Today’s medical device and equipment designs are highly influenced by continuous technological advances that affect their size, power consumption, and communication capabilities. But patient safety and comfort are also critical considerations, forcing designers to balance the demands of new technology with the demands of new forms of patient care. From addressing industry-wide concerns over bioburden to adhering to strict noise limitations and biocompatibility needs, the list of requirements is becoming increasingly long. In order to provide the best possible solutions, medical device designers need a working knowledge of the thermal management technologies available to them. Armed with a thorough understanding of both passive and active thermal management systems, including their components, benefits, and applications, designers can more effectively address all the needs of the medical industry.
Bioburden, Just the Beginning
Amidst all of the conditions impacting healthcare, one in particular is currently demanding a cure: bioburden. Bio - burden, also known as fouling, refers to the potential for bacteria to form on a given surface. In a hospital setting, bioburden can lead to hospital acquired infections (HAIs), which are new infections patients contract while in the hospital. The U.S. Department of Health and Human Services (http://www.hhs.gov/ash/initiatives/hai/ ) reports about 1 in 20 patients develops an infection related to hospital care, and the agency has made the reduction of HAIs a priority goal. Further emphasizing the issue, a 2013 Frost & Sullivan study (http://medicaldevicesummit.com/Main/News/2013-Global-Medical-Devices-Outlook-1497.aspx ) revealed mitigating HAI risk is one of the top five growth factors influencing healthcare innovation. Of even greater concern is that more and more of these infections are antibioticresistant, earning them the name “superbugs.” All of these factors place immense pressure on healthcare providers and equipment designers to identify and eliminate potential sources of pathogens, especially in places like the OR, ICU, ER, and recovery rooms.
The primary method of addressing bioburden through thermal management is with the use of passive system components. When considering the use of a passive versus an active thermal management system within a medical device or piece of equipment, passive components provide the greatest advantage when it comes to controlling bioburden. Passive systems may comprise any number of components, including: heat sinks, k-Core annealed pyrolytic graphite (APG) heat spreaders, heat pipe assemblies, vapor chamber assemblies, and advanced thermal materials.
What all of these components have in common is they have no moving parts, do not require a power source, and can be configured so they do not require fans for cooling the medical device or equipment electronics. Many of these components can even be produced in materials with inherent antimicrobial properties. Thermoelectric coolers (TECs), known as Peltier devices, can also be useful in addressing fouling concerns. While these solid-state refrigerators do require electrical power input and have a lower useful service life, they have no moving air or liquid to collect debris. These are frequently integrated into passively cooled systems to maintain lower component temperatures.
Another key benefit of these passive thermal management solutions described above is that they allow the designer to isolate the electronics from the medical treatment area to avoid the introduction of pathogens to the patient.
Removing fans is critical in addressing fouling and bioburden because air flow can cause dust and debris to collect inside warm medical devices and equipment. Once present, the dust and debris can incubate, ultimately causing proliferation into the fan air current and into the room. Eliminating the fan means reducing the airflow through the device, thereby also reducing the chance of debris collection and the spread of infectious material.
Surprisingly, many designers are still unaware that using fans within an active thermal management system can exacerbate this bioburden by collecting, incubating, and distributing pathogens, especially in mobile medical equipment that moves from room to room. These same designers are also often surprised by the capabilities of passive systems, which can dissipate kilowatts (kWs) of heat and manage heat fluxes of over 700 W/cm². (See Figure 1)
Tech Trends in Healthcare
In addition to battling bioburden and improving reliability, current technological trends and healthcare reforms are dictating requirements for devices to perform minimally invasive procedures, reduce procedure times, and continue to improve patient outcomes. To satisfy many of these expectations, devices must often be smaller (miniaturization) and in some cases portable—two more trends impacting device design.
Miniaturization is the driving force behind minimally invasive surgeries and procedures that allow for shorter hospital stays and quicker recovery times that benefit both patients and providers. Portability is also important. For hospitals portability allows providers to move devices and equipment from room to room to increase 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 incredibly reliable while requiring little in terms of service or repair. Patient-centered design criteria also include specifications for limited noise, biocompatibility, and accuracy, particularly in cases where a patient may be using a device at home.
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.
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)
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, Click Here .