By Andrew Dereka, Product Director, Laird Technologies
Medical lasers are designed for use in hospitals, outpatient surgical centers, and physician offices. They combine cutting, ablation, and coagulation properties for precise, virtually bloodless procedures, minimizing thermal damage to the surrounding tissue and reducing recovery time. They also sanitize the area through the heat of the laser, destroying any microbiological bodies that could lead to infection. Although medical lasers are valuable in many medical treatment applications, they do generate waste heat that affects their performance when in operation. They also have size constraints, power consumption requirements, and noise restrictions that make thermal management difficult.
Thermoelectric assemblies (TEAs) are cooling and heating systems that use thermoelectric modules (TEMs) to transfer heat by air, liquid, or conduction methods that include integrated temperature controls. TEAs remove the passive heat load generated by the ambient environment in order to stabilize the temperature of sensitive components used in medical lasers.
TEMs are solid-state heat pumps that require a heat exchanger to dissipate heat via the Peltier effect. During operation, DC current flows through the TEM to create heat transfer and a temperature differential across the ceramic surfaces, causing one side of the TEM to be cold while the other side is hot. A single-stage TEM can achieve temperature differentials of up to 70°C and transfer heat at a rate of up to 150 watts. In order to increase the amount of heat-pumping capacity, the TEM’s modular design allows for the use of multiple TEMs mounted side-by-side, which is called a TE array.
TEMs are composed of two ceramic substrates that serve as electrically insulating materials and house P-type and N-type semiconductor elements. Heat is absorbed at the cold junction by electrons as they pass from a low-energy level in the P-type element onto a higher-energy level in the N-type element. At the hot junction, energy is expelled to a thermal sink as electrons move from the high-energy element to the lower-energy element.
Reversing the polarity changes the direction of heat transfer. TEMs are rated at maximum parameters (∆Tmax, Imax, Vmax, and Qmax) under no-load conditions, with temperature control accuracy achieving ±0.01°C under steady-state conditions. They can cool to -100°C (6-stage) and pump up to 15 watts per cm2 of heat, with higher heat-pumping capacities achieved by wiring TEMs into an array. Their geometry can vary from 2mm x 2mm to 62mm x 62mm and are much more efficient in heating mode than resistant heaters. They also fit into tight geometric space constraints and can be mounted in any orientation that cannot accommodate a much larger compressor-based system.
Thermoelectrics provide good temperature stabilization to maintain peak performance of a medical laser and offer solid-state operation, low maintenance, and long service life. Thermoelectrics also make an excellent thermal management solution due to compact size no vibration and low total cost of ownership. This cannot be accomplished by any other means without a complex heating and cooling system.