Thermal Management for Today’s Medical Devices and Equipment

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The impressive capabilities of 21st century medical technology, from imaging equipment to surgical instruments and automated immunoassays, are in many ways a tribute to the advances in microprocessor computing power. However, more power means more heat, generally in a smaller space. And as greater demands for precision and reliability are placed on medical equipment, thermal control becomes more critical.

Because of the unique requirements for medical devices, where it’s common to encounter requirements for accuracy and precise temperature change coefficients (∆T°) that are more stringent than anything found outside military applications, medical thermal engineering can be a juggling act for designers. Designers are faced with challenges of efficiency and size vs. cost, and increasingly, thermal performance vs. low noise (quieter fans produce less volumetric air flow).



Medical equipment designers are turning to passive thermal control systems, such as heat pipes and vapor chambers, for simplicity, design flexibility, manageable cost and quiet operation, among other benefits.


To balance all these factors successfully, medical equipment designers are turning to passive thermal control systems that include heat pipes and vapor chambers. These devices offer high reliability (fewer moving parts to maintain), design flexibility, manageable cost, and quiet operation, among other benefits. Here are some examples of passive thermal management concepts incorporated in key medical equipment applications.

Diagnostic Imaging

Enclosure cooling is critical for electronics-rich technologies such as magnetic resonance imaging (MRI), computerized tomography (CAT scan), ultrasound, and radiography (X-ray). Slight fluctuations in temperature can impact calibration and results, leading to costly downtime and maintenance. The FDA also demands near-perfect repeatability and reproducibility of results from these devices, making thermal control a key aspect of electronic design. Designers typically have a narrow window of temperature change (∆T) to work with, typically 10° C between cabinet interior and exterior ambient temperature.

Multiple heat sources, such as the device power supply as well as electronic components, can produce a total power output of 1200 watts or more, with a potential heat load of 400W of waste heat to dissipate. The desire for quiet operation complicates matters further by limiting fan size and speed.



Enclosure cooling is critical for electronics-rich technologies such as magnetic resonance imaging (MRI), computerized tomography (CAT scan), ultrasound, and radiography (X-ray).


These challenges are often best met through a heat pipe heat exchanger, in which heat spreads through a heat pipe and is dissipated through a finned heat sink, where it is rejected to the ambient atmosphere. The large fin area of the heat exchanger and the efficiency of the heat pipes allow the use of smaller quieter fans than would otherwise be required while meeting the stringent thermal demands of the regulatory and clinical environments.

A similar use of heat pipe technology is used to cool monitors found in critical care monitoring devices. Here, a rack-mounted heat pipe assembly can provide complete thermal reliability with low maintenance. The heat pipe’s time-to-failure performance is in the millions of hours, leaving virtually no chance of failure during a critical care operation.

Assays and Sample Screening

Automated serum and urine screening assays use lasers and advanced optical systems to ensure complete calibration and measurement consistency across thousands of samples. These systems must be protected against heat produced by mechanical systems such as conveyors (to move samples and reagents) as well as other electronics and power sources.

Previous thermal management approaches for automated assays used thermoelectric coolers (TECs), which often dissipate heat inconsistently from the sides of a sample calibration area. However, a passive heat transfer approach relies on a copper thermal reservoir, moving heat through heat pipes into a finned heat sink. Extremely precise machining is necessary to achieve the same levels of thermal control as thermoelectric devices — but passive systems meet the challenge of consistent, isothermal performance.



In a heat pipe heat exchanger, heat spreads through a heat pipe and is dissipated through a finned heat sink, where it is rejected to the ambient atmosphere.


Some automated assay devices are ideal settings for heat pipe assemblies. In other applications, a vapor chamber, using convective cooling and condensation, provides optimum isothermalization. Some large and powerful automated assay devices may require a centralized liquid cooling system, which provides reliable thermal control but can be costly and space-consuming. Whichever solution is chosen, assay devices generally require specialty engineered thermal components designed for extremely narrow ∆T windows. Both assay designers and regulatory requirements often calculate the acceptable ∆T and then narrow it further to provide an extra margin of safety. Heat pipes and other passive devices can also be used to cool design elements such as conveyors, with the heat being rejected to outside air using few or no moving parts.

Biotechnology and Research

Cycling devices to facilitate polymerase chain reaction (PCR) technology can produce impressive thermal management challenges. The device temperature must be kept at exactly the right range for efficiency while cycling between hotter and cooler conditions thousands of times per minute to provide optimum conditions for PCR reactions to unfold.

PCR cycler thermal control has traditionally relied on up to six TECs. However, complex and expensive electronics and software are required to produce consistent thermal control across all TECs. Differing levels of degradation over time among the TECs can reintroduce thermal inconsistencies and prevent isothermalization.



A similar use of heat pipe technology is used to cool monitors found in critical care monitoring devices. A rack-mounted heat pipe assembly can provide complete thermal reliability with low maintenance.


A recent solution developed for a leading PCR device manufacturer connects TECs to a vapor chamber via a graphite interface (using a proprietary method with 32 precision-drilled holes to mount the vapor chamber assembly base to thermoelectronic controllers) so heat is dissipated consistently across three dimensional axes. This provides instantaneous isothermalization that matches the constant fluctuation between hotter and cooler cycles, using the laws of thermodynamics in place of complex electronic algorithms.

Surgical

Small diameter heat pipes contribute to revolutionary forceps designs. Several experiments have shown that when the temperature of cauterizing forceps exceed about 80°C, the tissue being cauterized tends to stick to the forceps during procedures such as brain surgery. The forceps’ proprietary design in this case employs active heat transfer to draw heat from the very small (one-millimeter diameter and smaller) forceps tip (and tissue). A heat pipe is a two phase device meaning that the working fluid exists as both a liquid and a vapor inside the heat pipe. The working fluid is evaporated by the heat generated during cauterization. The vapor then travels to the coolest part of the heat pipe where it condenses releasing the heat to be dissipated more efficiently. The condensed fluid then returns to the forceps tip by capillary action, even if it must travel against gravity. Precision engineering on a micron scale makes this ingenious application of passive thermal technology possible.

In the development of medical and biomedical devices, passive thermal management is clearly a factor in helping ensure the accuracy and advanced capabilities of today’s medical devices, and extending these capabilities still further.

 

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