While technological advancements continue to enable medical devices to become more capable and more compact, the use of advanced electronics has also created thermal management issues. Laser diodes, LEDs, transistors, transducers, electrodes, motors, and other advanced electronics increase heat loads in handheld surgical instruments, imaging machines, diagnostic equipment and many other medical devices.
As a result, medical device manufacturers must meet strict standards to ensure that their devices do not unintentionally harm patients or care providers (in accordance with IEC 60601). In addition, thermal devices used to manage the temperature of patients must adhere to strict safety, reliability, and temperature control requirements (IEC 80601 & ASTM F2196-02).
While designers of electronically intensive devices face the challenge of dissipating heat more effectively, they must also deal with a specific bioburden known as fouling. Biofouling occurs when pathogens are allowed to collect in a given area that is not disinfected and could expose humans to infection risks. A significant competitive advantage could be obtained by medical device original equipment manufacturers (OEMs) who can offer devices that address both touch temperature and hygiene challenges.
Hygienic design is especially important given recent efforts to reduce hospital- acquired infections (HAIs). According to the U.S. Centers for Disease Control and Prevention (CDC), HAIs cost the healthcare system more than $25 billion in treatment per year.
The CDC has recently sponsored programs to improve staff awareness and cleaning methods in healthcare facilities. Medicare is also incentivizing hospitals to take action by penalizing those with infection rates above the allowable limits. The improved cleaning regiments appear have been effective, since mortality rates in the US have dropped from 100,000 to 75,000 per year. However, it is also important to consider the design of medical equipment itself—especially devices that use fans and heat sinks, which can collect and distribute pathogens.
Focusing on Hygienic Equipment Design
Devices employing fan-cooled heat sinks can collect and transmit bacteria and viruses. HEPA filters are sometimes used to remove airborne pathogens that pass through heat sinks in medical devices. These filters are assumed to be impermeable to microbes since they have the ability to capture 99.97 percent of particles that are 0.3 microns in size. However, the manufacturers of these types of filters make no claims for particles smaller than this, since their efficiencies are dependent on airflow velocity and pressure. These filters rely on diffusion to capture particles below 0.2 microns. (See Figure 1)
The effectiveness of most filter capture methods decreases as airflow velocity increases, especially for the diffusion capture method. In addition, as airflow velocity increases, airborne droplets that contain pathogens break into smaller droplets and can even disagglomerate microbes that are bound together. This enables the pathogens to penetrate the filter and potentially infect people that are in proximity of the device. (See Table 1)
In the past, it was assumed that airborne transmission was limited to just a few feet. However recent studies have shown that airborne microbes can travel much further distances.
Understanding the Role of Thermal Management Technology in Hygiene
To dissipate heat from medical devices, medical device OEMs employ a variety of thermal management solutions that primarily fall into two categories: active and passive technologies. Most active cooling systems utilize forced air convection by incorporating fans that direct air through aluminum heat sinks. These components can function as fomites, surfaces that harbor viral and bacterial pathogens associated with HAIs. The airflow can eject these pathogens into the air and infect new patients, especially when these devices are moved from room to room. Unfortunately, many hospitals are still unaware of how fans within a device can contribute to HAIs.
In contrast, passive thermal management solutions use technologies that can reduce infection vector potentials. By definition, passive components have no moving parts and do not require a power source. Optimized thermal management systems can reduce airflow requirements to improve filter efficiency, thus reducing the bioburden associated with pathogens that can pass through medical device cooling systems. Passive technologies include heat pipe assemblies and vapor chambers, as well as the use of Annealed Pyrolytic Graphite (APG) components, which do not require fans or pumps.
Passive heat spreaders quickly transfer heat from concentrated, high-heat flux (W/cm2) sources inside the device to external heat sinks, remote liquid cooling lines or even to the outer wall of the enclosure. This isolates the interior of the device from the environment, eliminating the flow of air and microbes inside the device. The external components can be easily cleaned with disinfectants or even with UV light systems to prevent biofouling. Heat sinks can also be composed of or plated with antimicrobial materials.