Managing Heat in Medical Device Applications

With the proliferation of miniaturized and implantable devices and rapid advances in microprocessor computing power, the medical device designer and developer must pay increasing attention to heat-management concerns. John Bilski, senior thermal engineer at Thermacore Inc. (Lancaster, PA), highlights how to select the right thermal-management solution for the right medical device.

MPMN: How are evolving medical device technologies forcing medical device designers and developers to address and solve thermal-management issues?

Bilski: As medical devices continue to become smaller and more compact, designers are often challenged with meeting their project's size, performance, operating temperature, noise, and budget requirements. Heat-management technologies can often move, spread, and dissipate heat efficiently. While this helps improve system reliability, speed, precision, and service life, it can also help designers reduce their devices' packaging size, weight, energy consumption, noise, and fouling or bioburden concerns.

MPMN: What steps should medical device manufacturers take to select a thermal-management technology for a given medical device application?

Bilski: The manufacturer should begin by laying out all of the known thermal design requirements, including the number and location of heat sources, the total power of the heat sources, the available volume, system limitations, maximum allowable heat source temperatures, the maximum ambient temperature, and the available airflow if the device is air cooled. The manufacturer must then determine the thermal technology to be used to solve the heat problem, such as an all-metal heat sink, a heat-pipe assembly, a vapor chamber assembly, liquid cooling, Thermacore's k-Core annealed pyrolytic graphite (APG), or enclosure heat exchangers.

Of course, the optimal thermal-management tool depends on the application and the application-specific requirements. Ultimately, the solution depends on the heat loads, temperature requirements, form factors, available airflow, noise considerations, and other factors. In the medical device industry, solutions can range from a simple piece of aluminum to an air-liquid heat exchanger for cooling enclosures.

For basic applications or applications in which the thermal-solution technology is obvious or understood, the next step is analysis. Basic application analysis can be accomplished using a simple hand or spreadsheet calculation. For more-complex designs, a thermal model of the system can be created. For example, Thermacore might use one or more of the seven different CFD programs available to it to develop thermal models for helping manufacturers to determine which technology best meets a project's requirements. The ultimate objective is to balance the device's thermal/mechanical performance, reliability, speed, precision, service life, size, weight, energy consumption, noise, bioburden concerns, and system cost.

MPMN: What thermal-management concerns should be considered when balancing among airflow, fin size, and fan noise in a medical device application?

Bilski: The concerns will vary with each application. For lower-power devices, it may be possible to dissipate heat to the walls of the metal enclosure using heat pipes, allowing the medical device designer to bypass concerns associated with airflow, fins, or fans. Natural convection methods may also be an option, but they often have a large footprint and add weight to the overall medical device design. In addition, integrating one of many heat-spreading technologies can enable the medical device manufacturer to optimize the size of the heat sinks and modulate the fin pitch to best suit the allowable airflow. For example, more fin area does not necessarily mean better performance. If the fin spacing is too tight for the available airflow, performance can actually decrease.

MPMN: What materials should the medical device designer or developer consider for ensuring effective heat management?

Bilski: Because of its favorable thermal properties and relatively low cost, aluminum is a common choice for managing heat through metal conduction. It has good thermal conductivity, can be anodized for hardness, can be dyed various colors, and is relatively lightweight. While plastic offers lower mass and cost than aluminum, it also has poor thermal conductivity. Copper is another good thermal conductor, but it is usually more expensive and heavier than aluminum. Another thermal-management material is Thermacore's APG. It is 20% lighter than aluminum and three to four times more conductive than copper. Although it is more expensive than other materials, APG is often suitable for medical device applications in which size and mass are critical, such as handheld devices and thermal cyclers.