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Thermal Management: Solving Bigger Challenges in Smaller Devices

Wed, 07/25/2012 - 4:19pm
Michael Bucci

Developing medical devices that are smaller yet offer more functionality is challenge enough. Designers, however, also have to deal with the additional heat that is generated from these devices. This article looks at a number of heat related concerns in today’s medical device designs and outlines a number of available solutions that can be used to address the problem.

Thermacore solutions provide the temperature uniformity needed to maintain the levels of precision and reproducibility demanded by FDA, NEMA, and other requirements.

The medical equipment of today, like nearly all electronics, is getting smaller while capabilities continue to expand. This trend toward miniaturization is driven primarily by the need for portability, ergonomics, improved surgical precision, and smaller overall footprints to accommodate limited hospital space. The electronic components at the heart of medical devices—detectors, diodes, transistors, transducers, integrated circuits (microprocessors, GPUs, FPGAs, LEDs), among others—are steadily shrinking and becoming more compact. This, in turn, leads to designs that permit greater accuracy with access to smaller spaces, particularly in procedures such as endoscopy, ablation, electrosurgery, interventional medicine, and robotic surgery. Assay equipment and analyzers are also trending toward smaller sizes. Finally, there is the growing use of handheld products, from laser and arthroscopic shavers to ablation tools using heat, microwaves, RF signals, or cryogenic freezing.

Miniaturization, coupled with growing electronic power, means more heat in smaller spaces. That makes it more difficult to ensure that medical devices operate within appropriate operating temperature ranges. Proper thermal management can extend the useful life of these components exponentially, as described in the Arrhenius Equation.

Unique Thermal Challenges
While all electronics are trending in the direction of higher power densities with concentrated heat loads, there are some major thermal challenges that are unique to the medical industry. These include:

  • Biocompatibility - Many commonly-used thermal materials, such as copper, are not biocompatible with human tissue.
  • Precise Temperature Control - Applications, such as PCR thermal cycling, have requirements for very uniform temperatures (Δ T) across samples while heating-up and cooling-down as quickly as possible.
  • Orientation Insensitive - The need to operate independently of gravity in many applications, particularly surgical devices and CT scanners.
  • Sterile Environments – Electrical components are often cooled by fans that can collect dust and harbor bacteria, also known as fouling. The spread of hospital-acquired infections is of increasing concern to the medical community.
  • Mortality - Lives and health are at stake, so the margin for error is absolutely critical.

Vapor chambers use evaporative cooling to move heat out of medical devices without moving parts, making maintenance almost completely unnecessary.

Some medical device applications, such as military medical equipment, present unique challenges over and beyond the factors outlined above, often operating in extreme temperature and humidity. The risk of dust, dirt, shock, vibration, and other factors is much greater than in civilian applications, and serviceability is crucial because it can be more difficult to repair or replace parts. This is why many military medical applications use passive thermal systems that have few or no moving parts to potentially fail.

Medical Hot Spots and Thermal Solutions
Surgical Devices
Difficult thermal challenges often arise with handheld surgical devices used in applications such as; electrosurgery, cryosurgery, neurosurgery, robotic surgery, laser surgery, and ablation, as well as with ultrasound transducers and high-RPM arthroscopic shavers, among many others. Thermal solutions for these applications must be rugged (to stand up to high heat, rapid thermal cycles, and sometimes, long run times), accurate, biocompatible, ergonomic, and reliable. Proper thermal management prolongs service life by reducing thermal stress, preventing unintended tissue damage, reducing procedure time, and improving precision.

Interestingly, heat pipes can assist in this trend toward miniaturization because they can transfer heat loads from small spaces, reduce the size of heat sinks, and potentially eliminate the need for cooling fans. Sintered powder wick heat pipes, often used in these applications, offer the advantages of two-phase passive cooling (evaporation of a working fluid) with no moving parts and can operate in any orientation. Also, sintered powder wick heat pipes developed and commercialized by Thermacore can offer high flux capabilities of up to 350 W/cm2. Heat pipes can be formed to fit complex real estate spaces and can be less than 2.0 mm in diameter. There are, however, many factors to consider, such as biocompatibility, working fluid selection, fluid charge, pipe cross section, and wick structure (mesh, sintered, and nanowicks).

Vapor chambers are planar versions of heat pipes. They consist of an evacuated metal plate with a small fluid charge inside and a wick structure that lines the internal surfaces. Vapor chambers are fantastic heat spreaders. Basic vapor chambers offer heat flux capabilities of 350 W/cm2 and thermal conductivities of 5,000 W/mK. Specially designed vapor chambers, also known as Therma-Bases, can offer heat flux capabilities of 700 W/cm2.

Vapor chambers are planar versions of heat pipes. They consist of a hollow metal plate with a wick structure on the internal walls. The plate is evacuated, then filled with a small amount of working fluid. Thermacore vapor chambers are fantastic heat spreaders. Basic vapor chambers offer heat flux capabilities of 350 W/cm2 and thermal conductivities of 5,000 W/mK. Specially designed vapor chambers, also known as Therma-Bases®, can offer heat flux capabilities of 700 W/cm2. They can include thru-holes for easy installation, and larger plates can include thermal vias to provide support and improve the flow of the working fluid.

One of the most promising materials for medical applications is encapsulated Annealed Pyrolytic Graphite (APG), used in Thermacore’s line of k-Core thermal products. APG is quite versatile, as it can be paired with many different biocompatible materials, offering light weight and high thermal conductivity—at least three times the conductivity of copper with less mass than aluminum. The solid state design allows it to be used in any orientation relative to gravity. This gives designers considerable flexibility in creating solutions to address complex thermal challenges. Encapsulated APG is proven effective in maximizing electronic device accuracy, reliability, repeatability, and lifecycle.

Medical Imaging Equipment
The effectiveness and efficiency of diagnostic imaging—from LED lighting and ultrasound through X-ray, CT, MR, PET, and SPECT scans—require proper thermal management. The challenges involved are multi-faceted. Precision and reproducibility are vital for creating accurate images. Uptime is critical because imaging devices are in high demand and are often essential to rapid diagnosis. Improved accuracy can result in faster runtimes, which means reduced radiation exposure and increased throughput. Excess heat can cause calibration errors, and delay vital procedures. “Thermal noise” can interfere with the performance of electronic sensors, resulting in a loss of fidelity. Finally, the need for quiet operation limits the size and speed of cooling fans.

Increasing medical device capabilities means rising heat in smaller spaces. So, thermal protection is more critical than ever for protecting accuracy and reliability in applications from imaging to DNA analysis and PCR cycling.

Heat pipes and APG are great tools for removing concentrated heat at local points, like detectors, X-Ray generators, and boards. Vapor chambers are used to efficiently spread heat across an area, creating uniform temperature distributions that can provide consistent component performance across a detector array. Vapor chambers can also improve system accuracy and improve the efficiency of heat sinks. When heat is spread across a wide area in an enclosed space, air-to-air, or air-to-liquid heat exchangers can be used to pull the heat from the enclosure without having to transfer air into the enclosure.

Heat exchangers are effective and efficient cooling solution for medical equipment enclosures such as MRI, CT, PET, X-ray, and other medical equipment enclosures. Heat pipe based heat exchangers and impingement heat exchangers by Thermacore enable medical equipment electronics to be isolated and cooled without introducing the external environment (moisture, dirt, and other contaminants); these heat exchangers collect heat through internal fins, then transfer the heat outside of the compartment through conduction, so no air is transferred through the enclosure wall. The heat is then dissipated from the system through sets of external fins. These are typically active systems using fans and/or pumped liquid systems. They can be easily configured to accommodate a variety of thermal demands and packaging space limitations.

Refractory metal alloys, specially engineered for minimum size and maximum strength, represent another creative solution. Their primary advantage is high radiodensity for radiography. Refractory metals (tantalum, rhenium, molybdenum, niobium, hafnium, tungsten, and titanium) are radiopaque and enhance device visibility under X-ray, thus allowing lower doses of radiation. Refractory metal alloys continue to evolve, and some facilities, such as Thermacore’s PMT Division, perform laboratory research to create new alloys and produce existing alloys more efficiently. The development, characterization, testing, and processing of these high temperature alloys is extremely complex, requiring a high degree of expertise, precision, and above all, experience.

Bioscience Equipment
The crucial need for sample integrity, in applications such as DNA sequencing, hematology, and bioassays, has an impact on thermal management needs. Reliable results require consistency across all samples during calibration and processing.

Heat pipe/heat sink assemblies, vapor chambers, and heat exchangers are often used to cool assay devices. If an automated assay device is particularly large and powerful, designers can use a centralized liquid cooling system.

For imaging devices such as MRI, CT, ultrasound, and radiography, a rack-mounted heat pipe exchanger can be an ideal solution, with heat dissipated from inside the machine to the air via a finned heat sink.

Polymerase chain reaction (PCR) thermal cycling or DNA amplification involves precise, rapid cycling between hotter and cooler conditions to provide the ideal temperature range for PCR replications. Improving throughput requires accelerating the heating-cooling ramp times to increase the number of replications, while maintaining uniform temperatures across the entire microwell array to ensure the same conditions for each sample.

Traditional thermal approaches involve peltier thermoelectric cooler (TEC) arrays, which do not ensure uniform temperature distribution. Integration of the proper thermal management system is necessary to accommodate the variations associated with TEC devices, especially since they degrade at different rates. Vapor chambers can be combined with TECs to improve thermal uniformity, while dissipating heat more quickly and consistently. This produces a kind of “natural” isothermalization. Some thermal cyclers require a controlled thermal gradient across the sample tray. Embedded Heat Pipes Cold Plates can provide temperature uniformity across a row of samples, while allowing a thermal gradient across a column of samples. Vapor chamber/TEC combinations can also help reduce the overall size of thermal cycler units.

Whatever the solution, the cooling of biotechnology and bioanalysis equipment usually requires specially engineered thermal components designed to support narrow ΔT windows.

Conclusion
The trends toward greater power in smaller spaces will undoubtedly continue, bringing special thermal management challenges not seen in other applications. The response to these challenges involves the integration of advanced thermal management devices, such as heat pipes and vapor chambers; but for any solution, there are some key factors to consider when specifying thermal management for today’s medical technology. These factors include:

  • Temperature uniformity is critical in attaining the levels of precision and reproducibility demanded by regulatory and clinical requirements, with accurate control and monitoring.
  • Consider regulatory requirements. Medical devices must meet NEMA and UL safety specifications as well as FDA regulations, including NEMA guidelines regarding the device’s resistance to dust, moisture, and other factors.
  • Consider that medical device environments are also strictly regulated. Thermal solutions must have minimal impact on these environments, while meeting industry quality standards and safety regulations.
  • Recognize the need for biocompatibility and be willing to explore solutions such as encapsulated APG and refractory metal alloys.

While thermal challenges are becoming greater, so is the range of potential solutions, with the introduction of new materials and efficient heat transfer technologies that ultimately improve patient outcomes.

Michael Bucci is the market development manager for Thermacore, bringing more than ten years of technical sales and five years of manufacturing and design engineering experience to the position.

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