Date: Nov 18, 2014 Author: Michael Bucci Source: Medical Developments (
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Advances in thermal management allow medical designers to keep products compact and quiet while addressing biological contamination concerns.
A governing principle of health care is "First, do no harm." For centuries, this saying has guided decisions between procedural risks versus potential benefits. The continuing drive toward increased surgical instrument functionality, reliability, and miniaturization has helped to mitigate the risk of harm, allowing more procedures to become justifiable.
Many of these cutting-edge medical tools are built on the advances in microprocessor computing power, laser precision, ultrasound transducer fidelity, and digital imaging technologies. However, with miniaturization packing these electronics into smaller spaces, technological advances can produce more waste heat. This creates a fundamental challenge for designers: to make electrosurgical instruments more effective in the operating room, waste heat must be dealt with safely to prevent inaccuracies or complications during and after procedures.
To reduce surgeon fatigue and improve surgical outcomes, designers are seeking to make their surgical tools lighter, easier to manipulate, and more reliable, especially for long or complex procedures. However, concentrated heat loads can cause performance variability, premature component failure, and even harm the patient.
To address these safety issues, the International Electrotechnical Commission (IEC) has published and updated the IEC 60601 third edition technical standard, which sets standards for the safety and effectiveness of medical electrical (ME) equipment. Included in the updated IEC 60601 standard are allowable temperature limits for medical equipment components that are likely to be in contact with patients or medical personnel.
The IEC 60601 standard defines both the maximum temperature for different types of materials and the amount of time that they can be in contact with the skin. This standard protects collateral tissue and the skin of the hospital staff, should they come into contact with the device. Thermal management solutions enable medical device designers to meet these temperature requirements under the most challenging conditions.
Thermal management
Thermal management, or heat transfer, is the engineering discipline that allows heat to be efficiently controlled to improve system reliability, speed, and precision while minimizing equipment dimensions, energy consumption, and noise levels.
Devices that require large amounts of airflow can contribute to biofouling, exacerbating the risk of infection by collecting pathogens in the device and then distributing them throughout the facility, becoming a source of health care-associated infections (HAIs). Advanced thermal management allows designers to maintain consistent temperature control and increase the power level of a device without having to increase airflow through the device.
Thermal solutions fall into two primary categories, active and passive, which have their own corresponding advantages.
Active thermal management generally includes pumped liquid cooling, refrigeration, and, most commonly, forced convection or fan-cooled technologies. Active systems provide effective cooling and are relatively easy to control.
Design advantages and considerations include:
Forced convection systems allow for adjustable cooling points, but fans can generate noise, accelerate fouling, require filter servicing, and are a potential failure point that can cause complications or downtime.
Liquid cooling systems (Figure 1) can handle high heat fluxes, enabling remote heat dissipation, and can be flexible to allow for maneuverability. However, they employ pumps that can generate noise and eventually fail. Precautions must also be made to mitigate the risk of leakage, as this could severely damage the electronics or cause contamination.
Most refrigeration systems use compressors to circulate a refrigerant or thermoelectric devices to lower temperatures below ambient and usually offer adjustable cooling rates. However, designers must keep in mind that refrigeration systems are potential failure points, may cause condensation issues, and actually generate more heat than they remove -- requiring the resultant heat to be dissipated through other means.
Passive thermal management can solve complex cooling problems in small spaces by minimizing the required airflow. This reduces the amount of dust, moisture, and debris that enters medical devices, thereby resolving many bioburden and noise concerns. Passive cooling technologies generally include heat pipe assemblies, vapor chamber assemblies, phase change materials (PCMs), and advanced solid conduction materials, such as encapsulated annealed pyrolytic graphite (APG).
Heat pipe assemblies transfer high heat loads in small spaces, typically reducing the size of heat sinks, providing remote heat transport and enclosure isolation. They also make natural convection practical by eliminating the need for cooling fans. Sintered powder-wick 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 freeze-thaw tolerant. To enable more flexible design choices, heat pipes can be less than 2mm in diameter and can be formed to fit in complex spaces.
Vapor chambers are planar or flat versions of heat pipes. Shown in Figure 2, vapor chambers feature 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 1mm 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.
PCMs begin cooling at specific temperatures, so they can temporarily prevent a component from exceeding its temperature limit. They are also easy to integrate into medical device designs because they absorb heat and do not require access to ambient air or liquid cooling lines. However, they eventually become saturated and begin to act as insulators, which can aggravate thermal issues even further.
Cooling compact instruments
While large-scale diagnostic medical devices generate high heat loads that generally use active thermal management, smaller portable or mobile devices are in most cases best served with passive thermal management.
Electrosurgical instruments: Surgical tools often have electrode tips that create or cauterize incisions within a patient during a procedure. These electrode tips generate heat, which must be properly dissipated. In addition, designers of these types of tools seek to minimize size and weight to improve maneuverability. Use of heat pipes or encapsulated APG provides an effective solution.
Lasers: Laser-based systems often incorporate an active fan-cooled heat sink assembly to dissipate heat from the diode, but they also require quiet operation and typically need to be portable. Heat-pipe heat-sink assemblies reduce the required airflow, minimize noise levels, improve laser diode performance, and extend equipment life. For high power lasers, advanced liquid cooling technologies such as micro-channel cold plates and Therma-Cubes can manage heat fluxes of up to 12,000W/cm².
Diagnostic equipment: Automated biological analysis tools require precisely controlled operating temperatures. 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. The use of embedded heat pipes, vapor chambers, and APG cold plate technologies have enabled this type of control.
A promising new material for medical applications is encapsulated APG, which offers design versatility because it can be paired with many different biocompatible materials. APG 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 provides consistent performance, both with and against gravity, giving designers considerable flexibility in creating medical device solutions.
Design advantages and considerations include:
Complex cooling problems in small spaces are easily handled, offering medical device OEMs greater design freedom. However, the conductivities that these solutions offer can vary with length and direction.
A reduction in the risk of bioburden issues associated with fans and other active devices aids in preventing HAIs infections. However, sufficient surface area on the device must be available to conduct heat away locally or at a remote point.
Eliminating power sources reduces a device's size, weight, and power consumption (SWAP). The lack of moving parts reduces service costs and extends equipment life, yielding long-term savings. Consistent levels of isothermality enable designers to satisfy the precision requirements of the most cutting-edge medical devices.
Choosing the solution
There is no panacea that solves all thermal management issues. Ultimately, the performance requirements, operating conditions, size/weight constraints, noise limitations, and touch temperature limits provide the correct framework for choosing between available thermal technologies.
In an effort to contain costs and shorten development time, many designers initially search for off-the-shelf products or try to develop a thermal solution on their own. This may work in some cases but often involves spending a lot of time evaluating a multitude of variables and options. A time-consuming trial and error process may or may not lead to a suitable solution.
Partnering with experts in their respective fields allows designers to get it right the first time, saving time, reducing development risks, and meeting project requirements. This process also identifies design provisions early enough to accommodate changes.
An expert in designing and manufacturing thermal management solutions can help resolve whether an off-the-shelf or custom solution is the most appropriate option for a medical device and can provide insight on the quality management systems and good manufacturing practices that can turn a design into reality.
Thermacore Inc.
www.thermacore.com
About the author: Michael Bucci is a medical market development manager at Thermacore Inc. and can be reached at m.s.bucci@thermacore.com.
