For hospitals, devices used to perform electro-surgery are some of the best examples of how passive thermal management systems can reduce procedure time and improve patient outcomes through reliable device performance. For instance, surgical devices and tools often require cool down periods during a procedure. Optimized thermal management solutions can reduce or even eliminate the need for delays associated with cool-down cycles, often passively.

Fig. 3 – Active, variable speed liquid cooling system.
Depending on the device and application, heat pipe assemblies or APG-based thermal straps, as shown in Figure 2, can reduce warm up and cool down times by moving heat from its source to the desired point. Other methods include using a heat sink with a vapor chamber that uses convective cooling via threedimensional spreading. Ultimately, the effective management of heat means surgeons can focus more on the needs of their patients and less on the needs of the surgical device.

Increased Power, Not Temperature

Optimized thermal management also means designers can increase the power level of a device without increasing the operating temperature, thus improving reliability, service life, and precision to prevent damage to collateral tissues for applications such as ablation and coagulation of tissues. Maintaining touch point temperature limits is also becoming increasingly important with the introduction of the International Electrotechnical Commission (IEC) 60601-1 3rd edition standard, which places strict temperature limitations on medical electrical equipment that makes direct contact with human tissue and skin.

In minimally invasive procedures like an endoscopy or laparoscopic surgery, doctors are working within a small area of the body with a device that generates heat. The IEC 60601 3rd edition regulation specifies a temperature range for the device to protect collateral tissue not involved in the procedure, as well as to protect the skin of the doctors and nurses should he or she come in contact with certain surface areas of the device.

In contrast to smaller devices usually served by passive systems, active thermal management components, including fan-cooled heat sinks, liquid cooling systems, heat exchangers, pumps, and compressors, are still necessary when there is not enough surface area available to allow for natural convection. Active systems are commonly found in applications such as MRI, CT, ultrasound, digital X-ray, and other electronics- rich imaging devices where thermal loads are larger. In this case, a heat pipe exchanger aided by fans may be used to transfer heat from the inside of a machine to the outside of a machine. Unlike passive systems, active systems require a power source and may contain a number of moving parts, such as fans, pumps, compressors, and TECs, which in the medical industry means an increase in the potential for failure and/or fouling.

Though active components also tend to increase the size, noise, and service costs of devices and equipment, proper thermal optimization can reduce these concerns to manageable levels. Many applications like CAT scan machines, use both passive systems to reach compact heat sources and active systems for areas of the machine that extend further out. (See Figure 3)

Treatment Plan

With all of these variables to consider, it would be a mistake to prescribe just one type of thermal management system for a certain type of application. Passive thermal management systems receive a lot of attention for their ability to satisfy miniaturization and portability trends, but active systems still have their place in addressing higher heat loads for larger medical equipment. The decision as to how and where to use passive or active thermal management solutions typically comes down to a few key factors. First, the designers need to assess any noise level and fouling concerns. Second, designers must consider the heat load versus the geometry/space limitations. For a passive system to function, the power demand usually has to align with the available natural convection surface area to efficiently manage and dissipate the heat.

The best solutions begin with an understanding of the capabilities of both passive and active thermal management systems, and many designers find it is often a combination of the two that produces the best outcome. A good strategy also includes partnering with vetted thermal management experts who can apply medical design considerations to their components and capabilities to create an optimized thermal management solution.

This article was written by Michael Bucci, Market Development Manager, Thermacore, Inc., Lancaster, PA. For more information, visit http://info.hotims.com/45609-160.

