In an increasing number of medical device applications, thermal issues limit the overall performance and reliability of the system. Basic thermal management strategies such as liquid cold plates, air cooled heat sinks, and thermal interface materials are becoming insufficient as stand-alone solutions. In many new medical applications, implementation of advanced thermal technologies such as heat pipes and vapor chambers are becoming an integral part of the thermal management solution. These technologies offer excellent heat transfer and heat spreading performance. Furthermore, they are passive (no energy, no moving parts), quiet, and reliable. Several medical devices, such as powered surgical forceps, skin/tissue contacting devices, and polymerase chain reaction (PCR)/thermocyclers already use these technologies, and more applications are emerging. A discussion of heat pipe and vapor chamber operation and selected medical device applications follows.

Heat Pipes

Fig. 1 – Heat Pipe Schematic.

Heat pipes are vacuumsealed, two-phase devices that transfer heat by evaporation and condensation of a working fluid. From a thermal behavior perspective, a heat pipe is analogous to a very high thermal conductivity solid; it is a superconductor of heat. A schematic is shown in Fig. 1.

The driving force of heat pipe operation is the temperature difference between an external evaporator and condenser. Heat from the evaporator (“Heat In” in fig. 1) causes the working fluid inside the heat pipe to vaporize. Pressure pushes the vapor to the cooler condenser (“Heat Out”), where it becomes liquid. A wick structure inside the heat pipe enables the liquid to return to the evaporator end, where the cycle repeats. Heat pipes can be made in a variety of different sizes and materials. The most common system is a copper envelope/copper wick with water as the working fluid. The typical maximum heat flux is ~50–75 W/cm2, but can be higher with specially designed wicks. The power capability for a heat pipe is ~100W, but its performance is dependent on a number of design factors, including: heat pipe diameter, length, internal wick structure, as well as evaporator and condenser orientation with respect to gravity. Some advantages of heat pipes are that they can be designed to work against gravity, and water freezing issues can be solved with fluid inventory control. In addition, heat pipes can be bent or flattened to accommodate different geometries.

Fig. 2 – Example of heat pipes used in medical devices.

Heat pipes can be used to effectively transport both heat and cold. Some electrosurgery devices generate heat from the intense friction of the blade contacting tissue during the procedure. The blade temperature may often exceed several hundred degrees Celsius; excessive blade tip temperatures can force surgeons to temporarily halt procedures, waiting for the blade tip to cool in order to prevent burning of unintended tissue. To accelerate heat dissipation from the tips, heat pipes are ideal to spread the heat from the blade axially to the rest of the device.

Cryogenic heat pipes can also safely and precisely transfer “cold” for various medical applications. For example, a cryogenic heat pipe “pen” can be used to freeze tissue both inside the body and on the skin surface. Figure 2 shows an example of cryogenic heat pipes developed for this application. In all of these examples, insulation is important to assure that the cold heat pipe does not cool undesired locations.

Vapor Chambers

Fig. 3 – Vapor Chamber Schematic.

Vapor Chambers are a subset of heat pipes that spread heat in two dimensions rather than one. More specifically, vapor chambers are planar heat pipes that spread heat from concentrated heat source(s) to a large area heat sink with effective thermal conductivities greatly exceed ing copper. In the most basic configuration, as seen in Fig. 3, the vapor chamber consists of a sealed container, a wick formed on the inside wall of the container, and a small amount of fluid that is in equilibrium with its own vapor. As the heat is applied to one side of the vapor chamber (evaporator), the working fluid vaporizes and the vapor spreads to the entire inner volume and condenses over a much larger surface (condenser). The condensate is returned to the evaporator via capillary forces developed in the wick. Vapor chamber are capable of handling higher heat fluxes (300–500 W/cm2), and are used to transport heat away from high-powered chips and lasers. Another important feature of vapor chambers is that the low flux (condenser) region provides an essentially isothermal surface. This provides an ideal surface to attach heat sinks, because it enables maximum fin efficiency. A view of the key vapor chamber elements can be seen in Fig. 4. Vapor chambers can be used to increase throughput in Polymerase Chain Replicator (PCR) machines. In this process, DNA is replicated through a series of enzymatic reactions. PCRs require fast and precise thermal cycling. Throughput is governed by the rate of uniform temperature change across the device platform. In this application, thermoelectrics are often used to drive the temperature to the set points, typically from 50–95 °C. Vapor chambers can help improve performance by delivering a faster and more uniform thermal response in comparison to conventional metal blocks. Weight optimized vapor chambers can reduce the thermal mass of the sample holder, which also promotes higher cycling rates.

Summary

Fig. 4 – Cut away of a standard vapor chamber. Bottom section shows base wick structure with pedestals for both support and wicking. Top shows flat surface to be attached to heat sink or other isothermal surface.

As medical devices become more powerful and compact, thermal management solutions must evolve to ensure optimum performance. Traditional cooling methods such as heat sinks, fans, and pumped liquid cooling systems require augmentation to meet increasingly challenging thermal requirements. Heat pipes and vapor chambers are effective technologies that will play a more frequent role in future medical device thermal management system solutions.

This technology was done by Advanced Cooling Technologies, Lancaster, PA. For more information about thermal technologies for medical applications, visit http://info.hotims.com/40432-165.