regulations and manufacturing costs.
Regulations, Manufacturing, and More
Although the user is paramount, there are other key considerations for a product and thermal solution design. These concerns include regulations, development and manufacturing costs, and maintenance.
Does the device include high power electronics? As medical devices continue to become smaller, the electronics become more compact resulting in more concentrated heat loads. This poses an additional challenge for designers since conventional thermal solutions may not be adequate. Heat pipes are usually the first line of defense, as they can transfer heat away from the source to a point where it can be dissipated more easily. Vapor chamber heat pipes can spread concentrated heat loads so that they can be managed more easily. Similarly, these heat spreaders can also be used to keep a series of components at a uniform temperature.
Liquid cooling is an alternative to air-cooled systems, although integrating liquid systems into devices is often avoided due to complication and risk. When heat fluxes are exceedingly high, usually above 500 W/cm2, microchannel or porous metal liquid cold plates may be required. These types of cold plates increase the amount of surface area in contact with the liquid over a given area, making them more space efficient. These are ideal for cooling high power electronics, lasers, thermoelectric coolers, x-ray targets, and other components with densely packed heat loads.
Does it meet regulations, including IP Rating, IEC-60601, ISO 10333, or additional necessary requirements? In general, medical devices must meet higher safety and reliability standards than most other types of electronics because of their impact on users’ health and safety. IP ratings (ingress protection) are crucial if devices are going to be used in an environment with limited control. Fans and enclosure openings are determining factors for the IP rating of a device. If there are any solid particle protection or liquid ingress requirements, fan selection and establishing opening sizes of the enclosure are critical.
The thermal solution affects how large the enclosure openings need to be, which directly affects the IP rating. If larger openings are needed to enable enough heat transfer, finger guards may need to be designed in to maintain an IP rating of 2X.
For sensitive equipment or devices, a higher IP rating may be required, which would necessitate the use of filters to protect from dust. Filters directly impact flow resistance and pressure drop within an air-cooled system, and the fan must be chosen appropriately to function in those conditions.
For the even higher IP 6X ratings, which require a vacuum tight seal, the device must be fully within an enclosure, which might mean that the external surfaces need to dissipate the heat. This is often a requirement in the operating room, where open incisions must not be exposed to flowing air from cooling fans that can carry pathogens and lead to post surgery infections, often referred to as biofouling. And, if those external surfaces are to exceed IEC 60601 touch temperature limits, protection should be provided to protect the end user from accidental contact.
Depending on the application, full waterproofing may or may not be a key concern, but in any case, liquidingress due to cleaning and sterilization must be factored in. If a device is simply wiped down for cleaning, lower IP ratings would be acceptable. If there are full and extensive cleaning processes with powerful waterjets, any sort of openings devoted to heat transfer should be heavily scrutinized. If liquid is able to get into the device, and the device requires an IP rating or X5 or above, some sort of waterproofed coating should be considered for the electronic devices. Conformal coatings, such as thermally conductive silicones or epoxies, can seal the electronics from liquid, but still enable heat transfer away from the device. Additionally, take-home medical treatment devices should have a minimum IP rating of X4. Day-to-day activities, at-home cleaning, and spills are all legitimate concerns.
Material selection is critical in considering USP Class ratings (if the device is used in manufacturing any ingestible products, such as medicines, food ingredients, supplements, etc.) or ISO 10993 for physical devices that will be implanted, mounted, or in contact with the patient in any way.
Plastics are of great concern in these two standards due to outgassing and potential chemical leeching. If part of the solution utilizes a thermally conductive plastic, the plastic's properties in relation to biocompatibility must be determined prior to finalizing the product design.
Metals may also be an issue, depending on the level of corrosion resistance as well as potential allergies patients may have to particular metals, especially nickel, which is a common plating for corrosion resistance. Certain types of surface treatments, such as hard anodize for aluminum products, clear coats for copper, or conformal coating for most other components, can be potential resolutions that will still work in conjunction with a thermal solution.
Not only is it important to consider the design of a medical device, but how it is manufactured as well. The control of foreign objects and debris (FOD) is much more critical for medical applications because it can have negative impacts to either the function of the medical device or to the health and safety of the patient. In each of these products, high quality and sterile environments are required during production with the use of cleanrooms. The level of cleanliness is dependent on the overall usage requirements and final product designation.
Is affordability a key concern? If the costs of the device are going to be passed off to either the facility or patient, affordability of the end product can be a determining factor for thermal solution selection. Some technologies are more expensive than others, such as a liquid cooling system compared with a standard extruded heat sink. A fully custom system may also be out of the question if the product requires low costs.
Finding the balance between meeting strict thermal and regulatory requirements and keeping costs feasible is much easier when the thermal solution is considered very early in the design process. The earlier cooling is considered, the more efficient and cost-effective the solution is. When thermals are considered too late in the process, more expensive or less-effective cooling might be the only remaining option. Some may even find that they need to go back and completely redesign the product, costing more money and time.
The thermal solution design complexity can also add unseen costs if the assembly requires multiple components sourced at different vendors. While the solution itself may fit all of the thermal and performance requirements, it may make the solution impractical with extra assembly and sourcing costs. Take the time to consider where all components of the thermal solution are sourced to prevent expensive surprises later.
How will the device be serviced? A thermal solution should not hinder the maintenance of the product. Therefore, it should either be placed out of the way or designed to be easily removed and replaced without risk to the integrity of the device. If it will need to be removed, mounting hardware selection becomes more important if the heat sink needs to be mounted repeatedly. For example, spring loaded pins wear down with repeated removal and become less viable.
As noted earlier, cleaning may also be an issue and must be considered.
As discussed in Part 1 of this article, the thermal solution has a significant impact on a device's safety and success, but cooling is often considered too late in the design process. Part 2 has reviewed additional concerns from regulatory to maintenance. In all cases, 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.
This article was written by Julie Strachan, Thermal Systems Design Engineer, Aavid, Thermal Division of Boyd Corporation, Laconia, NH. For more information, visit here.