Electronic devices used in the medical industry have thermal management needs similar to those in other fields. Their electronics must stay cool enough to run continuously and correctly within their operating temperature range. Sound thermal management allows excess heat to be efficiently moved, spread, and dissipated. The result is improved system reliability and service life while, in many cases, it also helps designers reduce device package size, weight, energy consumption, and noise.
Many cutting-edge medical tools leverage the computing power of advanced microprocessors. These include lasers, ultrasound equipment and digital imaging technologies. However, these devices can be challenged by higher amounts of waste heat, particularly as the demand increases for smaller equipment sizes. Thus, as higher capabilities are designed into smaller instruments, waste heat must be safely dealt with to prevent inaccuracies or malfunctions.
Basic Cooling Approaches
Medical device designers have a range of thermal management methods to resolve excess heat issues. This includes the use of active or passive technologies, or a combination of both.
Active thermal management generally includes pumped liquid cooling, refrigeration, and, most commonly, forced convection or fan-cooled technologies. Liquid cooling can manage high heat fluxes and provide remote heat dissipation. Refrigeration systems have adjustable cooling rates and can lower temperatures below ambient. Variable speed fans allow for adjustable cooling rates.
But, active cooling technologies have their own requirements. They need power, occasional replacement parts, and typically a control system. Refrigerators generate more heat than they remove, so the heat they generate must also be dissipated. This means energy requirements, parts and performance maintenance, and added space.
Passive thermal management has fewer parts and no power requirements. A passive design can solve complex cooling problems in small spaces by minimizing the required airflow. Passive methods include heat sinks, heat pipe assemblies, vapor chamber assemblies, and phase change materials (PCMs). To provide heat transfer, a heat pipe uses a phasechanging fluid that absorbs heat from the enclosure as a liquid, converts to a vapor, and releases heat to the outside air via a heat sink or a heat exchanger attached to its condenser.
At the component level, heat pipes can transfer high heat loads in small spaces and reduce the size of heat sinks. Heat pipes, often used in these applications, offer the advantages of two-phase passive cooling (working fluid evaporation and condensation cycle) with no moving parts, and can operate in any orientation.
More recently, heat pipes are being employed with fans in active cooling systems that can provide cooling to large packaging areas. (See Figure 1)
Heat pipes offer the advantages of two-phase passive cooling (working fluid evaporation and condensation cycle) with no moving parts, operate better against gravity, and are freezethaw tolerant.
Vapor chambers are planar or flat versions of heat pipes. They include an evacuated vessel with a small amount of fluid inside and a capillary wick structure that lines the internal surfaces. Vapor chambers offer excellent heat spreading ability in all directions, allowing them to handle high heat fluxes and rapid thermal cycling. Vapor chambers can be designed to be less than 1 mm thick. For some applications, they can be matched to the coefficient of thermal expansion (CTE) for direct die attachment to the electronics, making this technology especially valuable for medical devices that require compact configurations.
Most PCMs function via the change in enthalpy when they’re heated from solid to liquid. This is known as the enthalpy of fusion, which for this condition is a latent heat, because during the melting the temperature remains constant. (See Figure 2) PCMs will maintain a temperature constant until they completely melt. This feature can temporarily prevent a component from exceeding its temperature limit. PCMs are easy to integrate into medical device designs because they do not require access to ambient air or liquid cooling lines.
Downsizing Medical Devices
Medical devices are trending smaller, driven by demands for portability, fast, accurate performance, and safety factors. As instruments get smaller, design engineers face new challenges in meeting their project’s performance, size, weight, operating temperature, noise, and budget requirements. Each of these factors also impacts thermal management technology choices. An effective thermal management solution will help engineers meet all project requirements and design a better all around medical device.
In one case, a major medical laser company saw that their competitor was producing a hand-held laser diode probe that was twice the cost of theirs, but just half the size. It was becoming a big seller with doctors. An analytical simulation using computational fluid dynamics (CFD) identified available heat transfer paths. The laser device was repackaged and its cooling system was replaced with smaller but more efficient components. The redesigned probe was lighter weight with a heat load distribution which ensured that the laser would not overheat or impact its light spectrum. (See Figure 3)
Cooling with Lasers
While laser systems themselves may require thermal management, lasers may one day have a role in cooling hot electronic components in medical electronics. A research team at Singapore’s Nanyang Technological University successfully used a laser to cool down a semiconductor material known as cadmium sulfide. The results of the study could lead to the development of self-cooling computer chips and smaller, more energy efficient air conditioners and refrigerators that don’t produce greenhouse gases.
Automated biological analysis tools, such as blood analyzers and the latest DNA sequencing tools, are now broadly used in many healthcare labs. This equipment requires precisely controlled operating temperatures. For example, to properly replicate DNA samples, the temperatures of hundreds of samples are cycled repeatedly across a wide temperature range, yet must be kept within 0.5°C of each other.
Though still in development, the breakthrough in laser cooling (or optical refrigeration) technology could lead to compact, cost effective, vibration-free and cryogen-less cooling systems in many different applications. It would mean that medical devices requiring extreme cooling, such as an MRI, which uses liquid helium, could do away with bulky refrigerant systems with just an optical refrigeration device in its place. CPUs could reduce their reliance on external cooling systems like fans and incorporate built-in laser-controlled systems instead. The potential for minimized heat and prolonged battery life in items such as tablets and smartphones is another example.
Displays in the Field
Outside the clinical offices, medical electronics can involve signage and other displays specially designed for patients, practitioners, and others. Most electronics operate better in cooler, drier environments, which include modern climate-controlled hospitals. Medical devices used outdoors, e.g., at public events or with emergency response teams, can be exposed to high ambient temperatures and direct sun-load. High humidity and air-borne contaminants may also pose equipment threats.
In such conditions, an essential need is proper function of digital displays. Used outdoors, digital displays will be exposed to high ambient temperatures and direct sun-load at some point throughout the year. Considering that these displays need to be sealed to protect them from rain, snow, and dust, placing these displays inside a sealed enclosure further compounds the problem of heat build-up.
A patented thermal management system was developed by the MRI Company using engineering fluid dynamics to model air flow and cooling performance based on ambient temperature conditions and the impact of direct sun load on the LCD screen, running at 2000+ nit brightness. The system is designed for digital displays up to several feet in size, for temperatures up to 50°C (122°F) and it uses a dual-loop air-circulation/cooling system. All electronics, display surfaces, optical films, and backlight assemblies are contained in a sealed, cool, dry, and clean environment thereby prolonging the life of the electronics and significantly reducing field failures and associated maintenance costs.
Many cooling methods are available to bring effective thermal management to medical electronics. Off the shelf products, such as heat sinks, can provide effective solutions at agreeable prices, although a careful evaluation period is usually needed given all the choices on the market. For such evaluations, or for solutions where heat sinks aren’t the answer, partnering with thermal experts can allow designers to save time, meet the design challenges and often save money by implementing a reliable solution the first time. This approach helps identify any necessary design changes at stages early enough to allow changes.
This article was written by Bahman Tavassoli, Chief Technologist, Advanced Thermal Solutions, Inc., Norwood, MA. For more information, Click Here .