Thermal Control Designs for Medical Devices

As miniaturized electronic technology is increasinglyincluded in medical devices, passive heat transfer devices - including heatpipe, vapor chamber and annealed pyrolytic graphite- (APG) based heat sinks -help to improve thermal control of the intense heat associated withmicroprocessors in medical devices. The power of microprocessors contributes tothe high performance of many machines in the medical design industry, includingimaging equipment, surgical instruments and automated immunoassays.

However, there are many challenges that these medicaldevices with microprocessors present to thermal engineers. Materials likecopper that can be used in other thermal applications cannot be used in medicaldevices because they cause damage to the human body. In addition, the miniaturizationof technology and precise nature of medical devices may mean little or no spaceis available for thermal control.

The combination of accuracy, reliability needs, spaceconstraints and material selection can make it difficult for designers tocreate cooling solutions for medical devices.

Understanding Heat Transfer Technology
Inside a heat pipe, a working fluid transfers heat by undergoing a phasechange from liquid to vapor within a vacuum-sealed vessel. Heat pipes can beused effectively and efficiently in diagnostic imaging medical devices becausethey have no moving parts, require no energy input, and their simple design - avacuum-sealed tube injected with a working fluid - can be easily miniaturized. Heatpipes are often integrated into heat sink assemblies to improve thermal performanceor reduce size and mass.

Heat sinks coolelectronic devices by absorbing and dissipating heat into air via extendedsurfaces such as fins through either forced (fan-driven air movement) or naturalconvection.

Heat pipe heat exchanger (HPHX)cores are also used to cool electronics within an enclosure. HPHX cores consistof an array of heat pipes with plate fins that transfer heat from within theenclosure to the external environment without introducing external air into theenclosure. A manifold plate separates the core into two halves, with one halfresiding 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 aheat sink, is a flat or planar heat pipe that allows the heat to spread inthree dimensions, improving conductivity and heat spreading. Vapor chambers areoften integrated into heat sink assemblies and provide a highly conductive heatsink base with highly uniform temperatures.

A number of new materials are becoming available to thermalengineers who are designing solutions for medical devices. Annealed pyrolyticgraphite (APG), for example, is a lighter, more efficient solid conductor ofheat when compared with raw metal such as aluminum or copper. APG has greaterconductivity than both aluminum and copper and is typically encapsulated withina metal. By encapsulating APG within a biocompatible metal, APG is versatileenough to be used in solutions designed for surgical instruments and othermedical 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. Anoptimal solution for cooling these devices is a heat pipe exchanger. In theheat pipe exchanger, the pipe transfers the heat from inside the machine to theoutside of the equipment, where it is released into the air via the fins of theheat sink.

Heavily regulated medical devices like scanners,biotechnology equipment and laboratory microassays must perform withnear-perfect repeatability and reproducibility of results. Heat pipe heatexchanger technology, with high reliability due to no moving parts, is an idealthermal solution for critical care monitoring devices, which simply cannot failduring an operation or procedure.

Automated serum and urine screening assays are calibratedwith lasers and advanced optical systems to maintain consistency acrossthousands of samples. As a result, heat from conveyors, electronics and powersources can damage these intricate systems.

There are a variety of passive thermal solutions for thesesystems, including a copper thermal reservoir, in which heat is moved from heatpipes into a heat sink with fins. Ensuring the devices have maximum contactwith each other results in reduced interface resistance and improved levels ofthermal control.

Depending on the device and the application, heat pipe assemblies can be used to improve thermalperformance. Sometimes the heat simply can be moved by a heat pipe to a thermalsink, where it is spread into the air outside the casing. Another approach isthe use of a heat sink with an embedded vapor chamber that uses more efficientconvective cooling via a three-dimensional spreading.

Devices used for polymerase chain reaction (PCR) mustmaintain a certain temperature for maximum efficiency while cycling betweenhotter and cooler conditions thousands of times per minute. Connecting athermoelectric cooler (TEC) to a vapor chamber with a graphite interfaceprovides consistent thermal control by ensuring that the heat is spreadconsistently across three dimensions. This reduces the cost and complexity of theelectronics and software that are used to control the TECs.

Small-diameter heat pipes can be used to remove the intenseheat in a forceps design used during brain surgery; this design includes thesmallest mass-produced heat pipe. After using effective heat transfer to removeheat from the forceps tip, vapor from the working fluid generated towardcauterization travels to the coolest part of the heat pipe, where it condensesinto liquid and returns to the forceps tip.

Passive thermal solutions offer many benefits to thermalengineers working on cooling alternatives for medical devices. Besides helping ensurereliability and accuracy of medical devices in many applications, passivethermal solutions do not include pumped liquids and therefore are safer on theenvironment. As miniaturization and the microprocessing technology continue toevolve and influence the development of medical devices, passive heat transferdevices will continue to be cost-effective, reliable and effective solutionsfor thermal engineers.

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

Click here for more information.