What is the application?
Is the device heating air/gas, fluid or solid material?
What is the process temperature?
What is the temperature ramp rate?
What are the dimensional requirements?
Are there any known material compatibility issues?
Are there any voltage limitations?
Are there any special agency approval requirements?
What is the heater life expediency?    

Depending on the design requirements, a decision can be made as to whether the best thermal solution is convection, conduction, radiated or a combination of heat transfer methods. Convection heating is a mode of heat transfer in gas or liquid in which heat is transferred through movement of masses of the fluid from a region of higher temperature to one of lower temperature. Conduction heating is a mode of heat transfer within a body or between bodies that are in contact. Radiated heating is a mode of heat transfer where energy is emitted in the form of waves or particles. A heater needs to be sized correctly for optimum performance. Dimensional requirements are usually governed by the allowable space in the device. Portability and space requirements of equipment are factors driving many of today’s design decisions. For the heater designer, this usually means less space is available for the thermal solution and that drives design decisions toward an integrated solution rather than just a traditional heater component. For example, an integrated syringe heater assembly might include a foil heater element embedded in Lexan® (polycarbonate) housing, temperature sensors, temperature controller and a high limit controller in a compact reusable assembly that can be snapped on to a fluid delivery system.  
Designing heaters for intend use is extremely impor tant. Some application requirements are for 24/7 operation while others are disposable and intended for one-time usage. What both applications have in common is patient well being, including safety, comfort and performance results.    

Reducing size and cost
In respiratory therapy equipment, the heater(s) may be used to heat air, water for humidification and medication. The heaters are part of the base unit and expected to have years of reliable operation. Conversely, heaters used in colonoscopy procedures may be part of the point of use device and therefore disposable. While patient well being is critical in both applications, the robustness, long-term reliability and cost of the respiratory therapy heater will be different than the one-time use colonoscopy heater.    

For obvious reasons, patient safety is paramount in equipment and heater design (syringe heater)

By understanding the product and agency requirements, there are many heater alternatives that simplify medical device design and procurement while reducing size and cost. A good example is meeting the safety requirements for low leakage current in medical devices. To summarize: U.S. leakage current deviation is based on the values and requirements of NFPA 99, “Health Care Facilities” and the ANSI/AAMI “Safe Current Limits for Electromedical Apparatus” standards. The differences from IEC 60601-1 modify the acceptable passing limits for the earth and enclosure leakage tests, and maintain the same values for the patient leakage tests.
The base IEC 60601-1 standard does not directly differentiate between inside and outside the patient environment. IEC 60601-1-1, “Medical Electrical Systems,” which addresses a combination of several pieces of equipment, does make a distinction between inside the patient environment and outside it with respect to leakage current testing. UL® 60601-1 differentiates between patient-care equipment (6 ft around and 7.5 ft above the patient) and non-patient-care equipment for these leakage current tests. In UL® 60601-1, the leakage current values are specified in Tables 19.5DV.1 and 19.5DV.2. These values are given as:    

Class I product (typical value) = 300 µA patient-care area
Class I product (typical value) = 500 µA non-patient-care area    

UL® 60601-1 allows opening of the ground conductor and one of the supply connections simultaneously for non-patient-care equipment. In most cases, the following is true: The earth leakage current test per UL® 60601-1 provides the worst-case conditions within the patient area, whereas the enclosure leakage current test per IEC 60601-1 is the worst-case test in the normal condition.
To meet these low current leakage safety requirements, many device manufacturers have resorted to using a step down isolation transformer and low voltage supply. The step down isolation transform adds significant cost, space and weight to the equipment.  An alternative design approach would be to use a low current leakage heater that could eliminate the isolation transform and save cost and space. Advanced ceramic heaters with aluminum nitride (AlN) as a base material and a dielectric strength of >15KV/mm can be designed to meet the leakage current requirements.  Medical equipment such as renal dialysis, insufflators, lithotripters and thermal abrasion could benefit from the performance and cost benefits of AlN heaters.  Royce Payton, Watlow    

German Summary
Elektrische Heizelemente können zur Verbesserung des Betriebes von verschiedenen Arten medizinischer und diagnostischer Gerätschaften beitragen. Aus offensichtlichen Gründen hat die Sicherheit des Patienten bei Geräte- und Heizelementkonstruktion erste Priorität. Zu beachten sind dabei unter anderem sicherheitsrelevante Aspekte der Elektrizität (geringer Leckstrom), von Feuer, Gas oder Ausgasungen, sowie die Temperaturstabilität. Medizinische und diagnostische Geräte sind durch Zulassungen wie VDE, FDA, UL® und CE abgesichert. Das Ziel aller Beheizungslösungen ist es, die bestmögliche Leistung für das medizinische Gerät zu liefern, für welches es verwendet werden soll. Um die optimale Konstruktion liefern zu können, muss die thermische Anforderung und das Gewerbe verstanden werden. Nur so kann das beste Servicepaket für den Gerätehersteller geschnürt werden. Der deutschsprachige Beitrag ist nachzulesen auf: www.meditec-international.com/medi0611heat