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.
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
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.
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
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
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.
W. John Bilski is
senior engineer for Thermacore Inc., and John Broadbent is sales &
marketing manager for Thermacore Europe.
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