Medical Device Miniaturization and Thermal Management

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By Michael Bucci


Medical devices are trending toward miniaturization, driven by portability, 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. The right thermal management solution can play an important role in helping engineers meet all project requirements and design the best medical device.

Miniaturization is the driving force behind less invasive surgeries and procedures, and the shorter hospital stays and quicker recovery times afforded by these procedures are beneficial for both patient and provider. Smaller devices also offer important portability benefits. For hospitals, portability allows providers to use a single device for multiple patients, saving floor space, reducing capital equipment cost, improving equipment uptime utilization, and increasing patient throughput. For patients, portability offers the option of in-home therapies, such as blood pressure monitoring, glucose monitoring and dialysis. These smaller, portable devices must also meet greater power demands to enable communication as well as performance, and they must be consistent and reliable while requiring little in the way of service or repair.

Innovative medical devices must also play a role in minimizing the risk of healthcare-associated infections (HAIs). A 2013 Frost & Sullivan study lists mitigating HAI risk as one of the top five growth sectors influencing healthcare innovation. Providers must strive to identify and eliminate potential sources of these microbes, especially in places like the OR and ICU. Bioburden refers to the potential for bacteria to form on a given surface, leading to HAIs. Advanced thermal management technologies can help eradicate bioburden by eliminating fan-cooled heat sinks, which can exacerbate the problem by collecting, incubating and distributing these pathogens, especially in mobile medical equipment.

 

Importance of Cooling Medical Devices

Thermal management allows heat to be efficiently moved, spread and dissipated, improving system reliability, speed, precision and service life while also helping designers reduce device packaging size, weight, energy consumption, noise and bioburden concerns. Advanced thermal management also allows designers to maintain consistent temperature control and increase the power level of a device without increasing the operating temperature.

The new International Electrotechnical Commission (IEC) 60601 third edition standard highlights the importance of appropriate thermal management in medical devices. This standard places strict temperature limitations on medical equipment to prevent unintentional burns or discomfort to both the patient and the caregiver. For example, in a minimally invasive procedure like an endoscopy or ablation, doctors are working within a small area of the body with a device that generates heat. The regulation specifies a temperature range for the device to protect collateral tissue not involved in the procedure, and also protects the doctor’s skin, should she/he come into contact with certain surface area of the device. Thermal management solutions enable medical device designers to meet those temperature requirements.



 

Active vs. Passive Cooling

Medical device designers have a number of thermal management options to explore with the above considerations in mind. There are active and passive cooling technologies and a multitude of materials and functionalities within each. Active thermal management includes forced convection or fan-cooled technologies, liquid cooling and refrigeration systems. Forced convection systems offer variable speed fans that allow for adjustable cooling rates, but these can be noisy, contribute to fouling and are potential failure points. Liquid cooling can handle high heat fluxes and allows remote heat dissipation, but pumps can fail and may also generate noise. Refrigeration systems have easily adjustable cooling rates and can lower temperatures below ambient, but are also potential failure points and actually generate more heat than they remove, meaning the resultant heat must be dissipated by other means. Another design consideration is that active cooling technologies require power and usually some type of control system, which impacts space and energy requirements.



Figure 1. Passive heat pipe heat sink assembly


Passive thermal management can solve complex cooling problems in small spaces by minimizing airflow to reduce the amount of dust, moisture and debris, thereby resolving many bioburden issues. Passive cooling technologies include heat pipe assemblies, vapor chamber assemblies, phase change materials (PCM), and advanced solid conduction materials such as k-Core® encapsulated Annealed Pyrolytic Graphite (APG). Heat pipes transfer high heat loads in small spaces, reducing the size of heat sinks and eliminating the need for cooling fans. Sintered powder wick 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. Sintered powder wick heat pipes also manage high fluxes of up to 350 W/cm² and are usually freeze-thaw tolerant. Heat pipes can be formed to fit complex spaces and can be less than 2 mm in diameter.

Vapor chambers are planar versions of heat pipes. They consist of an evacuated metal plate with a small fluid charge inside and a wick structure that lines the internal surfaces. Vapor chambers are fantastic heat spreaders. Basic vapor chambers offer heat flux capabilities of 350 W/cm², while specially designed vapor chambers, also known as Therma-Bases, can offer heat flux capabilities of 700 W/cm². They can be made from a variety of materials for CTE matching and can be made less than 2 mm thick.



Figure 2. Vapor chambers can handle high heat fluxes and rapid thermal cycling, and can often operate in any orientation, making this technology especially valuable for medical devices.



Thermal Management Materials

Common materials used in medical devices include aluminum, copper, stainless steel, titanium and plastic. Aluminum is relatively inexpensive, lightweight and has good thermal conductivity. Copper is highly conductive, but is more expensive and heavier than aluminum. Stainless steel offers good corrosion resistance, but is a relatively poor conductor and tends to be heavy. Titanium is strong, lightweight and is a decent conductor, but is expensive and often difficult to work with. Plastic offers lower mass and is inexpensive, but it is permeable and its poor conductivity makes it act like a thermal insulator.

A promising new material for medical applications is encapsulated Annealed Pyrolytic Graphite (APG), used in Thermacore’s line of k-Core® thermal products. APG is versatile, as it can be paired with many different biocompatible materials. It is lightweight and has high thermal conductivity – up to four times the thermal conductivity of copper, with less mass than aluminum. The solid state design means it can be used in any orientation relative to gravity, giving designers considerable flexibility in creating solutions to address complex thermal challenges. Encapsulated APG is proven effective in maximizing electronic device accuracy, reliability, repeatability and lifecycle.

With each of these solutions, designers must also consider the thermal interface between each component to ensure good conductivity within the system. The interface materials most commonly used are thermal greases, gap pads, foils, PCM pads, epoxies and solders. These fill in tiny gaps between mating components to help ensure consistent contact. Each option offers its own set of advantages; however, these interfaces also come with limitations that designers must consider to accurately predict and optimize performance.



Making a Choice

Because of the many design objectives small medical device engineers must consider, passive thermal management solutions are often an ideal solution for these projects. They solve complex cooling problems in small spaces, reduce the opportunity for spreading or harboring microbes and do not require a power source. Passive technologies have long-term reliability and can offer serious savings on service and maintenance costs – plus the cost benefits that come along with extended equipment lifetime.

However, when it comes to thermal management, there is no “one size fits all” approach. For example, for passive thermal management, allowable heat load capability is typically dependent on the surface area available – and on a small medical device, this surface area may be quite small. Thus, active cooling systems are often the necessary choice when high heat loads are involved. To ensure you’re using the most appropriate thermal management technology for your project, partner with a thermal solutions expert that shares a mutual goal of achieving a desired outcome.



 

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