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 presents five considerations to enable the design engineer to develop the right thermal-management solution for the right medical device.
1. How are evolving medical device technologies making it incumbent on medical device designers and developers to address and solve thermal-management issues?
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.
2. How should the medical device manufacturer select a thermal-management technology for a given medical device application?
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.
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 them 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, and bioburden concerns along with the system’s cost. Heat-management solutions can include extruded aluminum heat sinks, heat-pipe assemblies, vapor chamber assemblies, k-Core® APG assemblies, liquid cooling systems, or heat exchangers, among many others.
3. In developing a medical device, what thermal-management concerns should be considered when balancing among airflow, fin size, and fan noise?
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.
4. What materials should the medical device designer or developer consider for ensuring effective heat management?
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. Though more expensive, APG is often suitable for medical devices where size and mass are critical; such as handheld devices and thermal cyclers.
5. If the medical device manufacturer intends to develop a diagnostic device, a surgical hand tool, or imaging equipment, what is the optimal thermal-management tool for the application?
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. The design considerations for a surgical hand tool are far different from those for a molecular imaging scanner.
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