Thermal Control Designs for Medical Devices
By W. John Bilski and John Broadbent, Originally Posted in Design News, April 6, 2011
As miniaturized electronic technology is increasingly included in medical devices, passive heat transfer devices - including heat pipe, vapor chamber and annealed pyrolytic graphite- (APG) based heat sinks - help to improve thermal control of the intense heat associated with microprocessors in medical devices. The power of microprocessors contributes to the high performance of many machines in the medical design industry, including imaging equipment, surgical instruments and automated immunoassays.
However, there are many challenges that these medical devices with microprocessors present to thermal engineers. Materials like copper that can be used in other thermal applications cannot be used in medical devices because they cause damage to the human body. In addition, the miniaturization of technology and precise nature of medical devices may mean little or no space is available for thermal control.
The combination of accuracy, reliability needs, space constraints and material selection can make it difficult for designers to create cooling solutions for medical devices.
Understanding Heat Transfer Technology
Inside a heat pipe, a working fluid transfers heat by undergoing a phase change from liquid to vapor within a vacuum-sealed vessel. Heat pipes can be used effectively and efficiently in diagnostic imaging medical devices because they have no moving parts, require no energy input, and their simple design - a vacuum-sealed tube injected with a working fluid - can be easily miniaturized. Heat pipes are often integrated into heat sink assemblies to improve thermal performance or reduce size and mass.
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.
Heat sinks cool electronic devices by absorbing and dissipating heat into air via extended surfaces such as fins through either forced (fan-driven air movement) or natural convection.
Heat pipe heat exchanger (HPHX) cores are also used to cool electronics within an enclosure. HPHX cores consist of an array of heat pipes with plate fins that transfer heat from within the enclosure to the external environment without introducing external air into the enclosure. A manifold plate separates the core into two halves, with one half residing within an enclosure - such as magnetic resonance imaging (MRI) housing - and the other half outside the housing.
A vapor chamber, which can be used at the base of a heat sink, is a flat or planar heat pipe that allows the heat to spread in three dimensions, improving conductivity and heat spreading. Vapor chambers are often integrated into heat sink assemblies and provide a highly conductive heat sink base with highly uniform temperatures.
A number of new materials are becoming available to thermal engineers who are designing solutions for medical devices. Annealed pyrolytic graphite (APG), for example, is a lighter, more efficient solid conductor of heat when compared with raw metal such as aluminum or copper. APG has greater conductivity than both aluminum and copper and is typically encapsulated within a metal. By encapsulating APG within a biocompatible metal, APG is versatile enough to be used in solutions designed for surgical instruments and other medical devices that come into close contact with the human body.
Heat Transfer Applications
To maintain their performance, MRI, computed tomography (CT), ultrasound and radiography machines must be cooled to prevent failure. An optimal solution for cooling these devices is a heat pipe exchanger. In the heat pipe exchanger, the pipe transfers the heat from inside the machine to the outside of the equipment, where it is released into the air via the fins of the heat sink.
Heavily regulated medical devices like scanners, biotechnology equipment and laboratory microassays must perform with near-perfect repeatability and reproducibility of results. Heat pipe heat exchanger technology, with high reliability due to no moving parts, is an ideal thermal solution for critical care monitoring devices, which simply cannot fail during an operation or procedure.
Automated serum and urine screening assays are calibrated with lasers and advanced optical systems to maintain consistency across thousands of samples. As a result, heat from conveyors, electronics and power sources can damage these intricate systems.
There are a variety of passive thermal solutions for these systems, including a copper thermal reservoir, in which heat is moved from heat pipes into a heat sink with fins. Ensuring the devices have maximum contact with each other results in reduced interface resistance and improved levels of thermal control.
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.
Depending on the device and the application, heat pipe assemblies can be used to improve thermal performance. Sometimes the heat simply can be moved by a heat pipe to a thermal sink, where it is spread into the air outside the casing. Another approach is the use of a heat sink with an embedded vapor chamber that uses more efficient convective cooling via a three-dimensional spreading.
Devices used for polymerase chain reaction (PCR) must maintain a certain temperature for maximum efficiency while cycling between hotter and cooler conditions thousands of times per minute. Connecting a thermoelectric cooler (TEC) to a vapor chamber with a graphite interface provides consistent thermal control by ensuring that the heat is spread consistently across three dimensions. This reduces the cost and complexity of the electronics and software that are used to control the TECs.
Small-diameter heat pipes can be used to remove the intense heat in a forceps design used during brain surgery; this design includes the smallest mass-produced heat pipe. After using effective heat transfer to remove heat from the forceps tip, vapor from the working fluid generated toward cauterization travels to the coolest part of the heat pipe, where it condenses into liquid and returns to the forceps tip.
Passive thermal solutions offer many benefits to thermal engineers working on cooling alternatives for medical devices. Besides helping ensure reliability and accuracy of medical devices in many applications, passive thermal solutions do not include pumped liquids and therefore are safer on the environment. As miniaturization and the microprocessing technology continue to evolve and influence the development of medical devices, passive heat transfer devices will continue to be cost-effective, reliable and effective solutions for thermal engineers.
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