Feel the Heat:
Thermal Design Trends in Medical Devices
By: W. John Bilski and John Broadbent
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.
The impressive capabilities of twenty-first 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, for thermal engineers, these advances come with a price. 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.
Another challenge arises from the fact that medical devices have unique requirements because of the high stakes involved. For example, some common materials used in thermal solutions, such as copper, cannot be used in many medical applications because of concerns about the proximity of these materials to the human body (in addition to causing tissue irritation, copper has been implicated in serious, irreversible neurodegenerative conditions). The precise nature of some medical applications can reduce the available space for cooling solutions almost to the vanishing point—some surgical instruments that need thermal management to avoid causing tissue damage, offer designers as little as 0.5 mm in which to place heat transfer technology.
Another field requiring ultraminiaturized thermal management is the design of implantable devices, which combine tiny size with the need for precise temperature change coefficients (∆T°) to protect human organs. Finally, rapid temperature cycling, with variations of up to 50°C within milliseconds, is a common feature of many laboratory devices such as DNA splicers.
All of these elements add up to accuracy requirements, reliability needs, size constraints, and restrictions on material selection that can make medical thermal engineering a juggling act for designers. Thermal engineers must balance efficiency and size versus cost, and increasingly, thermal performance versus low noise (which means fans cannot be used in some applications, even when their high volumetric airflow would make them the best performing option).
Increasingly, thermal engineers have turned to passive heat transfer devices, such as heat pipes, to meet these challenges. Heat pipes are considered two-phase cooling devices because the working fluid exists as both a liquid and a vapor inside the heat pipe. The transition from the liquid to the vapor phase provides heat transfer. Heat pipes enable a continuous cycle of vaporization, transport, condensation, and return of the condensate to the area of evaporation. They can do so with no moving parts to fail—a key consideration in applications in which reliability is essential for precise results or patient health. Their straightforward design, generally involving a vacuum-sealed tube injected with a working fluid, is relatively easy to miniaturize. Continual advances in wicking structure technology have helped ensure that cooled, condensed fluid can efficiently and reliably return to the heat input portion of the heat pipe even against gravity. This allows the heat pipe to operate in any orientation. Designers can even turn to flexible heat pipes in some situations for additional design freedom.
Another favored thermal approach is the heat sink, which can operate via forced or natural convection. But again, either approach means trade-offs. More airflow, for greater cooling, means that a heat sink can have fewer or smaller fins. However, fans that produce more airflow also produce more noise. Using quieter and smaller fans, which produce less airflow, means that the heat sink must have more or larger fins. It’s difficult therefore to make thermal assemblies both smaller and quieter within the same device.
But it can be done. The way to reduce both size and noise is to make the heat sink more isothermal. A heat sink previously cooled by a single thermoelectric cooler (TEC) can be redesigned to use multiple TECs to spread the heat evenly across the face of the heat sink, rather than rely on pure conduction to spread the heat. However, this adds electronic complexity and costs in addition to maintenance requirements.
A rack-mounted heat pipe assembly can provide complete thermal reliability with low maintenance.
A simpler approach is to use passive thermal techniques by pairing a heat sink with an embedded vapor chamber (essentially, a heat pipe flattened to form a planar heat pipe) or a heat sink with heat pipes incorporated into the surface of the heat sink. Either approach can be used to spread heat evenly and quickly via the vapor formed by the working liquid in the embedded heat pipe or the vapor chamber. Vapor transports heat uniformly across the entire base plate surface of the heat sink and past fins, preventing hot spots. Because the heat sink is isothermal, the air moving across the fins removes a maximum amount of heat.
The trend toward passive heat transfer devices such as heat pipes, heat sinks, and vapor chambers in medical devices reflects the ongoing evolution of smaller, more powerful and more miniaturized electronics in general. While more traditional cooling approaches (refrigeration, TECs, liquid cold plates, etc.) are still the most appropriate choices for some medical equipment, designers are finding passive cooling technology to be more and more attractive as this technology develops.
A number of advances in material fabrication have made passive thermal options more attractive for designers of medical equipment. For example, the development of annealed pyrolytic graphite (APG) enables the development of thermal components that are smaller, lighter, and more efficient than traditional heat sinks using aluminum or copper. Higher-conductivity materials help designers as products trend toward greater miniaturization and smaller electronic enclosures. APG offers effective thermal conductivity of 1000 W/m•K, 5 times greater than that of solid aluminum and 2.5 times greater than that of solid copper. APG can also be encapsulated for applications such as surgical instruments, for which it is critical to avoid contact between APG and human tissue due to concerns about tissue damage, scarring, or contamination.
Developments like APG help explain why medical device designers are choosing passive thermal control systems more often. These systems offer not only a wider range of choices, but in many cases better choices, for thermal management. Compared with traditional liquid cooling approaches, passive thermal systems offer higher reliability (fewer moving parts means less failure risk), reduced maintenance, greater design flexibility, quieter operation, and in many cases, more manageable cost. Here are some examples of passive thermal management concepts incorporated in some key medical equipment applications.
Because the capabilities of electronics decline quickly after a critical temperature is reached, enclosure cooling is critical for electronics-rich technologies such as magnetic resonance imaging (MRI), computed tomography (CT), ultrasound, and radiography (x-ray). Slight fluctuations in temperature can influence calibration and results, leading to costly downtime and maintenance. FDA, of course, plays a critical role in the drive toward near-perfect (=95%) repeatability and reproducibility of results from medical devices such as scanners, biotechnology equipment, and laboratory microassays. The regulations for one diagnostic imaging machine alone (21 CFR 900.12) mandate 31 separate tests, many of which are affected by thermal performance, to ensure accuracy. A competitive diagnostic market makes exacting thermal control an even more important aspect of electronic design.
Designers typically have a narrow window of temperature change (∆T) to work with, generally 10°C between the cabinet interior and exterior ambient temperature. Multiple heat sources, such as the device power supply as well as other discrete electronic components, can produce a total power output of 1200 W or more, with a potential heat load of 400 W of waste heat to dissipate. The desire for quiet operation complicates matters further by limiting fan size and speed.
These challenges are often best met through a heat pipe heat exchanger, in which heat is transferred from inside the equipment to outside the equipment via a heat pipe and is dissipated through a finned heat sink, where it is ejected 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. In some cases, heat pipe technology can be used on its own, allowing the laws of thermodynamics, rather than electronic devices or fans, to provide heat transfer.
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 lack of moving parts puts the heat pipe’s time-to-failure performance in the millions of hours, leaving virtually no chance of failure during a critical-care operation.
Assays and Sample Screening
Some of the most challenging demands for accuracy and reproducibility are associated with automated serum and urine screening assays. These devices 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 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 of the heat pipe groove or vapor chamber pocket is necessary, because surface roughness can reduce the efficiency of heat transfer. Precision machining minimizes the peaks and valleys profile (at the micron level) of surfaces, thus ensuring that heat transfer surfaces have maximum actual contact with each other. That reduces interface resistance and helps achieve the same levels of thermal control as thermoelectric devices. At the same time, passive systems also meet the challenge of consistent, isothermal performance.
A passive heat transfer approach relies on a copper thermal reservoir, moving heat through heat pipes into a finned heat sink.
Some automated assay devices provide the ideal settings for heat pipe assemblies, which are most commonly used in one of two different ways. In the first approach, heat is moved by the heat pipe to a metal casing or other thermal sink. The heat then dissipates into the ambient air outside the casing. This is the simplest approach and has the benefit of flexibility because heat pipes can be manufactured in many different shapes and sizes to meet individual equipment needs.
In other applications, a second approach, a heat sink with an embedded vapor chamber, using convective cooling, provides optimal isothermalization for more efficient cooling. This approach can deliver higher thermal performance than a traditional heat sink by alleviating spreading resistance found in solid heat sink construction, via three-dimensional spreading which produces lower device temperature and greater component reliability. It should be noted that this option might require changing the geometry of the heat sink and the electronic component’s base.
Heat pipe assemblies are not always feasible. Some large and powerful automated assay devices could require a centralized liquid cooling system in which hot air from inside an electronics cabinet is transported over cooling liquid. This method provides reliable thermal control but can be costly and space-consuming. There is also the potential for leakage and the need for maintenance to ensure optimum system performance.
Whichever configuration 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 a thermal solution that features few or no moving parts.
Biotechnology and Research
Cycling devices to facilitate polymerase chain reaction (PCR) technology have emerged as real workhorses in biotechnology and research, but they can also produce impressive thermal management challenges. The device temperature must not only be kept at exactly the right range for efficiency, it must do so while cycling between hotter and cooler conditions thousands of times per minute to provide optimal conditions for PCR reactions to unfold.
Passive thermal control systems offer simplicity, design flexibility, manageable cost, and quiet operation.
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 recent process developed for a 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 that heat is dissipated consistently across three dimensional axes. This process provides instantaneous isothermalization that matches the constant fluctuation between hotter and cooler cycles, using the laws of thermodynamics in place of complex electronic algorithms.
Vapor heat dissipation via small-diameter heat pipes contributes significantly to a forceps design used in procedures such as brain surgery. This design includes precise temperature control to improve surgical efficiency. Several experiments conducted by surgical products manufacturers 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. The forceps’ proprietary thermal design in this case employs active heat transfer to draw heat from the very small (1 mm diameter and smaller) forceps tip (and tissue). 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 application of passive thermal technology possible. This surgical cooling system utilizes the smallest known mass-production heat pipe assembly (2.34 mm in diameter) to allow precise thermal control of forceps tips that range in size from 0.5 to 4 mm wide.
In the development of medical devices, passive thermal management is clearly a major factor in helping ensure the accuracy and advanced capabilities of today’s medical devices, and extending these capabilities still further. Passive thermal approaches also offer valuable advantages in terms of saving space, minimizing weight, and reducing maintenance costs, and they produce less environmental impact than cooling systems that rely on technologies such as pumped liquids. Because increases in electronic capabilities and computing power invariably produce increased heat to be dissipated, and because miniaturization is steadily reducing the amount of space available for thermal management, innovative thermal technologies will play a major role in the evolution of tomorrow’s medical devices.
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