New Thermal Management Strategies for Medical Devices
- Sunday, 01 April 2012
Heat pipes and vapor chambers are being utilized to address challenging thermal management requirements.
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 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.
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.