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Passive Thermal Management Options for EMS Devices

High-frequency pulsed electromagnetic stimulation (EMS) devices are more powerful and effective than ever before. These devices are finding applications in many areas, including as treatments for stress and depression, osteoporosis, and soft tissue injuries. Electromagnetic therapies stimulate tissue and cell mass to recuperate faster. The base technology for pulsed electromagnetic field (PEMF) is to input electrical energy into copper windings to create a series of electromagnetic waves. The waves offer a non-invasive anti-inflammatory and accelerated healing treatment option. In many cases, these devices have a large metal content and need to dissipate hundreds of watts of heat to effectively generate and deliver pulsed electromagnetic waves.

altMany of these PEMF devices require direct contact to the patient’s skin to function. With repeated pulsing, the copper windings generate waste heat. The more power that is input, the more heat there is to be dissipated. The FDA mandates that these devices cannot exceed 41°C at the patient contacting surface. From the medical practitioner’s perspective, it would be beneficial to be able to use these devices as frequently as needed, without concern for exposing the patient (or themselves) to a dangerously hot device. The most cost-effective thermal management solution is to use a heat sink for natural convection. Device packaging requirements and/or large metal coils may exceed natural convection capabilities and demand higher performance thermal management solutions. There are a number of both active and passive cooling technologies available to the medical device designer. More traditional cooling solutions, such as pumped liquid and forced convection air cooling, work well in many applications. But, these technologies require power to operate, have a risk of leaking, and generate noise. Fortunately, there are passive thermal management solutions, such as heat pipes and phase change materials, that provide excellent thermal management performance while requiring no input power and generating no noise.

As mentioned initially, EMS stimulation devices have a copper coil structure consisting of a series of copper wire windings wrapped around a solid metal core. The devices are pulsed, generating electromagnetic waves when turned on. With repetitive pulsing, the temperature of the device increases. This has a direct impact on the patient contact surface temperature. In many cases, the device must be turned off for several minutes to allow heat to dissipate to a safe level prior to using again. An effective thermal management system can resolve extended downtime issues and passive technologies are a great option to do that. An overview of heat pipes and phase change materials is provided below.

■ Heat Pipes

Heat pipes are well established, reliable heat transfer devices. Developed by Sandia National Laboratories in the early 1960s, heat pipes are ubiquitous, found in laptop computers, satellites, high-powered electronics, as well as solar and other alternative energy applications. They are sealed vacuum devices that contain only a wicking material and a small amount of a working fluid, typically water. A heat pipe is a two-phase (liquid/vapor) device that efficiently transfers heat from an external evaporator—the copper coils, in this case—to an external condenser, a heat sink of some kind. Heat pipes transport heat through boiling and condensation of the working fluid. The heat from the evaporator causes the working fluid to vaporize. Pressure pushes the vapor to the colder condenser end, where it re-forms as a liquid and is absorbed by a wick structure. The liquid is returned to the evaporator by capillary action from the wick. A simple diagram showing how a heat pipe operates is shown in Figure 1.

Heat pipes can be made in a variety of different sizes and materials, and can accommodate many different power levels. The most common system is a copper envelope/copper wick with water as the working fluid. The typical maximum heat flux for these copper water heat pipes is ~50-75 W/cm2, but can be higher with specially designed wicks. The power capability for a heat pipe is ~100W, and is dependent on heat pipe diameter, length, wick structure, orientation vs. gravity, and other factors. Heat pipes can be packaged together to handle larger heat loads. Some of the advantages of heat pipes are that they can be designed to work against gravity, and the freezing issue can be solved with fluid inventory control. In addition, they can be bent or flattened to accommodate different geometries.

In the case of EMS devices, heat pipes could be used to either transport excess heat away from the metal core to a remotely located heat sink, or to help spread the heat out over a larger area, such as the device package envelope. Another option is to combine heat pipes with phase change materials.

■ Phase Change Materials for Thermal Storage

Phase change materials (PCMs) are materials that can store large amounts of heat when undergoing phase change from solid to liquid. These materials have a high heat of fusion, which is defined as the amount of heat (energy) required to convert a solid at its melting point into a liquid, without an increase in temperature. In PCM operation, when a pulsed, powered device is turned “on”, heating causes the PCM to begin melting. As the device continues to operate, the PCM, now at its melting point, absorbs additional heat but does not increase in temperature. When the device is turned “off”, the PCM will dissipate heat and return to its original solid state. PCMs are ideal for thermal management scenarios where transient changes occur, such as in pulsing EMS devices. An important design consideration is to ensure sufficient PCM is present so that not all of it is melted during the “on” cycle.

The real benefit of PCMs is observed when examining multiple cycles. Under repetitive cycling, the peak temperature can be controlled to the melting point of the PCM, and dangerous high peak temperatures can be avoided.

■ Choosing the Right PCM Material

To implement a PCM solution, it is critical to select the right material. Desirable characteristics of a solid-liquid PCM include high heat of fusion per volume and corresponding melting and freezing characteristics to the specific application. An interesting and effective thermal management solution for electromagnetic stimulation devices is to combine heat pipes and phase change materials. In this scenario, the heat from the EMS device would be absorbed by the PCM. The stored heat would then be transported by a heat pipe to a remotely located heat sink. One of the many benefits of this approach is that use of the PCM will also insulate the metal heat pipes from the copper winding in the EMS, ensuring minimal distortion of the electromagnetic wave generation.

Of course, specific package requirements will dictate if this solution is feasible, and there are numerous other possible combinations. The key point is that passive thermal management solutions offer benefits that can greatly improve the duty cycle of electromagnetic stimulation devices without requiring any input power. All of these solutions can be used independently, or with other technologies, and all offer the additional benefits of long reliable life and silent operation.

This article was written by Peter Ritt, Vice President, Technical Services; Richard W. Bonner III, Manager, Custom Products; and Sanjida Tamanna, R&D Engineer; Advanced Cooling Technologies, Inc., Lancaster, PA, a proven expert in the medical device industry, helping designers and manufacturers solve key thermal problems. For more information, visit http://info.hotims.com/45602-163.