Many medical applications require heat for optimal performance. For patient comfort, effective treatment, and a variety of diagnostic processes, equipment and fluids must be raised to, or kept at, specific operating temperatures. Medical heating applications can have strict requirements including fast warm up, uniform heating both over time and across a surface, compact design, and high reliability.
Traditional approaches to thermal control couple some sort of heating technology with external sensors and controllers. While these multicomponent solutions can provide the necessary heat and thermal equilibrium, they can be expensive, bulky, and complicated to assemble. They can also have potentially vulnerable points of failure that negatively impact efficacy of the end product. In addition to the risks of heating loss due to breakdown, these designs can be subject to dangerous overheating in the case of malfunctions such as delamination.
The ideal heater for many of these medical applications would be one that is self-regulating without the need for external sensors or controllers. It would heat quickly, reduce power as the target temperature is neared, and cut power entirely before the target temperature is exceeded to eliminate the risk of overshoot. It would do all this in a compact package without additional connections that could complicate assembly or break down in the field.
Self-Limiting Heater Technology
There is a technology that can do all of this, eliminating the need for both sensors and controllers by integrating those functions into the thin film of the heater itself, controlling temperature at the molecular level by modulating power at an infinite number of points across the heating surface. The heater film consists of a thin silicone matrix loaded with conductive carbon particles. Electrical current moves between the carbon particles via quantum tunneling of electrons through the non-conductive silicone material.
The electrical resistance of the material and the current it can carry at any point over its surface is determined by the spacing between the carbon particles, and that spacing changes with temperature.
As the silicone warms, thermal expansion drives the carbon particles farther apart and increases resistance point-by-point over the surface of the heater; at a designed set point temperature, the heater effectively becomes an electrical isolator, draws negligible current, and no longer produces heat. Conversely, if the silicone cools via environmental or load variations, the carbon particles pull closer together, reducing resistance, allowing current to flow and the heater to produce heat as needed to maintain thermal equilibrium.
As a result, the technology works without external instrumentation and control systems. By controlling the composition of the carbon-silicone matrix in production, the heater is designed to approach but not exceed a specific temperature set point. When the heater is powered, it quickly warms to its designed temperature set point and maintains that temperature within a narrow band.
Simpler in Design. This technology is of particular value in medical device development for several reasons. First, it is far simpler than traditional heating technologies. With no external sensor or controller, it performs both of those functions at the molecular level within the heater itself. The heater is just 0.40 mm thick and can be integrated into an end product using just two connections.
Figure 1 shows that with fewer connections and no need for external components, this technology simplifies a product developer’s bill of materials. At the same time, with fewer potential points of failure, it provides a more robust installation. As an all-in-one heater, this technology requires virtually no maintenance and is far less likely to require repair in the field than a traditional heater and control system. No user programming is required because the technology is manufactured to maintain a specific temperature set point within the heater itself.
Stable in Operation. In medical applications, reliable temperature control can be critical, and temperature control with this technology is extremely reliable. From a cold start, the carbon-silicone matrix initially provides maximum power to speed warm-up to its designated operating temperature. As the heater approaches its set point, its resistance increases and current draw approaches zero to slow heating.
Once thermal equilibrium is achieved, the unit responds to any changes in the environment to maintain the target temperature across the entire device surface. The heater adjusts power at each point across its surface to maintain uniform temperature. These intrinsic features allow the heater to quickly achieve and maintain thermal equilibrium while preventing undesirable temperature overshoot that can occur in traditional control systems.
Set point temperature is reliably maintained as power is automatically adjusted in response to variations in ambient environment temperature. The technology is self-tuning in dynamic environments and maintains the target temperature across the entire device surface even if the heat sink exhibits variation. If, for example, one region of the heat sink surface is cooled more than the rest, the heater increases power output in that region only to maintain even temperature.
This is very different from a conventional resistive heater, the power output of which is typically controlled by a single sensor feedback loop. Similarly, where a conventional heater would have to be “zoned” (designed with specific regions of differing power density) to handle differences in heat loss across its surface, the carbon-silicone matrix responds to these differences via point-wise power modulation.
Safer in Use. In many medical applications, failure is not an option. When heating is lost, critical, even life-saving, processes can be affected. This risk is greatly reduced with a this type of self-regulating heater. Even if the heater is physically damaged, it will rarely be affected at any point beyond the location of the damage. Figure 2 shows a heater with a hole punched through the body. The accompanying infrared photo shows that the heater is continuing to function over most of its surface. In most cases, this provides enough heat to maintain the device’s designed function.
The opposite of heating loss can also happen; a heater can delaminate or otherwise fail resulting in dangerous overheating that can damage equipment or hurt patients, for instance, in a medical fluid warming application in which fluid may not always be present. When there is no fluid to warm, a traditional heater can easily overheat. A self-regulating heater, on the other hand, simply reduces its power output wherever a thermal load (i.e., fluid) is not present.
Overheat protection applies to each point across the heater surface. Because current flow is controlled locally at the molecular level, even if the heater delaminates — pulling away from the heat sink — current to the delaminated segment is reduced to prevent even local overheating, protecting both the heater itself and the surrounding materials. These capabilities make this technology a safe solution for medical applications in which heater failure or overheating must be avoided.
Heater Construction. Design construction of the self-limiting heater does not differ greatly from that of a traditional thin film heater. The difference is the material used for a self-limiting, “smart” heating element. The multilayer construction of a SmartHeat SLT heater, for example, begins with the carbon-silicone matrix itself. By adjusting the density of carbon particles within the silicone layer, the material is manufactured to have a specific temperature set point. When a set point temperature is reached, the carbon particles are spaced such that the heater becomes an electrical isolator, preventing any chance of thermal runaway. If temperature drops below the set point, current flow to that area resumes maintaining the set point temperature.
Figure 3 shows how the self-limiting matrix layer is sandwiched between a polyester layer, which provides mechanical support, and an etched copper element that carries current to the carbon-silicone layer. These layers are then encapsulated with polyimide dielectric layers and dry-film adhesive to protect and electrically insulate the heater from its surroundings.
In operation, each point on the carbon-silicone layer draws current as needed from the etched copper element to maintain temperature set point. The resulting stack of layers is only about 0.40 mm thick and can be produced in a variety of shapes, sizes, and temperature set points. While incorporating the functions of heat, sensing, and control, the heater requires just two leads to connect to the power supply.
Operating Parameters. These self-limiting heaters can operate in a wide range of environments from –40° to 100 °C. Actual operating temperature of the heater itself can range from 10° to 70 °C, and typical batch temperature set point variation is ±7 °C. Heaters can be produced in sizes up to 7 × 22 in.; larger heating surfaces can be achieved using a stitching manufacturing method.
Advantages over Traditional Heating Technologies
If the application can accommodate a ±7 °C temperature control tolerance of a self-limiting heater, this heating option offers significant advantages over traditional heating technologies. First, it greatly simplifies design, thereby shortening the product development process.
Medical device product developers need to make sure that the heater size, shape, and temperature set point meet their design requirements; there’s no need to accommodate additional sensing or control components or calibrate heat loss in the design process. The heater maintains the target temperature both over time and across the heater surface regardless of varying rates of heat loss, making it an ideal technology for creating compact, reliable, highly responsive end products.
Second, manufacturing is simplified without the need for additional components or connections. This integrated heater design reduces the costs in both component purchasing and assembly. Additionally, this technology eliminates complexity that can lead to breakdown or the risk of overheating, which can damage the heater and surrounding materials.
Moving maintenance of thermal equilibrium from external control to self-regulation within the heater itself simplifies front-end design, guarantees operational stability, and ensures safety, making such heating technology ideal for high-stakes medical device applications.
This article was written by Jack Stangl, Applications Engineer, and Joe Wagner, Design Engineer, for Minco, Minneapolis, MN. For more information, visit here .